Vapor Pressure Osmometry, Volumetry, and Compressibility

Article Cite This: J. Chem. Eng. Data 2017, 62, 4073−4082

pubs.acs.org/jced

Vapor Pressure Osmometry, Volumetry, and Compressibility Properties for Solutions of Several Imidazolium Based Ionic Liquids in (Glycine + Water) Solutions Sajjad Noshadi and Rahmat Sadeghi* Department of Chemistry, University of Kurdistan, Sanandaj, Iran S Supporting Information *

ABSTRACT: In this study, the effect of glycine on the thermodynamic properties of aqueous solutions containing ionic liquids 1-butyl-3-methylimidazolium chloride, [C4mim]Cl, 1-hexyl-3-methylimidazolium chloride, [C6mim]Cl, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [C4mim][CF3SO3], has been studied. For this purpose, first with help of the vapor pressure osmometery (VPO) method the water activity measurements have been performed on the ternary aqueous systems of (glycine + [C4mim]Cl, [C6mim]Cl, and [C4mim][CF3SO3]) at T = 308.15 K. The influence of the IL nature on the vapor− liquid equilibria behavior of the investigated systems has been studied. The VPO measurements of these solutions show that due to the unfavorable IL-glycine interactions, the negative deviations from the semi-ideal state and in conclude the soluting-out effects have been observed in these solutions. The soluting-out phenomena appears by aqueous biphasic system formation in the case of aqueous [C4mim][CF3SO3] + glycine system and precipitation of the glycine from the solution in the case of ternary [C4mim]Cl/[C6mim]Cl + glycine + water systems. In the second part of this work, density and sound velocity of solutions of the ILs in pure water and aqueous glycine solutions have been measured at four different temperatures. These data were used to obtain the apparent molar volume, Vϕ, isentropic compressibility, Kϕ, and those in the infinite dilution.

1. INTRODUCTION Ionic liquids (ILs), a novel class of green solvents, have received considerable investigations in different fields due to the specific properties indicated by these liquid electrolytes, such as their good solubility, negligible volatility, lack of inflammability, and the feasibility of fine-tuning the IL chemical structure to achieve a desired purpose.1−3 Recently, ILs are being investigated in a variety of industrial applications, and their considerable importance for the formation of aqueous biphasic systems (ABS) in purification and separation process is highlighted.4−7 The ABS have a biocompatible environment for purification and separation of biological products, as both equilibrium phases are aqueous, thus these systems have obtained significant attention in recent years in biotechnology.8−10 In the past few years, it has found that aqueous solutions of a number of ILs in the presence of some amino acids could be used to form ABS.11−13 Therefore, systematic investigations on the thermodynamic properties of different ILs in aqueous solutions of amino acids can be used for a better understanding © 2017 American Chemical Society

of the molecular-level IL−amino acid interactions and also for optimum design of industrial separation and purification process regarding the proposed IL−amino acid-based ABS. For this goal, volumetric, compressibility, and vapor pressure osmometry studies of IL−amino acid−water ternary mixtures can establish some new data for gaining information about the molecular interactions between their components. In this work, we studied the volumetric and compressibility properties at four different temperatures and vapor pressure osmometery (VPO) measurements at T = 308.15 for three imidazolium-based ILs in aqueous solutions of 0.7041 ± 1·10−3 mol·kg−1 glycine. The investigated ILs are 1-butyl-3-methyl-imidazolium trifluoromethanesulfonate, [C4mim][CF3SO3], 1-butyl-3-methylimidazolium chloride, [C4mim]Cl, and 1-hexyl-3-methylimidazolium chloride, [C6mim]Cl. However, relatively little data has been Received: March 28, 2017 Accepted: November 20, 2017 Published: December 4, 2017 4073

DOI: 10.1021/acs.jced.7b00297 J. Chem. Eng. Data 2017, 62, 4073−4082

Journal of Chemical & Engineering Data

Article

Table 1. Names, Sources, Water Content Along with the Physical Properties of the Chemicals Studiedb

a

chemical

CAS. no

source

purification method

mass fraction purity

water content/ppm

d/g·cm−3 (at 298.15 K and 845 hPa)

[C4mim]Cl [C6mim]Cl [C4mim][CF3SO3] glycine

79917-90-1

Merck Synthesis Aldrich Merck

none recrystallization none none

≥0.98 ≥0.98 ≥0.98 ≥0.997

2330 2110 3820

1.042861, 1.03967b, 1.044202c 1.297838 1.2983d, 1.3015e

174899-66-2 56-40-6

The standard uncertainties u are u(T) = 0.005 K, u(p) = 10 hPa, and ur(d) = 0.002. bRef 18. cRef 19. dRef 20. eRef 21.

Figure 1. Chemical structures of the chemicals studied.

