Vapor Pressure Measurements for Binary Mixtures Containing Ionic

Feb 10, 2016 - The vapor pressures for binary systems containing the ionic liquid (IL) 1-ethyl-3-methylimidazoliun tetrafluoroborate ([EMIM][BF4]) and...
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Vapor Pressure Measurements for Binary Mixtures Containing Ionic Liquid and Predictions by the Conductor-like Screening Model for Real Solvents Published as part of The Journal of Chemical and Engineering Data special issue “Proceedings of the 6th International Congress on Ionic Liquids” Jingli Han, Zhigang Lei,* Chengna Dai, and Jiangsheng Li State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing, 100029, China ABSTRACT: The vapor pressures for binary systems containing the ionic liquid (IL) 1-ethyl-3-methylimidazoliun tetrafluoroborate ([EMIM][BF4]) and a solute were measured with a modified equilibrium still, and the solutes investigated were benzene, toluene, thiophene, and water. The results indicate that the IL [EMIM][BF4] produces an obvious effect on the vapor pressures of binary systems, and the vapor pressures increase with the temperature. The vapor pressures of binary mixtures (i.e., [EMIM][BF4] + benzene, [EMIM][BF4] + thiophene, and [EMIM][BF4] + toluene) first increase with the increase of mole fraction of the solute, and then almost keep stable. However, the vapor pressures of the mixture of water and [EMIM][BF4] increase linearly with the increase of mole fraction of water. The vapor pressure experimental data for the binary systems were compared with the predicted values by the conductor-like screening model for real solvents (COSMO-RS) model. The results demonstrate that the COSMO-RS model gives quantitative prediction for the vapor pressure of the [EMIM][BF4]−water system, but only qualitative prediction for other systems investigated in this work.

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INTRODUCTION In the past few years, ionic liquids (ILs) have received much attention in extraction process technology. Because of their peculiarities such as negligible vapor pressure and potential as “designer solvents”, ILs are treated as a very promising alternative for the replacement of volatile organic solvents in chemical and biochemical separation processes, that is, extraction distillation,1−5 liquid−liquid extraction,6−9 solid−liquid extraction,10 membrane extraction,11 and gas separation.12−17 Compared with the use of conventional solvent, application of IL in the extraction not only makes the existing processes highly efficient, but also allows running processes that are safe for people and friendly for the environment.18−21 Extraction distillation as the coupling of extraction and distillation is commonly used to remove thiophene from benzene or the gasoline containing benzene, toluene, and water. From the environmental and economic point of view, ILs can be easily recovered from the purification process of benzene, because of their thermal stability and low vapor pressure. However, vapor− liquid equilibrium (VLE) data are highly needed for the design and optimization of purification and separation processes as well as for the extension of thermodynamic models. Thus, in this work the vapor pressure experiments of the binary systems of [EMIM][BF4] (1-ethyl-3-methylimidazoliun tetrafluoroborate) + water/toluene/benzene/thiophene were conducted, because to the best of our knowledge these data have not been reported over a wide concentration range. © XXXX American Chemical Society

The conductor-like screening model for real solvents (COSMO-RS) model, independent of any experimental data, is a powerful thermodynamic and quantum mechanical approach for predicting the thermodynamic properties of mixtures containing ILs,22−30 such as activity coefficients, vapor pressure, excess properties, density, and phase equilibrium data. Banerjee et al.28 used the COSMO-RS model to predict the vapor pressures for 13 binary systems including six solutes and five imidazolium ionic liquids (ILs). The solutes were benzene, cyclohexane, acetone, 2-propanol, water, and tetrahydrofuran; and the ILs were 1-methyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [MMIM][(CF3 SO 2 ) 2 N], 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMIM][(CF3SO2)2N], 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl) imide [BMIM][(CF3SO2)2N], 1-methyl-3-methylimidazolium dimethylphosphate [MMIM][(CH3)2PO4], and 1-ethyl-3-methylimidazoliumethoxysulfate [EMIM][C2H5OSO3]. It was found that the root-mean-square deviation of vapor pressure prediction by the COSMO-RS model is 6.00%, compared to 4.00%, 1.45%, and 3.13% for the Wilson, nonrandom two liquid (NRTL), and universal quasichemical (UNIQUAC) models, respectively. This indicates that the vapor pressure prediction of binary systems by the COSMO-RS model Received: September 6, 2015 Accepted: January 28, 2016

A

DOI: 10.1021/acs.jced.5b00760 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Specifications of Chemical Materials Used in This Work

may be reliable. Thus, in this work the vapor pressures of solutes in IL will also be predicted by the COSMO-RS model using the COSMOthermX software package (vision C30_1301). The aim of this work is to measure the vapor pressure of the binary systems of [EMIM][BF4] with benzene, toluene, thiophene, and water at different temperatures and concentrations, and to study the effect of IL on VLE behavior of these binary systems. Moreover, the COSMO-RS model was used to predict the activity coefficients, and then the vapor pressures were deduced. The experimental data were compared with the COSMO-RS prediction.

