Density and Derived Thermodynamic Properties of 1-Ethyl-3

Article pubs.acs.org/jced

Density and Derived Thermodynamic Properties of 1-Ethyl-3-methylimidazolium Alkyl Sulfate Ionic Liquid Binary Mixtures with Water and with Ethanol from 288 K to 318 K Esther Rilo,*,† Luis M. Varela,†,‡ and Oscar Cabeza† †

Mesturas Group, Departamento de Física, Facultad de Ciencias, Universidade da Coruña, Campus da Zapateira s/n, 15008 A Coruña, Spain ‡ Nanomaterials and Soft Matter Group, Departamento de Física de la Materia Condensada, Facultad de Física, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain ABSTRACT: We present in this paper experimental measurements of density for pure ionic liquids of the 1-ethyl-3-methylimidazolium alkyl sulfate family, with the alkyl group being ethyl ([EMIM][ES]), butyl ([EMIM][BS]), hexyl ([EMIM][HS]), and octyl ([EMIM][OS]), and their binary mixtures with water and with ethanol in the whole composition range, at 288 K, 298 K, 308 K, and 318 K and under atmospheric pressure. The IL family chosen allows us to analyze the influence of the alkyl chain length of the anion. We compare results with those previously published by us and by other research groups, where the influence of the alkyl chain length of the cation in volumetric properties is studied. We observed that the density for both kind of binary mixtures decreases with the increase of the alkyl chain length of the anion and with the decrease of the molar fraction of ionic liquid. From density data we extract molar volumes and excess molar volumes. Molar volumes can be adjusted to a straight line versus the molar fraction, and the excess molar volumes values are very small (less than 0.5 % of the molar volume value) for all of the measured systems. This fact allows us to state that these binary systems are quasi ideal from the molar volume point of view. Finally we study the influence of temperature on this property by means of isobaric expansivity, finding very different results depending on the solvent used.

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Table 1. Chemical Formula, Molar Mass (Mw), and Water Content for Pure ILs

INTRODUCTION Ionic liquid (IL) physical properties, mainly negligible vapor pressure and great solution capacity, make them appropriate for substituting traditional damaging and volatile organic solvents used in industry. For this reason a great number of industrial processes are being redesigned, and new environmentally friendly ones are being developed using ILs. In addition, one of the most attractive features of ILs is their ability to be tailored for a specific purpose by careful selection of the cation and the anion. An intensive investigation on IL physical properties, including density, viscosity, surface tension, refractive index, electrical conductivity, and so forth, is necessary to develop those industrial processes and to design new ILs for specific purposes. It has been published1−4 that the addition of different solvents, such as water and ethanol, strongly affects IL physical properties, and due to this fact this investigation includes measurements for both pure ILs and mixed with other solvents. However, given that there are about 106 possible ILs, the selection or design of the most suitable IL for a given application would be greatly assisted by theoretical models and numerical simulations, and experimental data are essential for developing and checking both of them. Although a great number of research groups are working on this subject and the number of published papers on physical © XXXX American Chemical Society

Mw IL [EMIM][ES] [EMIM][BS] [EMIM][HS] [EMIM][OS]

C8H16N2O4S C10H20N2O4S C12H24N2O4S C14H28N2O4S

water content (%)

g·mol−1

dealer

Karl Fisher

236.29 264.35 292.40 320.46

< 0.05 ≤1 ≤1 ≤1

0.02 0.12 0.06

properties of ILs has increased exponentially during the past decade, systematic studies that analyze the influence on physical properties of anion and cation size, temperature, or pressure are scarce. We present experimental density data for the 1-ethyl-3methylimidazolium alkyl sulfate family (being alkyl group ethyl [EMIM][ES], butyl [EMIM][BS], hexyl [EMIM][HS], and octyl [EMIM][OS]) and their binary mixtures with water and with ethanol in the whole range of concentrations, at 288 K, 298 K, 308 K, and 318 K, always at atmospheric pressure. We choose ILs from this family because they have halogen-free Received: December 2, 2011 Accepted: June 20, 2012

