Protic Ionic Liquids: Evolving Structure–Property Relationships and

Review pubs.acs.org/CR

Protic Ionic Liquids: Evolving Structure−Property Relationships and Expanding Applications Tamar L. Greaves* and Calum J. Drummond School of Applied Sciences, College of Science, Engineering and Health, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia S Supporting Information *

8.10. Biosensor 8.11. Electrochemical Method for Water Detection in ILs 8.12. Other Electrochemical Applications of Protic Ionic Liquids 9. Chromatography 10. Liquid−Liquid Extraction 11. Gas Capture 12. PILs as Solvents for Biological Media 12.1. Biomass Conversion 12.2. Extraction of Bioactives 12.3. Biocatalysis 12.4. Protein Crystallization 12.5. Protein Stabilization 13. Pharmaceutically Active PILs 14. Toxicology 15. Inorganic Synthesis 16. Inorganic Particle Stability in PILs 17. Catalysis and Organic Synthesis 17.1. Protection/Deprotection Reactions 17.1.1. BOC Protection of Amines 17.1.2. Deprotection of Acetals and Ketals 17.2. Addition Reactions 17.2.1. Hetero-Michael Addition 17.2.2. Synthesis of Diphenylmethyl Ethers 17.3. Cyclic Forming Reactions 17.3.1. Diels−Alder 17.4. Ring Cleavage 17.5. Heterocyclic Synthesis 17.5.1. Friedländer Annulation 17.5.2. Biginelli Reaction 17.5.3. Beckmann Rearrangement 17.5.4. Three-, Four-, or Five-Component Heterocyclic Forming Reactions 17.5.5. Other Heterocyclic Forming Reactions 17.6. Condensation Reactions 17.6.1. Baeyer Condensation 17.6.2. Knoevenagel Condensation 17.6.3. Aldol Condensation 17.6.4. Three-Component Condensation 17.6.5. Other Acid-Catalyzed Reactions 17.7. Desulfurization 17.8. Baylis−Hillman Reaction 17.9. Oxidation

CONTENTS 1. Introduction 2. PIL Formation and Intermolecular Forces 2.1. Synthesis, Ionicity, and Proton Transfer 2.2. Intermolecular Forces 2.3. Vapor Phase 3. Physicochemical Characterization 3.1. Proton Activity 3.2. Polarity 3.3. Conductivity 3.4. Activity Coefficients 4. New Subcategories of PILs 4.1. Fluorinated PILs 5. Solvent Mixtures Containing PILs 5.1. Effect of Molecular Solvents on Applications of PILs 5.2. Mixtures of PILs with ILs 6. Nanostructure of Protic Ionic Liquids 6.1. Bulk Nanostructure/Mesostructure of Ionic Liquids 6.2. Nanostructure of Ionic Liquids at Interfaces 7. Amphiphile Self-Assembly 7.1. Micelles 7.1.1. Micelles in EAN 7.1.2. Micelles in Other PILs 7.2. Lyotropic Liquid Crystal Phases 7.3. Microemulsions 8. Electrochemistry 8.1. Effect of Water 8.2. Electrochemical Windows 8.3. Electrolytes for Capacitors and Supercapacitors 8.4. Electrolytes for Batteries 8.5. Electrolytes for Fuel Cells 8.6. Ionic Conductive Membranes Containing PILs for Fuel Cells 8.7. Catalysts for Fuel Cells 8.8. Electrodeposition 8.9. Electrochemical Exfoliation © 2015 American Chemical Society

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Received: March 25, 2015 Published: October 1, 2015 11379

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Chemical Reviews 17.10. Baeyer−Villiger Oxidation 17.11. Reduction 17.11.1. Pinacole Rearrangement 17.11.2. Deprotection 17.11.3. Decarboxylation 17.11.4. Aromatic Nitration 17.11.5. Formylation 17.11.6. PILs as Intermediates 17.11.7. Substitution 17.11.8. PIL Modified Catalysts 17.12. Synthesis of Carbon Materials 18. Polymers 18.1. Polymerization in PILs 18.2. Polymerizable PILs 19. Lubricants 20. Use of PILs as Explosives 21. ILs Used for Space Propulsion Applications 22. High-Throughput Techniques 23. Concluding Remarks Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations Cations Anions Surfactant References

Review

use of high-throughput methodologies for synthesis and characterization of PILs, characterization of their toxicology, their use for green house gas capture, liquid−liquid extraction, and in polymer synthesis as solvents or precursors. There has been a continued strong interest in the use of PILs as solvents and catalysts for organic synthesis, and as nonaqueous amphiphile self-assembly solvents. There has also been a broad range of ab initio and molecular dynamics computational techniques used. A significant increase in the number of publications has occurred on PILs in a variety of electrochemical fields, such as their use as electrolytes for fuel cells, batteries, capacitors or supercapacitors, and in a variety of biological applications, such as biocatalysis, protein stability, and protein crystallization. The thermal and physico-chemical bulk properties were rigorously covered in the last review, and while we have provided all of the experimental data, extensive structure−property relationships for these fundamental properties have not been discussed because it would largely be a repeat of what was previously covered. Instead, in this Review, we have focused on characterization that includes their intramolecular forces, proton-activity, ionicity, and polarity, along with highlighting new categories of PILs. There have been a few reviews including PILs, although generally the focus is on aprotic ILs with a subsection on PILs. These include an account that focuses on the work of Angell et al. comparing PILs and AILs,2 a perspective into interfacial IL nanostructure,3 a perspective into potential industrial applications of PILs,4 a review of Brønsted acids in ILs, and their role in organic reaction,5 extensive reviews into catalysis in ILs6 and biocatalysis in ILs,7 a small section on PILs as nonaqueous solvents in membrane fuel cells,8 a review into the use of ILs for protein stability, protein crystallization, and protein extraction,9 a short review on nonaqueous micelles and microemulsions containing ILs,10 and a short review into a broader range of selfassembled amphiphile structures in ILs.11 The field of PILs is no longer dominated by ethylammonium nitrate (EAN), and instead there are a plethora of cations and anions that have been used, leading to PILs with a very broad range of properties. In this Review, we have included descriptions of the more unusual new classes of PILs, such as those containing phosphonium cations, diamino cations, allyl chains on the cation, fluorocarbon carboxylate anions, or super acids or super bases. There is also now a stronger focus on the use of PILs for specific applications, and in part this has led to an interest in the properties of ionic liquids when mixed with an additional solvent, or other solute. While we predominantly focus on PILs throughout this Review, where relevant we have also included protic molten salts with melting points above 100 oC, and protic salts where there is a nonstoichiometric proportion of acid to base. In addition, for many applications, the PILs are used as additives, and in these situations can most accurately be considered as a protic salt. In this Review, we primarily cover the literature between early 2007 and mid 2014, which follows on from our last review on PILs. Some key properties of neat PILs are covered initially, along with those of mixtures of PILs with other solvents. The liquid nanostructures of bulk PILs and of PILs at interfaces are described, along with those of various PIL−solvent and PIL− solute systems. This is followed by applications where PILs have been utilized, including in organic and inorganic synthesis, biological applications, electrochemical applications, lubrication, and as amphiphile self-assembly media.

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1. INTRODUCTION The field of ionic liquds is growing significantly, with an increased focus on developing ionic liquds for specific applications, along with fundamental research. By definition, ionic liquids (ILs) are salts with a melting point below 100 oC, whereas salts with higher melting points are classified as molten salts. It has become well accepted that there is a very large number of cations and anions that have the potential to be used in ionic liquids, and hence they are highly tailorable solvents, and through careful selection of ions a broad range of properties can be accessed. Protic ionic liquids (PILs) are a subset of ILs that are prepared through the stoichiometric neutralization reaction of certain Brønsted acids and Brønsted bases. A key feature of PILs is that they have an available proton on the cation. By convention, the rest of the ILs can be classified as aprotic ILs (AILs), although there are some other subsets that are similar to the PILs such as Brønsted acidic ILs, which are functionalized such that they have an available proton, typically on the anion. In 2008, we published a review titled “Protic Ionic Liquids: Properties and Applications”.1 Since then, there has been an extensive increase in the number of publications, including many new areas of research, and it was considered timely to provide an updated comprehensive review. We have endeavored to not duplicate what was previously covered in the 2008 review, but instead to focus on new areas of research. Emerging areas within the field of ionic liquids in the last few years include the characterization of their liquid nanostructure in the neat form, and in mixtures with other solvents or solutes, the 11380

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2. PIL FORMATION AND INTERMOLECULAR FORCES An inherent characteristic of PILs is the simplicity of their synthesis through proton transfer from a Brønsted acid to a Brønsted base. However, depending on the specific acid and based used, this proton transfer may be only partial, such that there is a portion of neutral molecular precursors remaining in the solution. Consequently, there has been considerable effort conducted into quantifying and understanding fundamental characteristics of PILs, such as their degree of proton transfer, polarity, ionicity, and related properties such as conductivity.

present, and for some acid−base pairs the biphasic mixture could be converted to a homogenous product by heating above the melting point. The biphasic mixture of triethylamine or tributylamine with acetate was characterized as consisting of an acid-rich salt solution on the bottom with a predominantly unreacted amine layer on top.16,18 It appears that the acetate anion does not always favor the 1:1 stoichiometry, which is consistent with it forming biphasic mixtures with various amines under certain synthesis conditions as outlined above. In addition, the combination of N-methylpyrrolidine with acetic acid was shown to have low ionicity for the 1:1 salt and highest ionicity for the 3:1 mole ratio of acid:base, which we tentatively suggest may be similar in composition to the bottom layer of the biphasic solutions.19 This raises the question: Can these salts with acid:base ratios other than 1:1 be classified as a protic ionic liquid or are they only protic salts? While ILs are often described as consisting solely of ions, their ionic conductivity is often less than expected if this was true. Instead, a proportion of the ions may be present as ionpairs or aggregates, and for PILs there may be some neutral molecular precursors present, all of which lead to a lower ionic conductivity. The ionicity of ILs has been proposed by MacFarlane et al. to be “the effective fraction of ions available to participate in conduction”.20 The ionicity was discussed by Watanabe et al. for ionic liquids, specifically in comparison to their solvent polarity and physicochemical properties, although there was limited information regarding PILs.21 A comparison for protic and aprotic imidazolium ILs showed that the ionicity followed EMIm TFSI > EMIm TfO ≈ EIm TFSI > EIm Tfo, where the lower ionicity in the PILs was suggested to be due to stronger ion−ion interactions, and on addition of water the ionicity of the PILs increased.22 The distinction between PILs and protic salt solutions is discussed by Canongia Lopes and Rebelo in terms of azeotropy, where the proportion of acid:base in the vapor phase is identical to that in the solution phase.23 In particular, for “weak” PILs, it can be expected that there will be a significant proportion of neutral species, which upon drying or degasification of the solution will lead to a solution that is unlikely to be stoichiometric.23 The reversible proton transfer process present for PILs can be exploited, and it has been shown that some PILs can be developed into ionic media buffers through addition of a nonionic neutral species, which can be converted into the IL through exchange of the counterion,24 for example, through the addition of the protonated form of the anion.24 A new class of PIL-like compounds was reported by Johansson et al., which were described as anion amphiprotic and involve the acid−base neutralization of methylcarbonate with trifluoromethanesulfonamide.25 These do not contain an acidic proton, and are more chemically stable as compared to conventional PILs, through not having a reversible reaction back to the neutral precursors.

2.1. Synthesis, Ionicity, and Proton Transfer

In general, the synthesis of PILs is a simple neutralization reaction with addition of a Brønsted acid to a Brønsted base. The resulting solution is then dried to remove any solvents present, such as water, to form the PIL. It is possible for the proton transfer to be incomplete, leading to some remaining molecular species. In addition, the drying processes are likely to remove more of one ion than the other in the PIL, leading to a nonstoichiometric mixture, enriched in the less volatile precursor species. There is a strong correlation between proton transfer from the acid to the base and the pKa difference between the acid and base precursors.12 However, when the dye of phenol red was used to determine the protonation state of some amine precursors, it was observed that the primary amines were able to deprotonate the indicator acid to a greater extent than the tertiary amines, despite all having comparable pKa values.13 The effect of using a weak or strong acid was investigated using the HMIm cation paired with acids of various acidities. The resulting stoichiometric mixtures were nearly completely ionic for strong acids such as Tf2N and TfO. The less acidic TFA led to a predominantly ionic mixture, whereas the weaker acids of formic and acetic had poor proton transfer,14 while still having proton conductive behavior, and it was proposed that they should be defined as “pseudo-protic ILs”.15 An alternative synthesis method has been reported by Burrell et al., where the acid and base precursors were dried before use, and added stoichiometrically to a rapidly stirred reaction vessel.16 It was noted that conducting the synthesis in this manner minimized the exothermic heat, and through sufficiently slow addition of the precursors enabled heat dissipation during the reaction.16 These dry PILs were investigated using 15N NMR to show that proton transfer had occurred, and that the species were all ionic. However, many were in the low-ionicity region of the Walden plot, which consists of the log (equivalent conductivity) plotted against the fluidity, and hence it was suggested there must be a significant proportion present that are electrically neutral, such as ionpairs.16 This is a good example that the determination of the ionicity and proton transfer within PILs is complicated by the hydrogen bonding, ion-pairs, and mesostructure present in PILs. Similarly, in the series of ethyl-, propyl-, and butylammonium nitrate, the propylammonium nitrate was determined to have a higher ionicity than expected, which was attributed to the odd number of carbons in the chains not segregating to the same extent as would be expected for the even numbers of carbons present in EAN and butylammonium nitrate.17 Of the series of 48 acid−base combinations prepared by Burrell et al. from dry precursors, only 18 formed homogeneous liquids and 12 formed biphasic mixtures.16 The biphasic mixtures did not occur when an additional solvent was

2.2. Intermolecular Forces

The interactions between the cations and anions in PILs consist of a mixture of hydrogen bonding, Coulomb charge, and dispersion forces, and have been investigated using a variety of spectroscopic techniques and theoretical calculations. A mini review on hydrogen bonds in ionic liquids by Dong and Zhang in 2012 has a focus on aprotic ILs, although it also includes PILs.26 11381

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the protic IL TMG acetate, along with the aprotic ILs of BMIm acetate and 1-butylpyridinium acetate, it was shown that the PIL had the highest cohesive energy density and strongest ion packing, along with more restricted motion of the ions.37 The anions and cations were shown to move together, indicating that they may travel as ion pairs or in clusters.37 Similarly, the NMR relaxation and diffusion of a variety of PILs indicated there was ion correlation between the cation and anions, although no evidence of significant alkyl chain aggregation was observed for PILs containing alkyl chains with less than four carbons.38 A series of PILs containing an imidazolium-[1,2a]-pyridine hydrogen cation paired with various anions were observed to have an ionicity based on the anions that followed hydrogen sulfate > oxalate > phthalate > pimelate > benzene-1,2dithiolate. The presence of the aromatic ring attached to the imidazolium was observed to increase the cohesive energy of these PILs as compared to their imidazolium counterparts.39 The stability of hydrogen-bonded aggregates within some protic ionic liquids has been studied by using electrospray ionization mass spectroscopy,40 and through thermochemistry calculations.41 The number of ions present for the most stable aggregates varied for different PILs, with EAN having the cluster of 8 cations and 7 anions being the most stable.40,41 MD simulations and high energy X-ray diffraction were used to investigate ethyl-, propyl-, and butylammonium nitrate. These were all observed to have distorted NH···O hydrogen bonding with an angle of ∼120°, which was attributed to the nitrate anion preferring to bond with the ammonium headgroup on the cation, even though the alkyl chain aggregation caused the bond distortion.42 The solvation of fullerene, as an uncharged nonpolar solute, was compared in EAN and BMIm BF4, showing that the fullerene weakened the ionic interactions for both ILs, although more strongly in BMIm BF4. Both ILs formed highly structured layers of solvent around the fullerene molecules, with these extending for larger distances in EAN or >1.8 nm.43 The different solvation properties of water and EAN toward ZnCl2 were investigated using EXAFS. In this study, it was observed that the ZnCl2 is fully dissociated in NaCl aqueous solutions, forming hydration complexes, whereas in EAN the interactions between the cation and anion are maintained, and not influenced by the presence of ZnCl2, and it is proposed that there is little change to the EAN structure on dissolving the salt.44

The combination of far infrared spectroscopy (FIR) and DFT calculations has been used to gain insight into the interactions between ions in PILs. PILs containing the singly hydrogen-bonding triethylammonium cation were used to confirm that the band at 150 cm−1 provides an estimate of the cation−anion interaction along the +N−H···O− bond.27 The FIR of EAN, propylammonium nitrate, and dimethylammonium nitrate indicates that all three have comparable interaction strengths.28 Similarly, the N−H proton chemical shift using 1H NMR and FTIR for the N−H bonds can be used to provide information on the hydrogen bonds between the cation and the anion. A series of PILs containing a range of ΔpKa values between their precursor acid and base were characterized, and a correlation was found between the strength of the hydrogen bonds present and the temperature, as well as the ΔpKa.29 FIR of EAN has also shown that while EAN can form a three-dimensional structure through hydrogen bonding, it is unable to form a tetrahedral structure like water.28 Dielectric relaxation spectroscopy and femtosecond IR of the N−H vibration in EAN indicate that the EA+ ion rotations are through angular jumps, consistent with it being a strongly hydrogen-bonded liquid.30 For monomethylammonium nitrate, Car−Parinello MD simulations showed that 1.8 of the 3 possible hydrogen bonds possible are present, with the free acceptor and donor sites making it similar to water, as is the fast fluctuating hydrogen-bond network.31 Like the triethylammonium PILs, monomethylammonium nitrate has a different structural arrangement of hydrogen bonds to water.31 Quantum chemical calculations were used to assess the various hydrogen bonds possible for MIm dicyanide, which has multiple bonding sites. It was determined that the N−H···N interactions are much stronger than the C−H···N interactions between the cation and the anion.32 Investigations of MIm Tf2N using FTRaman and FTIR showed that in the liquid state it probably forms ion pairs that are connected through N−H···O bonds from the nitrogen in the imidazolium ring on the cation to the oxygen on the anion.33 The hydrogen bonds present in PILs vary between strong linear hydrogen bonds and weaker bent ones. The angles of the hydrogen bonds are a consequence of the PIL nanostructure due to alkyl chain segregation. As the proportion of linear hydrogen bonds increases, the ionic liquid becomes more solidlike.34 It was shown that the hydrogen bond strength followed EASCN > EAHS > EAN > EAF, which correlated well to their melting and glass transitions.34 The cation−anion interaction was also investigated using far FTIR, in conjunction with DFT calculations. These showed that the hydrogen bond had a significant effect on the interaction energy, contributing over one-half of the interaction energy for propylammonium nitrate, and lesser amounts for aprotic imidazolium ILs also investigated.35 The Coulomb interactions contributed the larger proportion of the interaction energy for the aprotic ILs, and it was clearly shown that depending on the cation and anion, this ratio between hydrogen bonding and Coulombic interactions could be modified substantially.35 Ab initio calculations were reported by Watanabe et al. for 11 ion pairs of protic and aprotic ILs, where the PILs were observed to have stronger interactions, which were more directional and electrostatic in origin. The most stable structure present for the PILs involved the anion in contact with the N− H of the cation.36 In a molecular dynamics investigation into

2.3. Vapor Phase

While ionic liquids are often referred to as nonvolatile or having negligible vapor pressure, this is not always true. For example, some PILs can be distilled, as reported by Angell et al. in 2003,12 and many boiling points have been reported for PILs,12,45,46 which is evident in Table S1. However, there is conflicting evidence in the literature for whether the gas phase of PILs consists only of the molecular neutral species, or whether molecular and ionic aggregates coexist. For aprotic ionic liquids, of which some can be distilled under vacuum without decomposition,47 the gas phase consists of neutral ion pairs.48,49 MIm nitrate was rigorously investigated using ab initio calculations, DSC, and combustion calorimetry, all of which showed that in the gas phase it most likely was almost completely present as the molecular species of methylimidazole and nitric acid.50 Similarly, Fourier transform ion cyclotron 11382

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alkoxymethyl group on imidazolium,62 and the temperature dependence of alkylammonium nitrates.17 The group of Lemordant et al. has reported extensively on the physicochemical properties of a broad range of PILs, and series of PILs, including those containing various alkylammonium cations,63−65 pyrrolidinium cation,66−70 and long chained carboxylate anions.68,71,72 The physicochemical properties for a series of stoichiometric and nonstoichiometric combinations of heterocyclic amines and TFA have been reported.73,74 As a related part of this investigation, it was determined that the colored impurities present in the PILs and protic salts studied did not affect their electrochemical behavior, whereas even small changes in their stoichiometry had a significant effect.75 The melting points of the PILs cover a broad range, with many liquid at or below room temperature. In some ways, this is unexpected due to many of the PILs not meeting the conventional requirements for low melting points of aprotic ILs such as being flexible ions, having delocalized charges, or steric shielding of the charges. Instead, PILs such as EAN have relatively simple cations and anions. A recent investigation was reported into the low melting point of EAN as compared to the higher melting salts of EACl, MAN, and MACl.76 It was proposed that, as compared to the other salts, EAN in the solid phase did not have the same capacity toward orientational disorder. Therefore, on heating, the hydrogen bonds were weakened, and dynamic ion rotation disrupted the crystal packing leading to the relatively lower melting point of EAN.76 A particularly important parameter for PILs for many applications is their ability to conduct protons. The ΔpKa difference between the precursor acid and base of a PIL has been used as an indication of the degree of proton transfer. Interestingly, a correlation was reported by Watanabe et al., which showed that the open circuit potential for hydrogen fuel cells increased with increasing ΔpKa to around ΔpKa ≈ 17, then decreased.77 This behavior was attributed to the PILs that have a low ΔpKa having more neutral species present, and hence poorer electrochemical properties. The poor electrochemical behavior for the PILs with very high ΔpKa values was attributed to the strong hydrogen bonds between the nitrogen and the “available” hydrogen leading to poor proton activity.77 Consequently, there appears to be an optimal range of ΔpKa values for PILs to have maximum electrochemical activity.

resonance mass spectrometry was used to show that only molecular precursor acid and base species were present in the vapor phase for MIm ethanoate51,52 or 1,1,3,3-tetramethylguanidinium (TMG) chloride.53 MD simulations and ab initio calculations for TMG with the anions of lactate, TFA, and formate all showed proton transport in the gas phase to form neutral molecules. In the gas phase, proton transfer then could occur, leading to some trimers or tetramers where an ion pair was stabilized through the presence of a third or fourth species.54 Investigations into PILs with a range of pKa differences between their precursor acids and bases indicates that it is possible for the gas phase of some PILs to have molecular and ionic aggregates coexisting.55,56 Electron ionization mass spectrometry was used to characterize the gas phase of the 12 PILs produced through the combinations of the cations of TMG, triethylammonium, and 1-methylimidazolium paired with the anions of formate, lactate, trifluoroacetate, and trifluoromethylsulfonate.55 The evaporation of PILs consisting of the same cations paired with acetate, Tf2N, and OTf was investigated using FTIR and quantum calculations.56 The PILs with the highest ΔpKa between the acid and base had molecular and ionic aggregates coexisting.55,56 The other PILs had only molecular species present, consistent with the general expectation for PILs. The investigations described above correlate well with the expected strength of proton transfer, with only PILs with particularly large pKa differences between the precursor acids and bases leading to both molecular and ionic aggregates, and hence the implication that they are evaporating, at least to some extent, as ion pairs. This is suggesting that these PILs are the most like aprotic ILs, which have a vapor phase consisting of neutral ion pairs with no free ions or larger clusters detected.47,48,52 It seems that for the majority of PILs it can be expected that the vapor phase will consist of the molecular acid and base species, which may or may not be present in a partially aggregated state.

