Physical and Excess Properties of Eight Binary Mixtures Containing

Article pubs.acs.org/jced

Physical and Excess Properties of Eight Binary Mixtures Containing Water and Ionic Liquids Emilio J. González,† Á ngeles Domínguez,‡ and Eugenia A. Macedo†,* †

LSRE - Laboratory of Separation and Reaction Engineering - Associate Laboratory LSRE/LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal ‡ Advanced Separation Processes Group, Department of Chemical Engineering, University of Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain S Supporting Information *

ABSTRACT: In this paper, the density, speed of sound, and refractive index of eight binary systems (water + ionic liquid) were measured, along the whole composition range, at T = (288.15 to 308.15) K and atmospheric pressure. All binary mixtures were completely miscible in water at the studied temperatures. The ionic liquids used in this work are constituted by different cations (imidazolium, pyridinium and pyrrolidinium) and anions (trifluoromethanesulfonate, dicyanamide, methylsulfate, or ethylsulfate). From the experimental data, excess molar volumes and excess molar isentropic compressions were calculated and satisfactorily fitted using the Redlich−Kister equation. Finally, the effect of the ions and temperature on the physical and excess properties was analyzed and discussed. The obtained results show that the physical and excess properties studied in this work are dependent on water content, temperature, and structure of the ILs, especially of the anion.

■

INTRODUCTION One of the principles of green chemistry is the design of new chemicals and processes in order to reduce or eliminate the use of hazardous substances.1 In this way, ionic liquids (ILs) are a new class of liquid salts considered as a possible alternative to the typical volatile organic solvents due to their negligible vapor pressure under normal conditions, among other interesting properties. In recent years, ILs are finding a place in many fields such as their use as lubricants2 or electrolytes for Li-ion batteries3 and their application in bioreactor technology,4 in synthesis of nanoobjects,5 or in separation technology.6 In order to fully understand the behavior of these compounds, pure or mixed with other solvents, the experimental determination of their physical properties and the corresponding excess properties are of great importance. Moreover, the knowledge about the physical properties of ILs may help to find new applications for these kinds of substances. Although several works were published about the physical properties of ILs, pure or mixed with molecular solvents, only few papers contain physical properties data for the ILs included in this work.7−16 Most of them deal with to density, speed of sound, viscosity, and refractive index for some pure ILs, whereas references concerning deviations from ideality for the binary systems (water + ionic liquid) at the studied temperature range are still very scarce.7,9,10,15 Specifically, Vercher et al.7 reported refractive indices and deviations in refractive indices for several trifluoromethanesulfonate-based ILs in water from (288.15 to 338.15) K, Ge et al.9 measured density and viscosity data and calculated the corresponding excess molar volumes and the viscosity deviations, for the binary © 2012 American Chemical Society

mixture (water +1-butyl-3-methylimidazolium trifluoromethanesulfonate) over the whole mole fraction range from (303.15 to 343.15) K at atmospheric pressure, and García-Miaja et al.10 experimentally determined excess molar enthalpy at T = 303.15 K, and density and isobaric molar heat capacity within the temperature range T = (293.15 to 318.15) K for several binary mixtures (water + ionic liquid); from the experimental data, excess molar volumes and excess molar isobaric heat capacities were calculated. Finally, Carvalho et al.15 studied the effect of water on the viscosities and densities of 1-butyl-3-methylimidazolium dicyanamide and 1-butyl-3-methylimidazolium tricyanomethane at atmospheric pressure. In this work, new and accurate data on physical and excess properties for the binary sytems (water + IL) are reported at several temperatures and atmospheric pressure. The ILs included in this article are the following: 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMim][TfO], 1-butyl-1methylpyrrolidinium trifluoromethanesulfonate, [BMpyr][TfO], 1-butyl-3-methylpyridinium trifluoromethanesulfonate, [BMpy][TfO], 1-butyl-3-methylimidazolium dicyanamide, [BMim][dca], 1-hexyl-3-methylimidazolium dicyanamide, [HMim][dca], 1-butyl-1-methylpyrrolidinium dicyanamide, [BMpyr][dca], 1-methylpyridinium methylsulfate, [Mpy][MSO4], and 1,2diethylpyridinium ethylsulfate, [EEpy][ESO4]. Densities, speeds of sound and refractive indices of the binary systems (water + IL) were measured at T = (288.15 to 308.15) K. Received: January 10, 2012 Accepted: June 20, 2012 Published: July 6, 2012 2165

dx.doi.org/10.1021/je201334p | J. Chem. Eng. Data 2012, 57, 2165−2176

Journal of Chemical & Engineering Data

Article

Table 1. Purity, Water Content, Density, ρ, Speed of Sound, u, and Refractive Index, nD, o Pure Components at T = 298.15 Ka ρ/g·cm−3

water content compound

a

purity, in mass fraction

ppm

exp.

[Mpy][MSO4] [EEpy][ESO4] [BMim][TfO]

>0.99 >0.99 >0.99

475 561 602

1.34612 1.21926 1.29955

[BMim][dca]

>0.98

851

1.06017

[HMim][dca] [BMpyr][dca] [BMpyr][TfO] [BMpy][TfO] Water

>0.97 >0.98 >0.99 >0.99

1242 1497 836 1983

1.02856 1.01353 1.25298 1.27941 0.99704

nD lit. 8

1.34483 1.220208 1.30289 1.297610 1.2995711 1.303812 1.2988513 1.297437 1.0649114b 1.063115 n.a. 1.1143114b 1.252017 n.a. 0.9970517

exp.

lit.

1.51340 1.51350 1.43755

1.512968 1.513288 1.4365711 1.436812 1.437297

1.50893

n.a.

1.50415 1.49681 1.43289 1.46155 1.33255

n.a. n.a. 1.432677 n.a. 1.3325017

Standard uncertainty: ρ is ±0.00003 g·cm−3, nD is ±0.00004. bInterpolated value from a linear fitting.

Table 2. Densities, ρ, speeds of sound, u, refractive indices, nD, excess molar volumes, VE, and excess molar isentropic compressions, KES,m, of the binary mixtures water (1) + ionic liquid (2) at T = 288.15 Ka ρ x1

VE

u −3

g·cm

0.0000 0.1088 0.1227 0.2017 0.2943 0.3952 0.5030 0.5985 0.7032 0.8050 0.9006 0.9503 1.0000

1.06658 1.06559 1.06544 1.06462 1.06346 1.06188 1.05983 1.05737 1.05363 1.04726 1.03592 1.02407 0.99910

0.0000 0.1348 0.2028 0.2056 0.3083 0.4985 0.6062 0.7015 0.8064 0.8998 0.9309 0.9548 1.0000

1.30750 1.30364 1.30130 1.30120 1.29670 1.28457 1.27342 1.25859 1.23130 1.18010 1.14984 1.11626 0.99910