Table 2. Experimental Water Activity Data for Solutions of [C4mim]Cl, [C6mim]Cl, and [C4mim][CF3SO3] in Aqueous Solution of 0.7041 mol·kg−1 Glycine at 308.15 K and 845 hPaa

a

m ([C4mim]Cl)/mol·kg−1

aw

m ([C6mim]Cl)/mol·kg−1

aw

m ([C4mim][CF3SO3])/mol·kg−1

aw

0.0000 0.1497 0.3031 0.5269 0.7243 0.8996

0.9883 0.9813 0.9755 0.9678 0.9616 0.9563

0.0000 0.1557 0.3012 0.5200 0.7416 0.8993

0.9883 0.9810 0.9756 0.9687 0.9629 0.9594

0.0000 0.1513 0.3030 0.5284 0.7462 0.8972

0.9883 0.9814 0.9765 0.9711 0.9677 0.9659

The standard uncertainties u are u(m) = 2 × 10−4 mol·kg−1, u(aw) = 2 × 10−4, u(T) = 0.005 K, and u(p) = 10 hPa.

s), 7.62 (s, 1H), 10.47 (1H, s) and 13C NMR (250 MHz, CDCl3), 13.23, 21.59, 25.08, 29.53, 30.31, 35.76, 49.18, 121.69, 123.30, 136.64. 2.3. Apparatus and Procedures. 2.3.1. Vapor Pressure Osmometry Measurements. The vapor pressure osmometry method was used to measure the water activity of the investigated aqueous solutions. Water activity measurements were performed by using a K-7000 vapor pressure osmometer (Knauer Inc.). The details and procedure of vapor pressure osmometry measurements were as described in our previous work.22 The calibration of instrument was performed with the help of the aqueous solutions of NaCl as reference in the proper concentration range, yielding in a function that correlates the panel readings to the corresponding concentrations of the NaCl aqueous solutions. The water activity (aw) for a known aqueous IL + glycine solution, which has the same instrument reading as a NaCl solution with concentration mNaCl, was measured according to the following equation:

found in the literature regarding the thermodynamics properties of aqueous IL + amino acids solutions.11,12,14−17

2. EXPERIMENTAL SECTION 2.1. Materials. The sources and purities of the chemicals used in this work are presented in Table 1. Figure 1 shows the chemical structure of the studied chemicals. Since, [C4mim]Cl and glycine are solid at room temperature, their pure density values are not reported in Table 1. All the experiments were carried out with deionized and double distilled water. The water contents in the ILs were measured by Karl Fischer titration with precision of 3 ppm and it was taken into account in the calculation of the compositions of the investigated solutions. 2.2. Synthesis of Ionic Liquid. [C6mim]Cl was synthesized by carrying out a direct reaction between 1methylimidazole with an excess amount of 1-chlorohexane in ethyl acetate under reflux at 333 K for 2 weeks. The reactions were placed under an argon blanket and stirred vigorously in a round-bottom flask. This resulted in a two-phase mixture consisting of a viscous ionic liquid and ethyl acetate. The solvent was removed by decanting. The pale yellow product was purified by recrystallization from ethyl acetate at least five times and then dried under vacuum at 333 K for 1 day. The structure and purity of the synthesized IL was verified by 1 HNMR and 13CNMR spectroscopy (see Supporting Information). 1H and 13C NMR characterizations of [C6mim]Cl were measured using a BRUKER DRX-250 AVANCE spectrometer with CDCl3 as solvent. The chemical shifts of [C6mim]Cl are 1 H NMR (250 MHz, CDCl3) δ/ppm, 0.73 (3H, t), 1.175(6H, m), 1.76−1.78 (2H, m), 4.00 (3H, s), 4.22 (2H, t), 7.46 (1H,

a w = exp( −0.001νNaClmNaCl ΦNaCl M w )

(1)

Where ΦNaCl and νNaCl represent the osmotic coefficient for aqueous solution of NaCl with certain molality mNaCl obtained from the correlation of Colin et al.23 and the stoichiometric number of NaCl, respectively, while Mw is the molar mass of water. The standard uncertainty in the measurement of water activity was found to be better than ±2 × 10−4. It should be noted that the corrected mole fractions have been used as the water activity (activity in mole fraction scale), therefore it is dimensionless. 2.3.2. Density and Sound Velocity Measurements. Density and sound velocity measurements were performed with the 4074



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corresponding binary solutions. This equation has been obtained for the pseudoideal solutions in which two solutes have similar chemical structure and then the solutes-water interactions in the ternary aqueous solutions are the same as those in the constituent binary aqueous solutions. Therefore, the deviations from the eq 2 can be related to the presence of new interactions between solute 1 and solute 2 different from the interactions between the same solutes. The more favorable solute 1-solute2 interactions (as compare to the solute 1solute1 or solute 2-solute2 interactions) lead to the weakness of the water−solute1 interactions in the presence of the solute2 and then increase the amount of free water molecules as comparing to the semi-ideal behavior. Therefore, a positive deviation from the semi-ideal state can be expected. In these systems, the soluting-in behavior and then the mutual solubilities of two solutes (as cosolvent) occur. As example, amino acid-(NH4)2HCit29 and PEG-carbohydrate26 aqueous solutions, which are completely miscible, positive departures from the semi-ideal state have been observed. However, unfavorable solute 1-solute2 interactions (as compared to the interactions between the same solutes) lead to the strength of the solute−water interactions in the presence of the other solute and then reducing the amount of free water molecules as comparing to the semi-ideal behavior and then the negative departure from the semi-ideal state is observed. These systems show the soluting-out behavior in which the solubility of solute1 in water decreases in the presence of solute2 (as antisolvent). In these systems, each solute excludes itself from the vicinity of the other solute, and ultimately above the critical concentration due to entropic reasons (depending on the state of the soluted out agent) ABS formation (liquid−liquid equilibria) or precipitation of one of the solutes (solid−liquid equilibria) can be favorable.7,26,30−32 In Figure 3, the plots of aw

