Viscosity of Ionic LiquidâIonic Liquid Mixtures - Journal of Chemical

Article pubs.acs.org/jced

Viscosity of Ionic Liquid−Ionic Liquid Mixtures Joseph J. Fillion and Joan F. Brennecke* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The viscosities of 23 ionic liquid (IL)−IL mixtures were measured at multiple temperatures and mole fractions. Many of the mixtures involved a dialkylimidazolium bis(trifluoromethylsulfonyl)imide ([Tf2N]) IL paired with a tetra-alkylphosphonium IL containing an aprotic heterocyclic anion (AHA). As a result, most of the mixtures contained both unlike anions and cations. The mixtures ranged from strongly positive deviations from the Arrhenius model (the mole fraction weighted logarithm of the viscosity), even having mixture viscosities greater than the viscosity of either pure IL, to negative deviations. Studies of complementary pairs ([C1][A1] + [C2][A2] and [C2][A1] + [C1][A2] mixtures) have identical viscosities at overall mole fractions of 0.50, clearly indicating that the anions and cations of the mixtures exchange freely. These mixtures cannot be wellrepresented by the Arrhenius model assuming a random distribution of the four possible cation/anion combinations (e.g., 0.25 mole fraction of each possibility at an overall IL mole fraction of 0.50). Rather, if one assumes preferential formation of some cation/anion pairs to match the experimental viscosity data at a 0.50 IL mole fraction, a much better representation of the data can be obtained. However, additional IL−IL interactions are needed to explain maxima in the experimental viscosity data. Excess molar volumes are very small for all of the systems measured.

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because the AlCl3 is converted to [AlCl4]− and [Al2Cl7]−, with the speciation determined by the overall composition of the mixture.23 This mixture shows the melting point depression. Pure 1-ethyl-3-methylimidazolium chloride ([emim][Cl]) has a melting point of 357 K. With the addition of AlCl3, some overall compositions have melting points lower than 273 K, while other do not have a melting point at all, but a glass transition temperature instead.24,25 A mixture of 1,4-dimethyl-l,2,4triazolium chloride ([mm 4-Triz][Cl]) with AlCl3 is another mixture that shows this behavior.26 Many IL−IL mixtures have melting points that are lower than either of the pure ILs or the melting point is eliminated entirely, being replaced by a glass transition temperature.27−35 For instance, mixtures of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim][TfO]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [emim][Tf2N]) do not have melting points, even though the pure ILs have melting points of 258 and 255 K.33 Mixtures of 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and 1-ethyl-2-methylpyrazolium tetrafluoroborate ([empyra][BF4]) have lower melting points than either [emim][BF4] or [empyra][BF4].34 A reduction in the melting point or an elimination of the melting point increases the temperature range where the ionic liquid (mixture) can be used as a liquid. One common motivation for mixing ILs is to increase the ionic conductivity. If mixing the two ILs results in the reduction of the melting point, then the increases in ionic conductivity can be

INTRODUCTION There are many reasons why one might be interested in mixtures of ionic liquids (ILs). The most obvious is to combine the desirable properties of two different ILs, if one cannot identify a single IL that satisfies all requirements. Another is the possibility of forming eutectic mixtures, where the melting point of the mixture can be lower than the melting point of either pure IL. Uses for IL−IL mixtures previously investigated include as electrolytes in dye-sensitized solar cells1 and in liquid−liquid extraction, which take advantage of the properties of both of the pure ILs to enhance their properties.2,3 Key properties that have been measured for IL mixtures are conductivity and viscosity, and these are discussed below. In addition, a few groups have measured other properties of IL−IL mixtures, including the distribution ratio of Pd(III),4 liquid−liquid equilibrium,5,6 gas chromatographic stationary phase separation ability,7 density and excess molar volume,8,9 surface tension,10−12 CO2 solubility and Henry’s law constants,13−17 optical Kerr effect,18 and Tonset.12 There have also been some molecular simulations of IL mixtures.19,20 Two reviews have appeared that discuss IL mixtures in detail.21,22 An important concept in considering IL−IL mixtures is whether to think of them as binary or multicomponent solutions. A mixture of two ILs with a common anion ([C1][A] + [C2][A]) or a common cation ([C][A1] + [C][A2]) may appear to be binary mixtures. However, a mixture of two ILs with different cations and different anions ([C1][A1] + [C2][A2]) is likely better envisioned as multicomponent mixtures since the ions are free to exchange ([C1][A1], [C1][A2], [C2][A1], and [C2][A2]). 1,3-Dialkylimidazolide chloride mixed with AlCl3 can be thought of as one of the first IL−IL mixtures investigated, © XXXX American Chemical Society

Received: February 26, 2017 Accepted: April 25, 2017

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DOI: 10.1021/acs.jced.7b00221 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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dramatic since the ionic conductivities of liquids are significantly higher than the ionic conductivities of solids. The [emim][BF4]/ [empyra][BF4] mixture mentioned above is an example.34 In general, the ionic conductivity of mixtures of liquid ILs tend to be between the conductivities of the corresponding pure ILs.32,35−38 Nonetheless, it is not uncommon for some mixture compositions to have ionic conductivities slightly higher than either of the pure ILs.27,39,40 An extreme example is the ionic conductivity of [emim][TfO]/[emim][Tf2N] mixtures at temperatures where both pure ILs are liquid. They are greater than the conductivity of either [emim][TfO] or [emim][Tf2N]. This has been explained as a decrease in ion association.33 Likewise, the molar conductivities of mixtures of N-butyl-N-methylpyrrolidinium bis(pentafluoroethanesulfonyl)imide ([bmpyrro][BETI]) and N-butyl-N-methylpyrrolidinium (trifluoromethanesulfonyl) (nonafluorobutanesulfonyl)imide ([bmpyrro][IM14]) are greater than either of the pure ILs.38 Conversely, the conductivities of [emim][Tf2N]/[emim][BF4] mixtures are lower than the conductivity of either of the pure ILs.36 Another common measurement made for IL−IL mixtures is the viscosity. Of course, viscosity and ionic conductivity are related. Ionic conductivity should be roughly proportional to ion diffusivity and diffusivity should be inversely proportional to the viscosity, according to the Stokes−Einstein equation. One typical class of IL−IL mixtures uses a common cation. Examples of such mixtures are [bmpyrro][Tf2N]/[bmpyrro][IM14],28 [bpyr][BF 4 ]/[bpyr][Tf 2 N], 41 [bmim][BF 4 ]/[bmim][MeSO 4 ], 42 [bmim][BF4]/[bmim][PF6],42 [emim][BF4]/[emim][DCA],37 [emim][Tf 2 N]/[emim][EtSO 4 ], 10 [bmim][BF 4 ]/[bmim][PF 6],43 [hmim][BF 4]/[hmim][PF 6],43 [emim][acetate]/ [emim][Tf 2 N], 44 [bmpyrro][Tf 2 N]/[bmpyrro][IM 14 ], 38 [bmpyrro][Tf 2 N]/[bmpyrro][BETI], 38 and [bmpyrro][BETI]/[bmpyrro][IM14].38 The IL abbreviations are defined in Table 1. The viscosities of all of the above mixtures are between the viscosities of the corresponding pure ILs. However, for the [emim][Tf2N]/[emim][acetate] mixture, the viscosity does not decrease as much as expected when a small amount of [emim][Tf2N] is added to [emim][acetate]. The explanation was that the electrostatic interactions are quite different, leading to acetate ions forming complexes with [emim]+ in greater than 1:1 stoichiometries.44 The four most common anions used are [BF4]−, [Tf2N]−, [PF6]−, and [IM14]−, and the typical cations are dialkylpyrrolidium, alkylpyridinium, and dialkylimidazolium. Another typical class of IL−IL mixtures use a common anion. Examples of such mixtures are [hmim][BF4]/[emim][BF4],42 [hmim][BF4]/[bmim][BF4],42 [emim][BF4]/[pmim][BF4],45 [pmim][BF4]/[hmim][BF4],45 [emim][BF4]/[hmim][BF4],45 [P 66614 ][Tf 2 N]/[pmpyrro][Tf 2 N], 29 [mmpyrro][Tf 2 N]/ [bmpyrro][Tf 2 N], 32 and [empyrro][Tf 2 N]/[pmpyrro][Tf2N].32 The above mixtures have viscosities between the viscosities of the corresponding pure ILs. The typical anions are [BF4]− or [Tf2N]−, paired with dialkylpyrrolidium and dialkylimidazolium cations. However, for the [P66614][Tf2N]/ [pmpyrro][Tf2N] mixture, the viscosity does not decrease as much as expected when a small amount of [pmpyrro][Tf2N] is added to [P66614][Tf2N].29 Chatel et al. found that there were 102 different double salt ionic liquids (called IL−IL mixtures herein) reported as of 2014, with 42% involving mixtures with the same cation and two different anions and another 42% involving mixtures with the same anion but two different cations. The remaining 16% involve mixtures with more than three ions.22 It is possible that some of these mixtures involve one cation and three different anions, like

Table 1. Full Name and Abbreviation for Ionic Liquid Cations and Anions cation name

cation abbreviation

trihexyl(tetradecyl)phosphonium triethyl(decyl)phosphonium octyltriethylphosphonium heptyltriethylphosphonium triethyl((2-methoxyethoxy)methyl)phosphonium 1-hexyl-2-methyl-3-methylimidazolium 1-hexyl-3-methylimidazolium 1-butyl-3-methylimidazolium 1-propyl-3-methylimidazolium 1-ethyl-3-methylimidazolium 1-ethyl-2-methylpyrazolium N-butyl-N-methylpyrrolidinium N-propyl-N-methylpyrrolidinium N-ethyl-N-methylpyrrolidinium N-methyl-N-methylpyrrolidinium N-butylpyridinium anion name acetate 1,2,3-triazolide 1,2,4-triazolide 2-(cyano)pyrrolide 4-nitropyrazolide tetrazolide bis(trifluoromethylsulfonyl)imide chloride 1,4-dimethyl-l,2,4-triazolium trifluoromethanesulfonate tetrafluoroborate bis(pentafluoroethanesulfonyl) imide (trifluoromethanesulfonyl) (nonafluorobutanesulfonyl) imide methylsulfate hexafluorophosphate dicyanamide ethylsulfate

[P66614] [P22210] [P2228] [P2227] [P222(1O2O1)] [hmmim] [hmim] [bmim] [pmim] [emim] [empyra] [bmpyrro] [pmpyrro] [empyrro] [mmpyrro] [bpyr] anion abbreviation [acetate] [3-Triz] [4-Triz] [2-CNpyr] [4-NO2pyra] [tetrazolide] [Tf2N] [Cl] [mm 4-Triz] [TfO] [BF4] [BETI] [IM14] [MeSO4] [PF6] [DCA] [EtSO4]

