Thermodynamic properties of mixtures containing ionic liquids

Thermodynamic properties of mixtures containing ionic liquids

Fluid Phase Equilibria 236 (2005) 222–228 Thermodynamic properties of mixtures containing ionic liquids Vapor pressures and activity coefficients of ...

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Fluid Phase Equilibria 236 (2005) 222–228

Thermodynamic properties of mixtures containing ionic liquids Vapor pressures and activity coefficients of n-alcohols and benzene in binary mixtures with 1-methyl-3-butyl-imidazolium bis(trifluoromethyl-sulfonyl) imide Sergey P. Verevkin a , Javid Safarov b , Eckard Bich a , Egon Hassel c , Andreas Heintz a,∗ b

a Abteilung Physikalische Chemie, Institut f¨ ur Chemie, Universit¨at Rostock, Hermannstraße, 14, D-18055 Rostock, Germany Department of Heat and Refrigeration Techniques, Azerbaijan Technical University, H. Javid Avn. 25, AZ1073 Baku, Azerbaijan c Lehrstuhl f¨ ur Technische Thermodynamik, Fakult¨at Maschinenbau und Schiffstechnik, Universit¨at Rostock, Albert-Einstein-Str. 2, D-18059 Rostock, Germany

Received 4 July 2005; accepted 5 July 2005 Available online 26 August 2005

Abstract Vapor–liquid equilibria (VLE) of binary mixtures containing methanol, ethanol, propanol-1 and benzene in the ionic liquid [BMIM] [NTf2 ] were studied by using a static method. VLE measurements were carried out over the whole concentration range at four different temperatures in the range from 298.15 to 313.15 K. Activity coefficients γ i of these solvents in the ionic liquid have been determined from the VLE data and are described formally by using the NRTL equation. © 2005 Elsevier B.V. All rights reserved. Keywords: Ionic liquids; Non-aqueous mixtures; Activity coefficients; Vapor pressure

1. Introduction Ionic liquids (IL) have gained large interest during the last years. They have no detectable vapor pressure, most of them are thermally stable and therefore exhibit ideal systems which can be used for different promising purposes in chemical catalysis, separation processes and electrochemistry. More recently the application of ILs as heat transfer media is discussed using pure ILs and suitable mixtures of ILs with other solvents instead of traditional water soluble salts [1–5]. The large number of different combinations of anions and cations offers the possibility to prepare ILs which can be tailored to the special purpose of their application. Many of them are commercially available (e.g. Solvent Innovation, Aldrich, or Merck). Recently ∗

Corresponding author. Tel.: +49 381 498 6500; fax: +49 381 498 6502. E-mail address: [email protected] (A. Heintz).

0378-3812/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2005.07.008

published review articles [6–11] reflect the impressive diversity of possible applications in different fields of chemical technology. As a consequence the rate of published research papers has increased remarkably during the last years. It is obvious that a systematic knowledge of thermophysical and thermodynamic properties becomes necessary since the optimized design of chemical and separation processes as well as application as heat transfer media requires data of thermodynamic properties in mixtures containing ionic liquids such as activity coefficients, liquid–liquid phase equilibria, gas solubility, surface and interphase tensions as well as transport properties like viscosity, diffusion coefficients, thermal and electrical conductivities. We have contributed to this series of required thermophysical data in the past by determining infinite dilution activity coefficients [12–15], LLE of binary systems [16], viscosity and densities of binary systems [17]. In continuation of this work we present new data of activity coefficients covering the whole

S.P. Verevkin et al. / Fluid Phase Equilibria 236 (2005) 222–228

range of concentration in binary mixtures containing ionic liquids. In this work we have performed vapor pressure measurements of four solutes in the ionic liquid 1-methyl3-butyl-imidazolium bis(trifluoromethyl-sulfonyl) imide [BMIM][NTf2 ]. In contrast to previous work where VLE data [18] of low-volatile solutes in ILs were measured using the transpiration method, in the present work a series of binary mixtures of alcohols or benzene with the ionic liquid [BMIM] [NTf2 ] has been studied using a static vapor pressure method. From the pressure data activity coefficients γ i have been evaluated. Vapor pressures of pure methanol and of the mixture C6 H6 + n-C14 H30 have been measured to check the experimental procedure by comparing the results with reliable data available from the literature [18,19].