0.0000 0.0960 0.1277 0.2116 0.3077 0.3942 0.4952 0.5023

1.01917 1.01892 1.01884 1.01860 1.01831 1.01808 1.01778 1.01776

m·s

−1

nD

ρ

KES,m −1

cm ·mol 3

Water + [BMim][dca] 1764.3 1.51213 0.000 1762.0 1.50990 0.037 1761.9 1.50953 0.043 1760.9 1.50758 0.059 1759.9 1.50476 0.077 1758.3 1.50075 0.094 1756.8 1.49536 0.089 1755.5 1.48855 0.078 1751.4 1.47799 0.046 1738.5 1.46004 0.025 1694.1 1.42867 −0.011 1630.6 1.39733 −0.021 1466.6 1.33349 0.000 Water + [BMim][TfO] 1414.3 1.44032 0.000 1419.7 1.43903 −0.003 1422.9 1.43815 −0.012 1423.2 1.43810 −0.013 1429.3 1.43654 −0.006 1445.0 1.43224 −0.024 1458.0 1.42835 −0.031 1473.8 1.42301 −0.051 1496.3 1.41332 −0.100 1523.9 1.39564 −0.105 1533.1 1.38516 −0.110 1540.0 1.37365 −0.101 1466.6 1.33349 0.000 Water + [BMpyr][dca] 1836.9 1.49968 0.000 1836.1 1.49811 0.012 1835.8 1.49752 0.013 1835.3 1.49570 0.017 1835.6 1.49322 0.015 1834.9 1.49044 0.000 1835.1 1.48621 −0.023 1835.2 1.48580 −0.025

−1

m ·TPa ·mol 3

−1

x1

0.0000 −0.0015 −0.0019 −0.0036 −0.0058 −0.0081 −0.0110 −0.0136 −0.0161 −0.0172 −0.0151 −0.0111 0.0000

0.0000 −0.0024 −0.0032 −0.0056 −0.0087 −0.0113 −0.0147 −0.0149 2166

g·cm

VE

u −3

0.6047 0.6137 0.7061 0.7177 0.7571 0.8208 0.8468 0.9034 0.9040 0.9492 0.9507 0.9517 1.0000

1.01745 1.01741 1.01691 1.01681 1.01639 1.01522 1.01446 1.01196 1.01192 1.00827 1.00810 1.00798 0.99910

0.0000 0.0559 0.1084 0.2076 0.2997 0.4080 0.5038 0.6003 0.7052 0.8095 0.9018 0.9533 1.0000

1.26033 1.25904 1.25780 1.25512 1.25205 1.24741 1.24191 1.23421 1.22139 1.19877 1.15590 1.10438 0.99910

0.0000 0.0794 0.1517 0.2502 0.3515 0.4528 0.5628 0.6561

1.28707 1.28523 1.28328 1.27999 1.27576 1.27021 1.26190 1.25155

−1

m·s

nD

cm ·mol 3

KES,m −1

Water + [BMpyr][dca] 1835.9 1.47970 −0.060 1835.7 1.47901 −0.063 1832.9 1.47041 −0.089 1832.0 1.46905 −0.091 1827.3 1.46368 −0.096 1810.9 1.45158 −0.093 1798.5 1.44481 −0.086 1750.6 1.42419 −0.066 1749.8 1.42386 −0.066 1671.5 1.39597 −0.043 1667.9 1.39472 −0.042 1665.4 1.39390 −0.042 1466.6 1.33349 0.000 Water + [BMpyr][TfO] 1484.6 1.43557 0.000 1486.9 1.43506 0.016 1489.1 1.43458 0.013 1494.5 1.43345 −0.003 1500.7 1.43221 −0.021 1509.8 1.43031 −0.047 1520.1 1.42809 −0.075 1533.3 1.42501 −0.107 1552.3 1.41987 −0.142 1576.2 1.41085 −0.170 1595.4 1.39398 −0.155 1592.8 1.37399 −0.113 1466.6 1.33349 0.000 Water + [BMpy][TfO] 1436.1 1.46446 0.000 1439.2 1.46358 −0.012 1442.3 1.46266 −0.023 1447.7 1.46111 −0.025 1454.3 1.45910 −0.038 1463.0 1.45651 −0.053 1475.6 1.45257 −0.086 1489.9 1.44770 −0.116

m ·TPa−1·mol−1 3

−0.0186 −0.0188 −0.0212 −0.0214 −0.0218 −0.0213 −0.0205 −0.0173 −0.0173 −0.0124 −0.0122 −0.0121 0.0000 0.0000 −0.0028 −0.0053 −0.0106 −0.0155 −0.0209 −0.0252 −0.0289 −0.0315 −0.0314 −0.0264 −0.0199 0.0000



Article

Table 2. continued

a

ρ

u

x1

g·cm−3

m·s−1

0.7494 0.8383 0.9201 0.9603 1.0000

1.23548 1.20829 1.15576 1.10357 0.99910

0.0000 0.0987 0.1089 0.2048 0.3037 0.3979 0.4922 0.6018 0.7084 0.8029 0.8984 0.9493 1.0000

1.22593 1.22431 1.22406 1.22240 1.21981 1.21682 1.21276 1.20578 1.19496 1.17739 1.13840 1.09223 0.99910

0.0000 0.0785 0.1191 0.2124 0.3055

1.03476 1.03448 1.03431 1.03388 1.03338

nD

VE

KES,m

cm3·mol−1

m3·TPa−1·mol−1

Water + [BMpy][TfO] 1508.6 1.44008 −0.151 1530.4 1.42747 −0.165 1548.9 1.40357 −0.139 1553.2 1.37998 −0.103 1466.6 1.33349 0.000 Water + [EEpy][ESO4] 1774.5 1.51631 0.000 1778.2 1.51478 −0.073 1778.4 1.51458 −0.071 1781.1 1.51266 −0.185 1787.5 1.51023 −0.246 1794.1 1.50723 −0.323 1803.5 1.50325 −0.388 1817.3 1.49668 −0.441 1836.4 1.48656 −0.483 1854.8 1.47121 −0.460 1846.6 1.43938 −0.316 1759.2 1.40430 −0.135 1466.6 1.33349 0.000 Water + [HMim][dca] 1713.4 1.50729 0.000 1713.4 1.50599 0.008 1713.8 1.50525 0.013 1714.3 1.50333 0.022 1715.0 1.50096 0.026

0.0000 −0.0054 −0.0058 −0.0104 −0.0162 −0.0212 −0.0264 −0.0318 −0.0365 −0.0386 −0.0353 −0.0258 0.0000 0.0000 −0.0018 −0.0030 −0.0054 −0.0079

ρ

u

x1

g·cm−3

m·s−1

0.4019 0.5096 0.6069 0.7123 0.8067 0.9031 0.9499 1.0000

1.03276 1.03190 1.03086 1.02908 1.02634 1.02046 1.01451 0.99910

0.0000 0.0574 0.0730 0.1111 0.1927 0.2941 0.4005 0.4996 0.6077 0.7064 0.8058 0.9010 0.9497 0.9645 1.0000

1.35286 1.35036 1.34961 1.34774 1.34332 1.33661 1.32754 1.31650 1.29941 1.27609 1.23751 1.16889 1.10724 1.08191 0.99910

nD

VE

KES,m

cm3·mol−1


Water + [HMim][dca] 1715.8 1.49784 0.025 1717.0 1.49312 0.014 1717.9 1.48721 −0.002 1717.1 1.47716 −0.016 1709.4 1.46233 −0.028 1672.0 1.43179 −0.035 1618.9 1.40179 −0.034 1466.6 1.33349 0.000 Water + [Mpy][MSO4] 1901.1 1.51603 0.000 1900.5 1.51462 −0.005 1901.4 1.51423 −0.003 1901.9 1.51315 −0.006 1903.2 1.51059 −0.021 1906.5 1.50679 −0.037 1909.5 1.50177 −0.052 1912.2 1.49567 −0.075 1913.2 1.48632 −0.085 1906.8 1.47388 −0.085 1878.0 1.45378 −0.053 1782.3 1.41884 0.003 1670.4 1.38778 0.019 1622.5 1.37506 0.016 1466.6 1.33349 0.000

−0.0105 −0.0134 −0.0158 −0.0176 −0.0178 −0.0144 −0.0103 0.0000 0.0000 −0.0020 −0.0029 −0.0045 −0.0080 −0.0126 −0.0172 −0.0212 −0.0250 −0.0274 −0.0275 −0.0223 −0.0152 −0.0120 0.0000

Standard uncertainty: x is ±0.0001, ρ is ±0.00003 g·cm−3, u is ±0.3 m·s−1, nD is ±0.00004, VE is ±0.007 cm3·mol−1, and KES,m is ±0.0001 m3·TPa−1·mol−1.