[bmpyrro][Tf2N] + [bmpyrro][IM14] + [bmpyrro][BETI].38 However, the most common mixtures with more than three ions are IL−IL mixtures where both the cations and anions are different. Examples of such mixtures are [emim][Tf2N]/ [beim][EtSO4],10 [emim][CH3CO2]/[dmim]Cl,46 and [bpyr][BF4]/[4bmpyr][Tf2N].47 The above mixture have cations that differ only by the length of the alkyl chain. Alternatively, the variation may involve the addition of an alkyl group to the ring, as is the case for [bpyr]+ and [4bmpyr]+. The above mixtures have viscosities between the viscosities of the corresponding pure ILs. An example where one of the compositions measured had a viscosity that was lower than either of the pure ILs is [emim][TfO]/[pmpyrro][Tf2N].40 Researchers have also dissolved inorganic salt in ILs,48 but it is impossible to know if the viscosity of the mixture is between the viscosity of the two pure salts since the inorganic salt is solid at the temperatures investigated. The majority of the IL−IL mixtures presented here are ones where both the cations and anions are different. We focus on phosphonium and imidazolium cations and [Tf2N]− and aprotic heterocyclic anions (AHAs), which we have developed for postcombustion CO2 capture and cofluid vapor-compression refrigeration applications.49−51 All of the viscosity measurements B



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for the IL−IL mixtures are fit with the equation proposed by Grunberg and Nissan, eq 1,52 which modifies the simple model proposed by Arrhenius53 with an additional term (x1x2G). log(μm ) = x1 log μ1 + x 2 log μ2 + x1x 2G

temperatures. As mentioned above, the viscosity of all of the pure ILs as a function of temperature have been reported previously.49 An example of the results for the pure ILs is shown in Figure 1,

(1)

Previously, we have shown that the G value is dependent on temperature50 for mixtures of ionic liquids with tetraglyme. For those IL/tetraglyme mixtures, if G was negative (meaning a mixture viscosity lower than predicted from the Arrhenius model), G tended to increase with increasing temperature, whereas if G was a high positive number (mixture viscosities higher than predicted from the Arrhenius model), G tended to decrease with increasing temperature.50 In addition, the G value seemed to be dependent on the molecular weight of the ionic liquid, where mixtures of tetraglyme with higher molecular weight ionic liquids (typically, [P66614]+) gave higher G values.50 The goal of this manuscript is to investigate the viscosity of IL− IL mixtures, especially those containing anions and cations that are both different, analyzing the results in terms of the G value obtained from fitting the data to eq 1. In addition, we provide a more molecular analysis in terms of a random or nonrandom distribution of the four possible cation−anion combinations that could possibly exist in the mixtures. The physical properties, including viscosities, of most of the pure ILs have been published previously.49

Figure 1. Viscosity of [hmmim][4-NO2pyra], red ■; [P66614][4NO2pyra], green ◆; [hmim][4-NO2pyra], blue ●; and [P2228][4NO2pyra], yellow ▲.49

which compares the viscosity of four pure ILs with the [4NO2pyra]− anion. The tetra-alkylphosphonium IL with the longer alkyl chains ([P66614][4-NO2pyra]) is more viscous than the one with shorter chains ([P2228][4-NO2pyra]) due to increased van der Waals interactions. The imidazolium IL with the extra methyl group at the C2 position ([hmmim] [4NO2pyra]) is more viscous than [hmim][4-NO2pyra]. More importantly, the imidazolium [AHA] ILs have a much stronger temperature dependence than the tetra-alkylphosphonium [AHA] ILs, with viscosities increasing rapidly at lower temperatures. Similar results were observed for ILs with [4Triz]−, [2-CNpyr]−, [tetrazolide]−, and [acetate]− anions, as shown in the Supporting Information, Figure S1a−c. However, the temperature dependence of [hmmim][Tf2N] is not as great as the imidazolium AHA ILs, yielding a slope similar to [P66614][Tf2N] and [P66614][AHA] ILs (Figure S1c). The new IL−IL viscosity data are primarily for mixtures of AHA ILs with imidazolium or phosphonium [Tf2N] ILs. The initial motivation for this work was to use a lower viscosity [Tf2N]− IL to reduce the viscosity of the higher viscosity [AHA]− IL but evolved into a more general investigation of the viscosity behavior of IL−IL mixtures. The measured values are shown in Table 2a−x. The graphs of all IL−IL mixtures are located in the Supporting Information (Figures S2a−x), and the list of all IL− IL mixtures investigated are located in Table 3 and in the Supporting Information (Table S3), along with the fit at each different temperature using eq 1. Data are organized in Table 3 from the highest to the lowest value of G at 278.15 K, the lowest temperature investigated. The best fit values of G were determined at each temperature by minimizing the percent error between experimental viscosity data and the viscosity from using eq 1. Viscosity Trends and Fits Using the Grunberg and Nissan Model. The Grunberg and Nissan model (eq 1) provides a reasonably good representation of the experimental mixture viscosity data. An example is shown in Figure 2a for mixtures of [P66614][acetate] and [hmim][Tf2N]. This is an example of a system whose viscosities are greater than a straight line between the two pure IL viscosities on a log scale (which would be the Arrhenius model, i.e., G = 0). The value of G at 278.15 K is 2.04, and it decreases to 1.5 when the temperature increases to 303.15 K. The fit tends to underestimate the

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EXPERIMENTAL SECTION The source, materials used, and synthesis procedure followed have been reported previously49 for most of the ILs used, along with 1H NMR spectra demonstrating purity. The structure, name, abbreviation, and purity for those ILs are reproduced from that reference in the Supporting Information, Tables S1 and S2. In addition, the procedures to measure density (an oscillating Utube Anton Paar 4500 densitometer) and viscosity (a cone and plate ATS viscoanalyzer), along with the method to determine experimental uncertainties, can be found in the previous publication.49 The densitometer requires approximately 1.7 mL of sample, and we estimate the uncertainty in the density measurements of ionic liquids that are 99% and 97% pure to be ±0.002 g·cm−3 and ±0.006 g·cm−3, respectively. The viscoanalyzer requires 0.3−0.4 mL of sample, and the estimated uncertainty is ±6%. The uncertainty increases greatly below 50 mPa·s, so no measurements were made below this value. The mixtures were made gravimetrically under nitrogen in a glovebox. Typically, enough of each ionic liquid is added so there was about 1 mL of the mixture. Due to the high viscosity of the ionic liquids, the samples were stirred for at least a half hour. The mixture was visually inspected to make sure it was a single phase mixture. Cloudiness is an indication of a phase split, whereas a clear one phase sample is a single phase mixture. The mixture was examined again when it was extracted into the needle prior to the viscosity measurements to confirm it was single phase. After the viscosity measurements, the mixture was examined again when it was pulled into the syringe to confirm it was still single phase. All single phase mixtures remained single phase for the duration of the measurements. The uncertainty in the compositions is estimated to be 0.001 mole fraction.

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RESULTS AND DISCUSSION Viscosity Measurements. The viscosities of 23 different IL−IL mixtures were measured over the entire composition range for which the mixtures were single phase and at multiple C



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Table 2. Experimental Values of Viscosity, μ, of Various Mixtures (a−x) from 278.15 to 323.15 K at p = 0.1 MPaa μ (mPa·s) mole fraction (x1)

b

c

water content (weight fraction × 102)

150 0.899 0.799 0.699 0.600 0.499 0.400 0.300 0.199 0.101 049

0.0705/0.3100 0.0342/0.2006 0.0278/0.1783 0.0288/0.1118 0.0270/0.0728 0.0252/0.0497 0.0320/0.1074 0.0390/0.0991 0.0376/0.0717 0.0315/0.0515 0.0031/0.0134

149 0.900 0.800 0.699 0.612 0.278 0.200 0.100 0

0.0294/0.1516 0.0385/0.1225 0.0350/0.1058 0.0371/0.1298 0.0360/0.0956 0.0211/0.0665 0.0165/0.0653 0.0123/0.0608 0.0013/0.0187

149 0.865 0.700 0.652 0.550 0.420 0.302 0.151 049

0.0294/0.1516 0.0278/0.2135 0.0153/0.1390 0.0122/0.0898 0.0450/0.1989 0.0178/0.0843 0.0188/0.0872 0.0257/0.0733 0.0031/0.0134

1 0.850 0.699 0.600 0.498 0.401 0.301 0.150 0

0.1270/0.3510 0.0240/0.1810 0.0181/0.1233 0.0256/0.0489 0.0327/0.0543 0.0204/0.0337 0.0139/0.0404 0.0153/0.0300 0.0031/0.0071

150 0.900 0.805 0.718 0.622 0.507 0.400 0.307 0.208 0.107 049

0.0091/0.0830 0.0288/0.1800 0.0162/0.1800 0.217/0.1316 0.0514/0.1533 0.0369/0.1130 0.0940/0.1361 0.0272/0.0900 0.0253/0.0994 0.1550/0.1400 0.0031/0.0134

1 0.898 0.800 0.698

0.0257/0.0706 0.0400/0.0772 0.0440/0.1134 0.0331/0.0920

278.15 K

283.15 K

288.15 K

293.15 K

298.15 K

(a) [P66614][Acetate] + [hmim][Tf2N] 1817 1204 814 566 406 2415 1575 1047 717 505 2877 1850 1219 827 578 2721 1763 1160 785 530 2588 1669 1100 746 523 2057 1345 901 621 440 1403 939 640 449 324 946 644 447 319 235 579 404 286 209 156 353 251 182 136 105 203 150 113 87 68 (b) [P66614][3-Triz] + [bmim][Tf2N] 1970 1299 878 611 438 2285 1490 994 685 486 2422 1582 1055 723 507 2219 1448 967 663 468 1952 1291 870 602 428 706 481 336 242 178 432 302 219 164 123 242 175 130 98 77 138 104 80 63 51 (c) [P66614][3-Triz] + [hmim][Tf2N] 1970 1299 878 611 438 2499 1630 1087 744 527 2448 1593 1061 725 511 2031 1329 891 614 437 1597 1057 717 498 358 1211 819 567 402 294 755 522 369 268 200 417 295 214 159 122 203 150 113 87 68 (d) [P66614][3-Triz] + [hmim][Tf2N] under CO2 2286 1496 1005 696 496 2694 1751 1161 790 553 2214 1453 969 663 467 1930 1264 849 586 416 1435 950 644 448 321 1039 701 484 342 249 738 510 359 258 191 419 295 212 157 120 194 143 108 83 65 (e) [P66614][4-Triz] + [hmim][Tf2N] 4635 2982 1959 1321 918 5162 3302 2162 1458 1013 4777 3062 2000 1349 932 4094 2612 1719 1162 811 3093 1992 1317 901 633 2209 1445 975 677 485 1397 936 641 452 329 910 619 431 308 227 612 424 301 220 165 325 235 174 132 103 203 150 113 87 68 (f) [P66614][2-CNpyr] + [hmim][Tf2N] 1764 1179 804 563 404 1844 1223 830 578 414 1749 1160 785 546 391 1684 1117 755 525 376 D

303.15 K

308.15 K

313.15 K

318.15 K

323.15 K

298 366 414 381 376 320 239 177 120 82 55

224 271 304 279 277 238 181 136 94 66

172 204 228 208 207 181 140 107 75 53

135 158 175 159 159 141 110 85 61

107 123 137 125 124 112 88 70

321 353 368 339 311 135 96 61

242 263 273 252 231 107 76 49

185 199 205 190 174 84 62

145 154 158 147 135 68 50

116 123 125 115 107 56

321 382 369 317 262 219 152 96 55

242 284 274 236 197 168 119 77

185 215 206 180 151 131 94 62

145 166 159 139 118 104 76 51

116 131 125 109 94 84 62

361 400 337 302 235 185 144 93 52

270 293 248 225 176 141 112 75

220 220 186 171 136 109 88 61

159 168 144 133 105 87 71

126 131 113 105 80 70 58

654 720 664 579 456 355 245 171 127 81 55

477 524 484 422 337 267 186 132 99 65

354 389 359 314 253 204 145 104 79 54

269 296 273 239 194 159 115 83 64

209 229 211 185 152 127 93 67 53

297 303 286 275

223 228 214 207

170 173 163 158

133 135 128 124

106 108 102 99



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Table 2. continued μ (mPa·s) mole fraction (x1)

b

c


278.15 K

0.600 0.496 0.400 0.300 0.200 0.100 049

0.0284/0.0733 0.0284/0.0825 0.0252/0.0554 0.0232/0.0799 0.0161/0.0683 0.0103/0.0222 0.0031/0.0134