2. Experimental 2.1. Materials The alcohols (from Aldrich) and benzene (from Acros), were of commercial origins. The purities of these substances were 99.9% according to specification. The water content of the purchased solutes was less then 0.01% also according specification. All chemicals were used without further purification, but were carefully degassed. The [BMIM][NTf2 ] was supplied by the research group of Prof. Wasserscheid in Erlangen. Before using, the IL was subjected to vacuum evaporation at 333 K over 24 h to remove possible traces of

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solvents and moisture. The water concentration (<100 ppm) was checked by Karl Fischer titration. The VLE measurements of binary solutions of [BMIM][NTf2 ] with (CH3 OH, or C2 H5 OH, or C3 H7 OH, or C6 H6 ) have been performed in a glass cell by using a static method. The apparatus is shown schematically in Fig. 1. The experimental set up consisted of a bolted-top cell with an internal volume of 95.64 cm3 surrounded by a water bath (1) which was kept at constant temperature (±0.02 K) using a thermostat (6). The measuring cell is equipped with an injection port containing a septum which is mounted on the inlet of the one way metal valve (18). Its outlet is connected directly to the measuring cell. The outlet of the valve (18) is connected to the vacuum pump (17) through the valves (14). This construction allows evacuation of the injection port before and after injecting a liquid sample and prevents pressure fluctuation in the measuring cell during the injection procedure. The temperature inside the cell was measured by a platinum resistance thermometer (9) PT-100 (Type 42441-V100), connected to the signal conditioner Kelvimat Type 4303 (10), with an accuracy of ±0.01 K. The pressure was measured using a calibrated high accuracy sensor head (2) (Type 615A, MKS Baratron) connected (7) to the signal conditioner (8) (Type 670A, MKS Baratron) attached to the top of the cell. The sensor head and the connecting line (5) from the cell to the sensor were thermostated (3) at 333.15 ± 0.01 K. This temperature is always kept above the temperatures of the measuring cell in order to avoid any condensation in the pressure head. The experimental uncertainties were ±0.01 K for the temperature and ±10 Pa for the pressure.

Fig. 1. VLE apparatus: (1) bolted-top cell; (2) pressure sensor head; (3) pressure sensor thermostat; (4) magnetic coupling; (5) connecting line; (6) thermostat; (7) pressure sensor connection; (8) pressure signal conditioner; (9) platinum resistance thermometer; (10) temperature signal conditioner; (11) injection cell (details see Fig. 2); (12) magnetic stirrer; (13) volume buffers; (14) valves; (15) cooling trap; (16) liquid nitrogen supply; (17) vacuum pump; (18) metal valve.

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A starting amount of [BMIM][NTf2 ] was placed into the cell using a weighted syringe and was additionally degassed using a rotary vacuum pump (17). The cell with the loaded IL is kept at room temperature under vacuum for ca. 12 h (until the pressure sensor indicate zero point). Exactly known amounts of the degassed solvent were injected stepwise into the thermostated equilibrium cell with the help of special glass injectors. Phase equilibrium was reached in each step by using a magnetic stirrer (4) with a Teflon-coated magnet inside the cell. Equilibration in the cell is a rapid process and a constant pressure was reached within 15 min. Equilibrium pressure readings were registered in 10-min intervals. Prior to the filling and the injection process, the pure solvent was degassed in a specially designed unit (11) which is equipped with two Teflon valves on the bottom and top of the init. The valve on the top is connected to the vacuum line (not shown in Fig. 1) and the valve on the bottom of unit (11) contains a long stainless steel needle. After the portion of solvent was filled into the measuring cell (1) by condensation, the unit (11) was removed from the injection port (18) and was weighted. The length of the needle is determined by the distance between the septum in the injection port and the inlet orifice in the wall of the measuring cell, so that an injected sample is introduced directly into the cell (1). The injection cell was weighted before and after injection using a BP 221 S analytical balance (Sartorius AG) with resolution of ±0.0001 g. Details of the injection cell and injection port construction are shown in Fig. 2. After each solvent injection the mole fraction in the liquid phase was calculated taking in account the amount of solvent being in the vapor phase which could be calculated from the vapor pressure and the knowledge of the vapor volume of the cell. This amount changes with the temperature due to increasing vapor pressure with temperature. However, the corrections turned out to be 0.0003 for the mole fraction in the worst case, this is inside the experimental error of the mole fraction which is estimated to be ±0.0005. Therefore a single value of the mole fraction of the solvent was given in Table 3 for each temperature. The method was checked by measuring the vapor pressure of pure methanol [19] as well as the vapor–liquid equilibrium of the binary mixture (benzene + tetradecane), where reliable VLE data exist in the literature [20]. The experiments were