Table 3. Densities, ρ, Speeds of Sound, u, Refractive Indices, nD, Excess Molar Volumes, VE, and Excess Molar Isentropic Compressions, KES,m, of the Binary Mixtures Water (1) + Ionic Liquid (2) at T = 298.15 Ka ρ

u

x1

g·cm−3

m·s−1

0.0000 0.1088 0.1227 0.2017 0.2943 0.3952 0.5030 0.5985 0.7032 0.8050 0.9006 0.9503 1.0000

1.06017 1.05916 1.05901 1.05816 1.05698 1.05535 1.05324 1.05071 1.04689 1.04050 1.02960 1.01875 0.99704

0.0000 0.1348 0.2028 0.2056 0.3083 0.4985 0.6062 0.7015 0.8064 0.8998 0.9309

1.29955 1.29568 1.29330 1.29322 1.28866 1.27645 1.26525 1.25044 1.22333 1.17298 1.14339

nD

VE

KES,m

cm3·mol−1

m3·TPa−1·mol−1 0.0000 −0.0015 −0.0017 −0.0029 −0.0045 −0.0063 −0.0086 −0.0107 −0.0127 −0.0134 −0.0115 −0.0083 0.0000

Water + [BMim][dca] 1738.1 1.50893 0.000 1736.9 1.50673 0.049 1736.8 1.50631 0.056 1735.9 1.50443 0.083 1735.0 1.50161 0.111 1734.0 1.49756 0.140 1733.0 1.49223 0.148 1732.1 1.48541 0.147 1729.1 1.47495 0.124 1718.6 1.45694 0.106 1682.2 1.42613 0.057 1630.5 1.39520 0.025 1497.4 1.33255 0.000 Water + [BMim][TfO] 1391.4 1.43755 0.000 1396.7 1.43626 0.012 1400.2 1.43535 0.015 1400.9 1.43530 0.012 1406.7 1.43373 0.036 1422.8 1.42940 0.042 1436.4 1.42546 0.046 1453.1 1.42021 0.032 1478.8 1.41072 −0.019 1513.4 1.39323 −0.041 1527.6 1.38290 −0.058

2167

ρ

u

x1

g·cm−3

m·s−1

0.9548 1.0000

1.11062 0.99704

0.0000 0.0960 0.1277 0.2116 0.3077 0.3942 0.4952 0.5023 0.6047 0.6137 0.7061 0.7177 0.7571 0.8208 0.8468 0.9034 0.9040 0.9492 0.9507 0.9517 1.0000

1.01353 1.01325 1.01316 1.01289 1.01256 1.01227 1.01189 1.01186 1.01142 1.01137 1.01072 1.01061 1.01013 1.00891 1.00819 1.00598 1.00593 1.00312 1.00299 1.00291 0.99704

nD

VE

KES,m

cm3·mol−1


Water + [BMim][TfO] 1540.1 1.37157 −0.060 1497.4 1.33255 0.000 Water + [BMpyr][dca] 1811.1 1.49681 0.000 1810.6 1.49519 0.024 1810.4 1.49459 0.029 1810.1 1.49278 0.043 1810.0 1.49032 0.051 1810.0 1.48751 0.048 1810.8 1.48329 0.037 1810.9 1.48284 0.036 1811.8 1.47674 0.014 1811.8 1.47610 0.012 1809.6 1.46748 −0.005 1808.9 1.46612 −0.006 1804.9 1.46078 −0.009 1790.5 1.44870 −0.006 1779.7 1.44172 −0.003 1738.3 1.42145 0.004 1737.2 1.42118 0.005 1670.7 1.39362 0.004 1667.7 1.39249 0.005 1665.5 1.39161 0.005 1497.4 1.33255 0.000

0.0000 −0.0020 −0.0027 −0.0046 −0.0070 −0.0093 −0.0124 −0.0126 −0.0156 −0.0159 −0.0178 −0.0179 −0.0181 −0.0175 −0.0167 −0.0137 −0.0136 −0.0096 −0.0094 −0.0093 0.0000



Article

Table 3. continued ρ

u

x1

g·cm−3

m·s−1

0.0000 0.0559 0.1084 0.2076 0.2997 0.4080 0.5038 0.6003 0.7052 0.8095 0.9018 0.9533 1.0000

1.25298 1.25178 1.25057 1.24782 1.24468 1.23995 1.23436 1.22656 1.21367 1.19113 1.14895 1.09873 0.99704

0.0000 0.0794 0.1517 0.2502 0.3515 0.4528 0.5628 0.6561 0.7494 0.8383 0.9201 0.9603 1.0000

1.27941 1.27762 1.27563 1.27228 1.26800 1.26240 1.25401 1.24359 1.22752 1.20056 1.14907 1.09818 0.99704

0.0000 0.0987 0.1089 0.2048 0.3037 0.3979 0.4922 0.6018

1.21926 1.21763 1.21736 1.21583 1.21323 1.21011 1.20604 1.19892

nD

VE

KES,m

cm3·mol−1

m3·TPa−1·mol−1 0.0000 −0.0030 −0.0053 −0.0102 −0.0147 −0.0195 −0.0233 −0.0265 −0.0286 −0.0282 −0.0234 −0.0172 0.0000

Water + [BMpyr][TfO] 1460.1 1.43289 0.000 1462.7 1.43240 0.005 1464.9 1.43187 0.002 1470.5 1.43075 0.008 1476.9 1.42951 0.008 1486.1 1.42762 0.002 1496.5 1.42536 −0.010 1510.0 1.42221 −0.028 1529.6 1.41708 −0.054 1555.8 1.40814 −0.083 1582.1 1.39161 −0.087 1589.4 1.37210 −0.068 1497.4 1.33255 0.000 Water + [BMpy][TfO] 1412.3 1.46155 0.000 1415.5 1.46073 −0.013 1418.7 1.45982 −0.011 1424.3 1.45822 0.005 1431.2 1.45623 0.008 1440.0 1.45363 0.005 1452.9 1.44968 −0.012 1467.6 1.44482 −0.033 1487.4 1.43724 −0.064 1512.5 1.42474 −0.083 1540.1 1.40118 −0.081 1553.5 1.37782 −0.065 1497.4 1.33255 0.000 Water + [EEpy][ESO4] 1745.2 1.51350 0.000 1749.1 1.51196 −0.064 1749.5 1.51176 −0.058 1753.9 1.50984 −0.185 1760.5 1.50743 −0.236 1767.4 1.50440 −0.290 1776.8 1.50042 −0.348 1790.5 1.49381 −0.383

0.0000 −0.0051 −0.0055 −0.0106 −0.0159 −0.0204 −0.0251 −0.0297

ρ

u

x1

g·cm−3

m·s−1

0.7084 0.8029 0.8984 0.9493 1.0000

1.18792 1.17026 1.13151 1.08665 0.99704

0.0000 0.0785 0.1191 0.2124 0.3055 0.4019 0.5096 0.6069 0.7123 0.8067 0.9031 0.9499 1.0000

