Diversity Observed in the Nanostructure of Protic Ionic Liquids - The

Jul 19, 2010 - The nanostructure of a series of 20 protic ionic liquids (PILs) has been investigated using small- and wide-angle X-ray scattering (SAX...
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J. Phys. Chem. B 2010, 114, 10022–10031

Diversity Observed in the Nanostructure of Protic Ionic Liquids Tamar L. Greaves,† Danielle F. Kennedy,† Stephen T. Mudie,‡ and Calum J. Drummond*,†,§ CSIRO Molecular and Health Technologies, Bag 10, Clayton VIC 3169, Australia, Australian Synchrotron, 800 Blackburn Rd, Clayton VIC 3169, Australia, and CSIRO Materials Science and Engineering, Bag 33, Clayton VIC 3169, Australia ReceiVed: April 28, 2010; ReVised Manuscript ReceiVed: June 2, 2010

The nanostructure of a series of 20 protic ionic liquids (PILs) has been investigated using small- and wideangle X-ray scattering (SAXS and WAXS). The PILs contained alkylammonium, dialkylammonium, trialkylammonium, and cyclic ammonium cations combined with organic or inorganic anions. The presence of hydroxyl and methoxy substituents on the alkyl chains of the cations was also explored. Many of the PILs showed a nanostructure resulting from segregation of the polar and nonpolar components of the ionic liquid. It was found that this segregation was enhanced for longer alkyl chains, with a corresponding increase in the length scale, whereas the presence of hydroxyl groups on the alkyl chains led to much less ordered liquids. The broad range of protic ionic liquids studied allowed several structure-property relationships to be established. The solvophobic effect was shown to be dependent on the nanostructure of the PILs. These PILs support amphiphile self-assembly, and it was shown that the less structured PILs had more “water-like” behavior in the diversity of lyotropic liquid-crystal phases supported, and the thermal stability ranges for these phases. Introduction In recent years, it has been established that many ionic liquids (ILs) are nanostructured, possessing populations of cations and anions that are ordered on specific length scales.1-3 This nanostructure typically occurs due to segregation of the ionic liquid into polar and nonpolar domains. Ionic liquid cations usually consist of an ionic and an alkyl component which, similar to amphiphile self-assembly, drives the segregation. Typically, the length scale of this order is around 5-15 Å, and hence can be described as intermediate range order. These alkyl chains are usually incorporated into the cations to impart asymmetry, which is required to obtain the low melting points characteristic of ionic liquids (Tm < 100 °C) through decreasing the packing efficiency of the ions.4 Ionic liquids are often referred to as “designer” or “tailorable” solvents, since the cations and anions can be selected to produce an IL with specific chemical or thermal properties. As researchers gain a better understanding of the nanostructure present within ILs, it is apparent that this nanostructure is an important property for governing their interaction with other solvents and solutes. The presence of large nonpolar domains greatly enhances the ability of an IL to dissolve nonpolar species.5 Consequently, it is important to understand the nanostructure present in ILs and the effect that changes to the chemical structure of the cation or anion have on the nanostructure. The presence of nanostructure within ILs, particularly those containing imidazolium-derived cations, has been investigated using theoretical simulations,6-11 and experimental techniques such as SWAXS3,12-15 and neutron scattering.16,17 It was established that a minimum alkyl chain length of n ) 3 was required to obtain intermediate range order for [CnMIm][NTf2].14 Similarly, an alkyl chain length of at least n ) 3 was required for the * To whom correspondence should be [email protected]. † CSIRO Molecular and Health Technologies. ‡ Australian Synchrotron. § CSIRO Materials Science and Engineering.

addressed.

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[(Cn)2MIm][NTf2] double-chained ILs to contain intermediate range order.15 The polar and nonpolar segregated domains within the structured ILs are the result of intermolecular organization. These are expected to have a localized order with significant defects in the structure. The effect of adding alkoxy groups into alkyl chains on imidazolium or quaternary cations was shown using molecular dynamics simulations to weaken the interactions present, but to still maintain nanoscale segregation.10,11 A subset of the ionic liquids which has been receiving significant interest in recent years are the protic ionic liquids (PILs).18 These are prepared through proton transfer from a Brønsted acid to a Brønsted base. The most frequently studied PIL is ethylammonium nitrate (EAN) which has been utilized for a wide range of applications.18 The small-angle neutron scattering (SANS) pattern of EAN and PAN showed that both PILs containing the ethyl- and propylammonium chains were nanostructured liquids. Structure peaks were observed corresponding to intermediate range order at q values of 0.66 and 0.54 Å-1, respectively.16 The liquid structure of EAN was also investigated by Umebayashi et al. using a combination of X-ray scattering over a wider angular range and molecular dynamics simulations.19 In addition to a peak at 0.62 Å-1 from the intermediate range order, they reported a more intense peak at 1.7 Å-1, then a smaller peak at 2.6 Å-1, followed by oscillations for the extent of their q range to 18 Å-1. The peaks within the radial distribution functions for the X-ray scattering and molecular dynamic simulations were assigned to intramolecular bonding and nonbonding atom-atom correlations. An ab initio molecular dynamics simulation of methylammonium nitrate (MAN) showed that there was a hydrogen-bonded network present, but no nanoscale segregation.20 A series of PILs containing the N-methyl-2-hydroxyethylammonium cation with various carboxylic acid anions was reported by Alvarez et al.21 The self-diffusion coefficients obtained from NMR diffusion-ordered spectroscopy were different for the cation and anions, except for the formate-containing PIL. They interpret this as due to the existence of nanoscale segregation,