Fig. 2. The measuring cell with the injection port and injection unit (numbers are the same as in Fig. 1).

Table 1 Experimental and literature values [19] of vapor pressure p of CH3 OH T (K)

pexp (Pa)

plit (Pa)

(pexp − plit ) (Pa)

298.15 303.15 308.15 313.15 318.15 323.15

16971 21877 27958 35441 44537 55576

16958 21880 27960 35450 44550 55597

13 −3 −2 −9 −13 −21

carried out in the temperature range T = 298.15 to 313.15 K. The experimental vapor pressure is assessed to be reliable to within ±1% according to the test measurements. Comparison of our experimental results with those from the literature [19,20] is shown in Tables 1 and 2.

Table 2 Experimental and literature [20] values of vapor pressure of solution [xC6 H6 + (1 − x)C14 H30 ] x

T = 298.15 K

0.23627 0.34585 0.58016 0.66452 0.75466 0.8444 0.87346 a

T = 303.15 K

pexp (Pa)

plit (Pa)



3432 5028 8083 9137 10113 11034 11328

3442 5014 8102 9101 10108 11061 11362

−0.29 0.28 −0.23 0.39 0.05 −0.25 −0.30

∆% = 100(pexp − plit )/pexp .

(%)a

T = 313.15 K

pexp (Pa)

plit (Pa)



4253 6206 10027 11342 12625 13782 14194

4241 6192 10058 11321 12601 13821 14209

0.28 0.23 −0.30 0.19 0.19 −0.28 −0.11

(%)a

pexp (Pa)

plit (Pa)

∆ (%)a

6298 9179 15112 17033 19012 21071 21603

6277 9204 15109 17077 19094 21035 21655

0.33 −0.28 0.02 −0.26 −0.43 0.17 −0.24

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Table 3 Experimental vapor pressure values of xCH3 OH + (1 − x)[BMIM][NTf2 ] (γ = γ Methanol ) x

0.0000 0.0454 0.0816 0.0992 0.1400 0.2537 0.3349 0.4658 0.5852 0.6878 0.7840 0.8526 0.9007 0.9321 0.9786 0.9900 1.0000

T = 298.15 K

T = 303.15 K

T = 308.15 K

T = 313.15 K

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

0 748 1356 1588 2332 4287 6126 8826 10875 12730 14086 15001 15609 16001 16597 16772 16958

0.945 0.964 0.980 0.987 1.004 1.047 1.072 1.101 1.109 1.098 1.071 1.045 1.026 1.014 1.002 1.000 1.000

0 928 1670 1972 2899 5303 7574 11085 13765 16060 17878 19045 19953 20491 21342 21607 21880

0.911 0.931 0.946 0.954 0.970 1.014 1.041 1.073 1.085 1.079 1.059 1.038 1.021 1.012 1.001 1.000 1.000

0 1113 1989 2404 3482 6421 9382 13737 17180 20065 22370 24104 25260 26003 27198 27540 27960

0.850 0.873 0.891 0.899 0.919 0.970 1.002 1.042 1.061 1.060 1.045 1.029 1.016 1.009 1.001 1.000 1.000

0 1299 2399 2844 4174 7780 11439 17045 21347 24766 27947 30239 31701 32702 34391 34850 35450

0.806 0.830 0.849 0.859 0.880 0.937 0.973 1.022 1.049 1.054 1.043 1.029 1.016 1.009 1.001 1.000 1.000