3. PHYSICOCHEMICAL CHARACTERIZATION A summary of the thermal characterization that has been published for PILs is provided in Table S1. This includes the glass transition, Tg, melting point, Tm, boiling point, Tb, and decomposition temperature, Td. Similarly, a summary of the physicochemical characterization of PILs that have been reported is provided in Table S2. These parameters include the density, ρ, refractive index, nD, surface tension at the liquid−vapor interface, γLV, viscosity, η, and ionic conductivity, κ. The water content of the PILs has been included in the tables where available. For completeness, the data in Tables S1 and S2 incorporate data prior to 2007. A selection of protic molten salts with melting points above 100 °C has been included, which have structural similarities to many of the PILs. The structure−property relationships for the various thermal and physicochemical properties described in the tables were discussed in our 2008 review. Since then, there have been a considerable number of new PILs reported, including large series exploring systematic changes of various cations and anions. Frequently, these new PILs contained the same anion or cation as previous PILs, and hence fit into the trends previously identified and discussed in our 2008 review.1 These include investigations into the effect of substitution of the ammonium cation,16,46,57,586 the effect of methoxy or hydroxyl moieties,16,46,58−60 cyclic ammonium cations,57,61 length of

3.1. Proton Activity

In aqueous solutions, the pH scale is routinely used to measure the proton activity. However, the pH scale is not valid in neat ionic liquids or highly concentrated ionic liquid solutions. Consequently, alternative measures for the proton activity have been developed, although currently there is no standard method. These techniques include using acid/base probe molecules that are sensitive to the proton activity present. The probes that have been used include the indicator phenol red,24 the dye 4-nitroanilin,e78 and Reichardt’s dye,79 all of which had their level of protonation determined using UV/vis spectra. The dye 4-nitroaniline was used to provide a measure of the Hammett acidity functions for the addition of formic, acetic, or propionic acid into PILs that contain a carboxylate anion. The acidity of the acids in the PILs was propionic > acetic > formic, which was opposite to what is observed in water, and was attributed to the stabilization of the PIL cation toward the conjugate base of the acid.78 A short review of the Brønsted acidity of various solvents, including some considerations for the characterization of the acidity of ILs, highlighted the 11383

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values have been reported for a few ammonium-containing PILs. It was clearly shown that PILs such as EAN have high polarities as compared to molecular solvents, and that the presence of a hydroxyl on the cation or anion led to a significant increase.91−93 For example, the dielectric constant, ε, for EAN and EOAN was 30 and 60, respectively, with the highest value reported for ethanolammonium lactate with ε = 85.6.92 It was reported that the hydrogen bonding has a comparable effect on the polarity for PILs and molecular solvents.91 The high polarities of the PILs as compared to molecular solvents were due in part to the dipole densities being significantly lower in the PILs.92 A computational investigation into the electrostatic properties of a few imidazolium AILs and the protic methylammonium nitrate indicated that they all had short ranged electrostatic interactions, with a broad electric dipole moment and large fluctuations, and did not find evidence of screening occurring on a molecular length scale.94

inherent differences from aqueous systems, and that it cannot be assumed that aqueous data are relevant for IL systems.80 The δ(N−H) proton chemical shift as measured using NMR has been proposed by Byrne and Angell et al. to provide a measure of the proton activity.81,82 This technique was noted to require anhydrous conditions, with water contents less than 1000 ppm.82 A measure of the proton activity has also been obtained from the hydrogen-redox potential for hydrogen saturated PILs by Angell et al. where the difference between the potentials required to remove a proton from the acid (cation) and to reprotonate the base (anion) correlated well to the ΔpKa of the precursor acid and base, and gave a good measure of the proton activity for the PILs.83 The acidity of a few PILs has been described by the selfdissociation constant, Ks, or by pKs, where pKs = −log(Ks) and is analogous to pH in aqueous systems. The pKs of EAN was determined to be 5 at 25 °C.84 The pKs values for PILs containing the MIm cation paired with Tf2N and TfO are 8.58 and 8.93, respectively, and the solution was determined to have few neutral species present.14 In contrast, MIm paired with CF3COO, HCOO, or CH3COO had pKs values of 3.14, −1.4, and −1.4, respectively, with more neutral species present as compared to when it was paired with Tf2N or TfO, but still consisting mostly of ionic species.14 Similarly, the pKs values for C2Im Tf2N and C4Im Tf2N were 12.3 and 12.6, respectively, with little effect from changing the alkyl chain length.85

3.3. Conductivity

The ionic conductivity of protic ionic liquids in the last few years has predominantly been investigated for electrochemical applications such as in batteries, capacitors, supercapacitors, and fuel cells. These end-uses are extensively covered in section 8. In addition, the conductivity of PIL−water solutions has been characterized for many systems, and is covered in section 5.1. In this section, we report on a few fundamental aspects of conductivity, which have been reported since our previous review. A collection of protic ionic liquids in a supercooled state were investigated by Paluch et al. to explore the conductivity mechanisms present, Grotthuss (proton hopping) and vehicle (molecular transport).95−97 The conductivity and structural relaxation processes in the PILs lidocaine hemisuccinate and lidocaine di(hydrogen phosphate) were able to be decoupled through decreasing the temperature,96,97 and similarly by increasing the pressure in the supercooled state for other pharmaceutical-containing PILs.95 Near the glass temperature, the conductivity relaxation was significantly faster for lidocaine hemisuccinate than its structural relaxation.96 High pressure conductivity and rheology measurements of supercooled PILs of carvedilol chloride and carvedilol dihydrogen phosphate increased the conductivity significantly at the same viscosity, which was attributed to increased proton hopping due to deformation of the hydrogen-bonded network.95 It was proposed on cooling that the conductivity transitioned from a vehicle to a Grotthuss mechanism to explain the high proton conductivity and decoupling of the conductivity relaxation from the structural relaxation.97 To the best of our knowledge, the highest conductivity reported to date for a PIL was 88.2 mS/cm at 170 °C for allyldimethylammonium TfO, which was higher than its fully alkyl counterpart of dimethylpropylammonium TfO, that had a conductivity of 76.8 mS/cm at the same temperature.98 The allyl chain led to a more compact structure than its fully alkyl counterparts, thus increasing the number of ions present. It was determined that having a short alkyl chain present (methyl) led to lower viscosities and higher ionicity as compared to having longer alkyl chains.98

3.2. Polarity

The high polarity of PILs depends on the Coulombic and van der Waal interactions, which are present between the cation and anions. Solvatochromic probes have been used to obtain the Kamlet−Taft parameters and the electronic transition energy, ET(30), which provide a measure of the polarity. The Kamlet−Taft parameters consist of hydrogen-bond donor acidity, α, hydrogen-bond acceptor basicity, β, and the polarity index, π*. The polarity of ILs has previously been reviewed in depth,21,86−88 with a focus on the Kamlet−Taft parameters, and we refer the readers to these for a more in-depth discussion.86 These reviews have nearly exclusively focused on aprotics, which is representative of the general literature on the polarity of ILs. However, the Kamlet−Taft parameters have been determined for a series of PILs consisting of hydrogen sulfate, formate, acetate, or propionate paired with the imidazolium cations of HMIM or HBIM.89 The hydrogen-bond acceptor basicity, β, had a strong correlation to the polarity, whereas none was observed for the hydrogen-bond donor acidity, α. It was determined that for the PILs, the polarity was far more dependent on the anion than on the cation, whereas the reverse is generally observed for AILs.89 For either cation, the polarity followed the same trend dependent on the anion of hydrogen sulfate > formate > acetate > propionate.89 In related work, the solvatochromic probe of Nile red was used to classify the polarity of a series of PILs containing cyclic cations, and it was determined that they all had high polarities, which were between those of water and methanol or ethanol.61 A selection of nine PILs with a variety of cations and anions was determined to be highly hygroscopic, more so than aprotic ILs. The hygroscopicity correlated well to their polarity, with six of the nine categorized as superhydrophilic. The initial water sorption rate was strongly dependent on the polarity, whereas the total water capacity of the PILs was not.90 Alternatively, the dielectric constant, or static dielectric permittivity, provides a good measure of the polarity of ILs, and

3.4. Activity Coefficients

The activity coefficient provides a measure of the interaction between a solvent and a solute, with large values corresponding to small solute−solvent interactions. Activity coefficients for a 11384

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amine site. This PIL was able to form a chelate complex through coordination of copper(II) ions to the cation moiety.107 The first phosphonium PILs consist of the cation tributylphosphonium,103 trioctylphosphonium,108 or triphenylphosphonium108 with various anions. Tributylphosphonium Tf2N is shown in Figure 1b. In comparison to their ammonium counterparts, they had higher melting points, higher decomposition temperatures, and higher ionic conductivities, which was attributed in part to their weaker hydrogen bonding.103,108 Previously, Brønsted acidic aprotic ILs that incorporated SO3Hfunctionalized phosphonium cations were developed for use as catalysts and solvents in organic synthesis, with good results.109 There have been a few reports of PILs that have been synthesized from super bases and/or super acids, such as those shown in Figure 1c.105,110−113 Hydrophobic PILs containing superbases of phosphazenes or guanidines paired with superacids such as Tf2N or BETI were shown to have very low vapor pressures, even at 150 °C.112 The polarities were determined to be lower than those of conventional PILs, but higher than those of aprotic ILs.112 Similarly, 18 hydrophobic PILs from organic superacids and fluorinated β-diketone superbases were reported, which had a wide liquid temperature range, and thermal stabilities that are comparable to aprotic ILs.105 The super strong base of 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) was paired with various Brønsted acids.113 The PILs had good thermal stability and high ionicity, which correlated well with the ΔpKa of the acid−base pair, with ionicity increasing as ΔpKa increased before reaching a plateau at ΔpKa ≈ 15.113 A new superacid of hexafluoroisopropoxysulfuric, hfipOSO3, acid was reported, which is related to the commonly used anions in ILs of OTf and NTf2.110 This acid was paired with either imidazole amines, producing PILs that could be classified as “good ILs” according to their Walden plot. The first synthesis that we are aware of that uses an amide as the base was in 2010 with acetamide paired with various acids in stoichiometric and nonstoichiometric proportions.114 A later series of PILs from different amides paired with the Tf2N anion were more thermally stable, and used in proton exchange membrane fuel cells. The structure of one of these, isobutyramide paired with trifluoromethanesulfonate, is shown in Figure 1e. A possible proton conduction mechanism was proposed with the proton hopping between the NH3+ of one cation to the oxygen present as a CO on a neighboring cation.106 PILs containing one or more allyl chains on an ammonium cation, paired with TfO, were developed and compared to their alkyl counterparts. Both series had similar ion-pair interactions, although more compact structures were present in the allylcontaining PILs, which led to higher conductivities.98 A series of 25 combinations of amino-acid derived cations paired with various anions resulted in 21 amino-acid derived PILs.115 Related amino-acid derived PILs were first reported in 2005,116,117 although this later study significantly increased the number of known PILs of this type, with only four having been previously reported. Aprotic ILs that are related to PILs were reported, which were referred to as “anion amphiprotic ILs”, and are more closely related to previously reported Brønsted acidic aprotic ILs.25 Unlike conventional PILs, these are not formed through proton transfer from a Brønsted acid to a Brønsted base, and instead use aprotic ILs that contain diacids or amides as a

broad range of solutes have previously been reported in extensive series of aprotic ILs by Domanska et al. (for examples, see refs 99 and 100). Recently, they have extended their investigations to include the protic ionic liquid made from the super acid−base combination of 1,3,4,6,7,8-hexahydro-1methyl-2H-pyrimido[1,2-a]pyrimidine bis(pentafluoroethyl)sulfonylimide, [MTBDH][BETI], and have reported the activity coefficients for 54 solutes in this PIL.101 Many of the activity coefficients were comparable to those for the aprotic IL of BMIm BETI, indicating similar solvent properties for these two ILs. This PIL was investigated for potentially being useful for industrial separations/extractions, but generally had selectivities comparable to or lower than those of conventional solvents and aprotic ILs.101

4. NEW SUBCATEGORIES OF PILs There have been many recent reports of significantly different types of PILs, and the most notable of these are outlined below with representative examples provided in Figure 1.

Figure 1. Structures of new subclasses of PILs including (a) ammonium dication paired with anion,102 (b) tributylphosphonium Tf2N,103 (c) butylammonium pentadecafluorooctanoate,104 (d) super acid−base pair of 7-methyl-1,5,7-triazabicyclo[4,4,0]dec-5-ene (MTBD) paired with 1,1,1,5,5,5-hexafluoroacetylacetone (Hhfac),105 (e) isobutyramide paired with trifluoromethanesulfonate,106 and (f) Nhexylethylenediaminium TFSA.

A new class of ionic liquids was reported, which can be considered in many ways as an intermediate between protic and aprotic ILs. These ILs contained cations with one aprotic charge and one protic charge, as shown in Figure 1a, and were synthesized using imidazolium, quaternary ammonium, and pyrrolidinium cations.102 Interestingly, the resulting ILs had hydrogen-bonding behavior similar to that of conventional PILs. A related type of PIL is shown in Figure 1, where the protonation preferentially occurs at the secondary amine as shown; however, there is some protonation of the primary 11385

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contain an alkyl chain on the cation and a fluorocarbon chain on the anion.104 These fluorinated PILs can form complex structures consisting of polar, hydrocarbon, and fluorocarbon segregated domains.104 These are an interesting class of solvents, which have some significantly different properties as compared to their fully hydrocarbon counterparts. For example, the melting point of the fluorinated PILs is higher than that of their analoguous hydrocarbon counterparts, and the fluorinated PILs have higher thermal stability.104 The presence of three domains has also been reported for aprotic ionic liquids, although it appears to be a much weaker fluorinated domain.122 The interaction of the fluorinated PILs resulting from the hydrocarbon amines with the perfluorinated carboxylic acids with various solvents has been reported.123,124 In particular, these solvents are miscible with ethanol and butanol, and some are miscible with water while others are not.123 The nanostructure of the fluorinated PILs, and the effect of solvent addition on their nanostructure, are discussed further in section 6.

starting material.25 Consequently, these ILs do not have an equilibrium between neutral and ionic species. 4.1. Fluorinated PILs

Commonly used fluorine-containing anions such as TFSA, BETI, PF6, and BF4 have been extensively used in PILs. However, more recently fluorinated carboxylate anions were paired with alkylammonium cations to produce analogues of conventional alkylammonium carboxylate PILs. These PILs had a complex liquid nanostructure, which is discussed in section 6, which consisted of hydrocarbon, fluorocarbon, and polar domains.104 The lowest ionic conductivity was observed for butylammonium pentadecafluorooctanooate (BAOF), shown in Figure 1c, which had the most pronounced nanostructural segregation of the domains, and hence most “rigid” structure.104 These fluorinated PILs are potentially capable of providing a solvent environment suitable for hydrocarbon, fluorocarbon, and polar solutes. There has been an extensive use of fluorinated anions in ionic liquids, predominantly using the anions of BF4, PF6, and Tf2N, with their use being to delocalize the charge and hence lead to ionic liquids with more desirable properties. However, in recent years, ionic liquids containing longer fluorocarbon chains have been specifically investigated for their unusual properties, which arise due to the fluorinated component. This section is focused on PILs, which contain a substantial amount of fluorine, particularly in the form of fluorinated alkyl chains, where the presence of the fluorine is fundamental to the properties of the PIL. A developing niche application for PILs is their use as specialized lubricants, particularly for those that are fluorinated. This section does not include PILs that contain commonly used small fluorinated anions such as BF4, PF6, or Tf2N.118 There have been only a few series of fluorinated PILs reported. These are a series of hydrocarbon amines with perfluorinated carboxylic acids of which only 11 of the 24 acid− base combinations formed PILs,104 a series of perfluoropolyethercarboxylates with primary, secondary, or tertiary ammonium cations,118 and a series of 1,2,4-oxadiazoles with either two pyridines or one pyridine and one fluorinated alkyl chain combined with a carboxylic acid with one or two fluorinated alkyl chains, where of the 18, five formed PILs and four formed protic molten salts.119 The thermal properties for the PILs and protic molten salts are provided in Table S1, and the physicochemical properties for the two PILs from the first series that were liquid at room temperature are provided in Table S2. There appears to be a weaker driving force for proton transfer between the fluorinated acids and hydrocarbon amines as compared to their nonfluorinated counterparts. Consequently, it is essential to characterize the final product to detect whether proton transfer has occurred, and if so to what extent. Two methods have been reported, using either 13C, 1H, and 19F NMR104 or infrared spectroscopy, specifically the band associated with the carboxylate or carboxylic acid stretch.118,119 Both of these techniques are sensitive to the molecular and ionic species present. The presence of a fluorocarbon chain in these PILs enables the formation of a fluorinated domain through segregation of the fluorocarbon chains. This is analogous to the wellcharacterized hydrocarbon segregation, which has been reported for many aprotic and protic ionic liquids.120,121 Small- and wide-angle X-ray scattering (SAXS) has been used to characterize the mesostructure of the fluorinated PILs, which

5. SOLVENT MIXTURES CONTAINING PILs There is recent interest in optimizing the solvent properties of PILs for specific applications through mixing them with a molecular solvent. In particular, the addition of solvents generally decreases the melting point and viscosity and increases the conductivity. While there have been a good number of papers on the interactions between PILs and molecular solvents, this field is still limited to a handful of PILs and solvent combinations. A broader range of ions has been investigated for aprotic ILs and water mixtures, and some of these have been covered in a review by Ohno et al., focusing on their research in this field.125 In this section, the fundamental properties of PILs−molecular solvents mixtures are reviewed, where most commonly this has been reported for solvents such as water, alcohols, or acetonitrile. Last, the applications that have been shown to benefit through addition of water or molecular solvents to PILs are briefly discussed. The interaction of PILs with various solvents is dependent on the specific PIL and solvent, and a greater understanding on the interactions occurring will be of benefit for many applications. Predominantly, the interactions depend on hydrogen bonds, and how much they are disrupted in the neat IL and neat solvent, as compared to the new ones forming between the IL ions and the solvent. Other factors are the steric hindrance of the IL or solvent, which may inhibit some interactions, and the hydrophilic/hydrophobic balance of the IL that affects which solvents it can mix with, and how it interacts with different solvents. The available information about these interactions for PIL−solvent mixtures is discussed in this section. The liquid nanostructure of PILs is described in section 6, and includes details on how the nanostructure is modified on addition of water, molecular solvents, or other solutes. The effect of molecular solvents on the hydrogen bonding present in PILs, as measured by their pKa values from Reichardt’s dye, was investigated.79 Methanol was used as a protic polar solvent, dimethyl sulfoxide (DMSO) as a polar aprotic solvent, and acetonitrile as a low hydrogen-bond acceptor solvent. For all of the PILs, DMSO increased the pKa values, indicating a strong cation−anion interaction and a more basic nature of the PILs. The addition of methanol had an opposite effect with the PILs becoming more acidic.79 The addition of water to butylammonium nitrate, BAN, was investigated using Raman spectroscopy. It was determined 11386

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Table 1. Summary of Molar Excess Volumes for Mixtures of ILs and Molecular Solventsa ionic liquid

water

EAN propylammonium nitate butylammonium acetate butylammonium nitate EOAA EOAF EOAP EOAL m-2-HEAB pyrrolidinium formate pyrrolidinium nitrate pyrrolidinium HSO4 pyrrolidinium octanoate imidazolium octanoate collidinium formate DIPEAF DIPEA heptanoate DIPEA octanoate C B

N133 N133

methanol

ethanol

propanol

nbutanol

acetonitrile

methyl acetate

ethyl acetate

propyl acetate

γbutyrolactone

1octanol

A137

N139 N139 N134 N140

N134,135 N140 N135 N135

N134 N140

A138

A138

A138

70

N P70 P70 N68

N68

N72 N70 N70 N65 N65

A68

A68

N72

N68 N72

P72

N65 N65 N132

N132

N132 A136

A136

A136

a

The behavior is classified as positive (P) or negative (N) across the entire composition range, or as asymmetric (A) with regions of both. C = bis(2hydroxyethyl)methylammonium formate. B = n-methyl-2-hydroxyethylammonium hexanoate. m-2-HEAB = N-methyl-2-hydroxyethylammonium butyrate.

weaker, and less water is required to disrupt the interaction.130 The strength of the cation−anion interaction in PILs is ion dependent. This interaction can be disrupted through addition of a molecular solvent, although the amount required depends on the strength of the cation−anion interaction, and on which molecular solvent, and in particular its polarity. The strong anion−cation interactions present in triethylammonium methylsulfonate required a minimum of four water molecules to begin to be disrupted.130 However, when the anion of trifluoromethylsulfonate is paired with the triethylammonium, then the cation−anion interaction is weaker, and less water is required to disrupt the interaction.130 Similarly, for EIm TFSI and EIM TfO, the addition of water increased the ionicity through disruption of cation−anion interactions.22 The contact ion pairs present in triethylammonium trifluoromethylsulfonate had little change when low polarity solvents were added, such as chloroform, THF, or acetone regardless of solvent concentration, although DMSO had a sufficiently high polarity to disrupt the cation−anion interaction.131 The measurement of excess molar volumes for the mixtures of PILs and water or other molecular solvents has been used to provide insight into the interactions. The molar excess volume depends on competing contributions of the interactions between the cation and the anion of the PIL, interactions between the PIL and the solvent, size differences, and van der Waal interactions.132 A summary is provided in Table 1 of the PIL−solvent systems reported, and these are classified as either being positive or negative across the entire composition range, or as being asymmetric where they are positive or negative depending on the composition. It should be noted that for many of these systems, the excess molar volume was only about 1−3% of the total molar volume.

that the strong anion−cation hydrogen bonds present in BAN were replaced by water−anion hydrogen bonds as the water content increased.126 A comparison between aqueous solutions of structurally similar protic and aprotic ILs containing the BF4 anion showed different interactions occurred for these two classes between the ILs and water. The PILs had noticeably stronger interactions with water, with exothermic dissolution of water, whereas for the aprotic ILs it was an endothermic process. For comparison, the dissolution of NaCl is also endothermic. Exothermic dissolution was observed for water and to a weaker extent for ethylene glycol in MIm BF4, whereas DMSO and DMF displayed endothermic behavior in MIm BF4.127 Very dilute solutions of methanol in MIm BF4 were exothermic, and then were endothermic. In contrast, the mixing of these same solvents in either 2-methylpyridinium BF4 or N-methylpyrrolidinium BF4 was always endothermic.127 The exothermic behavior in the PILs was attributed predominantly to the ability of the PILs to hydrogen bond with the water or ethylene glycol, with the exothermicity increasing with number of hydrogenbonding sites present.127,128 The hydrogen bonding between triethylammonium PILs and water was investigated using 1H NMR. It was observed that the kosmotropic anions of H2PO4− and HSO4− caused a large downfield shift, indicative of strong interactions between the PILs and water, the fluorinated Tf and BF4 had only weak interactions with water, attributed to their hydrophobicity, and no interaction was observed for chaotropic anions.129 An FIR investigation showed that the strong anion−cation interactions present in triethylammonium methylsulfonate required a minimum of four water molecules to begin to be disrupted.130 When the anion of trifluoromethylsulfonate is paired with the triethylammonium cation, then the cation−anion interaction is 11387

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on a pseudo-lattice of Bahe−Varela have been reported for modeling PIL−water interactions, which treats them as structured concentrated electrolyte solutions.141,147 From this model, it was determined that for a 1:1 mixture of EAN with water, there is a localized disruption to the solvent structure. The addition of organic or inorganic electrolytes to EAN− water solutions indicated that the EAN was dominant in governing the interactions with these solutes, although the size and nature of the solutes effected the local interactions.141 Solvatochromic probes were used to obtain the Kamlet−Taft parameters for the mixtures of ethylammonium propionate with the alcohols of methanol, ethanol, 1-propanol, and 2-propanol. All of these mixtures showed higher normalized polarity (ETN) parameters than expected for ideal behavior, which was attributed to hydrogen-bond donor acidity and the polarizability of the mixtures.148 The effect of water, ethanol, and methanol on the structure of EAN has been investigated using molecular dynamics (MD) simulations.149 All of these solvents were accommodated homogeneously within the existing hydrogen-bonded network of EAN, with no cosolvent network present. The water had a preference to bond with the cations, and around 55% water expelled the nitrate anions from the first solvation layer around the ethylammonium cations. The water was more tightly bound into the network due to being able to form more hydrogen bonds.149 MD simulations of DEMA triflate (DEMA Tfl) with water have shown that the water is present in the coordination shells of the cation and anions, and weakens the cation−anion interaction through electrostatic shielding of the ions.142 With increasing water concentration, the water is present as monomers, then dimers, then small clusters, and finally as a percolated network.142 The water interacts with the IL structure and modifies it; however, long-range spatial correlations of DEMA Tfl are present even at high water concentrations of 0.8 mole fraction of water.142 MD simulations of methylammonium nitrate (MAN) with water showed that there was good incorporation of water into the MAN hydrogen-bonded network, with the water generally retaining its tetrahedral hydrogen-bonded coordination.150 The methylammonium cation formed stronger hydrogen bonds than the anion. Calculations for an ion pair of diethylmethylammonium trifluoromethanesulfonate (DEMA TfOH) with a water molecule have shown that the lowest energy conformation for this trimer occurs with the water hydrogen bonded to the N−H along with the anion.143 Hydrogen bonds between the methyl or ethyl groups on the cation are likely to be weaker. Quantum chemical calculations showed that the PIL MIm dicyanide can form a number of different cyclic complexes using five ion pairs. The addition of water, acetonitrile, or DMSO modified which of these cyclic structures was most likely, due to the different polarities of the added solvents changing the binding energy of the ion pairs.32 The rotational dynamics of two organic solutes in PAN was investigated. The solutes were structurally similar, although one was hydrogen-bond accepting and the other was hydrogenbond donating and accepting. The latter had 60% slower rotational diffusion, clearly showing the specific interactions present, and that they have a significant effect.144 Mixtures of EAN and acetonitrile were investigated using broadband dielectric relaxation spectroscopy.145 For EAN-rich compositions, the solvent mixture had ionic liquid-like dynamics, with the acetonitrile present bound to the EAN, with strong cation−

It is evident from Table 1 that the majority of PIL−solvent systems studied have involved the addition of polar solvents, and that generally the molar excess volumes are negative across the full range of compositions. The excess molar volume is negative when the interactions between the IL and the solvent are stronger than those present for the neat IL or neat solvent. Therefore, it can be considered that the hydrogen bonds forming between the IL and the solvents are stronger than the cation−anion or the solvent−solvent bonds. Conversely, positive excess molar volumes correspond to weak interactions between the PIl−solvent, which are not sufficient to compensate for the loss of interactions due to disruption of the solvent−solvent or PIL−PIL hydrogen-bonded network. The PILs consisting of DIPEA with either heptanoate or octanoate anions had negative molar excess volumes on mixing with either water or acetonitrile, with a negligible difference between the two anions, and a minima around 0.7 mole fraction of water.65 It can be assumed that the polar solvents such as water and acetonitrile will primarily have interactions with this cation as compared to with anions that are largely nonpolar. The molar excess curves for pyrrolidinium octanoate with water, ethanol, and acetonitrile were negative across the full concentration range, and had the largest magnitude for the mixtures with water, which was attributed to segregation of the hydrocarbon portion of the PILs.68 In addition to Table 1, the magnitude of the molar excess volumes indicated that the IL−IL interactions were stronger in PAN than in EAN,133 and that the solubility of the water and the alcohols was primarily due to interactions with the cation.134 However, for the series of ethanolammonium containing PILs of EOAA, EOAF, and EOAL with methanol, the lactate anion led to excess molar volumes with a greater magnitude that was attributed to the stronger interactions due to hydrogen bonding from the OH on the anion.135 There are three PIL−solvent combinations in Table 1, which have positive excess molar volumes across the full composition range. These were pyrrolidinium nitrate or pyrrolidinium hydrogen sulfate with water,70 and imidazolium octanoate with 1-octanol.68 In contrast, the pyrrolidinium cation with formate or octanoate anions was negative across the full range. This highlights that both the types of cation and anion affect how the ILs mix with water. The more structured anions such as sulfate and nitrate led to a greater disruption of the water structure than the other anions when paired with the pyrrolidinium cation.70 However, it is clear that there is also a dependence on the cation, because the nitrate anion in ethylammonium nitrate has a negative excess molar volume. There were several PIL−solvent combinations in Table 1, which had asymmetric molar excess volume curves, that contained positive and negative regions depending on the composition. For all of these, the positive feature was of smaller intensity and covered a much smaller concentration range as compared to the negative region.68,136−138 The negative excess volumes were attributed to solvation of the ILs by the added solvent, such as through ion−dipole interactions, ion−ion association, or hydrogen bonds. The positive values at high IL concentrations were attributed to a breakdown of the hydrogen bonds in the neat solvents, leading to free interstitial spacing, and consequently positive excess molar volumes.137 The interactions present for PIL−molecular solvent systems have also been investigated in a few other ways, such as through modeling,141−143 rotational dynamics,144 dielectric spectroscopy,145 and polarity probes78,146 as outlined below. Models based 11388

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water, the conductivity decreases, with the decrease in the number of charge carriers becoming the dominant effect. The self-diffusion coefficient for protons in triethylammonium containing PILs was shown using pulsed field gradient stimulated echo (PFG-STE) NMR to increase significantly with even just 1000 ppm of water present.154 The addition of the solvent 1H-1,2,4-triazole to the protic molten salt of imidazolium methanesulfonate did not modify the ionic conductivity, but did beneficially decrease the melting point from 188.2 °C for the neat PIL to 80.5 °C when 0.333 mole fraction of the solvent was present.155 The ionic conductivity was effectively unchanged from that of the neat protic molten salt with up to a mole fraction of 0.750 of the solvent added. The added solvent dissociated the molten salt through weakening electrostatic interactions, specifically hydrogen bonds, thus increasing the mobility of the ions.155 A local anesthetic drug of 2-diethylamino-N-(2,6dimethylphenyl)acetamide hydrochloride monohydrate has been referred to as a protic ionic liquid, and this salt has a melting point of 72 °C. At room temperature, it is present in a crystalline state, although addition of water leads to a stable amorphous mixture, which possibly has higher bioavailability.156 The ionic networks present in the PILs can be stable up to significant dilutions of water, such as for tris(2hydroxyethyl)methylammonium methylsulfate, where this PIL retains a strong interacting network for water additions less than 0.8 mole fraction of water, between 0.8 and 0.95 mole fraction of water there is ionic clustering, whereas only when there is greater than 0.95 mole fraction of water present are the ions solvated by water.157 The use of PILs in amphiphile self-assembly has included a variety of roles of water or other molecular solvents. These include the use of PILs containing long alkyl chains as amphiphiles in water,71,158 surfactant self-assembly in PIL− water mixtures,159 and microemulsions, typically containing the PIL as the polar phase, an amphiphile, and an alkane solvent.10 Further details on these are provided in section 7. ILs can be classified for how amphiphilic they are on addition to water. For example, the aprotic ILs of EMIm BF4 and BMIm BF4 are highly amphiphilic, with a strong preference to locate at the air−water interface, forming a monolayer, before going into solution. In contrast, EAN showed a weak amphiphilic behavior, and EOAF had none observed, and readily mixed in the bulk of water without going to the air−liquid interface.160 The physicochemical properties of 6 PILs and 3 silvercontaining AILs were investigated, and the differences between proton and silver-containing ILs were compared.161 The AILs consisted of a double-chained alkylethylenediamine silver(I) cation paired with the Tf2N anion, and the PILs of singlechained alkylethylenediammonium salts also paired with the Tf2N anion. The alkyl group on the PILs varied with increasing alkyl chain length between butyl to dodecyl, along with the branched 2-ethylhexyl. Because of the hydrophobic anion and large amount of hydrocarbon present in the cation, the highest water solubility in the PILs was 0.1 wt % for the alkyl group being 2-ethylhexyl, whereas the lowest was for the alkyl groups being decyl or dodecyl with water solubility acetic > propionic, whereas in PILs the order was reversed. The acidity of the formate-containing PILs decreased with added water in a nonlinear manner, which indicated that there was an intermediate solvent/pseudosolvent effect present for mixtures of these PILs with water.78 The potential use of PILs in separations or extractions is based on their interacting differently with different species, such as for the separation of aromatics from aliphatics151 this is discussed further in section 10. PILs have been used as solvents and/or catalysts for a large variety of organic synthesis reactions, and these are highly dependent on the interactions of the PILs with various solutes (see section 17 on organic synthesis). As discussed in section 15 on the use of PILs in inorganic synthesis, zinc oxide has been prepared from EAN− water.152 A three-solvent solution mixture consisting of an ionic liquid and ethyl acetate combined with either ethanol or acetic acid was investigated for MIm HSO4, EMIm HSO4, and BMIm HSO4. All of the ILs were miscible with water, and all were immiscible with either ethanol or acetic acid. 153 The immiscibility region on the ternary phase diagram increased in size with increasing alkyl chain length on the IL, such that it was smallest for the PIL. It was proposed that both MIm HSO4 and EMIm HSO4 have potential for use in extracting compounds from esters.153 5.1. Effect of Molecular Solvents on Applications of PILs

For some applications, the presence of any water is detrimental. However, it is increasingly becoming accepted that for many applications the addition of small or large amounts of water leads to desirable solvent properties, while frequently retaining properties associated with ionic liquids. For many of the uses of PILs in organic and inorganic synthesis, biological applications, and electrochemical applications, there is an additional solvent present with the PIL, and these are discussed in sections 17, 15, 12, and 8, respectively. A brief outline and some specific examples are provided next. Perhaps the main potential application of PILs that has extensively utilized PIL−solvent mixtures is their use as electrolytes for fuel cell and other applications based on their high conductivity. Many of the neat PILs have relatively high conductivities, as can be seen in Table S2. However, the conductivity of the PILs has been shown to be significantly increased through addition of water or other solvents, with 2− 30 times larger conductivities achieved.69,139 This effect is due to two main competing factors, which are the decrease of charge carriers through dilution and the increase in charge mobility through the decrease in viscosity. On addition of water, the conductivity increases until reaching a maximum with the viscosity effect dominant. For further additions of 11389

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Figure 2. Characterization of the nanostructure of EAN using different techniques, including (a) SANS for pure D-EAN at 45 °C170 and (b) SAXS/ WAXS for EAN at 25 °C.175 Reproduced with permission from refs 170 and 175. Copyright 2008 and 2010 American Chemical Society.