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EXPERIMENTAL SECTION Chemicals. Benzene, toluene, thiophene, and [EMIM][BF4] were purchased from the chemical market as summarized in Table 1. Prior to the experiment, the IL was pretreated to remove traces of water and volatile impurities using a vacuum rotary evaporator at 353.2 K for 12 h. The water content was kept at less than 400 ppm after drying. The water content of [EMIM][BF4] was determined by Karl Fischer titration (type KLS701). Deionized water was obtained from a Milli-Q reverse osmosis purification system. Apparatus and Procedure. The experimental vapor− liquid equilibrium (VLE) data of binary systems were measured by vapor pressure method with a modified equilibrium still. The experimental apparatus were shown in Figure 1. The accuracies of experimental apparatus in temperature and pressure were 0.01 K and 0.01 kPa, respectively. Before the experiment, the equilibrium still (U-type tube with a vacuole) was washed thoroughly using deionized water and then dried. All binary mixtures with different compositions were obtained by weighing the solutes and IL with an electronic balance (type FA2104B, Shanghai Precision Scientific Instrument Co., China), with a precision of 0.0001 g. About 40 mL of binary mixture of benzene/toluene/thiophene/water and [EMIM][BF4] was added into the equilibrium still at a given water-bath heating temperature. The volume of vapor headspace is about 25 mL, and thus the mole fraction of the solute in the liquid phase can remain almost constant. The vacuum pump was opened until the liquid boiled in the still. Then the needle valve was slowly opened, making the contents of the U-type tube at the same level height on both sides. If the liquid

Figure 1. Experimental apparatus for the measurement of vapor pressure. 1, intelligent temperature controller; 2, constant temperature bath glass sink; 3, U-type equilibrium still; 4, condenser; 5, thermocouple thermometer; 6, buffer air tank; 7, vent value; 8, needle value; 9, precision digital pressure gauge; 10, pressure buffer tank; 11, vacuum pump.

level did not change within 20 min, it was considered that the system maintained equilibrium. To ensure the reliability of experimental apparatus, the vapor pressure data of water were first measured from 322.2 to 358.4 K. The experimental data were compared with the calculated value using the Antoine equation. B log ps /kPa = A − (1) T /K + C where the Antoine constants (Ai, Bi, and Ci) for the solutes water, toluene, benzene, and thiophene are listed in Table 2.31−33 The comparison of vapor pressures of four solutes between experimental data and calculated values by Antoine equation was made, as shown in Figure 2 where Pexp and Pantoine represent the experimental data and calculated values by the Antoine equation, respectively. It is clear that experimental data and calculated value are in good agreement, with the relative deviation ΔP/P = (Pexp − Pantoine)/Pexp between −1.98% and 1.74%. Moreover, the comparison of VLE experimental data for the binary system of [EMIM][BF4] + benzene between this work and that in ref 34 was also made in this work. As shown in Figure 3, the measured B

DOI: 10.1021/acs.jced.5b00760 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 2. Parameters of the Antoine Equation Used in This Work Antoine parameters compounds water31 toluene32 benzene33 thiophene33

Table 3. Experimental and Predicted Vapor Pressure for the Binary Mixture of Benzene (1) + [EMIM][BF4] (2)a

temperature range

T

Pexp

Ppred

U(T)

U(P)

K

kPa

kPa

γexp 1

γpred 1

K

kPa

U(x)