1407 1216 900 662 520 337 203

149 0.898 0.797 0.695 0.501 0.300 0.200 0.099 049

0.0114/0.0276 0.0333/0.1160 0.0290/0.1210 0.0261/0.1163 0.0374/0.0885 0.0336/0.0884 0.0350/0.0682 0.0291/0.0614 0.0031/0.0134

3558 3071 2704 2474 1594 831 528 338 203

149 0.898 0.799 0.701 0.651 0.400 0.299 0.200 0.100 049

0.0396/0.1332 0.0532/0.1199 0.0478/0.0944 0.0447/0.1259 0.0348/ 0.0428/0.0884 0.0202/0.0485 0.0153/0.0474 0.0107/0.0299 0.0031/0.0134

2741 2587 2353 2047 1920 985 704 485 298 203

1 0.902 0.799 0.699 0.601 0.500 0.399 0.300 0.201 0.100 050

0.0257/0.0706 0.0552/0.1630 0.0557/0.1339 0.0389/0.0744 0.0389/0.1380 0.0293/0.0594 0.0293/0.0461 0.0202/0.0396 0.0180/0.0325 0.0161/0.0291 0.0011/0.0170

1764 1930 1868 1745 1658 1594 1356 1056 895 701 485

149 0.903 0.748 0.596 0.398 0.200 049

0.0061/0.0169 0.0040/0.0057 0.0041/0.0052 0.0045/0.0050 0.0046/0.0076 0.0044/0.0081 0.0031/0.0134

1208 1114 1019 873 652 390 203

150 0.900 0.751 0.600 0.500 0.401 0.250 0.124 050

0.0080/0.0872 0.0342/0.2006 0.0278/0.1783 0.0320/0.1074 0.0288/0.1118 0.0390/0.0991 0.0270/0.0728 0.0252/0.0497 0.0011/0.0170

1337 1386 1382 1244 1156 1011 771 602 485

283.15 K

288.15 K

293.15 K

298.15 K

(f) [P66614][2-CNpyr] + [hmim][Tf2N] 938 638 445 319 815 558 392 284 608 420 298 217 455 319 229 170 363 259 190 144 240 176 132 101 150 113 87 68 (g) [P66614][4-NO2pyra] + [hmim][Tf2N] 2285 1501 1018 710 1971 1297 880 614 1750 1158 788 554 1600 1061 726 512 1055 715 499 357 565 393 282 208 367 261 191 144 241 175 132 101 150 113 87 68 (h) [P66614][Tetrazolide] + [hmim][Tf2N] 1809 1219 844 600 1711 1155 800 567 1557 1055 732 523 1361 923 643 459 1285 869 607 434 662 461 331 245 489 346 253 189 342 247 183 139 215 158 120 93 150 113 87 68 (i) [P66614][2-CNpyr] + [hmmim][Tf2N] 1179 804 563 404 1279 867 602 430 1221 820 566 403 1141 764 527 375 1093 734 507 361 1048 705 488 350 897 607 422 304 699 474 332 240 598 411 290 212 472 328 234 173 335 236 172 129 (j) [P66614][Tf2N] + [hmim][Tf2N] 841 594 430 318 774 547 396 293 706 500 362 270 611 434 316 236 459 329 242 183 280 205 153 118 150 113 87 68 (k) [P2228][4-NO2pyra] + [hmmim][Tf2N] 868 578 398 282 890 589 404 285 883 580 396 279 795 524 359 254 742 491 338 241 658 442 307 220 510 348 246 179 409 283 204 151 335 236 172 129 E

303.15 K

308.15 K

313.15 K

318.15 K

323.15 K

236 210 162 129 111 80 55

177 160 125 100 88 64

136 123 97 79 71 52

107 98 78 64 58

85 79 63 52

510 441 400 371 264 158 111 80 55

375 325 296 276 199 122 88 65

280 244 222 209 153 97 70 52

214 187 171 162 120 78 57

167 146 134 128 96 64

437 414 382 337 318 185 145 108 73 55

326 308 286 253 239 143 114 86 59

247 232 217 194 181 112 91 70

191 179 168 151 141 90 74 57

151 141 133 120 113 74 61

297 315 294 273 264 257 224 178 159 131 99

223 237 219 204 197 193 169 136 122 102 78

170 180 167 155 151 148 130 105 96 81 63

133 141 130 120 117 116 103 83 77

106 112 103 96 94 93 82 67 63

241 221 204 180 141 92 55

185 171 158 140 111 74

145 134 124 111 89 60

115 107 100 90 72

92 86 81 73 60

207 208 203 185 177 163 135 115 99

155 156 152 139 134 124 104 90 78

119 119 117 106 103 97 82 72 63

94 94 91 83 82 77 65 58

75 75 73 67 66 63 53



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b

c


278.15 K

150 0.898 0.750 0.499 0.250 0.126 049

0.0080/0.0872 0.0260/0.1604 0.0217/0.1011 0.0221/0.0716 0.0198/0.0558 0.0246/0.0673 0.0031/0.0134

1337 1242 1046 680 358 267 203

149 0.898 0.749 0.596 0.400 0.200 050

0.0061/0.0169 0.0048/0.0071 0.0071/0.0129 0.0068/0.0108 0.0075/0.0123 0.0064/0.0090 0.0011/0.0170

1208 1130 1015 1000 932 707 485

149 0.750 0.500 0.250 049

0.0437/0.4520 0.0336/0.1150 0.0278/0.1840 0.0210/0.0870 0.0031/0.0134

260 254 238 235 203

149 0.901 0.797 0.700 0.599 0.500 0.401 0.300 0.201 0.098 049

0.0294/0.1516 0.0585/0.1827 0.0519/0.0914 0.0543/0.1026 0.0477/0.0847 0.0464/0.0923 0.0424/0.0650 0.0423/0.0619 0.0383/0.0582 0.0327/0.0406 0.0061/0.0169

1970 1767 1720 1657 1661 1528 1400 1454 1378 1213 1208

1 0.900 0.800 0.700 0.600 0.499 0.399 0.300 0.200 0.100 049

0.0300/0.1800 0.0279/0.1528 0.0401/0.1311 0.0387/0.0674 0.0333/0.0696 0.0287/0.0813 0.0354/0.0636 0.0282/0.0780 0.0251/0.0533 0.0243/0.0401 0.0061/0.0169

1540 1461 1397 1410 1379 1275 1299 1258 1261 1166 1208

149 0.850 0.700 0.500 0.300 0.15 0

0.0031/0.0134 0.0129/0.0188 0.0151/0.0253 0.0098/0.0270 0.0129/0.0251 0.0061/0.0202 0.0147/0.0191

203 181 153 120 95 89 80

149 0.900 0.750 0.499

0.0683/0.1500 0.0717/0.2279 0.0563/0.1663 0.0430/0.1249

1616 1198 716 334

283.15 K

288.15 K

293.15 K

298.15 K

(l) [P2228][4-NO2pyra] + [hmim][Tf2N] 868 578 398 282 804 535 369 263 682 457 317 227 456 314 223 164 251 180 133 102 191 140 106 82 150 113 87 68 (m) [P66614][Tf2N] + [hmmim][Tf2N] 841 594 430 318 786 555 400 297 698 490 353 261 683 477 342 252 634 442 317 234 483 336 242 179 335 236 172 129 (n) [P222(10201)][3-Triz] + [hmim][Tf2N] 186 137 104 80 184 136 103 80 171 127 96 75 171 127 98 76 150 113 87 68 (o) [P66614][3-Triz] + [P66614][Tf2N] 1299 878 611 438 1170 793 553 397 1144 779 547 393 1110 760 533 385 1114 764 539 390 1030 709 502 363 946 655 465 338 995 693 495 362 945 660 473 348 839 591 425 313 841 594 430 318 (p) [hmim][2-CNpyr] + [P66614][Tf2N] 902 553 355 239 893 566 374 257 877 571 387 271 910 603 416 295 905 612 423 304 846 575 404 292 876 602 426 310 851 589 419 307 867 605 435 320 806 568 410 303 841 594 430 318 (q) [hmim][Tf2N] + [emim][Tf2N] 150 113 87 68 135 102 79 63 116 89 70 57 92 72 58 74 58 70 56 63 50 (r) [P22210][4-NO2pyra] + [P222(10201)][Tf2N] 1031 678 461 325 788 531 370 265 486 337 241 178 239 174 131 101 F

303.15 K

308.15 K

313.15 K

318.15 K

323.15 K

207 192 168 124 79 66 55

155 145 127 96 63 53

119 110 99 73 52

94 83 77 59

75 67 62

241 224 196 190 177 136 99

185 173 151 146 137 106 78

145 135 119 115 107 84 63

115 108 95 92 86 68

92 80 77 75 71 55

321 291 289 284 288 270 251 270 261 236 241

242 220 217 214 218 204 191 207 200 181 185

185 168 167 165 168 158 148 160 156 142 145

145 132 131 129 132 124 117 127 124 112 115

116 106 104 103 106 100 94 102 100 91 92

167 183 196 216 224 216 231 230 241 229 241

122 135 146 162 169 164 177 176 185 176 185

91 101 112 125 130 127 137 137 145 138 145

70 79 87 97 102 100 109 109 115 110 115

55 62 69 78 82 81 88 87 93 89 92

175 149 104 64

133 115 82 52

104 91 66

82 73 55

64 64 60 61 55

55 51

236 195 135 79



Article


b

c


278.15 K

0.250 0.100 0

0.0288/0.1181 0.0155/0.0739 0.0033/0.0345

160 103 84

149 0.901 0.750 0.500 0.248 0.101 0

0.0516/0.1696 0.0739/0.2445 0.0232/0.1788 0.0189/0.1214 0.0130/0.0691 0.0120/0.0489 0.0033/0.0345

1003 761 474 234 135 103 84

149 0.900 0.700 0.500 0.299 0.101 049

0.0470/0.2253 0.0924/0.2454 0.0633/0.1430 0.0546/0.0810 0.0252/0.1220 0.0168/0.0546 0.0061/0.0169

3314 2649 2039 1582 1360 1201 1208

149 0.900 0.699 0.600 0.500 0.403 0.303 0.101 050

0.0730/0.2345 0.0212/0.1180 0.0178/0.0922 0.0163/0.0652 0.0408/0.0848 0.0160/0.0409 0.0261/0.0515 0.0120/0.0214 0.0015/0.0137

2178 1778 989 903 696 591 499 404 379

149 0.900 0.701 0.499 0.299 0.100 049

0.0470/0.2253 0.0904/0.2280 0.0634/0.2004 0.0431/0.1177 0.0219/0.0505 0.0123/0.0318 0.0015/0.0137