3. Results and discussion

of the activity coefficients. Eq. (1) has been used to determine activity coefficients γ 1 using the NRTL equation from experimental data of partial pressures p1 including the vapor pressure of the pure solutes p10 :

Experimental vapor pressures p of binary mixtures of [BMIM][NTf2 ] with (CH3 OH, or C2 H5 OH, or C3 H7 OH, or C6 H6 ) at T = 298.15 to 313.15 K are listed in Tables 3–6. Binary mixtures of ILs with non-electrolyte components belong to the class of electrolyte solutions covering the whole range of composition including the pure liquid electrolyte. Since there exist no reliable theoretical models for the Gibbs energy of mixing of this kind of mixtures we have tried to describe the results of activity coefficients using purely empirical expressions well known in thermodynamics of nonelectrolyte mixtures. After several attempts with the simple Margules equation, the Wilson equation and the UNIQUAC equation it turned out that the NRTL equation gives the best empirical description

p1

ϕ1 = p10 x1 γ1NTRL ϕ10

(1)

Corrections due to fugacity coefficients ϕ1 and ϕ10 have been accounted for by:   −(V 1 − B11 )(p1 − p10 ) ϕ1 = exp (2) ϕ10 RT The second virial coefficients B11 of the alcohols and benzene have been taken from [21], the molar liquid volume V1 from [22]. It turned out that Eq. (2) is only a small correction for the values of γ 1 which is within ±1%. The expression for

Table 4 Experimental vapor pressure values of xC2 H5 OH + (1 − x)[BMIM][NTf2 ] (γ = γ Ethanol ) x

0.0000 0.0631 0.0824 0.1995 0.2690 0.3622 0.5088 0.5584 0.6777 0.7116 0.8008 0.8768 0.9365 1.0000

T = 298.15 K

T = 303.15 K

T = 308.15 K

T = 313.15 K

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

0 852 1136 2626 3392 4294 5621 6059 6979 7154 7546 7733 7785 7876

1.819 1.756 1.738 1.638 1.584 1.517 1.418 1.384 1.298 1.272 1.193 1.115 1.048 1.000

0 1006 1325 3223 4247 5471 7242 7789 8937 9169 9742 10108 10262 10468

1.556 1.541 1.536 1.501 1.476 1.438 1.363 1.332 1.248 1.222 1.145 1.077 1.028 1.000

0 1141 1552 3993 5202 6797 9196 9952 11431 11773 12527 13051 13337 13768

1.359 1.374 1.378 1.390 1.389 1.375 1.322 1.296 1.216 1.190 1.117 1.057 1.019 1.000

0 1334 1827 4850 6342 8331 11603 12510 14336 14767 15875 16679 17193 17928

1.209 1.244 1.254 1.301 1.317 1.322 1.288 1.265 1.191 1.166 1.099 1.046 1.014 1.000

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Table 5 Experimental vapor pressure values of xC3 H7 OH + (1−x)[BMIM][NTf2 ] (γ = γ Propanol ) x

T = 298.15 K

0.0000 0.1155 0.2119 0.3339 0.4673 0.5733 0.6716 0.7370 0.8178 0.8675 0.8910 0.9358 1.0000

T = 303.15 K

T = 308.15 K

T = 313.15 K

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

0 789 1309 1849 2356 2582 2703 2746 2772 2779 2782 2784 2786

2.707 2.420 2.215 1.986 1.761 1.592 1.438 1.335 1.209 1.134 1.101 1.043 1.000

0 976 1633 2373 3001 3361 3589 3696 3772 3803 3812 3820 3826

2.437 2.192 2.021 1.833 1.652 1.517 1.392 1.306 1.198 1.130 1.099 1.044 1.000

0 1241 1994 2912 3790 4331 4702 4879 5031 5102 5132 5168 5199

2.314 2.042 1.868 1.693 1.540 1.435 1.340 1.276 1.190 1.132 1.104 1.050 1.000

0 1594 2543 3714 4875 5560 6066 6332 6584 6714 6771 6866 6986

2.241 1.960 1.785 1.615 1.471 1.374 1.290 1.234 1.159 1.110 1.086 1.041 1.000

Table 6 Experimental vapor pressure values of xC6 H6 + (1 − x)[BMIM][NTf2 ] (γ = γ Benzene ) x