15.63 24.45 36.60 53.12 74.82

15.90 24.36 36.16 52.19 73.43

1.000 1.000 1.000 1.000 1.000

1.000 1.000 1.000 1.000 1.000

0.01 0.01 0.02 0.01 0.01

0.33 0.51 0.75 1.08 1.52

0.001 0.001 0.001 0.001 0.001

15.87 36.00

16.56 36.53

1.161 1.164

1.212 1.182

0.01 0.01

1.69 2.62

0.002 0.002

15.92 35.50

20.59 43.80

1.391 1.369

1.799 1.689

0.01 0.01

1.69 2.51

0.002 0.002

34.81

51.15

2.407

3.536

0.02

2.58

0.002

28.16 35.10 44.14

48.79 57.24 66.72

2.225 2.301 2.417

3.855 3.752 3.653

0.02 0.01 0.01

2.92 2.61 2.51

0.002 0.002 0.002

25.62 29.89 36.40

45.20 53.02 61.79

2.362 2.286 2.325

4.167 4.055 3.947

0.02 0.01 0.01

2.66 2.09 2.74

0.002 0.002 0.002

19.77 23.33 29.44

40.33 47.32 55.15

2.187 2.141 2.257

4.462 4.343 4.227

0.02 0.01 0.01

2.08 2.43 2.04

0.002 0.002 0.002

16.73 19.32 23.00

34.04 39.95 46.58

2.313 2.216 2.204

4.707 4.583 4.463

0.02 0.01 0.01

1.77 2.03 2.40

0.003 0.003 0.003

13.11 15.00 19.52

26.82 31.51 36.76

2.417 2.294 2.494

4.945 4.819 4.696

0.01 0.02 0.01

1.41 1.60 2.05

0.003 0.003 0.003

10.99 12.90 15.94

18.70 21.98 25.67

3.039 2.960 3.054

5.171 5.044 4.919

0.01 0.01 0.02

1.20 1.39 1.69

0.003 0.003 0.003

Ai

Bi

Ci

K

7.204 7.077 6.224 5.990 6.104

1733.926 1659.793 1432.925 1187.685 1259.054

−39.485 −45.854 −43.929 −55.071 −49.915

304.0−333.0 334.0−363.0 320.3−383.7 287.7−354.1 312.2−392.9

x1 = 1.000 303.15 313.15 323.15 333.15 343.15 x1 = 0.855 303.15 323.15 x1 = 0.717 303.15 323.15 x1 = 0.400 323.15 x1 = 0.350 323.15 328.15 333.15 x1 = 0.300 323.15 328.15 333.15 x1 = 0.250 323.15 328.15 333.15 x1 = 0.200 323.15 328.15 333.15 x1 = 0.150 323.15 328.15 333.15 x1 = 0.100 323.15 328.15 333.15

Figure 2. Relative deviations of vapor pressure ΔP/P = (Pexp − Pantoine)/Pexp between experimental data and calculated values by Antoine equation: ▲, benzene; ●, toluene; ★, thiophene; ■, water; error bars representing the expanded uncertainty (0.95 level of confidence).

a

The expanded uncertainties with 0.95 level of confidence of experimental temperature T, pressure Pexp, and solute mole fraction x1 are U(T), U(P), and U(x), respectively. Ppred is the predicted vapor pred are the experimental and pressure by COSMO-RS. γexp 1 and γ1 predicted activity coefficients by COSMO-RS, respectively.

Figure 3. Comparison of VLE experimental data for the binary system of [EMIM][BF4] + benzene between this work and that in ref 34. Scattered points, (■) this work, (□) ref 34 at 323.2 K; (▲) this work, (△) ref 34 at 333.2 K; solid lines, extrapolated from ref 34 data.

VLE experimental data correspond well with the reference values, with the average relative deviation (ARD) of 8.57% and 7.51% for T = 323.2 and 333.2 K, respectively. Thus, it proves that the experimental apparatus and measurement method presented in this work are reliable.

mixture (solute + IL); P is the system pressure; and Psi is the vapor pressure of pure solute and can be estimated by the Antoine equations as mentioned above. As the IL is nonvolatile, yi can be treated as 1. Since the experiment was carried out at low pressure, the gas phase was assumed as ideal vapor phase (ϕi = 1). Thus, eq 2 can be rewritten as

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COSMO-RS CALCULATION The equilibrium equation for solute i and IL can be expressed as ϕiyP = i

γixiPis

P = γixiPis

(3)

where i stands for the component benzene, toluene, thiophene, or water. The activity coefficient of solute i in [EMIM][BF4] γi can be derived by the COSMOthermX program package, in which an IL molecule may be described as a discrete cation and anion.