Table 2. Small- and Wide-Angle X-ray and Neutron Scattering Data of the Nanostructure of Protic Ionic Liquids, Where q1, q2, and q3 Are the Scattering Vectors, q, at the Peaks of Maximum Intensity and d1, d2, and d3 Are Their Corresponding Correlation Distances PIL

water (wt %)

EAN EAN EAN EAN EAF EAG PAN PAN PAN BAN BAN PeANa BAF PeAF EOAF EOAN PeOAF PeOAN DEAF TEAF DEOAF TEOAFa,e 2MEANa PyrrOF PyrrNd PyrrON EA pentadecafluorooctanoatea BA pentadecafluorooctanoatea EOA pentadecafluorooctanoatea 1-methylimidazolium heptafluorobutyratea butylammonium heptafluorobutyratea butylammonium pentadecafluorooctanoatea diethylammonium heptafluorobutyratea 2-pyrrolidinonium heptafluorobutyratea pyrrolidinium heptafluorobutyratea ethanolammonium heptafluorobutyratea triethylammonium heptafluorobutyratea

107

88 to >107 91−104

CTAB

71 to >106

78−106

CTAB CTAB CTAB CTAB CTAB CTAB CTAB

67 to >107 54 to >105 79−96 74 to >106 73 to >107 114 to >132

CTAB Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K Myverol 1899K phytantriol phytantriol phytantriol phytantriol phytantriol

89−96 80−106 88−107 116−132 94 to >108

Lα

V2

H2

ref

86−97 94−106 88 to >107 98 to >104 88 to >107 88−104 83 to >106

115 115 46 46 46 46 46

82 to >106

46 46 46 46 46 46 46 46

88 to >106 92 to >124 81−102 87 to >107

83 to >108 32−56

46 115

30−43

115

31−39

115

17−43

46

22−61

31−63

46

22−35

46

8−47

26−59

46

23−42b

28−47

46 22−51

46

24−52

46

37−46

46

4−18 4−22c 8−42 10−31

8−17

46 46 46 46 46

phytantriol

6−20

46

phytantriol phytantriol phytantriol C16MPyrrBr C16MPyrrBr P123 P65

13−43 10−79

46 46 46 227 227 228 229

L121 L81 C16E4 C16E5 C16E6 C16E4 C18E6

28−42

37−41

25 >25>80d

70 70 25 >25 to >80d

25 >25 to >80d

35 to >70 30 to >70 25 32 to >80d

>63 >65 >65

22 42 >65 >65 11396

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Table 4. continued protic ionic liquid

surfactant

I1

H1

PAN EAN EAN EAN

C18E8 C16MImCl Brij 97 C14MIMCl

44 20−122d PAN,207 which is consistent with the free energy of transfer of a methylene group into a micelle, that was determined to be −0.973 kJ/mol in EAN and −0.691 kJ/mol in PAN.207 A series of CnEm nonionic micelles in PAN were investigated using SANS.230 The micelles of C12E2 and C18E4 fitted well to disks (short cylinders), while micelles of C14E4 and C16E6 fitted well to polydisperse core−shell spheres.230 All of the micelles were smaller than their aqueous or EAN counterparts. It is expected that the PA+ ions would act as cosurfactants in the micelles, which is attributed as the main cause of the larger head group solvated areas in PAN and the smaller radii of micelles.230 The C12Em (m = 3−8) surfactants were investigated in a large series of alkylammonium nitrate and formate PILs.245 There was a strong correlation between the alkyl chain length present on the PIL and the CMC, and whether or not micelles formed. Micelles were detected in EAN, PAN, diethylammonium formate (DEAF), triethylammonium formate (TEAF), butylammonium formate (BAF), and ethanolammonium nitrate (EOAN).245 The CMC increased significantly with alkyl chain length with micelles detected from 1 wt % in EOAN, 5 wt % in EAN, and only at 20 wt % in BAF. No micelles were detected below 20 wt % in PeAF or PeAN, attributed to hydrocarbon solubility being too high in these PILs. In contrast, EOAF did not support any micelles due to having insufficient solubility of the surfactants to reach the CMC.245 Micelles of nonionic Brij 97 have been reported in the PILs of EAN, ethylammonium butyrate (EAB), and pyrrolidinium nitrate (PyrrN), along with in the aprotic ILs BMIm BF4 and BMIm PF6.232 Similarly, a quaternary ammonium Gemini surfactant was found to form micelles in EAN and PAN, but not in BAN due to the higher hydrocarbon solubility, and hence weaker solvophobic effect.225

High-throughput penetration scans using cross polarized optical microscopy (POM) have been used to identify the lyotropic liquid crystal phases supported by a broad range of PILs, and to determine their thermal stability ranges. Of a series of 21 amino-acid derived PILs, only three were observed to support the self-assembly of nonionic Myverol 18-99K, and only two supported CTAB, with mostly lamellar phases being formed.115 Of a series of 22 PILs and 6 protic molten salts, all of which contained alkyl ammonium cations, 14 were found to support the self-assembly of amphiphiles.46 The amphiphiles of Myverol 18-99K, phytantriol, and CTAB were used, and the phases identified included lamellar, inverse hexagonal, and an isotropic band assumed to be a bicontinuous phase. The phase diversity was found to be poorer for the PILs that contained more substitution on the ammonium cations, and generally followed that the PILs with a higher cohesive energy density, as measured by the Gordon parameter, were richer self-assembly media.46 The cationic Gemini surfactants of type [CmH2m+1(CH3)2N(CH2)2N(CH3)2CnH2n+1]Br2, m = 12 + n = 12, m = 14 + n = 10, and m = 16 + n = 8, in EAN form a reverse hexagonal phase,235 in addition to forming micelles.223 In contrast, these surfactants formed normal hexagonal and lamellar phases in water. The differences were attributed to the weaker solvophobic effect and greater charge screening in EAN, where the later led to higher critical packing parameter, enabling the reverse hexagonal phase to form.235 Increasing the alkyl chain length of the PIL to have PAN or BAN led to substantial changes to the lyotropic liquid crystal phases of the 12-2-12 Gemini surfactant, with normal hexagonal and bicontinuous cubic phases present in PAN and bicontinuous cubic and lamellar phases present in BAN.225 These differences were attributed to a combination of the lower solvophobic effect with increasing alkyl chain length on the PIL, and to the longer alkyl chains on the PILs causing a larger effective headgroup area of the surfactants, and thus leading to lyotropic liquid crystal phases with lower critical packing parameters, and to the surfactants behaving as cosurfactants.225 The AmILs of C14MImCl233 or C16MImCl231 in EAN formed the lyotropic liquid crystal phases of normal micellar, normal hexagonal, lamellar, and reverse bicontinuous phases, which were reported as phase diagrams over the temperature range from 20 to 140 °C.231 The normal hexagonal phase of C14MImCl in EAN was used to disperse multiwalled carbon nanotubes, MWCNT, and the lyotropic liquid crystal phase was retained, although with an increase in the lattice spacing, indicating there is swelling, and with an increase in the viscosity.233 MWCNTs were also dispersed in the hexagonal phase formed by the nonionic surfactant C16E6 in EAN, forming a highly viscoelastic composite that has potential as a lubricant.251 A series of nonionic CnEm surfactants in PAN were investigated, and compared to their aggregation behavior in water and EAN.230 The solvophobic effect was weaker in PAN as compared to EAN, due to the increased hydrocarbon solubility in PAN. This led to less lyotropic liquid crystal phase diversity for the surfactants in PAN, higher curvature lyotropic liquid crystal phases, and no phases being observed for C14E4, C14E6, C14E8, or C18E3 in PAN.230 The surfactants of type C12E4, C14E4, and C16E4 were investigated at the EAN−air interface using neutron reflectivity and vibrational sum frequency spectroscopy.252 It was found that above the CMC the surfactants orientated as a monolayer, with the layer

7.2. Lyotropic Liquid Crystal Phases

It has become commonly accepted that protic ILs can support the main lyotropic liquid crystal phases that are observed in aqueous systems, as shown in Table 4, for cubic, hexagonal, and lamellar phases. Less common phases are beginning to be reported, such as the first sponge phase in an IL that was reported for didodecyldimethylammonium bromide (DDAB) in EAN,248 inverse bicontinuous cubic phases (and cubosomes) of monoolein supported in EOAF,249 and an ionogel of sodium laurate in EAN.250 The majority of the publications have focused on using EAN to date, but there are many new PILs being introduced in this field. Approximate partial phase diagrams between 25 and >100 °C have been reported for the cationic surfactants of CTAC and hexadecylpyridinium bromide (HDPB) in the PILs EAN, EOAN, and diethanolammonium formate (DEOAF).234 Greater phase diversity was observed in EOAN or DEOAF than in EAN, which reflects their higher solvophobic effect due to the presence of the hydroxyls that decrease the hydrocarbon solubility in these solvents and increase their cohesive energy density. 11400

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part of review by Zech and Kunz in 2011.10 The field is not limited to PILs, with for example the aprotic IL BMIm BF4256,257 and N-methyl-N-propylpyrroldinium Tf2N258 having been reported as the polar phase in microemulsions. An extensive series of microemulsions was reported by Warr and Atkin, which included EAN259 or PAN230 with a nonionic alkyl oligoethylene oxide CnEm surfactant and an alkane such as octane, decane, dodecane, tetradecane, or hexadecane. The surfactant chain needed to have around 4−6 more methylene groups to produce microemulsions in EAN, which were comparable to those in water.259 Much higher surfactant concentrations were required in PAN to enable the microemulsions to form due to the lower solvophobic effect.230 Similarly, an emulsion was reported of perfluoromethyldecalin in a hexagonal lyotropic liquid crystal phase formed by C16E6 in EAN, where more than 80 wt % of the fluorocarbon could be emulsified.260 A series of nonionic Brij surfactants of Brij 52, Brij 56, Brij 58, and Brij 93 were used in IL-in-oil microemulsions of EAN and benzene.261 It was found that increasing the polar head group of the surfactants led to larger reverse micelle sizes, whereas increasing the hydrocarbon chain polarity decreased the micelle size.261 The size of the micelles decreased with increasing temperature, which was attributed to be due to a decrease in the hydrogen bonding between the EAN and the polar head groups. Zech and Kunz et al. have focused on the use of cationic amphiphiles in their microemulsions to increase the thermal stability range. They have prepared microemulsions using combined surfactants of AmIL C16MImCl with decanol, EAN or BMIm BF4 as the polar phase, and dodecane as the nonpolar continuous phase.257,262 An IL-in-oil microemulsion formed, which had a spherical shape, and a sharp interface present between the polar and apolar components.257,262 The microemulsion of EAN, C16MImCl, decanol, and dodecane was stable over the entire temperature range tested of 30−150 °C.262 For BMIm BF4, the spheres were repulsive, although not for EAN.257 The droplets swelled with increasing EAN concentration until aggregates formed for concentrations above 18 wt % of EAN at 30 °C.262 A closely related microemulsion was reported where biodiesel was used instead of dodecane as the apolar phase, and again with EAN as the polar phase and C16MImCl and decanol.263 The microemulsion was stable over the tested temperature range from 30 to 150 °C, although a smaller single phase region was present with biodiesel as compared to dodecane.263 Microemulsions consisting of the nonionic surfactant C12E3 in EAN and various n-alkanes were observed to be similar to aqueous systems for oil-in-EAN droplets and bicontinuous microemulsions. However, for EAN-in-oil droplets, there was very little EAN present, which was attributed to the sponge-like nanostructure of EAN making the confinement of EAN energetically unfavorable where it prevents the EAN from having an intermediate-range order. It was questioned whether the EAN-in-oil microemulsion actually formed at all.264 Microemulsions were formed for a series of imidazolium-type aprotic ionic liquids combined with BMIm, AOT, and benzene. However, the use of EAN or propylammonium formate did not result in microemulsions.265 The C6Mim Tf2N aprotic IL successfully formed a microemulsion using the PIL surfactant of N,N-dimethylethanolammonium 1,4-bis(2-ethylhexyl) sulfosuccinate and the oil phase of isopropyl myristate.266

thickness of the alkyl tails and the head groups both thinner than at the air−water interface, and the polar headgroups are more poorly solvated in EAN than in water.252 The triblock copolymer, Pluronic P123, consists of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) in a PEO−PPO−PEO configuration. When this surfactant is mixed with EAN, the EAN solvates the PEO groups in Pluronic P123, leading to the lyotropic liquid crystal phases at 25 °C of micellar, normal micellar cubic, normal hexagaonal, lamellar, and a reverse bicontinuous cubic phase. These are similar to those observed in water or BMIm PF6.228,234 It was proposed from the FTIR data that there is hydrogen bonding occurring between the NH3+ of the EAN and the PEO groups, with little change to the NO3− anions.228 More recently, the phases present for the EAN−P123 system were investigated at higher temperatures to form a more complete phase diagram.234 It was reported that P123 was immiscible with the PILs EOAN and DEOAF.234 The Pluronic amphiphiles of P65, L81, and L121 were investigated in EAN. The lyotropic liquid crystal phases of P65 in EAN are provided in Table 4 and were similar to those reported for P123 in EAN.229 L121 at 10 wt % in EAN formed unilamellar vesicles at 25 °C, and at 1 or 10 wt % in EAN forms lamellae stacks at 63 °C. For L81 in EAN the SANS data were interpreted as due to dissolving droplets of the polymer, and no lyotropic liquid crystal phases were observed at 1 or 10 wt %, at 25 or 63 °C.229 The nonionic amphiphile oleyl polyoxyethylene (10), Brij 97, was investigated in EAN, EOAN, and DEOAF, with approximate partial phase diagrams constructed for 25 and >100 °C.234 Similarly, the aggregation of Brij 97 in the PILs of EAN, ethylammonium butyrate (EAB), and pyrrolidinium nitrate (PyrrN) was compared to the aggregation in the aprotic ILs BMIm BF4 and BMIm PF6.232 The PILs of EAB and PyrrN, and the aprotic ILs of BMIm BF4 and BMIm PF6, only supported micelles. EAN was reported to support normal and reverse micelles and a normal hexagonal phase. In contrast, other polyoxyethylene amphiphiles of Brij 35, Triton X-100, and NP-10 did not form any lyotropic liquid crystal phases in EAN.232 Manganese(II) polyoxometalate was dispersed in a hexagonal lyotropic liquid crystal phase of the nonionic amphiphile C16E6 in EAN, and the liquid crystal phase was not removed.253 Polarized optical microscopy and SAXS was used to confirm the resulting material had a hexagonal structure. The EAN− C12E6 hexagonal phase was found to be highly viscoeleastic, whether or not the manganese(II) polyoxometalate was present.253 Lyotropic liquid crystal phases have also been used in PILs in the sol−gel synthesis of titanium dioxide254 and silica.174 These are discussed further in section 15 on inorganic synthesis. The thermotropic liquid crystal phases of amphiphilic PILs consisting of fatty carboxylic acids (oleic or stearic) paired with ethanolammonium or diethanolammonium were investigated in excess acid and excess amine.255 Normal hexagonal, lamellar, and inverse hexagonal phases were observed for certain compositions and temperature ranges. 7.3. Microemulsions

The use of PILs in microemulsions is usually as a replacement for water as the polar phase, and predominantly involves the use of EAN, although PAN has also been reported. There have only been a few papers on this field, and it has been included as 11401

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Table 6. Cathodic and Anodic Limits and Electrochemical Window for Each Specified PIL, Working Electrode Material, and Reference Electrodea PIL EAN pyrrolidinium NO3 pyrrolidinium HSO4 pyrrolidinium formate pyrrolidinium acetate pyrrolidinium TFA pyrrolidinium C7H15COO 1-methylpyrrolidinium BF4 1-methyl-2-oxopyrrolidinium BF4 1-methylpiperidinium BF4 1-ethylpiperidinium BF4 4-methylmorpholin-4-ium BF4 4-ethylmorpholin-4-ium BF4 diisopropylethylammonium formate diisopropylethylammonium acetate diisopropylmethylammonium formate diisopropylmethylammonium acetate diisopropylethylammonium HF diisopropylmethylammonium HF diethanolammonium acetate diethanolammonium acetate diethanolammonium acetate diethanolammonium acetate diethanolammonium acetate diethanolammonium acetate diethanolammonium HSO4 diethanolammonium HSO4 diethanolammonium HSO4 diethanolammonium HSO4 diethanolammonium HSO4 diethanolammonium HSO4 di-n-propylammonium formate di-n-propylammonium formate di-n-propylammonium formate di-n-propylammonium formate di-n-propylammonium formate di-n-propylammonium formate triethylammonium acetate triethylammonium acetate triethylammonium acetate triethylammonium acetate triethylammonium acetate triethylammonium acetate diethanolammonium formate diethanolammonium formate diethanolammonium formate diethanolammonium formate diethanolammonium formate diethanolammonium formate triethylammonium formate triethylammonium formate triethylammonium formate triethylammonium formate triethylammonium formate triethylammonium formate diethanolammonium sulfamate diethanolammonium sulfamate diethanolammonium sulfamate diethanolammonium sulfamate

cathodic limit (V)

anodic limit (V)

−0.85 −0.3 −1.2 −0.3 −0.3 −0.6 −0.2 −2.75 −2.45 −2.72 −2.68 −2.69 −2.66 −2.6d −2.6d −2.4d

1.96 1.2 1.8 1.2 1.2 1.5 1.2 0.45 0.51 0.47 0.51 0.48 0.50 0.7d 0.9d 0.8d

−2.1d

0.75d

−1.84 −0.63 −1.53 −0.73 −0.93 −1.24 b b −1.05 −0.48 −0.38 −1.79 −2.46 −1.26 −1.95 c −0.93 −1.38 −2.77 −1.49 −2.39 −0.58 −1.17 −1.33 −3.39 −2.05 −2.73 c −1.00 −1.42 −3.04 −1.70 −2.45 c −1.08 −1.26 b b −2.41 −0.58

0.59 1.80 0.90 0.77 0.99 1.15 b b 1.11 1.45 1.44 1.11 0.23 1.43 0.74 c −0.21 1.10 0.65 1.93 1.03 0.95 0.89 0.99 0.21 1.55 0.87 c 0.16 0.93 0.39 1.73 0.98 c −0.15 −0.87 b b 1.84 0.89 11402

electrochemical window 2.81 1.5 3.0 1.5 1.5 2.1 1.4 3.2 2.96 3.19 3.19 3.17 3.16 2.7 2.6 2.7 2.6 2.3 2.2 2.43 2.43 2.43 1.50 1.92 2.39 b b 2.16 1.93 1.82 2.90 2.69 2.69 2.69 c 1.14 2.48 3.42 3.42 3.42 1.53 2.06 2.32 3.60 3.60 3.60 c 1.16 2.35 3.43 3.43 3.43 c 1.23 2.13 b b 4.25 1.47

working electrode material GC carbon carbon carbon carbon carbon carbon GC GC GC GC GC GC vitreous vitreous vitreous vitreous vitreous vitreous GC GC GC Pt Au BDD GC GC GC Pt Au BDD GC GC GC Pt Au BDD GC GC GC Pt Au BDD GC GC GC Pt Au BDD GC GC GC Pt Au BDD GC GC GC Pt

carbon carbon carbon carbon carbon carbon

reference electrode Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Fc/Fc+ Fc/Fc+ Fc/Fc+ Fc/Fc+ Fc/Fc+ Fc/Fc+ Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Fc/Fc+ Cc+/Cc Ag wire Ag wire Ag wire Ag wire Fc/Fc+ Cc+/Cc Ag wire Ag wire Ag wire Ag wire Fc/Fc+ Cc+/Cc Ag wire Ag wire Ag wire Ag wire Fc/Fc+ Cc+/Cc Ag wire Ag wire Ag wire Ag wire Fc/Fc+ Cc+/Cc Ag wire Ag wire Ag wire Ag wire Fc/Fc+ Cc+/Cc Ag wire Ag wire Ag wire Ag wire Fc/Fc+ Cc+/Cc Ag wire Ag wire

ref 271 66 66 66 66 66 66 61 61 61 61 61 61 64 64 64 64 64 64 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58

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Table 6. continued PIL diethanolammonium sulfamate diethanolammonium sulfamate diethanolammonium Cl diethanolammonium Cl diethanolammonium Cl diethanolammonium Cl diethanolammonium Cl diethanolammonium Cl diethylmethylammonium trifluoromethanesulfonate 1H-1,2,4-triazolium methanesulfonate triethylammonium TFSI triethylammonium TFSI dimethylethylammonium triflate butylimidazolium Tf2N triethylmethylammonium triflate triethylmethylammonium OMs HBIm OMs 2-MPy TFA 2-MPy TFA DEMA triflate 2-MePy TFA 2-MePy TFA 2-EtPy TFA 2-EtPy TFA 2-pentylPy TFA 2-pentylPy TFA 3-EtPy TFA 3-EtPy TFA 2-MePy Fm 2-MePy Fm pyrrolidinium nitrate

cathodic limit (V) −0.97 −1.98 b b −2.54 0.10 −0.10 −1.11 −0.05 −1.3d −1.3d 0 0 −2.5d −2.2d −2.3d −0.4d −1.3d 0 −0.4 −1.4 −0.3 −1.0 −0.3 −0.7 −0.5 −1.0 none none −0.35d

pyrrolidinium formate

−1.65d

pyrrolidinium formate pyrrolidinium formate pyrrolidinium formate pyrrolidinium formate pyrrolidinium acetate pyrrolidinium acetate pyrrolidinium acetate pyrrolidinium acetate ethylenediammonium formate ethylenediammonium formate ethylenediammonium formate ethylenediammonium formate ethylenediammonium acetate ethylenediammonium acetate ethylenediammonium acetate ethylenediammonium acetate diethanolammonium di-n-butylphosphate diethanolammonium di-n-butylphosphate diethanolammonium di-n-butylphosphate diethanolammonium di-n-butylphosphate bis(2-methoxyethyl)ammonium formate bis(2-methoxyethyl)ammonium formate bis(2-methoxyethyl)ammonium formate bis(2-methoxyethyl)ammonium formate bis(2-methoxyethyl)ammonium acetate bis(2-methoxyethyl)ammonium acetate

−1.89 −1.51 −0.87 −0.37 −2.33 −1.78 −0.97 −0.68 −2.56 −1.73 −0.88 −0.50 e −1.87 −0.90 −1.03 e −2.05 −0.93 −0.62 −2.06 −1.40 −0.62 −0.72 −2.00 −1.37

anodic limit (V) 1.00 1.05 b b 1.47 1.24 0.73 1.39 1.94 2.8d 2.5d 1.3 1.3 2.7d 2.1d 1.8d 2.1d 1.7d 1.3 2.0 1.6 2.0 1.8 2.1 1.9 2.0 2.5 −1.0 to 1.2 −1.0 to 1.2 1.15d 0.65d 0.66 1.04 0.95 1.08 0.32 0.87 0.90 0.92 0.19 1.02 0.18 1.23 e 0.89 0.58 1.42 e 0.79 0.64 1.01 0.18 0.84 0.10 −0.06 0.37 1.00

11403

electrochemical window

working electrode material

1.97 3.03 b b 4.01 1.14 0.83 2.50 1.3 2.0 4.1d 3.8

Au BDD GC GC GC Pt Au BDD Pt Pt Pt Pt

5.2d 4.3d 4.1d 2.5 3.0 1.3 2.4 3.0 2.3 2.8 2.4 2.6 2.5 3.5

1.5

GC GC GC Pt GC Pt wire Pt GC Pt GC Pt GC Pt GC Pt GC GC

2.3

GC

2.55 2.55 1.82 1.45 2.65 2.65 1.87 1.60 2.75 2.75 1.06 1.73 e 2.76 1.48 2.45 e 2.84 1.57 1.63 2.24 2.24 0.72 0.66 2.37 2.37

GC GC Au Pt GC GC Au Pt GC GC Au Pt GC GC Au Pt GC GC Au Pt GC GC Au Pt GC GC

reference electrode Ag wire Ag wire Fc/Fc+ Cc+/Cc Ag wire Ag wire Ag wire Ag wire RHE RHE Ag Pt wire RHE RHE Ag wire Ag wire Ag wire

RHE AgQRE AgQRE AgQRE AgQRE AgQRE AgQRE AgQRE AgQRE AgQRE AgQRE Ag/AgClsat, KClsat Ag/AgClsat, KClsat Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire

ref 58 58 58 58 58 58 58 58 272 273 274 275 272 272 276 276 276 75 75 272 74 74 74 74 74 74 74 74 74 74 277 277 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278

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Table 6. continued PIL bis(2-methoxyethyl)ammonium acetate bis(2-methoxyethyl)ammonium acetate diethylammonium di-n-butylphosphate diethylammonium di-n-butylphosphate diethylammonium di-n-butylphosphate diethylammonium di-n-butylphosphate triethylammonium MsOH triethylammonium MsOH triethylammonium MsOH triethylammonium MsOH triethylammonium di-n-butylphosphate triethylammonium di-n-butylphosphate triethylammonium di-n-butylphosphate triethylammonium di-n-butylphosphate EANf triethylammonium methylsulfonatef bis(2-methoxyethyl)ammonium acetatef Im methanesulfonate: 1H-1,2,4-triazole, 0.25:0.75 molar ratio tributylphosphonium BF4 in acetonitrile

cathodic limit (V)

anodic limit (V)

electrochemical window

working electrode material

reference electrode

ref

−0.79 −0.59 −2.79b −2.09 −0.89 −0.68 −3.04 −2.70 −0.90 −0.42 e −1.76 −0.93 −0.69 −1.2d −1.72d −1.14d −0.00086