A

dx.doi.org/10.1021/je2012633 | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 4. Density, ρ, for Aqueous Binary Mixtures of [EMIM][HS] and [EMIM][OS] at Different Temperatures

Table 2. Experimental and Literature Values of Density for Pure Compounds at Four Temperatures T/K [EMIM][ES]

ρexp/g·cm−3

288 298

[EMIM][BS] [EMIM][HS] [EMIM][OS]

water ethanol

1.2453 1.2383

308 318 298 298 288 298

1.2316 1.2249 1.1757 1.1296 1.1007 1.0938

308 318 298 298

1.0870 1.0805 0.9971 0.7858

ρlit/g·cm−3

ρ/g·cm−3

8

1.24446 1.238811 1.237638 1.2373313 1.230888 1.224158 1.185 1.135 1.1008217 1.0941517 1.105 1.0875217 1.0809717 0.997023 0.785468

x1 1.0000 0.9346 0.7976 0.7243 0.5820 0.3982 0.2075 0.0965 0.0501 0.0000

ρ/g·cm−3 T = 288 K

1.0000 0.8726 0.7975 0.6798 0.5829 0.4939 0.3950 0.2988 0.1961 0.1022 0.0700 0.0319 0.0000

1.2453 1.2426 1.2408 1.2371 1.2333 1.2284 1.2204 1.2085 1.1861 1.1419 1.1141 1.0645 0.9991

1.0000 0.9416 0.8922 0.7872 0.6909 0.5868 0.4866 0.3957 0.2973 0.2007 0.0987 0.0526

1.1825 1.1818 1.1814 1.1796 1.1779 1.1750 1.1717 1.1672 1.1593 1.1461 1.1143 1.0810

T = 298 K

T = 308 K

[EMIM][ES] + Water 1.2383 1.2358 1.2339 1.2302 1.2264 1.2214 1.2134 1.2014 1.1789 1.1353 1.1080 1.0599 0.9971 [EMIM][BS] + Water 1.1757 1.1750 1.1745 1.1729 1.1711 1.1684 1.1648 1.1602 1.1523 1.1391 1.1075 1.0758

T = 318 K

1.2316 1.2290 1.2271 1.2234 1.2194 1.2144 1.2063 1.1943 1.1718 1.1285 1.1018 1.0544 0.9941

1.2249 1.2223 1.2203 1.2166 1.2125 1.2074 1.1993 1.1872 1.1646 1.1215 1.0951 1.0484 0.9902

1.1690 1.1683 1.1679 1.1662 1.1643 1.1615 1.1580 1.1532 1.1453 1.1319 1.1005 1.0696

1.1624 1.1617 1.1612 1.1595 1.1576 1.1547 1.1511 1.1463 1.1383 1.1247 1.0934 1.0632

1.1366 1.1358 1.1345 1.1335 1.1312 1.1258 1.1114 1.0865 1.0614 0.9991

1.0000 0.9371 0.8545 0.7747 0.7373 0.5963 0.3923 0.1002 0.0053

Table 3. Density, ρ, for Aqueous Binary Mixtures of [EMIM][ES] and [EMIM][BS] at Four Temperatures x1

T = 288 K

T = 298 K

T = 308 K

[EMIM][HS] + Water 1.1296 1.1290 1.1277 1.1268 1.1245 1.1190 1.1048 1.0802 1.0560 0.9971 [EMIM][OS] + Water 1.0938 1.0935 1.0930 1.0924 1.0921 1.0905 1.0863 1.0606 1.0053

