Phase Transitions, Decomposition Temperatures, Viscosities, and

Article pubs.acs.org/jced

Phase Transitions, Decomposition Temperatures, Viscosities, and Densities of Phosphonium, Ammonium, and Imidazolium Ionic Liquids with Aprotic Heterocyclic Anions Joseph J. Fillion, Han Xia, M. Aruni Desilva, Mauricio Quiroz-Guzman, and Joan F. Brennecke* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Ionic liquids (ILs) with aprotic heterocyclic anions (AHAs) have been developed primarily for CO2 capture applications. However, they have also been considered for cofluid CO2/IL vapor compression refrigeration cycles and for various electrochemical applications. In all of these cases, reducing the viscosity of the IL is of primary importance. Therefore, the focus of this work is tuning the cation to produce AHA ILs with both low viscosities and low melting points. Toward this goal we have synthesized 40 AHA ILs paired with phosphonium, ammonium and imidazolium cations, as well as a number of ILs with the bis(trifluoromethylsulfonyl)imide anion to use for comparison. The azolide anions investigated were 2-cyanopyrrolide, 4-nitropyrazolide, various substituted imidazolides, 1,2,3-triazolide, and tetrazolide. Melting points, glass transition temperatures, and decomposition temperatures were measured for all ILs. Viscosities and densities were measured from 278.15 to 343.15 K and 283.15 to 353.15 K, respectively, for all ILs except those with high melting points or excessively high viscosities. Shortening the alkyl chains on tetra-alkylphosphonium and tetra-alkylammonium cations reduces viscosity, but eventually results in unacceptably higher melting points. For equivalent alkyl chain lengths and anions, ammoniums have higher melting points and lower decomposition temperatures than phosphoniums. The introduction of an ether chain on a phosphonium cation lowers viscosity but reduces thermal stability. Di- and trialkylimidazolium with sufficiently low melting points have relatively high viscosities. [BF4]− yields a salt, [Na][BF4], with Tm = 657 K,8 a lowering of the melting point by more than 400 K. The size of the anion can be further increased to give [Na][B(OC(H)(CF3)2)4], which has a melting point of just 329 K.9 Replacing the cation of NaCl with 1-butyl-2,3-dimethylimidazolium ([bmmim]+), 1butyl-3-methylimidazolium ([bmim]+), tributyldodecylphosphonium ([P 44412 ] + ), or trihexyltetradecylphosphonium ([P66614]+) yields [bmmim][Cl], [bmim][Cl], [P44412][Cl], and [P66614][Cl], which have melting points of 376 K,10 314 K,11 295 K,12 and 223 K,13 respectively. In addition, ILs with more symmetric ions have higher melting points. For instance, [bmmim][Cl] has a higher melting point than [bmim][Cl]. This is even clearer when the total number of CH2 or CH3 groups in the ion is kept the same: the more symmetric [Cn−1mmim][Tf2N] has a smaller molar volume, higher melting point, and larger viscosity than [Cnmim][Tf2N].14 Series of tetra-alkylphosphonium ILs, such as [P222n][Tf2N], where n = 1, 4, 5, 8, and 12, show the same effect. The melting

1. INTRODUCTION Since the report of ethylammonium nitrate (Tm = 286−287 K) in 1914,1 which is now considered an ionic liquid (salts with melting points below 373.15 K),1 physical properties of these materials have been key. Emphasis switched to solution speciation for many of the chloroaluminate ILs (e.g., ethylpyridinium chloride/AlCl31 and 1,3-dialkylimidazolium chloride/AlCl3 mixtures2) but squarely returned to physical properties with the seminal publication by Graetzel’s group on low viscosity and low melting point ILs containing the bis(trifluoromethylsulfonyl)imide ([Tf2N]−) anion.3 In the ensuing decades, much has been learned about what controls melting points and viscosities of salts. The first is size, i.e., charge density. High melting salts, such as NaCl (Tm = 1074 K),4 are composed of small (spherical) cations and anions, which allow the formation of low energy lattices, even at relatively high temperatures, due to the strong Coulombic forces between the cations and anions. In order to reduce the melting point, the small cation or anion can be replaced with a larger cation or anion, reducing the charge density and increasing the distance between the ions. These salts make less energetically favorable lattices, which can only form at lower temperatures where there is less thermal energy to melt the lattice.5−7 For example, replacing [Cl]− in NaCl with © XXXX American Chemical Society

Special Issue: In Honor of Kenneth R. Hall Received: March 28, 2016 Accepted: June 21, 2016

A

DOI: 10.1021/acs.jced.6b00269 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Structure of the Family of Cations and Anions Investigated in This Studya

a Ri indicates a particular pendant group, which is usually an alkyl chain. A bracket indicates the specific ion used in this study, rather than a family of ions.

[PF3(CnF2n+1)3]−)26,27 anions are the next most popular (roughly 14% and 7%, respectively). While ILs in general have been explored for a wide variety of electrochemistry, spectroscopy, synthesis,28 aluminum battery,29 photovoltaic,3 electroplating, capacitor,17,30 separation,31 supercritical CO2 separation,32,33 organic and catalytic reaction,34 and many other1,35−38 applications, the AHA ILs have been developed to react with CO2 so that they can be used for CO2 capture, even when the CO2 is present at relatively low partial pressure. AHA ILs are particularly attractive since they do not exhibit large increases in viscosity upon reaction with CO2,39 which is a common problem. Most of the AHA ILs reported in our previous work on CO2 capture applications have been based on the bulky [P66614]+ cation.39−41 This is because phosphoniums provide good thermal stability and [P66614][Br], which is used as a precursor in the synthesis, is available commercially. However, the very long chain tetraalkylphosphonium ILs have relatively high viscosities, due to van der Waals interactions between the alkyl chains, and they have high heat capacities. For CO2 capture applications with AHA ILs, lowering viscosity in order to reduce mass transfer resistance is certainly one of our goals. We are also interested in the potential use of AHA ILs for a variety of electrochemical applications,42 which might include

point of these ILs are 370 K, 328 K, 290 K, 223 K* (*Tg), and 286 K, respectively. [P2221][Tf2N] has the most symmetric cation and highest melting point. Increasing the length of one of the alkyl chains, which increases asymmetry, lowers the melting point to the extent of eliminating it entirely for [P2228][Tf2N]. The melting point reappears for longer alkyl chains, such as in [P22212][Tf2N], likely due to ordering from van der Waals interactions between the very long alkyl chains. Note that [Tf2N]− is a very bulky anion that is known for melting point suppression.15 While the trends for melting points and viscosities of ILs described above are well-known, it is interesting that the diversity of IL cations and anions for which thermophysical properties have been investigated is really not very great and certainly does not adequately cover ILs with azolide anions. A database we compiled of viscosity measurements for 3250 ILs from 450 articles reveals that imidazolium,2,3,16 ammonium,17,18 phosphonium,19,20 pyridinium,21,22 and pyrrolidinium18,23 cations are the most popular, accounting for roughly 46%, 12%, 8%, 7%, and 7% of the data, respectively. [Tf2N]− is the most common anion, being present in 38% of the ILs.3,17 Boron-based (e.g., [BF 4 ] − , [BF 3 (C n F 2n+1 ) 3 ] − , and [B(CN)4]−)16,24,25 and phosphorus-containing (e.g., [PF6]− and B



Article

Table 2. Full Names and Abbreviations for All of Cations and Anions Investigated in This Study ionic liquid trihexyl(tetradecyl)phosphonium triazolide trihexyl(tetradecyl)phosphonium nitropyrazolide trihexyl(tetradecyl)phosphonium nitroimidazolide trihexyl(tetradecyl)phosphonium dichloroimidazolide trihexyl(tetradecyl)phosphonium dicyanoimidazolide trihexyl(tetradecyl)phosphonium nitroimidazolide trihexyl(tetradecyl)phosphonium

purity (%)

1,2,3-

[P66614][3-Triz]

99

4-

[P66614][4NO2pyra] [P66614][4NO2imid] [P66614][4,5Climid] [P66614][4,5CNimid] [P66614][2-CH3,5NO2imid] [P66614] [Tetrazolide] [P66614][Tf2N]

99

44,54,52-methyl-5tetrazolide

trihexyl(tetradecyl)phosphonium bis (trifluoromethylsulfonyl)imide tributyl(dodecyl)phosphonium 1,2,3-triazolide tributyl(octadecyl)phosphonium 1,2,3-triazolide tributyl((2-methoxyethoxy)methyl) phosphonium 1,2,3-triazolide tributyl(methoxymethyl)phosphonium 1,2,3triazolide butyltriethylphosphonium 2-methyl-5nitroimidazolide butyltriethylphosphonium 4-nitropyrazolide heptyltriethylphosphonium 4-nitropyrazolide octyltriethylphosphonium 4-nitropyrazolide nonyltriethylphosphonium 4-nitropyrazolide triethyl(decyl)phosphonium 4-nitropyrazolide octyltriethylphosphonium 4-nitroimidazolide octyltriethylphosphonium 2-methyl-5nitroimidazolide octyltriethylphosphonium 1,2,3-triazolide octyltriethylphosphonium bis (trifluoromethylsulfonyl)imide hexyltriethylammonium 1,2,3-triazolide heptyltriethylammonium 1,2,3-triazolide triethyl((2-methoxyethoxy)methyl) phosphonium 1,2,3 triazolide triethyl(methoxymethyl)phosphonium 1,2,3triazolide (ethoxymethyl)triethylphosphonium 1,2,3triazolide triethyl((2-methoxyethoxy)methyl) phosphonium 4-nitropyrazolide

ionic liquid triethyl((2-methoxyethoxy)methyl) phosphonium 2-methyl-5-nitroimidazolide (cyclopentyl)triethylphosphonium 4nitropyrazolide (butene)triethylphosphonium 4-nitropyrazolide

98 98

(butene)triethylphosphonium bis (trifluoromethylsulfonyl)imide S-hexyl-1,1,3,3-tetramethylthiouronium bis (trifluoromethylsulfonyl)imide S-octyl-1,1,3,3-tetramethylthiouronium bis (trifluoromethylsulfonyl)imide 1-butylthianium bis(trifluoromethylsulfonyl) imide 1-butyl-2-methylpyrazolium bis (trifluoromethylsulfonyl)imide butylthiolanium bis(trifluoromethylsulfonyl) imide 1-hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide 1-hexyl-3-methylimidazolium 4-nitropyrazolide

99 99 99 99

[P44412][3-Triz] [P44418][3-Triz] [P444(1O2O1)][3Triz] [P444(1O1)][3-Triz]

99 99 99

[P2224][2-CH3,5NO2imid] [P2224][4NO2pyra] [P2227][4NO2pyra] [P2228][4NO2pyra] [P2229][4NO2pyra] [P22210][4NO2pyra] [P2228][4NO2imid] [P2228][2-CH3,5NO2imid] [P2228][3-Triz] [P2228][Tf2N]

95

96

1-hexyl-2-methyl-3-methylimidazolium 4nitropyrazolide 1-hexyl-3-methylimidazolium 1,2,3-triazolide 1-hexyl-3-methylimidazolium 1,2,4-triazolide 1-hexyl-3-methylimidazolium tetrazolide

99 99 99

1-hexyl-3-methylimidazolium 2-(cyano) pyrrolide 1-propyl-2-methyl-3-methylimidazolium 4nitropyrazolide 1-cyclopentyl-2-methyl-3-methylimidazolium 4nitropyrazolide 1-butene-2-methyl-3-methylimidazolium 4nitropyrazolide 1-ethyl-2-(ethoxymethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-(methoxymethyl)-2,3-dimethyl-imidazolium 1,2,3-triazolide 1-((2-methoxyethoxy)methyl)-2,3-dimethylimidazolium 1,2,3-triazolide 1-butyl-2-((methoxymethoxy)methyl)-3-methyl imidazolium 1,2,3-triazolide 1-butyl-2-(((2-methoxyethoxy)methoxy) methyl)-3-methylimidazolium 1,2,3-triazolide trihexyl(tetradecyl)phosphonium acetate 1-hexyl-3-methylimidazolium acetate triethyl((2-methoxyethoxy)methyl) phosphonium bis(trifluoromethylsulfonyl) imide