0.87 1.03 0.15b 0.85 0.79 0.93 1.69 2.03 1.93 2.05 e 0.90 0.83 0.93 2.1d 2.0d 1.0d 1.39

1.66 1.62 2.94 2.94 1.68 1.61 4.73 4.73 2.83 2.47 e 2.66 1.76 1.62 3.30 3.72 2.14 1.4

Au Pt GC GC Au Pt GC GC Au Pt GC GC Au Pt GC GC GC Pt

Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Fc0/+ Ag wire Ag wire Ag wire Ag wire Ag wire Ag wire RHE

278 278 278 278 278 278 278 278 278 278 278 278 278 278 279 279 279 155

−2.6d

3.3d

5.9d

GC

Ag/AgCl

280

Temperatures at 25 °C unless otherwise stated. Glassy carbon (GC), boron-doped diamond (BDD), platinum (Pt), gold (Au), ferrocene (Fc), silver (Ag), cobaltocenium hexafluorophosphate (Cc), reversible hydrogen electrode (RHE). bNot established. cNo well-defined potential window. d Estimated from figure in reference. eFerrocene not sufficiently soluble. fAppreciable amounts of water present. a

alkyl chain.69 The maximum conductivities occurred at water contents between 0.41 and 0.74 weight fraction of water.69 It was found that the highest conductivities were obtained using nitrate or formate anions, and that these occurred at lower water concentrations as compared to the other alkylcarboxylate anions. More hydrophobic cations or anions required higher proportions of water to reach their conductivity maxima, and did not have as significant an increase in their conductivity as compared to the more hydrophilic ions.69 This was evident for the pyrroldinium cation paired with the hydrophilic HSO4 or more hydrophobic TFA anions where Pyrr HSO4 had a maximum conductivity of 187 mS/cm at 60 wt % water, whereas Pyrr TFA had a maximum of 44.20 mS/cm at 50−55 wt % water.269 For Pyrr HSO4, the higher conductivity is attributed to it forming a stronger bond between the anion and water, which leads to a greater effect on the viscosity, and hence conductivity. In addition, Pyrr HSO4 appears to have Grotthuss proton conduction for water concentrations above 50 wt % and a vehicle-type mechanism for lower water concentrations.269 In these pyrrolidinium PILs, for water concentrations above 8 wt %, the proton was solvated by the water as a H3O+ ion, while for lower concentrations it was located between the cation and anion and not solvated.269 The diisopropylethylammonium cation (DIPEA)63,65,268 has received attention, along with a broad range of anions. For DIPEA formate, the maximum conductivity occurs between 0.6−0.75 wt fraction of water,63 as compared to 0.707 wt fraction of water for DIPEA methanesulfonate.268 The DIPEA methanesulfonate PIL combined with water was determined to be promising as an electrolyte for supercapacitors. This PIL− water electrolyte had much higher power density than conventional aqueous electrolytes, due to the PIL’s larger electrochemical window, while still having comparable energy densities. Because of its lower level of hydration as compared to some systems, it had faster rates in and out of pores in the activated carbon electrode, leading to better charge/discharge

8. ELECTROCHEMISTRY The inherent nature of PILs of having protons available for proton conduction, without needing to be acidic, makes them highly suitable as electrolytes, such as in fuel cells and batteries. A perspective article by MacFarlane et al. on the potential use of aprotic and protic ILs in various energy applications highlighted the broad range of possible applications, which are due to fundamental properties associated with ILs such as high thermal stability, high proton conductivity, and low vapor pressures.267 As outlined below, the properties of PILs can be tailored for specific applications to achieve wide electrochemical windows, high conductivity, and/or other desirable properties. This can be done through changes to the structure of the cation or anion, or through the presence of an additional solvent. 8.1. Effect of Water

Anouti et al. have characterized the effect of added water in a variety of PILs that are of interest as electrolytes for energy storage devices.63,65,69,268,269 In particular, this body of work has been focused on the beneficial conductivity properties that water provides for PILs, and also on what effect the byproduct water will have on anhydrous PIL electrolytes in fuel cells. The conductivity properties can be greatly enhanced through addition of water for the PILs trialed. The addition of water decreases the viscosity of the PILs, which increases the charge mobility, and increases the conductivity. In opposition to this, the added water also decreases the number of charge carriers through dilution. Typically there is a maximum in the conductivity at medium to high levels of water where the increase in charge mobility is a significantly greater effect than the dilution. This has been reported as 2−30 times larger as compared to that of the neat PILs.69 The optimal water content, and increase in the conductivity, is highly dependent on the specific ions in the PIL. A range of pyrrolidinium containing cations were paired with nitrate or alkylcarboxylate anions with 5−8 carbons in their 11404

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properties.268 The PILs of DIPEA with heptanoate or octanoate anions were non-Newtonian with shear thickening behavior. These PILs had an increase in their viscosity on addition of up to 0.8 mole fraction of water.65 DIPEA or diisoproylmethylammonium cations paired with formate, acetate, or hydrogenbisfluoride anion all had a density maximum at 20−30 wt % in water. In contrast, BMIm BF4 has a constant decrease in density with addition of water. The protic molten salt imidazolium methanesulfonate when combined with 0.75 mole fraction of 1H-1,2,4-triazole has a eutectic composition with a melting point of 79.5 °C, which is considerably lower than the melting point of the neat protic molten salt of 188.2 °C.155 This system was developed as a potential electrolyte for polymer electrolyte membrane fuel cells for use under nonaqueous conditions. A high conductivity was maintained for the eutectic, with the decrease in carrier concentration due to dilution with the 1H-1,2,4-triazole balanced by the increase in mobility through reduction of electrostatic interactions through the solvation shells decreasing the interaction between ions. The 1H-1,2,4-triazole behaves as a solvent to dissolve and dissociate the PIL, but does not appear to behave as a proton solvent.155 There was no evidence from IR for any protonation of the 1H-1,2,4-triazole. The electrolyte properties for mixtures of the organic solvents dimethylcarbonate or formamide added to EAN were reported.270 The dimethylcarbonate was selected as a weakly polar, aprotic solvent and formamide as a strongly polar, protic solvent. The solvent structure of dimethylcarbonate was disrupted on addition of EAN, whereas the structure of formamide was retained. The addition of the solvents to EAN increased its conductivity with maxima of 33.25 mS/cm at 25 °C for 6−7 mol/L of dimethylcarbonate, and ∼37 mS/cm at 25 °C for 4−5 mol/L of formamide.270 These two systems followed Arrhenius-type behavior, which is typical for salt solutions.270 The maxima in conductivity arose due to the addition of solvents decreasing Coulombic interactions and ionassociation present in EAN, thus lowering the viscosity, and having this effect greater than that of the decrease in charge carriers due to dilution.

Zhao et al. clearly showed that there is a significant difference in the anodic and cathodic limits depending on what material the working electrodes are made from.58 The largest electrochemical windows were obtained using glass carbon, followed by boron-doped diamond, then gold, and last platinum.58,278 The largest change in electrochemical windows for different electrode material occurred for diethylammonium chloride, which varied more than 3 V from 0.83 V using gold to 4.01 V for glassy carbon.58 The electrochemical windows obtained for the PILs using glassy carbon working electrodes had the highest values of 4.25 for diethylammonium sulfamate58 and 4.73 for triethylammonium MsOH.278 These values are lower than those generally reported for AILs, which can be up to 6.0 V,281 but larger than protic molecular solvents. The electrochemical window of a solvent mixture of 3.4 mol/L of [Bu3HP][BF4] in acetonitrile had an electrochemical window of about 6 V, which was highly similar to that of the aprotic [Et4N][BF4] under the same conditions.280 Changes to the stoichiometry of the acid and base were found to have a negligible effect on the electrochemical window for 2-methylpyridinium TFA in the 1:1 (PIL) and the 1:2 (base:acid) ratios.75 In addition, it was determined that the colored impurities for this IL had no effect on the electrochemical window of stability.75 The reduction reactions that occur at the negative electrode for the common cations used in ionic liquids have been reviewed.282 The mechanisms through which the different cations break down are dependent on the type of cation, with the aprotic ILs being stable to much lower potentials, particularly quaternary ammonium and phosphonium cations. In PILs, the limiting negative potential is generally where hydrogen gas is evolved from the acidic protons.282 In addition, the trace amounts of water that are frequently present in PILs will decrease their electrochemical stability. 8.3. Electrolytes for Capacitors and Supercapacitors

A variety of PILs, nonstoichiometric protic salts, and PILsolvents have been trialed as potential electrolytes in supercapacitors by Anouti and Timperman.73,74,274,275,277,280,283,284 In general, the nonstoichiometric protic salts and PIL-solvents had properties superior to those of the PILs for use as electrolytes in supercapacitors, and in comparison to aqueous electrolytes could be used at much lower temperatures.277,280 A series of protic salts consisting of heterocyclic amines73 and alkyl-pyridine74 with TFA in ratios of 1:1 and 1:2 were prepared and characterized as potential electrolytes for metal− oxide supercapacitor applications. In general, the protic salts with a 1:2 base:acid ratio had better properties than their 1:1 counterparts, attributed predominantly to the lower viscosities.73,74 Initial structure−property relationships indicated that shorter alkyl chains lengths led to higher conductivities.74 The protic salts were trialed with a RuO2 electrode to assess their capacitance properties, with specific capacitance values obtained for the heterocylic ammonium TFA salts between 26−42 F/g,73 and for the alkyl-pyridinium TFA salts between 44−50 F/g.74 In comparison, the aprotic BMIm PF6 has a specific capacitance of 4.1 F/g. The conductivity was mainly due to a vehicle-type mechanism, and overall the data suggest that these PILs are enabling pseudocapacitance. These include pyrrolidinium nitrate mixed with 0.608 molar fraction of γ-butyrolactone, which led to an operating voltage of 2 V, rapid charging and discharging, and the ability to be used at temperatures down to −60 °C.283 Triethylammonium Tf2N

8.2. Electrochemical Windows

The electrochemical window of ionic liquids is governed by the oxidation of the anion, which sets the anodic limit, and the reduction of the cation, which sets the cathodic limit. A summary of the electrochemical windows, and the anodic and cathodic limits for PILs, are provided in Table 6. From the limited data available, it appears that the trend for the electrochemical windows in PILs based on the anions follows the trend of HF < nitrate, formate < acetate < chloride < sulfamate, HSO4. The carboxylate anions become oxidized at relatively low anodic potentials.64 There is insufficient data to determine trends between the electrochemical window of the PILs and the cations. The presence of electron-withdrawing groups on the cation led to increased cathodic limits, which corresponds to smaller electrochemical windows.58,61 A broad range of reference electrode and working electrodes have been used, which is evident in Table 6. The reference electrode potential scales of ferrocene, cobaltocenium hexafluorophosphate (Cc), and silver wire showed no difference in the electrochemical window. The ferrocene and Cc are internal standards and IUPAC recommended; however, these had undesirable behavior in high viscosity PILs due to solubility issues, leading to adsorption/precipitation.58,278 In contrast, 11405

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electrolyte in a fuel cell, it is required to have a high ionic conductivity, thermal stability, low volatility, and electrochemical stability. The commonly used Nafion requires water to be present for high proton conductivity, whereas due to their protic nature PILs do not. Consequently, they could be used in fuel cells at higher temperatures than is achievable with water, which is particularly beneficial for the performance of the Pt catalyst, which can become poisoned at lower temperatures. Aprotic ionic liquids with available protons have also been used as electrolytes for PEMFCs, such as 1-methyl-3-(4-sulfobutyl)imidazolium Tf2N.292 A report of the use of PILs in fuel cells as electrolytes and in membranes highlights the potential used of PILs in this field, and identifies a few promising candidates to date.291 A schematic of a PEMFC is provided in Figure 6.

with acetonitrile had a maximum conductivity of 50 mS/cm, as compared to 5 mS/cm for the neat PIL.274 The mixture of the PIL tributylphosphonium BF4 (Bu3HP BF4) with 0.86 molar fraction of acetonitrile produced an electrolyte with a large electrochemical window of 6 V, an operating voltage of 1.5 V, good stability for 1000 charge−discharge cycles, and an operating temperature between −40 and 80 °C.280 At 80 °C, the conductivity was 5.42 mS/cm for Bu3HP BF4, whereas the Bu3HP BF4−acetonitrile mixture was 46.8 mS/cm.280 Further characterization of the Bu3HP BF4−acetonitrile electrolyte transport properties clearly showed the significant increase in conductivity for the mixtures of the two solvents, with a maximum of 29.3 mS/cm at 0.809 molar fraction of acetonitrile at 25 °C.285 On addition of acetonitrile to the PIL, the increase in the conductivity up to the maximum is attributed to the decrease in the viscosity being greater than the decrease of ionic species through dilution.285 At higher acetonitrile concentrations, the dilution effect dominates. Pyrrolidinium nitrate or pyrrolidinium formate combined with graphite and a polyvinyl fluoride binder were trialed as working electrodes in capacitors, and as electrolytes for conventional activated carbon electrodes.277 These PILs enabled higher voltage windows as compared to aqueous electrolytes, and an ability to be used at lower temperatures.277 Adjusting the pH of pyrrolidinium nitrate from 7 to around 11 improved the specific capacitance of a supercapacitor with porous activated carbon electrodes from 121 to 208 F/g.284 The presence of water has also been shown to have a detrimental effect due to decreasing the cell voltage.284 The mixtures of 2-methoxypyridine and TFA in a 1:1 and 1:2 ratio were trialed as electrolytes for capacitors containing MnO2 electrodes, and for the electrodeposition of the MnO2 films. The nonstoichiometric composition resulted in a significantly lower viscosity, and hence faster ion transport, and enabled metal oxide electodes to be used above 100 °C.286 The stability of PIL-based supercapacitors was investigated, and promising results were obtained, with cycling stabilities over 30 000 cycles, and various temperature ranges over which they could be used, including as low as −20 °C for a mixture of trimethylammonium TFSI with propylene carbonate, and across the broad range of 10−60 °C for triethylammonium TFSI.275

Figure 6. Schematic of a polymer membrane fuel cell.

A great deal of work has been conducted in this field by Watanabe et al.,272,293−295 who have screened large libraries of protic ILs and nonstoichiometric protic salts, identifying potentially useful PILs, and developing structure−property relationships. A series of aliphatic amines were combined with either an oxoacid or imide-acid to produce protic salts, of which many were liquid below 100 °C and hence can be classified as PILs.272 Diethylmethylammonium (DEMA) TfO was identified as the most promising due to an ionic conductivity of 0.430 mS/cm at 120 °C, low melting point of −13.1 °C, and decomposition temperature of 360 °C.272 DEMA TfO was also trialed as an electrolyte in direct methanol fuel cells.296 The methanol oxidation had a high overpotential in the neat PIL, which could be lowered by addition of water; however, that prevents the use of the fuel cell above 100 °C, which is the key potential advantage of using PILs.296 Further investigations of this PIL showed that for positive potentials above 1.0 V an oxide layer was formed on the Pt surface due to trace amounts of water present, which slowed the oxidation reaction.297 Proton conduction was observed to occur between DEMA TfO and water in solutions with a 1:1 M composition. A mixture of NMR and quantum chemical calculations was used to outline two possible proton exchange pathways, such as through dimer or trimer complexes, with the presence of the water increasing the proton conduction.143 Closely related PILs shared some promising properties with DEMA TfO, but overall did not have equally good character-

8.4. Electrolytes for Batteries

Electrolytes containg 0.5 M LiNO3 in a 1:1 solution by weight of pyrrolidinium nitrate and propylene carbonate,287 and 1 M LiTFSI in triethylammonium TFSI,288 have been developed as electrolytes for lithium-ion batteries. These proof of concept investigations showed that the electrolytes had a sufficiently wide electrochemical window and good conductivity for use in lithium ion batteries.287,288 The lithium ion coordination of LiTFSI was compared between a series of pyrrolidinium PILs and aprotic ILs.289 The interactions were different for PILs and AILs predominantly due to the steric shielding present for aprotic ILs. For PILs, the TFSI ion interacts with both lithium ions and the anion of the PIL, whereas in the aprotic ILs the TFSI mostly interacts with the lithium ion. Consequently, the lithium ions are more mobile in the protic ionic liquids.289 8.5. Electrolytes for Fuel Cells

A niche application of PILs is as electrolytes for fuel cells, particularly polymer electrolyte membrane fuel cells (PEMFC), under non-humid conditions.290,291 For a PIL to be of use as an 11406

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isatics. Specifically, DEMA TF2N also had a high ionic conductivity, of around 0.5 mS/cm at 150 °C, but a lower open circuit voltage of 0.7 V as compared to 1.03 V for DEMA TfO.294 A higher ionic conductivity of 0.56 mS/cm was reported for dimethylethylammonium (DMEA) TfOH, but its higher melting point of 41.6 °C made it less favorable overall.272 Triethylammonium MsOH was trialed as an electrolyte for fuel cells in the neat form, and with additional water. The neat PIL had greater electrochemical stability, whereas the addition of water led to much higher conductivity, but also had the detrimental formation of platinum oxide at 1.2 V.278 A series of PILs consisting of the DEMA cation paired with fluoroalkylsulfonates were investigated for use in fuel cells. Increasing the length of the fluoroalkyl chain on the anion led to increased oxygen solubility, although the diffusion decreased. Of the PILs trialed, the DEMA HfO had the most promising properties for use as an electrolyte in fuel cells.298 EAN was shown to possess a moderately favorable oxygen reduction rate (ORR), diffusion and dissolution of oxygen, and was suggested to be a promising electrolyte for fuel cells.299 In related work, the ORR was determined to be about 10−6 cm2/s in a wide range of PILs.299 It was identified that there was a maximum in the ORR with temperature for some PILs. The ΔpKa difference between the precursor acid and base of a PIL in water can be used with caution to obtain an indication of how strong the proton transfer may be. A correlation was reported by Watanabe et al., which showed that the open circuit potential (OCP) for hydrogen fuel cells increased with increasing ΔpKa to around ΔpKa ≈ 17, then decreased.77 Consequently, there appears to be an optimal range of ΔpKa values for PILs to have maximum electrochemical activity. Three PILs had ΔpKa ≈ 17, all with similar OCP values where ethylmethylpropylammonium(EMPA) C4F9SO3 > DEMA TfO > EMPA TfO.77 For the PILs that have very high ΔpKa values, the poor electrochemical behavior was attributed to the strong hydrogen bonds between the nitrogen and the “available” hydrogen leading to poor proton activity. The PILs that had lower ΔpKa’s will have more neutral species present, through incomplete proton transfer, causing a decrease in the OCP such as through oxidation of the neutral species.77 Electrocatalytic reactions, such as those required in fuel cells, have also been investigated using PILs. During electrochemcial use, PILs containing DEMA or DMEA cations with a TfO anions300 led to a monolayer of hydrogen adsorbed to the Pt electrode, and good behavior toward CO oxidation with a prepeak present at lower potentials. In contrast, the PILs containing the DEMA or DMEA cations with the Tf2N anion had the Tf2N anion adsorbed from low potentials, which hindered the hydrogen absorption and resulted in poorer CO oxidation due to blocking of the catalytic sites.300 The hydrophobic MTBD beti, shown in Figure 7, was incorporated into high surface area NiPt alloy nanoporous particles for use as a catalyst for oxygen reduction in fuel cells.301 These PIL incorporated nanoparticles had high oxygen solubility, and showed very promising catalytic behavior as compared to Pt/C and other commercial catalsyts for fuel cells. The benefit of using a hydrophobic IL was that there was no detrimental buildup of water at the catalyst surface, although small amounts of water were present, which aided in the proton transfer.301 Investigations into the cathodic oxygen reduction rate (ORR) in DEMA TfO on a rotating Pt disk electrode highlighted the

Figure 7. Structure of MTBD beti, which is the product of the acid− base reaction of bis(perfluoroethylsulfonyl)imide) with (7-methyl1,5,7-triazabicyclo[4.4.0]dec-5-ene).303

important kinetic and potential information that can be obtained using Tafel plots, which relates the rate of an electrochemical reaction to the overpotential. These data can then be used for optimizing future systems.302 Protic salts with nonstoichiometric proportions of the acid and base precursors have been trialed to optimize their performance as electrolytes. A variety of salts were prepared from 1H-1,2,4-triazole with methanesulfonic acid, and the conductivity was lowest for the PIL, while there were two local maxima for 0.1 and 0.8 mole fractions of the base of 149 and 128 ms/cm at 200 °C, respectively.273 In a series of protic salts prepared from benzimidazole with TF2N, the stoichiometric salt (PIL) had the highest H+ conductivity of 0.83 mS/cm and a thermal stability above 350 °C.293 The use of this PIL, and its nonstoichiometric counterpart with extra benzimidazole, worked well as electrolytes for fuel cells, with both H2 oxidation and O2 reduction occurring at a Pt electrode, with behavior comparable to that of anhydrous H3PO4 or aqueous H2SO4 solutions. 8.6. Ionic Conductive Membranes Containing PILs for Fuel Cells

A key component of polymer electrolyte membrane fuel cells (PEMFC) is the proton conducting membrane.8 Currently materials such as Nafion are used as the electrolyte membranes under a high level of water hydration, which limits their use to temperatures below 80 °C. Unfortunately, CO poisoning of the Pt catalyst during cell operation is increased with decreasing temperature. There has been significant interest in PILs as a nonaqueous protic solvent to replace the water in these membranes, thus enabling PEMFC to be operated at higher temperatures, and this was the focus of a review by MacFarlane and Forsyth,304 and considered along with other nonaqueous sovlents in a review by Pourcelly.8 PILs have also been used in these membranes in a variety of ways, including doped or adsorbed into a polymer or zeolite matrix, or with the cationic group of the PIL incorporated into the polymer matrix and the polymer neutralized with the Brønsted acid. A summary has been provided of these PIL-containing membranes in Table 7, along with their maximum reported conductivity. However, it should be noted that PILs and AILs can still lead to poisoning of the Pt/C catalyst in fuel cells, and the extent depends on which IL.305 It is difficult to directly compare the behavior of different PILs in this role because the specific PIL and membrane combinations will affect the final conductivity, as will other additives in the system, particularly water. It is evident from Table 7 that most of the membranes had conductivities in the range of n-pentyl-benzene > methyl-propionate > methanol.362 As expected, the more polar solutes had significantly stronger interactions with EAN. As compared to a broad range of imidazolium and quaternary AILs, the interactions were nearly all weaker in EAN. The large difference in the activity coefficients values for n-dodecane and methanol in EAN suggests that the separation of polar and nonpolar solutes may be feasible using EAN, or related PILs.362 Since then, Domanska et al. have recently extended on their widespread reporting of activity coefficients for AILs to include the PIL containing a bicyclic guanidine cation paired with a superbase of 1,3,4,6,7-hexahydro-1-methyl-2H-pyrimidol[1,2a]pyrimidine bis(pentafluoroethyl)sulfonylimide, MTBDH

11. GAS CAPTURE There is a strong need for efficient solvents that are capable of absorbing CO2 and SO2 and other gases, and some ionic liquids are promising candidates for gas capture.364−366 So far, predominantly aprotic ILs have been trialed, although gas solubilities in a few PILs have also been reported,194,195,367−370 and are described in this section. The CO2 solubility was determined in a series of eight hydroxyl-containing ammonium PILs, with the highest solubility observed in triethanolammonium lactate and tri-(2-hydroxy ethoxy)ammonium acetate.371 Similarly, a series of PILs with multiple amine moieties on the cations paired with formate, acetate, or chloride anions were trialed for CO2 capture. The highest CO2 capture obtained for the neat PILs, and one aqueous PIL solution, was ∼13 w/v %, which corresponds to a mole fraction comparable to that achieved with the commonly used methylamine solution with 30 wt % water.369 An added advantage was that the amine groups present in these PILs were less basic than in their molecular counterparts, leading to lower binding energies to the CO2, overall less alkaline and less corrosive solutions, and lower energies required for desorption of the CO2.369 The PILs resulting from the cation of N-methyl-2hydroxyethylammonium paired with formate or acetate anions were shown to have good CO2 solubility under high pressure of up to 80 MPa, with slightly higher solubility in the acetate PIL.372 In contrast, methane had poor solubility in the acetate containing PIL, indicating that N-methyl-2-hydroxyethylammonium acetate is potentially useful for separating carbon dioxide out of methane. The acetate anion was observed to form chemical interactions with the carbon dioxide through electron donor−acceptor complexes, whereas there were no specific interactions present for the formate containing PIL.372 Porous nitrogen doped carbon materials were prepared from PIL and 11411

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that should neither be bulky nor hydrophobic, and the ability to offer several H-bond acceptor sites.”382 The use of ILs in the pretreatment of lignocellulosic biomass was reviewed in 2013,379 and while there were many ILs that were useful, the cost, difficulty to recycle, and viscosity were identified as challenges to the use of ILs at an industrial scale.379 Protic ILs have the key benefit as compared to their aprotic counterparts of being simple to synthesize, and generally much cheaper. In particular, the PIL BIm HSO4 was trialed as a comparison to the effective lignocellulose dissolving aprotic BMIm HSO4. The PIL was trialed due to being significantly cheaper and easy to synthesize, and successfully dissolved the lignocellulose of Miscanthus giganteus.383 Nonstoichiometric proportions of the BIm HSO4 between acid-rich and base-rich compositions modified the final products; where slightly acid-rich compositions led to increased glucose yields, highly acid-rich compositions were detrimental to the process overall. The presence of water was essential for a good digestion of the cellulose.383 There have only been a few other PILs that have been reported capable of dissolving these biopolymers. The PIL of MIm Cl dissolved cellulose and was also able to support cellusase enzymes with good hydrolysis rates, although not as good as the aprotic IL of tris(2-hydroxyethyl)methylammonium methylsulfate (HEMA).384 PILs consisting of the MIm cation paired with the anions of Cl, Br, hydrogen sulfate, and BF4 were able to degrade lignin through hydrolysis of the β-O4 ether linkage.385 MIm Cl was found to be a better solvent for corn straw and soybean straw lignocellulosic materials than the PILs consisting of MIm with acetate, H2PO4, or HSO4.386 MIm HSO4 was used in an optimization study into isolating cellulose and other components from the lignocellulosic feedstock of sugarcane bagasse. Specifically, the PIL was used as a catalyst to hydrolyze the filtrate obtained after the cellulose had been removed, resulting in the synthesis of furfural.387 There was no correlation identified between the Hammett acidity of the PILs and their ability to catalyze the hydrolysis. However, the anions that were more capable of forming stronger hydrogen bonds had a greater ability to dissolve the biomass, and to degrade the lignin.385 Triethylammonium methanesulfonate was selected as a low viscosity, high conductivity solvent for dissolving lignocellulosic biomass.388 Electro-oxidative cleavage of this solution resulted in a wide range of aromatic fragments. The ability of the PIL to dissolve the lignin was attributed to the sulfate component of the anion. PILs containing the cations of pyrridinium, MIm, or pyrrolidinium paired with acetate were able to dissolve the lignin to levels greater than 50 wt %, although they were poor solvents for cellulose, and hence are promising solvents for lignin extraction, and furthermore the PILs could be recycled.389 PILs containing alkanolammonium cations of ethanolammonium, diethanolammonium, triethanolammonium, propan-1olammonium, and diallylammonium paired with anions of formate, acetate, malonate, and citrate were all observed to not dissolve cellulose.59 However, ethanolammonium formate and ethanolammonium acetate were both reported to be good solvents for dissolving the protein polymer from corn processing, known as zein, which is water insoluble.390 Both PILs performed similarly, with up to 20 % zein able to be dissolved at 95 °C, or higher, using microwave radiation. These PILs were proposed as economical and environmental

protic salt precursors, and were reported to absorb up to 2.63 mmol/g of CO2 at 25 °C under standard atmospheric conditions. This highest uptake was measured for the carbon material resulting from the precursor of DEMA HSO4.370 The solubility of H2S and CO2 was determined in the four PILs from the combinations of the cations methyldiethanolammonium (MDEA) and dimethylethanolammonium (DMEA) paired with the anions of formate and acetate.373 These PILs have potential for selectively adsorbing H2S due to having a solubility of H2S, which is an order of magnitude as compared to CO2. Of these PILs, the highest H2S adsorption and selectivity occurred in DMEA formate. The protons in H2S enable it to directly interact with the hydroxyls in the DMEA cation, which leads to the high solubility, and fast adsorption rates. However, CO2 requires hydration before it can interact with the PILs, and hence has a significantly slower adsorption rate and lower solubility levels.373 The TMG cation of 1,1,3,3-tetramethylguanidinium (TMG) has been used with a variety of ions to make PILs suitable for SO2 capture. The absorption and desorption of SO2 in PILs containing the TMG cation paired with the anions of phenol, 2,2,2-trifluoroethanol, or imidazole was observed to be via both physical and chemical interactions, with the preferred physical absorption favored at lower temperatures. For TMGIm the physical absorption dominated over chemical interactions up to 50 °C.374 A MD simulation of the solubility of SO2 and CO2 in TMGL showed that, while both gases where soluble in TMGL, the solubility of the SO2 was significantly greater, and hence there is selectivity toward the SO2.375 This greater solubility was attributed to the strong association between the SO2 and the lactate anion.375,376 Both the CO2 and the SO2 interacted with the TMG cation, with stronger binding energies between the nitrogen of the TMG cation with the SO2 than with the CO2.376