1.02856 1.02825 1.02807 1.02761 1.02708 1.02641 1.02546 1.02431 1.02240 1.01960 1.01413 1.00912 0.99704

0.0000 0.0574 0.0730 0.1111 0.1927 0.2941 0.4005 0.4996 0.6077 0.7064 0.8058 0.9010 0.9497 0.9645 1.0000

1.34612 1.34361 1.34285 1.34098 1.33656 1.32985 1.32080 1.30979 1.29274 1.26951 1.23117 1.16331 1.10267 1.07785 0.99704

nD

VE

KES,m

cm3·mol−1


Water + [EEpy][ESO4] 1809.2 1.48367 −0.408 1827.2 1.46831 −0.378 1821.6 1.43661 −0.239 1746.2 1.40198 −0.087 1497.4 1.33255 0.000 Water + [HMim][dca] 1687.7 1.50415 0.000 1688.1 1.50284 0.020 1688.3 1.50210 0.030 1688.9 1.50016 0.051 1689.7 1.49780 0.066 1690.6 1.49467 0.076 1692.1 1.48997 0.081 1693.4 1.48405 0.078 1693.2 1.47400 0.074 1687.6 1.45924 0.063 1659.1 1.42902 0.038 1618.6 1.39945 0.014 1497.4 1.33255 0.000 Water + [Mpy][MSO4] 1875.8 1.51340 0.000 1876.0 1.51199 0.001 1876.4 1.51159 0.004 1877.4 1.51052 0.005 1879.2 1.50803 −0.004 1882.6 1.50426 −0.013 1886.2 1.49921 −0.022 1889.5 1.49314 −0.039 1891.0 1.48384 −0.042 1885.4 1.47142 −0.038 1859.2 1.45141 −0.004 1772.7 1.41670 0.044 1673.5 1.38593 0.048 1633.3 1.37336 0.039 1497.4 1.33255 0.000

−0.0335 −0.0349 −0.0310 −0.0219 0.0000 0.0000 −0.0016 −0.0024 −0.0043 −0.0063 −0.0082 −0.0106 −0.0125 −0.0137 −0.0137 −0.0109 −0.0077 0.0000 0.0000 −0.0020 −0.0026 −0.0042 −0.0074 −0.0115 −0.0156 −0.0193 −0.0225 −0.0244 −0.0240 −0.0189 −0.0125 −0.0099 0.0000

a Standard uncertainty: x is ±0.0001, ρ is ±0.00003 g·cm−3, u is ±0.3 m·s−1, nD is ±0.00004, VE is ±0.007 cm3·mol−1, and KES,m is ±0.0001 m3·TPa−1·mol−1.

content and volatile compounds to negligible values. Once dried, the ILs were kept in a bottle under argon gas, and their purity was periodically checked by density measurements. The purity, water content and physical properties (density and refractive index) for all the pure components at T = 298.15 K are shown in Table 1. In this table, values of these properties found in literature for the pure components are also included for comparison purposes.8−15,17 As it can be seen, the density and refractive index for the ILs reported in this work are quite consistent with those available in the literature; the larger difference was found for the density value of [BMpyr][dca] ionic liquid. In order to minimize this difference, this compound was subjected to a longer drying period than the rest of the ILs, showing that its density remained constant. The differences between experimental and literature data can be attributed to the presence of water or nonvolatile impurities in the different samples. Apparatus and Procedure. The binary mixtures were prepared by weighing known masses of ionic liquid and

From the experimental measurements, excess molar volumes, and excess molar isentropic compressions were calculated and fitted using the Redlich−Kister equation. Moreover, a discussion about the influence of composition, structure of the ILs, and temperature on the excess properties was carried out.

■

EXPERIMENTAL SECTION

Chemicals. The studied ILs containing trifluoromethanesulfonate and dicyanamide anions were supplied by Iolitec GmbH (Germany), while the alkylsulfate-based ILs were synthetized in the laboratory following the procedure published by Gómez et al.8 To confirm their purity, 1H NMR was carried out and their density, refractive index and water content were measured. All binary mixtures were prepared using deionized water (Milli-Q quality). Since the physical properties of ILs are sensitive to impurities and water content, prior their use, these compounds were subjected to vacuum (p = 2 × 10−1 Pa) at moderate temperature (T = 323.15 K) with the aim of reducing the water 2168



Article

Table 4. Densities, ρ, Speeds of Sound, u, Refractive Indices, nD, Excess Molar Volumes, VE, and Excess Molar Isentropic Compressions, KES,m, of the Binary Mixtures Water (1) + Ionic Liquid (2) at T = 308.15 Ka ρ x1

VE

u −3

g·cm

0.0000 0.1088 0.1227 0.2017 0.2943 0.3952 0.5030 0.5985 0.7032 0.8050 0.9006 0.9503 1.0000

1.05383 1.05279 1.05264 1.05177 1.05055 1.04888 1.04670 1.04409 1.04015 1.03369 1.02311 1.01306 0.99402

0.0000 0.1348 0.2028 0.2056 0.3083 0.4985 0.6062 0.7015 0.8064 0.8998 0.9309 0.9548 1.0000

1.29162 1.28774 1.28533 1.28523 1.28065 1.26832 1.25708 1.24222 1.21523 1.16556 1.13656 1.10451 0.99402

0.0000 0.0960 0.1277 0.2116 0.3077 0.3942 0.4952 0.5023 0.6047 0.6137 0.7061 0.7177 0.7571 0.8208 0.8468 0.9034 0.9040 0.9492 0.9507 0.9517 1.0000

1.00793 1.00763 1.00752 1.00723 1.00685 1.00650 1.00603 1.00599 1.00542 1.00535 1.00454 1.00440 1.00386 1.00257 1.00184 0.99984 0.99981 0.99762 0.99753 0.99747 0.99402

0.0000 0.0559 0.1084 0.2076 0.2997 0.4080 0.5038

1.24578 1.24455 1.24330 1.24051 1.23730 1.23247 1.22679

m·s

−1

nD

ρ

KES,m −1

cm ·mol 3

Water + [BMim][dca] 1713.1 1.50578 0.000 1712.4 1.50348 0.061 1712.2 1.50321 0.069 1711.6 1.50114 0.104 1710.7 1.49821 0.142 1710.0 1.49443 0.181 1709.4 1.48897 0.201 1709.0 1.48225 0.209 1706.9 1.47147 0.195 1698.2 1.45386 0.180 1668.8 1.42382 0.117 1627.4 1.39322 0.065 1522.3 1.33131 0.000 Water + [BMim][TfO] 1369.1 1.43475 0.000 1374.8 1.43343 0.025 1378.1 1.43259 0.038 1379.4 1.43253 0.037 1384.6 1.43090 0.072 1400.9 1.42655 0.102 1414.8 1.42268 0.116 1432.0 1.41759 0.109 1460.0 1.40803 0.057 1500.5 1.39103 0.018 1519.2 1.38091 −0.011 1536.7 1.36969 −0.023 1522.3 1.33131 0.000 Water + [BMpyr][dca] 1781.9 1.49392 0.000 1782.4 1.49222 0.032 1783.1 1.49162 0.042 1783.8 1.48982 0.063 1784.2 1.48710 0.082 1785.7 1.48432 0.089 1786.7 1.47987 0.090 1786.7 1.47973 0.090 1788.0 1.47345 0.080 1788.0 1.47289 0.080 1786.1 1.46424 0.071 1785.6 1.46281 0.071 1782.1 1.45746 0.069 1769.4 1.44577 0.071 1756.5 1.43884 0.073 1724.2 1.41909 0.066 1723.6 1.41891 0.066 1666.7 1.39175 0.046 1664.3 1.39055 0.046 1662.5 1.38970 0.045 1522.3 1.33131 0.000 Water + [BMpyr][TfO] 1436.7 1.43018 0.000 1439.7 1.42970 0.014 1441.5 1.42918 0.022 1447.2 1.42808 0.040 1453.6 1.42683 0.055 1462.8 1.42490 0.065 1473.2 1.42262 0.064