T = 318 K

1.1231 1.1223 1.1212 1.1203 1.1178 1.1122 1.0978 1.0737 1.0502 0.9941

1.1166 1.1158 1.1147 1.1138 1.1112 1.1054 1.0908 1.0669 1.0441 0.9902

1.0870 1.0869 1.0864 1.0857 1.0855 1.0838 1.0796 1.0540 1.0018

1.0805 1.0804 1.0799 1.0792 1.0791 1.0772 1.0729 1.0474 0.9973

Table 5. Density, ρ, for Binary Mixtures of [EMIM][ES] and [EMIM][BS] with Ethanol at Four Temperatures ρ/g·cm−3 x1

anions and are suitable for large-scale applications due to the reasonable prices of these ILs.5 Moreover they are stable against hydrolysis, especially [EMIM][BS], [EMIM][HS], and [EMIM][OS], although [EMIM][ES] is only prone to hydrolysis under acidic conditions and elevated temperatures. We compare results obtained for this work with those previously measured by us for other imidazolium-based ILs.6 From density data we have calculated molar volumes (Vm), excess molar volumes (VEm), and the coefficient of thermal expansion (α).

T = 288 K

1.0000 0.9211 0.8270 0.7421 0.6483 0.5667 0.4727 0.3485 0.2469 0.1233 0.0494 0.0000

1.2453 1.2348 1.2205 1.2053 1.1853 1.1649 1.1368 1.0884 1.0353 0.9436 0.8649 0.7944

1.0000 0.8794 0.7858 0.6866 0.5699 0.4993 0.4691 0.3812 0.2904 0.1938 0.0989 0.0498

1.1825 1.1699 1.1584 1.1437 1.1220 1.1059 1.0982 1.0721 1.0374 0.9870 0.9151 0.8645

T = 298 K

T = 308 K

[EMIM][ES] + Ethanol 1.2383 1.2316 1.2279 1.2211 1.2138 1.2068 1.1983 1.1913 1.1782 1.1712 1.1577 1.1506 1.1295 1.1222 1.0809 1.0734 1.0276 1.0199 0.9356 0.9276 0.8566 0.8481 0.7858 0.7771 [EMIM][BS] + Ethanol 1.1757 1.1690 1.1631 1.1563 1.1516 1.1447 1.1367 1.1297 1.1149 1.1079 1.0988 1.0911 1.0910 1.0839 1.0648 1.0576 1.0300 1.0224 0.9793 0.9716 0.9071 0.8989 0.8562 0.8479

T = 318 K 1.2249 1.2143 1.2000 1.1844 1.1642 1.1435 1.1150 1.0660 1.0123 0.9195 0.8397 0.7683 1.1624 1.1496 1.1380 1.1228 1.1010 1.0837 1.0769 1.0504 1.0148 0.9638 0.8909 0.8395

At our knowledge only density for two of the binary systems studied in this work has been measured previously:7 [EMIM][ES] + water4,8−10 and [EMIM][ES] + ethanol,8,11−13 and although some more papers about pure [EMIM][ES] have B



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Table 6. Density, ρ, for Binary Mixtures of [EMIM][HS] and [EMIM][OS] with Ethanol at Four Temperatures ρ/g·cm−3 x1

T = 288 K

1.0000 0.9063 0.7455 0.7252 0.6214 0.4947 0.4377 0.3246 0.2202 0.0977 0.0000

1.1366 1.1285 1.1132 1.1109 1.0973 1.0755 1.0632 1.0317 0.9900 0.9112 0.7944

1.0000 0.9047 0.7920 0.6844 0.5975 0.4964 0.3955 0.2985 0.1901 0.0980

1.1007 1.0944 1.0857 1.0756 1.0656 1.0509 1.0316 1.0063 0.9641 0.9064

T = 298 K

T = 308 K

[EMIM][HS] + Ethanol 1.1296 1.1231 1.1218 1.1152 1.1064 1.0997 1.1041 1.0974 1.0904 1.0835 1.0684 1.0614 1.0560 1.0489 1.0243 1.0171 0.9824 0.9749 0.9034 0.8953 0.7858 0.7771 [EMIM][OS] + Ethanol 1.0938 1.0870 1.0875 1.0810 1.0789 1.0723 1.0688 1.0621 1.0587 1.0519 1.0439 1.0370 1.0244 1.0174 0.9989 0.9918 0.9565 0.9490 0.8985 0.8907