10.1021/jp103863z  2010 American Chemical Society Published on Web 07/19/2010

Nanostructure of Protic Ionic Liquids perhaps as a lamellar or micellar structure. X-ray scattering data from the PIL containing the butyrate anion showed correlation peaks corresponding to sizes between 9 and 15.5 Å.21 Pott et al. investigated the nanostructure of a series of quaternary trialkylmethylammonium aprotic ILs (AILs) containing the NTf2 or Cl anions and alkyl chain lengths of 4, 6, or 8.13 These samples are chemically similar in structure to the primary ammonium PILs, EAN and PAN, in that they contain an ammonium cation, and do not contain heterocyclic cations like the imidazolium AILs. The SAXS profiles for the quaternary ammonium AILs investigated displayed three correlation peaks for q < 2 Å-1. The three peaks observed were attributed to intermediate range order for the peak between 0.41 and 0.63 Å-1, anion-anion interactions for the peak at ∼0.78 Å-1, and interactions between the cation aliphatic chains at ∼1.37 Å-1 in a direction perpendicular to the intermediate range order. The alkyl chains were found to be highly interdigitated, with all the systems having 75% overlap of the chains. This intermediate range order from intermolecular organization can be observed by X-ray and neutron scattering techniques in the q range 99%, Fluka Chemika), butylamine (>99%, BDH), pentylamine (>98%, Fluka Chemika), ethanolamine (>99%, Fluka Chemika), pentanolamine (95 wt %, Aldrich, solid, stored and used under argon), diethylamine (99.5%, Aldrich), diethanolamine (Merck, was distilled, fraction collected at 122 °C and 6.8 × 10-1 mmHg), triethylamine (45% solution in water, Fluka), triethanolamine (BDH, was distilled, fraction collected at 152 °C/2 × 10-1 mmHg), 2-MEA (>98%, MERCK), pyrrolidine (>99%, SigmaAldrich), and 2-pyrrolidinone (>99%, Sigma-Aldrich). The organic acids used were formic acid (98%, Ajax Chemicals) and glycolic acid (>99%, BDH). The inorganic acid used was nitric acid (69% in water, Merck). The reaction with strong inorganic acids is extremely aggressive and care should be taken. Anhydrous primary alcohols used were methanol (99.8%), ethanol (200 proof anhydrous 99.5%), propanol (99.7%), and butanol (99.8%) (alcohols all Sure/Seal, Sigma Aldrich). The protic ionic liquids ethylammonium nitrate (EAN), propylammonium nitrate (PAN), butylammonium nitrate (BAN), ethanolammonium nitrate (EOAN), 2-methoxyethylammonium nitrate (2MEAN), ethylammonium formate (EAF), butylammonium formate (BAF), pentylammonium formate (PeAF), ethanolammonium formate (EOAF), diethylammonium formate (DEAF), triethylammonium formate (TEAF), diethanolammonium formate (DEOAF), triethanolammonium formate (TEOAF), ethylammonium glycolate (EAG), pyrrolidinium nitrate (PyrrN) 2-pyrrolidinonium nitrate (PyrrON), and 2-pyrrolidinonium formate (PyrrOF) were prepared and dried according to previously reported methods.36 The chemical structure of the protic ionic liquids and their abbreviations are illustrated in Figure 1. The water contents were determined using Karl Fischer titration. Typically 1 mL of 20 w/w % solution of the ionic liquid in diluted anhydrous methanol was injected for analysis. Pentylammonium nitrate (PeAN), pentanolammonium nitrate (PeOAN), and pentanolammonium formate (PeOAF) were synthesized by an adaptation of the method reported for the synthesis of other PILs; equimolar amounts of the acid were added slowly while stirring the appropriate amine solution contained in a round-bottom flask over ice. The temperature

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Greaves et al. from the scattering profiles. The capillaries were nominally 2 mm diameter; however, there were some small deviations in size which may lead to changes in the intensities of a few percent. The SAXS and WAXS data were carefully combined to obtain profiles with the q range of 0.02-3 Å-1. The X-ray wavelength used was 1.033 Å. In order to investigate the radiation sensitivity of the samples, repeated measurements on the same sample and acquisitions with longer exposure times were taken. These scattering patterns were equivalent, showing that the samples did not undergo degradation due to radiation damage during the time scales used in these experiments. Results and Discussion

Figure 1. Chemical structures and abbreviations of the cation and anion components of the protic ionic liquids (PILs) investigated by SAXS/ WAXS.

during reaction was maintained below 0 °C. The PIL was dried in vacuo. The final product was hygroscopic and required vacuum freeze-drying prior to use. Small- and wide-angle X-ray scattering experiments (SWAXS) were performed on the SAXS/WAXS beamline at the Australian Synchrotron, Clayton, Australia.38 Samples were loaded into 2 mm capillaries and sealed with a silicon rubber to prevent water absorption. Samples were housed in a 60-sample capillary holder mounted on a 3-axis sample stage. The temperature of the sample holder was controlled using a Huber recirculating water bath. Scattering patterns were acquired at temperatures of 25 and 50 °C using an exposure time of 1 s. The SAXS q range was 0.02-0.62 Å-1, and the WAXS q range was from 0.54 to 3 Å-1. The contribution from an empty capillary was subtracted