help of an Anton Paar DSA 5000 model high precision densimeter and sound velocity measuring device. The instrument equipped with a built-in thermostat that can maintain the samples at working temperature within ±10−3 K. For all samples, first the instrument was calibrated with doubly distilled degassed water and dry air at atmospheric pressure. The uncertainties in density and sound velocity were obtained were obtained as 0.2% of the measured values and 0.5 m·s−1, respectively.

3. RESULTS AND DISCUSSION 3.1. Vapor Pressure Osmometry Measurements. Water activity measurements by VPO method were performed for solutions of [C4mim]Cl, [C6mim]Cl, and [C4mim][CF3SO3] in aqueous solutions of 0.7041 mol·kg−1 glycine at T = 308.15 K and the measured water activity data are given in Table 2 and are shown in Figure 2.

Figure 2. Plot of water activity, aw, against IL molality, mIL, for solutions of the investigated ILs in aqueous solution of 0.7041 mol· kg−1 glycine at 308.15 K: ▲, [C4mim][CF3SO3]; ●, [C6mim]Cl; □, [C4mim]Cl.

Figure 1 depicts that in the same concentration of solute, the water activities of the investigated ternary aqueous solutions increase in the order: [C4mim]Cl < [C6mim]Cl < [C4mim][CF3SO3], which is the same as those observed in the case of binary IL + water solutions.24 As can be seen, [C6mim]Cl presents weaker interactions with water than [C4mim]Cl, which can be attributed to the longer alkyl side chain of the cation of the IL. Furthermore, the higher hydrophobicity of [C4mim][CF3SO3] than [C4mim]Cl can be attributed to the lower charge density of CF3SO3− than Cl−. Khan et al.25 showed that the IL-water interactions become weaker by fluorination of IL anion. For study the soluting-effect occurring in these ternary systems, we calculated deviations from the semi-ideal behavior according to equation:26−28 ° + a w,gl ° )=0 a w + 1 − (a w,IL

Figure 3. Plot of aw + 1 − (a°w,IL + a°w,gl) against IL molality, mIL, for solutions of the investigated ILs in aqueous solution of 0.7041 mol· kg−1 glycine at 308.15 K: ▲, [C4mim][CF3SO3]; ●, [C6mim]Cl; □, [C4mim]Cl.

+ 1 − (a0w,gl + a0w,IL) against IL molality have been shown. As can be seen from Figure 3, all the investigated IL+ glycine + water systems show the negative departures from the eq 2. Figure 3 also shows that the negative deviations from the eq 2 increase with increasing the hydrophobic nature of the IL, and follows the order: [C4mim]Cl < [C6mim]Cl < [C4mim][CF3SO3]. According to our observations, the aqueous

(2)

where aw,gl ° (= 0.9883) and aw,IL ° (taken from our previous work24) are the activities of water, respectively, in IL + H2O and glycine + H2O solutions with molalities mIL and mgl (= 0.7041 mol·kg−1) and aw is the activity of water in IL + glycine + water ternary solution with the same solutes molality as the 4075



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Table 3. Experimental Density d/(g·cm−3) and Speed of Sound u/(m·s−1) of the Solutions of [C4mim]Cl, [C6mim]Cl, and [C4mim][CF3SO3] in Pure Water and in Aqueous Solution of 0.7041 mol·kg−1 Glycine at Different Temperatures and 845 hPaa T = 288.15 K mIL mol·kg−1

d g·cm−3

0.0344 0.0526 0.0926 0.1181 0.1806 0.2371 0.4790 0.7374

0.999577 0.999828 1.000374 1.000720 1.001564 1.002326 1.005590 1.009021

0.0346 0.0581 0.0748 0.1166 0.1697 0.2300 0.4140 0.7522

1.020160 1.020397 1.020556 1.020946 1.021442 1.022043 1.023814 1.027201

0.0096 0.0296 0.0585 0.0734 0.0999 0.1212 0.1542 0.2244 0.2863 0.4690 0.8901

0.999203 0.999412 0.999713 0.999867 1.000148 1.000374 1.000726 1.001473 1.002124 1.004080 1.008376

0.0224 0.0370 0.0547 0.0720 0.1075 0.1526 0.2056 0.3033 0.5376 0.8470

1.020759 1.020844 1.020958 1.021066 1.021280 1.021562 1.021906 1.022555 1.024162 1.026218

0.0164 0.0259 0.0420 0.0699 0.1072 0.1455 0.3021 0.4645 0.6651 0.8671

1.000268 1.000932 1.002060 1.003987 1.006541 1.009121 1.019308 1.029211 1.040599 1.051208