3314 2181 1045 639 470 402 379

149 0.898 0.700 0.500 0.303 0.098 049

0.0160/0.2065 0.0440/0.1870 0.0415/0.1117 0.0341/0.1017 0.0220/0.0686 0.0160/0.0445 0.0061/0.0169

14620 7584 3692 2212 1562 1280 1208

149 0.899 0.701 0.499 0.299 0.101 049

0.0160/0.2065 0.0340/0.1880 0.0258/0.1452 0.0245/0.0998 0.0196/0.0635 0.0162/0.0650 0.0015/0.0137

14620 6554 2252 992 620 401 379

283.15 K

288.15 K

293.15 K

298.15 K

(r) [P22210][4-NO2pyra] + [P222(10201)][Tf2N] 120 91 72 57 79 62 49 65 51 (s) [P2227][4-NO2pyra] + [P222(10201)][Tf2N] 657 443 309 223 510 350 249 182 328 231 168 126 170 126 96 75 101 78 61 50 80 62 50 65 51 (t) [hmim][4-NO2pyra] + [P66614][Tf2N] 1806 1038 633 407 1540 935 597 400 1290 840 566 395 1045 707 492 352 914 629 445 323 816 570 410 302 841 594 430 318 (u) [hmim][Tetrazolide] + [P2228][Tf2N] 1335 848 561 384 1117 725 489 341 664 453 316 227 608 414 290 211 478 334 240 175 409 289 209 156 347 246 179 135 286 206 153 116 268 194 145 111 (v) [hmim][4-NO2pyra] + [P2228][Tf2N] 1806 1038 633 407 1262 765 488 326 660 432 295 209 428 294 209 154 325 230 168 127 284 205 152 116 268 194 145 111 (w) [hmmim][4-NO2pyra] + [P66614][Tf2N] 6910 3494 1909 1113 4009 2241 1330 830 2223 1385 897 605 1418 933 636 447 1040 707 495 357 871 606 434 319 841 594 430 318 (x) [hmmim][4-NO2pyra] + [P2228][Tf2N] 6910 3494 1909 1113 3416 1886 1108 689 1345 836 543 369 636 423 292 209 421 293 211 156 284 204 151 115 268 194 145 111

303.15 K

308.15 K

313.15 K

318.15 K

323.15 K

165 137 97 59

126 105 76

97 84 61

77 66 50

62 54

275 278 285 259 241 228 241

192 200 211 195 184 175 185

139 149 160 150 142 138 145

102 113 124 118 113 110 115

79 89 99 94 91 90 92

273 247 166 156 129 119 104 90 87

200 183 125 119 100 93 82 72 73

150 139 97 93 78 74 66 58 57

115 108 77 74 63 61 53

91 86 63 61 52

275 227 154 116 98 90 87

192 164 116 90 77 72 73

139 122 90 71 62 58 57

102 93 71 57 51

79 74 57

691 545 422 324 264 239 241

450 373 304 240 200 184 185

303 263 223 182 154 144 145

213 192 169 142 121 115 115

147 144 131 112 97 93 92

691 450 259 154 120 90 87

450 307 188 118 93 72 73

303 216 140 91 74 57 57

213 157 107 72 60

147 119 84 59

a Standard uncertainties u are u(T) = 0.1 K, u(p) = 0.005 MPa, u(x1) = 0.001 and relative standard uncertainty ur is ur(μ) = 0.06. bThe mole fraction is for the first part of each mixture: (a) [P66614][acetate]; (b) [P66614][3-Triz]; (c) [P66614][3-Triz]; (d) [P66614][3-Triz]; (e) [P66614][4-Triz]; (f) [P66614][2-CNpyr]; (g) [P66614][4-NO2pyra]; (h) [P66614][tetrazolide] (mixtures with mole fractions of 0.599, 0.499, and 0.447 of [P66614][tetrazolide] are two phases at room temperature); (i) [P66614][2-CNpyr]; (j) [P66614][Tf2N]; (k) [P2228][4-NO2pyra]; (l) [P2228][4NO2pyra]; (m) [P66614][Tf2N]; (n) [P222(10201)][3-Triz]; (o) [P66614][3-Triz]; (p) [hmim][2-CNpyr]; (q) [hmim][Tf2N]; (r) [P22210][4-NO2pyra];

G



Article

Table 2. continued (s) [P2227][4-NO2pyra]; (t) [hmim][4-NO2pyra]; (u) [hmim][tetrazolide]; (v) [hmim][4-NO2pyra]; (w) [hmmim][4-NO2pyra]; (x) [hmmim][4NO2pyra]. cThe water content was determined by Brinkman 831 Karl Fischer coulometer both before and after the viscosity measurements.

Table 3. Effect of Temperature on the Value of G (eq 1) for All IL−IL Mixtures from 278.15 to 303.15 K G IL1

IL2

278.15 K

283.15 K

288.15 K

293.15 K

298.15 K

[P66614][acetate] [P66614][3-Triz] [P66614][3-Triz] [P66614][3-Triz]/ CO2 [P66614][4-Triz] [P66614][2-CNpyr] [P66614][4-NO2pyra] [P66614][tetrazolide] [P66614][2-CNpyr] [P66614][Tf2N] [P2228][4-NO2pyra] [P2228][4-NO2pyra] [P66614][Tf2N] [P222(10201)][3-Triz] [P66614][3-Triz] [hmim][2-CNpyr] [hmim][Tf2N] [P22210][4-NO2pyra] [P2227][4-NO2pyra] [hmim][4-NO2pyra] [hmim][tetrazolide] [hmim][4-NO2pyra] [hmmim][4-NO2pyra] [hmmim][4-NO2pyra]

[hmim][Tf2N] [bmim][Tf2N] [hmim][Tf2N] [hmim][Tf2N] [hmim][Tf2N] [hmim][Tf2N] [hmim][Tf2N] [hmim][Tf2N] [hmmim][Tf2N] [hmim][Tf2N] [hmmim][Tf2N] [hmim][Tf2N] [hmmim][Tf2N] [hmim][Tf2N] [P66614][Tf2N] [P66614][Tf2N] [emim][Tf2N] [P222(10201)][Tf2N] [P222(10201)][Tf2N] [P66614][Tf2N] [P2228][Tf2N] [P2228][Tf2N] [P66614][Tf2N] [P2228][Tf2N]

2.04 1.85 1.53 1.39 1.36 1.21 1.01 0.97 0.89 0.72 0.58 0.39 0.34 0.11 −0.04 −0.08 −0.09 −0.19 −0.30 −0.43 −0.45 −0.95 −1.21 −1.53

1.93 1.73 1.45 1.31 1.28 1.13 0.94 0.91 0.83 0.68 0.51 0.33 0.32 0.10 −0.04 −0.01 −0.07 −0.15 −0.27 −0.31 −0.38 −0.82 −1.01 −1.33

1.81 1.63 1.37 1.23 1.20 1.04 0.87 0.85 0.78 0.64 0.46 0.28 0.30 0.10 −0.05 0.06 −0.05 −0.13 −0.24 −0.21 −0.33 −0.71 −0.82 −1.15

1.70 1.52 1.29 1.15 1.14 0.97 0.80 0.80 0.72 0.60 0.42 0.24 0.28 0.09 −0.05 0.11

1.61 1.41 1.23 1.07 1.09 0.91 0.76 0.76 0.67 0.58 0.37 0.23 0.26 0.09 −0.06 0.16

1.15 1.00 1.02 0.84 0.70 0.71 0.63 0.54 0.35 0.20 0.24 0.09 −0.06 0.19

−0.13 −0.30 −0.62 −0.67 −1.01

−0.05 −0.28 −0.54 −0.55 −0.89

0.00 −0.27 −0.46 −0.44 −0.78

303.15 K 1.50

percent error 8.3 4.8 7.1 5.3 9.2 3.3 3.7 2.2 3.6 3.4 2.0 4.7 5.6 2.3 3.5 2.6 3.3 2.1 2.3 3.9 3.4 1.8 3.6 4.1

the applications of the AHA ILs is for CO2 capture. When the anion is reacted with CO2, this is essentially a new IL, and we were interested in determining whether the viscosity reduction observed when [hmim][Tf2N] is added to [P66614][3-Triz] is also observed for the reacted IL product. In addition, both the [P66614][4-Triz] + [hmim][Tf2N] and the [P66614][2-CNpyr] + [hmim][Tf2N] systems have small ranges of mole fraction where the mixture viscosity is greater than the viscosity of either of the pure ILs. Figure 3a shows the viscosity of these five IL−IL mixtures, where the viscosity is greater than the viscosity of either pure ILs for some compositions, at 278.15 K. To fit these systems, G values range from 1.2 to 2.0 at 278.15 K. The most pronounced increase in viscosity occurs for the [P66614][acetate] + [hmim][Tf2N] system. Five mixtures of an AHA IL and a [Tf2N]− IL have positive G values at 278.15 K (from 0.35 to 1.0) but do not show any mixture viscosities that are significantly greater than either of the pure component viscosities, as shown in Figure 3b. [P66614][Tf2N] + [hmim][Tf2N] mixtures also fit into this category. The viscosities of mixtures with some mole fractions of [P66614][2CNpyr] + [hmmim][Tf 2 N] and [P 2228 ][4-NO 2 pyra] + [hmmim][Tf2N] are slightly higher than the viscosity of the pure AHA IL. However, for the [P66614][4-NO2pyra] + [hmim][Tf2N], [P66614][tetrazolide] + [hmim][Tf2N], and [P2228][4-NO2pyra] + [hmim][Tf2N] mixtures, the viscosity is always between the two pure ILs. Of these five IL−IL mixtures, [P2228][4-NO2pyra] + [hmim][Tf2N] has the least curvature, consistent with its low G value. Even though [P66614][4NO2pyra] + [hmim][Tf2N] and [P66614][tetrazolide] + [hmim][Tf2N] only have viscosities between the viscosities of the pure

viscosity slightly at higher concentrations of [P66614][acetate] but overestimate the viscosity slightly at lower concentrations of [P66614][acetate], as shown in Figure 2a. However, overall the fits with eq 1 are very good. When the mole fraction of [P66614][acetate] is higher than 0.5, the viscosity of the mixture is higher than the viscosity of either of the two pure ILs. In fact, at 278.15 K and a [P66614][acetate] mole fraction of 0.799 the viscosity of the mixture is 58% higher than the viscosity of pure [P66614][Acetate], which is the more viscous of the two pure ILs. The effect is less dramatic with increasing temperature, with the viscosity of the mixture just 28% higher than the pure [P66614][acetate] at 323.15 K. Figure 2b and c shows the data, and eq 1 fits for two other mixtures: [P66614][Tf2N] + [hmmim][Tf2N] and [hmmim][4-NO2pyra] + [P66614][Tf2N]. There are a few other IL−IL mixtures that have viscosities greater than either of the pure ILs. The graphs of these mixtures are shown in Figure S2a−f,i of the Supporting Information. The viscosity is greater than either of the pure ILs at [P66614][3-Triz] mole fractions of 0.612 or greater in the [P66614][3-Triz] + [bmim][Tf2N] system. Note that two phases were observed for this system at [P66614][3-Triz] mole fractions of roughly 0.4 and 0.5. To avoid the complication of the formation of two liquid phases, the alkyl chain on the imidazolium cation was increased from butyl to hexyl. Indeed, the mixture of [P66614][3-Triz] + [hmim][Tf2N] is one phase throughout the entire composition region, and at [P66614][3-Triz] mole fractions of 0.652 and greater the viscosity of the mixture is greater than either of the two pure ILs. Likewise, a mixture of [P66614][3-Triz] + [hmim][Tf2N], where the mixture was saturated with 1 bar CO2, gives similar results to the measurement under N2. One of H