T = 318 K 1.1166 1.1088 1.0930 1.0907 1.0768 1.0544 1.0420 1.0098 0.9674 0.8874 0.7683 1.0805 1.0744 1.0658 1.0554 1.0452 1.0301 1.0103 0.9844 0.9414 0.8828

Figure 2. Molar volume versus molar fraction for binary mixtures with (a) water and (b) ethanol at 298 K: □, [EMIM][ES]; ●, [EMIM][BS]; ▲, [EMIM][HS]; ×, [EMIM][OS].

have been carried out taking into account the viscosity of the sample (viscosity correction). ILs used were purchased to Merck except for [EMIM][ES] whose dealer was IoLitec. Their purities are better than 98 % for [EMIM][BS] and [EMIM][HS] and better than 99 % for [EMIM][ES] and [EMIM][OS], and they were not further purified. The ethanol used was from Panreac with a purity better than 99.5 %, and the water employed for preparing mixtures was Milli-Q grade. Due to the hygroscopic character of the ILs, chemicals were opened from their original tin in a dry chamber with a relative humidity grade lower than 10 %. This humidity grade, and the speed of the process, ensures that our original ILs were not contaminated with moisture as we recently studied quantitatively.18 Samples of pure ILs were bottled and sealed before taking them out of the dry chamber. The water mass fraction certified by the dealer and those obtained experimentally by means of a Karl Fisher 701 KF Titrino coulometer are compared in Table 1 for three of the ILs used. For [EMIM][OS] the manufacturer certifies that the water mass fraction is lower than 0.01, and no additional analysis were done. It is important to note that [EMIM][OS] becomes a jelly for IL molar fractions between 0.12 and 0.28, and consequently no experimental data are presented in the mentioned concentration range.

Figure 1. Comparison of experimental density values for binary mixtures of [EMIM][ES] at 298 K with data found in literature. ◊, aqueous experimental; ○, aqueous from ref 8; +, aqueous from ref 9; □, with ethanol experimental; ∗, with ethanol from ref 8; △, with ethanol from ref 13.

been published.14−16 Also we found only one paper for pure [EMIM][BS] and [EMIM][HS]5 and two for [EMIM][OS].5,17 For the ILs and their mixtures mentioned we compare results published with those experimentally obtained by us, presenting both sets of data with a very good agreement.

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EXPERIMENTAL PROCEDURE Density (ρ), was measured in an Anton Paar SVM 3000 Stabinger viscodensimeter, which was thermostatted with a Peltier cell with an uncertainty of ± 0.02 °C. The resolution in the measurement of density is 1·10−4 g·cm−3, and its uncertainty is 5·10−4 g·cm−3. Each measurement were performed at least three times to ensure reproducibility. These measurements C



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Figure 4. Isobaric expansivities calculated from eq 5 for (a) aqueous binary mixtures and (b) mixtures of ILs with ethanol.

studied. This trend was also observed with the increase of the alkyl chain length of the cation as we published before,6 and it is in agreement with the tendency found in literature for other IL families.19,20 From density data we extract molar volumes and excess molar volumes using the usual expressions:

Figure 3. Excess molar volume versus molar fraction for binary mixtures with (a) water and (b) ethanol at 298 K: □, [EMIM][ES]; ●, [EMIM][BS]; ▲, [EMIM][HS]; ×, [EMIM][OS]. Solid lines are the best fit of eq 3 with parameters given in Table 7.