0.0110 0.0237 0.0353 0.0443 0.0574

1.021600 1.022444 1.023208 1.023794 1.024659

u m·s−1

T = 298.15 K d g·cm−3

u m·s−1

T = 308.15 K d g·cm−3

u m·s−1

[C4mim]Cl in water 0.997479 1501.54 0.994434 1523.93 0.997710 1503.98 0.994648 1526.07 0.998209 1509.32 0.995113 1530.78 0.998524 1513.08 0.995403 1534.05 0.999300 1520.67 0.996124 1540.71 0.999998 1528.03 0.996773 1547.13 1.002982 1557.12 0.999537 1572.51 1.006112 1585.96 1.002435 1597.63 [C4mim]Cl in aqueous solution of 0.7041 mol·kg−1 glycine 1507.75 1.017602 1535.16 1.014247 1555.77 1511.32 1.017816 1538.23 1.014441 1558.45 1513.78 1.017959 1540.39 1.014574 1560.35 1519.80 1.018310 1545.59 1.014892 1564.88 1527.35 1.018760 1552.16 1.015301 1570.61 1535.73 1.019296 1559.44 1.015784 1576.99 1559.94 1.020887 1580.46 1.017232 1595.19 1601.55 1.023928 1616.56 1.019995 1626.52 [C6mim]Cl in water 1468.21 0.997128 1498.23 0.994101 1521.10 1471.67 0.997304 1501.33 0.994252 1523.68 1476.81 0.997562 1505.68 0.994471 1527.39 1479.43 0.997692 1507.91 0.994583 1529.31 1483.86 0.997932 1511.69 0.994787 1532.56 1487.42 0.998125 1514.76 0.994950 1535.17 1492.97 0.998421 1519.49 0.995204 1539.19 1503.95 0.999058 1528.86 0.995748 1547.26 1513.81 0.999620 1537.32 0.996227 1554.30 1540.24 1.001266 1559.66 0.997628 1573.20 1591.40 1.004888 1602.41 1.000699 1608.73 [C6mim]Cl in aqueous solution of 0.7041 mol·kg−1 glycine 1507.94 1.018190 1535.4 1.014827 1555.99 1510.65 1.018253 1537.68 1.014870 1557.97 1513.81 1.018340 1540.39 1.014937 1560.31 1516.76 1.018425 1542.90 1.015000 1562.53 1522.95 1.018591 1548.25 1.015124 1567.05 1530.55 1.018809 1554.74 1.015289 1572.57 1539.39 1.019075 1562.28 1.015492 1578.97 1554.79 1.019581 1575.37 1.015875 1590.05 1588.07 1.020832 1603.43 1.016826 1613.54 1624.15 1.022423 1633.14 1.018032 1637.91 [C4mim][CF3SO3] in water 1468.67 0.998176 1498.52 0.995134 1521.09 1469.82 0.998823 1499.43 0.995765 1521.77 1471.85 0.999922 1501.03 0.996836 1523.00 1475.00 1.001795 1503.45 0.998662 1524.80 1479.24 1.004279 1506.62 1.001081 1527.24 1483.76 1.006783 1510.15 1.003519 1529.84 1499.64 1.016656 1522.13 1.013111 1538.42 1513.44 1.026224 1532.30 1.022387 1545.40 1526.73 1.037208 1541.46 1.033026 1551.08 1535.56 1.047439 1546.89 1.042937 1553.82 [C4mim][CF3SO3] in aqueous solution of 0.7041 mol·kg−1 glycine 1505.67 1.019034 1533.34 1.015672 1554.19 1507.17 1.019857 1534.48 1.016477 1555.02 1508.53 1.020602 1535.53 1.017203 1555.80 1509.54 1.021173 1536.30 1.017762 1556.37 1511.01 1.022013 1537.43 1.018578 1557.19 1471.98 1474.71 1480.82 1484.95 1493.82 1502.23 1535.67 1568.82

4076

T = 318.15 K d g·cm−3

u m·s−1

0.990589 0.990791 0.991228 0.991504 0.992177 0.992785 0.995370 0.998079

1540.08 1541.95 1546.09 1548.87 1554.79 1560.47 1582.65 1604.46

1.010207 1.010387 1.010508 1.010798 1.011173 1.011620 1.012945 1.015478

1570.83 1572.87 1574.53 1578.45 1583.43 1588.99 1604.82 1632.00

0.990268 0.990400 0.990580 0.990681 0.990859 0.990996 0.991215 0.991681 0.992089 0.993281 0.995899

1537.61 1539.79 1542.96 1544.56 1547.33 1549.56 1552.97 1559.80 1565.76 1581.57 1610.92

1.010777 1.010809 1.010853 1.010904 1.010991 1.011108 1.011252 1.011529 1.012229 1.013122

1570.85 1572.47 1574.44 1576.32 1580.11 1584.83 1590.20 1599.58 1619.07 1638.88

0.991289 0.991906 0.992952 0.994732 0.997093 0.999470 1.008807 1.017825 1.028161 1.037796