(2)

where ϕi is the gas-phase fugacity coefficient of pure solute; yi and xi are the mole fractions of solute i in the gas and liquid phases, respectively; γi is the activity coefficient of solute i in the C

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Table 4. Experimental and Predicted Vapor Pressure for the Binary Mixture of Toluene (1) + [EMIM][BF4] (2)a T K

Pexp

Ppred

kPa

x1 = 1.000 323.16 12.29 325.16 13.37 327.15 14.53 329.15 15.77 332.15 17.80 334.25 19.35 337.65 22.08 339.65 23.83 342.30 26.32 342.45 26.47 x1 = 0.853 323.15 12.13 325.35 13.48 x1 = 0.500 323.15 12.16 x1 = 0.350 323.15 12.36 x1 = 0.300 323.15 10.45 328.15 14.05 333.15 16.18 x1 = 0.200 323.15 9.03 328.15 10.96 333.15 13.17 x1 = 0.150 323.15 8.43 328.15 9.67 333.15 12.26 x1 = 0.100 323.15 6.06 328.15 7.52 333.15 9.22 x1 = 0.050 323.15 5.22 328.15 6.75 333.15 8.57

kPa

γexp 1

γpred 1

U(T)

U(P)

K

kPa

Table 5. Experimental and Predicted Vapor Pressure for the Binary Mixture of Thiophene (1) + [EMIM][BF4] (2)a

U(x)

12.37 13.45 14.61 15.85 17.88 19.42 22.16 23.90 26.39 26.54

1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.01 0.01

0.27 0.29 0.31 0.34 0.38 0.41 0.46 0.50 0.55 0.55

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

12.76 13.95

1.157 1.172

1.218 1.213

0.01 0.01

1.31 1.45

0.002 0.002

24.22

1.980

3.945

0.01

1.32

0.002

26.87

2.876

6.251

0.01

1.34

0.002

26.35 31.28 36.86

2.836 3.093 2.911

7.152 6.886 6.633

0.01 0.02 0.01

1.15 1.51 1.72

0.002 0.002 0.002

22.02 26.13 30.78

3.676 3.619 3.554

8.965 8.629 8.307

0.01 0.02 0.01

1.00 1.20 1.42

0.003 0.003 0.003

18.21 21.62 25.48

4.576 4.258 4.412

9.885 9.519 9.168

0.01 0.02 0.01

0.94 1.07 1.33

0.003 0.003 0.003

13.30 15.80 18.64

4.935 4.967 4.977

10.826 10.437 10.061

0.01 0.01 0.01

0.71 0.85 1.02

0.003 0.003 0.003

7.12 8.47 10.00

8.501 8.917 9.252

11.592 11.190 10.801

0.01 0.01 0.01

0.62 0.78 0.96

0.003 0.003 0.003

The expanded uncertainties with 0.95 level of confidence of experimental temperature T, pressure Pexp, and solute mole fraction x1 are U(T), U(P), and U(x), respectively. Ppred is the predicted vapor pred are the experimental and pressure by COSMO-RS. γexp 1 and γ1 predicted activity coefficients by COSMO-RS, respectively.

Therefore, the mixture of solute and IL will be a hypothetical ternary system consisting of solute, cation, and anion. The activity coefficient of solute i in the binary mixture (solute + IL) in the dilution state is calculated by γi ternxitern xibin

=

γi tern 2 − xibin

Pexp

Ppred

U(T)

U(P)

K

kPa

kPa

γexp 1

γpred 1

K

kPa

U(x)