−1

m ·TPa ·mol 3

−1

x1

0.0000 −0.0012 −0.0012 −0.0022 −0.0032 −0.0046 −0.0064 −0.0081 −0.0097 −0.0100 −0.0084 −0.0059 0.0000

0.0000 −0.0022 −0.0032 −0.0052 −0.0072 −0.0096 −0.0119 −0.0120 −0.0143 −0.0145 −0.0157 −0.0158 −0.0157 −0.0147 −0.0132 −0.0110 −0.0109 −0.0074 −0.0072 −0.0071 0.0000 0.0000 −0.0030 −0.0048 −0.0095 −0.0136 −0.0179 −0.0214 2169

g·cm

VE

u −3

0.6003 0.7052 0.8095 0.9018 0.9533 1.0000

1.21889 1.20588 1.18337 1.14173 1.09263 0.99402

0.0000 0.0794 0.1517 0.2502 0.3515 0.4528 0.5628 0.6561 0.7494 0.8383 0.9201 0.9603 1.0000

1.27180 1.26998 1.26796 1.26458 1.26024 1.25458 1.24608 1.23557 1.21946 1.19269 1.14204 1.09230 0.99402

0.0000 0.0987 0.1089 0.2048 0.3037 0.3979 0.4922 0.6018 0.7084 0.8029 0.8984 0.9493 1.0000

1.21265 1.21114 1.21087 1.20922 1.20665 1.20343 1.19929 1.19205 1.18085 1.16305 1.12448 1.08072 0.99402

0.0000 0.0785 0.1191 0.2124 0.3055 0.4019 0.5096 0.6069 0.7123 0.8067 0.9031 0.9499 1.0000

1.02240 1.02207 1.02188 1.02138 1.02081 1.02008 1.01904 1.01778 1.01571 1.01281 1.00762 1.00334 0.99402

0.0000 0.0574 0.0730 0.1111 0.1927 0.2941 0.4005 0.4996

1.33937 1.33686 1.33611 1.33423 1.32982 1.32312 1.31409 1.30309

−1

m·s

nD

cm ·mol 3

KES,m −1

Water + [BMpyr][TfO] 1486.7 1.41943 0.055 1506.6 1.41430 0.036 1534.6 1.40557 0.002 1566.6 1.38937 −0.022 1582.8 1.37023 −0.027 1522.3 1.33131 0.000 Water + [BMpy][TfO] 1389.6 1.45868 0.000 1392.5 1.45784 −0.002 1395.8 1.45694 0.010 1401.5 1.45535 0.038 1408.5 1.45331 0.054 1417.4 1.45069 0.063 1430.3 1.44673 0.060 1445.3 1.44183 0.049 1465.8 1.43429 0.021 1493.4 1.42192 −0.006 1528.6 1.39863 −0.026 1550.4 1.37570 −0.030 1522.3 1.33131 0.000 Water + [EEpy][ESO4] 1718.4 1.51067 0.000 1722.4 1.50914 −0.078 1723.0 1.50896 −0.071 1727.5 1.50705 −0.174 1734.4 1.50461 −0.224 1741.1 1.50156 −0.261 1750.6 1.49759 −0.308 1763.7 1.49094 −0.329 1782.1 1.48071 −0.339 1799.4 1.46515 −0.303 1795.6 1.43375 −0.171 1731.8 1.40013 −0.045 1522.3 1.33131 0.000 Water + [HMim][dca] 1662.7 1.50100 0.000 1663.1 1.49962 0.029 1663.3 1.49894 0.043 1664.0 1.49698 0.076 1664.8 1.49462 0.102 1665.8 1.49131 0.124 1667.6 1.48679 0.142 1668.9 1.48083 0.150 1669.1 1.47076 0.157 1665.3 1.45590 0.146 1644.5 1.42645 0.103 1614.6 1.39679 0.057 1522.3 1.33131 0.000 Water + [Mpy][MSO4] 1852.4 1.51077 0.000 1852.3 1.50937 0.004 1852.7 1.50895 0.008 1853.7 1.50794 0.012 1855.8 1.50539 0.006 1859.2 1.50170 0.003 1863.4 1.49665 −0.001 1867.0 1.49057 −0.012

m ·TPa−1·mol−1 3

−0.0241 −0.0258 −0.0253 −0.0207 −0.0150 0.0000

0.0000 −0.0049 −0.0054 −0.0102 −0.0152 −0.0193 −0.0235 −0.0275 −0.0308 −0.0316 −0.0273 −0.0187 0.0000 0.0000 −0.0012 −0.0018 −0.0032 −0.0047 −0.0062 −0.0081 −0.0095 −0.0103 −0.0100 −0.0078 −0.0055 0.0000 0.0000 −0.0016 −0.0022 −0.0036 −0.0066 −0.0104 −0.0143 −0.0175



Article

Table 4. continued ρ

u

x1

g·cm−3

m·s−1

0.6077 0.7064 0.8058 0.9010

1.28607 1.26291 1.22476 1.15751

Water + [Mpy][MSO4] 1868.9 1.48134 −0.010 1863.4 1.46886 −0.002 1837.7 1.44897 0.034 1760.4 1.41447 0.076

nD

VE

KES,m

cm3·mol−1

m3·TPa−1·mol−1 −0.0204 −0.0218 −0.0209 −0.0160

ρ

u

x1

g·cm−3

m·s−1

0.9497 0.9645 1.0000

1.09768 1.07327 0.99402

Water + [Mpy][MSO4] 1673.4 1.38397 0.070 1637.2 1.37158 0.057 1522.3 1.33131 0.000

nD

VE

KES,m

cm3·mol−1

m3·TPa−1·mol−1 −0.0102 −0.0079 0.0000

Standard uncertainty: x is ±0.0001, ρ is ±0.00003 g·cm−3, u is ±0.3 m·s−1, nD is ±0.00004, VE is ±0.007 cm3·mol−1, and KES,m is ±0.0001 m3·TPa−1·mol−1.

a

uncertainties for density and speed of sound are ±3 × 10−5 g·cm−3 and ±0.3 m·s−1, respectively, and the combined expanded uncertainties (k = 2) for density and speed of sound are ±5 × 10−5 g·cm−3 and ±0.6 m·s−1, respectively. The experimental determination of refractive index of pure compounds and of the studied binary mixtures was carried out with an automatic refractometer Abbemat-HP Dr. Kernchen. For the refractive index, the experimental uncertainty is ±4 × 10−5 and the combined expanded uncertainty (k = 2) is ±8 × 10−5. More detailed information about the densimeter and the refractometer used in this work can be found in a previous publication.8 The water content of the pure ILs was measured with a Metrohm 870 KF Titrino using Titran 2, supplied by Merck, as titrant. Finally, a Metter Toledo differential scanning calorimeter (DSC822e) was used to determine the molar isobaric heat capacities for pure ILs, which were necessary to calculate the excess molar isentropic compressions for the binary mixtures at the studied temperatures. The experimental uncertainty for the molar isobaric heat capacity is ±1 J·mol−1·K−1 and the corresponding combined expanded uncertainty (k = 2) is ±2 J·mol−1·K−1.