Article

Figure 2. Viscosity of binary mixtures of (a) [P66614][acetate] (1) + [hmim][Tf2N] (2) at 278.15 K, purple ∗; 283.15 K, blue ◇; 288.15 K, green ◆; 293.15 K, yellow ●; 298.15 K, orange ▲; and 303.15 K, red ■; (b) [P66614][Tf2N] (1) + [hmmim][Tf2N] (2) at 278.15 K, black □; 283.15 K, purple ∗; 288.15 K, blue ◇; 293.15 K, blue ×; 298.15 K, green ◆; 303.15 K, yellow ●; 308.15 K, orange ▲; and 313.15 K, red ■; (c) [hmmim][4-NO2pyra] (1) + [P66614][Tf2N] (2) at 278.15 K, black □; 283.15 K, purple ∗; 288.15 K, blue ◇; 293.15 K, blue ×; 298.15 K, green ◆; 303.15 K, green ○; 308.15 K, yellow ●; 313.15 K, orange ▲; 318.15 K, red ■, and 323.15 K, red △. Lines are the best fit with the Grunberg and Nissan model (eq 1). The values of G are located in Table 3. Figure 3. Viscosity of IL (1) + IL (2) mixtures at 278.15 K. (a) [P66614][Acetate] (1) + [hmim][Tf2N] (2), red ■; [P66614][3-Triz] (1) + [bmim][Tf2N] (2), blue ●; [P66614][3-Triz] (1) + [hmim][Tf2N] (2), green ◆; [P66614][4-Triz] (1) + [hmim][Tf2N] (2), yellow ▲; and [P66614][2-CNpyr] (1) in [hmim][Tf2N] (2), purple ∗, at 278.15 K. The values of G for these mixtures range from 1.2 to 2.0. (b) [P66614][2CNpyr] (1) + [hmmim][Tf2N] (2), yellow ▲; [P2228][4-NO2pyra] (1) + [hmim][Tf2N] (2), purple ∗; [P2228][4-NO2pyra] (1) + [hmmim][Tf2N] (2), green ◆; [P66614][4-NO2pyra] (1) + [hmim][Tf2N] (2), blue ●; and [P66614][tetrazolide] (1) + [hmim][Tf2N] (2), red ■. The values of G for these mixtures range from 0.4 to 1.0. (c) [hmim][Tf2N] (1) + [emim][Tf2N] (2), purple ∗; [P2227][4-NO2pyra] (1) +

ILs, their G values are higher than the [P66614][2-CNpyr] + [hmmim][Tf2N] and [P2228][4-NO2pyra] + [hmmim][Tf2N] systems, which exhibited slightly higher viscosities than the pure AHA ILs for some mole fractions. This is because the difference in the pure component viscosities of the two pure ILs is greater for [P 66614][4-NO2pyra] + [hmim][Tf2N] and [P66614][tetrazolide] + [hmim][Tf2N] than for [P66614][2-CNpyr] + [hmmim][Tf2N] and [P2228][4-NO2pyra] + [hmmim][Tf2N]. When the difference is the pure component viscosities is greater, a higher G value is needed to create the same amount of I



Article

imidazolium AHA ILs mixed with phosphonium [Tf2N]− ILs. The range of G values needed to represent the IL−IL mixtures shown in Figure 3d are between −1.5 and −0.1 at 278.15 K. For a given system, the G values decrease in magnitude with increasing temperature, as the viscosities of the two pure ILs become more similar. Recall that the temperature dependence of the viscosity of the different types of pure ILs are not the same. For instance, the G value for [hmmim][4-NO2pyra] + [P66614][Tf2N] mixtures is −1.21 at 278.15 K but increases to −0.06 at 323.15 K. All temperatures for this system are shown in Figure 2c. The [hmim][2-CNpyr] + [P66614][Tf2N] system is particularly interesting because at lower temperatures the viscosity of [hmim][2-CNpyr] is greater than that of [P66614][Tf2N], whereas at higher temperatures the opposite is true. Correspondingly, the G value at lower temperatures is slightly negative, whereas at higher temperatures the G value is positive. For [hmim][4-NO2pyra] + [P66614][Tf2N], the G values trend from slightly negative to slightly positive with increasing temperature. For the other three mixtures the G values are negative over the entire temperature range but decrease in magnitude with increasing temperature as the viscosities of the two ILs become more similar. Viscosities of Complementary Mixtures. Eight of the 23 IL−IL mixtures are complementary pairs, where both [C1][A1] + [C2][A2] and [C2][A1] + [C1][A2] mixtures have been investigated: [P66614][2-CNpyr] + [hmim][Tf2N] and [hmim][2-CNpyr] + [P66614][Tf2N]; [P66614][4-NO2pyra] + [hmim][Tf2N] and [hmim][4-NO2pyra] + [P66614][Tf2N]; [P2228][4NO2pyra] + [hmmim][Tf2N] and [hmmim][4-NO2pyra] + [P2228][Tf2N]; [P2228][4-NO2pyra] + [hmim][Tf2N] and [hmim][4-NO2pyra] + [P2228][Tf2N]. The data for these four sets of mixtures at 278.15 K are shown in Figure 4a−d. All four figures clearly show that equal molar mixtures of [C1][A1] + [C2][A2] and [C2][A1] + [C1][A2] have the same viscosities, when taking the experimental uncertainty into account. This is a very important result. It means that anions and cations are freely exchanged in the mixtures. Mixtures of [C1][A1] + [C2][A2] or [C2][A1] + [C1][A2] are really a sea of anions ([A1] and [A2]) and cations ([C1] and [C2]). This is true at all temperatures. Graphs of all four complementary mixture pairs at 303.15 K are shown in the Supporting Information (Figure S3a−d). Note that the uncertainty in the data for mixtures containing [P2228][4NO2pyra] is a bit higher than other mixtures since the purity of the sample used for most of the measurements was just 97%. As noted above, that large batch gave viscosities about 18% higher than previous batches with a higher (99%) purity. From a very practical standpoint, Figure 4a−d shows why the G values for phosphonium [AHAs] mixed with imidazolium [Tf2N] have to be positive and why the G values for phosphonium [Tf2N] ILs mixed with imidazolium [AHAs] are negative. For example, in Figure 4a, the pure IL viscosities of [hmim][2-CNpyr] and [P66614][Tf2N] at 278.15 K are very similar, so the most likely trend for the mixture data is something close to a straight line on the log plot. The pure IL viscosity of [P66614][2-CNpyr] is also similar to that of [hmim][2-CNpyr]; however, the viscosity of [hmim][Tf2N] is much lower than all three of the other ILs. In order for the viscosity of the [hmim][2CNpyr]0.5/[P66614][Tf2N]0.5 mixture to be equal to the viscosity of the [P66614][2-CNpyr]0.5/[hmim][Tf2N]0.5 mixture, the [hmim][Tf2N] + [P66614][Tf2N] data must be concave down (a positive G value) or else the viscosity would fall below the viscosity of the [P66614][2-CNpyr]0.5/[hmim][Tf2N]0.5 mixture. In fact, the experimental data and the fit with eq 1 actually show a

Figure 3. continued [P 222(10201) ][Tf 2N] (2), green ◆ , [P 22210 ][4-NO 2pyra] (1) + [P222(10201)][Tf2N] (2), blue ●; [P66614][3-Triz] (1) + [P66614][Tf2N] (2), red △; [P222(10201)][3-Triz] (1) + [hmim][Tf2N] (2), blue ×; [P66614][Tf2N] (1) + [hmmim][Tf2N] (2), red ■; and [P66614][Tf2N] (1) + [hmim][Tf2N] (2), yellow ▲. The values of G for these mixtures range from −0.3 to 0.7. (d) [hmmim][4-NO2pyra] (1) + [P2228][Tf2N] (2), red ■; [hmmim][4-NO2pyra] (1) + [P66614][Tf2N] (2), red △; [hmim][4-NO2pyra] (1) + [P2228][Tf2N] (2), green ◆; [hmim][tetrazolide] (1) + [P2228][Tf2N] (2), purple ∗; [hmim][4-NO2pyra] (1) + [P66614][Tf2N] (2), blue ●; and [hmim][2-CNpyr] (1) + [P66614][Tf2N] (2), yellow ▲. The values of G for these mixtures range from −1.5 to −0.1. Lines are the best fit with the Grunberg and Nissan model (eq 1). The values of G are located in Table 3.

“curvature” in the model (eq 1). Note that all of these systems have higher viscosities than predicted by the Arrhenius model (where G = 0). The ten IL−IL mixtures shown in Figure 3a and b involves a phosphonium [AHA] IL mixed with an imidazolium [Tf2N] IL. With one exception ([P222(10201)][3-Triz] + [hmim][Tf2N]), the next group, shown in Figure 3c, involves mixtures of two ILs that have either the same or similar cations or the same anion. [P2227]+, [P222(10201)]+, and [P22210]+ are very similar in structure so [P2227][4-NO2pyra] + [P222(10201)][Tf2N] and [P22210][4NO2pyra] + [P222(10201)][Tf2N] mixtures are included in Figure 3c. The main difference between [P2227][4-NO2pyra] and [P222(10201)][4-NO2pyra] is that the viscosity of [P222(10201)][4NO2pyra] is much lower than [P2227][4-NO2pyra]. All of the mixtures in Figure 3c have small (either negative or positive) values of G, which means that the logarithm of the mixture viscosities is close to a mole fraction weighted linear combination of the logarithm of the pure IL viscosities, i.e., the Arrhenius model. The overall observation is that keeping the anion or cation the same (or similar) leads to G values near 0. A few clarifications and explanations are needed to support this statement. First, the one mixture without a common anion or cation, [P222(10201)][3-Triz] + [hmim][Tf2N], has a small G value simply because the viscosity of the two pure ILs are very similar. As noted above, a larger G value is needed to capture a particular curvature if the two pure viscosities are very different. Likewise, an anomalously small value of G can fit a particular curvature if the pure viscosities are very similar. Second, we have included in the graph the data for [P66614][Tf2N] + [hmim][Tf2N], even though the G value is a relatively large 0.72 at 278.15 K. The interpretation is that this system really does not deviate substantially from the Arrhenius model. The higher molecular weight and viscosity of [P66614][Tf2N] compared to [hmim][Tf2N] is why the G value appears high compared to other IL−IL mixtures that share a common cation or anion. This is supported by the fact that the G value for [P66614][Tf2N] + [hmmim][Tf2N] (which is more viscous than [hmim][Tf2N] but differs only in the addition of one methyl group) at 278.15 K is just 0.34. Figure 2b shows the very slight curvature in the data for this system at all temperatures. With the exception of these two rather asymmetric systems ([P66614][Tf2N] + [hmim][Tf2N] and [P66614][Tf2N] + [hmmim][Tf2N]), the range of G values for the IL−IL mixtures shown in Figure 3c are between −0.3 and 0.1 at 278.15 K. Thus, the conclusion that keeping the anion or cation the same (or similar) leads to G values near 0 is justified. The six IL−IL mixtures shown in Figure 3d have G values that are negative, which means that the mixture viscosities are lower than predicted by the Arrhenius model. They all involve J