The SAXS and WAXS profiles were obtained for 20 PILs and combined to form a single SWAXS profile for each PIL over the q range of 0.02-3 Å-1. The cations and anions used are shown in Figure 1. These PILs were carefully chosen to enable structureproperty relationships between the chemical structure of the PILs and their nanostructure to be explored. In the following sections the effect of increasing alkyl chain lengths, including hydroxyl or methoxy groups on the cation, having primary, secondary and tertiary ammonium cations, organic and inorganic anions, and pyrrolidinium cations, is discussed. The SWAXS profiles of the PILs contained up to three correlation peaks in the q range used. The peak positions were determined through fitting each profile to three Pearson VII line shapes, using the method of Pott et al.13 The data fitted in this manner can be found in the Supporting Information. While physically meaningless, these line shapes enable a simple and consistent method to determine the peak positions. Three peaks were required to fit all the data, and the peak positions are given in Table 1. Peak 3, at approximately 2.5 Å-1, was a very broad peak and was not assigned to a specific correlation distance. The q values for the peak positions of peaks 1 and 2 were converted into correlation distances using Bragg’s law, d ∼ 2π/ qmax. The profiles are shown in Figures 2-10. Profiles with the intensity normalized to that of peak 2 are also provided in the Supporting Information. For comparison, the SWAXS profiles of the primary alcohols methanol, ethanol, propanol, and butanol were also acquired

TABLE 1: Protic Ionic Liquids (PILs) with Their Water Content, Peak Positions, and Corresponding Distancesa PIL

water (wt %)

peak 1 (Å-1)

peak 2 (Å-1)

peak 3 (Å-1)

d1 (Å)

d2 (Å)

EAN PAN BAN PeANa EAF BAF PeAF EOAF EOAN PeOAF PeOAN DEAF TEAF DEOAF TEOAFa,e EAG 2MEANa PyrrOF PyrrNd PyrrON

0.33 0.39 1.00 1.40 4.15 0.66 1.13 0.43 0.81 0.45 0.94 1.10 0.21 1.39 1.41 5.00 0.43 1.67 1.94 1.47

0.70 0.54 0.45 0.39 0.75 0.48 0.41 b b 0.59c 0.57 1.02 1.15 1.15 c 1.12 0.75 0.57 b 0.67 b

1.68 1.63 1.58 1.55 1.71 1.60 1.57 1.70 1.69 1.57 1.57 1.74 1.79 1.73 1.79 1.62 1.67 1.54 1.52 1.56

2.57 2.47 2.58 2.59 2.80 2.73 2.79 2.68 2.57 2.76 2.64 2.91 3.28 2.81 3.15 2.62 d 2.47 2.55 d

9.03 11.71 14.02 15.94 8.37 13.17 15.22

3.73 3.87 3.97 4.05 3.67 3.94 4.01 3.69 3.73 4.00 3.99 3.61 3.50 3.63 3.51 3.87 3.76 4.09 4.14 4.03

11.04 6.14 5.49 5.63 8.41 11.02 9.35

a Data acquired at 50 °C, since sample solid at 25 °C. b No peak detectable. c Very weak peak intensity. d q range only to 2.3, hence peak 3 out of range. e TEOAF had only partly melted, and had overlapping sharp crystalline peaks, and broad liquid state peaks. a Values from profiles acquired at 25 °C unless otherwise stated.

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TABLE 2: Peak Positions and Corresponding Distances for Primary Alcoholsa peak 1 (Å-1) peak 2 (Å-1) peak 3 (Å-1) d1 (Å) d2 (Å) methanol ethanol propanol butanol a

1.01 0.76 0.65 0.59

1.75 1.56 1.47 1.45

2.97 2.98 2.77 3.01

6.20 8.22 9.60 10.68

3.59 4.02 4.26 4.32

Values from profiles acquired at 25 °C.

Figure 2. SWAXS profile for EAN, including the assignment of the three peaks.

TABLE 3: Estimated Cation Lengths for Ethyl-, Propyl-, Butyl-, and Pentylammonium Cations Using the Tanford Equation for the Alkyl Chain40 and a C-N Bond Length of 1.5 Å cation EA PA BA PeA

length (Å) 5.5 6.8 8.1 9.3

been reported for primary alcohols.24 Previously, schematic representations have been given by Triolo et al. for the structure of [CnMIm][PF6] ILs.39 These schematic representations provide a visualization aid for the structures, though note that without extensive modeling it is not possible to describe the specific structures contained in ILs. In the aforementioned representation, the cations are arranged similar to surfactants in micelles, with the alkyl chains segregated inside a roughly spherical shape (rodand disk-shaped aggregates are also legitimate structures for consideration) which is surrounded by the polar head groups and anions.39 The correlation distance, d1, obtained using Bragg’s law, is considered to give the distance between the polar head groups, and hence to give a measure of the domain size. This peak is not present in all the PILs, as can be seen from Table 1. The distance between the alkyl chains corresponds to d2 and is perpendicular to the intermediate range ordering. The most commonly used PIL, ethylammonium nitrate (EAN), has been used as an example to assign the peaks observed in the SWAXS profiles. The SWAXS profile obtained for EAN is displayed in Figure 2. The peak positions for peak 1 and peak 2 were 0.70 and 1.68 Å-1, respectively. These values compare favorably with the intermediate range order reported for EAN using SANS of 0.66 Å-1,16 and by X-ray scattering of 0.62 Å-1,19 and was the same as the value of 0.7 Å-1 obtained by Umebayashi et al. in their molecular dynamics simulation.19 Effect of Alkyl Chain Length of the Cation. The effect of increasing the alkyl chain length of the primary ammonium cation was studied for the PILs containing a nitrate or formate anion for ethyl, propyl, butyl, and pentyl chains. Propylammonium formate was not included in this investigation due to its higher melting point. The combined SWAXS profiles are shown in Figure 3. It is evident from Figure 3 that increasing the alkyl chain length of the PILs containing a primary ammonium cation causes an increase in the amount of ordering, as shown by the intensity increase of peak 1. It can also be observed from Figure