1537.41 1537.89 1538.81 1540.05 1541.73 1543.56 1549.20 1553.41 1556.15 1556.53

1.011628 1.012411 1.013122 1.013667 1.014464

1569.18 1569.74 1570.28 1570.67 1571.22



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Table 3. continued T = 288.15 K −1

mIL mol·kg 0.0927 0.1098 0.2271 0.2978 0.4749 0.6634 0.8531 a

d g·cm

−3

1.026953 1.028061 1.035396 1.039674 1.049862 1.059864 1.069125

T = 298.15 K −1

−3

u m·s

d g·cm

1514.92 1516.73 1528.48 1534.56 1547.93 1557.68 1562.53

1.024244 1.025322 1.032443 1.036586 1.046430 1.056088 1.065038

T = 308.15 K −1

−3

u m·s

d g·cm

1540.38 1541.77 1550.45 1554.93 1564.38 1570.40 1572.57

1.020755 1.021803 1.028728 1.032748 1.042294 1.051649 1.060328

T = 318.15 K −1

d g·cm−3

u m·s−1

1559.35 1560.35 1566.50 1569.58 1575.47 1578.45 1578.41

1.016588 1.017610 1.024353 1.028262 1.037528 1.046624 1.055069

1572.66 1573.32 1577.20 1579.01 1581.94 1582.27 1580.33

u m·s

The standard uncertainties u are u(m) = 2 × 10−4 mol·kg−1, u(u) = 0.5 m·s−1, u(T) = 0.005 K, u(p) = 10 hPa, and ur(d) = 0.002.

Figure 4. Comparison of the measured densities and speeds of sound with literature values for aqueous solutions of [C4mim][CF3SO3] and [C4mim]Cl at 288.15 K (dashed line), 298.15 K (dotted line) and 308.15 K (solid line): ▲, (this work); ○, (ref 40); □, (ref 46).

from the solution. The solid−liquid equilibria behavior of [C4mim]Cl/[C6mim]Cl + glycine aqueous systems was studied in our previous work.24 It was found that, the solubility of glycine in the aqueous [C4mim]Cl and [C6mim]Cl solutions are less than that in pure water. The ability of ILs to solutingout of glycine in aqueous solutions increases by increasing the hydrophobicity of IL ([C6mim]Cl > [C4mim]Cl). This behavior is in agreement with that shown in Figure 3, in which, the negative departure from the semi-ideal state increases by alkyl-chain length of the cation of IL.

[C4mim][CF3SO3] + glycine solutions become biphasic above a certain critical concentration. In the case of ternary [C4mim][CF3SO3] + glycine aqueous system in which the glycine is more hydrophilic than the IL (the glycine acts as soluting-out agent), the soluting-out effect appears by ABS formation. The liquid−liquid equilibria behavior of [C4mim][CF3SO3]-glycine aqueous system was studied in our previous work.24 However, in the case of ternary [C4mim]Cl/[C6mim] Cl + glycine aqueous systems in which the glycine is more hydrophobic than the ILs (the IL acts as soluting-out agent), the soluting-out effect appears by precipitation of the glycine 4077



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3.2. Volumetric and Compressibility Measurements. In this work, in order to achieve a better understanding of the different interactions that exist in the investigated solutions, we also performed the volumetric and compressibility studies on the solutions of [C4mim]Cl, [C6mim]Cl, and [C4mim][CF3SO3] in aqueous solution of 0.7041 mol·kg−1 glycine by accurate measurement of density and speed of sound at different temperatures. The experimental density, d and speed of sound, u data measured at T = 288.15, 298.15, 308.15, and 318.15 K are reported in Table 3. As can be seen from Figure 4, the values of density and speed of sound for binary aqueous solutions of [C4mim][CF3SO3] and [C4mim]Cl at T = (288.15, 298.15, and 308.15) K are in good agreement with those taken from the literature. The apparent molar volumes, Vϕ, and apparent molar adiabatic compressions, Kϕ, were calculated for the ILs in the investigated solvents (pure water and aqueous solutions of 0.7041 mol·kg−1 glycine) by means of the following equations: Vϕ =

1000(d − d0) MIL − d mIL dd0

(3)

Kϕ =

MILκs 1000(dκs0 − d0κs) − d mIL dd0

(4)

The concentration and temperature dependence of Vϕ of the ILs in water and aqueous glycine solutions are presented in Figure 6. This figure shows that the Vϕ values of the investigated ILs in water and aqueous glycine solutions slightly decreased with molality of the IL. The similar behavior was observed for other ILs solutions.22,33−37 Furthermore, the values of Vϕ in aqueous glycine solutions are larger than those in the absence of glycine. In Figure 7a, the temperature and concentration dependence of apparent molar adiabatic compressibility are presented for [C6mim]Cl in pure water and in aqueous glycine solutions. The Kϕ values of [C6mim]Cl, which are negative in low temperature and IL concentration, increase by increasing both temperature and IL molality, so that the Kϕ values become positive in higher IL molality or temperatures. The other investigated ILs show the similar behavior. Figure 7b shows that the Kϕ values of the studied ILs increase in the presence of glycine and in each case they increased by increasing the hydrophobicity of the ILs ([C4mim]Cl < [C6mim]Cl < [C4mim][CF3SO3]). At infinite dilution, the solute−solute interactions are negligible and then the values of the Kϕ and Vϕ at infinite dilution are important thermodynamic properties of solutions which provide some useful information about the solute− solvent interactions.38 The infinite dilution partial molar volume, Vϕ0 , and infinite dilution partial molar adiabatic compressibility, K0ϕ, can be obtained by correlation of the Kϕ and Vϕ data with mIL using the following Redlich−Mayer equation:39

where κs0 and κs are the isentropic compressibility of the solvent and solution, respectively. The value of κs is obtained by κs =