99 99 95 99 98 98

[N2226][3-Triz] [N2227][3-Triz] [P222(1O2O1)][3Triz] [P222(1O1)][3-Triz]

97 99 99

[P222(1O2)][3-Triz]

99

[P222(1O2O1)][4NO2pyra]

99

98

[P222(1O2O1)][2CH3,5NO2imid] [P222(cy-C5)][4NO2pyra] [P222(butene)][4NO2pyra] [P222(butene)] [Tf2N] [C6thiour][Tf2N]

purity (%) 96 97 98 99 97

[C8thiour][Tf2N]

97

[bthain][Tf2N]

99

[bmpyra][Tf2N]

99

[bthiol][Tf2N]

99

[hmim][Tf2N]

99

[hmim][4NO2pyra] [hmmim][4NO2pyra] [hmim][3-Triz] [hmim][4-Triz] [hmim] [Tetrazolide] [hmim][2CNpyr] [pmmim][4NO2pyra] [mm(cy-C5)im] [4-NO2pyra] [mm(butene)im] [4-NO2pyra] [e(1O2)mim] [Tf2N] [(1O1)mmim][3Triz] [(1O2O1)mmim] [3-Triz] [b(1O1O1)mim] [3-Triz] [b(1O1O2O1) mim][3-Triz] [P66614][Acetate] [hmim][Acetate] [P222(1O2O1)] [Tf2N]

99 97 99 99 96 96 97 97 98 95 99 97 96 96 99 98 98

have been constructed and tested with CO2 and organic liquids that physically absorb CO2, the coefficients of performance were not attractive.44 Better performance should be achievable with AHA ILs that weakly react with CO2.45 The critical thermophysical properties of AHA ILs for this application are low viscosity, even at temperatures as low as 278.15 K, Tm or Tg below 273.15 K, and low heat capacity (i.e., reduced molecular weight). In an effort to improve AHA ILs for CO2 capture, electrochemical, and cofluid vapor-compression refrigeration applications, we have synthesized 40 AHA ILs paired with phosphonium, ammonium, and imidazolium cations. The azolide anions investigated were 2-cyanopyrrolide, 4-nitro-

batteries and supercapacitors. In these cases, high conductivity is of primary importance, and the easiest way to increase the conductivity of ILs is to lower viscosity. Another application of interest for AHA ILs is in cofluid vapor-compression refrigeration, which would use a mixture of IL with CO2. Nontoxic, nonflammable CO2 is the refrigerant, but the enthalpy of evaporation and condensation is essentially replaced by the enthalpy of absorption and desorption or reaction with the liquid. The main advantage over a pure CO2 refrigeration cycle is that the required operating pressure is reduced from >10 MPa to around 3 MPa, which is similar to conventional refrigeration systems.43 While cofluid vaporcompression refrigeration systems using wet compression C



Article

Table 3. Materials Used for Synthesis of the Ionic Liquidsa material

source

Cation Starting Material [P66614][Br] Santa Cruz Biotechnology Cation Base tributylphosphine Sigma-Aldrich triethylphosphine Sigma-Aldrich triethylamine Sigma-Aldrich tetramethylthiourea Sigma-Aldrich thiane (tetrahydrothiopyran) Alfa Aesar 1-methylpyrazole Sigma-Aldrich tetrahydrothiophene Sigma-Aldrich methylimidazole Sigma-Aldrich dimethylimidazole Sigma-Aldrich Cation Side Group 1-bromopropane Sigma-Aldrich 1-bromobutane Sigma-Aldrich 1-bromohexane Acros Organics 1-bromoheptane Sigma-Aldrich 1-bromooctane Sigma-Aldrich 1-bromononane Sigma-Aldrich 1-bromodecane Sigma-Aldrich 1-bromododecane Sigma-Aldrich 1-iodooctadecane Sigma-Aldrich bromocyclopentane Sigma-Aldrich 1-bromobutene Sigma-Aldrich bromomethyl methyl ether Sigma-Aldrich chloromethyl methyl ether Sigma-Aldrich chloromethyl ethyl ether Sigma-Aldrich 2-methoxyethoxymethyl chloride Sigma-Aldrich Anion Starting Material 1,2,3-triazole Sigma-Aldrich 1,2,4-triazole Acros Organics 4-nitropyrazole Sigma-Aldrich 4-nitroimidazole Sigma-Aldrich 4-chloro,5-chloroimidazole Sigma-Aldrich 4-cyano-5-cyanoimidazole Sigma-Aldrich 2-methyl-5-nitroimidazole Sigma-Aldrich pyrrole-2-carbonitrile Alfa Aesar tetrazole (0.45 M in acetonitrile) Alfa Aesar lithium bis(trifluoromethylsulfonyl)imide 3M acetic acid Sigma-Aldrich Solvents acetonitrile Sigma-Aldrich CH2Cl2 BDH water MilliPore toluene Sigma-Aldrich hexane BDH formaldehyde solution (37% wt. in H2O, 10−15% methanol) Sigma-Aldrich methanol BDH tetrahydrofuran (THF) EMD ethyl acetate BDH dimethylformamide (DMF) Sigma-Aldrich sodium bicarbonate Fisher Scientific magnesium sulfate Sigma-Aldrich sodium hydride Sigma-Aldrich sodium chloride (to make brine) Fisher Scientific a

purity 97% 97% 99% 99% 98% 98% 99.0% 99% 99% 98% 99% 99% 99% 99% 99% 98% 98% 97% 95% 98% 97% technical grade, 90% technical grade 95% technical grade 97% 99.5% 97% 97% 99% 99% 99% 99% 99.6% trace ions 99.7% 99.9% 99.5% 18.2 MΩ·cm 99.8% 98.5% 99.9% 99.8% 99.9% 99.5% 99.9% 99.9% 99.5% 95% 99.7%

All materials were used without further purification.

enthalpies of reaction with CO2 that are appropriate for the cofluid vapor-compression refrigeration application. We present melting temperatures, glass transition temperatures, and

pyrazolide, various substituted imidazolides, 1,2,3-triazolide, and tetrazolide. ILs with the 4-nitropyrazolide and 1,2,3triazolide were a particular focus because these anions have D



Article

Figure 1. General synthesis of C-2 substituted imidazolium bromides.

water. This is possible because the [Tf2N]− based ILs are not completely miscible with water. As mention above, the IL is dried on a vacuum line and the purity determined with NMR spectroscopy. Synthesis of [b(1O1O1)mim][Br] and [b(1O1O2O1)mim][Br] requires three steps instead of one, as can be seen in Figure 1. The first step involves making 2-(hydroxymethyl)-1methylimidazole from 1-methylimidazole in (HCHO)n/acetonitrile at 398 K. The second step involves adding charged sodium hydride, that was sealed and purged with argon, to dry degassed dimethylformamide (DMF) and 2-(hydroxymethyl)1-methylimidazole at 273 K. 2-(Hydroxymethyl)-1-methylimidazole was added in four portions over 1 h, and the reaction was stirred for an additional hour until no more evolution of gas was observed. Chloromethyl methyl ether or 2-methoxyethoxymethyl chloride was added, which converts the C-2 position to (1O1O1) or (1O1O2O1), respectively (see definitions in Table 2). These reagents were added dropwise over 15 min, the reaction was warmed slowly to room temperature and then stirred for over 2 h. The reaction was cooled to 273 K and NaHCO3 was added slowly to neutralize excess (1O1)Cl or (1O2O1)Cl and NaH. Then at 298 K, the mixture was stirred and filtered through a plug of Celite. The aqueous layer was extracted with CH2Cl2 (3 × 40 mL); the organic fractions were combined and washed with brine (NaCl, 3 × 100 mL), dried with MgSO4, and concentrated under reduced pressure. The third step in the procedure is the same as the first step for the synthesis of normal AHA ILs, which involves making the cation paired with a halide anion. 2.2. Melting Temperature, Glass Transition Temperature, and Decomposition Temperature. The melting point (Tm) and glass transition temperature (Tg) were measured with a model DSC 822 Mettler-Toledo differential scanning calorimeter (DSC), which is equipped with the Mettler-Toledo STARe software version 9.10. The samples are run in aluminum crucibles (Mettler Toledo Al-Crucibles standard 40 μL) with nitrogen (Airgas, high purity, grade 4.8) as the purging gas at 50 mL/min. 10−20 mg of sample was used. For samples that are liquid at room temperature, thermograms were recorded from 153 to 323 K after cooling from 298 to 153 K. For samples that were solid at room temperature, thermograms were recorded from 153 to 383 K, after cooling from 383 to 153 K. Both heating and cooling rates were 10 K/min. The melting point (Tm) was determined to be the onset temperature of an endothermic peak upon heating, and the glass transition temperature (Tg) was determined as the temperature at the midpoint of a small heat capacity change. The uncertainties of Tm and Tg are ±2 K. The decomposition temperatures were measured with a Mettler-Toledo TGA/SDTA 851e/SF/1100 °C thermal gravimetric analyzer, using the same software as above. All samples were run in aluminum crucibles (Mettler Toledo AlCrucibles standard 40 μL) with nitrogen (same as above) as the purging gas at 50 mL/min. 15−25 mg of sample was used. In a typical experiment, the sample was dried in situ at 383 K for 45 min and then heated to 773 at 10 K/min. Relative thermal stabilities were described by the onset temperatures (Tonset),

decomposition temperatures of these AHA ILs, as well as a number of ILs with the bis(trifluoromethylsulfonyl)imide anion that are included for comparison. This information gives the lower and upper operating temperature limits of each IL. The viscosity and density are measured for all ILs with sufficiently low melting points and viscosities.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. The ILs synthesized for this study contain the phosphonium, ammonium and imidazolium cations shown and listed in Tables 1 and 2, paired with pyrrolide, pyrazolide, imidazolide, triazolide and tetrazolide anions. Several [Tf2N]− ILs were also synthesized for comparison purposes. The general structures of the cations and anions are shown in Table 1. The full names of the ILs, abbreviations used and purities are shown in Table 2. 1H NMR spectra for the ILs are giving in Supporting Information. The starting materials with their purities and sources are shown in Table 3. All chemicals were used without further purification. The synthesis of AHA ILs generally involves a three-step procedure. The first step involves mixing the cation base, such as triethylphosphine, with an alkylating agent, such as 1bromooctane, containing the desired cation side group. The reaction normally takes place in a solvent, typically acetonitrile or toluene, between 298 and 333 K, under N2. When making [(1O2O1)mmim][Cl], the solvent was tetrahydrofuran (THF), and the reaction was done at 273 K. When making [P222(1O1)][Cl] and [P222(1O2)][Cl], the reaction was done at 273 K, or the temperature was increased slowly up to 323 K if the reaction did not take place at the lower temperature. In all cases, the product from the first step was washed three times with hexane or ethyl acetate to remove residual solvent and alkyl halide. The product from this first step is the desired cation paired with a halide anion. [P66614][Br] was purchased instead of being synthesized so the first step in the procedure was skipped for ILs containing the [P66614] cation. The second step involves replacing the halide anion with hydroxide, which is done by contacting the halide salt with SBR LG NC(OH) ion-exchange resin or Amberlite IRN78 in methanol at room temperature. The cation paired with hydroxide is removed from the ion-exchange resin or Amberlite by filtration. The third step involves adding an equimolar amount of the anion precursor to [cation][OH] in methanol at room temperature. Once the mixture fully reacts, the methanol and water byproduct are evaporated at room temperature. The ionic liquid is put on the vacuum line (∼10 mmHg at 323 K) for two or more days to reduce the water content below 0.001 weight fraction. The water content was measured using a Brinkman 831 Karl Fischer coulometer. The purity of each of the ILs, based on the NMR spectroscopy, is shown in Table 2. The synthesis of [Tf2N]− based ILs involves a two-step procedure. The first step is the same as the first step in the synthesis of AHA ILs, which involves synthesis of the appropriate halide (bromide in the particular case of [Tf2N]− based ILs). The second step involves reacting the [cation][Br] with LiTf2N in CH2Cl2/water and washing the LiBr out with E