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Figure 4. continued [hmim][Tf2N] (2), red ■. (c) [hmim][4-NO2pyra] (1) + [P2228][Tf2N] (2), blue ● and [P2228][4-NO2pyra] (1) + [hmim][Tf2N] (2), red ⧫. (d) [hmmim][4-NO2pyra] (1) + [P2228][Tf2N] (2), blue ● and [P2228][4-NO2pyra] (1) + [hmmim][Tf2N] (2), red ■. Lines are the best fit with the Grunberg and Nissan model (eq 1). The values of G are located in Table 3.

slight maximum in order for [P66614][2-CNpyr]0.5/[hmim][Tf2N]0.5 to match the viscosity of [hmim][2-CNpyr]0.5/ [P66614][Tf2N]0.5. The situation shown in Figure 4b is very similar. To make the viscosity of [P66614][4-NO2pyra]0.5/[hmim][Tf2N]0.5 match the viscosity of [hmim][4-NO2pyra]0.5/[P 66614][Tf2N]0.5, the [hmim][Tf2N] + [P66614][4-NO2pyra] mixture must decrease in viscosity slowly (i.e., concave down corresponding to a positive G value) whereas the mixture of [P66614][Tf2N] + [hmim][4-NO2pyra] mixture viscosity must decrease significantly with the addition of small amounts of [P66614][Tf2N] (i.e., a negative G value producing a curve that is concave up). The mixtures in Figure 4c are [P2228][4-NO2pyra] + [hmim][Tf2N] and [hmim][4-NO2pyra] + [P2228][Tf2N]. At 278.15 K, the viscosity of [P2228][4-NO2pyra] is lower than the viscosity of [hmim][4-NO2pyra], but the viscosity of [P2228][Tf2N] is higher than that of [hmim][Tf2N]. As a result, the [hmim][4-NO2pyra] + [P2228][Tf2N] mixtures have higher viscosities than the [P2228][4-NO2pyra] + [hmim][Tf2N] mixtures. However, they have to meet at [P 2228 ][4NO2pyra]0.5/[hmim][Tf2N]0.5 and [hmim][4-NO2pyra]0.5/ [P2228][Tf2N]0.5. The only way for this to occur is for [hmim][4-NO2pyra] + [P2228][Tf2N] to be concave up (negative G value) and [P2228][4-NO2pyra] + [hmim][Tf2N] to be concave down (positive G value). The mixtures in Figure 4d are the same as in Figure 4c, except that the imidazolium cation is [hmmim]+ instead of [hmim]+. The viscosity of [hmmim][Tf2N] is slightly greater than the viscosity of [P2228][Tf2N], whereas the viscosity of [hmmim][4NO2pyra] is much greater than the that of [P2228][4-NO2pyra] at 278.15 K. Like the mixtures in Figure 4a−c, the G value for the phosphonium [AHA] mixed with the imidazolium [Tf2N] is positive, and the G value for phosphonium [Tf2N] mixed with imidazolium AHA is negative, which allows the viscosity of [P2228][4-NO2pyra]0.5/[hmmim][Tf2N]0.5 to match that of [hmmim][4-NO2pyra]0.5/[P2228][Tf2N]0.5. The above arguments are for the data at 278.15 K, but the same results are obvious at other temperatures. Equivalent figures at 303.15 K are supplied in the Supporting Information (Figure S3a−d). Arrhenius Random Distribution Model. Although the above discussion of the necessary curvatures to satisfy the requirement that the complementary mixtures meet at overall compositions of 0.5 is useful, it does not provide any molecular understanding of the viscosity behavior of the IL−IL mixtures (except at overall compositions of 0.5). We attempt to provide that insight below. First, as pointed out above, the fact that the viscosities of the [C1][A1]0.5/[C2][A2]0.5 mixtures equal the viscosities of the [C2][A1]0.5/[C1][A2]0.5 mixtures means that the cations and anions are freely mobile in the solutions and can be thought of as a four-component mixture, made up of [C1]+, [C2]+, [A1]−, and [A2]−. Of course, ion neutrality must always be obeyed; i.e., the number of cations will always be equal to the number of anions in the mixture. However, this means that in any mixture of [C1]+,

Figure 4. Viscosity of IL (1) + IL (2) mixtures at 278.15 K. (a) [hmim][2-CNpyr] (1) + [P66614][Tf2N] (2), blue ● and [P66614][2CNpyr] (1) + [hmim][Tf2N] (2), red ■. (b) [hmim][4-NO2pyra] (1) + [P66614][Tf2N] (2), blue ● and [P66614][4-NO2pyra] (1) + K



Article

[C2]+, [A1]−, and [A2]−, i.e., all four IL combinations ([C1][A1], [C2][A2], [C2][A1], and [C1][A2]), is possible. An obvious question is whether the viscosity of the IL−IL mixture can be represented by the Arrhenius model (i.e., G = 0) with some combination (that satisfies the mass balance) of the four IL possibilities. This possibility is motivated by the observation that the viscosities of IL−IL mixtures that share a common cation or a common anion (so that the only possible ILs in the mixture are the two pure ILs that are original mixed) can be represented with G values close to zero. In particular, what would be the viscosity of the mixture using the Arrhenius model if one assumed a completely random distribution of the four IL combinations? This can be done for the four complementary pairs of IL−IL mixtures shown in Figure 4a−d because, for each complementary pair, the viscosities of all four possible pure ILs are known. A random distribution assumes that there is no preference of either of the anions for either of the cations, eq 2.

0.8, 0.2, and 0.2, respectively. For a random distribution, the mole fraction of each of the four possible ILs is the product of the mole fraction of the cation and anion. Thus, the mole fraction of [hmim][2-CNpyr] in the random mixture would be 0.64, which is 0.8 × 0.8. For an overall 0.5 mixture, a random distribution would give 0.25 mole fraction of [C1][A1], [C2][A2], [C2][A1], and [C1][A2]. Using this model, the results for the [hmim][2CNpyr] + [P66614][Tf2N] and [P66614][2-CNpyr] + [hmim][Tf2N] mixtures at 278.15 K are shown in Figure 5a. The corresponding graphs for all four complementary pairs of IL−IL mixtures are shown in the Supporting Information, Figure S4a− d. Clearly, the Arrhenius model assuming a random distribution of the four possible IL combinations ([C1][A1], [C2][A2], [C2][A1], and [C1][A2]) seriously underestimates the viscosities at all compositions for both of the binary IL−IL mixtures. This is also the case for [hmim][4-NO2pyra] + [P66614][Tf2N] and [P66614][4-NO2pyra] + [hmim][Tf2N] (Figure S4b in the SI). The Arrhenius model with random distribution of the four possible IL combination overestimates the viscosities for the [hmim][4-NO2pyra] + [P2228][Tf2N] and [P2228][4-NO2pyra] + [hmim][Tf2N] (Figure S4c in the SI) and [hmmim][4NO 2 pyra] + [P 2228 ][Tf 2 N] and [P 2228 ][4-NO 2 pyra] + [hmmim][Tf2N] (Figure S4d in the SI) mixtures. Thus, we conclude that [C1][A1] + [C2][A2] and [C2][A1] + [C1][A2] mixtures should not be thought of as quaternary mixtures of a random distribution of the four possible IL combinations that can be represented by the Arrhenius model (eq 2). This leads to two possible explanations. First, [C1][A1] + [C2][A2] and [C2][A1] + [C1][A2] mixtures are composed of a random distribution of the four possible IL combinations, but they simply cannot be described by the Arrhenius model (eq 2). We note that this model is entirely empirical and that there is no fundamental reason that any particular mixture should follow it. Arrhenius Nonrandom Distribution Model. An alternative explanation is that the mixtures of [C1]+, [C2]+, [A1]−, and [A2]− do not form random distributions of the four possible IL combinations. One could then use the experimental data to estimate what the nonrandom distribution of the four ILs might be. For the [hmim][2-CNpyr] + [P66614][Tf2N] and [P66614][2CNpyr] + [hmim][Tf2N] mixtures shown in Figure 5a, to increase the overall mixture viscosity there would have to be a preference for the formation of [hmim][2-CNpyr] and [P66614][Tf2N] (which has a higher combined viscosity) rather than [hmim][Tf2N] and [P66614][2-CNpyr] (which has a lower combined viscosity). The combined viscosity, using eq 1, is the viscosity when G is zero at a mole fraction of 0.5. To match the experimental viscosity at 0.5, using a nonrandom Arrhenius model (eq 3),

log(μm ) = xC1xA1 log μC1A1 + xC1xA2 log μC1A2 + xC 2xA1 log μC2A1 + xC2xA2 log μC2A2

(2)

where the sum of the mole fraction of cations equals 1 and the sum of the mole fraction of the anions equals 1. For Figure 5a, if the mole fraction of [hmim][2-CNpyr] was 0.8 and the mole fraction of [P66614][Tf2N] was 0.2, the mole fractions of [hmim]+, [2-CNpyr]−, [P66614]+, and [Tf2N]− would be 0.8,

log(μm ) = xC1A1 log μC1A1 + xC1A2 log μC1A2 + xC2A1 log μC2A1 + xC2A2 log μC2A2

(3)

at ionic neutrality and satisfying the ion mass balances, one would have to have the following mole fraction distribution of the four possible ILs: [hmim][2‐CNpyr]: 0.45

[hmim][Tf 2N]: 0.05

Figure 5. Viscosity of [hmim][2-CNpyr] (1) + [P66614][Tf2N] (2) [data, blue ●; Arrhenius random distribution model purple (a) and green (b)] and [P66614][2-CNpyr] (1) + [hmim][Tf2N] (2) [data, red ■; Arrhenius random distribution model orange (a) and dark red (b)] mixtures at 278.15 K.