Figure 3. SWAXS profiles for the ethyl-, propyl-, butyl-, and pentylammonium cations with (a) the nitrate anion and (b) the formate anion.

and treated in the same manner as the PILs. The peak positions and corresponding distances for the primary alcohols are given in Table 2, and the profiles are shown in Figure 5. Assigning the Peaks. We anticipate that these PILs contain a variety of intermediate range ordered structures, such as has

Figure 4. Intermediate range order, d1, for the ethyl-, propyl-, butyl-, and pentylammonium formates and nitrates. The calculated comparison of 1 and 2 times the cation length is included as dotted and solid lines, respectively.

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Greaves et al. 3 that the intensity of the intermediate range order for the PILs containing the nitrate anion was significantly greater than for those containing the formate anion, relative to peak 2.

Figure 5. SWAXS profiles of primary alcohols.

In addition to the intensity change, there was also a shift of peak 1 to lower q values with increasing alkyl chain lengths. Each additional -CH2- group contributed 2.3 Å to the intermediate range order distance, as shown in Figure 4. For comparison, the cation length from the nitrogen atom to the end of the alkyl chain was approximated. The Tanford equation gives as estimate of the alkyl chain length for a fully extended chain of n carbon atoms as length (Å) ) 1.5 + 1.265n.40 This was added to the average C-N bond length of 1.5 Å, and the resulting lengths are given in Table 3. The intermediate range order is the distance between two polar groups, separated by the alkyl chain domain. Hence, it can be expected that the intermediate range order must be somewhere

Figure 6. SWAXS profiles showing the effect of hydroxyl groups on the intermediate range order for (a) EAN/EOAN, (b) EAF/EOAF, (c) PeAN/ PeOAN, (d) PeAF/PeOAF, (e) DEAF/DEOAF, and (f) TEAF/TEOAF.

Nanostructure of Protic Ionic Liquids between 1 and 2 times the estimated lengths, depending on the degree of interdigitation of the alkyl chains. Figure 4 shows the intermediate range order for the alkylammonium nitrate and formate series, along with 1 and 2 times the calculated cation length for comparison. It is suggested that there is some interdigitation of the alkyl chains for the PILs, or that the chains were not fully extended. A similar result has previously been reported for [CnMIm][PF6] ILs.39 Both the alkylammonium nitrates and formate PIL series plotted in Figure 4 showed that each additional -CH2- in the alkyl chain contributed 2.3 Å to the intermediate range order. This corresponds to a contribution of 1.15 Å from each alkyl chain since the intermediate range order is a measure over two cations. The relationship is nearly identical to what was reported by Triolo et al. for [CnMIm][Cl] ILs with n ) 4, 6, 8, and 10, and [CnMIm][BF4] with n ) 4, 6, and 8,3 and larger than the contribution to d1 of 1.96 Å per -CH2- which was seen for [CnMIm][NTf2] ILs,14 or of 2.1 Å per -CH2- which was seen for [CnMIm][PF6] ILs.39 Comparison to Primary Alcohols. The primary alcohols can be considered to be molecular solvent analogues of the primary alkylammonium PILs which were shown in Figure 3. The SAXS/WAXS profiles of the primary alcohols ethanol, propanol and butanol are shown in Figure 5, and the corresponding peak values in Table 2. There are notable differences and similarities between the PILs and alcohols in Figures 3 and 5, respectively. Peak 2 displays similar behavior with a shift to smaller q (larger distances) with increasing alkyl chain length. The most notable difference is in the intermediate range order, as shown by peak 1. In the PILs shown in Figure 3, on increasing the alkyl chain length there was a significant increase in the intensity of peak 1. In contrast, the alcohols show a slight decrease in the intensity of peak 1 with increasing alkyl chain length. The intermediate range order distance, d1, in the alcohols increases by 1.5 Å for each additional -CH2- group, compared to 2.3 Å in the PILs. This is consistent with the alcohols forming a highly interdigitated structure.39 Simulations of the structure of primary alcohols have shown that they form many different aggregate structures including linear, cyclic, and branched.24 Effect of Hydroxyl Groups on the Cation Chain. A number of the PILs were carefully selected to investigate the effect of hydroxyl substituents on alkylammonium cations on the nanostructure of the PILs. These were EAN and EOAN, EAF and EOAF, PeAN and PeOAN, PeAF and PeOAF, DEAF and DEOAF, and TEAF and TEOAF. For all of these PILs the presence of the hydroxyl groups caused the intermediate range order to virtually disappear. For the six examples shown in Figure 6, only PeOAN retained any appreciable intermediate range order. The data for TEOAF in Figure 6e was acquired at 50 °C; however, TEOAF was still a semicrystalline solid at this temperature, as shown by the sharp crystalline peaks as well as the broad liquid-state peaks. Effect of Methoxy Groups. The effect of methoxy groups within the alkyl chain was investigated by comparing the nanostructure observed within the PIL 2MEAN to that of EAN and BAN. EAN and BAN were chosen for comparison since the methoxy-containing chain for 2MEAN can either be treated as an ethyl chain with the methoxy substituent, or as a butyl chain with a methylene unit replaced with an oxygen. The profiles of 2MEAN, EAN, and BAN are shown in Figure 7. The intermediate range ordering was still present in 2MEAN, and it corresponded to a correlation distance between that seen in EAN and BAN. It is proposed that the methoxy group toward

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Figure 7. Comparison of SWAXS profiles of 2MEAN to EAN and BAN.