12. PILs AS SOLVENTS FOR BIOLOGICAL MEDIA The use of protic ionic liquids in biological applications has undergone significant growth since our last review. In 2000, Summers and Flowers reported that EAN suppressed aggregation of lysozyme, and improved its folding and refolding ability.377,378 Since then, the stability and activity of a few proteins in a range of PILs have been explored, and many PILs have been shown to be good stabilizers. PILs have also been shown to be beneficial in biocatalysis, as additives for protein crystallization, and to a lesser extent as solvents for lignin and cellulose for biomass conversion. There have been numerous reviews on the use of ILs in biological applications, although these have predominantly focused on aprotic ILs. Recently, these reviews have included the role of ILs in biomass conversion,379 the use of ILs in protein assays,9 solvents for biopolymers,125 and the stability of proteins.380,381 12.1. Biomass Conversion

The conversion of cellulose and lignin into a useable biomass requires the dissolution and hydrolysis of the biopolymers into smaller fragments. Ionic liquids have been shown to be capable of dissolving cellulose, and some general characteristics of the cation and anion likely to lead to dissolution have been identified. Specifically good cation characteristics are “an aromatic heterocycle with delocalized positive charge, a second heterocycle atom in the aromatic ring that results in a dipolar character of the cation.”382 For the anion, the promising characteristics are “small sized H-bond acceptors, substituents 11412

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alternatives to organic solvents, or to aprotic ILs, such as BMIm Cl.390 The best dissolution was obtained at 150 °C, although it was observed that both PILs boiled at this temperature. Previously, we have attributed the DSC peaks at 192 and 210 °C as the boiling points of ethanolammonium formate and ethanolammonium acetate,45 so it is interesting that the addition of zein significantly lowered the boiling point to less than 150 °C for these PILs. Both of these PILs, although especially ethanolammonium formate, readily form an amide through a condensation reaction, where the rate is increased on heating.45 Depending on the reaction durations used in these experiments, it could be expected that there will be an appreciable amount of amide present. Some solvation properties of monosaccharides D(+)glucose and D(−)ribose dissolved in an aqueous solution of 3hydroxypropylammonium formate were investigated, as part of an understanding on cellulose dissolution in ILs. Overall, the solvation was found to predominantly have hydrophilic−ionic interactions, which were stronger for the glucose. Parameters related to the taste quality of apparent massic volume and apparent massic isentropic compressibility indicate that the sweet taste of these two monosaccharides is maintained within this PIL solution.391

Figure 8. Kinetic resolution of 1-phenylethanol by Candida antarctica lipase B (CaLB)-catalyzed acylation with vinyl acetate to obtain (R)acetate and (S)-1-phenylethanol.396

to good reaction rates and high enantioselectivities, with PILs containing bulkier ions generally leading to higher activities, except for the PIL containing 1-methylpiperidinium, which led to slower acylation.396 The activity and structure of the enzymes subtilisin and chymotrypsin were tested using a model substrate in six hydroxyl containing PILs, which had water concentrations typical for “dry” PILs.397 The PILs used were diethanolammonium chloride or methanesulfonate, dimethylethanolammonium acetate or glycolate, bis(2-methoxyethyl)ammonium sulfamate, and N-butyldiethanolammonium trifluoromethanesulfonate. The subtilisin retained activity in diethanolammonium chloride with a lower rate than the aqueous buffer, but higher than in hexane.397 No activity was observed for any of the other PILs, or with the other protease of chymotrypsin. In contrast, the aprotic IL of BMIm PF6 supported activity of chymotrypsin but not subtilisin. The secondary and tertiary structures of the subtilisin were retained in diethanolammonium chloride, whereas the chrmotrypsin was either unfolded or misfolded and inactive.397 It was commented that, while this PIL was not a viable alternative solvent for these enzymes, screening a wider selection of PILs could identify one, and could lead to a greater understanding of the important solvent features for protein stability and biocatalysis. Tyrosinase-type enzymes are capable of converting monophenols into diphenols, and diphenols into quinines, as shown in Figure 9. The activity and selectivity for the tyrosinases of B.

12.2. Extraction of Bioactives

The extraction of bioactive lactones from plants using PILs and microwave-assisted extraction was investigated, and the same level of extraction was obtained using the PILs of N,Ndimethyl-N-(2-hydroxyethoxyethyl)ammonium propionate and N,N-dimethyl(cyanoethyl)ammonium propionate, as is obtained using conventional molecular organic solvents.392 The PILs could be recovered using hexane, and reused. The protic ionic liquid of 1-[(nonyloxy)methyl]-1Himidazol-3-ium salicylate was able to obtain higher amounts of extracts from the fungus Cantharellus cibarius than the molecular solvent of ethyl acetate and significantly more than the other PILs used in this investigation of MIm lactate and the aprotic ILs of 1-methyl-3-[(pentyloxy)methyl]-1H-imidazol-3ium tetrafluoroborate and 1,3-bis(butoxymethyl)-1H-imidazol3-ium tetrafluoroborate.393 The extracts obtained had good insecticidal and microbicidal properties.393 12.3. Biocatalysis

Biocatalysis in ionic liquids has been extensively reviewed, with a focus on aprotic ILs as they have been predominantly used.7,394,395 The review by Sheldon et al. in 2007 is comprehensive up to that date, and highlights the potential benefits of having the enzymes in ILs as compared to water for biocatalysis, such as improving the solubility of hydrophobic solutes, the unconventional solvent properties of ILs, and the ability to tailor the reactions to favor synthesis over hydrolysis.7 There was limited PILs papers cited, although it was mentioned that triethylammonium MeSO4 deactivated enzymes, attributed to its Brønsted acidity, whereas the quarternary aprotic IL of triethylmethylammonium MeSO4 did not.7 While the use of aprotic ILs still dominates the field of ILs used as solvents for biocatalysis, there has been an increasing interest in the use of PILs, as described below. The enzyme Candida antarctica lipase B (CaLB) can be used for the bioresolution of 1-phenylethanol-catalyzed acylation with vinyl acetate, as shown in Figure 8.396 This reaction was trialed using either free enzyme or cross-linked enzyme aggregates in a variety of PILs, which consisted of a ternary ammonium cation and a carboxylate anion. The use of PILs led

Figure 9. Representative reactions for tyrosinase reactions with (a) monophenolase activity and (b) diphenolase activity.398

megaterium and Agaricus bisporus toward some monophenols or diphenols improved through addition of EAN or BMIm BF4, and for some it decreased.398 Addition of the aprotic ILs of EMIm EtSO4 or BMIm Cl led to negligible enzyme activity or to a decrease in activity, respectively, for all reactions trialed. From the variation in behavior, the ILs appear to affect the enzyme structure and not the substrate solubility. Previously, it was reported that CaLB can catalyze a broad range of reactions in BMIm BF4 and BMIm PF6m, but with rates only slightly higher than can be achieved in organic solvents.399 The main benefits of the PILs in comparison to aprotic ILs as solvents for biocatalysis are that they are comparably inexpensive, and generally have better biodegradability and lower ecotoxicity.396 In addition, the hydrogendonating ability should stabilize enzymes, and they can be made self-buffering through using alkanoate anions.396 11413

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The addition of EAN to aqueous solutions of lysozyme causes an initial decrease in the lysozyme solubility followed by an increase with a maximum around 75 wt % of EAN. In one investigation, it was noted that the lysozyme solubility decreased by two orders of magnitude when 100 g/L of EAN was added, down to 4 g/L, as compared to 415 g/L in demineralized water.404 A later investigation into the solubility of lysozyme in EAN showed that, while the solubility decreased for up to 30 wt % of EAN, it then increased, with a maximum lysozyme solubility of 400 g/L when there was 75 wt % EAN present, as compared to 100 g/L406 for pure water. It is difficult to directly compare these two investigations because the solubility of lysozyme in pure water is stated as either 400 g/ L404 or 100 g/L.406 The lysozyme had good stability in the EAN−water mixtures, and could fold and refold.406 Similarly, investigations using other PILs showed that the solubility of lysozyme in aqueous systems was significantly decreased on addition of 100 g/L of either EOAF, bis(2methoxyethyl)ammonium acetate, N,N-dimethylethanolammonium glycolate, or choline dihydrogen phosphate.404 When the PILs were present at high concentrations, there was an increase in the lysozyme solubility for the PILs consisting of the triethylammonium cation with either the methanesulfonate or the triflate anion.406 In a related study, lysozyme was crystallized in the presence of the aprotic ILs of BMIm BF4, BMIm Cl, BMIm Br, and MMIm iodine. The aprotic ILs increased the relative activity of the lysozyme crystals through modifying the lysozyme structure.408 The increased lysozyme solubility in EAN−water solutions containing approximately 75 wt % EAN was exploited to obtain lysozyme crystals through a “rehydration” method. The addition of water decreased the lysozyme solubility, leading to the crystallization.406 The converse method of dehydration was faster and only led to aggregates or small, poor-quality crystals. The crystals of lysozyme produced through the rehydration method were of sufficient quality for X-ray diffraction.406 A similar method has been employed to crystallize lysozyme from a water−AIL solution, where the presence of the AIL of 1,3-butylimidazolium chloride increased the lysozyme solubility. A hanging-drop vapor diffusion protocol was used to crystallize the lysozyme from the supersaturated solution, leading to large single crystals being obtained.409 Other investigations showed improved lysozyme crystallization through addition of various PILs, leading to larger crystals,404 improved kinetics,404 and reduced crystal polymorphism.404 It was reported that the PILs containing the formate or glycolate anions were more beneficial for lysozyme than those containing nitrate anions.404 In a broader high-throughput investigation, 10 PILs were trialed as additives for protein crystallization, using the globular proteins of lysozyme, glucose isomerase, and trypsin.405 The JCSG+ screen was used, which is a matrix screen frequently used for protein crystallization studies where the crystallization conditions are unknown. The PILs were present at concentrations of 20 or 200 mM, and it was evident that the protein crystallization was modified differently for each protein−PIL combination, and hence there is unlikely to be one ideal PIL for all proteins.405 Ethylammonium acetate led to the best crystals of trypsin, through fewer nucleation sites and slower crystal growth.405 In contrast, EAA was a detrimental additive for lysozyme crystallization, with EAN, ethanol-, diethanol-, and triethanolammonium nitrate, leading to better quality crystals as compared to the water control system.405 It

It has been shown by Byrne et al. that PILs can be advantageously used as additives in aqueous systems to improve biotransformation reactions.400 The addition of 48 wt % of triethylammonium mesylate into the aqueous system containing Thermomyces lanuginosus (TLL) improved the thermal stability of the enzyme and its hydrolytic and selectivity toward concentrating omega 3 fatty acids from anchovy oil.400 The secondary and tertiary structures of the enzyme were modified on addition of the PIL, and highly dependent on the PIL:water ratio. At the optimal 48 wt % of this PIL, the tertiary structure was proposed to have changed to a more compact structure due to the decrease in available water, and the secondary structure to be predominantly helical.400 The addition of various cosolvents to the trans-sialylation of lactose using the biocatalyst of Trypanosoma rangeli was trialed to increase the yield of 3′-sialyllactose as compared to sialic acid. For the cosolvents trialed, the improvement in yield was greatest for tert-butanol followed by EAN and 1-(2-hydroxyethyl)-3-methylimidazolium PF6, with a detrimental effect on addition of MMIm MeSO4. Consequently, for this select group of ILs, there appears no clear fundamental benefit for using PILs as compared to aprotic ILs, and instead the molecular solvent was the best cosolvent, with an increase of 40% of the desired product with 25 v/v % tert-butanol added.401 The bioresolution of tertiary amino alcohols via their protic ionic liquid form was conducted using a hydrolase enzyme of subtilisin. It had high selectivity for the synthesis of quinuclidin3-ol in the ionic mixture, although poorer selectivities for the other compounds.402 PILs have also been used in low concentrations, between 0.5 and 3.0% (w/v), to protect the Burkholder cepacia lipase when it is immobilized in a silica matrix using sol−gel techniques.403 This enzyme is of use for biocatalysis, and immobilization has the potential to improve the enzyme behavior and its reuse. The PILs trialed consisted of the ethanolammonium cation paired with the anions of acetate, propionate, butyrate, and pentanoate. The presence of the PILs enhanced the yield of lipase immobilized, and led to higher lipase activities, with higher values for the PILs with longer alkyl chains. The ethanolammonium pentanoate at 1.0 % (w/v) had the best effect as an additive, leading to an increase in the activity recovery yield of 35 times as compared to no PIL present.403 The PILs also increased the surface areas and pore size of the silica sol−gel matrix. 12.4. Protein Crystallization

The studies to date on PILs used as additives for protein crystallization have shown that often the PILs increase the solubility of water-soluble proteins in aqueous systems. However, there has been limited investigation in this area, usually with lysozyme used,404−406 although one investigation included glucose isomerase and trypsin.405 A related investigation using three AILs as additives to protein solutions of canavalin, β-lactoglobulin B, xylanase, and glucoise isomerase determined that ILs are potentially useful for the solubulization and crystallization of proteins.407 It was highlighted that not all ILs will affect proteins in the same manner, and that the addition of ILs often has a significant effect on the solubility of the proteins.407 In general, for PILs and aprotic ILs used as additives in protein crystallization, the improved protein crystals are obtained when the added ILs increase the protein solubility, and hence decrease the rate of crystallization, and increase the amount that can be dissolved. 11414

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enzyme Photinus pyralis luceiferase, although only the lactate anion led to an enhancement of its activity toward light emission from luciferin. In contrast, when the propionate,416 trichloroacetate,417 or trifluoroacetate417 anions were paired with the TMG cation, the enzyme had either no change or decreased stability and activity. The beneficial properties of the lactate anion were attributed to the additional hydroxyl moiety enabling greater coordination with the enzyme, a higher polarity, and hydrophilicity. The luminescence decay of the firefly enzyme was decreased in all of these PILs.416,417 In a comparison of the effect of added EAN and aprotic BMIm NO3 toward the stability of bovine milk β-lactoglobulin in aqueous solutions, it was observed that EAN inhibited protein aggregation, whereas BMIm NO3 induced it, and that above 15 mol % of either IL the tertiary structure of the protein was disrupted, with an α-helix structure forming.418 It was noted that these effects were comparable to those observed on addition of alcohols, such as 2,2,2-trifluoroethanol, to water as a cosolvent, where it is attributed to the low polarity of the alcohols, which is consistent with these ILs also having a relatively low dielectric constant. It was proposed that EAN, and potentially other ILs, could be of use for studying kinetics and intermediate protein states, because a metastable intermediate was observable during the unfolding of this protein.418 The proportion of alkyl chains present in the PILs has been shown to have a significant effect. This was clearly shown for lysozyme in a series of PIL−water solutions with PIL concentrations of 25, 50, and 75 wt %.419 The PILs contained the formate anion paired with ethylammonium, propylammonium, ethanolammonium, or 2-methoxyethylammonium cations. High concentrations of PAF denatured the lysozyme, which was attributed to the increased hydrophobic portion of the PIL adsorbing to the protein hydrophobic core, preventing it from refolding.419 Conversely, ethanolammonium formate, which was the least hydrophobic of the PILs, provided the greatest stability, with near perfect refolding, and led to the highest activity.419 The proton activity of PILs has been developed by Angell and Byrne et al. as an analogous scale to pH, and has been quantified by the N−H shift with a more acidic medium corresponding to a greater upfield resonance.81,82 It was shown that there is a correlation between the proton activity of PILs and the denaturation temperature of lysozyme or ribonuclease A in the PILs of EAN, and triethylammonium paired with triflate, trifluoroacetate, or formate anions.81 The combinations of PILs−PILs and PILs with molecular solvents have been successfully tailored to produce useful solvent systems, partially through modifying the proton activity of the solvent mixture.81,82 Some of these have increased the protein stability,81,82 its ability to refold,82 and of the folding and unfolding intermediates.81 These highlight the multidimensional possibilities of solvent combinations and concentrations that can be optimized for each protein. Lysozyme in concentrations >200 mg/mL in PIL−water− sugar solutions was observed to have a good shelf-life, stability against aggregation and hydrolysis, and to retain the ability to denature and refold.420 Isopropylammonium formate (IPAF) was used to stabilize cytochrome c, and led to good stability up to about 70 vol % IPAF in water. For comparison, it is stable in methanol−water up to 40 vol % methanol in water. A previous study showed that it was stable in MAF− or EAF−water solutions up to 70−80 vol % PIL, where the stabilization effect

should be noted that the addition of PILs also changed the pH, density, and viscosity of the solutions. 12.5. Protein Stabilization

A broad range of alkylammonium PILs in aqueous solutions has been clearly established as good protein stabilizers, and a smaller subset as enhancers for protein folding. There are a number of factors that influence each PIL−protein combination, such as the hydrophilic/hydrophobic groups on both the PIL and protein, pH, viscosity, concentration of the PIL in water, protein concentration, and ionic species. Growing interest in this field is evident based on the increasing number of publications; however, there is a limited number of PILs that have been investigated. Hence, while a few PILs have been identified that are particularly good for protein stability, investigations that cover a broader range of PILs are needed for furthering our understanding of what PIL solvent features are beneficial for stabilizing proteins. The Hofmeister series describes ions as kosmotropes or chaotropes, which are water structure makers and breakers, respectively. It is well known that the Hofmeister series is relevant for protein stability and enzyme activity with kosmotropic ions being beneficial for protein folding, whereas chaotropic ions lead to unfolding of proteins. A recent review by Kumar and Venkatesu showed that for ILs the anions present in ILs do not always follow the Hofmeister series for protein stability.381 Specifically for PILs, four PILs were prepared from the chaotropic cations of trimethylammonium and triethylammonium, paired with the anions having different kosmotropicity of hydrogen sulfate > dihydrogen phosphate > acetate.410 The stability of succinylated concanavalin A was increased in all of these PILs, and followed the trend of triethylammonium dihydrogen phosphate > TMAHS > TMA dihydrogen phosphate > TMAA for the four PILs used. Only triethylammonium dihydrogen phosphate was a refolding enhancer. For this series, the trimethylammonium PILs followed the expected behavior according to the Hofmeister series based on the kosmotropicity of the anions.410 In contrast, an investigation into the ability of PILs to inhibit amyloid fibril formation of lysozyme used a series of PILs as additives, which consisted of the tetramethylguinidium cation paired with formate, acetate, trifluoroacetate, trifluoromethanesulfonate, tetrafluoroborate, nitrate, chloride, and perchlorate anions. No trend was identified between the ability of the PILs to inhibit fibril formation and the anionic extent of kosmotropicity.411 A variety of PILs have been shown to have a stabilizing effect on the enzyme of α-chymotrypsin. These include triethylammonium acetate, which counteracts the denaturating effect from urea.412 The stability of this enzyme was tested in a range of ammonium PILs where it was found that the PILs with small amounts of alkyl chains on the cations were stronger stabilizers than more hydrophobic counterparts. 413 A variety of ammonium and imidazolium ILs protect the α-chymotrypsin enzyme against reactive oxygen species produced by atmospheric pressure plasma jet,414 with triethylammonium dihydrogen phosphate leading to the greatest protection. In a related study, triethylammonium phosphate could be used as a refolding additive for the enzymes of α-chymotrypsin and succinylated Con A after they had been denaturated using urea.415 The PILs containing the TMG cation paired with lactate416 or acetate417 anions had a good stabilizing effect on the firefly 11415

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were sufficient to inhibit amyloid formation of lysozyme using PILs containing the tetramethylguinidium cation paired with formate, acetate, trifluoroacetate, trifluoromethanesulfonate, tetrafluoroborate, nitrate, chloride, or perchlorate anions.411 There was no trend for fibril inhibition with anionic size or kosmotropicity, although the carboxyl group was identified as beneficial. The fibril formation was inhibited because the intermediate oligomers were made more stable with the PILs present, and hence were not converted to protofibrils.411 The Thioflavin T binding assay was found to be sensitive for monitoring amyloid formation, with tetramethylguinidium acetate having the greatest inhibition effect. Tetramethylguinidium acetate decreased amyloid formation by nearly 50% in vitro, and led to thinner fibrils.411 The conversion of Aβ monomers to Aβ amyloid fibrils was conducted in PIL−water solutions using the PILs consisting of the cations ethylammonium, diethylammonium, or triethylammonium paired with the methanesulfonate anion.425 Increasing the substitution on the ammonium cation decreased the fibril formation, and it was suggested that this was due to differences in the hydrogenbonded network.425 Amyloid fibrils of hen egg white lysozyme (HEWL) were formed either using the PILs of EAN, triethylammonium triflate (TEATf), and triethylammonium mesylate or through the more conventional method of adding ethanol.426 These fibrils could then be dissolved using the PILs, and the final activity of the lysozyme was determined using a standard biological assay for the lysing of Micrococcus lysodeikticus.426 The fibrils formed using ethanol led to higher final activities then those formed from PILs. Dissolving the fibrils in the PILs had highest activity in EAN. A maximum activity of 72% was obtained in the dissolved lysozyme fibrils when the fibrils were formed through addition of ethanol and then dissolved in EAN.426 The lower critical solution temperature (LCST) of poly(Nisopropylacrylamide) has been used as a model phase transition for describing the protein folding/refolding process. PILs containing the triethylammonium cation with a variety of anions were added to aqueous solutions of the polymer. Kosmotropic anions on the PIL lowered the LCST more than chaotropic ions, which was attributed to the interactions between the ions and water. Conversely, the fluorinated anions of Tf and BF4 led to an increase in the LCST.129

of the PIL was attributed mainly to its behavior as a pH buffer.421 At higher PIL or methanol concentrations, the protein began to unfold. It has been proposed that these aqueous solutions of isopropylammonium formate, EAF, and MAF have the potential to be used as solvents for chromatographic separation of proteins.345 An interesting comment by Angell et al. on the ability of proteins to be stable at high concentrations in highly concentrated PILs solutions is in regards to biogenesis and the primordial soup: “We conjecture that the stability of proteins in the extreme ionic concentrations of our studies is not an esoteric finding but an indication of the sort of low water-activity media in which biomolecules might have existed on early earth.”82 Alkylammonium salts and polyamines were trialed as additives for increasing the solubility of membrane proteins in buffer solutions with a detergent present. These included the PIL EAN and the protic molten salts of EACl and PACl, along with the polyamines of putrescine, spermidine, and spermine.422 All of these additives increased the solubility and activity of polygalecturonic acid synthase, from the plant Golgi membrane, with the greatest enhancement by spermidine. It was also shown that EAN and spermidine were useful as additives for solubilizing a range of membrane proteins, with increased total activity also reported for plant membrane enzymes of NADH-dependent cytochrome c reductase from the endoplasmic reticulum, cytochrome c oxidase from the mitochondria, and γ-glutamyl transpeptidase from the plasma membrane, and for the animal membrane protein of bovine liver b-glucoside a1,3-xylosyltransferase.422 The enhanced solubilization was proposed to be due to enhancing the release of the membrane protein from the membrane through the PIL binding to the anionic sites on the protein, resulting in it being less lipid-like.422 A series of PILs were beneficial additives to insulin in an aqueous buffer solution.423 The PILs of trimethylammonium (TMA) hydrogen sulfate, triethylammonium (TEA) hydrogen sulfate, TMA dihydrogen phosphate, TEA dihydrogen phosphate, and TMA acetate were used. All of these PILs were beneficial in that they decreased the amount of insulin associating into an inactive form, and had a thermal stabilizing effect.423 In addition, the PILs led to a more folded structure with a smaller hydrodynamic radius, and did not alter the functional groups of the insulin.423 In comparison to phosphate buffer solutions, the stability of the tobacco mosaic virus was enhanced in the PILs of ethylammonium methanesulfonate, diethylammonium methanesulfonate, propylammonium methanesulfonate, and dipropylammonium methanesulfonate.424 The increased stability was attributed to the low water activity present. In contrast, the PILs of triethylamonium methanesulfonate and tripropylamonium methanesulfonate changed the secondary structure of the virus. The shelf life of the tobacco mosaic virus was significantly enhanced to up to 4 months in ethylammonium methanesulfonate as compared to less than 3 weeks in the aqueous phosphate buffer solution.424 It is not clear why these PILs and amines are increasing the solubility; however, it was reported that these additives all had a denaturing effect on the proteins, and hence the increased solubility is not due to stabilization of the proteins.424 Likewise, the additives are not activating the enzymes to lead to the increased activities.424 Various PILs have been shown to inhibit amyloid fibril formation of lysozyme 411 and Aβ monomers. 425 Low concentrations of around 400 μM of PILs in aqueous solutions

13. PHARMACEUTICALLY ACTIVE PILs An emerging field are PILs that are synthesized from cations and/or anions, which are known to be pharmacologically active. The pharmaceutical activity of these ions includes antiinflammatory, analgesic, antifungal, and anti-viral properties. Extensive collections of potentially pharmaceutically active PILs, protic molten salts, and complexes have been developed by Mirksov et al., such as in refs 427−430, and generally their synthesis, IR, NMR, and elemental analysis have been reported. For example, a series of water-soluble aspirin derivatives were synthesized with potential anti-inflammatory properties. These consisted of an aspirin-derived anion of salicylate paired with the cations of tris(2-hydroxyethyl)ammonium, bis(2hydroxyethyl)ammonium, N,N-bis(2-hydroxyethyl)-N-methylammonium, and N-(2-hydroxyethyl)-N,N-dimethylammonium.431 These four protic salts could be injected, and had good anti-inflammatory activity, reduced fever, and inhibited clot formation.431 MacFarlane et al. investigated the permeation of pharmaceutically active PILs through a model membrane, to further 11416

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understand how some PILs may permeate the skin.432 The PILs investigated were two that had pharmaceutically active cations and anions of tuammoniumheptane salicylate and bromohexinium ibuprofenate, and two more conventional PILs of butylammonium acetate and heptylammonium acetate.432 The membrane does not enable ionic species transfer, and hence only the ILs that formed clusters of ions (neutral species) were able to permeate. Consequently, PILs with lower ionicity are preferable for drug delivery applications due to their ability to cross the membrane barrier more easily.432,433 The tuammoniumheptane salicylate and bromohexinium ibuprofenate readily crossed the membrane, and while slower, butylammonium acetate and heptylammonium acetate also permeated the membrane.432 When the PILs were mixed with propylene glycol, there was very little IL permeation of the barrier due to the disruption to the IL association, and hence a significant decrease in neutral species. This was further explored for a series of pharmaceutically active PILs and protic molten salts with both pharmaceutically active cations and anions.433 These were prepared in stoichiometric and 2:1 acid:base ratios, and their melting points and/or glass transitions are provided in Table S1. A broad range of melting points were observed from PILs liquid at room temperature up to a melting point of 256 °C, reflecting the different proton transfer and ionicity present.433 It must be noted that not all Brønsted acid/Brønsted base pairs will lead to protic ionic liquids, and this was shown for pharmaceutically active precursors using a theoretical DFT method. The investigation included the amines of triethanolamine, methyldiethanolamine, dimethylethanolamine, and trimethylamine, combined with acetic acid, hydrochloric acid, and three different arylheteroacetic acids.434 On the basis of the interactions and binding energies, predictions were made for whether there would be proton transfer leading to PILs, or the formation of hydrogen-bonded complexes. The data suggest that protic salts or hydrogen-bonded complexes would be formed with the triethanolamine, depending on which acid it was paired with, whereas the other amines would only lead to hydrogen-bonded complexes.434 The nontoxic and biologically active PIL of tris(2hydroxyethyl)ammonium 4-chlorophenylsulfanylacetate, with a melting point of 90−92 °C, was used as a precursor for the synthesis of a biologically active metalated aprotic molten salt, zinc di(4-chlorphenylsulfanyl)acetate dehydrate, which has a melting point of 202 °C.435 There are currently used drugs that are PILs, or closely related compounds. These include a local anesthetic drug of 2diethylamino-N-(2,6-dimethylphenyl)acetamide hydrochloride (lidocaine hydrochloride) that has been referred to as a protic ionic liquid, and this salt has a melting point of 72 °C.436−438 A related compound is the drug verapamil hydrochloride (Tm = 149 °C), which can be considered a protic molten salt, and is used for reducing blood pressure and to correct irregular heartbeats.438,439 Procainamide hydrochloride (Tm = 169 °C) is a cardiac depressant and a protic molten salt.440 Similarly, carvedilol phosphate, which is used for treating hypertension and heart abnormalities, and procaine hydrochloride can be considered as protic molten salts.440

increased the toxicity, whereas the presence of a COOH or ether group decreased it for certain ILs. The toxicity was strongly dependent on the anion, although this investigation only included aprotic ILs.441 Since then, the toxicology of 10 PILs containing select combinations of ethanol-, diethanol-, or triethanolammonium cations with formate, acetate, propionate, butyrate, isobutyrate, or pentanoate has been reported and compared to commonly used imidazolium and pyridinium-type aprotic ILs.442,443 Aquatic toxicity, biodegradability, and tests on enxyme and leukemia rat cells were conducted, with the PILs having EC50 values >100 mg/L and a good biodegradability rate.442 Terrestrial ecotoxicity of these PILs and AILs through bioassays on various plants and soil microorganisms showed no toxic effects from the PILs, with the PILs having potential biodegradation in soil.443 In contrast, the aprotic ILs were generally more toxic, with EC50 values orders of magnitude lower for all of the tests, and a greater resistance to biodegradation.442,443 In general, shorter alkyl chains and less complex structures on the ILs led to lower toxicities.