13.29 16.61 20.59 25.31 30.87 37.40 44.98

13.38 16.75 20.72 25.51 31.06 37.61 45.23

1.000 1.000 1.000 1.000 1.000 1.000 1.000

1.000 1.000 1.000 1.000 1.000 1.000 1.000

0.01 0.01 0.01 0.01 0.02 0.01 0.01

0.29 0.35 0.43 0.53 0.64 0.77 0.92

0.001 0.001 0.001 0.001 0.001 0.001 0.001

30.31

32.08

1.390

1.471

0.01

2.12

0.002

13.10 20.55 29.94

14.96 22.46 32.66

1.632 1.653 1.607

1.863 1.807 1.753

0.01 0.01 0.01

1.41 2.16 1.98

0.002 0.002 0.002

12.34 19.62 29.46 11.12 18.02 27.84 10.14 15.10 25.42 9.16 13.38 23.11

14.80 22.20 32.23 14.44 21.63 31.38 13.84 20.74 30.07 12.94 19.40 28.14

1.677 1.722 1.725 1.662 1.740 1.793 1.684 1.620 1.819 1.712 1.615 1.860

2.012 1.948 1.887 2.159 2.089 2.021 2.299 2.224 2.152 2.417 2.341 2.265

0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01

1.33 2.06 2.09 1.21 1.90 2.88 1.11 1.61 2.64 1.02 1.44 2.41

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002

8.23 12.01 20.76

11.80 17.72 25.73

1.757 1.656 1.910

2.520 2.444 2.367

0.01 0.01 0.01

0.92 1.30 2.18

0.002 0.002 0.002

7.39 10.96 17.87

10.45 15.72 22.87

1.841 1.764 1.918

2.603 2.530 2.454

0.01 0.01 0.01

0.84 1.20 1.89

0.002 0.002 0.002

6.53 9.88 14.38

8.92 13.45 19.60

1.952 1.908 1.852

2.665 2.597 2.525

0.01 0.02 0.01

0.75 1.09 1.54

0.002 0.002 0.002

5.73 8.83 12.02

7.22 10.92 15.96

2.141 2.131 1.935

2.698 2.636 2.569

0.01 0.01 0.01

0.67 0.98 1.30

0.003 0.003 0.003

5.01 7.91 11.45

5.45 8.28 12.13

2.496 2.546 2.458

2.717 2.663 2.603

0.02 0.01 0.01

0.60 0.89 1.25

0.003 0.003 0.003

4.23 7.05 10.09

3.64 5.54 8.15

3.161 3.403 3.249

2.720 2.676 2.625

0.01 0.01 0.01

0.52 0.81 1.11

0.003 0.003 0.003

x1 = 1.000 303.15 308.15 313.15 318.15 323.15 328.15 333.15 x1 = 0.702 323.15 x1 = 0.600 303.15 313.15 323.15 x1 = 0.550 303.15 313.15 323.15 303.15 313.15 323.15 303.15 313.15 323.15 303.15 313.15 323.15 x1 = 0.350 303.15 313.15 323.15 x1 = 0.300 303.15 313.15 323.15 x1 = 0.250 303.15 313.15 323.15 x1 = 0.200 303.15 313.15 323.15 x1 = 0.150 303.15 313.15 323.15 x1 = 0.100 303.15 313.15 323.15

a

γi bin =

T

(4)

bin where xbin i and γi are the mole fraction and activity coefficient of solute i in the binary system of solute + [EMIM][BF4], respectively; xtern and γtern are the mole fraction and activity i i coefficient of solute i in the hypothetical ternary system of solute + [EMIM]+ + [BF4]−, respectively. Therefore, it is necessary to convert the molar fractions xbin in the binary i

a

The expanded uncertainties with 0.95 level of confidence of experimental temperature T, pressure Pexp, and solute mole fraction x1 are U(T), U(P), and U(x), respectively. Ppred is the predicted vapor pred are the experimental and pressure by COSMO-RS. γexp 1 and γ1 predicted activity coefficients by COSMO-RS, respectively. D

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Table 6. Experimental and Predicted Vapor Pressure for the Binary Mixture of Water (1) + [EMIM][BF4] (2)a T K x1 = 1.000 322.13 329.07 337.68 341.81 347.45 350.90 355.11 358.38 x1 = 0.990 328.15 333.61 339.20 344.61 348.70 351.81 355.18 363.78 x1 = 0.978 325.19 329.76 333.35 338.64 343.09 345.79 350.72 354.88 x1 = 0.963 323.14 328.70 334.47 337.99 342.87 345.64 347.94

Pexp kPa

Ppred kPa

γexp 1

γpred 1

U(T)

U(P)

K

kPa

U(x)