[P66614 ][2‐CNpyr]: 0.05

[P66614 ][Tf 2N]: 0.45 L



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Table 4. Density of Various Binary Mixtures from 283.15 to 353.15 K at p = 0.1 MPaa (a) [P66614][3-Triz] (1) + [hmim][Tf2N] (2) ρ/g·cm−3

T/K mole fractionb

049

0.150

0.300

0.496

0.700

0.847

149

water content (weight fraction × 10 )

0.0020

0.0160

0.0170

0.0157

0.0356

0.0153

0.0098

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

1.3866 1.3821 1.3775 1.3757 1.3729 1.3684 1.3638 1.3593 1.3548 1.3503 1.3458 1.3413 1.3368 1.3324 1.3279 1.3235

1.2649 1.2606 1.2564 1.2548 1.2523 1.2482 1.2441 1.2400 1.2359 1.2319 1.2278 1.2237 1.2197 1.2156 1.2116 1.2076 (b) [P66614][3-Triz] (1)

1.0732 1.0697 1.0662 1.0648 1.0626 1.0591 1.0556 1.0522 1.0488 1.0455 1.0421 1.0387 1.0353 1.0319 1.0285 1.0251 (2)

0.9948 0.9913 0.9881 0.9868 0.9848 0.9816 0.9784 0.9752 0.9720 0.9688 0.9657 0.9625

0.9488 0.9453 0.9422 0.9408 0.9389 0.9358 0.9327 0.9296 0.9265 0.9236

0.9075 0.9042 0.9012 0.9000 0.8982 0.8952 0.8922 0.8892 0.8863 0.8834 0.8804 0.8775 0.8746 0.8717 0.8687 0.8658

2

1.1702 1.1663 1.1625 1.1609 1.1586 1.1548 1.1511 1.1473 1.1436 1.1398 1.1361 1.1324 1.1287 1.1250 1.1213 1.1176 + [bmim][Tf2N]

ρ/g·cm−3

T/K mole fraction

0

0.100

0.278

0.612

0.699

0.800

0.900

1


0.0013

0.0123

0.0211

0.0360

0.0371

0.0350

0.0385

0.0159

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

1.4511 1.4463 1.4415 1.4396 1.4368 1.4320 1.4273 1.4227 1.4180 1.4133 1.4087 1.4041 1.3994 1.3948 1.3902 1.3857

0.9984 0.9949 0.9917 0.9904 0.9884 0.9851 0.9819 0.9786 0.9755 0.9723 0.9692 0.9660 0.9628 0.9596 0.9564 0.9533

0.9646 0.9612 0.9581 0.9568 0.9549 0.9517 0.9486 0.9454 0.9423 0.9393 0.9362 0.9331 0.9300 0.9270 0.9239 0.9209

0.9341 0.9308 0.9277 0.9265 0.9246 0.9216 0.9185 0.9154 0.9124 0.9094 0.9064 0.9034 0.9004 0.8974 0.8944 0.8914

0.9066 0.9035 0.9004 0.8992 0.8974 0.8944 0.8914 0.8884 0.8855 0.8826 0.8796 0.8767 0.8737 0.8708 0.8678 0.8649

1.3458 1.2054 1.0317 1.3414 1.2013 1.0283 1.3370 1.1973 1.0250 1.3352 1.1957 1.0236 1.3326 1.1933 1.0216 1.3283 1.1894 1.0182 1.3239 1.1856 1.0149 1.3195 1.1817 1.0116 1.3152 1.1778 1.0083 1.3109 1.1739 1.0050 1.3065 1.1701 1.0018 1.3022 1.1662 0.9985 1.2979 1.1624 0.9952 1.2936 1.1585 0.9919 1.2893 1.1547 0.9886 1.2851 1.1509 0.9854 (c) [P2228][4-NO2pyra] (1) + [hmim][Tf2N] (2)

ρ/g·cm−3

T/K mole fraction

0

0.126

0.250

0.499

0.750

0.898

1


0.0012

0.0125

0.0151

0.0151

0.0186

0.0198

0.0089

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15

1.3856 1.3810 1.3765 1.3746 1.3719 1.3673 1.3628 1.3583 1.3537 1.3492 1.3447 1.3403 1.3358 1.3314 1.3269

1.3400 1.3355 1.3311 1.3294 1.3267 1.3224 1.3181 1.3138 1.3096 1.3054 1.3012 1.2970 1.2928 1.2887 1.2845

1.2965 1.2923 1.2882 1.2865 1.2841 1.2800 1.2759 1.2718 1.2677 1.2637 1.2596 1.2555 1.2515 1.2475 1.2435

1.2096 1.2058 1.2019 1.2004 1.1982 1.1945 1.1909 1.1872 1.1835 1.1799 1.1762 1.1726 1.1690 1.1653 1.1617

1.1243 1.1209 1.1175 1.1161 1.1141 1.1107 1.1075 1.1042 1.1009 1.0976 1.0943 1.0911 1.0878 1.0846 1.0813

1.0744 1.0712 1.0680 1.0668 1.0648 1.0617 1.0586 1.0555 1.0525 1.0494 1.0463 1.0433 1.0402 1.0372 1.0341

1.0407 1.0377 1.0346 1.0334 1.0315 1.0285 1.0255 1.0226 1.0196 1.0167 1.0138 1.0108 1.0079 1.0050 1.0021

M



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Table 4. continued (c) [P2228][4-NO2pyra] (1) + [hmim][Tf2N] (2) ρ/g·cm−3

T/K mole fraction

0

0.126

0.250

0.499

0.750

0.898

1


0.0012

0.0125

0.0151

0.0151

0.0186

0.0198

0.0089

1.3225 1.2803 1.2395 1.1581 (d) [P2228][4-NO2pyra] (1) + [hmmim][Tf2N] (2)

1.0781

1.0311

0.9992

353.15

ρ/g·cm−3

T/K mole fraction

0

0.124

0.250

0.500

0.751

0.900

1


0.0011

0.0168

0.0183

0.0194

0.0224

0.0258

0.0089

1.3730 1.3315 1.2900 1.2067 1.3684 1.3271 1.2859 1.2030 1.3639 1.3228 1.2818 1.1992 1.3621 1.3210 1.2801 1.1977 1.3595 1.3185 1.2776 1.1955 1.3551 1.3143 1.2737 1.1918 1.3508 1.3101 1.2697 1.1881 1.3464 1.3060 1.2657 1.1845 1.3420 1.3018 1.2617 1.1809 1.3376 1.2976 1.2577 1.1773 1.3332 1.2935 1.2538 1.1737 1.3289 1.2893 1.2498 1.1702 1.3245 1.2851 1.2458 1.1666 1.3202 1.2810 1.2419 1.1630 1.3158 1.2769 1.2379 1.1595 1.3115 1.2727 1.2340 1.1559 (e) [hmim][4-NO2pyra] (1) + [P66614][Tf2N] (2)

1.1238 1.1204 1.1170 1.1156 1.1136 1.1102 1.1069 1.1037 1.1004 1.0971 1.0939 1.0906 1.0874 1.0841 1.0809 1.0777

1.0742 1.0710 1.0678 1.0665 1.0646 1.0614 1.0583 1.0553 1.0522 1.0491 1.0461 1.0430 1.0400 1.0369 1.0339 1.0309

1.0407 1.0377 1.0346 1.0334 1.0315 1.0285 1.0255 1.0226 1.0196 1.0167 1.0138 1.0108 1.0079 1.0050 1.0021 0.9992

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

ρ/g·cm−3

T/K mole fraction water content (weight fraction × 10 ) 2

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

049

0.250

0.500

0.0057

0.0058

0.0162

0.0159

0.0470

1.0761 1.0818 1.0907 1.0724 1.0782 1.0871 1.0688 1.0746 1.0836 1.0673 1.0731 1.0822 1.0651 1.0710 1.0801 1.0615 1.0674 1.0766 1.0579 1.0638 1.0731 1.0544 1.0603 1.0696 1.0508 1.0568 1.0662 1.0473 1.0534 1.0628 1.0437 1.0499 1.0594 1.0401 1.0464 1.0560 1.0366 1.0429 1.0526 1.0330 1.0394 1.0492 1.0295 1.0359 1.0458 1.0260 1.0325 1.0424 (f) [hmmim][4-NO2pyra] (1) + [P66614][Tf2N] (2)

1.1056 1.1020 1.0985 1.0972 1.0952 1.0917 1.0883 1.0849 1.0816 1.0783 1.0750 1.0717 1.0683 1.0650 1.0617 1.0585

1.1336 1.1300 1.1266 1.1253 1.1233 1.1200 1.1166 1.1134 1.1102 1.1070 1.1038 1.1006 1.0974 1.0942 1.0911 1.0879

0.749

149

0.749

149

ρ/g·cm−3

T/K 049

mole fraction

0.250

0.501

water content (weight fraction × 10 )

0.0057

0.0196

0.0312

0.0436

0.0160

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15

1.0761 1.0724 1.0688 1.0673 1.0651 1.0615 1.0579 1.0544 1.0508 1.0473

1.0818 1.0782 1.0746 1.0732 1.0711 1.0675 1.0639 1.0604 1.0570 1.0535

1.0902 1.0867 1.0832 1.0817 1.0797 1.0762 1.0727 1.0693 1.0659 1.0626

1.1042 1.1005 1.0970 1.0957 1.0936 1.0903 1.0869 1.0835 1.0802 1.0770

1.1294 1.1260 1.1224 1.1210 1.1189 1.1157 1.1124 1.1092 1.1060 1.1028

2

N



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Table 4. continued (f) [hmmim][4-NO2pyra] (1) + [P66614][Tf2N] (2) ρ/g·cm−3

T/K mole fraction water content (weight fraction × 10 ) 2

328.15 333.15 338.15 343.15 348.15 353.15

049

0.250

0.501

149

0.0057

0.0196

0.0312

0.0436

0.0160

1.0437 1.0500 1.0592 1.0401 1.0465 1.0558 1.0366 1.0430 1.0524 1.0330 1.0395 1.0490 1.0295 1.0361 1.0457 1.0260 1.0326 1.0423 (g) [hmim][Tf2N] (1) + [emim][Tf2N] (2)c

1.0737 1.0705 1.0672 1.0640 1.0607 1.0575

1.0997 1.0966 1.0935 1.0904 1.0874 1.0843

0.749

ρ/g·cm−3

T/K mole fraction

0

0.150

0.300

0.500

0.700

0.850

149


0.0013

0.0059

0.0067

0.0080

0.0050

0.0143

0.0020

1.5341 1.5071 1.4818 1.4510 1.5290 1.5021 1.4769 1.4463 1.5239 1.4971 1.4720 1.4415 1.5219 1.4951 1.4701 1.4396 1.5189 1.4922 1.4672 1.4367 1.5139 1.4872 1.4623 1.4319 1.5089 1.4823 1.4575 1.4272 1.5039 1.4774 1.4527 1.4225 1.4990 1.4725 1.4479 1.4178 1.4941 1.4677 1.4431 1.4131 1.4891 1.4628 1.4383 1.4084 1.4842 1.4580 1.4335 1.4037 1.4793 1.4532 1.4288 1.3990 1.4744 1.4484 1.4241 1.3944 1.4696 1.4436 1.4194 1.3898 1.4647 1.4388 1.4147 1.3852 (h) [C6-thiourea][Tf2N] (1) + [bmpyra][Tf2N] (2)

1.4229 1.4182 1.4135 1.4116 1.4088 1.4041 1.3995 1.3948 1.3901 1.3855 1.3809 1.3762 1.3716 1.3670 1.3623 1.3577

1.4036 1.3990 1.3944 1.3925 1.3897 1.3851 1.3805 1.3759 1.3713 1.3668 1.3622 1.3577 1.3532 1.3486 1.3441 1.3397

1.3866 1.3821 1.3775 1.3757 1.3729 1.3684 1.3638 1.3593 1.3548 1.3503 1.3458 1.3413 1.3368 1.3324 1.3279 1.3235

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

ρ/g·cm−3

T/K mole fraction

49

0

0.251

0.502

0.748

149


0.0019

0.0061

0.0044

0.0058

0.0019

283.15 288.15 293.15 295.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

1.4654 1.4607 1.4560 1.4541 1.4512 1.4465 1.4418 1.4371 1.4324 1.4277 1.4230 1.4184 1.4138 1.4091 1.4046 1.4000

1.4312 1.4266 1.4219 1.4201 1.4173 1.4127 1.4081 1.4035 1.3989 1.3943 1.3897 1.3852 1.3806 1.3761 1.3716 1.3671

1.4004 1.3957 1.3914 1.3899 1.3874 1.3820 1.3776 1.3732 1.3688 1.3647 1.3604 1.3560 1.3517 1.3473 1.3428 1.3384

1.3758 1.3714 1.3669 1.3651 1.3624 1.3579 1.3535 1.3490 1.3446 1.3402 1.3358 1.3314 1.3270 1.3226 1.3183 1.3139

1.3522 1.3478 1.3434 1.3416 1.3390 1.3346 1.3302 1.3258 1.3215 1.3171 1.3128 1.3085 1.3042 1.2998 1.2956 1.2913

Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.005 MPa, and u(x1) = 0.001. In parts a, b, e, and g, u(ρ) = 0.002 g·cm−3. In parts c, f, and h, u(ρ) = 0.002 g·cm−3 for (2) and 0.006 g·cm−3 for (1). bThe mole fraction listed in the table is for (a) [P66614][3-Triz]; (b) [P66614][3-Triz]; (c) [P2228][4-NO2pyra]; (d) [P2228][4-NO2pyra]; (e) [hmim][4-NO2pyra]; (f) [hmmim][4-NO2pyra]; (g) [hmim][Tf2N]; (h) [C6-thiourea][Tf2N]. c Two different batches of [hmim][Tf2N] were used. One batch was used to measure the density of pure [hmim][Tf2N], while the other was used for the mixtures with [emim][Tf2N]. This could lead to a greater error in the calculation for excess molar volume. a

is entirely reasonable. This is especially likely for dialkylimidazolium cations, which contain an acidic proton on the carbon between the two nitrogen atoms. [Tf2N]− is well-known as a nonassociating anion.