15. INORGANIC SYNTHESIS To the best of our knowledge, there have been few reports of inorganic synthesis in PILs, since the last extensive PILs review in 2008.1 The limited number of publications is perhaps due to difficulties in modifying the conventional routes that are typical in aqueous media, and due to the relative expense of using ILs as solvents where large volumes would be required. Nanostructured zinc oxide, ZnO, has been synthesized in EAN−water−NaOH solutions, forming small particles with a wurtzite phase.152 Significant differences in the ZnO particle shapes were obtained through varying the EAN:water ratio, with a spherical morphology preferred with increasing EAN content. This was attributed to the EAN at higher concentrations forming small aggregates that were preferentially binding to the charged planes of the ZnO, inducing growth in the other planar directions, and leading to ZnO spherical superstructures.152 The resulting ZnO particles have potential as photocatalysts, with improved photocatalytic properties cas ompared to commercially available ZnO. Crystalline zinc phosphate-triethylenetetramine has been synthesized using the PIL of triethylenetetraammonium acetate as the solvent and template.444 Two different crystalline solids were obtained, which had either an open framework or a ladder structure, where the structure depends on the phosphate to zinc ratio. A portion of the triethylenetetramine cations was incorporated into the crystalline structure. The protonation state of the phosphoric acid precursor was complex due to the interplay between it and the cation of the PIL.444 The surfactant-like PILs of octylammonium formate and bis(2-ethyl-hexyl)ammonium formate were used in water or dimethylformamide as redox-active structuring media to synthesize monodispersed nanoparticles of gold,445,446 and other metals from MCl3 precursors.446 The structure of the PILs, and their aggregation in the solvents, affected the size and shape of the metal nanoparticles. The addition of EIm TFSI was observed to accelerate the sol to gel process for the nonaqueous sol−gel synthesis of silica. Specifically, this was due to the EIM TFSI enabling a faster reaction rate of the condensation reaction of short chain silica into small rings, which then act as nucleation sites for larger rings, than the polymeric silica network.447 The sol−gel synthesis of titanium dioxide, TiO2, using various cationic

14. TOXICOLOGY The in vitro cytotoxicity of an extensive range of over 80 ionic liquids was evaluated using human colon cancerous cells (CaCo-2). In general, increasing the alkyl chain on the cation 11417

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of use for many organic reactions, the IL is required to be a Brønsted acid, and less commonly to be a Lewis acid or base. The PILs are fundamentally a Brønsted acid due to the available proton, with the HMIm cation the most commonly used. In contrast, for aprotic ionic liquids, the Brønsted acidity requires more careful design of the ions, either through the presence of a Brønsted acidic group on the cation, such as SO3H, or an available proton on the anion, such as HSO4. Because of these structural requirements, the ionic liquids used in organic synthesis are frequently referred to as “task specific” or “functionalized” even though many of the ILs can be considered as “common ILs”, and are used in a broad range of fields. It is apparent that for many organic reactions the determining factor for the optimal PIL trialed is the acidity or basicity required for the reaction and of the pIL, with some ions frequently being found to be beneficial. In particular, it has been found that the HSO4 anion is a good source of an active proton, EAN is a good acidic PIL, and that hydroxyl-containing cations and/or the lactate anion tend to provide a good basic environment. A large driving force for the use of ILs in organic synthesis is their potential as greener solvent/catalysts as compared to existing methods that use molecular solvents. Reviews of the green chemistry aspects of organic synthesis have discussed the role of ionic liquids, where there are potential benefits, but only if the ILs are recoverable and reusable.453−456 There are an increasing number of publications that are using the knowledge gained from using ILs as solvents/catalysts for organic synthesis to use more benign molecular solvents, which share certain similarities to ILs such as glycerol,457 or low melting natural compounds.455 The use of PILs is promising for a broad range of organic synthesis reactions. In particular, the ability to conduct reactions with the PIL as the catalyst with no additional solvents added, and under generally mild temperature, is highly advantageous. The most commonly used PILs for organic synthesis are those containing HMIm or HBIm cations. However, there is an increasing use of nonfluorinated and cheaper PILs, such as those containing primary ammonium cations458−466 or the 1,1,3,3-tetramethylguanidine (TMG) cation467−469 with anions such as formate, acetate, or nitrate. These PILs have the advantage that they can be designed to be distillable.465 In the vast literature on ILs used in organic synthesis, there has been inconsistent terminology as noted by Welton.452 In particular relevance for this Review, it is relatively uncommon for the term “protic ionic liquids” to be used in the field of organic synthesis, and instead PILs are mostly referred to by more general terms such as “ionic liquids”, “Brønsted acidic ILs”, or “task-specific ILs”. There are numerous reviews on various aspects of ionic liquids in organic synthesis, and most include protic ionic liquids to some extent.6,452,456,470−477 A few reviews specialize in a narrow subset of ILs such as chiral ILs,478−480 and ether- or alcohol-functionalized ILs,93 or on microwave synthesis in ILs.481 In this section, we anticipate that we have not been exhaustive, predominantly due to the inconsistent terminology in this field. The ability of PILs to be added to molecular solvents to tailor the reaction medium was reported using EAN added to acetonitrile as an example.482 The EAN−acetonitrile medium was trialed for nucleophilic aromatic substitution between 1fluoro-2,4-dinitrobenzene and 1-butylamine or piperidine, and

surfactants in EAN or BMIm BF4 led to up to 60% nanocrystalline TiO 2 in coexistence with anataste TiO 2 crystals.254 The surfactants used included the amphiphilic aprotic ILs of C16MIm BF4, C4MIm Cl, and hexadecyltrimethylammonium chloride (CTAC). The use of EAN as the solvent led only to anatase TiO2, whereas BMIm BF4 led to either all anatase or a mixture of anatase and nanocrystalline TiO2. The materials prepared in EAN had surface areas up to 400 m2/g, which were generally higher than those prepared in BMim BF4, and smaller pores in EAN.254 Hierarchically nanostructured silica was obtained through sol−gel synthesis using the nonionic block copolymer Pluronic P123 in EAN. After removal of the EAN and surfactant, the resulting silica contained mesopores of 4.5 nm due to the P123 cylindrical micelles, and micropores of 1.0 nm due to the mesostructure of EAN (see section 6.1 for more information).

16. INORGANIC PARTICLE STABILITY IN PILs Silica suspensions of 1 μm spheres were reported to remain suspended without aggregation in EAN, water, and briefly mentioned to also be stable in EAF and diethylmethylammonium formate.448 However, for EAN−water mixtures with water contents between 0.5 and 95 wt %, the solutions were unstable, with aggregation occurring. The stability in EAN is unusual as the charges on these particles are efficiently screened due to the ionic strength of EAN, with a molarity of 11 M, and in neat water this charge maintains the particle stability. Instead in EAN it was proposed that solvation layers of the EAN around the silica particles lead to the particle stability, with these decreasing in strength and thickness on addition of water.448 Stable ferrofluids were reported in EAN, which consisted of superparamagnetic iron oxide nanoparticles of 8−12 nm with an additional thin coating of short acrylic acid-b-acrylamide copolymer (AA10-b-AM14).449 These polymer-coated particles also formed stable ferrofluids in the aprotic IL of EMIm acetate but not in EMIm SCN or BMIm BF4. In contrast, the bare iron oxide nanoparticles were unstable in EAN, formed ferrofluids stable for several months in EMIm acetate or EMIm SCN, and settled over 2−4 days in BMIm BF4.449 Maghemite nanoparticles prepared in aqueous solution were transferred to ethylammonim nitrate to investigate their colloidal stability.450,451 Because of the magnetic properties of maghemite, the resulting dispersions can be considered as ferrofluids. The surface charge of the nanoparticles was varied, and balanced with either nitrate, perchlorate, or benzenesulfonate anions450,451 or with lithium, nitrate, potassium, or ammonium derivative cations.451 Uncharged nanoparticles would not redisperse in EAN. In contrast, the charged nanoparticles maintained a surface charge on transferance to EAN, and all dispersed. The minor exception was those particles containing lithium as a counterion, where they initially aggregated, then slowly redispersed over a few days.450,451 The nanoparticles with sodium counterions were observed to be well dispersed, whereas small clusters were present for those with lithium counterion.451 17. CATALYSIS AND ORGANIC SYNTHESIS The use of ionic liquids, including protic ionic liquids, as solvents and/or catalysts for a wide variety of organic synthesis reactions has been extensively studied, in particular in the comprehensive review by Hallett and Welton452 in 2011. To be 11418

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17.2. Addition Reactions

for nucleophilic addition of amines to carbonyl compounds to form imines. It was reported that the EAN could participate in the reactions as a nucleophile and as a Brønsted acid, leading to an improved medium with the presence of the EAN important for both trial reactions.482 Previously, PILs and protic salts such as butylammonium nitrate, butylammonium chloride, and pyridinium nitrate were combined with ethanol or chloroform to modify the reaction medium for a trial organic reaction, but with little effect due to the salt.483 The broad range of solvent properties within PILs (and aprotic ILs) enables good solvent optimization for each reaction. For example, the HSO4 anion is a favorable anion for reactions requiring an acidic solvent environment, EAN for relatively neutral to slightly acidic, and the lactate anion for basic conditions.

17.2.1. Hetero-Michael Addition. The hetero-Michael reaction enables the addition of sulfur, nitrogen, or oxygen nucleophiles to α,β-unsaturated carbonyl or nitrile compounds, as shown in Figure 11. Typically this reaction requires an excess

Figure 11. Hetero-Michael addition reaction, where EWG is the electron-withdrawing group and X can be S, N, or O.

of reagents, long reaction times, harsh conditions, expensive heavy metal salts, and toxic solvents. ILs are an alternative solvent/catalyst system, which have been successful for a broad range of nucleophile and olefin precursors.459,489,490 The use of ILs for the hetero-Michael reaction led to faster reaction times and good to excellent yields. In addition, the ILs could be recycled and reused with no significant loss in activity.459,489,490 A limited range of protic ILs, Brønsted acidic ILs, and aprotic ILs has been trialed for the hetero-Michael reaction. The aprotic ILs BMIm BF4 and BMIm PF6 did not support any reaction, whereas the Brønsted acidic and protic ILs all did, although with variation in the yields. In a comparative study, it was shown that the PIL HMIm OTs led to significantly better yields as compared to the PILs HMIm HSO4 or HMIm BF4, or the Brønsted acidic ILs 1-propyl sulfonic acid-3-methylimidazolium HSO4 or 1-propyl sulfonic acid pyridinium HSO4.490 HMIm OTs was useable for a broad range of nitrogen, oxygen, or sulfur nucleophiles;490 likewise, HMIm TFA was trialed successfully for a broad range of nitrogen and sulfur nucleophiles.489 As cost-effective alternatives, the PILs of 1,1,3,3-tetramethylguanidine (TMG) cation with a variety of anions,468 and ethanolammonium formate,459 EOAF, were trialed. EOAF led to excellent yields with a variety of secondary nitrogen nucleophiles, aromatic sulfur nucleophiles, and activated olefins, but slow reaction rates for long-chain acrylates, and no reactions for alcohols, aliphatic thiols, or aromatic amines.459 PILs containing the TMG cation with Tfa, acetate, lactate, npropionate, n-butyrate, iso-butyrate, and TFA were trialed as catalysts for the addition of aliphatic and aromatic amines to electron-deficient olefins, with the lactate leading to the best performance.468 17.2.2. Synthesis of Diphenylmethyl Ethers. The PILs containing the triethylammonium cation paired with hydrogen sulfonate (TEAHS), formate (TEAF), methanesulfonate (TEAMS), and trifluoroacetate (TEATFA) were trialed as cosolvents and catalysts for the synthesis of diphenylmethyl ethers from diphenylmethanol in ethanol under microwave irradiation. TEAHS and TEAMS both led solely to the desired product and in high yields, although TEAHS required a relatively large amount of additional ethanol, making TEAMS the best PIL of these four trialed. In contrast, the reaction did not progress in TEAF and TEATFA, which was attributed to their low proton activity. Yields between 60−98% were obtained for a broad range of diphenylmethyl ethers using TEAMS as the cosolvent.491

17.1. Protection/Deprotection Reactions

17.1.1. BOC Protection of Amines. The protection of amine groups during organic synthesis can be conducted using N-Boc protection. This reaction can be conducted under basic or acidic conditions; however, in molecular solvents under acidic conditions the N-Boc protection requires harmful catalysts and highly acidic conditions. Under basic conditions, the reactions are slow and often result in toxic chemicals formed as side reactions. The N-Boc protection has successfully been conducted using di-tert-butyl pyrocarbonate, (Boc)2O, in the PILs MIm BF4,484 MIm TFA in dichloromethane,485 BIm TFA in dichloromethane,485 and 1,1,3,3-tetra-methylguanidinium acetate [TMG][Ac],467 and the Brønsted acidic IL 1,3-disulfonic acid imidazolium HSO4486,487 as shown in Figure 10. The PILs and

Figure 10. N-Boc protection of amines.

BAILs performed very well as catalysts for N-Boc protection of a broad range of heterocyclic and aliphatic amines with no side reactions, high yields, and good selectivity. The ILs appear to catalyze the reaction through activating the carbonyl oxygen atoms of (Boc)2O, which then have a nucleophilic reaction with the amine groups. The reactions were conducted at ambient temperatures with no additional solvents or catalysts required. All of the ILs could be separated from the reaction products and reused with no significant loss of activity. The PILs containing the TMG cation with HSO4, NO3, or CF3SO3 anions and the BAIL 1,3-disulfonic acid imidazolium Cl487 were also trialed but had lower yields and longer reaction times. A key advantage of the TMG Ac as compared to the other ILs is that it is significantly cheaper, does not contain fluorine, and performed as well as the others as a catalyst for N-Boc protection. 17.1.2. Deprotection of Acetals and Ketals. Imidazolium PILs of BIm BF4, BIm TFA, and 1-butyl-2-methylimidazolium TFA along with similar aprotic ILs were trialed as catalysts for the chemoselective deprotection of acyclic and cyclic acetals, ketals, benzylidene acetals, and tetrahydropyranyl ethers. The protic ILs led to yields of 81−99%, whereas the aprotic ILs for the same reaction only led to yields of 20−50%, and needed a much longer reaction time.488

17.3. Cyclic Forming Reactions

17.3.1. Diels−Alder. The Diels−Alder reaction in ionic liquids was thoroughly reviewed in 2010 by Chiappe et al., with mostly aprotic ILs being reported, along with the PILs EAN, HMIm Tf2N, and HBIm.492 The reaction of cyclopentadiene 11419

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Figure 12. Reaction of cyclopentadiene with alkyl acrylates.

Figure 13. Friedländer annulation to form substituted quinolines.

Figure 14. Biginelli reaction.504

acid, base, or high temperatures to occur. A representative reaction is shown in Figure 13. EAN has been used as the catalyst and solvent for the synthesis of quinolines with yields between 46% and 95% depending on the precursors.460,461 EAN could be recovered by extraction with diethyl ether, and reused three times with no loss of yield.461 A Friedlander annulation was performed in conjunction with a Knoevenagel condensation reaction using HMim TFA with yields between 78% and 87%.497 The Brønsted acidic HSO3−BMIm CF3SO3498 and 1,3disulfonic acid imidazolium hydrogen sulfate499 were successfully trialed for this reaction, with yields between 85−98%498 and 88−94%,499 respectively, and these ILs could be recycled and reused.498,499 Similarly, the Brønsted acidic n-butanesulfonic acid pyridinium HSO4 dispersed on MCM-41 silica nanoparticles performed well for the Friedlander synthesis of quinolines with yields between 82−93% for the precursors trialed. The solid substrate enabled the IL to be reused after washing with ethanol and drying.500 The aprotic IL of butylpyridinium tetrachloroindate used as a solvent and catalyst led to yields between 83−97%.501 In comparison for nonionic liquid solvent/catalyst systems, prolinate potassium in DMSO has led to yields between 73−97 % with short reaction times.502 Acidic nickel oxide nanoparticles in ethanol led to high conversions of 89−94%, whereas the same nanoparticles in acetonitrile, THF, and dichloromethane led to conversions of 58−68%, and negligible conversion was reported when toluene was used as the solvent.503 17.5.2. Biginelli Reaction. The Biginelli reaction involves the synthesis of 3,4-dihydropyrimidin-2(1H)-one and 3,4dihydropyrimidine-2(1H)-thione derivatives through a cyclocondensation reaction of three components, as shown in Figure 14. These components are a substituted aromatic or

with methyl acrylate, as shown in Figure 12, is a common Diels−Alder reaction. The selectivity toward the endo product can be increased by including strongly interacting groups on the IL such as hydroxyl, carboxyl, nitrile, or benzyl, or N−H bonds, such as those present in PILs. Therefore, increasing the cohesive energy density of the IL increases the selectivity. For example, the endo:exo ratios for EAN and HMim Tf2N were 6.7 and 6.1, respectively, whereas for aprotic ILs with no hydroxyl groups the ratios were typically between 3.5 and 5.492 The reaction between nitropyrrole derivatives and various nucleophilic dienes to produce indoles has been investigated by Mancini et al. using the PILs HMIm BF4 and EAN.493−495 The ILs led to improved yields as compared to molecular solvents chloroform493,494 or benzene493 when using nitropyrrole derivatives, but only comparable yields to molecular solvents for nitroindoles.495 Theoretical studies indicate that the global electrophilicity of nitripirroles followed EAN > HMIm BF4 > chloroform, which reflects the strength of the hydrogen bond between the IL and the nitripirrole, and was consistent with generally higher yields in EAN as compared to HMIm BF4 for the same reaction time.494 17.4. Ring Cleavage

The task-specific PIL of MIm azide was used as a solvent, reagent, and activator for the ring-opening reaction of epoxides to form 1,2-azidoalcohols. Good yields and selectivities were achieved in short times.496 17.5. Heterocyclic Synthesis

The synthesis of a diverse range of heterocyclic compounds has been achieved in protic ILs, where these compounds are cyclic with more than one atom type in the ring. The Friedländer annulation and Biginelli reaction are outlined below, followed by a variety of other heterocyclic reactions. 17.5.1. Friedländer Annulation. The Friedländer annulation is a heterocyclic reaction thattypically requires a strong 11420

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Other heterocyclic compounds were synthesized using a three-component condensation reaction in protic ionic liquids. These included the synthesis of 2-amino-4,6-diphenylpyridine3-carbonitrile, which occurred from chalcones, malonitriole, and ammonium acetate in EAN. This reaction led to yields of 90% at 60 °C after 2 h, as compared to yields between 70% and 80 % in molecular solvents with reaction times between 8 and 10 h.462 In addition, the EAN could be reused several times. Similarly, a series of 2,4,5-trisubstituted imidazoles were synthesized from aldehydes, benzyl, and ammonium acetate using HMIm HSO4 as catalyst with acetonitrile, chloroform, methanol, or ethanol present as solvent. Yields between 81% and 97% were obtained, and the IL could be recovered and reused.509 The synthesis of 1,8-dioxo-decahydroacridines derivatives occurred through a three-component reaction of a primary amine, aromatic aldehyde, and 5,5-dimethyl-1,3-cyclohexanedione in HMIm TFA at 80 °C with yields between 78% and 89%.510 When this reaction did not include the primary amine, then 1,8-dioxo-octahydroxanthenes derivatives could be produced under the same conditions with yields of 80−94%.510 For both of these reactions, the yields decreased significantly with decreasing temperature, with virtually no reaction at 35 °C or less. Lower yields were obtained using HMIm HClO4, HMIm HSO4, or HMIm BF4.510 HMIm TFA was also used in 10 mol % in water to enable the three-component reaction of benzil or benzoin, aliphatic or aromatic aldehydes, and ammonium acetate to form 2-alkyl and 2-aryl-4,5-diphenyl1H-imidazoles. The reaction was optimized for solvent, concentration, and catalyst, with significantly lower rates and yields obtained for solvents other than water, and for other acid catalysts. The reaction yield increased with increasing concentrations of HMIm TFA, reaching a plateau at 10 mol %.511 The synthesis of dispiropyrrolidines through a threecomponent reaction between ninhydrin, sarcosine, and 1benzyl/methyl-3,5-bis[(E)-arylidene]-piperidin-4-one was successfully conducted in TMG acetate, with no additional solvents or catalysts.512 Yields were achieved between 86% and 92%. In related work, TMG acetate was used as a basic catalyst for a four-component synthesis of 2H-indazolo[2,1b]phthalazinetriones and dihydro-1H-pyrano[2,3-c]pyrazol-6ones.513 The highest yields and fastest reaction times were obtained when no additional solvents were present, and the reaction was conducted at 80−100 °C. The PIL could be recovered and reused at least four times for all of these reactions.512,513 A four-component reaction was developed to produce triazolyl methoxy phenylquinazolines from aromatic propargylated aldehydes, azides, 2-aminobenzophenone derivatives, and ammonium acetate, with catalytic quantities of HMIm TFA, Cu(OAc)2, and sodium ascorbate present.514 Yields between 75% and 95% were achieved.

heterocyclic aldehyde, methyl acetoacetate, and urea or thiourea. Typically the three-component Biginelli reaction has only poor to moderate yields, and instead multistep methods have been used. However, the PIL HMIm HSO4 with added NaNO3 enabled this reaction, with yields of 55−97% achieved at 80 °C in 2−4 h.504 The PIL HMIm NO3 did not support the reactions, and HMIm H2PO4 with NaNO3 only led to low yields.504 The reaction has also been successfully conducted in the Brønsted acidic IL BMIm HSO4, using microwave radiation for 4.4−8 min, with good yields achieved.505 Similarly, this reaction had high yields and short reaction times in HMIm Tfa, high yields in moderate reaction times in HMIm HSO4, and poorer performance in HMIm OTs, BMIm Br, BMIm PF6, or BMIm BF4.506 The benefit of the HSO4 anion was attributed to a need for an acidic hydrogen to be present to catalyze the reaction.504 17.5.3. Beckmann Rearrangement. The Beckmann rearrangement of cylcohexanone oxime to produce εcaprolactam, Figure 15, using tosyl chloride in various PILs

Figure 15. Beckmann rearrangement of cylcohexanone oxime to produce ε-caprolactam.507

led to high conversions and selectivities in HMIm TFA, HMIm TsO, TMG TFA, TMG acetate, and TMG TsO and TMG lactate but no yields in HMIm PF6 or TMG lactate.507 However, anionic exchange occurred in TMG Ac and TMG TFA leading to a solid salt, and there was only partial solubility of the product in HMIm TFA and HMIm TsO, which led to impurities.507 The highest yield of 98% was obtained using tosyl chloride in a mixture of acetone and TMG TsO at 60 °C.507 The Beckmann reaction was also highly successful in a range of other oximes using ILs as promoters. 17.5.4. Three-, Four-, or Five-Component Heterocyclic Forming Reactions. The synthesis of quinazolines is similar to the Biginelli reaction in that it is a three-component condensation reaction, as shown in Figure 16.508 This reaction was conducted in HMIm TFA with no additional solvent under aerobic conditions leading to good yields of 80−95%. In comparison, the use of p-TSOH as a catalyst in various molecular solvents, or HMIm TFA combined with molecular solvents, led to yields of 17% at best.508 The HMIm TFA could be reused for at least three runs with less than 4% loss of activity.