11.12 16.75 24.18 29.68 37.91 43.56 51.68 58.49

11.91 16.68 24.76 29.66 37.63 43.33 51.24 58.17

1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01

0.24 0.36 0.50 0.61 0.78 0.89 1.05 1.19

0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

15.55 20.31 26.21 33.15 39.25 43.88 50.11 70.12

15.62 20.17 25.91 32.72 38.79 44.01 50.31 69.82

0.984 0.996 1.000 1.002 1.001 0.986 0.985 0.994

0.988 0.989 0.989 0.989 0.989 0.989 0.989 0.989

0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01

1.66 2.13 2.72 2.85 2.49 2.71 2.82 2.96

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002

12.36 15.79 18.80 24.27 28.29 31.96 39.36 46.81

13.20 16.44 19.44 24.67 29.94 33.56 41.13 48.56

0.913 0.937 0.944 0.961 0.923 0.931 0.936 0.943

0.975 0.976 0.976 0.977 0.977 0.977 0.978 0.978

0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.01

1.34 1.68 1.98 2.53 2.93 2.30 2.81 2.78

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002

11.62 15.28 19.34 23.38 28.64 32.58 35.77

11.64 15.25 19.96 23.39 28.94 32.55 35.82

0.964 0.968 0.937 0.967 0.958 0.969 0.967

0.965 0.966 0.967 0.968 0.968 0.969 0.969

0.01 0.01 0.01 0.01 0.02 0.01 0.01

1.26 1.63 2.03 2.44 2.96 2.36 2.68

0.002 0.002 0.002 0.002 0.002 0.002 0.002

T

Pexp

Ppred

K

kPa

kPa

γexp 1

γpred 1

K

kPa

U(x)

39.36 43.23 47.37

38.92 42.32 47.27

0.980 0.990 0.971

0.969 0.969 0.969

0.01 0.01 0.01

2.04 2.42 2.84

0.002 0.002 0.002

12.70 14.92 17.97 20.54 23.72 26.62 33.04 37.87 42.17

12.98 14.96 17.92 20.56 23.49 26.86 32.38 36.59 41.31

0.955 0.973 0.978 0.974 0.985 0.966 0.994 1.008 0.994

0.976 0.976 0.976 0.976 0.975 0.975 0.974 0.974 0.973

0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.01

1.37 1.59 1.90 2.15 2.47 2.76 2.40 2.89 2.32

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002

9.66 12.40 15.91

10.87 13.76 17.27

0.964 0.971 0.986

1.085 1.077 1.070

0.01 0.01 0.01

1.07 1.34 1.69

0.002 0.002 0.002

8.39 10.97 13.74

10.37 13.04 16.26

0.957 0.981 0.973

1.183 1.167 1.151

0.01 0.02 0.01

0.94 1.20 1.47

0.002 0.002 0.002

7.58 9.81 11.72

9.57 11.96 14.80

1.009 1.024 0.968

1.274 1.248 1.222

0.01 0.01 0.01

0.86 1.08 1.27

0.002 0.002 0.002

7.28 9.27 11.57 14.22 17.75

8.49 10.53 12.95 15.79 19.10

1.163 1.161 1.147 1.125 1.130

1.355 1.319 1.283 1.249 1.216

0.01 0.01 0.01 0.02 0.01

0.83 1.03 1.26 1.52 1.88

0.002 0.002 0.002 0.002 0.002

5.66 7.30 9.37

7.20 8.87 10.83

1.130 1.143 1.161

1.437 1.388 1.342

0.01 0.01 0.01

0.67 0.83 1.04

0.002 0.002 0.002

349.96 352.03 354.81 x1 = 0.917 326.15 329.11 332.99 336.02 339.02 342.11 346.54 349.52 352.54 x1 = 0.800 323.15 328.15 333.15 x1 = 0.700 323.15 328.15 333.15 x1 = 0.600 323.15 328.15 333.15 x1 = 0.500 323.15 328.15 333.15 338.15 343.15 x1 = 0.400 323.15 328.15 333.15

U(T)

U(P)

a The expanded uncertainties with 0.95 level of confidence of experimental temperature T, pressure Pexp, and solute mole fraction x1 are U(T), U(P), pred are the and U(x), respectively. Ppred and Ppred are the experimental and predicted vapor pressures by COSMO-RS, respectively. γexp 1 and γ1 experimental and predicted activity coefficients by COSMO-RS, respectively.

solute−IL system to the calculated molar fractions xtern in the i hypothetical ternary system according to the following equation (for a 1:1 IL): xibin =

xiterm =

2xitern 1 + xitern

(benzene + [EMIM][BF4], toluene + [EMIM][BF4], thiophene + [EMIM][BF4], and water + [EMIM][BF4]), and the expanded uncertainties with 0.95 level of confidence of experimental temperature, pressure, and solute mole fraction, that is, U(T), U(P) and U(x), are shown in Tables 3−6, along with the experimental and predicted activity coefficients of solutes. As shown in Figure 4, vapor pressure increases with the increase of temperature for the four binary systems investigated in this work. In addition, water and [EMIM][BF4] in the concentration range investigated are miscible. The [EMIM][BF4]− water interactions are stronger than the pure water−water interaction, thus the vapor pressure of binary mixture is lower than that of pure water at the same temperature. Meanwhile, other binary mixtures containing benzene/toluene/thiophene are not completely miscible at middle solute concentration, leading to the vapor pressure also being lower than that of pure solutes at the same temperature. Influence of Solute Concentration on the Vapor Pressure of Binary Systems. The profiles of vapor pressure versus solute concentration are shown in Figure 5. It can be