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Vm =

RESULTS AND DISCUSSION Table 2 shows experimental density data (ρ) for the pure compounds studied in this work together with those previously reported by other authors.5,8,11,13,17 Comparing density data measured here with others published in the literature, it can be observed that both sets of data are in good agreement, with deviations lower than 0.08 % between them. Tables 3 and 4 include the density data measured for the aqueous binary systems studied (x1 IL + x2 water), and in Tables 5 and 6 we present the same magnitude for mixtures with ethanol (x1 IL + x2 ethanol). For all of the systems studied measurements were done at 288 K, 298 K, 308 K, and 318 K, except for [EMIM][OS] + water that was measured at 298 K, 308 K, and 318 K, because at 288 K it jellifies in a broad range of concentrations. In Figure 1 we show the good agreement between our experimental data for the two binary systems [EMIM][ES] + water and [EMIM][ES] + ethanol and data published in literature.8,9,13 As can be observed in Tables 2, 3, and 4, density decreases as the alkyl chain length of the anion and temperature increase in all binary systems

x1M1 + x 2M 2 ρ

⎛x M xM ⎞ VmE = Vm − ⎜⎜ 1 1 + 2 2 ⎟⎟ ρ2 ⎠ ⎝ ρ1

(1)

(2)

where Vm is the molar volume, VEm is the excess molar volume, ρ is the mixture density, ρ1 is the pure IL density, and ρ2 is the solvent density (water or ethanol). The uncertainty for excess molar volume values given are lower than 0.05 cm3·mol−1. Figure 2a,b plots molar volumes for aqueous mixtures and those with ethanol, respectively, at 298 K, as observed they follow nearly a straight line versus molar fraction. Excess molar volumes at 298 K for both kinds of mixtures are presented in Figure 3a,b, and as expected they are very low. These last were fitted to the Redlich−Kister equation:21 n

VmE = x1x 2 ∑ Ai (2x1 − 1)i − 1 i=1

(3)

where Ai are the polynomial coefficients. The degree (n − 1) of the polynomial Redlich−Kister equation was optimized by D



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Table 7. Best Fitting Parameters for the Redlich−Kister Type Equation 3 Used to Reproduce Excess Molar Volumes and Standard Deviations Calculated with Equation 4 T/K

A1

A2

A3

288 298 308 318 288 298 308 318 288 298 308 318 298 308 318

−1.5831 −1.4402 −1.2173 −1.0436 −1.3774 −1.2382 −1.0493 −0.8406 −0.8235 −0.7662 −0.5332 −0.3522 −0.4452 −0.3762 −0.1853

1.3462 1.1431 0.9824 0.8649 1.1496 0.7469 0.6573 0.5205 0.7499 0.5206 0.2534 0.0470 0.0933 −0.0805 −0.2995

−1.3061 −1.1107 −1.2748 −1.2555 −0.9231 −0.8174 −0.6526 −0.5199 −0.3673 −0.3150 −0.4709 −0.2559 −0.2111 0.5027 0.0717

288 298 308 318 288 298 308 318 288 298 308 318 288 298 308 318

−2.4823 −2.7022 −2.9622 −3.2247 −1.8178 −2.0396 −2.1715 −2.3572 −1.7178 −1.9273 −2.1305 −2.3237 −1.1376 −1.3984 −1.6846 −1.8684

0.8461 0.9377 1.0729 1.2429 0.8486 0.9928 1.2134 1.3330 0.8699 1.3339 1.5022 1.6568 1.4018 1.2100 1.2620 1.2251

−0.7519 −1.1380 −1.0727 −1.3229 −1.0509 −1.3584 −1.7080 −1.7126 −0.7620 −1.5322 −1.0301 −1.4073 −0.6540 −0.9618 −1.2334 −1.4549

A4

A5

s/cm3·mol‑1

−0.0731 −0.6171 −0.7844 −1.1080 0.6144 0.6323 0.0018 −0.2204 0.6232 0.1682 0.2604 0.0371 0.0643 −0.7333 −0.8623

1.8806 1.8921 2.7004 3.0931 −0.8089 −0.3542 −0.2428 0.1712 0.3960 0.5380 1.4015 1.4253 0.0474 −1.0357 −0.1056