Figure 16. Synthesis of quinazoline derivatives.508 11421

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The PIL 2-methylpyridinium trifluoromethanesulfate was used at 1 mol % in a three-component Hantzsch condensation reaction of aromatic aldehydes, 1,3-dione, and aniline derivatives to form 1,8-dioxodecahydroacridine derivatives.515 The use of this PIL led to high yields, and is greener than conventional catalysts for this reaction. 17.5.5. Other Heterocyclic Forming Reactions. The synthesis of 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles by reacting o-phenylenediamines and aromatic aldehydes in TFA, acetic acid, or various PILs containing the HMIm cation with ClO4, HSO4, BF4, TFA, or p-toluene solfunate was trialed.516 Of these solvents, HMIm TFA gave the highest yields, and excellent selectivity of 99% toward the desired product. The addition of water to HMIm TFA led to comparable yields and selectivities by significantly decreasing the required reaction time from 3 to 0.15 h.516 Yields between 76% and 95% were obtained using a HMIm TFA−water mixture, and the IL could be reused with little loss of yield. The dehydration of D-fructose to produce 5-hydroxymethylfurfural is shown in Figure 17. This reaction was trialed in PILs

Figure 18. Multiply substituted 1,2,4-triazoles from 1,3,4-oxadiazoles and organoamines.520

pyridinium cation with either TFA or Ac performed best, with efficiency for arylamines and alkylamines, respectively. PILs containing HDABCO with OAc or TFA, HMIm with OAc, OTs, TFA, HSO4, or Cl, and the PIL triethylammonium TFA were all trialed with yields between 28% and 85%. The aprotic IL BMIm Br led to negligible yield, whereas butylpyridinium Br with added acid of TFA, acetic, or TsOH had yields between 17% and 46%. It was determined that the cation and anion were both important for the catalysis process. A series of 4,6-disubstituted-3-cyano-2-pyridone compounds was prepared from cyanoacetamides with chalcones or acetyl acetone, such as in Figure 19, with TMG containing PILs trialed as the catalyst with no additional solvents present.521 Specifically, the PILs trialed consisted of the TMG cation paired with lactate, acetate, propionate, n-butyrate, or trifluoroacetate (TFA). Of these PILs, the TMG lactate led to the best yield, and was recycled and reused four times with no loss of activity. When TMG TFA was used, there was no reaction, and the other three PILs only led to moderate yields. The high yield in TMG lactate was attributed to this PIL being the most basic and having a low viscosity.521 The heterocyclic 6-aminouracils were formed from cyanoacetylureas using TMG lactate for the ring closure.522 Yields were obtained between 84% and 92%, with the IL able to be recovered and reused at least five times with little loss of yield. The precursor cyanoacetylureas were formed prior by a threecomponent condensation of aliphatic or aromatic amines with potassium cyanate using acetic acid or cyanoacetic acid as the catalyst. 1,5-Benzothiazepiens and 1,5-benzodiazepines were prepared by a cyclocondensation reaction from o-aminothiophenol or ophenylenediamine with α,β-unsaturated carbonyl compounds, using MIm NO3 as a catalyst. Yields between 75% and 98% were obtained, in relatively short reaction times.523 MIm HSO4 was used as the catalyst in methanol for the reaction of 2,5hexanedione with amines to form 2,5-dimethyl-N-substituted pyrroles under ultrasonic irradiation at room temperature. Yields between 77% and 91% were obtained, and MIm HSO4 could be recycled and reused at least three times with little loss in yield.519 In contrast, the use of BMIm Cl, EMIm Br, or EMIm Cl as catalysts only led to trace yields.519 The superbase 1,8-diazabicyclo[5.4.0] was paired with trifluoroethanol to create a CO2 reactive PIL, which was used as the solvent and catalyst for the reaction of CO2 with 2aminobenzonitriles. Quinazoline-2,4(1H,3H)-diones were produced in excellent yields at room temperature and atmospheric pressure, and the PIL could be recovered and reused.524

Figure 17. Dehydration of D-fructose.517

containing the HMIm or N-methylmorpholinium cation with HSO4 or CH3SO3 anions. The PILs were present in catalytic quantities of 10 mol % in either water, DMF, DMF−LiBr, DMF−LiCL, DMF−NaBr, or DMF−KBr. N-Methylmorpholinium CH3SO3 in DMF−LiBr led to the best yields of 74.8% and 47.5% from the starting materials of D-fructose and sucrose, respectively.517 The Hammett acidity H0 value for the PILs had a good correlation to the yields, with higher proton acidities leading to higher yields. 2,4,6-Triarylpyridines were synthesized from benzyl alcohols and acetophenones using microwave radiation in a solvent mixture of HMIm NO3 and BMIm BF4 with added ammonium acetate.518 Yields of 88−99% were achieved. This reaction used ammonium acetate as the promoter, HMIm NO3 as the oxidizing agent, which converted the aryl alcohols into aldehydes, and BMIm BF4 as the catalyst. For the initial optimization reaction, the use of HMIm NO3 without BMIm BF4 led to a yield of 28%, and the use of aprotic ILs instead of BMIm BF4 led to yields of 39−71%. HMIm HSO4 was used as the catalyst for the formation of 2,5-dimethyl-N-substituted pyrroles from 2,5-hexanedione and various amines.519 The optimal amount of the PIL was determined to be 20 mmol %, with higher quantities not increasing the yields. In contrast, the use of the aprotic ILs of BMIm Cl, EMIm Br, or EMIm Cl led only to trace amounts of the product.519 The formation of 1,2,4-triazoles has been successfully conducted from reacting organoamines and oxadiazoles in protic ILs, as shown in Figure 18.520 The PILs consisting of the

17.6. Condensation Reactions

17.6.1. Baeyer Condensation. The Baeyer condensation reaction enables the formation of triarylmethanes from a variety of aromatic aldehydes and dimethyl- or diethyl- aniline, as shown in Figure 20.525 Conventionally, this reaction requires a strong Brønsted or Lewis acid as catalyst, which is non11422

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Figure 19. Reaction of cyanoacetamides with acetyl acetone.

Figure 20. Baeyer condensation.525

Figure 21. Knoevenagel condensation of salicylaldehyde with Meldrum’s acid.529

recyclable, corrosive, and needs to be present in high concentrations. More recently, the Baeyer condensation reaction has been conducted using microwave radiation. It was shown that yields of 70−96% were achieved with catalytic amounts of ZrOCl2,526 or p-toluene sulfonic acid,527 with no additional solvent required. Protic ILs of HMIm with HSO4, CH3COO, and CF3COO2 were trialed as acidic catalysts. HMIm HSO4 had the best catalytic behavior of these three, leading to moderate to excellent yields (72−96% yields), and this was attributed to it being the strongest acid.525 In addition, HMIm HSO4 could be recycled and reused, with 92% of its activity retained after six uses.525 17.6.2. Knoevenagel Condensation. The Knoevenagel reaction is a carbon−carbon bond forming reaction that can be conducted under acidic or basic conditions.528 A wide range of solvents can be used, including water, although typically the reaction requires long reaction times, harsh conditions, and large amounts of catalysts. The potential benefit of using ILs is their use as both solvent and promoter.

The PIL HMIm TFA and aprotic ILs BMIm PF6, BMIm BF4, and C6MIm BF4 were trialed for the Knoevenagel condensation of Meldrum’s acid with ortho-hydroxylaryl aldehydes to form coumarin-3-carboxylic acids.529 A representative example is shown in Figure 21. Of the ILs used, HMIm TFA had superior properties, leading to yields between 65% and 90 %, and conversions between 75% and 99% in 45−60 min. In contrast, the other ILs led to lower conversions even after significantly longer reaction times of 16 or 36 h.529 In a related study, a series of aprotic ILs was trialed, which included various imidazolium cations with Cl, BF4, or PF6 anions.530 It was found that the 1-benzyl-3-methylimidazolium Cl led to yields of 90−96%, which was higher than the others that ranged between 80% and 90% for their test reaction.530 A combined Friedlander annulation and Knoevenagel condensation reaction was conducted to produce 2-styrylquinolines from 2-aminoarylketones and methyl ketones using HMIm TFA to promote both reactions.497 This reaction proceeded well, and let to yields of 78−87%. 11423

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Figure 22. Knoevenagel condensation between aromatic aldehydes and methylene active compounds. E represents electron-withdrawing groups.458

Figure 23. Aldol reaction between benzaldehyde with acetone to produce (a) β-hydroxyl ketone and (b) α,β-unsaturated ketone (benzylideneacetone).464,533

Figure 23. The best conversion and selectivity toward the βhydroxyl ketone was achieved using TMEA OH. It was shown to be a useable catalyst for a range of aldedhydes and ketones with yields between 45% and 92%, able to be reused at least three times, and the most environmentally benign of the ILs trialed.533 Structure−property relationships were developed for alkylammonium PILs between their catalytic activity for aldol reactions and the degree of substitution on the cation, and the alkyl chain length on either the cation or the anion. The PILs used contained 2-hydroxy (mono-, di-, or tri-) ethylammonium cations with either butanoate, isobutanoate, or pentanoate anions.464 Two aldol reactions were used to test the catalytic behavior of the PILs. These were between citral and acetone to produce pseudoionones, where the highest conversion of 37.6% was in 2-HEAPE with 49.5% selectivity, and between benzaldehyde with acetone to produce benzylideneacetone (product (b) in Figure 23), where 2-HEAB had the highest conversion and selectivity of 98.8% and 86.5%, respectively.464 Di- and trisubstituted ammonium cations had lower catalytic behavior, which was attributed to the steric hindrance of the catalytic sites. The PILs could be reused three times with no significant loss of activity.464 These ILs have lower catalytic activity than conventional catalysts for these reactions; however, they are greener, easily separated, and more environmentally friendly. A series of ethanolammonium carboxylate PILs were trialed as catalysts for the condensation reactions between citral and acetone, and between benzaldehyde and acetone. Good conversions and selectivities were obtained. In contrast, the condensation reaction between benzaldehyde and heptanaol had poor conversion or selectivity in most of the PILs.534 A later study immobilized these PILs on analine, which dramatically improved their recovery rate and ability to be reused.535 17.6.4. Three-Component Condensation. A variety of three- or four-component cyclocondensation reactions in PILs, including the Biginelli reaction, were previously described in sections 17.5.2 and 17.5.3. Further examples are outlined in this section. Ethylammonium nitrate (EAN)463 and N-methyl-2pyrrolidone HSO4 ([NMP][HSO4])536 have been used as the

Ethanolammonium formate, EOAF, was trialed as a much cheaper alternative to the imidazolium ILs, and due to its low viscosity and low melting point.458 It worked well as a catalyst and solvent for the Knoevenagel condensation for a range of aromatic aldehydes and methylene active compounds, Figure 22, with yields between 75% and 98%.458 The EOAF could be reused, with 6.5% loss of activity after five uses.458 The reaction was faster than in the aprotic ILs BMIm BF4 or BMIm PF6. A protic ionic liquid produced from hexamethylenetetramine and acetic acid was used as a catalyst for the Knoevenagel condensation reaction. The model reaction of benzaldehyde with ethyl cyanoacetate was fast in the neat PIL, but had a low yield of 46%, due to the mixture solidifying during the reaction.531 Adding water to obtain a 1:3 molar ratio of the PIL:water led to yields up to 96%, while maintaining the fast reaction times. The water helped through lowering the viscosity and has previously been reported as beneficial for the Knoevenagel reaction.531 This PIL−water solvent was used for the reaction of a range of aldehydes with ethyl cyanoacetate with yields obtained between 87% and 98%.531 The Knoevenagel condensation of substituted phenyl acetontitriles with 1H-pyrrole-2-carbaldehyde in the presence of catalytic piperidene had significantly higher yields in the aprotic ILs of BMImBr and BMImOH as compared to the protic ILs of EAN, EOAN, or PAN and as compared to other aprotic ILs containing the BMIm cation.532 The lower yields in the nitrate-containing PILs were possibly due to small quantities of free nitric acid interacting with the piperidine catalyst.532 17.6.3. Aldol Condensation. Aldol condensation reactions are typically catalyzed by alkaline bases such as KOH or NaOH, which are corrosive, environmentally damaging, and difficult to separate. A series of Brønsted acidic or basic ILs were trialed for the aldol reaction shown in Figure 23. The acidic ILs were the protic HMIm BF4, HMIm TFA, NMP HSO4, and the aprotic HSO3−BMIm HSO4 and TOBSA HSO4, while the basic ILs were the protic TMG Lac and the aprotic trimethylethanolammonium OH (TMEA OH), also known as choline hydroxide.533 Of the protic ILs, there was no reaction detected in HMIm TFA or TMG Lac and low conversions in HMIm BF4 and NMP HSO4 of 20.8 and 12.6 toward product (a) in 11424

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Figure 24. Synthesis of 1-amido- and 1-carbamato-alkyl napthol/phenol compounds.463,536

Figure 25. Synthesis of functionalized piperidine derivatives.541

17.6.5. Other Acid-Catalyzed Reactions. The condensation reaction of indoles with aldehydes or ketones to form bisindolyl methanes was successfully conducted in an aqueous solution containing the PIL of prolinium triflate as a catalyst. The inclusion of water in the system led to increased yields and decreased reaction times.542

solvent and catalyst for the multicomponent synthesis of 1amido- and 1-carbamato-alkyl napthols/phenols, as shown in Figure 24. The three precursors were various aldehydes, with either an amide, carbamate, or urea, along with a napthol or phenol. The condensation reaction in EAN had yields between 85% and 95% at room temperature for 1 h, with negligible loss of activity after five uses.463 For a different set of precursors, [NMP][HSO4] had yields between 60% and 95% at 125 °C, in typically less than 20 min.536 The PILs of HMIm TFA, HMIm NO3, and HMIm HSO4 were all trialed and led to lower yields than [NMP][HSO4].536 Comparable yields were obtained for these reactions with BMIm Br,537 BMIm HSO4,536,538 HMIm p-TSA,536 or aprotic ILs containing the HSO4 anion and sulfonic acid groups on the cation,539,540 and slightly lower but still comparable yields for EAN.463 However, the benefit of using EAN is that it is significantly cheaper than imidazolium ILs, and the reaction in EAN was at room temperature, whereas in the aprotic ILs, or other PILs, the reactions were conducted at temperatures greater than 60 °C. The aprotic ILs of BMIm with BF4, PF6, or Cl all led to significantly lower yields and longer reaction times.538 Functionalized piperidine derivatives were prepared in a three (pseudo five)-component condensation reaction between aromatic aldehydes, substituted anilines, and ethyl/methyl acetoacetate as shown in Figure 25.541 Yields were obtained between 86−91% in HMIm HSO4 or TMG ClO4 and 87−91% in TMG TFA. All three ILs could be recycled and reused with little loss of activity.

17.7. Desulfurization

The desulfurization of oil can be achieved through oxidative desulfurization, among other methods, to remove compounds such as thiophenes and benzothiophenes. Using H2O2 as the oxidant and ammonium tungstate and N-methyl-pyrrolidinium BF4 as the catalyst worked very well for this process.543 Using other PILs with the same cation or HMIm cations with HSO4, SO3CH3, or BF4 anions led to significantly less desulfurization. Likewise, using the IL alone or with only the oxidant led to desulfurization levels below 30%.543 The catalyst system could be reused seven times with about 2% loss of activity. Two ammonium PILs were developed for the desulfurization process, as cost-effective, distillable ILs. These were 2-[2(dimethylamino)ethoxy] ethanol propionate and 3-(dimethylamino)-propanenitrile propionate.465 The proton present on the cation forms a hydrogen bond to the sulfur atoms, which enables the efficient removal of the sulfur compounds. The sulfur content in the model oil used could be decreased by 60% after a single extraction, and up to 98%, or 19 ppm, after five extractions.465 There was no change in behavior of the PILs after vacuum distillation and reuse. 11425

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Figure 26. Baeyer−Villiger oxidation reaction.

17.8. Baylis−Hillman Reaction

17.10. Baeyer−Villiger Oxidation

The Baylis−Hillman reaction for the carbon−carbon bond formation between an aldehyde and unsaturated carbonyl compound is conventionally conducted using a tertiary amine as a catalyst, although there are generally very low reaction rates. Consequently, PILs are a viable alternative as a solvent/ catalyst system where because the PIL is also the catalyst, it enables a very high catalyst concentration, and hence has the potential for faster reaction rates. A series of PILs were prepared from 1,4-diazabicyclic [2,2,2]octane reacted with the Brønsted acids of PhCO2H, AcOH, EtCO2H, and tert-BuCO2H.544 These PILs were used as the catalyst for the carbon−carbon bond forming Baylis− Hillman reaction in acetonitrile, DMF, methanol, or dioxane as the solvent. The HDABCO-AcO PIL with a small amount of additional water was the best catalyst, for the reaction of aromatic aldehydes, aliphatic aldehydes, and cinnamaldehydes with acrylates or acrylonitrile. Yields between 22% and 98% were obtained, which depended on the precursors.544

The Baeyer−Villiger oxidation reaction enables oxidation of cylic or linear ketones using an oxidant, and a representative reaction is shown in Figure 26. The use of a lipase in an IL was developed to enable epodixation and the Baeyer−Villiger oxidation reactions through a chemo-enzymatic approach.550 Hydrogen peroxide was used as the oxidant, and Candida antarctica B lipase (CaLB) as the enzyme. An extensive range of aprotic ILs were trialed, along with the protic IL of triethanolammonium nitrate. It was found that hydrogenbond-donating ILs were most suited for supporting this reaction, with the aprotic task-specific IL of 1-(3-hydroxypropyl)-3-methylimidazolium nitrate leading to the highest yields. In another investigation, a mixture of protic and aprotic ILs was trialed for the Baeyer−Villiger reaction using OxoneR as the oxidant. The protic HMIm OAc and the aprotic BMIm BF4 led to equally high yields of up to 99%, while lower or negligible yields were achieved with HMIm BF4, BMIm OAc, 2methylpyridinium OAc, BMIm TF2N, EMIm ethylsulfate, and PEG-5 cocomonium methylsulfate.551 The ILs could be reused at least three times with no loss of activity for the PIL, and about 2% for the aprotic IL.

17.9. Oxidation

The PIL HMIm NO3 was successfully used as solvent and promoter for the synthesis of a range of oximes from alcohols and hydroxylamine hydrochloride, under microwave irradiation with yields between 85% and 99%.545 No yields were obtained using nitric acid, or using aprotic ILs with the BMIm cation and PF6, Cl, HSO4, or NTf2 anions. The hydrothiocyanation or hydrosulfenylation of Baylis− Hillman alcohols to produce methyl β-thiocyanato (or βphenylsulfenyl)- α-formylhydrocinnamates was achieved in HMIm HSO4 with NaNO3. Yields of 74−87% were obtained in a one-pot method.546 The conversion of aliphatic, aromatic, and heterocyclic thiols into diphenylmethyl thioethers was conducted in the nonstoichiometric PIL of triethylammonium methanesulfonate, with 10% excess methane sulfuric acid. High yields of 63−99% were achieved in short times, with the solvent able to be recycled.547 A range of protic and aprotic imidazolium ILs were trialed as solvents for organic transformations using cerium(IV) salts as the oxidizing agent.548 The cerium salts with the highest solubility and stability were found to be cerium(IV) ammonium nitrate and cerium(IV) triflate. The ILs with the best solvent performance were the aprotic 1-alkyl-3-imidazolium triflates. The protic ILs contained either the HMIm or the HBIm cations and were found to be decomposed by the cerium(IV) ammonium nitrate salt.548 The epoxidation of styrene to styrene oxide was trialed in the protic ILs of TMG lactate, TMG TFA, and the aprotic ILs of BMIm BF4, BMIm PF6, and BMIm Cl.549 The yields obtained using BMIm BF4 and BMIm Cl of 69% and 34%, respectively, were significantly higher than the others with the PILs leading to yields of 0.3% and 2% for the TFA and lactate salts, respectively.

17.11. Reduction

HMIm BF4 has been used as the reaction media for the amination of aromatic aldehydes through a reduction reaction using NaBH4.552 Yields were obtained between 90% and 95% using isoxazole amines to produce N-benzyl-N-isoxazolyl amines. 17.11.1. Pinacole Rearrangement. The pinacole rearrangement typically requires a strong Brønsted acid, and as an alternative four PILs with different acidities were trialed, which consisted of the triethylammonium cation with Ms, TFA, HSO4, or TfOH. The reaction benefited through the use of microwave radiation at low power.553 The TEA HSO4 PIL performed significantly better than the others, which was attributed to its higher proton activity, consistent with this reaction needing an acidic environment. The PILs containing the other anions led to little or no conversion. Under optimal conditions, a yield of 91% with 83% selectivity was obtained. 17.11.2. Deprotection. The deprotection of a series of methoxymethyl and ethoxyethyl ethers into alcohols was conducted in HMIm HSO4 under microwave radiation, where this PIL was chosen due to being Brønsted acidic.554 The alcohols could then be converted to acetates to trimethylsilyl compounds. In general, slightly higher yields were obtained for the deprotection reaction using microwave irradiation compared to traditional thermal conditions. However, after microwave radiation the catalytic activity of the PIL was decreased, suggesting it has been partially degraded. 17.11.3. Decarboxylation. A series of ILs were screened for decarboxylation of N-heteroaryl and aryl carboxylic acid under microwave irradiation to form indoles, styrenes, stilbenes, and arene derivatives. The only protic IL in the investigation 11426

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with the catalyst stability attributed to the coordination ability of the TMG cations.

HMIm PTSA and the aprotic C6MIm Br were able to promote the decarboxylation without any added catalyst. C6MIm Br required a mild base to be added to perform well.555 A mechanism was suggested where the IL catalyzes a carboxylate anion formation of the precursors, followed by the IL absorbing microwave energy, leading to the decarboxylated product. 17.11.4. Aromatic Nitration. EAN was used successfully with added Tf2O or TFAA for the nitration of aromatic and heteroaromatic compounds. Very high yields were obtained for many of the precursors trialed. Overall the EAN with Tf2O was observed to lead to higher nitration yields and better selectivity for a broad range of precursors, especially for strongly deactivated systems.466 17.11.5. Formylation. PILs were used as the catalyst with no additional solvent for the N-formylation of amines and alcohols using formic acid.556,557 In one study, the HBIm TFA PIL led to the higher yields than HMIm TFA, HMIm p-TsO, and 1-butyl-benzimidazolium TFA, although all performed well with a broad range in yields from no reaction to 98% achieved. The lack of reaction in benzimidazole, carbazole, phenol, and benzoin was speculated to be due to the strong polar N−H and O−H bonds present in EAN hydrogen bonding to these amines and rendering them non-nucleophilic.556 PILs containing the cation of 1,5,7-triazabicyclo[4.4.0]dec-5-ene [TBD], paired with the anions of TFA, BPh4, and Cl− were trialed, with [TBD][TFA] providing the best results, and were reusable.557 When additional solvents of ethanol, acetonitrile, or THF were present, the reaction times increased and yields decreased. 17.11.6. PILs as Intermediates. Protic ionic liquids have formed as intermediates in organic synthesis, such as when the 1-methylimidazole cation has been used as an acid scavenger during the BASIL process of BASF,558 the synthesis of fluorinated alkoxytrimethylsilanes,559 and in the synthesis of unsymmetrical alkyl-substituted secondary phosphine oxides.560 17.11.7. Substitution. The synthesis of bis(indolyl)methane through electrophilic substitution of indoles with aldyhedes was conducted in EAN with yields between 83% and 95% at 25 °C.561 No additional catalyst or solvents were required, and hence EAN is advantageous as compared to previous methods that generally required toxic catalysts in high concentrations, expensive solvents, and led to lower yields. In the proposed mechanism, the catalytic behavior of EAN is toward the activation of the carbonyl group, which can then undergo reaction with the indoles through nucleophilic addition, dehydration, and aromatization. The good catalytic behavior of EAN toward this reaction has been attributed to its acidic nature, in addition to its ability to adsorb the water formed during the dehydration reaction.561 17.11.8. PIL Modified Catalysts. A catalyst was prepared using the mineral attapulgite as support with rhodium nanoparticles attached using the IL TMG lactate to mediate their immobilization.562 The final catalyst consisted of attapulgite with less than 5 nm Rh0 particles and TMG+ ions. This catalyst was used for the hydrogenation of cyclohexane to cyclohexane with conversions of about 99% even after five uses. The attapulgite with only rhodium nanoparticles provided a conversion of 51.5%, whereas the attapulgite with the IL only had 3.51%.562 Similarly, a palladium catalyst supported on a TMG modified molecular sieve was produced through immobilizing palladium nanoparticles on SBA-15 using TMG lactate.563 This catalyst system worked well for the Heck arylation of olefins with aryl halides for at least six times with no additional solvent required,

17.12. Synthesis of Carbon Materials

An extensive series of PILs and protic salts including a broad range of cations and anions were prepared by Watanabe et al. and used as precursors in the direct synthesis of nitrogen doped carbon structures.370,564 The synthesis of the carbon materials simply involved heating the PILs or protic salts to 1000 °C at 10 °C/min under Ar, and holding it at that temperature for 2 h.370 While this synthesis was energy intensive, it was rapid, and did not require additional catalysts or solvents. A broad variety of structures and compositions were obtained, enabling structure−property relationships to be developed; the final carbon materials with high nitrogen contents generally were more amorphous, with lower surface areas, and nonporous; the PILs and protic salts with stronger proton transfer led to higher yields, whereas those with weaker binding energies had a portion convert back to the precursor acid and bases; aliphatic chains and heterocyclic moieties on the PIL or protic salt precursors led to porous carbon structures with high surface areas.370 Mesoporous carbon was successfully obtained through an ionothermal carbonization reaction of the sugars of glucose and fructose in the PIL of N,N-dimethyl-N-formylammonium (DMFH) Tf2N. The reaction proceded via cyclic furanose or 5-hydroxymethylfurfural intermediates followed by a polymerization reaction to form the carbon.565

18. POLYMERS 18.1. Polymerization in PILs

The synthesis of polymers using atom transfer radical polymerization (ATRP) has been reported in a variety of PILs containing the MIm cation with carboxylic anions.566−569 The ATRP method is used to obtain polymers with a predictable molecular weight and narrow polydispersity, and usually requires a large amount of catalyst, and catalyst solubility to obtain fast polymerization rates.568 Three PILs consisting of the MIm cation with acetate, propionate, or butyrate anions were used as the solvent for ATRP of methyl methacrylate, with initiator ethyl 2-bromo-isobutyrate (EBiB)/ CuBr.566 In a similar study, these same PILs were used as solvents for the ATRP of acrylonitrile, using EBiB initiator, FeBr3 catalyst, and ascorbic acid as a reducing agent.567 For both of these polymerization reactions, the reaction rate followed the trend of MIm CH3COO > MIm CH3CH2COO > MIm CH3CH2CH2COO.566,567 It was suggested that the different reaction rates might be due to the PILs polarity, or due to formation of complexes between the carboxyl group of the ILs and the copper in the initiator.566 The polymerization of acrylonitrile proceeded slower, but with better molecular weight control, in air as compared to in the absence of oxygen.567 In a similar investigation, PILs consisting of the MIm cation with acetate, valerate, or caproate anions were used as solvents in the ATRP of acrylonitrile, initiated by EBiB with a FeBr2 catalyst.569 The fastest reaction rate, and best molecular weight control, were again achieved in MIm acetate. The single electron transfer-living radical polymerization (SET-LRP) of acrylotonitrile in the PILs of MIm acetate, propionate, or valerate was investigated using Fe(0) as a wire catalyst.570 The reaction rates were fastest in MIm acetate. The polymerization of methacrylonitrile through ATRP was reported in PILs containing the MIm cation with acetate, 11427