Article

Table 4. Melting Temperature, Tm, Glass Transition Temperature, Tg, and Decomposition Temperature, Tonset, of Various ILs

which is defined as the measured sample temperature at the intersection of the baseline before decomposition and the step tangent of weight vs time curve as decomposition occurs. The largest uncertainty is from manually determining the tangent point for the onset temperature, which results in an uncertainty in the onset temperature of ±2 K. 2.3. Viscosity Measurements. The viscosity of each IL, which was liquid at room temperature and did not have excessive high viscosity, was measured with a cone and plate ATS Viscoanalyzer under a flow of dry nitrogen to prevent absorption of water from the atmosphere. Nonetheless, the AHA ILs are relatively hydrophilic so the water content did change during the course of the experiments. Water contents of the samples both before and after the viscosity measurements are reported. The instrument requires 0.3−0.4 mL of sample, and the estimated uncertainty is ±6%. Based on testing with standards, the instrument underestimates the viscosity by about 3% when the viscosity is above 150 mPa·s. The best measurements are in the range between 100 mPa·s and 150 mPa·s. Viscosities between 50 mPa·s and 100 mPa·s are overestimated. The uncertainty increases greatly below 50 mPa· s, so no measurements were made below this value. The viscosity was normally measured between 278.15 and 343.15 K, in 5−10 K increments. 2.4. Density Measurements. The density of each IL was measured with an oscillating U-tube Anton Paar 4500 densitometer. The instrument requires approximately 1.7 mL of sample and reportedly has ±0.00005 g cm−3 uncertainty. However, when taking the sample impurities into account, we estimate the uncertainty in the density measurements to be ±0.002 g cm−3 for ionic liquids that are 99% pure. We estimate the uncertainty in the density measurements of ionic liquids that are 98%, 97%, 96%, and 95% pure to be ±0.004 g cm−3, ±0.006 g cm−3, ±0.008 g cm−3, and ±0.010 g cm−3, respectively. The density was normally measured from 283.15 to 353.15 K, in 5−10 K increments.

3. RESULTS AND DISCUSSION 3.1. Melting Temperature, Glass Transition Temperature, and Decomposition Temperature. The melting point, glass transition temperature, and decomposition temperature were measured in order to determine the operational range of each IL. The melting point, or glass transition temperature if no melting point is present, determines the lowest temperature the IL can be used as a liquid, whereas the decomposition temperature determines the highest possible temperature. However, it is well-known that decomposition temperatures, reported as the onset temperature (Tonset) from TGA, significantly overestimate, perhaps by 100 K or more, the actual temperature at which one could operate an IL for long periods of time without serious degradation. The ILs are organized in Table 4 to facilitate easy comparison of the Tm, Tg, and Tonset values. The ILs with [P66614]+ cations only have glass transition temperatures, all of which are very similar, ranging between 209 and 194 K. The lack of a melting point is typical for ILs with long alkyl chains that frustrate packing into a crystal lattice. The Tonset values for the [P66614]+ ILs are very different, ranging between 563 and 683 K. The Tonset and Tg reported here for [P66614][Tf2N] are 663 and 191 K, which differ slightly from the literature values of 673 and 197 K, respectively.20 This may simply be due to differences in the experimental methods. Thermal stabilities of ILs correlate well with nucleophilicity of anions;46 [Tf2N]− is

ionic liquid

Tma/K

Tga/K

Tonseta/K

[P66614][3-Triz] [P66614][4-NO2pyra] [P66614][4-NO2imid] [P66614][4,5-Climid] [P66614][4,5-CNimid] [P66614][2-CH3,5-NO2imid] [P66614][Tetrazolide] [P66614][Tf2N] [P44412][3-Triz] [P44418][3-Triz] [P444(1O2O1)][3-Triz] [P444(1O1)][3-Triz] [P2224][2-CH3,5-NO2imid] [P2224][4-NO2pyra] [P2227][4-NO2pyra] [P2228][4-NO2pyra] [P2229][4-NO2pyra] [P22210][4-NO2pyra] [P2228][4-NO2imid] [P2228][2-CH3,5-NO2imid] [P2228][3-Triz] [N2226][3-Triz] [N2227][3-Triz] [P222(1O2O1)][3-Triz] [P222(1O1)][3-Triz] [P222(1O2)][3-Triz] [P222(1O2O1)][4-NO2pyra] [P222(1O2O1)][2-CH3,5-NO2imid] [P222(cy-C5)][4-NO2pyra] [P222(butene)][4-NO2pyra] [P222(butene)][Tf2N] [C6thiour][Tf2N] [C8thiour][Tf2N] [bthain][Tf2N]47 [bthain][Tf2N] [bmpyra][Tf2N] [bthiol][Tf2N] [hmim][4-NO2pyra] [hmmim][4-NO2pyra] [hmim][3-Triz] [hmim][4-Triz] [hmim][Tetrazolide] [hmim][2-CNpyr] [pmmim][4-NO2pyra] [mm(cy-C5)im][4-NO2pyra] [mm(butene)im][4-NO2pyra] [e(1O2)mim][Tf2N] [(1O1)mmim][3-Triz] [(1O2O1)mmim][3-Triz] [b(1O1O1)mim][3-Triz] [b(1O1O2O1)mim][3-Triz]

none none none none none none none none none 297 none 280 290 317 289 278 289 284 245 none 312/339 333 342/356 302 302/350 292/306 258 none 324 312 322 none none 263 263 none none 306 none none 307 none none none 365 none none >353 331c none none

203 198 207 209 195 194 203 191 206 none 198 205 202 none 201 201 201 202 none 206 none none none 190 none 207 197 198 none none none 191 191

601 580 563 628 683 570 602 663 600 595 516 496 540 583 573 580 472 570 559 540b 596 439 440 522 499 516 535 527 572 533 703 602 623

191 191 190 217 226 210 217 193 208 223 none 220 195 none 221 236 238

566 621 569 487 502 475 490 506 479 503 502 492 689 NA NA 483 448

a

Standard uncertainties u are u(Tm) = 2 K, u(Tg) = 2 K, and u(Tonset) = 2 K. bBlack porous residue. cOverlap with crystallization.

known for its low nucleophilicity and, therefore, exhibits high thermal stability, with a Tonset of 663 K. In general, the AHA anions are stronger bases than [Tf2N]− so they are more nucleophilic and have lower Tonset values than the correspondF



Article

ing [Tf2N]− ILs. Adding nonenergetic electron-withdrawing substituents, such as −CN or −Cl, makes the substituted anions less nucleophilic, and therefore they have better thermal stability than other AHA ILs. Shortening of the alkyl chains on the tetra-alkylphosphonium cation introduces the possibility of crystallization. [P2228][4NO2pyra], [P2228][4-NO2imid], and [P2228][3-Triz] all have melting points. The planar, symmetric [3-Triz]− anion allows easy stacking and results in solid transitions at 312 and 339 K. Note that the higher temperature, 339 K, is assigned as the melting point, Tm; the lower temperature, 312 K, is likely a solid−solid transition. The asymmetry introduced by the nitro groups in [P2228][4-NO2pyra] and [P2228][4-NO2imid] lowers the Tm values to 278 and 245 K, respectively. The asymmetry resulting from adding two functional groups to the imidazolide anion ([P2228][2-CH3,5-NO2imid]) completely suppresses the melting point. Of the 16 ILs with the [3-Triz]− anion, ten of them have melting points. The cations of the six [3-Triz]− ILs that do not have a melting point are [P66614]+, [P44412]+, [P444(1O2O1)]+, [hmim]+, [b(1O2O1)mim]+, and [b(1O1O2O1)mim]+. The ten [3-Triz]− ILs with melting points have the following cations: [P44418]+, [P444(1O1)]+, [P2228]+, [N2226]+, [N2227]+, [P222(1O2O1)]+, [P222(1O1)]+, [P222(1O2)]+, [(1O1)mmim]+, and [(1O2O1)mmim]+. We attribute the presence of a melting point primarily to symmetry of the cation, which allows easier stacking and subsequent crystallization. The one notable exception is [P44418][3-Triz]. In this case the crystal structure must be dominated by alignment of the exceptionally long octadecyl chains through van der Waals forces. Simply shortening that one alkyl chain to a dodecyl ([P44412][3Triz]) completely eliminates the melting point. One can reduce the melting point by using ether groups. [P2228][3-Triz] has a melting point of 339 K, whereas [P222(1O2O1)][3-Triz] has a melting point of only 302 K. A disadvantage of the ether chains is that they can lower the decomposition temperature. [P 66614 ][3-Triz], [P 44412 ][3-Triz], [P 44418 ][3-Triz], and [P2228][3-Triz] all have roughly the same Tonset values (595 to 601 K), whereas replacing an alkyl with an ether chain causes Tonset to decrease by 75−100 K, to be between 499 and 522 K. Replacing a phosphonium cation with a similar ammonium cation can also reduce Tonset. For example, the Tonset values of [P2228][3-Triz] and [N2227][3-Triz] are 596 and 440 K, respectively. The glass transition temperature of phosphonium ILs with the [3-Triz]− anion are around 200 K. Imidazolium ILs with the [3-Triz]− anion have Tg values that are a bit higher, typically between 210 and 240 K. Of the 14 ILs synthesized with the [4-NO2pyra]− anion, only [P66614][4-NO2pyra], [pmmim][4-NO2pyra], [hmmim][4NO2pyra], and [mm(butene)im][4-NO2pyra] did not have melting points. Interesting enough, [hmim][4-NO2pyra] has a melting point of 306 K, whereas [hmmim][4-NO2pyra] only has a glass transition temperature. The trisubstituted imidazoliums are of interest because eliminating the acidic proton on the C2 carbon blocks the formation of carbene, which occurs with the AHA ILs. The carbene can react with CO2, which complicates the chemistry if dialkylimidazolium AHAs were to be used for CO2 capture applications. The absence of a melting point for the trisubstituted imidazolium AHAs is promising. However, as will be shown below, their viscosities are prohibitively high. The introduction of a cyclopentyl substituent resulted in melting points above room temperature: 324 K for [P222(cy-C5)][4-NO2pyra] and 365 K

for [mm(cy-C5)im][4-NO2pyra]. Replacing a butyl chain with a butylene chain does lower the melting point slightly, which is seen for both [4-NO2pyra]− and [Tf2N]− ILs: the Tm values for [P2224][4-NO2pyra], [P222(butene)][4-NO2pyra], [P 2224][Tf2N], and [P222(butene)][Tf2N] are 317, 312, 328, and 322 K, respectively. A series of [P222n][4-NO2pyra] ILs, where n = 4, 7, 8, 9, and 10, was investigated to determine how alkyl chain length affects the melting point, as shown in Figure 2.

Figure 2. Melting point of [P222n][4-NO2pyra] as a function of n, where n is 4, 7, 8, 9, and 10. Solid line is just to guide the eye.