Figure 8. SWAXS profiles showing the effect on the nanostructure due to different anions for the PILs EAN, EAF, and EAG.

the end of the alkyl chain is causing the chains to become either more interdigitated, or effectively shorter due to the methoxy groups potentially having an affinity to reside close to the ammonium moiety. The presence of the methoxy group significantly reduced the intensity of the intermediate range order compared to EAN and BAN. However, this effect is not as large as the decrease in intermediate range order caused by the hydroxyl substituents, which can be seen by comparing the change in intensity for 2MEAN in Figure 7 to the profiles in Figure 6. Effect of Anion. The effect of different anions on the nanostructure of PILs was explored for the series EAN, EAF, and EAG, as shown in Figure 8. The intensity of the intermediate range order was significantly greater for EAN and EAG than for EAF, which was consistent for all the PILs containing the nitrate anion compared to the formate anion. It was apparent that the anions had an impact on the nanostructure, with formate anions disrupting the intermediate range order. The q region between 2 and 3 Å-1 in Figure 8 is representative of what was seen for all the PILs containing the nitrate and formate anions, with a third peak present for the nitrates, and no peak for the formate PILs. Further experiments or simulations would be required to determine the source and nature of the specific changes observed. Primary, Secondary, and Tertiary Ammonium Cations. The nanostructure within a series of PILs containing an increasing number of alkyl substitutents on the ammonium cation was investigated for the series of EAF, DEAF, and TEAF

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Greaves et al.

Figure 10. SWAXS profiles for the PILs with cations containing pyrrolidinium groups.

Figure 9. SWAXS profiles showing changes to the nanostructure due to different numbers of alkyl chains substituted onto the ammonium cation for (a) EAF, DEAF, and TEAF and (b) EOAF, DEOAF, and TEOAF.

as well as EOAF, DEOAF, and TEOAF. The SWAXS profiles are shown in Figure 9 for these two series. For the EAF, DEAF, and TEAF series, shown in Figure 9a, increasing the number of alkyl chains leads to a greater intensity of the intermediate range order peak (peak 1), as well as peak 1 shifting to higher q values, which corresponds to smaller distances for the intermediate range order. The intermediate range order distances for EAF, DEAF, and TEAF were 8.37, 6.14, and 5.49 Å, respectively. One possible explanation is that DEAF and TEAF are forming structures with highly interdigitated chains. Using the length of an ethyl chain on the ammonium cation of 5.12 Å as a guide, the TEAF chains must be highly interdigitated to fit the ethyl chains from two cations between the nitrogen atoms over these short distances. In addition, the spacing between the alkyl chains was also observed to change, with peak 2 shifting to smaller q values with increasing numbers of alkyl chains, corresponding to increased distances of 1 Å for each additional chain. The series of EOAF, DEOAF, and TEOAF shown in Figure 9b displayed a similar trend. The increasing overlap for the di- and trialkylammonium PILs is consistent with what was reported by Pott et al.13 who reported that AILs containing tertiary ammonium cations with long alkyl chains were highly interdigitated with 75% overlap of the alkyl chains. Analogous behavior was reported for the double chained [(Cn)2MIm][NTf2] compared to the conventional single-chained [CnMIm][NTf2], with an increase in intensity and shift to higher q of the peak between 0.3 and 0.6 Å-1.15

Cyclic Ammonium Cations. Three PILs containing cyclic ammonium cations were studied, as shown in Figure 10. In comparison to the alkylammonium PILs, these all displayed no or little intermediate range order, with only PyrrN having any appreciable intermediate range order present. The presence of no, or little, intermediate range order is consistent with these cations not having clearly defined polar and nonpolar regions which can behave as small amphiphiles. We anticipate these would have analogous behavior to the imidazolium cations, where N-alkyl chains off the ring of at least three carbons are required to lead to the formation of polar and nonpolar domains.14 Effect of Temperature. The SAXS and WAXS profiles were taken for all of these PILs at 25 and at 50 °C (except for the PILs which were solid at room temperature). No change was observed for peak 1 on heating from 25 to 50 °C. Peak 2 showed a slight decrease in q with increased temperature, corresponding to a larger distance between the alkyl chains of 1 Å, and this was consistent across all the PILs studied. Comparison of the Aggregation Behavior Observed with Other Techniques. ESI-MS of neat PILs and of solutions of PILs in different solvents has shown that there is generally a dominant aggregate size for each PIL. As described in the Introduction, EAN favors a C8A7+ aggregate in the positive mode,22 which has been modeled and proposed to consist of two cubes joined by a shared anion.23 There will be some inherent differences between the aggregates observed using the gas-phase technique of ESI-MS compared to SWAXS which is a bulk-liquid-phase technique. In the positive mode ESI-MS studies, the PILs series of EAN, PAN, BAN, and PeAN all favored the C8A7+ aggregate, consistent with the strong peak present in the SWAXS representative of intermediate range order. This is further evidence that the intermediate range order present in this series of PILs is similar, with the additional -CH2- groups simply expanding the structure. In comparison, the presence of additional alkyl chains for the series of EAN, DEAN, and TEAN saw the observation of dominant aggregates of C8A7+, C3A2+, and C2H+, respectively, by ESI-MS. While this series was not studied using SWAXS, the closely related series of EAF, DEAF, and TEAF showed a reduction in size with increasing numbers of alkyl chains on the ammonium cation. Therefore, the structure observed by SAXS is consistent with the decreasing size of the clusters observed by ESI-MS.