Compared to the random distribution of 0.25 mole fraction of each, this indicates a very strong preference for [hmim][2CNpyr] compared to [hmim][Tf2N]. A preferred interaction between the basic [2-CNpyr]− anion and the imidazolium cation O



Article

more [P66614]+ cations in the mixture than [hmim]+ cations) and the [hmim]+ ILs in the mixture. In fact, all of the mixtures investigated here that show maxima and that require high positive values of G to fit the data involve a mixture of a phosphonium ILs with basic anions (AHA or acetate) mixed with an imidazolium [Tf2N]− IL. The maximum always occurs when there is a large concentration of the phosphonium cation and the basic anion. It is the attraction between the basic anions and the imidazolium cations that cause the viscosity increase. The situation is not ameliorated by replacing the acidic proton on the carbon between the two nitrogens of the imidazolium ring with a methyl group (i.e., [hmmim]+ instead of [hmim]+). For instance, the [P66614][2-CNpyr] + [hmim][Tf2N] mixture at 278.15 K requires G = 1.21 to fit the data, but the [P66614][2-CNpyr] + [hmmim][Tf2N] mixture still requires a positive G value of 0.89. We attribute this to the protons on the other two carbons (C4 and C5) also being somewhat acidic. Estimating Viscosities of Complementary Mixtures. Note that for six of the IL−IL mixtures ([P66614][acetate] + [hmim][Tf2N]; [P66614][3-Triz] + [hmim][Tf2N]; [P66614][4Triz] + [hmim][Tf2N]; [P66614][tetrazolide] + [hmim][Tf2N]; [P222(10201)][3-Triz] + [hmim][Tf2N]; [hmmim][4-NO2pyra] + [P66614][Tf2N]) we did not measure the viscosities of mixtures of the complementary pairs, although we have made the complementary ILs (e.g., [P66614][Tf2N] and [hmim][acetate]) and reported their pure IL viscosities.49 We used the complementary pure IL viscosities (e.g., [P66614][Tf2N] and [hmim][acetate]) and the viscosity at 0.5 mole fraction for the mixture that we did measure (e.g., [P66614][acetate] + [hmim][Tf2N]) to predict the viscosity as a function of composition for the pairs that we did not measure (e.g., [P66614][Tf2N] + [hmim][acetate]). This is explained and the results shown in Table S6 and Figures S7−S12 of the SI. Density Measurements. The densities of a few IL−IL mixtures were measured from 283.15 to 353.15 K, as shown in Table 4a−h. All of the densities decrease with an increase in temperature. In addition, the density of all mixtures were between the densities of the two pure ILs. The excess molar volumes were calculated, and the data are located in the Supporting Information, Table S4a−h. The VE values range from −0.5 to 0.9 cm3/mol and are plotted in Figure S6a−e. When the uncertainty in the density measurements from impurities in the samples are taken into account, one can safely say that the VE values are roughly zero for all of the mixtures measured. Freezing Point Depression. Some of the mixtures where tested for freezing point depression. After the viscosity measurements were done, the samples were placed in a sealed container in a freezer at 250 K for a few days. Some samples remained liquid even after two or more weeks in the freezer. The results are summarized in Table S5, located in the Supporting Information. Samples of triethylsulfonium bis(trifluoromethylsulfonyl) ([S222][Tf2N]), butylthiolanium bis(trifluoromethylsulfonyl)imide ([bthiol][Tf2N]), and S-octyl-1,1,3,3-tetramethylthiouronium bis(trifluoromethylsulfonyl)imide ([C8thiour][Tf2N]) were mixed with either a phosphonium [AHA] or an imidazolium [AHA]. The six mixtures were [P 2229 ][4NO2pyra]/[S222][Tf2N], [P66614][4-NO2pyra]/[bthiol][Tf2N], [hmmim][4-NO2pyra]/[bthiol][Tf2N], [P2228][4-NO2pyra]/ [bthiol][Tf 2 N], [P 66614 ][3-Triz]/[C 8 thiour][Tf 2 N], and [P66614][3-Triz]/[S222][Tf2N]. The mixtures decomposed over time with some mixtures producing a noticeable odor. This is not surprising because when [S222]+, [bthiol]+, and [othiourea]+

If one assumes this same preference for [hmim][2-CNpyr] over the entire composition range (limited by [2-CNpyr]− for half of the region and by [hmim]+ for the other half), along with eq 3, one obtains the viscosity predictions shown in Figure 5b. For the mixture of [hmim][2-CNpyr] with [P66614][Tf2N], the mole fractions of [hmim][Tf2N] and [P66614][2-CNpyr] have to be calculated first because they have the smallest nonrandom mole fractions at the original 0.5 mixture composition, as mentioned above. The mole fraction of [hmim][Tf2N] equals the mole fraction of [P66614][2-CNpyr]. For example, when the initial mole fraction is 0.2 for [hmim][2-CNpyr] and 0.8 for [P66614][Tf2N], the final nonrandom mole fractions of [hmim][Tf2N] and [P66614][2-CNpyr] would be 0.2 × 0.8 × 0.05 × 4 = 0.032. The 0.2 × 0.8 is the random distribution of the ions. Note that for the 0.5/0.5 mixture, 0.5 × 0.5 multiplied by 4 in the equation on the previous line yields unity, leaving the stipulated nonrandom mole fraction (0.05). For the initial mole fraction is 0.2 for [hmim][2-CNpyr] and 0.8 for [P66614][Tf2N], the mole fraction of [hmim][2-CNpyr] would be 0.2 − 0.032 = 0.168. The mole fraction of [P66614][Tf2N] would be 1 − 0.032 − 0.032 − 0.168 = 0.768. For the mixture of [P66614][2-CNpyr] with [hmim][Tf2N], the mole fraction of [P66614][2-CNpyr] has to be calculated first if the original mole fraction of [hmim][Tf2N] is greater than the original mole fraction of [P66614][2-CNpyr] and the mole fraction of [hmim][Tf2N] has to be calculated first if it has lower original mole fraction than the mole fraction of [P66614][2-CNpyr]. If the initial mole fraction was 0.2 for [hmim][Tf2N] and 0.8 for [P66614][2-CNpyr], the final nonrandom mole fraction of [hmim][Tf2N] would be 0.2 × 0.2 × 0.05 × 4 = 0.008. The mole fraction of [P66614][Tf2N] and [hmim][2-CNpyr] are 0.2 − 0.008 = 0.192. The mole fraction of [P66614][2-CNpyr] is 1 − 0.008 − 0.192 − 0.192 = 0.608. The lines are not smooth. They have “break points” when a particular anion or cation is completely used up. The real situation would probably have some transition as the composition of a particular anion or cation becomes very small, but we have chosen to use the simplest nonrandom model that we could envision. Obviously, these predictions (based on determining the nonrandom distribution of the four possible ILs in the mixture from the experimental viscosity at 0.5 mole fraction) provide a much better representation of the experimental viscosities over the entire composition range than the random distribution. Figure 5a is repeated in the SI as Figure S5a, and the equivalent calculations for the other three complementary pairs are shown in Figure S5b−d in the SI. These results suggest that the ions do arrange themselves in orientations that, on average, favor the formation of preferred cation/anion pairs. Of course, all of the ions are independent and mobile and should not be thought of as forming static or long-lived [C][A] species. However, the above interpretation will never produce a prediction of a maximum in the viscosity of any mixture. Therefore, at least in some IL−IL mixtures, there must be additional attractive interactions that cause the viscosity to be higher than one might expect, even higher than the viscosity of the most viscous of the four [C][A] possibilities. For instance, the [P66614][2-CNpyr] + [hmim][Tf2N] mixture at 278.15 K (Figure 5a and b) with an overall [P 66614 ][2-CNpyr] composition of 0.9 has an experimental viscosity of 1844 mPa· s, which is higher than the viscosity of pure [P66614][2-CNpyr], the most viscous of the four possible ILs with a viscosity of 1764 mPa·s. In this case, we attribute the high viscosity to attractions between the basic anions (i.e., [2-CNpyr]−) that are paired with phosphonium cations (necessary because there are so many P



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cations were paired with [AHAs]− they tended to decompose during the synthesis process. Thus, no results are reported for these mixtures.

CONCLUSIONS The viscosity of 23 different IL−IL mixtures were measured. The mixture of phosphonium [AHA] ILs with imidazolium [Tf2N] ILs showed positive deviations (G value is positive) from the modified Arrhenius model (eq 1), indicating higher than expected viscosities. By contrast, imidazolium [AHA] ILs mixed with phosphonium [Tf2N] ILs showed a negative deviation (G value is negative) from eq 1, indicating lower than expected viscosities. Mixtures with either a common cation or a common anion had G values near 0. Some mixtures had a maximum in the viscosity. Four complementary pairs ([C1][A1] + [C2][A2] and [C2][A1] + [C1][A2] mixtures) were studied, and they had identical viscosities at overall mole fractions of 0.50, clearly indicating that the anions and cations of the mixtures exchange freely. The Arrhenius model assuming a random distribution of the four possible cation/anion combinations (e.g., 0.25 mole fraction of each possibility at 50/50) did not provide a good representation of the data. On the other hand, assuming a nonrandom distribution of the four possible cation/anion combinations, fit to the 50/50 data point, provided a better representation. However, additional IL−IL interactions are needed to explain maxima in the experimental viscosity data. Excess molar volumes are very small for all of the systems measured. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00221. Details on the structure of the cations and anions and the names, abbreviations, and purities of the materials used can be found in SI. In addition, the SI contains a comparison of the pure IL viscosities, graph versions of Table 2a−x, G values from 308.15 to 323.15 K, graphs of the data in Table 4a−d at 303.15 K, graphs of all other mixtures using the random Arrhenius model and nonrandom Arrhenius model, as well as data and graphs of excess molar volumes of the mixtures, graphs of estimated viscosity profiles of complementary mixtures, and freezing point depression analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Joan F. Brennecke: 0000-0002-7935-2134 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Aruni Desilva, Dr. Mauricio QuirozGuzman, and Dr. Oscar Morales-Collazo for the synthesis of the ionic liquids, which we describe in more detail in a previous publication.49 This material is based upon work supported by the Department of Energy ARPAe under Award No. AR000019. Q



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