[P2224][4-NO2pyra] has the highest melting point. Increasing the alkyl chain length to [P2228][4-NO2pyra] causes the melting point to decrease to 278 K. Increasing the alkyl chain length further has little effect. However, we anticipate an increase in the melting point for very long chains, as discussed above for [P44418][3-Triz]. Asymmetry lowers the melting point but van der Waals forces between very long alkyl chains causes the melting point to increase. When comparing ILs with the same cation but different AHA anions, the first observation is that [4-NO2pyra]− ILs always have lower melting points than the equivalent [3-Triz]− ILs. [P2228][4-NO2pyra] has a melting point of 278 K, but [P2228][3Triz] is 339 K. Likewise, Tm of [P222(1O2O1)][4-NO2pyra] is just 258 K, while that of [P222(1O2O1)][3-Triz] is 302 K. We attribute this to the greater symmetry of the [3-Triz]− anion. Based on this feature, [4-NO2pyra] − ILs are better choices than [3Triz]− ILs for low temperature applications, such as cofluid vapor-compression refrigeration [P2224][4-NO2pyra] even has a lower melting point than [P2224][Tf2N] (317 and 328 K,15 respectively). A disubstituted imidazolide, [P2224][2-CH3,5NO2imid], has an even lower melting point of 290 K. Nonetheless, [Tf2 N] − ILs remain attractive for many applications due to their low viscosities (see below) and the absence of melting points entirely when the cation size is larger. Note that the decomposition of some ILs can be highly exothermic. This was the case for [P2228][2-CH3,5-NO2imid], which left a highly porous carbonaceous residue in the TGA pan. 3.2. Viscosity Measurements. The viscosity measurements for all of the ILs are shown in Table 5. Viscosities decrease with increasing temperature, in accordance with the Vogel−Fulcher−Tammann (VFT) equation. The solid lines in the figures are the VFT fits, and the VFT fitting parameters for all ILs with 30 K or higher temperature range are shown in Table 6. Some of the viscosities are reported at temperatures below the melting points of the ILs. This is because many of the ILs remain as subcooled liquids, especially when subjected to sheer in the rheometer. For a particular anion, the simplest way to reduce viscosity for the phosphonium AHAs is to shorten the alkyl chain length. This is shown for [4-NO2pyra]−, [3-Triz]−, and [2-CH3,5G


[P66614][3-Triz]41 [P66614][3-Triz] [P66614][4-NO2pyra] [P66614][4-NO2imid] [P66614][4,5-Climid] [P66614][4,5-CNimid] [P66614][2-CH3,5-NO2 imid] [P66614][Tetrazolide] [P66614][Tf2N], ref 50 ref 51 ref 5 ref 20 [P66614][Tf2N] [P44412][3-Triz] [P44418][3-Triz] [P444(1O1)][3-Triz] [P2224][2-CH3 5-NO2imid] [P2224][4-NO2pyra] [P2227][4-NO2pyra] [P2228][4-NO2pyra] [P2229][4-NO2pyra] [P22210][4-NO2pyra] [P2228][4-NO2imid] [P2228][2-CH3,5-NO2imid] [P2228][Tf2N], ref 15 ref 52 [P2228][Tf2N] [P222(1O2O1)][3-Triz] [P222(1O2)][3-Triz] [P222(1O2O1)][4-NO2pyra] [P222(1O2O1)][2-CH3,5NO2imid] [P222(butene)][4-NO2pyra] [C6thiour][Tf2N]49 [C6thiour][Tf2N] [C8thiour][Tf2N]49 [C8thiour][Tf2N] [bthain][Tf2N]47 [bthain][Tf2N] [bmpyra][Tf2N]53 [bmpyra][Tf2N]

ionic liquid

T/K =

H

268 356 398 214

0.0011/0.0146 0.0164/0.0252 0.0019/0.0098

155

279

254

193

216 381

309 567

0.0019/0.0119

0.0500/0.1730

268 186

379 260

801 1350 551 657 716 888 1030 804 2280

1310 2180 840 1000 1100 1370 1620 1220 3720

0.0015/0.0137 0.0437/0.4520 0.0824/0.7370 0.0183/0.1386 0.0214/0.1749

841 1660

1210 2660

0.0061/0.0169 0.0220/0.0420 0.0112/NA 0.0220/0.1600 0.0292/0.1871 0.0490/0.2650 0.0516/0.1696 0.0380/0.1525 0.0047/0.0685 0.0683/0.1500 0.0104/0.1600 0.0084/0.1628

283.15 1221 1300 2290 2220 2580 1630 3730 1810 871

1970 3560 3380 4040 2490 5890 2740 1250

278.15 0.0294/0.1516 0.0114/0.0276 0.0173/0.1055 0.0094/0.1109 0.0151/0.0764 0.0193/0.1286 0.0396/0.1332

water content before/after (weight fraction × 102)

116

200

186

143

156 264

194 137

509 858 372 443 480 593 678 547 1440

594 1060

878 1500 1500 1680 1090 2420 1220 622

288.15

88

148

140

109

116 189

145 104

341 569 261 309 333 409 461 382 940

450 430 706

293.15 595 611 1020 1040 1140 754 1620 844 454

134 19 85 23 108 68 112 67.1 70

88 140

237 391 189 223 238 291 325 276 637 113 129 111 80

318 484

298.15 433 438 710 735 789 535 1110 600 337 335.9

55

88

85

102 16 68

87 64 62 69 106

170 277 141 165 175 213 236 204 446

241 343

243

303.15 321 321 510 533 566 391 786 437 254

70

79 19 55 17 69

73 51 50 55 83

202 108 126 133 160 175 155 321

185 249

242 375 396 415 291 569 326 195

308.15

57

56

63 11

65

57

95 150 85 97 103 123 133 120 237

145 185

313.15 186 185 280 298 308 220 420 247 152

μ (mPa·s)

Table 5. Experimental Values of Viscosity, μ, of Various ILs from 278.15 to 343.15 K at Pressure p = 0.1 MPaa

12

52

53

115 67 77 81 96 104 95 179

115 141

145 214 230 234 171 317 191 120

318.15

92 110 111 59 90 55 62 65 77 82 76 138

323.15 115 116 167 180 182 135 244 151 96

53 63 66 62 109

72

76 88 90

133 144 144 108 192 121 78

328.15

52 54 52 80

58

63 71 73

108 117 116 88 153 99 63

333.15

59 61

73 125

88 96

338.15

51

74 80

343.15

Journal of Chemical & Engineering Data Article


a

T/K = 278.15 3310 14600 2610 3040 2180 1540 9610 6650 207 203 284 1820 12300 84

0.0470/0.2253 0.0160/0.2065 0.0600/0.3400 0.0460/0.2570 0.0730/0.2345 0.0299/0.1823 0.0390/0.2128 0.1073/0.1717 0.0049/0.0394 0.0031/0.0134 0.0047/0.0262 0.0705/0.3100 0.0440/0.4230 0.0030/0.0345

water content before/after (weight fraction × 102)

149 148 150 206 1200 6460 65

283.15 1810 6910 1560 1670 1340 902 4500 3260

113 153 814 3550 51

111

288.15 1040 3490 965 961 848 553 2270 1720 85 86 87 117 566 2060

293.15 633 1910 624 589 561 355 1240 973

298.15 407 1110 419 381 384 239 722 590 53.3 66 68 68 91 406 1270 53 55 55 73 298 816

303.15 275 691 294 258 273 167 447 380

59 224 537 172 372

313.15 139 303 156 132 150 91 200 179

μ (mPa·s) 308.15 192 450 211 182 200 122 294 255

Standard uncertainties u are u(T) = 0.1 K and u(p) = 0.005 MPa, and the relative standard uncertainty ur is ur(μ) = 0.06.

[hmim][4-NO2pyra] [hmmim][4-NO2pyra] [hmim][3-Triz] [hmim][4-Triz] [hmim][Tetrazolide] [hmim][2-CNpyr] [pmmim][4-NO2pyra] [mm(butene)im][4-NO2pyra] [e(1O2)mim][Tf2N]48 [e(1O2)mim][Tf2N] [hmim][Tf2N]22 [hmim][Tf2N] [bthiol[Tf2N] [P66614][Acetate] [hmim][Acetate] [P222(1O2O1)][Tf2N]

ionic liquid

Table 5. continued

135 265

318.15 102 213 118 98 115 70 143 131

107 194

323.15 79 147 92 76 91 55 106 99

113

65 62

81 77

147

61

333.15 51 87 60

328.15 63 112 74 61 74

90

53 50

51

69

338.15

56

343.15

Journal of Chemical & Engineering Data Article

I



Article

Table 6. VFT Parameters for Viscosity of the Various ILs ionic liquid [P66614][3-Triz] [P66614][4-NO2pyra] [P66614][4-NO2imid] [P66614][4,5-Climid] [P66614][4,5-CNimid] [P66614][2-CH3,5NO2imid] [P66614][Tetrazolide] [P66614][Tf2N] [P44412][3-Triz] [P444(1O1)][3-Triz] [P2224][2-CH3,5NO2imid] [P2224][4-NO2pyra] [P2227][4-NO2pyra] [P2228][4-NO2pyra] [P2229][4-NO2pyra] [P22210][4-NO2pyra] [P2228][4-NO2imid] [P2228][2-CH3,5NO2imid] [P2228][Tf2N] [P222(10201)][3-Triz] [P222(10201)][4-NO2pyra] [P222(10201)][2-CH3,5NO2imid] [C6thiour][Tf2N] [C8thiour][Tf2N] [bthain][Tf2N] [bmpyra][Tf2N] [hmim][4-NO2pyra] [hmmim][4-NO2pyra] [hmim][3-Triz] [hmim][4-Triz] [hmim][Tetrazolide] [hmim][2-CNpyr] [pmmim][4-NO2pyra] [mm(butene)im][4NO2pyra] [bthiol][Tf2N] [P66614][Acetate] [hmim][Acetate]

temperature range (K)

μo/mPa·s

b/K

To/K

278.15−323.15 278.15−333.15 278.15−338.15 278.15−333.15 278.15−338.15 278.15−333.15

0.0614 0.0412 0.0354 0.0388 0.0555 0.0241

1227 1383 1499 1411 1283 1607

159.9 156.5 147.4 156.0 158.4 148.6

278.15−328.15 278.15−333.15 278.15−333.15 278.15−323.15 278.15−333.15

0.0340 0.0417 0.0364 0.122 0.0528

1460 1382 1257 823 1105

148.9 143.7 165.9 189.5 174.2

278.15−323.15 278.15−323.15 278.15−328.15 278.15−333.15 278.15−333.15 278.15−333.15 278.15−333.15

0.158 0.106 0.110 0.0768 0.0760 0.0947 0.0371

819 934 927 1049 1043 1021 1277

182.7 176.1 177.5 171.0 173.5 170.2 167.3

278.15−313.15 278.15−308.15 278.15−308.15 278.15−318.15

0.367 0.273 0.266 0.164

651 661 655 788

184.4 181.8 185.4 181.4

278.15−308.15 278.15−313.15 278.15−313.15 278.15−303.15 278.15−323.15 278.15−323.15 278.15−328.15 278.15−323.15 278.15−328.15 278.15−323.15 278.15−313.15 278.15−318.15

0.351 0.280 0.231 0.267 0.0799 0.0344 0.0587 0.0905 0.0981 0.0784 0.0393 0.0837

635 715 733 659 870 1049 1046 841 961 856 945 831

182.5 178.1 179.8 179.6 196.3 197.2 180.4 197.5 182.1 191.6 202.0 204.5

278.15−308.15 278.15−323.15 278.15−338.15

0.273 0.0566 0.0201

712 1232 1300

175.7 159.5 180.6

Figure 3. Viscosities of [P66614][4-NO2pyra] blue ●, [P22210][4NO2pyra] □, [P2229][4-NO2pyra] green ◆, [P2228][4-NO2pyra] light blue ×, [P2227][4-NO2pyra] dark red −, [P2224][4-NO2pyra] yellow ▲, [P222(1O2O1)][4-NO2pyra] red ■.

Figure 4. Viscosities of [P44418][3-Triz] purple ∗, [P44412][3-Triz] □, [P66614][3-Triz] green ◆, [P444(1O1)][3-Triz] blue ●, [P222(1O2O1)][3Triz] yellow▲, and [P222(1O2)][3-Triz] dark red −.