(5)

xibin 2 − xibin

(6)

More details about how to calculate the activity coefficient of a solute in an IL by the COSMO-RS model can be found at the Web site http://www.scm.com/Doc/Doc2014/GUI/GUI_tutorial/ page195.html, which is provided by our research group.

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RESULTS AND DISCUSSION Influence of Temperature on the Vapor Pressure of Binary Systems. The vapor pressure of the binary mixture at different temperatures was measured. The experimental and predicted data of vapor pressure for the binary mixtures E

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Figure 4. Vapor pressures of binary mixtures versus temperature at the solute molar fraction range of 0.1 to 1: (a) benzene (1) + [EMIM][BF4] (2), (b) toluene (1) + [EMIM][BF4] (2), (c) thiophene (1) + [EMIM][BF4] (2), and (d) water (1) + [EMIM][BF4] (2).

Figure 5. Vapor pressures of binary mixtures versus solute concentration at 323.2 K: (a) benzene + [EMIM][BF4] (2), (b) toluene (1) + [EMIM][BF4] (2), (c) thiophene (1) + [EMIM][BF4] (2), and (d) water (1) + [EMIM][BF4] (2). Solid lines, predicted results by the COSMO-RS model; dashed lines, vapor pressures of pure solutes; scattered points, experimental data. F

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seen that with the increase of the mole fraction of solutes, the vapor pressures for the binary mixtures containing benzene/ toluene/thiophene first increase, and then almost remain at certain values. It seems that a vapor pressure peak arises at the middle solute concentration due to phase split. In this case, the COSMO-RS model only gives a good prediction of vapor pressure at low and high concentration ranges. But it is interesting to note that the COSMO-RS model can predict the vapor pressure peak which is, however, qualitatively consistent with the experimental data. On the other hand, for the binary mixture of water + [EMIM][BF4], the vapor pressure increases linearly with the increase of the mole fraction of water (see Figure 5d). It can be seen that the predicted values by the COSMO-RS model are quantitatively consistent with the experimental data in the total concentration range, indicating that the COSMO-RS model is a useful predictive model in this case. Moreover, the vapor pressure data of the water−[EMIM][BF4] system were also correlated by the NRTL model. The average relative deviation (ARD) is defined as ARD =

1 N

N

∑ 1

(7)

where Pexp is the experimental vapor pressure of the water− [EMIM][BF4] system, Pcal is the calculated vapor pressure data by the COSMO-RS (or NRTL) model, and N is the number of data points. The ARDs for the COSMO-RS and NRTL models are 6.11% and 3.89%, respectively. But it should be noted that the NRTL model is a correlative model, the model parameters of which depend on the experimental data, whereas the COSMO-RS model is a prior predictive model.

■

CONCLUSIONS The experimental vapor pressure data for binary systems consisting of [EMIM][BF4] with the solutes benzene, toluene, thiophene, and water were measured to investigate the effect of IL on solutes. These data are important but are lacking in the corresponding extraction or extractive distillation process design. We compared our experimental data with those predicted by the COSMO-RS model. It was found that for the binary systems of [EMIM][BF4] with benzene, toluene, and thiophene, the COSMO-RS model can only provide a correct qualitative trend of vapor pressure. This may be attributed to the phase split at the middle solute concentration, showing the complicated nonideal phase behavior. On the contrary, for the binary systems of [EMIM][BF4] with water, a good quantitative agreement was obtained. Therefore, care should be taken to use the COSMO-RS model, although it can be used as an a priori tool for the prediction of phase equilibrium behavior of the systems containing ILs.

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Pcal − Pexp Pexp

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-1064433695. E-mail: [email protected]. Funding

This work was financially supported by the National Natural Science Foundation of China under Grant Nos. 21476009, 21406007, and U1462104. Notes

The authors declare no competing financial interest. G

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H

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