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TABLE 4: Physicochemical and Thermal Properties Reported for PILs, Including Density (G), Viscosity (η), Conductivity (K), Molar Volume (Vm), Glass Transition (Tg), Melting Point (Tm), and Boiling Point (Tb)a PIL

F (g/cm3)

η (mPa · s)

κ (mS/cm)

Vm (cm3/mol)

Tg (°C)

Tm (°C)

EAN EAF BAF PeAF EOAF EOANb DEAF TEAF DEOAF TEOAFc EAG 2MEANc

1.216 1.039 0.968 0.95 1.184 1.265 1.039 1.028 0.988

32 32 70 78 220 113 5.4 5.8 494

26.9 12.16 3.1 1.53 3.4 9.35 13.13 13.05 0.77

97.99 87.69 123.1 133.19 90.47 105.29 120.37 143.21 153

0.9

88.42

a -106 -95 -93 -85 -82 -109 -119 -78 a -67 a

9 -15 2 12 a 51 4 a a 67 a 37

1.189

1200

Tb (°C) 180 179 179 192 126 202 182 197 130

a None detected. b EOAN can exist as a metastable liquid at room temperature despite the higher melting point. c Melting point above RT, and hence physicochemical properties not measured. a Data compiled from Greaves et al.36,37

The ESI-MS of the hydroxyl-containing PILs DEOAN and TEOAN did not indicate the presence of aggregates; only the dimer and monomer ions were observed of C2H+ and C+, respectively. The similar PILs DEOAF and TEOAF as studied using SWAXS showed virtually no intermediate range order present. In contrast, the methoxy-containing PIL of 2MEAN had a favored ESI-MS aggregate of C2H+ which was not consistent with the intermediate range order observed in the SWAXS of 11.02 Å. This anomalous behavior for 2MEAN may be related to the weak intermediate range order structure, as observed from the small peak in the SWAXS, such that aggregates are not stable in the gas phase and are not observed during the ESI-MS experiments. An alternative way that we have previously used to estimate the aggregation is based on the method reported by Kohler et al.,41,42 based on the comparison of the viscosity as estimated using the molar mass and the measured viscosity of the sample. The aggregation numbers estimated using this method have been previously reported for many of these PILs,37 and from the data contained within Greaves et al.36 The aggregation numbers varied between 3.6 and 6.2 for these PILs. There was not particularly good agreement between the aggregation numbers and the intermediate range order from SWAXS, which is probably related to other factors affecting the viscosity, such as the distribution of size and shapes of species and the interaction energies between species in the liquid. According to the viscosity-derived aggregation numbers, the aggregate sizes would decrease when the number of -CH2- groups in EAF, BAF, and PeAF is increased, which is not consistent with the SWAXS or ESI-MS data. There were slight decreases in the aggregate sizes for the hydroxyl-containing PILs, and decreases for increasing the number of alkyl chains for the series of EAF, DEAF, and TEAF. Comparison with Physicochemical Properties and Thermal Phase Behavior. The physicochemical properties and thermal phase behavior for many of these neat PILs which are liquid at room temperature have previously been reported,36,37 and are reproduced in Table 4, including their glass transition temperature, melting point, boiling point, density, viscosity, ionic conductivity, and the molar volume (which reflects the ion-ion spacing in the ILs43). The intensity of peak 1 in the SWAXS profiles provides information about how well structured the intermediate range order is. A weak peak or shoulder corresponds to a poorer structure with many defects present, and order only present over short distances. Intense peaks correspond to a more structured liquid, though still not a liquid-crystal phase. In this section, we have compared the intensity of peak 1 from

the SWAXS data with the physicochemical properties of these PILs. Previously, it was reported by Pott et al. that increasing the alkyl chain length for the trialkylmethylammonium AILs led to lower densities.13 The SWAXS data for EAF, BAF, and PeAF are shown in Figure 3b, and clearly the intensity increased significantly with increasing alkyl chain length, and hence the nanostructure is becoming more organized. The physicochemical properties for these three PILs correlates well in that increasing the alkyl chain length led to increased molar volumes, which suggests greater ion-ion spacings. The viscosity increased and ionic conductivity decreased with increasing alkyl chain length, indicating the ions are less freely able to move. The effect of increasing the number of alkyl chains on the ammonium cation was explored for the series EAF, DEAF, and TEAF, and EOAF and DEOAF, with the SWAXS data shown in Figure 9. The molar volume increased significantly with increased number of alkyl chains. The molar volume can be considered as giving an indication of the ion-ion spacing, and hence this suggests that as the order increased the average ion-ion spacing increased. There was little change in the density with increasing numbers of alkyl chains. The viscosity and ionic conductivity did not show consistent trends with the changes to the intermediate range order, perhaps reflecting the greater complexity of varying the degree of interdigitation of the chains. The effect that hydroxyl groups on the alkyl chains had on the physicochemical properties could not be directly related to changes in the intermediate range order. This was probably because of the additional hydrogen bonding possible for these hydroxyl-containing PILs, which is reflected in the significant increase in the viscosity. There was insufficient data to observe trends due to changes in the anion. The glass transition and melting point of these PILs are given in Table 4. There is a suggestion within the information that PILs with little intermediate range order are more resistant to crystallization and may be less likely to have a melting point. It was also observed that the glass transition temperature increased with increasing alkyl chain length, and increased due to the presence of hydroxyls. PILs Intermediate Range Order and Their Ability To Serve As Amphiphile Self-Assembly media. Previously, we reported on the self-assembly of CTAB (hexadecyltrimethylammonium bromide), Myverol 18-99K (main component is monoolein), and phytantriol in many of these PILs.35,37,44 There were systematic variations between the chemical structure of the PILs and the number of lyotropic liquid-crystal phases supported, and the temperature ranges for phase stability.