Table 5. Isobaric Expansibility, αp, and Molar Isobaric Heat Capacity, Cp, for the Pure ILsa Cp J·mol·K−1

αp K−1

ionic liquid [BMim][dca] [BMim][TfO] [BMpyr][dca] [BMpyr][TfO] [BMpy][TfO] [EEpy][ESO4] [HMim][dca] [Mpy][MSO4] a

6.01 6.11 5.54 5.81 5.97 5.45 6.01 5.01

× × × × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

T = 288.15 K

T = 298.15 K

T = 308.15 K

367

376

381

473 424

502 435

521 441

399 510 288

412 534 299

420 548 305

Standard uncertainty: Cp is ±1 J·mol·K−1

water, which were injected into a stoppered glass vial. All samples were prepared immediately prior to measurements using a Mettler AX-205 Delta Range balance with an uncertainty of ±3 × 10 −4 g. In order to avoid water adsorption, the ILs were manipulated inside a glovebox under argon. An Anton Paar DSA-5000 M digital vibrating-tube densimeter was used to measure density and speed of sound of the pure liquids and their binary mixtures. The experimental

Table 6. Fitting Parameters and Standard Relative Deviations, σ, for the Binary Mixtures Water (1) + Ionic Liquid (2) at T = 288.15 K excess property

B0

VE/cm3·mol−1 KES,m/m3·TPa−1·mol−1

0.358 −0.044

VE/cm3·mol−1

−0.094


−0.103 −0.061


−0.298 −0.102

VE/cm3·mol−1

−0.241


−1.587 −0.109


0.070 −0.052


−0.287 −0.086

B1

B2

Water + [BMim][dca] −0.108 −0.194 −0.047 −0.030 Water + [BMim][TfO] −0.064 −0.097 Water + [BMpyr][dca] −0.619 −0.454 −0.065 −0.038 Water + [BMpyr][TfO] −0.519 −0.421 −0.069 −0.022 Water + [BMpy][TfO] −0.453 −1.053 Water + [EEpy][ESO4] −1.358 −0.829 −0.088 −0.043 Water + [HMim][dca] −0.205 −0.311 −0.047 −0.032 Water + [Mpy][MSO4] −0.416 −0.198 −0.069 −0.044 2170

B3

B4

σ

−0.323 −0.077

−0.280 −0.076

0.181 0.051

−1.069

−1.405

0.406

0.089 −0.065

0.166 −0.075

0.213 0.036

−1.078 −0.139

−0.607 −0.164

1.253 0.063

−0.836

0.190

−0.298 −0.187

−0.229

0.093 0.030

−0.204 −0.062

−0.057

0.520 0.043

0.746 −0.098

0.946 −0.087

1.900 0.047



Article

Table 7. Fitting Parameters and Standard Relative Deviations, σ, for the Binary Mixtures Water (1) + Ionic Liquid (2) at T = 298.15 K excess property VE/cm3·mol−1 KES,m/m3·TPa−1·mol−1 VE/cm3·mol−1

B0 0.593 −0.035 0.174


0.143 −0.051


−0.039 −0.094

VE/cm3·mol−1

−0.009


−1.433 −0.102


0.324 −0.042


−0.140 −0.077

B1

B2

Water + [BMim][dca] 0.100 −0.041 −0.039 −0.025 Water + [BMim][TfO] 0.140 0.007 Water + [BMpyr][dca] −0.377 −0.261 −0.058 −0.034 Water + [BMpyr][TfO] −0.254 −0.313 −0.061 −0.022 Water + [BMpy][TfO] −0.245 −0.347 Water + [EEpy][ESO4] −1.028 −0.417 −0.077 −0.052 Water + [HMim][dca] 0.057 0.043 −0.038 −0.023 Water + [Mpy][MSO4] −0.317 −0.099 −0.062 −0.040

B3

B4

σ

−0.031 −0.051

−0.055

0.052 0.039

−0.807

−1.186

0.325

0.353 −0.040

0.419 −0.048

0.215 0.032

−0.632 −0.115

−0.593 −0.139

4.724 0.048

−0.428

−0.852

0.292

−0.098 −0.159

−0.164

0.142 0.028

0.027 −0.041

−0.043

0.096 0.030

0.948 −0.072

1.141 −0.060

1.534 0.034

Table 8. Fitting Parameters and Standard Relative Deviations, σ, for the Binary Mixtures Water (1) + Ionic Liquid (2) at T = 308.15 K excess property VE/cm3·mol−1 KES,m/m3·TPa−1·mol−1 VE/cm3·mol−1

0.7952 −0.0258 0.4159


0.3588 −0.0486


0.2555 −0.0860

VE/cm3·mol−1

■

B0

0.2517


−1.2686 −0.0955


0.5592 −0.0320


−0.0335 −0.0704

B1

B2

Water + [BMim][dca] 0.2907 0.2495 −0.0320 −0.0219 Water + [BMim][TfO] 0.3428 0.1332 Water + [BMpyr][dca] −0.1450 −0.0796 −0.0458 −0.0281 Water + [BMpyr][TfO] −0.0312 −0.1860 −0.0549 −0.0193 Water + [BMpy][TfO] −0.0050 −0.1910 Water + [EEpy][ESO4] −0.8105 −0.1975 −0.0707 −0.0525 Water + [HMim][dca] 0.2899 0.3443 −0.0287 −0.0138 Water + [Mpy][MSO4] −0.2252 −0.0211 −0.0570 −0.0342

RESULTS AND DISCUSSION

B3

B4

σ

0.2224 −0.0307

−0.0350

0.023 0.039

−0.5850

−1.0266

0.723

0.5717 −0.0239

0.6104 −0.0257

0.013 0.036

−0.4405 −0.0969

−0.3704 −0.1188

0.359 0.040

−0.2766

−0.6832

0.177

0.3977 −0.1293

−0.1246

0.161 0.020

0.2722 −0.0262

−0.0311

0.021 0.033

1.0989 −0.0521

1.2784 −0.0372

0.529 0.031

of sound, this property decreases with the temperature except at high concentrations of water, where it increases due to the fact that the speed of sound of the pure water increases with the temperature, showing an opposite behavior to the speed of sound of the pure ILs included in this work. On the other hand, the variation of this property with the water composition, x1, depends on the nature of the ILs. This property decreases with x1 for the systems containing dicyanamide- and alkylsulfatebased ILs, and it increases for the studied systems containing

Values of density, speed of sound, and refractive index for the binary systems (water + [BMim][TfO], [BMpyr][TfO], [BMpy][TfO], [BMim][dca], [HMim][dca], [BMpyr][dca], [Mpy][MSO4], or [EEpy][ESO4]) at T = (288.15 to 308.15) K and atmospheric pressure are shown in Tables 2−4. From these data, it is possible to observe that density and refractive index decrease with the water composition, x1, and with temperature for all of the studied binary mixtures. With regard to the speed 2171



Article

Regarding to the speed of sound, u, this property can be related to the isentropic compressibility, κs, by the Laplace equation: κs = −Vm−1(∂Vm/∂p)S = ρ−1u−2 = Vm/(M mu 2)

(2)

where Mm is the molar mass of the mixture. To achieve agreement with the other properties, it is more appropriate to shift from the volume-intensive, κs, to the moleintensive quantity, KS,m,18,19 calculated as follows: ΚS,m = −(∂Vm/∂p)S = Vmκs = Vm 2/(M mu 2)