T = 298.15 K

0.0000 0.2189 0.2351 0.3353 0.4401 0.5478 0.5970 0.6761 0.7116 0.7478 1.0000

T = 303.15 K

T = 308.15 K

T = 313.15 K

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

p (Pa)

γ

0 2348 2571 4218 6024 7935 8794 10088 10693 11288 12704

0.635 0.853 0.870 0.974 1.074 1.151 1.173 1.185 1.180 1.168 1.000

0 2924 3187 5208 7462 9843 10901 12514 13219 14000 15919

0.634 0.848 0.865 0.967 1.064 1.139 1.160 1.173 1.168 1.157 1.000

0 3576 3890 6320 9087 12014 13264 15275 16113 17061 19659

0.633 0.842 0.858 0.957 1.052 1.125 1.146 1.158 1.155 1.145 1.000

0 4394 4761 7681 11133 14741 16221 18669 19693 20874 24331

0.630 0.836 0.853 0.950 1.043 1.113 1.134 1.146 1.143 1.134 1.000

ln γ 1 is:  ln

γ1NTRL

=

x22

 τ21

G21 x1 + x2 G21

2

 τ12 G12 , + (x2 + x1 G12 )2 (3)

with Gij = exp(−αij τ ij ), τ ij = (gij − gjj )/RT and αij = αji = α (i, j = 1, 2; i = j). Table 7 contains the parameters αij and (gij − gjj ) obtained by fitting γ1NRTL to the experimental VLE data. The plots of Eq. (3) of [BMIM][NTf2 ] with alcohols and benzene versus the mole fractions x1 of solvent are shown in Figs. 3–6. Values of ln γ 1 decrease with temperature for all four mixtures. In case of methanol ln γ 1 passes a maximum in the positive region starting with negative values in diluted solutions of methanol in [BMIM][NTf2 ]. In the ethanol mixture only positive values of ln γ 1 are observed showing maximal values at 308.15 and 313.15 K, while at 303.15 and 298.15 K the slope of ln γ 1 with the mole fraction of ethanol is continuously negative. This is also the case for all temperatures of the propanol mixtures which exhibit the highest values of ln γ 1 of all alcohols studied. A special case is the mixture benzene + [BMIM][NTf2 ] where a mixing gap is observed at mole fractions of ben-

Table 7 Parameters of the NRTL equation g21 –g11 (kJ mol−1 )

α

[BMIM][NTf2 ] + CH3 OH 298.15 4.30 303.15 4.22 308.15 5.02 313.15 5.43

−1.47 −1.63 −2.45 −2.72

0.6778 0.6598 0.4620 0.4426

[BMIM][NTf2 ] + C2 H5 OH 298.15 7.54 303.15 5.58 308.15 5.64 313.15 8.24

0.507 −0.074 −1.07 −3.40

0.6719 0.6986 0.5035 0.2368

[BMIM][NTf2 ] + C3 H7 OH 298.15 8.10 303.15 7.88 308.15 8.06 313.15 6.83

0.764 0.798 1.05 1.12

0.4770 0.5421 0.6339 0.7400

T (K)

g12 –g22 (kJ mol−1 )

[BMIM][NTf2 ] + C6 H6 298.15 11.34 303.15 11.33 308.15 11.25 313.15 11.38

−4.57 −4.70 −4.82 −5.09

0.2604 0.2578 0.2562 0.2458

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Fig. 3. Plot of activity coefficient γ of methanol in CH3 OH + [BMIM][NTf2 ] mixture vs. mole fraction x of CH3 OH: () 298.15 K; () 303.15 K; () 308.15 K; (䊉) 313.15 K, points and solid lines: Eq. (3).

zene above x = 0.75. This mixing gap is extended to almost pure benzene. A possible small solubility of [BMIM][NTf2 ] in benzene could not be determined in the frame of our experimental technique. It is interesting that the negative values at lower values of x indicate an increasingly better solubility of benzene in [BMIM][NTf2 ] with decreasing mole fraction of benzene. Similar results also obtained from VLE data of saturated hydrocarbons and aromatics mixed with imidazolium based ILs have been reported recently by other authors [23]. Broad mixing gaps in the liquid phase have been observed extending in all cases practically to the pure liquid solvent (hexane, cyclohexane, cyclohexene, benzene) on the solvent rich side. This agrees with our results for [BMIM][NTf2 ] + benzene obtained between 298.15 and 313.15 K (see Table 6) which indicate that the width of the mixing gap remains almost unchanged (xbenzene ≈ 0.75).