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There is a strong link between lyotropic liquid-crystal phase formation in the PILs and the intermediate range order present. The highly nanostructured alkylammonium PILs such as EAF, BAF, and PeAF generally supported only one liquid-crystal phase (either lamellar or inverse hexagonal). The PILs containing hydroxyl groups (and hence no intermediate range order) generally supported more liquid-crystal phases, and were more “water-like” in the phases they supported. The PILs containing the methoxy group in 2MEAN and 2MEAF (some intermediate range order present) did not show more liquid-crystal phases than their alkyl-chained counterparts. The series of EOAN, DEOAN, and TEOAN with CTAB showed that as the number of alcohol chains increased the same lyotropic liquid-crystal phases were retained. A significant driving force for amphiphile self-assembly is the negative contribution to the free energy on moving amphiphile hydrocarbon chains to within micelles. Consequently, solvents with lower hydrocarbon solubilities will have a greater driving force, which may manifest as lower critical micelle concentrations (cmc). It was reported by Evans et al. that the hydrocarbon solubility in EAN is higher than in water,45 and higher cmc’s were seen as a consequence.46 The hydrocarbon solubility in PILs will be highly dependent on the intermediate range order. The presence of nonpolar domains will significantly increase the solubility of hydrocarbons and lower the free energy for amphiphile self-assembly. It was evident from comparing the nanostructure of the PILs with their self-assembly behavior that the PILs containing little intermediate range order generally corresponded to richer self-assembly media, as shown by greater liquid-crystal phase diversity, and greater temperature ranges for phase stability. It is anticipated that the PILs with negligible intermediate range order will lead to much lower cmc’s than their structured counterparts. It should be noted that the hydrocarbon solubility is not the only governing factor for micellization, and other properties such as having high cohesive energy densities are desirable to promote amphiphile selfassembly. Structure-Property Relationships. From this series of 20 PILs, we have identified the effect that simple changes to the chemical structure have on the nanostructure of the PILs using synchrotron SWAXS. The intermediate range order length scale can be increased through increasing the alkyl chain length while maintaining little interdigitation of the alkyl chains, such as was observed for the series EAN, PAN, BAN, and PeAN. Alternatively, the intermediate range order length scale can be decreased by increasing the number of alkyl chains on the ammonium cation through increasing the interdigitation of the alkyl chains, such as for the series EAF, DEAF, and TEAF. The extent of structure present in the PILs, i.e., the intensity of the intermediate range order, can be modified through changing the anion, with nitrates and glycolate anions leading to significantly greater structure than formates. Conversely, the structure can be effectively removed by the presence of hydroxyl groups on the end of cation alkyl chains. The presence of methoxy groups within the cation alkyl chain leads to significantly less intermediate range structure than simple alkyl chains, but more than is seen for chains with hydroxyls. The pyrrolidinium and 2-pyrrolidinonium cations displayed negligible intermediate range order present with either the formate or nitrate anions. Conclusion Structure-property relationships between the chemical structure of 20 different PILs and their nanostructure were investigated using SWAXS. This is the first report of the nanostructure