1 du 2

(5)

In these equations, MIL and mIL are the molecular mass and molality of the IL, respectively. d0 and d are the density of the solvent and solution, respectively, and u0 and u are the speed of sound of the solvent and solution, respectively. As expected, the Vϕ values of the investigated ILs increase in the order: [C4mim] Cl < [C6mim]Cl < [C4mim][CF3SO3]. However, as can be seen from Figure 5, the values of the apparent specific volume, Vϕ/MIL, increase in the order: [C4mim][CF3SO3] < [C4mim] Cl < [C6mim]Cl. Similar behavior was observed for both the binary ILs−water and ternary ILs−glycine−water systems. This trend shows that the apparent specific volume of CF3SO3− is smaller than Cl− and that of C4mim+ is smaller than C6mim+.

1/2 Vϕ = V ϕ0 + S VmIL + b V mIL

(6)

1/2 Kϕ = Kϕ0 + SK mIL + bK mIL

(7)

The V0ϕ, bV, K0ϕ, and bK values of the studied systems determined by a least-squares analysis of the eqs 6 and 7, together with the calculated standard deviations (σ) are given in Table 4. As can be seen, our data for V0ϕ of the ILs in water are in good agreement with the literatures values.34,36,40 In the above equations, SV and SK are the Pitzer−Debye−Hückel limiting slope respectively for volume and apparent molar isentropic compressibility, and whose values for aqueous 1:1 electrolyte solutions at different temperatures are given in the literature.39 bV and bK are empirical parameters related to the pair interactions, which show the departure from the limiting law caused by nonelectrostatic solute−solute interactions.41 As can be seen, for all of the investigated systems at all temperatures, the obtained values of bV and bK are negative and positive, respectively. The infinite dilution apparent molar volumes, ΔV0ϕ,tr, and adiabatic compressibilities, ΔK0ϕ,tr, for transfer of the investigated ILs from water to aqueous glycine solutions were determined by using the following equations: ΔV ϕ0 ,tr = V ϕ0(in aqueous glycine solution) − V ϕ0(in water) (8)

ΔKϕ0 ,tr = Kϕ0(in aqueous glycine solution) − Kϕ0(in water) (9)

ΔV0ϕ,tr

ΔK0ϕ,tr

Table 5 indicates that the values of both and are positive and decrease with increase in temperature. According to the cosphere overlap model,42 four types of different interactions can occur in these systems:15,17,43−45 (i) ionic− hydrophilic interactions between hydrophilic group (NH3+,

Figure 5. Plot of the apparent specific volume against molality of IL, mIL, for the investigated ILs in pure water at 298.15 K: ■, [C4mim]Cl; ●, [C6mim]Cl ; ▲, [C4mim][CF3SO3]. 4078



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Figure 6. Plot of apparent molar volume, Vϕ, against molality of IL, mIL. (a) [C4mim]Cl; (b) [C4mim][CF3SO3]) in pure water (filled symbol) and in aqueous solutions of 0.7041 mol·kg−1 glycine (empty symbol): ■, 288.15 K; ●, 298.15 K; ▲, 308.15 K; ⧫, 318.15 K. Lines were calculated using Redlich−Mayer equation.

Figure 7. (a) The temperature dependence of apparent molar adiabatic compressibility of [C6mim]Cl, Kϕ, in pure water (filled symbol) and in aqueous solutions of 0.7041 mol·kg−1 glycine (empty symbol) as a function of molality of [C6mim]Cl, mIL:■, 288.15 K; ●, 298.15 K; ▲, 308.15 K; ⧫, 318.15 K. (b) Plot of apparent molar adiabatic compressibility, Kϕ, against molality of IL, mIL, in pure water (filled symbol) and in aqueous solutions of 0.7041 mol·kg−1 glycine (empty symbol) at 298.15 K: ■, [C4mim]Cl; ●, [C6mim]Cl; ▲, [C4mim][CF3SO3]. Lines were calculated using Redlich−Mayer equation.