(3)

where KS,m is the molar isentropic compression. The excess molar isentropic compression, KES,m, is calculated by the next equation: E id ΚS,m = ΚS,m − ΚS,m

(4)

KidS,m

where is defined by the approach developed by Benson and Kiyohara20 id ΚS,m

⎡ ⎡ (∑ x E * )2 ⎤ (Ep,*i)2 ⎤ i i p, i ⎥ ⎢ ⎥ * = ∑ xi ΚS, i + T − T⎢ * ⎢ ⎥ ⎢ Cp, i ⎦ ⎣ ⎣ ∑i xiCp,*i ⎥⎦ i

(5)

where K*S,i is the product of the molar volume, V*i, and the isentropic compressibility, κ*S,i, of the pure component i. The molar isobaric expansion of pure component i, E*p,i, is the product of the molar volume and the isobaric expansibility, α*p,i(α*p,i = −1/ρ(∂ρ/∂T)p), and C*p,i is the molar isobaric heat capacity of the pure component i. The α*p,i values for the pure ILs were calculated from their density data at the studied temperatures and C*p,i values were experimentally determined using a differential scanning calorimeter. The C*p,i values were determined for the studied ILs except to [BMim][TFO] and [BMpyr][TfO] since these ILs present a melting point close to room temperature and their C*p,i values cannot be reliably determined in the studied temperature range. These data for the pure ILs are listed in Table 5.For water, α*p,i and C*p,i values were taken from literature.21 The excess molar volume and excess molar isentropic compression at several temperatures were fitted to a Redlich− Kister22 type equation M

ΔQ ij = xixj

Figure 1. Excess molar volumes, VE for the binary mixtures water (1) + ionic liquid (2) at T = 298.15 K. Ionic liquids: (a) (●) [BMim][dca], (■) [HMim][dca], (▲) [BMpyr][dca]; (b) (○) [BMim][TfO], (□) [BMpy][TfO], (△) [BMpyr][TfO]; and (c) (⧫) [Mpy][MSO4] and (◊)[EEpy][ESO4].

V = Vm −

∑ xiVi* i

(6)

p=0

where ΔQij is the excess property, x is the mole fraction, BP is the fitting parameter, and M is the degree of the polynomial expansion, which was optimized using the F-test.23 The corresponding fitting parameters for each studied temperature, together with the standard relative deviation, σ, are given in Tables 6−8. The equation used for the calculation of σ is the following:

ILs with the trifluoromethanesulfonate anion, where a maximum at high concentrations of water was observed. Excess Properties. From density and speed of sound, the excess molar volume, VE, and excess molar isentropic compression, KES,m, were calculated. These data were also included in Tables 2−4. The experimental densities were used to calculate the excess molar volume, VE, of the mixture using the following equation: E

∑ Bp(xi − xj)p

1/2 ndat ⎧ ⎫ ⎪ 2 σ=⎨ ∑ ((z − zcal)/zcal) /ndat⎬ ⎪ ⎭ ⎩ i ⎪

⎪

(7)

where z and zcal are the values of the experimental and calculated property and ndat is the number of experimental points. In order to study the effect of water and structure of the ILs on the excess molar volume, the variation of this excess magnitude with the molar fraction of water, x1, for the binary systems water (1) + ionic liquid (2) at T = 298.15 K and the corresponding

(1)

where Vm is the molar volume of the mixture, and xi and V*i represent the mole fraction and the molar volume of component i, respectively. 2172



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Figure 2. (a) Excess molar volumes, VE, for the binary mixture water (1) + [BMim][dca] (2) at the studied temperatures. Full and empty symbols represent experimental and literature data from Carvalho et al.,15 respectively. (b) Excess molar volumes, VE, for the binary mixture water (1) + [BMim][TfO] (2) at the studied temperatures. Full symbols represent experimental data from this work, empty symbols represent literature data published by Garcı ́a-Miaja et al.10 and gray symbols represent literature data from Ge et al.9 Circles, square, and triangles indicate T = 288.15 K, T = 298.15 K, and T = 308.15 K, respectively.

fitted curves using the Redlich−Kister equation were plotted in Figure 1. Figures 1a), 1b) and 1c) show the variation of VE versus x1 for the aqueous mixtures containing dicyanamide-, trifluoromethanesulfonate-, and alkylsulfate-based ILs, respectively. Moreover, excess molar volumes, VE, versus x1 for all the systems, at the three studied temperatures, were also plotted in Figure S1, available in the Supporting Information (SI). As it can be observed in these figures, all of the studied systems present asymmetrical curves, which are quite common in mixtures containing two components with a large molar volume difference, as is indeed the case. The studied aqueous systems containing dicyanamide-based ILs show a sinusoidal behavior at low temperatures, with a minimum at high concentration of water that disappears when the temperature increases. The binary mixtures with trifluoromethanesulfonatebased ILs show this same sinusoidal shape at the three studied temperatures. This change from positive to negative in the VE behavior was also observed by García-Miaja et al.,10 Rodriguez and Brennecke,24 and Vercher et al.25 studying aqueous mixtures containing trifluoromethanesulfonate-based ILs. Finally, a sinusoidal behavior (with a minimum close to x1 = 0.6 and a maximum at high concentration of water) was observed for the binary mixture water + [Mpy][MSO4], while negative VE values were obtained, over the whole composition range, for the system containing the studied ionic liquid with [ESO4]− anion. Negative VE values were also found in literature for the aqueous system containing a imidazolium-based ionic liquid with the [ESO4]− as anion.10,24,26 From these results, it is possible to conclude that the behavior of the VE for the binary systems containing ILs with a common anion (such as dicyanamide, or trifluoromethanesulfonate) is quite similar and different from the rest of the mixtures containing ILs with other anions. This fact is consistent with the results previously published by Rodriguez and Brennecke,24 who calculated excess molar volumes for binary mixtures of water and three ILs containing the same cation (1-ethyl-methylimidazolium) and different anions (ethylsulfate, trifluoroacetate, and trifluoromethanesulfonate), and with those obtained by García-Miaja et al.,10 comparing excess molar volumes for the binary mixtures water + 1-ethyl-3-methylimidazolium ethylsulfate or 1-ethyl-3-methylimidazolium triflate and water +1-butyl-3methylimidazolium methylsulfate or 1-butyl-3-methylimidazolium tetrafluoroborate. Furthermore, several papers with VE values