Greaves et al. for 18 of these PILs. Many of these PILs contain intermediate range order with segregation of polar and nonpolar groups. The determined structure-property relationships show that the cations and anions in these PILs can be tailored to achieve specific intermediate range order length scales, modified to increase or decrease the intensity of the ordering, or designed to have no intermediate range order present. The presence of intermediate range order was shown to have a significant effect on their ability to be used as amphiphile self-assembly media. PILs with less structure present provided more ‘water-like’ properties in the diversity of lyotropic liquid-crystal phases supported and the thermal stability ranges of those phases. Acknowledgment. C.J.D. is the recipient of an Australian Research Council (ARC) Federation Fellowship. This work was also partly supported by an ARC Discovery Project grant, DP0666961. Supporting Information Available: The fitted SWAXS profiles to three Pearson VII line shapes are available. The SWAXS patterns shown in the figures are also provided normalized to peak 2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Weingaertner, H. Angew. Chem., Int. Ed. 2008, 47, 654. (2) Lopes, J.; Padua, A. A. H. J. Phys. Chem. B 2006, 110, 3330. (3) Triolo, A.; Russina, O.; Bleif, H. J.; Di Cola, E. J. Phys. Chem. B 2007, 111, 4641. (4) Tokuda, H.; Ishii, K.; Susan, M.; Tsuzuki, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833. (5) Triolo, A.; Russina, O.; Keiderling, U.; Kohlbrecher, J. J. Phys. Chem. B 2006, 110, 1513. (6) Bhargava, B. L.; Balasubramanian, S.; Klein, M. L. Chem. Commun. 2008, 3339. (7) Wang, Y.; Jiang, W.; Yan, T.; Voth, G. A. Acc. Chem. Res. 2007, 40, 1193. (8) Chiappe, C. Monatsh. Chem. 2007, 138, 1035. (9) Maginn, E. J. J. Phys.: Condens. Matter 2009, 21. (10) Siqueira, L. J. A.; Ribeiro, M. C. C. J. Phys. Chem. B 2009, 113, 1074. (11) Smith, G. D.; Borodin, O.; Li, L. Y.; Kim, H.; Liu, Q.; Bara, J. E.; Gin, D. L.; Nobel, R. Phys. Chem. Chem. Phys. 2008, 10, 6301. (12) Gontrani, L.; Russina, O.; Lo Celso, F.; Caminiti, R.; Annat, G.; Triolo, A. J. Phys. Chem. B 2009, 113, 9235. (13) Pott, T.; Meleard, P. Phys. Chem. Chem. Phys. 2009, 11, 5469. (14) Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R.; Xiao, D.; Hines, L. G.; Bartsch, R. A.; Quitevis, E. L.; Pleckhova, N.; Seddon, K. R. J. Phys: Condens. Matter 2009, 21. (15) Xiao, D.; Hines, L. G.; Li, S. F.; Bartsch, R. A.; Quitevis, E. L.; Russina, O.; Triolo, A. J. Phys. Chem. B 2009, 113, 6426. (16) Atkin, R.; Warr, G. G. J. Phys. Chem. B 2008, 112, 4164. (17) Hardacre, C.; Holbrey, J. D.; Mullan, C. L.; Nieuwenhuyzen, M.; Youngs, T. G. A.; Bowron, D. T. J. Phys. Chem. B 2008, 112, 8049. (18) Greaves, T. L.; Drummond, C. J. Chem. ReV. 2008, 108, 206. (19) Umebayashi, Y.; Chung, W.-L.; Mitsugi, T.; Fukuda, S.; Takeuchi, M.; Fujii, K.; Takamuku, T.; Kanzaki, R.; Ishiguro, S. J. Comput. Chem. Jpn. 2008, 7, 125. (20) Zahn, S.; Thar, J.; Kirchner, B. J. Chem. Phys. 2010, 132, 124506. (21) Alvarez, V. H.; Dosil, N.; Gonzalez-Cabaleiro, R.; Mattedi, S.; Martin-Pastor, M.; Iglesias, M.; Navaza, J. M. J. Chem. Eng. Data 2010, 55, 625. (22) Kennedy, D. F.; Drummond, C. J. J. Phys. Chem. B 2009, 113, 5690. (23) Ludwig, R. J. Phys. Chem. B 2009, 113, 15419. (24) Tomsic, M.; Jamnik, A.; Fritz-Popovski, G.; Glatter, O.; Vlcek, L. J. Phys. Chem. B 2007, 111, 1738. (25) Hayes, R.; Warr, G. G.; Atkin, R. Phys. Chem. Chem. Phys. 2010, 12, 1709. (26) Greaves, T. L.; Drummond, C. J. Chem. Soc. ReV. 2008, 37, 1709. (27) Fisicaro, E.; Compari, C.; Braibanti, A. Phys. Chem. Chem. Phys. 2004, 6, 4156. (28) Maibaum, L.; Dinner, A. R.; Chandler, D. J. Phys. Chem. B 2004, 108, 6778. (29) Moya, M. L.; Rodriguez, A.; Graciani, M. D.; Fernandez, G. J. Colloid Interface Sci. 2007, 316, 787.

Nanostructure of Protic Ionic Liquids (30) Ruelle, P.; Kesselring, U. W. J. Pharm. Sci. 1998, 87, 987. (31) Southall, N. T.; Dill, K. A.; Haymet, A. D. J. J. Phys. Chem. B 2002, 106, 521. (32) Widom, B.; Bhimalapuram, P.; Koga, K. Phys. Chem. Chem. Phys. 2003, 5, 3085. (33) Beesley, A. H.; Evans, D. F.; Laughlin, R. G. J. Phys. Chem. 1988, 92, 791. (34) Evans, D. F. Langmuir 1988, 4, 3. (35) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. J. Phys. Chem. B 2007, 111, 4082. (36) Greaves, T. L.; Weerawardena, A.; Fong, C.; Krodkiewska, I.; Drummond, C. J. J. Phys. Chem. B 2006, 110, 22479. (37) Greaves, T. L.; Weerawardena, A.; Krodkiewska, I.; Drummond, C. J. J. Phys. Chem. B 2008, 112, 896. (38) Kirby, N. B., J. W.; Gentle, I.; Cookson, D. In Synchrotron Radiation Instrumentation, Parts 1 and 2; Choi, J. Y., Rah, S., Eds.; American Institute of Physics: Melville, New York, 2007; Vol. 879, p 887.

J. Phys. Chem. B, Vol. 114, No. 31, 2010 10031 (39) Triolo, A.; Russina, O.; Fazio, B.; Triolo, R.; Di Cola, E. Chem. Phys. Lett. 2008, 457, 362. (40) Tanford, C. The Hydrophobic Effect; 2nd ed.; John Wiley: New York, 1980. (41) Kohler, F.; Atrops, H.; Kalall, H.; Liebermann, E.; Wilhelm, E.; Ratkovics, F.; Salamon, T. J. Phys. Chem. 1981, 85, 2520. (42) Kohler, F.; Gopal, R.; Gotze, G.; Atrops, H.; Demeriz, M. A.; Liebermann, E.; Wilhelm, E.; Ratkovics, F.; Palagyl, B. J. Phys. Chem. 1981, 85, 2524. (43) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B 2003, 107, 6170. (44) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. Langmuir 2007, 23, 402. (45) Evans, D. F.; Chen, S.-H. J. Am. Chem. Soc. 1981, 103, 481. (46) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89.

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