NO2imid]− ILs in Figures 3, 4, and 5, respectively. For the [P222n][4-NO2pyra] ILs the viscosity increases in the order of [P2224]+ < [P2227]+ < [P2228]+ < [P2229]+ < [P22210]+ < [P66614]+. The viscosity of [P2224][4-NO2pyra] is almost four times lower than [P66614][4-NO2pyra]. Increasing viscosity with increasing alkyl chain length is clearly attributable to an increase in van der Waals interactions. A perhaps unexpected result is shown in Figure 4, where both [P44412][3-Triz] and [P44418][3-Triz] are slightly more viscous than [P66614][3-Triz]. However, it has been reported previously that [P44414][Tf2N] is slightly more viscous than [P66614][Tf2N].20 Thus, it may not be the total alkyl chain length, but the relative lengths, that matter. One possible explanation is that the hexyl side chain of [P66614] interact more effectively with the tetradecyl side chain than the butyl side chain of [P44414]+. The C6−C14 interactions reduce the C14−C14 interactions (compared to [P44414]+), reducing in the total van der Waals interaction, which in turn would reduce the viscosity.

Figure 5. Viscosities with VFT fitting of [P66614][2-CH3,5-NO2imid] green ◆, [P2228][2-CH3,5-NO2imid] blue ●, [P2224][2-CH3,5NO2imid] yellow ▲, and [P222(1O2O1)][2-CH3,5-NO2imid] red ■.

Another way to reduce viscosity is to replace alkyl chains with ethers. This reduces the viscosity (and the melting point) due to increased flexibility from the ether functional group and from electron donation to the cationic center, which reduces the positive charge on the phosphorus atom.15 This is seen easily in Figures 3, 4, and 5, where the ILs with the ether substituent on the cation always have the lowest viscosities. For instance, at 278.15 K the viscosity of [P222(1O2O1)][4-NO2pyra] is only 310 mPa·s, compared with 840 mPa·s for [P2224][4NO2pyra] and 1000 mPa·s for [P2227][4-NO2pyra]. Figure 6 shows the effect on viscosity of the total chain length on the J



Article

Figure 6. Viscosities at 289.15 K of [Pxxxy][2-CH3,5-NO2imid] red ■, [Pxxxy][4-NO2pyra] blue ●, [P222(1O2O1)][2-CH3,5-NO2imid] green ◆, and [P222(1O2O1)][4-NO2pyra] yellow ▲, where the chain length is the addition of all of the carbons and oxygens in the cation. The chain length of [P2224], [P222(1O2O1)], [P2227], [P2228], [P2229], [P22210], and [P66614] are 10, 12, 13, 14, 15, 16, and 32, respectively.

Figure 8. Viscosities with VFT fitting of [P2228][2-CH3,5-NO2imid] green ◆, [P2228][4-NO2imid] red ■, [P2228][4-NO2pyra] blue ●, and [P2228][Tf2N] yellow ▲.

lide]− < [4-NO2pyra]− ∼ [4-NO2imid]− ∼ [4,5-Climid]− < [2CH3,5-NO2imid]−. The trend is the same for the [P2228]+ AHA ILs tested, whose viscosities increase as follows: [4-NO2pyra]− < [4-NO2imid]− < [2-CH3,5-NO2imid]−. Note that [P2228][3Triz] has a melting point above room temperature so no viscosities are reported for this IL. For the tetra-alkylphosphonium AHAs, it is likely that the viscosity is determined by the charge distribution on the anion, which affects the strength of cation−anion interactions. Chloro-, nitro-, and cyano-groups are all electron-withdrawing, while methyl-groups are slightly electron-donating. However, some of the trends can be explained by differences in the molar volumes, which is discussed below. The viscosities of several of the imidazolium AHAs are shown in Figure 9. The most obvious feature is that the

cation for both tetra-alkylphosphonium cations and cations containing ether functional groups, paired with [4-NO2pyra]− and [2-CH3,5-NO2imid]− anions at 298.15 K. As mentioned above, longer alkyl chains result in higher viscosities for both of the anions. However, [P 222(1O2O1) ][4-NO 2 pyra] and [P222(1O2O1)][2-CH3,5-NO2imid] both have lower viscosities than [P2224][4-NO2pyra] and [P2224][2-CH3,5-NO2imid], even though the ether chain is longer than the butyl chain. The 1O2O1 chain (−CH2−O−CH2−CH2−O−CH3) is particularly effective in reducing viscosity and melting points. However, as mentioned above, incorporation of ether chains sacrifices thermal stability. Tonset is 535 K for [P222(1O2O1)][4-NO2pyra], compared to 583 and 573 K for [P2224][4-NO2pyra] and [P2227][4-NO2pyra], respectively. The effect of the different AHA anions on the viscosity of tetra-alkylphosphonium ILs is explored in Figures 7 and 8.

Figure 9. Viscosities of [hmmim][4-NO2pyra] red ■, [pmmim][4NO2pyra] green ◆ [mm(butene)im][4-NO2pyra] light blue ×, [hmim][4-NO2pyra] □, [hmim][4-Triz] purple ∗, [hmim][3-Triz] yellow ▲, [hmim][Tetrazolide] dark red −, and [hmim][2-CNpyr] blue ●.

Figure 7. Viscosities of [P66614][2-CH3,5-NO2imid] purple ∗, [P66614][4,5-Climid] □, [P66614][ 4-NO2imid] yellow ▲, [P66614][ 4NO2pyra] dark red −, [P66614][Tetrazolide] blue ●, [P66614][4,5CNimid] light blue ×, [P66614][3-Triz] green ◆, and [P66614][Tf2N] red ■.

trisubstituted imidazolium ILs (where there is a −CH3 rather than a −H attached to the carbon between the nitrogens) are more viscous than the disubstituted imidazolium ILs. When paired with the [hmim]+ cation, the viscosities of the [4-Triz]−, [3-Triz]−, [Tetrazolide]−, and [4-NO2pyra]− salts are all rather similar. Only [hmim][2-CNpyr] is significantly lower. An interesting observation is that the temperature dependence of the viscosities of the various imidazolium AHA ILs are very different. Note that the viscosity of [hmim][Tetrazolide], [pmmim][4-NO2pyra], and [mm(butene)im][4-NO2pyra] are very similar at 338.15 K (50−53 mPa·s) but differ greatly at 278.15 K (2178−9612 mPa·s). This is because the viscosity of

Figure 7 shows the viscosity of ILs with the [P66614]+ cation, and Figure 8 shows the viscosity of ILs with the [P2228]+ cation. In both figures the data for the corresponding [Tf2N]− ILs are shown for comparison. For both [P66614]+ and [P2228]+ cations, the [Tf2N]− ILs have the lowest viscosities, which we attribute to more effective delocalization of the negative charge on the [Tf2N]− anion than occurs for any of the AHAs. The viscosities of the AHA ILs are 2−10 times higher than the equivalent [Tf2N]− salts. For the [P66614]+ AHA ILs, viscosity increases in the following order: [3-Triz]− < [4,5-CNimid]− < [TetrazoK



Article

Table 7. Experimental Values of Density, ρ, of Various ILs from 283.15 to 353.15 K at Pressure p = 0.1 MPa ionic liquid

water content (weight fraction × 102) T/K =

[P66614][3-Triz]41 [P66614][3-Triz] [P66614][4-NO2pyra] [P66614][4-NO2imid] [P66614][4,5-Climid] [P66614][4,5-CNimid] [P66614][2-CH3,5-NO2imid] [P66614][Tetrazolide] [P66614][Tf2N], ref 50 ref 51 ref 20 [P66614][Tf2N] [P44412][3-Triz] [P2224][2-CH3,5-NO2imid] [P2224][4-NO2pyra] [P2227][4-NO2pyra] [P2228][4-NO2pyra] [P2229][4-NO2pyra] [P22210][4-NO2pyra] [P2228][4-NO2imid] [P2228][2-CH3,5-NO2imid] [P2228][Tf2N]54 [P2228][Tf2N] [P222(1O2O1)][4-NO2pyra] [C6thiour][Tf2N] [C8thiour][Tf2N] [bmpyra][Tf2N]53 [bmpyra][Tf2N] [hmim][4-NO2pyra] [hmmim][4-NO2pyra] [hmim][3-Triz] [hmim][4-Triz] [hmim][Tetrazolide] [hmim][2-CNpyr] [pmmim][4-NO2pyra] [mm(butene)im][4NO2pyra] [hmim][Tf2N], refs 55, 56 ref 51 ref 57 ref 58 ref 59 ref 14 [hmim][Tf2N] [bthiol][Tf2N] [P66614][Acetate], ref 57 ref 60 ref 61 [P66614][Acetate] [hmim][Acetate], ref 62 ref 63 [hmim][Acetate] [P222(1O2O1)][Tf2N]

ρa/g cm−3 283.15 0.90660 0.9075 0.9488 0.9494 0.9741 0.9289 0.9481 0.9116 1.0774

0.9042 0.9456 0.9463 0.9708 0.9257 0.9451 0.9085 1.0736

293.15 0.90070 0.9012 0.9423 0.9431 0.9675 0.9226 0.9419 0.9054 1.0698

0.0057 0.0220 0.0178 0.0410 0.0730 0.0743 0.0047 0.0503 0.0104 0.0308

1.0761 0.9211 1.0744 1.0879 1.0471 1.0401 1.0310 1.0200 1.0378 1.0378

1.0724 0.9179 1.0713 1.0847 1.0440 1.0370 1.0279 1.0170 1.0348 1.0346

1.080 1.0688 0.9149 1.0683 1.0815 1.0409 1.0339 1.0249 1.0139 1.0318 1.0315

1.0673 0.9137 1.0671 1.0803 1.0397 1.0327 1.0236 1.0127 1.0306 1.0303

0.0039 0.0352 0.0019 0.0011

1.2563 1.1492 1.3522 1.3096

1.2522 1.1458 1.3478 1.3052

1.2481 1.1424 1.3434 1.3010

1.2466 1.1410 1.3416 1.2993

0.0019 0.0470 0.0160 0.0414 0.0400 0.0730 0.0940 0.0390 0.1073

1.4654 1.1336 1.1294 1.0499 1.1327 1.0775 1.0310 1.1930 1.1915

1.4607 1.1300 1.1260 1.0468 1.1292 1.0743 1.0279 1.1894 1.1877

1.4560 1.1266 1.1224 1.0437 1.1258 1.0712 1.0249 1.1856 1.1842

1.4541 1.1253 1.1210 1.0425 1.1245 1.0699 1.0236 1.1843 1.1828

0.0098 0.0107 0.0173 0.0095 0.0151 0.0240 0.0324

288.15

295.15 0.89948 0.9000 0.9411 0.9418 0.9662 0.9215 0.9406 0.9042

1.376

0.0020/0.0070 0.0047

1.3866 1.4690

1.3821 1.4643

1.3775 1.4597 0.89458

1.3757 1.4578

0.0630

0.8950

0.8919

0.8889

0.8876

0.0440 0.0030

1.0257 1.3739

1.0227 1.3694

1.0196 1.3648

1.0184 1.3630

L

298.15 0.89770 0.8982 0.9393 0.9401 0.9644 0.9197 0.9387 0.9024 1.0661 1.065 1.0651 0.9119 1.0653 1.0784 1.0378 1.0309 1.0218 1.0109 1.0288 1.0286 1.244 (297.15 K) 1.2442 1.1390 1.3390 1.2967 1.40 1.4512 1.1233 1.1189 1.0407 1.1225 1.0681 1.0218 1.1822 1.1808 1.372 1.357 1.371 1.373 1.37200 1.3747 1.3729 1.4550 0.891 0.89101 0.89137 0.8858 1.01700 1.0606 1.0166 1.3603