for binary mixtures containing water and other imidazolium, pyrrolidinium, and pyridinium-based ILs were found in literature,10,27−33 in which the shape of the VE values is much related with the anion. As example, all of the VE data found in literature for aqueous systems containing ILs with tetrafluoroborate anion10,27−30 are positive in the whole composition range, while aqueous mixtures containing ILs with monomethylethersulphate31 or octanoate anions32,33 show negative VE values. All of these results suggest that the chemical structure of the anion plays an important role on the shape of the VE curves and, therefore, on the thermodynamic behavior of the studied systems. Comparing the dicyanamide-based ILs included in Figure 1a, it is possible to observe that an increase of the alkyl chain length of the cation leads to smaller VE values, which can be attributed to differences in size and shape of the studied ILs and/or to different intermolecular interactions in the mixtures. This behavior was also observed by García-Miaja et al.,10 comparing excess molar volumes for the aqueous systems containing 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-butyl-3-methylimidazolium trifluoromethanesulfonate. Another remarkable fact is that [BMim][dca] ionic liquid shows values of VE larger than the corresponding pyrrolidinium-based ionic liquid ([BMpyr][dca]). VE versus mole fraction of water at T = 298.15 K for the binary mixtures containing trifluoromethanesulfonate-based ILs are presented in Figure 1b. As it can be seen, although these systems show similar VE curves, since that all mixtures contain ILs with the same anion, the numerical VE values are different depending on the cation present: VE(imidazolium) > VE(pyridinium) ≅ VE(pyrrolidinium). This trend is in accordance with the results presented above for the binary systems containing dicyanamidebased ILs, and it is also consistent with the comparison made by García-Miaja et al.,10 using literature data for the binary systems (water + 1-butyl-3-methylimidazolium tetrafluoroborate)27 and (water + 1-butyl-3-methypyridinium tetrafluoroborate).28 A comparison between VE values obtained in this work and those found in literature was included in Figure 2. Figures 2, panels a and b, shows the variation of the VE values obtained in this work versus x1 and the corresponding data found in literature9,10,15 for the binary mixtures water (1) + [BMim][dca] (2) and water (1) + [BMim][TfO] (2), respectively. For the rest of the systems reported in this work, a comparison was 2173



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observed, a good agreement between our data and those reported by García-Miaja et al.10 were found at the two compared temperatures, while a small discrepancy for the numerical values was observed with the data published by Ge et al.9 Since the shape of the curves is similar in both cases, the authors consider that these little differences between experimental and literature values can be due to different levels of impurities in the samples, or the fact that the density data measured by Ge et al.9 were obtained with a pycnometer. On the other hand, as commented above, excess molar volumes, VE, versus x1 for all the studied systems, at the three temperatures, were also plotted in Figure S1, available in the Supporting Information (SI). From this figure, it is possible to analyze the effect of temperature on the excess molar volume. As it can be seen, VE values increase when the temperature increases; that is, it is more positive for VE > 0 and less negative for VE < 0. This dependence of VE with temperature for the binary mixtures water + IL, which is opposite to alcohol + ionic liquid,25,26,28,34 was also reported by other authors.10,24−28,30,31 Lehmann et al.26 explain that, for aqueous systems, the increasing excess molar volumes with increasing temperature may result from the temperature dependence of the strength of hydrogen bonds. As it is known, the excess molar volume depends mainly on the intermolecular forces between components of the mixtures, and on the packing due to the differences in size and shape of molecules. According to this, the negative excess volumes can be attributed to strong water-ionic liquid interactions and to an efficient packing of the components in the mixture. Following a similar procedure to that carried out with the excess molar volumes, excess molar isentropic compressions, KES,m, for binary systems water (1) + ionic liquid (2) at T = (288.15, 298.15 and 308.15) K were plotted in Figure S2, available in SI. The KES,m values for the systems containing [BMim][TfO] and [BMpy][TfO] were not calculated because the corresponding C*p,i values for the pure ILs are not available at the studied temperatures, as it was commented above. From this figure, it is possible to observe that all the studied systems show negative KES,m values in the whole composition range, showing a minimum at high concentration of water. These negative values imply that these mixtures are less compressible than the corresponding ideal mixtures due to a closer approach of unlike molecules and stronger interaction between the components of the mixtures. Moreover, in Figure S2 it is also possible to observe that KES,m values are less negative when the temperature increases. In order to analyze the effect of the structure of the ILs on the excess molar isentropic compression, the variation of this property versus x1 for the binary systems water (1) + ionic liquid (2) at T = 298.15 K together with the corresponding fitted curves, obtained from the Redlich−Kister equation, were plotted in Figure 3. Figure 3a shows the variation of KES,m versus x1 for the aqueous mixtures containing dycianamide-based ILs. As it can be seen, the aqueous system containing [BMpyr][dca] is the one which shows a behavior more far away from ideality. Moreover, comparing the systems containing [BMim][dca] and [HMim][dca] it is also possible to conclude that the values of this magnitude are quite similar for both systems, indicating that, in this case, the alkyl chain length does not seem to have a large effect on the KES,m values. In Figure 3b the KES,m values versus x1 for the binary mixture water (1) + [BMpyr][TfO] (2) at T = 298.15 K were plotted.

Figure 3. Excess molar isentropic compressions, KES,m for the binary mixtures water (1) + ionic liquid (2) at T = 298.15 K. Ionic liquids: (a) (●) [BMim][dca], (■) [HMim][dca], (▲) [BMpyr][dca]; (b) (△) [BMpyr][TfO]; and (c) (⧫) [Mpy][MSO4] and (◊)[EEpy][ESO4].

not possible because no data were found in literature at the studied temperatures. VE values included in this work and those reported by Carvalho et al.15 for the binary mixture water (1) + [BMim][dca] (2) at T = (288.15, 298.15, and 308.15) K are compared in Figure 2a. As it can be seen, a significant discrepancy between literature and experimental data was found for x1 > 0.3, where literature data increase remarkably, leading to values quite larger than those reported in this work. In Figure 2b, VE values for the aqueous system containing [BMim][TfO] published by Ge et al.9 at T = 308.5 K and García-Miaja et al.10 at T = (298.15 and 308.15) K are compared with the values obtained in this work. As it can be 2174



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Comparing the KES,m values plotted in Figure 3b for the binary system with [BMpyr][TfO] with those plotted in in Figure 3a for the aqueous mixture containing [BMpyr][dca], it is possible to analyze the effect of the anion on this excess magnitude. These results indicate that the system with [BMpyr][TfO] leads to more negative values of KES,m than that containing [BMpyr][dca]. Finally, the KES,m values versus x1 for the binary mixtures containing [Mpy][MSO4] and [EEpy][ESO4] ILs at T = 298.15 K were drawn in Figure 3c. In this case, both systems show negative KES,m values in the whole composition range, showing a minimum at x1 = 0.8. A comparison with published data for KES,m was not included since no data were found in the literature for binary systems {water + ionic liquid}.

CONCLUSIONS In this work, new data of density, speed of sound and refractive index for eight binary systems (water + ionic liquid) were measured at T = (288.15 to 308.15) K and atmospheric pressure. From these experimental data, excess molar volumes, VE, and excess molar isentropic compressions, KES,m, were calculated and fitted to a Redlich−Kister type equation. The results included in this paper show that the structure of the ILs plays an important role on the physical properties and on their corresponding excess magnitudes. The experimental data indicate that anions have a strong effect on the shape of excess properties curves, especially on excess molar volume. The experimental results show that an increase of the alkyl chain length of the cation leads to smaller VE values, and that excess molar volume for the studied ILs follow the trend: VE(imidazolium) > VE(pyridinium) ≅ VE(pyrrolidinium). On the other hand, the system with [BMpyr][TfO] leads to more negative values of KES,m than that containing [BMpyr][dca]. Regarding to the effect of temperature on the excess properties, VE values increase with temperature, and KES,m values, which are negative in the whole composition range, are closer to ideality when temperature increases. ASSOCIATED CONTENT

S Supporting Information *

Excess molar volumes and excess molar isentropic compressions versus water composition for the binary systems water (1) + ionic liquid (2) at studied temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +351225081674. Fax: +351225081669. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by project PEst-C/EQB/ LA0020/2011, financed by FEDER through COMPETE Programa Operacional Factores de Competitividade and by Fundaçaõ para a Ciência e a Tecnologia - FCT - (Portugal). E.J.G. is thankful to FCT for his postdoctoral grant (SFRH/ BPD/70776/2010). 2175



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Physical and Excess Properties of Eight Binary Mixtures Containing

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