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Fig. 5. Plot of activity coefficient γ of propanol in C3 H7 OH + [BMIM][NTf2 ] mixture vs. mole fraction x of C3 H7 OH: () 298.15 K; () 303.15 K; () 308.15 K; (䊉) 313.15 K, points and solid lines: Eq. (3).

Fig. 6. Plot of activity coefficient γ of benzene in C6 H6 + [BMIM][NTf2 ] mixture vs. mole fraction x of C6 H6 : () 298.15 K; () 303.15 K; () 308.15 K; (䊉) 313.15 K, points and solid lines: Eq. (3).

The results of Kato et al. [23] for the same system show that the mixing gap becomes smaller at higher temperature (353 K) with xbenzene ≈ 0.84. Values of γi∞ extrapolated from our VLE results agree well with our previous results obtained by GC-techniques [24] for ethanol and propanol (see Table 8). The deviation in case of benzene arises from an insufficient extrapolation based on VLE data with mole fractions not small enough to justify an extrapolation within the uncertainty of experimental error of the GC method [24] Table 8 Comparison of values of γi∞ at 298 K derived in this work with those measured by the GC method [24] and by the dilutor technique [25] Solute i

Fig. 4. Plot of activity coefficient γ of ethanol in C2 H5 OH + [BMIM][NTf2 ] mixture vs. mole fraction x of C2 H5 OH: () 298.15 K; () 303.15 K; () 308.15 K; (䊉) 313.15 K, points and solid lines: Eq. (3).

Benzene Ethanol Propanol

γi∞ at 298 K This work

Ref. [24]

Ref. [25]

0.64 1.8 2.7

0.88 1.9 2.7

0.86

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or the dilution method [25]. This is also the reason why we are not able to confirm the increase of γi∞ for benzene in [BMIM][NTf2 ] with temperature found by Krummen et al. [25]. Our VLE data in the range of finite concentrations indicate a decrease of ln γ i with temperature. This question will soon be answered by direct calorimetric measurements of the heat of dilution of methanol, ethanol, propanol, and benzene mixed with [BMIM][NTf2 ]. Work is being in progress. Finally, we would like to point out that the change of sign in ln γ i as observed for [BMIM][NTf2 ] + benzene and [BMIM][NTf2 ] + CH3 OH is not often encountered in liquid mixtures. However, ln γi∞ of benzene is definitely negative as confirmed by different authors [24,25]. On the other side (benzene + [BMIM][NTf2 ]) has a mixing gap at mole fraction x > 0.75 of benzene. Since ln γ i of benzene in the mixture being in liquid–liquid equilibrium with practically pure benzene must be positive, as a consequence ln γ i has to change its sign within the concentration range 0 < x < 0.75. The same trend is observed with methanol although there exist no two phase region in the liquid state at temperatures above 298 K. Obviously, this asymmetry is broken in case of higher alcohols like ethanol and propanol. List of symbols B11 second virial coefficient of the solvent (m3 mol−1 ) G12 and G12 interaction parameters of the NRTL equation vapor pressure of the pure solvent (Pa) p10 p1 partial pressure of the solvent (Pa) V1 molar volume of the liquid solvent (m3 mol−1 ) Greek letters αij interaction parameters of the NRTL equation (see text) γi activity coefficient of component i γi∞ activity coefficient at infinite dilution of component i ϕ1 and ϕ10 fugacity coefficients of the solvent in the mixture and the pure state, respectively τ ij interaction parameters of the NRTL equation (see text) Acknowledgments We are grateful to Prof. P. Wasserscheid (Technical University of Erlangen, Germany) for supplying the ionic liquid [BMIM][NTf2 ]. One of us (J.S.) acknowledges gratefully a research scholarship from the Alexander von Humboldt Foundation.

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