303.15 0.89471 0.8952 0.9364 0.9371 0.9613 0.9168 0.9357 0.8994 1.0624

308.15 0.8922 0.9334 0.9342 0.9583 0.9139 0.9328 0.8964 1.0588

313.15 0.88871 0.8892 0.9304 0.9313 0.9552 0.9110 0.9298 0.8935 1.0552

1.0615 0.9089 1.0623 1.0754 1.0348 1.0279 1.0188 1.0079 1.0258 1.0256

1.0579 0.9059 1.0593 1.0724 1.0318 1.0249 1.0158 1.0049 1.0229 1.0227

1.0544 0.9029 1.0565 1.0693 1.0288 1.0220 1.0129 1.0020 1.0201 1.0198

1.2402 1.1356 1.3346 1.2924

1.2362 1.1322 1.3302 1.2882

1.2322 1.1289 1.3258 1.2840

1.4465 1.1200 1.1157 1.0377 1.1192 1.0650 1.0188 1.1789 1.1775

1.4418 1.1166 1.1124 1.0347 1.1158 1.0619 1.0158 1.1755 1.1741

1.4371 1.1134 1.1092 1.0318 1.1127 1.0588 1.0128 1.1722 1.1708

1.3684 1.4504

1.3638 1.4457

1.3593 1.4411 0.88234

0.8828 1.01393

0.8798 1.01088

0.8768 1.00785

1.0135 1.3557

1.0104 1.3512

1.0074 1.3468



Article

Table 7. continued ionic liquid

water content (weight fraction × 102) T/K =

[P66614][3-Triz]41 [P66614][3-Triz] [P66614][4-NO2pyra] [P66614][4-NO2imid] [P66614][4,5-Climid] [P66614][4,5-CNimid] [P66614][2-CH3,5-NO2imid] [P66614][Tetrazolide] [P66614][Tf2N]50 [P66614][Tf2N] [P44412][3-Triz] [P2224][2-CH3,5-NO2imid] [P2224][4-NO2pyra] [P2227][4-NO2pyra] [P2228][4-NO2pyra] [P2229][4-NO2pyra] [P22210][4-NO2pyra] [P2228][4-NO2imid] [P2228][2-CH3,5-NO2imid] [P2228][Tf2N] [P222(1O2O1)][4-NO2pyra] [C6thiour][Tf2N] [C8thiour][Tf2N] [bmpyra][Tf2N] [hmim][4-NO2pyra] [hmmim][4-NO2pyra] [hmim][3-Triz] [hmim][4-Triz] [hmim][Tetrazolide] [hmim][2-CNpyr] [pmmim][4-NO2pyra] [mm(butene)im][4NO2pyra] [hmim][Tf2N] [bthiol][Tf2N] [P66614][Acetate] [hmim][Acetate]62 [hmim][Acetate] [P222(1O2O1)][Tf2N]

0.0098 0.0107 0.0173 0.0095 0.0151 0.0240 0.0324 0.0057 0.0220 0.0178 0.0410 0.0730 0.0743 0.0047 0.0503 0.0104 0.0308 0.0039 0.0352 0.0019 0.0011 0.0019 0.0470 0.0160 0.0414 0.0400 0.0730 0.0940 0.0390 0.1073 0.0020/0.0070 0.0047 0.0630 0.0440 0.0030

ρa/g cm−3 318.15 0.8863 0.9274 0.9284 0.9522 0.9081 0.9269 0.8905 1.0516 1.0508 0.9000 1.0536 1.0663 1.0260 1.0190 1.0099 0.9991 1.0172 1.0169 1.2282 1.1255 1.3215 1.2797 1.4324 1.1102 1.1060 1.0289 1.1095 1.0558 1.0099 1.1689 1.1676

323.15 0.88287 0.8834 0.9245 0.9254 0.9491 0.9052 0.9240 0.8876 1.0480 1.0473 0.8971 1.0507 1.0633 1.0231 1.0161 1.0070 0.9962 1.0143 1.0141 1.2242 1.1222 1.3171 1.2755 1.4277 1.1070 1.1028 1.0260 1.1062 1.0528 1.0069 1.1657 1.1644

328.15 0.8804 0.9216 0.9226 0.9462 0.9024 0.9211 0.8847 1.0444 1.0437 0.8942 1.0478 1.0603 1.0202 1.0132 1.0040 0.9933 1.0114 1.0112 1.2202 1.1189 1.3128 1.2713 1.4230 1.1038 1.0997 1.0231 1.1031 1.0498 1.0039 1.1625 1.1611

333.15 0.87700 0.8775 0.9187 0.9197 0.9432 0.8996 0.9182 0.8818 1.0409 1.0401 0.8912 1.0449 1.0573 1.0173 1.0102 1.0011 0.9904 1.0085 1.0084 1.2163 1.1156 1.3085 1.2671 1.4184 1.1006 1.0966 1.0202 1.0999 1.0468 1.0010 1.1593 1.1579

1.3548 1.4365 0.8737 1.00490 1.0045 1.3423

1.3503 1.4319 0.8707 1.00188 1.0015 1.3379

1.3458 1.4273 0.8677 0.99888 0.9985 1.3334

1.3413 1.4227 0.8647 0.99589 0.9955 1.3290

338.15

343.15

348.15

353.15

0.8746 0.9158 0.9169 0.9402 0.8967 0.9153 0.8789 1.0373 1.0366 0.8883 1.0421 1.0543 1.0145 1.0073 0.9982 0.9875 1.0057 1.0056 1.2123 1.1123 1.3042 1.2629 1.4138 1.0974 1.0935 1.0173 1.0967 1.0438 0.9980 1.1561 1.1547

0.8717 0.9128 0.9140 0.9372 0.8939 0.9125 0.8760 1.0337 1.0330 0.8854 1.0392 1.0513 1.0116 1.0044 0.9953 0.9846 1.0028 1.0027 1.2084 1.1091 1.2998 1.2587 1.4091 1.0942 1.0904 1.0144 1.0935 1.0408 0.9951 1.1528 1.1515

0.8687 0.9099 0.9112 0.9342 0.8910 0.9096 0.8731 1.0302 1.0295 0.8825 1.0364 1.0484 1.0088 1.0015 0.9924 0.9817 1.0000 0.9999 1.2044 1.1058 1.2956 1.2546 1.4046 1.0911 1.0874 1.0115 1.0903 1.0378 0.9921 1.1496 1.1483

0.8658 0.9070 0.9083 0.9313 0.8882 0.9068 0.8702 1.0266 1.0260 0.8795 1.0335 1.0454 1.0060 0.9986 0.9895 0.9788 0.9971 0.9971 1.2005 1.1026 1.2913 1.2504 1.4000 1.0879 1.0843 1.0086 1.0872 1.0348 0.9891 1.1464 1.1452

1.3368 1.4182 0.8617 0.99288 0.9926 1.3246

1.3324 1.4136 0.8587

1.3279 1.4091 0.8557

1.3235 1.4046 0.8527

0.9896 1.3202

0.9867 1.3158

0.9837 1.3115

Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.005 MPa, and u(ρ) = 0.002 g cm−3 for ILs that are 99% pure, 0.004 g cm−3 for ILs that are 98% pure, 0.006 g cm−3 for ILs that are 97% pure, 0.008 g cm−3 for ILs that are 96% pure, and 0.010 g cm−3 for ILs that are 95% pure.

a

viscosities previously reported for [C6thiour][Tf2N] and [C8thiour][Tf2N], just 19 and 23 mPa·s at room temperature, should be called into question. 3.3. Density Measurements. The densities of all of the ILs measured decrease, as anticipated, with increasing temperature, and the values are shown in Table 7. Linear fits of the data are shown in Table 8. Figure 10 shows the effect of alkyl chain length on the molar volume (molecular weight/density) for tetra-alkylphosphonium ILs with the [4-NO2pyra]− anion. Density decreases with increasing alkyl chain length, as the salts become more “alkanelike”. The molar volume increases with increasing alkyl chain length due to both increasing molecular weight and decreasing density. Only the [P66614]+ AHAs are dependably less dense than water. Alternatively, one can envision that the reduction in the size of the cation causes an increase in the density due to easier packing with the smaller cations. The replacement of an

[hmim][Tetrazolide] does not increase with decreasing temperature as quickly as some of the other ILs. By contrast, the temperature dependence of the viscosity is particularly large for [mm(butene)im][4-NO2pyra]. While we do not have explanations for these trends, it does emphasize the importance of measuring the viscosity at multiple temperatures. The viscosities reported here for [P66614][Tf2N], [P66614][3Triz], [P2228][Tf2N], [bmpyra][Tf2N], and [hmim][Tf2N] match literature values within experimental uncertainties, as shown in Table 5. Our value for [e(1O2)mim][Tf2N] (66 mPa·s at 298 K) does not match the one literature value (53.3 mPa·s at 298 K),48 even when taking our higher uncertainty at lower viscosities into account. We attribute this to the relatively low estimated purity (95%) of this particular sample. Our values for [C6thiour][Tf2N] and [C 8thiour][Tf2N] are significantly higher than those reported in the literature.49 In light of the results presented here, the exceptionally low M



Article

Table 8. Parameters for Fitting the Density of Various ILs to a Straight Line ionic liquid [P66614][3-Triz] [P66614][4-NO2pyra] [P66614][4-NO2imid] [P66614][4,5-Climid] [P66614][4,5-CNimid] [P66614][2-CH3,5-NO2imid] [P66614][Tetrazolide] [P66614][Tf2N] [P44412][3-Triz] [P2224][2-CH3,5-NO2imid] [P2224][4-NO2pyra] [P2227][4-NO2pyra] [P2228][4-NO2pyra] [P2229][4-NO2pyra] [P22210][4-NO2pyra] [P2228][4-NO2imid] [P2228][2-CH3,5-NO2imid] [P2228][Tf2N] [P222(10201)][4-NO2pyra] [C6thiour][Tf2N] [C8thiour][Tf2N] [bmpyra][Tf2N] [hmim][4-NO2pyra] [hmmim][4-NO2pyra] [hmim][3-Triz] [hmim][4-Triz] [hmim][Tetrazolide] [hmim][2-CNpyr] [pmmim][4-NO2pyra] [mm(butene)im][4NO2pyra] [hmim][Tf2N] [bthiol][Tf2N] [P66614][Acetate] [hmim][Acetate] [P222(10201)][Tf2N]

temperature range/K

b/g cm−3 m/g cm−3 K−1

283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15

1.0748 1.1162 1.1142 1.1458 1.0919 1.1145 1.0781 1.2784 1.0881 1.2390 1.2589 1.2126 1.2072 1.1985 1.1861 1.2021 1.2010 1.4815 1.3374 1.5986 1.5486 1.7303 1.3169 1.3102 1.2161 1.3156 1.2494 1.1997 1.3792 1.3765

0.0005922 0.0005930 0.0005836 0.0006081 0.0005773 0.0005891 0.0005890 0.0007151 0.0005908 0.0005825 0.0006051 0.0005861 0.0005913 0.0005923 0.0005875 0.0005809 0.0005781 0.0007960 0.0006656 0.0008707 0.0008448 0.0009361 0.0006491 0.0006408 0.0005880 0.0006476 0.0006081 0.0005965 0.0006601 0.0006560

283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15 283.15−353.15

1.6421 1.7293 1.0659 1.1953 1.6262

0.0009026 0.0009200 0.0006041 0.0005995 0.0008920

Figure 11. (a) Densities of [P66614][4,5-Climid] yellow ▲, [P66614][4NO2imid] light blue ×, [P66614][2-CH3,5-NO2imid] dark red −, [P66614][4-NO2pyra] □, [P66614][4,5-CNimid] red ■, [P66614][Tetrazolide] blue ●, and [P66614][3-Triz] green ◆. (b) Molar volumes of [P66614][4,5-CNimid] red ■, [P66614][2-CH3,5-NO2imid] dark red −, [P66614][4,5-Climid] yellow ▲, [P66614][4-NO2pyra] □, [P66614][4-NO2imid] light blue ×, [P66614][3-Triz] green ◆, and [P66614][Tetrazolide] blue ●.