COO−) of glycine and the anions and cation of the IL; (b) hydrophobic−hydrophobic interactions between the hydrophobic part of glycine and the alkyl chain of the imidazolium cation of IL; (c) ionic−hydrophobic interactions between the ions of IL and hydrophobic part of glycine; (d) hydrophilic− hydrophobic interactions between hydrophilic groups of glycine and the alkyl chain of the imidazolium cation of IL. According to this model, among the different interactions occurring between glycine and IL, only the interaction type (i) through the reduction of electrostriction effect of water in the hydration cosphere of IL will lead to positive values for ΔV0ϕ,tr, whereas the other interactions (ii−iv) through the enhancement of electrostriction effect of water in the hydration cosphere of IL will lead to negative ΔV0ϕ,tr. The overall positive ΔV0ϕ,tr obtained in this study show that the hydrophilic-ionic interactions dominate over the other interactions (ii−iv) in these systems. By considering the following equation for the temperature dependency of Vϕ0 , the infinite dilution apparent molar expansibilities, E0ϕ, were calculated as46 V ϕ0 = a + bT + cT 2

⎛ ∂V 0 ⎞ φ ⎟⎟ = b + 2cT Eφ0 = ⎜⎜ T ∂ ⎠P ⎝

(11)

As can be seen from Figure 8, for all the investigated systems, E0ϕ is positive and increase with increasing hydrophobicity of the ILs. Moreover, the values of E0ϕ for the investigated ILs in pure water are larger than those in aqueous glycine solutions. Positive expansibility is considered as a specific property of aqueous solutions with the hydrophobic hydration. This indicates that some water molecules released from the solvation layers of the ILs at higher temperatures. As expected, hydrophobic hydration of ILs increases with increasing degree of hydrophobicity and therefore the E0ϕ values become more positive either the elongation of the alkyl-side chain of the cation, or the size of the anion of IL. Hepler47 proposed the following thermodynamic equation as a basis for the classification of solutes as structure-making or structurebreaking in solution: ⎛ ∂ 2V 0 ⎞ ⎛ ∂C 0 ⎞ φ ⎟ ⎜ P ⎟ = −T ⎜⎜ 2 ⎟ ⎝ ∂T ⎠T ⎝ ∂T ⎠ P

(10) 4079

(12) DOI: 10.1021/acs.jced.7b00297 J. Chem. Eng. Data 2017, 62, 4073−4082


Article

Table 4. Values of V0ϕ, bV, K0ϕ, and bK (Coefficients of Eqs 6 and 7) and Standard Deviations, σ, for Different Systems Investigated in This Work at Different Temperatures V0ϕ cm3·mol−1

T/K

308.15 318.15

160.80 ± 162.08 ± 161.9b 163.37 ± 164.64 ±

288.15 298.15 308.15 318.15

161.89 163.01 164.22 165.39

± ± ± ±

0.05 0.05 0.05 0.05

288.15 298.15 308.15 318.15

192.00 193.98 195.77 197.54

± ± ± ±

0.06, 192.699c 0.07, 194.611c 0.06, 196.377c 0.06

288.15 298.15 308.15 318.15

192.83 194.53 196.23 197.87

± ± ± ±

0.12 0.10 0.10 0.09

288.15 298.15 308.15 318.15

217.07 219.25 221.35 223.51

± ± ± ±

0.05, 217.85a 0.05, 220.38a 0.04, 221.79a 0.03

288.15 298.15 308.15 318.15

217.90 219.96 221.88 223.97

± ± ± ±

0.08 0.08 0.07 0.06

288.15 298.15

a

bV cm3 kg·mol−2

σ cm3·mol−1 K0ϕ×105 cm3·mol−1·kPa−1 bk × 105 cm3·kg·mol−2·kPa−1 σ × 105 cm3·mol−1·kPa−1 [C4mim]Cl in water 0.03 −3.21 ± 0.05 0.05 −1.41 ± 0.04

0.02, 161.71a 0.03, 163.32a,

−2.82 ± 0.05 −3.26 ± 0.07

0.02, 164.63a 0.03

−3.25 ± 0.07 0.05 −0.20 ± 0.04 −3.32 ± 0.10 0.06 0.79 ± 0.04 [C4mim]Cl in aqueous solution of glycine 0.7041 mol·kg−1 −2.98 ± 0.14 0.09 −1.93 ± 0.03 −3.38 ± 0.15 0.09 −0.56 ± 0.02 −3.38 ± 0.15 0.09 0.47 ± 0.02 −3.39 ± 0.14 0.09 1.23 ± 0.07 [C6mim]Cl in water −3.5 ± 0.18 0.06 −2.81 ± 0.03 −3.43 ± 0.21 0.17 −0.96 ± 0.03 −3.25 ± 0.18 0.15 0.61 ± 0.03 −3.17 ± 0.16 0.15 1.86 ± 0.03 [C6mim]Cl in aqueous solution of glycine 0.7041 mol·kg−1 −4.13 ± 0.35 0.28 −2.15 ± 0.02 −3.78 ± 0.30 0.24 −0.46 ± 0.01 −3.57 ± 0.30 0.24 0.94 ± 0.03 −3.40 ± 0.26 0.21 2.06 ± 0.01 [C4mim][CF3SO3] in water −2.11 ± 0.15 0.12 −1.40 ± 0.08 −1.82 ± 0.12 0.11 0.94 ± 0.03 −1.57 ± 0.09 0.08 2.51 ± 0.02 −1.51 ± 0.08 0.07 3.84 ± 0.03 [C4mim][CF3SO3] in aqueous solution of glycine 0.7041 mol·kg−1 −2.39 ± 0.24 0.22 −0.07 ± 0.01 −2.17 ± 0.24 0.22 1.65 ± 0.01 −1.83 ± 0.20 0.19 3.08 ± 0.02 −1.81 ± 0.18 0.16 4.25 ± 0.01