0.0169 0.0176 0.0188 0.0220 0.0105 0.0038 0.0041 0.0043 0.0073 0.0043 0.0102 0.0135 0.0031 0.0088 0.0154

2.2052 2.2461 2.4700 2.7651 2.4691 2.4900 2.4927 2.8172 4.3562 1.8719 1.7610 1.8528 1.3824 1.9815 1.7655 2.1732

−2.4953 −2.5056 −2.8845 −2.8915 −1.7779 −1.7646 −1.7174 −2.1062 −0.4252

0.0183 0.0247 0.0188 0.0213 0.0254 0.0276 0.0365 0.0488 0.0171 0.0495 0.0251 0.0457 0.0071 0.0064 0.0083 0.0101

Water [EMIM][ES]

[EMIM][BS]

[EMIM][HS]

[EMIM][OS]

Ethanol [EMIM][ES]

[EMIM][BS]

[EMIM][HS]

[EMIM][OS]

applying the F-test. Coefficients of eq 3 and standard deviations are presented in Table 7 for molar volumes at 298 K. Standard deviations were calculated from ⎛ ∑N (V E − V E )2 ⎞1/2 m,cal m,exp i=1 ⎟ s = ⎜⎜ ⎟ − N 1 ⎝ ⎠

−2.6129 −1.8294 −0.9398 −0.7235 −1.3210 −1.4034

water value and consequently each ethanol molecule placed in a hole diminishes the molar volume more than a water molecule in an equivalent hole. We also observe that higher temperatures imply more negative excess molar volume values as usually happens. The variation of the molar volume with temperature can be expressed by the coefficient of thermal expansion or isobaric expansivity, α, defined as

(4)

where VEm,cal is the excess molar volume value fitted with eq 3, VEm,exp, is the experimental excess molar volume value, and N is the number of adjusted data. As it can be observed excess molar volume values are very small for all measured systems (less than 0.5 % of the molar volume value) and also for all of the temperatures measured. This fact makes the quasi ideal behavior of these mixtures clear as we previously reported for water and ethanol mixtures with [CnMIM][BF4] ILs.6 As observed excess molar volumes are negative for all of the mixtures. This could be explained using the pseudolattice theory recently adapted to explain IL properties:22 some of the solvent molecules are placed in the free holes of the pseudolattice formed by IL cations. These also explain why values are less negative for longer alkyl chain lengths. Mixtures with ethanol present excess molar volume values more negative than those calculated for aqueous mixtures because available holes are the same for both kind of mixtures, but the ethanol molar volume is higher than the

⎛ ∂ ln ρ ⎞ ⎟ α = −⎜ ⎝ ∂T ⎠ P

(5)

We obtain the α values from the slope of the linear relationship of ln ρ versus T. Values calculated for all mixtures measured are presented in Table 8 and plotted versus molar fraction in Figure 4a for mixtures with water and in Figure 4b for those with ethanol. Isobaric expansivity values were calculated for the temperature range between 288 K and 318 K. The calculated values of α for pure [EMIM][ES] are in agreement with data published by Matkowska et al.13 We observe that isobaric expansivities slightly increase with the number of carbon atoms, and they are significantly lower than those of typical organic liquids23 and closer to water isobaric expansivity values. As observed in Figure 4, the behavior of α with composition is completely different for both solvents; thus for aqueous mixtures its value is similar (but higher) to that of E



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Table 8. Isobaric Expansivity Values for Pure Compounds and Mixtures [EMIM][ES]

[EMIM][BS]

[EMIM][HS]

x1

α·103/K‑1

x1

α·103/K‑1

1.0000 0.8726 0.7975 0.6798 0.5829 0.4939 0.3950 0.2988 0.1961 0.1022 0.0700 0.0319

0.5498 0.5493 0.5551 0.5567 0.5675 0.5748 0.5815 0.5927 0.6092 0.6009 0.5726 0.5111

1.0000 0.9416 0.8922 0.7872 0.6909 0.5868 0.4866 0.3957 0.2973 0.2007 0.0987 0.0526