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temperature, and often high pressure for industry scales. This is an example of a protic molten salt being used to form a commodity polymer. A series of PIL-like polymers were produced through free radical polymerization of PIL monomers. 576 The PIL monomers consisted of the 2-(dimethylammonium)ethyl methacrylate cation (HDMAEMA) with acetate, butyrate, hexanoate, octanoate, benzoate, or oleate anions. The tertiary ammonium cation and carboxylic acid were retained in the polymer, giving it the PIL-like characteristics such as miscibility with a broad range of polar and nonpolar solvents, particularly those with intermediate polarity, low thermal stability, and low Tg values.576 Increasing the anion chain length decreased the thermal decomposition temperature of the polymer from 330 °C for poly(HDMAEMA-acetate) to 216 °C for the oleate containing polymer.576 Similarly, the PIL monomer of N,Ndiethyl-N-(2-methacryloylethyl) ammonium TFSI was polymerized using three different polymerization methods, ATRP, activators regenerated by electron transfer (ARGET)-ATRP, and organotellurium-mediated living radical polymerization (TERP), with the last two leading to controllable narrow molecular weight ranges and good monomer conversion.577 The polymerization of aniline to polyaniline was achieved through converting the aniline precursor to a molten salt of anilinium nitrate, and then electrodepositing the polymer.338 The preprotonation removed the need for a highly acidic solvent, which is usually a requirement for this polymerization reaction. The electrodeposition of the polyaniline from anilinium nitrate was trialed from aqueous media, EAN, and from the aprotic ionic liquid of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide. Significantly better results were achieved from the EAN as compared to the other solvents with thick, porous deposition of the polyaniline.338 PILs containing the MIm cation paired with sorbate578 or mercaptopropionate579 anions were used as additives into rubber/silica composites.578,579 The anions used were polymerized during vulcanization, and the polymerized PILs increased the hydrogen bonding within the polymer, leading to enhanced mechanical properties.578,579 In later work, MIm mercaptopropionate and bis(1-methylimidazolium) mercaptosuccinate were used in a similar manner to improve the mechanical properties of styrene butadiene rubber/halloysite nanotube composites, where the PILs increased the hydrogen bonding and improved the clay halloysite nanotube dispersion.580

butyrate, caproate, and heptylate anions. The reaction was initiated by azobis(isobutyronitrile) (AIBN) with FeCl3 catalyst in PILs.568 MIm acetate led to the fastest reaction rate and best molecular weight control, with the reaction rates following MIm acetate > MIm butyrate > MIm caproate > MIm heptylate.568 The ATRP and SET-LRP reactions in PILs did not require any additional ligand present, with the benefit that there was no ligand that had to be removed from the polymer. The ionic liquids and FeBr2 or FeCl3 catalysts could be recycled and reused.568,569 These recent investigations using PILs for ATRP reactions have all used the MIm cation with various carboxylate anions. MIm acetate was the most favorable PIL for these polymerization reactions, with the smaller, more polar anion leading to faster reaction rates, better molecular weight control, and better polydispersity of the polymers.566−570 A polypyrrole-modified platinum electrode was produced through polymerization of pyrrole from EIm Tfa. The polymerization of the pyrrole from EIm Tfa led to a denser and more homogeneous polypyrrole film as compared to the equivalent procedure from aqueous solution.571 The polypyrrole-EIm Tfa film on platinum nanoparticles was trialed for the electrooxidation of formaldehyde and gave improved results over just platinum nanoparticles or platinum nanoparticles with only the polypyrrole, indicating that a proportion of the PIL is incorporated beneficially into the electrode.572 The electrooxidation of small organic molecules is a potential fuel, but usually there is poisoning of the platinum catalyst by CO, which is a byproduct of the electrooxidation. The polypyrrole-EIm Tfa was shown to help oxidize the CO to CO2 and hence reduce the poisoning effect.572 Investigations have been conducted into poly(ethylene oxide) PEO and poly(propylene oxide) PPO in EAN,573,574 along with pluronic PEO−PPO−PEO surfactants.229 EAN has been reported to be a good solvent for PEO, and a poor solvent for PPO. The solvation of PEO in EAN has been attributed to hydrogen bonding between the ammonium on the ethylammonium cation with the ether oxygens in the PEO chain.575 This is in competition with the hydrogen bonding between the ethylammonium cation and the PIL anion. Consequently, anions with a greater hydrogen-bonding ability are expected to have lower PEO solubility, which was shown using ethylammonium formate where the PEO was not soluble.575 PEO adsorbs to silica surfaces in EAN, through the polymer displacing ethylammonium cations.229,573 In contrast, PPO does not adsorb, which was possibly due to the short chains present due to poor PPO solubility in EAN limiting the size of PPO molecules that could be dissolved.573 Films of PEO were prepared by drop casting 0.01 wt % 10, 35, and 100 kDa PEO from EAN. The PEO polymer films were proposed to consist of a mushroom or pancake configuration,573,574 and to be more compact than in water due to the EAN being a poorer solvent for PEO than water. The distance the mushrooms extended from the silica surface increased with increasing PEO molecular weight, and with increasing PEO concentration.

19. LUBRICANTS The use of ionic liquids as lubricants in specialized areas where their cost is not prohibitive is emerging as a niche application of interest. The advantages of using ILs as lubricants are their low or negligible volatility, high viscosity, which prevents spinning off surfaces, and liquid state, which enables them to replenish across a surface.118 While some aprotic ILs have good lubricant properties,581−583 a key benefit of the PILs is that they have the potential to be significantly cheaper, while still having desirable lubrication properties. Generally, the PILs that have been reported as potential lubricants have a long alkyl chain on an ammonium cation paired with an anion that is closely related to conventional lubricants.118,584,585 The PIL molecules are assumed to be oriented with their polar end toward the metal surface, forming a close packed layer.118,585 The lubrication properties of ethylammonium nitrate, as a representative IL, have been reported for surfaces of silica or

18.2. Polymerizable PILs

The condensation polymerization reaction to form nylon-6,6 involves the reaction of hexanedioic acid with 1,6-diaminohexane. The mechanism involves the protonation of the amine by the acid, thus leading to a protic salt intermediate. The polymer is then formed through heating the resulting salt at high 11428

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alumina against silica or polytetrafluoroethylene.586 The EAN decreased the friction significantly as compared to air through suppressing surface forces such as van der Waal, electrostatic double-layer, and capillary forces.586 The surface ordering, which is present in EAN as a lamellar-like structure, led to a significant decrease in the viscosity as compared to bulk EAN.586 EAN between silica and mica was observed to form structured layers, with cations predominantly attached to the mica and silica surfaces. During lateral applied forces these layers can be sheared, leading to low friction coefficients.587 A hexagonal liquid crystal phase of nonionic polyoxyethylene surfactant C16EO6 in EAN was reported to have significantly lower friction coefficient and lower wear volume for steel disks as compared to neat EAN.251 Up to 0.2 wt % of multiwalled carbon nanotubes (MWCNT) were incorporated into the hexagonal phase, and led to only a small decrease of the friction coefficient as compared to the hexagonal phase, and higher wear than the hexagonal phase or neat EAN.251 The combination of alkylammonium cations with perfluorinated carboxylate anions has led to PILs with very good lubricant properties, along with enhanced solvent properties as compared to fluorinated oils.118 The hydrocarbon chain on the ammonium cation had a significant effect on the tribological properties, with friction coefficients between 0.15 and 0.3.118 Lower friction coefficients and decreased wear volumes were obtained through increasing the alkyl chain length, and having a primary ammonium cation with a long saturated alkyl chain, of around 18 carbons.118 These PILs were good at uniformly covering the surfaces, which led to lower friction and reduced corrosion. A series of PILs were prepared using the dodecylammonium cation with S-(1-carboxylpropyl)-N,N-dialkyldithiocarbamate anions, where the alkyl group was either ethyl, butyl, or octyl.584 These PILs were used as additives between 2 and 5 wt % in a lithium containing grease, and decreased the friction with negligible difference between the three PILs.584 In a related study, a series of PILs consisting of octadecylammonium or dodecylammonium cations paired with alkylbenzenesulfonate were prepared and used either neat or as additives in a polyalphaolefin.585 The neat PIL had lower friction than the neat oil, although the best lubricant properties were achieved with 2 wt % of the PIL in the oil.585 The PILs appeared to decompose partially at the steel surface, forming a layer of sulfide and organic amine, which formed the tribological layer, and led to lower friction, but increased corrosion.584,585 PILs containing long-chained alkylammonium cations paired with long-chained perfluoro alkoxide, carboxylate, or sulfonate anions had good thermal stability, friction, and wear properties.588 The PIL containing the sulfonate worked well as a lubricant for metal thin film media. A few PILs have been specifically selected for use as lubricants for copper. A pin-on-disc tribological test found the lowest friction and wear rates followed di-[bis(2-hydroxyethyl)ammonium] adipate < bis(2-hydroxyethyl)ammonium salicylate < aprotic IL of C6MIm Tf2N < commercial lubricant PAO 6.589 The PIL of bis(2-hydroxyethyl)ammonium oleate had initially low friction, which increased after the running in period. The good properties of the PILs were attributed to their ability to form hydrogen bonds with the surfaces, and that no corrosion or reaction products formed.589 In a closely related study, four aprotic ILs and the protic IL of 2-hydroxyethylammonium succinate reacted with copper in different ways, whereas di-[bis(2-hydroxyethyl)ammonium] adipate had no

corrosion products detected.590 While the PIL of 2-hydroxyethylammonium succinate reacted with copper, it was shown to be a good lubricant for sapphire−stainless steel interfaces.591

20. USE OF PILs AS EXPLOSIVES The explosive nature of the PILs containing the nitrate, or similar, anion is an important safety consideration, and care needs to be taken on heating, especially because additives (particularly nitrogen- or oxygen-rich additives) can significantly lower the temperature where they explode. The explosive decomposition temperature varies considerably depending on the specific cation and anion, and the pressure, and, as was reported, pyrrolidinium nitrate exploded at 110 °C at 15 mbar.592 In addition to unwanted explosions, a few PILs have been specifically prepared for their explosive properties, including series of PILs containing 1-Cn-imidazolium and 1-Cn-2methylimidazolium related cations (n between 1−6) paired with nitrate or picrate anions.593 Structure−property relationshiops were identified for the effect of the alkyl chain length, and the presence of additional methyl or electron-withdrawing groups. The most energetic of these PILs were the MIm nitrate and 1,2-dimethylimidazolium picrate. The nitrates all had lower decomposition temperatures as compared to their picrate counterparts, and the presence of a methyl substituent in the C2 ring position increased the decomposition temperature by typically 20 °C. Additional −NO2 groups to the ring generally decreased the explosive temperature, with more of an effect in the C2 than the C4 ring position. In contrast, aprotic anologues of a select few of these PILs had similar melting points, but higher decomposition temperatures.593 21. ILs USED FOR SPACE PROPULSION APPLICATIONS A more unusual application of PILs has been the use of EAN as an electrospray space propellant, where an array of nozzles simultaneously emits droplets of EAN. An average propulsion efficiency of 62 % was obtained in EAN, as compared to 85% in a formamide−methylammonium formate system. However, the EAN overall was considered probably better, because it could be used with a broader applied current range, and would have less evaporative loss.594 22. HIGH-THROUGHPUT TECHNIQUES It is well recognized that there are vast number of potential ILs that can be produced through cation−anion combinations. However, for most ILs, the synthesis required is prohibitive to developing extensive libraries, and instead relatively small series are usually used. The use of combinatorial approaches has assisted in synthesis efficiency, such as for chiral aprotic ILs.595 Efficient routes for synthesis of functionalized aprotic ILs have been reported, such as a two-step method via protonation of an amine followed by a Michael reaction.596 Perhaps the most high-throughput aprotic IL synthesis has been for functionalized chiral ionic liquids via a “click” synthesis reaction from precursors of chiral cyclic sulfates and cyclic sulfamidates.597 A prediction model for the melting points of ionic liquids and molten salts has been proposed as a high-throughput computational screen to identify compounds likely to have useful melting points before synthesis.598 The initial “calibration” study did not include protic ILs due to the experimental melting points published for many PILs likely to be affected by 11429

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While there is still significantly more publications on aprotic ILs than their protic counterparts, there has been strong growth in the number of PIL publications, particularly describing many applications where the benefit of the PILs may include them being relatively cheap, protic, and easy to synthesize and tailor. There has been strong interest in modifying the properties of PILs through the addition of water or other molecular solvents, and it is anticipated that this will continue. For many applications, the presence of some water in the PILs is not detrimental, and instead leads to enhanced solvent properties such as lower viscosity, higher conductivities, and lower melting points. It remains an issue of definition though of how to refer to these resulting protic solutions. There is also an ongoing difficulty surrounding how to describe the proton activity in the PILs, analogous to pH in aqueous systems. For a broad range of applications, it has been reported that the acidity/basicity of the PIL or PIL−solvent system is crucial for their benefical properties, for example, for their use as solvents and/or catalysts for many oganic reactions and as additives for protein stabilization. It is expected that the fundamental properties of PILs will continue to be explored, along with continued interest in many existing and new applications, such as in electrochemistry, organic and inorganic synthesis, and biological applications. In particular, there has been a significant interest in a broad range of PILs for use as electrolytes and incorporation in polymer electrolytes for fuel cells, and other energy storage devices. We anticipate that this interest will continue to grow due to the specific properties of PILs being very well suited to this application, in that they are protic, nonaqueous, cost effective, and can be selected to have low viscosity or other required properties. There continues to be a strong interest in the application of PILs as solvents and/or catalysts for a broad range of organic reactions. To date, we are not aware of structure−property relationships that can be used in the selection of PILs for specific organic reactions. However, it is apparent that for reactions that require an acidic or basic solvent/catalyst media, there are certain ions that are frequently present in the PILs, such as HSO4 for acidic media and hydroxyl groups on the cation or anion for basic media. It should be noted that for many of these reactions, there have only been a relatively limited number of PILs trialed. An evolving field since our 2008 review has been the characterization of the nanostructure of protic and aprotic ionic liquids, with very few publications prior to 2008. Since our last review, a broad range of PILs, fluorous PILs, PIL−solvent solutions, and PIL−PIL mixtures have been characterized. It is evident that many PILs possess a complex liquid nanostructure consisting of polar and apolar domains, which influence their properties and interactions with solutes and solvents. Because of this, we anticipate that the nanostructure of PILs and PIL− solute mixtures will continue to be a field of significant interest, and to date there is only embryonic knowledge on the influence that liquid nanostructure has on many properties, and hence on the optimized use of PILs in many applications. One specific area that we have previously discussed in depth is the relationship between IL nanostructure, the solvophobic effect, and the ability of the ILs to support amphiphile selfassembly.121 For this area, it has been clearly established that the nanostructure of the PILs has a significant influence. An emerging area is the use of high-throughput and combinatorial approaches for the synthesis and characterization

small nonstoichiometric effects, and the presence of solvent molecules such as water. However, the resulting predictive scheme appears applicable to both protic and aprotic ILs.598 It has also been reported that ab initio calculations used to predict liquid range and conductivity for aprotic ILs could not be used for PILs; however, estimates for PILs could be obtained on the basis of the cation and anion sizes, with larger anions leading to higher melting points.599 A significant advantage of PILs as compared to their aprotic counterparts is the simplicity of their synthesis. However, to date we are only aware of one very recent report that has utilized this for high-throughput synthesis of PILs.600 It is evident from Tables S1 and S2 that there is a broad range of cations and anions that have been used in PILs, but even from the ions in these tables there are a significant number of combinations that have not been trialed, and a vast number of other acid−base combinations that could potentially result in PILs. A simple screen was recently reported for screening acid− base combinations for potential PILs. A series of 36 stoichiometric acid−base combinations were individually prepared using 4 acids paired with 9 primary, secondary, or tertiary alkylammonium cations that contained ethanol groups.601 The screen employed was to observe the state of the samples as liquid or solid at 25 and 100 °C. An artificial neural network has been reported for predicting the solubility of CO2 in the PIL propyl amine methyl imidazole alanine with various water contents.602 Very good agreement was obtained between the experimental CO2 capture rate and the predicted results from the neural network. While limited in extent, this study shows the potential of data analysis techniques such as artificial neural networks for making meaningful predictions beyond measured data. A few high-throughput screens have been reported for aprotic ILs for various applications, and it is assumed that these would be suitable for PILs. These include high-throughput, low volume screens for testing the dissolution kinetics of lignocellulosic biomass in ILs,603 a high-throughput screen to test the enantiomeric discrimination of aprotic chiral ILs toward the test compounds of racemic potassium Mosher’s salt and aromatic alcohols using 19F NMR spectroscopy,595 and a screen of aprotic ILs for use in membrane protein crystallization.407 The agar diffusion test is a fast and simple test as an initial screen of ecotoxicity of ILs,604 where the short-listed ILs can then be further assessed by a second different high-throughput test.605 A series of aprotic imidazolium ILs were screened using a response surface methodology, which is a statistical design of experiment approach for optimization, and it was used to assess their ability to be used in the extraction of flavonoids from Chamaecyparis obtuse leaves.606 The effect of various aprotic ILs as additives for the unfolding/folding of lysozyme was investigated using an assay for enzyme activity and an aggregation assay on samples in 96-well plates.607 Tests like these could be extended to PILs.

23. CONCLUDING REMARKS In this Review, the literature on protic ionic liquids (PILs), since our last major review in 2008, has been discussed. A summary has been provided of the thermal and physicochemical properties reported for PILs, and it is highly evident that there has been an extensive range of alkylammonium, imidazolium, and heterocyclic cations paired with many organic and inorganic anions that have been employed to prepare PILs. 11430

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of ionic liquids. PILs are well suited for these styles of experiments due to their simplicity of synthesis, and the vast number of potential combinations. While to date only a few publications have included high-throughput or combinatorial approaches, we envisage this will be an area of growth within the ionic liquid community in future years.

chemistry and advanced materials. He completed his higher education at The University of Melbourne (BScEd, H1, 1981; BScHons, H1, 1982; Ph.D., 1987; D.Sc., 2015). Prior to his roles at RMIT, he served as CSIRO Group Executive for Manufacturing, Materials and Minerals, CSIRO Divisional Chief for Materials Science and Engineering, and inaugural Vice President Research for CAP-XX (an Intel portfolio company). He has published more than 175 journal articles and is a Fellow of the Australian Academy of Technological Sciences and Engineering, Royal Australian Chemical Institute, Royal Society of Chemistry, and the Australian Institute of Company Directors. He has also been the recipient of an Australian Research Council (ARC) Queen Elizabeth II Fellowship and an ARC Federation Fellowship.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemrev.5b00158. Thermal phase behavior of the PILs in Table S1, and the physicochemical properties of PILs in Table S2; references 608−635 are not cited in the main text, but are referred to within the Supporting Information (PDF)

AUTHOR INFORMATION

ACKNOWLEDGMENTS Our early work in the field of PILs was supported by grants to C.J.D. from the Australian Research Council (FF 0348522 and DP 0666961) and through CSIRO awarding a post-doctoral fellowship to T.L.G.

Corresponding Author

ABBREVIATIONS

*E-mail: [email protected].

Cations

Notes

EA, PA, BA, etc. EOA, DEOA, TEOA

ethyl-, propyl-, butyl-ammonium ethanol-, diethanol-, triethanol-ammonium DMEA dimethylethylammonium DEMA diethylmethylammonium t-BuP1(dma) tert-butylimino-tris(dimethylamino)phosphorane t-BuP1(pyrr) tert-butylimino-tris(pyrrolidino)phosphorane BEMP 2-tert-butylimino-2-diethylamino1,3-dimethyperhydro-1,3,2-diazaphosphorane HP1(dma) imino-tris(dimethylamino)phosphorane EtP2(dma) ethylimino-tris(dimethylamino)phosphorane HNC(dma) 1,1,3,3-tetramethylguanidinium TMG 1,1,3,3-tetramethylguanidinium MTBD 7-methyl-1,5,7-triazabicyclo[4,4,0]dec-5-ene MTBDH 1,3,4,6,7,8-hexahydro-1-methyl-2Hpyrimido[1,2-a]pyrimidine bis(pentafluoroethyl)sulfonylimide HTBD triazabicyclodecene or 1,5,7triazabicycle[4,4,0]dec-5-ene BTBD 7-butyl-1,5,7-triazabicyclo[4,4,0]dec-5-ene P1-t-Bu tert-butylimino-tri(pyrrolidino)phosphorane P2-Et tetramethyl(tris(dimethylamino)phosphoranylidene)phosphorictriamid-ethyl-imin BuG5 1,3-dimethyl-2-(1-butylimino)imidazolidine BPS-10 ethoxylated phytosterol (10 carbons in ethoxy chain) m-2-n Gemini surfactants [ C m H 2 m + 1 ( C H 3 ) 2 N ( C H 2 ) 2 N (CH3)2CnH2n+1]Br2 CnMPyrrBr Cnmethylpyrrolidinium bromide CnPyrBr Cnpyridinium bromide DIPEA diisopropylethylammonium

The authors declare no competing financial interest. Biographies

Tamar Greaves is a senior research fellow at RMIT University. Tam completed her B.Sc. (Hons I) in 1999 and her Ph.D. in Physics in 2004 from Monash University. In 2005 she joined the group of Prof. Drummond at CSIRO as a Postdoctoral Fellow, followed by a Research Scientist position at CSIRO.

Professor Calum John Drummond is Deputy Vice Chancellor Research and Innovation and a Vice President at RMIT University. He is also an active research professor with interests in physical 11431

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(11) Hao, J. C.; Zemb, T. Self-Assembled Structures and Chemical Reactions in Room-Temperature Ionic Liquids. Curr. Opin. Colloid Interface Sci. 2007, 12, 129−137. (12) Yoshizawa, M.; Xu, W.; Angell, C. A. Ionic Liquids by Proton Transfer: Vapor Pressure, Conductivity, and the Relevance of CpKa from Aqueous Solutions. J. Am. Chem. Soc. 2003, 125, 15411−15419. (13) Stoimenovski, J.; Izgorodina, E. I.; MacFarlane, D. R. Ionicity and Proton Transfer in Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2010, 12, 10341−10347. (14) Kanzaki, R.; Doi, H.; Song, X. D.; Hara, S.; Ishiguro, S.; Umebayashi, Y. Acid-Base Property of N-Methylimidazolium-Based Protic Ionic Liquids Depending on Anion. J. Phys. Chem. B 2012, 116, 14146−14152. (15) Doi, H.; Song, X.; Minofar, B.; Kanzaki, R.; Takamuku, T.; Umebayashi, Y. A New Proton Conductive Liquid with No Ions: Pseudo-Protic Ionic Liquids. Chem. - Eur. J. 2013, 19, 11522−11526. (16) Burrell, G. L.; Burgar, I. M.; Separovic, F.; Dunlop, N. F. Preparation of Protic Ionic Liquids with Minimal Water Content and N-15 NMR Study of Proton Transfer. Phys. Chem. Chem. Phys. 2010, 12, 1571−1577. (17) Capelo, S. B.; Mendez-Morales, T.; Carrete, J.; Lago, E. L.; Vila, J.; Cabeza, O.; Rodriguez, J. R.; Turmine, M.; Varela, L. M. Effect of Temperature and Cationic Chain Length on the Physical Properties of Ammonium Nitrate-Based Protic Ionic Liquids. J. Phys. Chem. B 2012, 116, 11302−11312. (18) Lv, Y. Q.; Guo, Y.; Luo, X. Y.; Li, H. R. Infrared Spectroscopic Study on Chemical and Phase Equilibrium in Triethylammonium Acetate. Sci. China: Chem. 2012, 55, 1688−1694. (19) Johansson, K. M.; Izgorodina, E. I.; Forsyth, M.; MacFarlane, D. R.; Seddon, K. R. Protic Ionic Liquids Based on the Dimeric and Oligomeric Anions: [(AcO)xHx‑1]‑. Phys. Chem. Chem. Phys. 2008, 10, 2972−2978. (20) MacFarlane, D. R.; Forsyth, M.; Izgorodina, E. I.; Abbott, A. P.; Annat, G.; Fraser, K. On the Concept of Ionicity in Ionic Liquids. Phys. Chem. Chem. Phys. 2009, 11, 4962−4967. (21) Ueno, K.; Tokuda, H.; Watanabe, M. Ionicity in Ionic Liquids: Correlation with Ionic Structure and Physicochemical Properties. Phys. Chem. Chem. Phys. 2010, 12, 1649−1658. (22) Yaghini, N.; Nordstierna, L.; Martinelli, A. Effect of Water on the Transport Properties of Protic and Aprotic Imidazolium Ionic Liquids - an Analysis of Self-Diffusivity, Conductivity, and Proton Exchange Mechanism. Phys. Chem. Chem. Phys. 2014, 16, 9266−9275. (23) Canongia Lopes, J. N.; Rebelo, L. P. N. Ionic Liquids and Reactive Azeotropes: the Continuity of the Aprotic and Protic Classes. Phys. Chem. Chem. Phys. 2010, 12, 1948−1952. (24) MacFarlane, D. R.; Vijayaraghavan, R.; Ha, H. N.; Izgorodin, A.; Weaver, K.; Elliott, G. D. Ionic Liquid ‘‘Buffers’’pH Control in Ionic Liquid Systems. Chem. Commun. 2010, 46, 7703−7705. (25) Treskow, M.; Pitawala, J.; Arenz, S.; Matic, A.; Johansson, P. A New Class of Ionic Liquids: Anion Amphiprotic Ionic Liquids. J. Phys. Chem. Lett. 2012, 3, 2114−2119. (26) Dong, K.; Zhang, S. J. Hydrogen Bonds: A Structural Insight into Ionic Liquids. Chem. - Eur. J. 2012, 18, 2748−2761. (27) Fumino, K.; Fossog, V.; Wittler, K.; Hempelmann, R.; Ludwig, R. Dissecting Anion-Cation Interaction Energies in Protic Ionic Liquids. Angew. Chem., Int. Ed. 2013, 52, 2368−2372. (28) Fumino, K.; Wulf, A.; Ludwig, R. Hydrogen Bonding in Protic Ionic Liquids: Reminiscent of Water. Angew. Chem., Int. Ed. 2009, 48, 3184−3186. (29) Miran, M. S.; Kinoshita, H.; Yasuda, T.; Abu Bin, M.; Susanz, H.; Watanabe, M. Hydrogen Bonds in Protic Ionic Liquids and their Correlation with Physicochemical Properties. Chem. Commun. 2011, 47, 12676−12678. (30) Hunger, J.; Sonnleitner, T.; Liu, L. Y.; Buchner, R.; Bonn, M.; Bakker, H. J. Hydrogen-Bond Dynamics in a Protic Ionic Liquid: Evidence of Large-Angle Jumps. J. Phys. Chem. Lett. 2012, 3, 3034− 3038. (31) Zahn, S.; Thar, J.; Kirchner, B. Structure and Dynamics of the Protic Ionic Liquid Monomethylammonium Nitrate (CH3NH3 NO3)

1,8-diazabicyclo-[5,4,0]-undec-7ene imidazolium-[1,2a]-pyridine

ImPr Anions

N HF2 HSO4 H2PO4 TFA TfO BF4 PO3F CH3SO3 MsO Hhfac Htfac Hfod Hbta Htta Tf2N BETI hfipOSO3

nitrate bifluoride hydrogen sulfate dihydrogen phosphate trifluoroacetate triflate tetrafluoroborate monofluorophosphate methanesulfonate methanesulfonate 1,1,1,5,5,5-hexafluoroacetylacetone 1,1,1-trifluoroacetylacetone 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadione benzoyl-1,1,1-trifluoroacetone 2-thenoyltrifluoroacetone bis(trifluoromethanesulfonyl)imide bis(perfluoroethyl sulfonyl) imide hexafluoroisopropoxysulfuric

Surfactant

polyfluorinated-2-dodecenyl(3-sulfate)propyldimethylammonium CnTPB alkyltriphenylphonium bromide CnTABr alkyltrimethylammonium bromide CnTACl alkyltrimethylammonium chloride CnMPyrrB 1-alkyl-1methylpyrrolidinium bromide CnMImBr 1-alkyl-3-methylimidazolium bromide CnMImCl 1-alkyl-3-methylimidazolium chloride BPS-10 ethoxylated phytosterol surfactant, with 10 oxyethylene units PDSPDA polyfluorinated-2-dodecenyl (3-sulfate) propyl dimethylammonium (C 9 F 1 9 CFCHCH 2 N− (CH3)2(CH2)3OSO3)

PDSPDA

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