increase from low to high in the following order: [3-Triz]− < [Tetrazolide]− < [4,5-CNimid]− < [4-NO2pyra]− ∼ [2-CH3,5NO2imid]− ∼ [4-NO2imid]− < [4,5-Climid]−. The densities may, in fact, help to explain the viscosities presented in the previous section. [P66614][3-Triz] and [P66614][Tetrazolide] differ only by a −CH− in the ring structure being replaced with a −N−, yet the density of [P66614][Tetrazolide] is slightly higher (0.9116 g cm−3 at 283.15 K compared to 0.9075 g cm−3 for [P66614][3-Triz]). The smaller molar volume of [P66614][Tetrazolide] means that the ions are slightly closer, which would increase the intermolecular interactions and, subsequently, increase the viscosity. A similar argument can be made to explain the higher viscosity of [P66614][2-CH3,5-NO2imid]. [P66614][4-NO2pyra], [P66614][4-NO2imid], and [P66614][2CH3,5-NO2imid] all have very similar densities, but the molecular weight of [P66614][2-CH3,5-NO2imid] is larger (by the −CH3 group). The viscosities of [P66614][4-NO2pyra] and [P66614][4-NO2imid] are similar, but that of [P66614][2-CH3,5NO2imid], whose molar volume is larger, is much greater. This result is reinforced in Figure 12a and b, where the densities of [P2228][4-NO2pyra], [P2228][4-NO2imid], and [P2228][2-CH3,5NO2 imid] are all very similar. Yet, as with the [P66614]+ ILs, and the viscosity of [P2228][2-CH3,5-NO2imid] is higher. A comparison between the density of [Pxxxy][4-NO2pyra] and [Pxxxy][2-CH3,5-NO2imid] at 298.15 K can be seen in Figure 13. The more total carbons in the alkyl chains on the cation, the

Figure 10. Molar volumes of [P66614][4-NO2pyra] green ◆, [P22210][4-NO2pyra] yellow ▲, [P2229][4-NO2pyra] light blue ×, [P2228][4-NO2pyra] dark red −, [P2227][4-NO2pyra] purple ∗, [P222(1O2O1)][4-NO2pyra] red ■, and [P2224][4-NO2pyra] blue ●.

alkyl chain with an ether also causes an increase in its density. This follows the same trend when the anion is [Tf2N]−. The influence of the anion on density and molar volume is shown in Figure 11a and b for [P66614]+ AHA ILs. The densities N



Article

Figure 14. Molar volumes of [hmim][2-CNpyr] red ■, [hmim][4NO2pyra] □, [hmim][3-Triz] green ◆, [hmim][Tetrazolide] blue ●, and [hmim][4-Triz] dark red −.

All of the ILs with the [Tf2N]− anion have much higher densities than the AHA ILs, with increasing density of: [P66614]+ < [P2228]+ < [C8thiour]+ < [C6thiour]+ < [hmim]+ < [bmpyra]+ ∼ [bthiol]+. As anticipated, increasing alkyl chain length decreases density: [P2228][Tf2N] is more dense than [P66614][Tf2N] and [C6thiour][Tf2N] is more dense than [C8thiour][Tf2N]. As shown in Figure 15, when the anion is [Tf2N]−, the molar volume increase in the following order: [bmpyra]+ ∼ [bthiol] + < [hmim]+ < [C6thiour]+ < [C8thiour]+ ∼ [P2228]+ < [P66614]+. Figure 12. (a) Densities of [P2228][4-NO2pyra] yellow ▲, [P2228][4NO2imid] dark red −, and [P2228][2-CH3,5-NO2imid] □. (b) Molar volumes of [P2228][2-CH3,5-NO2imid] □, [P2228][4-NO2imid] dark red −, and [P2228][4-NO2pyra] yellow ▲.

Figure 15. Molar volumes of [P66614][Tf2N] purple ∗, [P2228][Tf2N] yellow ▲, [C8thiour][Tf2N] blue ●, [C6thiour][Tf2N] green ◆, [hmim][Tf2N] red ■, [bthiol][Tf2N] □, and [bmpyra][Tf2N] dark red −.

Figure 13. Densities at 289.15 K of [Pxxxy][2-CH3,5-NO2imid] red □, [Pxxxy][4-NO2pyra] blue ●, and [P222(1O2O1)][4-NO2pyra] yellow ▲, where the chain length is the addition of all of the carbons and oxygens in the cation. The chain length of [P2224], [P222(1O2O1)], [P2227], [P2228], [P2229], [P22210], and [P66614] are 10, 12, 13, 14, 15, 16, and 32, respectively.

The densities reported here for [P66614][Tf2N], [P2228][Tf2N], [hmim][Tf2N], [P66614][acetate], and [hmim][acetate] match literature values within experimental uncertainties in most cases, as shown in Table 5.50,51,54−63 Since the densities reported here for [P66614][Tf2N] match those reported by two separate groups,50,51 the single value in the third reference20 is probably in error. The densities reported here for [P2228][Tf2N] and [hmim][Tf2N] matched the literature values within experimental uncertainties.54−59 The densities reported here for [P66614][Acetate] are about 0.005 g cm−3 lower than those reported previously,57,60,61 which is slightly greater than our experimental uncertainty, but probably within the combined uncertainty of the various groups. The densities reported here for [hmim][Acetate] closely match the one value reported by Ma et al.,62 suggesting that the value reported by Guan et al. is incorrect.63

lower the density. The density of [P2228][4-NO2pyra] is similar to that of [P2228][2-CH3,5-NO2imid], and the density of [P66614][4-NO2pyra] is similar to that of [P66614][2-CH3,5NO2imid], as mentioned above. On the other hand, the density of [P222(1O2O1)][4-NO2pyra] is higher than that of [P2224][4NO2pyra], even though the ether chain is longer than the butyl chain; the replacement of an alkyl chain with an ether significantly increases the density. Figure 14 shows that, when the cation is [hmim], the molar volume increases in the following order: [4-Triz] − < [Tetrazolide]− < [3-Triz]− < [4-NO2pyra]− < [2-CNpyr]−. The density increases in the following order: [2-CNpyr]− < [3Triz]− < [Tetrazolide]− < [4-Triz]− ∼ [4-NO2pyra]−. O



Article

Ambient-Temperature Molten Salts. Inorg. Chem. 1996, 35, 1168− 1178. (4) Janz, G. J. Molten Salts Data as Reference Standards for Density, Surface Tension Viscosity and Electrical Conductance: KNO3 and NaCl. J. Phys. Chem. Ref. Data 1980, 9, 791−829. (5) Kilaru, P. K.; Condemarin, R. A.; Scovazzo, P. Correlations of Low-Pressure Carbon Dioxide and Hydrocarbon Solubilities in Imidazolium-, Phosphonium-, and Ammonium-Based Room-Temperature Ionic Liquids. Part 1. Using Surface Tension. Ind. Eng. Chem. Res. 2008, 47, 900−909. (6) Valderrama, J. O. Myths and Realities about Existing Methods for Calculating the Melting Temperatures of Ionic Liquids. Ind. Eng. Chem. Res. 2014, 53, 1004−1014. (7) Trohalaki, S.; Pachter, R.; Drake, G. W.; Hawkins, T. Quantitative Structure - Property Relationships for Melting Points and Densities of Ionic Liquids. Energy Fuels 2005, 19, 279−284. (8) Perry, D. L. Handbook of Inorganic Compounds, 2nd ed.; Taylor & Francis Group, LLC, 2011; p 378. (9) Bulut, S.; Klose, P.; Krossing, I. (2011). Na[B(hfip)4] (hfip  OC(H) (CF3)2): a weakly coordinating anion salt and its first application to prepare ionic liquids. Dalton Trans. 2011, 40, 8114− 8124. (10) Hunt, P. A. Why does a reduction in hydrogen bonding lead to an increase in viscosity for the 1-butyl-2,3-dimethyl-imidazolium-based ionic liquids? J. Phys. Chem. B 2007, 111, 4844−4853. (11) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. Thermophysical Properties of Imidazolium-Based Ionic Liquids. J. Chem. Eng. Data 2004, 49, 954−964. (12) Adamová, G.; Gardas, R. L.; Nieuwenhuyzen, M.; Puga, A. V.; Rebelo, L. P. N.; Robertson, A. J.; Seddon, K. R. Alkyltributylphosphonium chloride ionic liquids: synthesis, physicochemical properties and crystal structure. Dalton Trans. 2012, 41, 8316−8332. (13) Scovazzo, P.; Camper, D.; Kieft, J.; Poshusta, J.; Koval, C.; Noble, R. Regular Solution Theory and CO2 Gas Solubility in RoomTemperature Ionic Liquids. Ind. Eng. Chem. Res. 2004, 43, 6855−6860. (14) Chen, Z. J.; Lee, J.-M. Free volume model for the unexpected effect of C2-methylation on the properties of imidazolium ionic liquids. J. Phys. Chem. B 2014, 118, 2712−2718. (15) Tsunashima, K.; Sugiya, M. Physical and electrochemical properties of low-viscosity phosphonium ionic liquids as potential electrolytes. Electrochem. Commun. 2007, 9, 2353−2358. (16) Seddon, K. R.; Stark, A.; Torres, M.-J. Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl. Chem. 2000, 72, 2275−2287. (17) Sun, J.; Forsyth, M.; MacFarlane, D. R. Room-Temperature Molten Salts Based on the Quaternary Ammonium Ion. J. Phys. Chem. B 1998, 102, 8858−8864. (18) Matsumoto, H.; Kageyama, H.; Miyazaki, Y. Room temperature ionic liquids based on small aliphatic ammonium cations and asymmetric amide anions. Chem. Commun. 2002, 1726−1727. (19) Ito, N.; Arzhantsev, S.; Heitz, M.; Maroncelli, M. Solvation Dynamics and Rotation of Coumarin 153 in Alkylphosphonium Ionic Liquids. J. Phys. Chem. B 2004, 108, 5771−5777. (20) Del Sesto, R. E.; Corley, C.; Robertson, A.; Wilkes, J. S. Tetraalkylphosphonium-based ionic liquids. J. Organomet. Chem. 2005, 690, 2536−2542. (21) Sun, X.; Luo, H.; Dai, S. Ionic Liquids-Based Extraction: A Promising Strategy for the Advanced Nuclear Fuel Cycle. Chem. Rev. 2012, 112, 2100−2128. (22) Crosthwaite, J. M.; Muldoon, M. J.; Dixon, J. K.; Anderson, J. L.; Brennecke, J. F. Phase transition and decomposition temperatures, heat capacities and viscosities of pyridinium ionic liquids. J. Chem. Thermodyn. 2005, 37, 559−568. (23) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases. J. Phys. Chem. B 1999, 103, 4164−4170. (24) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Low-melting, Lowviscous, Hydrophobic Ionic Liquids: N-Alkyl(alkyl ether)-N-methyl-

4. CONCLUSIONS The purpose of this study was to develop low viscosity AHA ILs that are appropriate for CO2 capture, electrochemical, and/ or cofluid vapor-compression refrigeration applications. This can be achieved using short alkyl chains and/or incorporating ether groups. For [P222n][4-NO2pyra], n = 8 gives the lowest melting point and a lower viscosity than longer alkyl chains. However, for the [3-Triz]− ILs, shortening the alkyl chain to octyl raises the melting point too much. [P222(1O2O1)] salts have low melting points and low viscosities, but their thermal stability is compromised compared to equivalent tetraalkylphosophonium ILs. This may be acceptable if the IL is intended solely for low temperature operation. Some of the short chain dialkylimidazolium AHAs have sufficiently low melting points, low viscosities, and decent thermal stability. However, trisubstituted imidazoliums, where the C2 position is blocked to prevent carbene formation that would complicate use with CO2, raises the viscosity. Although likely less expensive, ammonium salts generally have higher melting points and poorer thermal stability so they are not good choices. Overall, we have presented some good strategies for viscosity reduction, but they usually involve trade-offs with operational range and thermal stability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00269. Details on the NMR characterization for each IL synthesized, description of viscosity and density fitting parameters, and representative DSC scans (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (574) 631-5847. Fax: (574) 631-8366. E-mail: jfb@nd. edu. Funding

This material is based upon work supported by the Department of Energy ARPAe under Award Number AR000019. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Professor Brandon Ashfeld and Christopher Meyer for the synthesis of [P444(1O2O1)][3-Triz], [P444(1O1)][3Triz], [P 22 2( 1 O 2) ][3-Triz], [b(1O1O1)mim][3-Triz], [b(1O1O2O1)mim][3-Triz], and [e(1O2)mim][Tf2N]. We acknowledge Dr. Oscar Morales-Collazo for the synthesis of [P66614][Acetate], [hmim][Acetate], and [P222(1O2O1)][Tf2N]. We also acknowledge Dr. Yong Huang for some Tg, Tm, Tonset measurements.