0.5715 0.5718 0.5729 0.5729 0.5798 0.5821 0.5894 0.6022 0.6093 0.6298 0.6324 0.5559

1.0000 0.9211 0.8270 0.7421 0.6483 0.5667 0.4727 0.3485 0.2469 0.1233 0.0494

0.5498 0.5578 0.5660 0.5834 0.5984 0.6178 0.6457 0.6949 0.7492 0.8626 0.9845

1.0000 0.8794 0.7858 0.6866 0.5699 0.4691 0.3812 0.2904 0.1938 0.0989 0.0498

0.5715 0.5833 0.5931 0.6151 0.6298 0.6529 0.6818 0.7349 0.7936 0.8948 0.9760

[EMIM][OS]

x1

α·103/K‑1

x1

α·103/K‑1

1.0000 0.9346 0.7976 0.7243 0.5820 0.3982 0.2075 0.0965 0.0501

0.5908 0.5925 0.586 0.5856 0.5949 0.6096 0.6248 0.6065 0.5481

1.0000 0.9371 0.8545 0.7373 0.5963 0.3923 0.1002 0.0053 0.0000

0.6094 0.6003 0.6029 0.5988 0.6136 0.6206 0.6262 0.3995 0.2987

1.0000 0.9063 0.7455 0.7252 0.6214 0.4947 0.4377 0.3246 0.2202 0.0977

0.5908 0.5873 0.6106 0.6118 0.6288 0.6601 0.6731 0.7147 0.7695 0.8852

1.0000 0.9047 0.7920 0.6844 0.5975 0.4964 0.3955 0.2985 0.1901 0.0980 0.0000

0.6162 0.6137 0.6163 0.6316 0.6443 0.6660 0.6950 0.7304 0.7920 0.8787 1.1124

Water

Ethanol

the pure IL down to a molar fraction of x1 ≈ 0.1, and then they decrease abruptly to the pure water value, following the same curve independently of the anion alkyl chain length. In contrast, for the systems with ethanol α varies uniformly with concentration following different curves for each IL compound, but at about x1 ≈ 0.25 values of the four systems converge, and they increase to the pure ethanol α value following the same curve. This completely different behavior of α depending on the solvent used resembles the behavior of the surface tension, σ, for the same systems.24 Surface tension values obtained for aqueous mixtures do not appreciably change from the pure IL one down to very diluted mixtures (then it increases abruptly to the σ pure water value). In contrast, for the systems with ethanol σ changes nearly linearly from the pure IL value to that of a given concentration (x1 ≈ 0.3), and then the four systems have the same value down to the pure ethanol σ one.24 This different behavior can be qualitatively explained using the Bahe−Varela theoretical model, which assumes that the cations of the IL forms a pseudolattice, with water clusters in the void space, so water do not appreciably contribute to thermal expansion but only to the pseudolattice net distances. This does not happen for ethanol mixtures because in this case solvent molecules do not cluster and are mixed uniformly with IL anions, so contributing to its temperature dilation.22

observe an equivalent trend when we study the influence of the alkyl chain length of the anion: density decreases when alkyl chain length increases. Molar volumes can be adjusted to a straight line versus the molar fraction, and the excess molar volumes values are very small for all of the measured systems.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +34 981 167065. E-mail: [email protected]. Funding

This work was supported by the Directorate General for R+D+i of the Xunta de Galicia (Grants No. 10-PXIB-103-294 PR and 10-PXIB-206-294 PR). Both research projects have been cofinanced with the European Regional Development Fund funds. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS We present experimental measurements of density for binary mixtures prepared with ILs of the 1-ethyl-3-methyl imidazolium alkyl sulfate family with water and with ethanol. The alkyl chain of the anion studied is ethyl, butyl, hexyl, and octyl, and most of data published here were not published before. The density behavior with concentration is very similar for both mixtures with water and those with ethanol. Also for both kinds of mixtures we F



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