1.63 ± 0.15 0.88 ± 0.13

0.10 0.08

0.63 ± 0.12 0.41 ± 0.11

0.08 0.07

1.27 0.91 0.69 0.73

± ± ± ±

0.09 0.06 0.05 0.21

0.06 0.04 0.03 0.13

2.27 1.95 1.57 1.33

± ± ± ±

0.08 0.08 0.08 0.09

0.06 0.07 0.07 0.07

2.18 1.83 0.49 1.34

± ± ± ±

0.06 0.03 0.09 0.03

0.05 0.03 0.07 0.02

3.58 2.51 2.11 1.81

± ± ± ±

0.21 0.07 0.06 0.09

0.19 0.06 0.06 0.08

3.13 2.63 2.18 1.93

± ± ± ±

0.03 0.02 0.04 0.04

0.02 0.02 0.04 0.04

Ref 40. bRef 36. cRef 34.

Table 5. Partial Molar Volume of Transfer, ΔV0ϕ,tr and Partial Molar Isentropic Compressibility of Transfer, ΔK0ϕ,tr, of ILs in Aqueous Solution of Glycine 0.7041 mol·kg−1 at Different Temperatures IL [C4mim]Cl

[C6mim]Cl

[C4mim][CF3SO3]

ΔV0ϕ,tr cm3·mol−1

T/K 288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15 288.15 298.15 308.15 318.15

1.09 0.93 0.85 0.75 0.83 0.55 0.46 0.33 0.83 0.71 0.53 0.46

± ± ± ± ± ± ± ± ± ± ± ±

0.05 0.06 0.05 0.06 0.13 0.12 0.12 0.11 0.09 0.09 0.08 0.07

ΔK0ϕ,tr·105 cm3·mol−1·kPa−1 1.28 0.85 0.67 0.44 0.66 0.50 0.33 0.20 1.33 0.69 0.57 0.41

± ± ± ± ± ± ± ± ± ± ± ±

0.06 0.09 0.04 0.08 0.04 0.03 0.04 0.03 0.08 0.02 0.03 0.03

⎛ ∂ 2V ϕ0 ⎞ Based on the eq 12, a negative value of ⎜ ∂T 2 ⎟ is related to a ⎝ ⎠P

Similar behaviors for the ILs in pure water have been observed previously.46

structure-breaker solute and a positive one is related to a structure-maker solute. Figure 6 shows that the values of ⎛ ∂Eϕ0 ⎞ ⎛ ∂ 2V ϕ0 ⎞ ⎜ ⎟ = ⎜ ⎟ for all the ILs investigated in this study in ⎝ ∂T ⎠ P ⎝ ∂T 2 ⎠ P

4. CONCLUSIONS Two experiments sets were performed in order to obtain useful information about the hydration of solutes and also to further the understanding about different interactions existing in the ILs−glycine−water systems. First, VPO measurements at 308.15 K were used for studying the vapor−liquid equilibria behaviors of solutions of [C4mim]Cl, [C6mim]Cl and

pure water are negative. Therefore, our results suggest that these solutes act as structure-breakers when dissolved in water. 4080



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Notes

The authors declare no competing financial interest.

■

Figure 8. Plot of the infinite dilution apparent molar expansibilities, E0ϕ, against temperature, T, for the investigated ILs in pure water (solid lines) and in aqueous solutions of 0.7041 mol·kg−1 glycine (dotted lines): ■, [C4mim]Cl; ●, [C6mim]Cl ; ▲, [C4mim][CF3SO3]. Lines and points were calculated using eq 11.

[C4mim][CF3SO3] in (glycine + water) solutions. The results indicate that the water activity of these systems increase in the order: [C4mim]Cl > [C6mim]Cl > [C4mim][CF3SO3]. The VPO measurements of these systems indicate that due to the unfavorable IL−glycine interactions, they show the negative departure from the semi-ideal state and then these systems indicate the soluting-out effects. For {[C4mim][CF3SO3] + glycine + water} system, the soluting-out phenomenon appears by the formation of aqueous two-phase system. However, the soluting-out phenomenon in {[C4mim]Cl + glycine + water} and {[C6mim]Cl + glycine + water} systems appears by precipitation of the glycine from the solution. The ability of the IL to soluting-out of the glycine in aqueous solutions increases with an increase in the hydrophobicity of the IL ([C6mim]Cl > [C4mim]Cl). In the second part of this work, the values of Vϕ and Kϕ for [C4mim]Cl, [C6mim]Cl and [C4mim][CF3SO3] in pure water and in aqueous solutions of 0.7041 mol·kg−1 glycine were obtained at different temperatures. The infinite dilution apparent molar volume, ΔV0ϕ,tr, and adiabatic compressibility, ΔK0ϕ,tr, for transfer of the ILs from water to aqueous glycine solutions were positive and decreased by increasing temperature. Finally, the infinite dilution apparent molar expansibility, E0ϕ, values were calculated from the first derivative of apparent molar volume with respect to the temperature. These values for the investigated ILs in pure water were positive and decreased with increasing temperature.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00297. 1 H NMR spectrum of [C6mim]Cl (PDF)

■

REFERENCES

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