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REFERENCES

(1) Plechkova, N. V.; Seddon, K. R. Applications of ionic liquids in the chemical industry. Chem. Soc. Rev. 2008, 37, 123−150. (2) Fannin, A. A.; Floreani, D. A.; King, L. A.; Landers, J. S.; Piersma, B. J.; Stech, D. J.; Vaughn, R. L.; Wilkes, J. S.; Williams, J. L. Properties of 1,3-Dialkylimidazolium Chloride-Aluminum Chloride Ionic Liquids. 2. Phase Transitions, Densities, Electrical Conductivities, and Viscosities. J. Phys. Chem. 1984, 88, 2614−2621. (3) Bonhôte, P.; Dias, A.-P.; Armand, M.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Hydrophobic, Highly Conductive P



Article

pyrrolidinium Perfluoroethyltrifluoroborate. Chem. Lett. 2004, 33, 1636−1637. (25) Chapeaux, A.; Simoni, L. D.; Stadtherr, M. A.; Brennecke, J. F. Liquid Phase Behavior of Ionic Liquids with Water and 1-Octanol and Modeling of 1-Octanol/Water Partition Coefficients. J. Chem. Eng. Data 2007, 52, 2462−2467. (26) Blanchard, L. A.; Gu, Z.; Brennecke, J. F. High-Pressure Phase Behavior of Ionic Liquid/CO2 Systems. J. Phys. Chem. B 2001, 105, 2437−2444. (27) Ignat’ev, N. V.; Welz-Biermann, U.; Kucheryna, A.; Bissky, G.; Willner, H. New ionic liquids with tris(perfluoroalkyl)trifluorophosphate (FAP) anions. J. Fluorine Chem. 2005, 126, 1150−1159. (28) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Dialkylimidazolium Chloroaluminate Melts: A New Class of RoomTemperature Ionic Liquids for Electrochemistry, Spectroscopy, and Synthesis. Inorg. Chem. 1982, 21, 1263−1264. (29) Vestergaard, B. Molten Triazolium Chloride Systems as New Aluminum Battery Electrolytes. J. Electrochem. Soc. 1993, 140, 3108− 3113. (30) Sato, T.; Masuda, G.; Takagi, K. Electrochemical properties of novel ionic liquids for electric double layer capacitor applications. Electrochim. Acta 2004, 49, 3603−3611. (31) Visser, A. E.; Holbrey, J. D.; Rogers, R. D. Hydrophobic ionic liquids incorporating N-alkylisoquinolinium cations and their utilization in liquid−liquid separations. Chem. Commun. 2001, 2484−2485. (32) Scurto, A. M.; Aki, S. N. V. K.; Brennecke, J. F. CO2 as a separation switch for ionic liquid/organic mixtures. J. Am. Chem. Soc. 2002, 124, 10276−10277. (33) Blanchard, L. A.; Brennecke, J. F. Recovery of Organic Products from Ionic Liquids Using Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 2001, 40, 287−292. (34) Olivier-bourbigou, H.; Magna, L. Ionic liquids: perspectives for organic and catalytic reactions. J. Mol. Catal. A: Chem. 2002, 182−183, 419−437. (35) Mizuuchi, H.; Jaitely, V.; Murdan, S.; Florence, A. T. Room temperature ionic liquids and their mixtures: potential pharmaceutical solvents. Eur. J. Pharm. Sci. 2008, 33, 326−331. (36) Chatel, G.; Pereira, J. F. B.; Debbeti, V.; Wang, H.; Rogers, R. D. Mixing ionic liquids − “simple mixtures” or “double salts”? Green Chem. 2014, 16, 2051−2083. (37) Niedermeyer, H.; Hallett, J. P.; Villar-Garcia, I. J.; Hunt, P. A.; Welton, T. Mixtures of ionic liquids. Chem. Soc. Rev. 2012, 41, 7780− 7802. (38) Brennecke, J. F.; Maginn, E. J. Ionic Liquids: Innovative Fluids for Chemical Processing. AIChE J. 2001, 47, 2384−2389. (39) Gurkan, B.; Goodrich, B. F.; Mindrup, E. M.; Ficke, L. E.; Massel, M.; Seo, S.; Senftle, T. P.; Wu, H.; Glaser, M. F.; Shah, J. K.; Maginn, E. J.; Brennecke, J. F.; Schneider, W. F. Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO2 Capture. J. Phys. Chem. Lett. 2010, 1, 3494−3499. (40) Gurkan, B. E.; Gohndrone, T. R.; McCready, M. J.; Brennecke, J. F. Reaction kinetics of CO2 absorption in to phosphonium based anion-functionalized ionic liquids. Phys. Chem. Chem. Phys. 2013, 15, 7796−7811. (41) Seo, S.; Quiroz-Guzman, M.; DeSilva, M. A.; Lee, T. B.; Huang, Y.; Goodrich, B. F.; Schneider, W. F.; Brennecke, J. F. Chemically Tunable Ionic Liquids with Aprotic Heterocyclic Anion (AHA) for CO2 Capture. J. Phys. Chem. B 2014, 118, 5740−5751. (42) Shi, C.; Quiroz-Guzman, M.; DeSilva, A.; Brennecke, J. F. Physicochemical and Electrochemical Properties of Ionic Liquids Containing Aprotic Heterocyclic Anions Doped With Lithium Salts. J. Electrochem. Soc. 2013, 160, A-1604−A1610. (43) Mozurkewich, G.; Greenfield, M. L.; Schneider, W. F.; Zietlow, D. C.; Meyer, J. J. Simulated performance and cofluid dependence of a CO2 -cofluid refrigeration cycle with wet compression. Int. J. Refrig. 2002, 25, 1123−1136.

(44) Yongming, N.; Jiangping, C.; Zhijiu, C.; Huanxin, C. Construction and testing of a wet-compression absorption carbon dioxide refrigeration system for vehicle air conditioner. Appl. Therm. Eng. 2007, 27, 31−36. (45) Wujek, S. S.; McCready, M. J.; Mozurkewich, G.; Schneider, W. F.; Elbel, S. Experimental and Modeling Improvements to a Co-Fluid Utilizing Ionic Liquids and Carbon Dioxide. 15th International Refrigeration and Air Conditioning Conference at Purdue 2014, 2501, 1−10. (46) Maton, C.; Vos, N. D.; Stevens, C. V. Ionic liquid thermal stabilities: Decomposition mechanisms and analysis tools. Chem. Soc. Rev. 2013, 42, 5963−5977. (47) Guo, L.; Pan, X.; Zhang, C.; Wang, M.; Cai, M.; Fang, X.; Dai, S. Novel hydrophobic cyclic sulfonium-based ionic liquids as potential electrolyte. J. Mol. Liq. 2011, 158, 75−79. (48) Jin, Y.; Fang, S.; Zhang, Z.; Zhang, J.; Yang, L.; Hirano, S.-I. C-2 Functionalized Trialkylimidazolium Ionic Liquids with Alkoxymethyl Group: Synthesis, Characterization, and Properties. Ind. Eng. Chem. Res. 2013, 52, 7297−7306. (49) Abai, M.; Holbrey, J. D.; Rogers, R. D.; Srinivasan, G. Ionic liquid S-alkylthiouronium salts. New J. Chem. 2010, 34, 1981−1993. (50) Neves, C. M. S. S.; Carvalho, P. J.; Freire, M. G.; Coutinho, J. A. P. Thermophysical properties of pure and water-saturated tetradecyltrihexylphosphonium-based ionic liquids. J. Chem. Thermodyn. 2011, 43, 948−957. (51) McHale, G.; Hardacre, C.; Ge, R.; Doy, N.; Allen, R. W. K.; MacInnes, J. M.; Bown, M. R.; Newton, M. I. Density-viscosity product of small-volume ionic liquid samples using quartz crystal impedance analysis. Anal. Chem. 2008, 80, 5806−5811. (52) Yamaguchi, T.; Mikawa, K.-I.; Koda, S.; Fukazawa, H.; Shirota, H. Shear relaxation of ammonium- and phosphonium- based ionic liquids with oxyethylene chain. Chem. Phys. Lett. 2012, 521, 69−73. (53) Chiappe, C.; Sanzone, A.; Mendola, D.; Castiglione, F.; Famulari, A.; Raos, G.; Mele, A. Pyrazolium- versus imidazoliumbased ionic liquids: structure, dynamics and physicochemical properties. J. Phys. Chem. B 2013, 117, 668−676. (54) Shirota, H.; Fukazawa, H.; Fujisawa, T.; Wishart, J. F. Heavy Atom Substitution Effects in Non-Aromatic Ionic Liquids: Ultrafast Dynamics and Physical Properites. J. Phys. Chem. B 2010, 114, 9400− 9412. (55) Mandai, T.; Masu, H.; Imanari, M.; Nishikawa, K. Comparison between cycloalkyl- and n-alkyl-substituted imidazolium-based ionic liquids in physicochemical properties and reorientational dynamics. J. Phys. Chem. B 2012, 116, 2059−2064. (56) Dzyuba, S. V.; Bartsch, R. A. Influence of Structural Variations in 1-Alkyl(aralkyl)-3-Methylimidazolium Hexafluorophosphates and Bis (trifluoromethylsulfonyl) imides on Physical Properties of the Ionic Liquids. ChemPhysChem 2002, 3, 161−166. (57) Shimizu, K.; Tariq, M.; Costa Gomes, M. F.; Rebelo, L. P. N.; Canongia Lopes, J. N. Assessing the dispersive and electrostatic components of the cohesive energy of ionic liquids using molecular dynamics simulations and molar refraction data. J. Phys. Chem. B 2010, 114, 5831−5834. (58) Shirota, H.; Mandai, T.; Fukazawa, H.; Kato, T. Comparison between Dicationic and Monocationic Ionic Liquids: Liquid Density, Thermal Properties, Surface Tension, and Shear Viscosity. J. Chem. Eng. Data 2011, 56, 2453−2459. (59) Seoane, R. G.; Corderi, S.; Gomez, E.; Calvar, N.; Gonzalez, E. J.; Macedo, E. A.; Dominguez, A. Temperature Dependence and Structural Influence on the Thermophysical Properties of Eleven Commercial Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51, 2492−2504. (60) Esperanca, J. M. S. S.; Guedes, H. J. R.; Blesic, M.; Rebelo, L. P. N. Densities and Derived Thermodynamic Properties of Ionic Liquids. 3. Phosphonium-Based Ionic Liquids over an Extended Pressure Range. J. Chem. Eng. Data 2006, 51, 237−242. (61) Tariq, M.; Forte, P. A. S.; Costa Gomes, M. F.; Canongia Lopes, J. N.; Rebelo, L. P. N. Densities and refractive indices of imidazoliumand phosphonium-based ionic liquids: Effect of temperature, alkyl chain length, and anion. J. Chem. Thermodyn. 2009, 41, 790−798. Q



Article

(62) Ma, X.-X.; Wei, J.; Zhang, Q.-B.; Tian, F.; Feng, Y.-Y.; Guan, W. Prediction of Thermophysical Properties of Acetate-Based Ionic Liquids Using Semiempirical Methods. Ind. Eng. Chem. Res. 2013, 52, 9490−9496. (63) Guan, W.; Ma, X.-X.; Li, L.; Tong, J.; Fang, D.-W.; Yang, J.-Z. Ionic Parachor and Its Application in Acetic Acid Ionic Liquid Homologue 1-Alkyl-3methylimidazolium Acetate {[Cnmim][OAc](n = 2,3,4,5,6)}. J. Phys. Chem. B 2011, 115, 12915−12920.

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