Determination of Activity Coefficients at Infinite Dilution of Organic

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Determination of Activity Coefficients at Infinite Dilution of Organic Solutes in the Ionic Liquid 1‑(2-Hydroxyethyl)-3-methylimidazolium Nonafluoro-1-butanesulfonate Using Gas−Liquid Chromatography Cheng Zhang* Chemistry Department, Long Island University (Post), 720 Northern Boulevard, Brookville, New York 11548, United States S Supporting Information *

ABSTRACT: Activity coefficients at infinite dilution (γ∞ i ) for a variety of organic solutes (alkanes, cycloalkanes, alkenes, alkyl benzenes, alcohols, 1,4-dioxane, acetone, acetonitrile, tetrahydrofuran, chloroform, and dichloromethane) in the ionic liquid (IL) 1-(2hydroxyethyl)-3-methylimidazolium nonafluoro-1-butanesulfonate ([C2OHmim][C4F9SO3]) have been determined by gas−liquid chromatography using the IL as the stationary phase. The measurements were conducted in the temperature range from 303 to 353 K. The partial E,∞ E,∞ molar excess Gibbs energies (ΔGE,∞ i ), enthalpies (ΔHi ), and entropies (ΔSi ) at infinite dilution of the solutes in the ionic liquid were calculated from the temperature dependence of ∞ ∞ the values of γ∞ i . Values of selectivity sij and capacity kj at T = 323.15 K for the IL [C2OHmim][C4F9SO3] were calculated for both cyclohexane/benzene and benzene/ methanol binary systems. The results obtained from this work were compared with reported literature data for other [C2OHmim] based ILs.

been broadly studied in the literature.1−4,20,22−41 Reviews on the properties and applications of such ionic liquids are available for details.42,43 In general, protic ionic liquids with Brønsted acidity are not preferred as separation media because they may catalyze undesired reactions for molecules of interest to be separated. ILs which have shown great potential for the separation of cyclohexane/benzene by extractive distillation or extraction are exemplified by those in the literature: s∞ ij = 38.9 (at 298.15 K) = 33.2 (at 298.15 K) for [EMIM]for [EMIM][BF4];22 s∞ ij 24 ∞ [SCN];23 s∞ ij = 21.7 (at 298.15 K) for [EMIM][C2H5SO4]; sij 25 ∞ = 19.7 (at 298.15 K) for [BMIM][BF4]; sij = 10.2 (at 298.15 K) for [BMIM][Tf2N];26 s∞ ij = 29.3 (at 298.15 K) for [BMIM][SCN];27 s∞ ij = 7.8 (at 298.15 K) for [HMIM]29 ∞ sij [CF3SO3];28 s∞ ij = 7.4 (at 298.15 K) for [HMIM][NTf2]; 30,31 ∞ = 53.6 (at 323.15 K) for [C2OHmim][BF4]; sij = 15.8 (at 323.15 K) for [C2OHmim][FAP].32 Generally, the selectivity for the separation of aromatic hydrocarbons/aliphatic hydrocarbons decreases with increasing length of the alkyl chain on the imidazolium, or anion of the IL. The best selectivity values for the separation of benzene/methanol (at 323.15 K) were reported for [EMIM][EtO(H)PO2] (sij∞ = 83.4);33 for 30,31 and for [BMIM][DCA] [C2OHmim][BF4] (s∞ ij = 9.9); 34 ∞ (sij = 4.73). These values were all determined from activity coefficients at infinite dilution measurements. Very recently, Domańska et al. reported the synthesis, the thermal properties of 1,3-didecyl-2-methylimidazolium dicya-

1. INTRODUCTION Ionic liquids (ILs) as green solvents have attracted tremendous attention in recent years. Due to their unique properties such as low vapor pressure, high thermal stability, wide range of solubility, and nonflammability, ILs have been found promising as novel solvent systems in the application of organic separation processes.1 The activity coefficient at infinite dilution (γ∞ i ) is an important thermodynamic parameter for the chemical and production process. It reflects the solubility and selectivity of solvent on solute and can be found in a wide variety of applications such as in extractive distillation, liquid−liquid extraction, and absorption in the chemical engineering process.1,2 The activity coefficient at infinite dilution can illustrate the interaction between solute and solvent molecules and is a key parameter to estimate the performance of the solvent separation. Studies on the determination of activity coefficients at infinite dilution by the gas−liquid chromatography (GLC) method for various ILs have been widely reported.1−21 However, no study has been reported for the IL 1-(2-hydroxyethyl)-3-methylimidazolium nonafluoro-1-butanesulfonate [C2OHmim][C4F9SO3]. The selectivity (s∞ ij ) at infinite dilution directly calculated from the γ∞ i values offers an important means to evaluate the performance of ILs as solvents in various separation problems. The larger the selectivity value is, the more effective the separation is for the components in the mixture. The capacity k∞ j indicates the solubility of the solutes in the solvent. The lower the value of the capacity, the smaller the amount of solute that dissolves in the solvent, causing the separation less efficiency. Disubstituted imidazolium based ionic liquids as a class of solvents for extraction and separation processes have © XXXX American Chemical Society

Received: January 24, 2018 Accepted: May 4, 2018

A

DOI: 10.1021/acs.jced.8b00080 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

namide [D2MIM][DCA], and the determination of γ∞ at i infinite dilution.35 At 358.15 K, strong interaction was observed ∞ for methanol (γ∞ i = 0.329) and benzene (γi = 0.734), resulting from the highly polar anion of the IL with two cyano groups and the imidazolium ring in the cation. Less strong interaction was observed for cyclohexane (γ∞ i = 2.10). Apparently, the selectivity for the separation of benzene/methanol and cyclohexane/benzene is low (s∞ ij = 2.23 and 2.86, respectively). Another recent report also showed low selectivity (s∞ ij = 1.18 and 1.97, respectively) at 328.15 K for the separation of benzene/methanol and cyclohexane/benzene in the trihexyltetradecyl-phosphonium tricyanomethanide ionic liquid.44 A deep eutectic solvent (tetramethylammonium chloride + ethylene glycol), which is not an ionic liquid, was reported to have suitable benzene/menthol separation but with very poor 45 cyclohexane/benzene separation capacity (k∞ j = 0.06). In this paper, we report the activity coefficients at infinite dilution (γ∞ i ) for 29 solutes, including alkanes, cycloalkanes, alkenes, alkyl benzenes, alcohols, 1,4-dioxane, acetone, acetonitrile, tetrahydrofuran, chloroform, and dichloromethane in the IL [C2OHmim][C4F9SO3] in the temperature range 303−353 K at 10 K intervals. The partial molar excess Gibbs E,∞ E,∞ energies (ΔGE,∞ i ), enthalpies (ΔHi ), and entropies (ΔSi ) at infinite dilution of the solutes in the ionic liquid were derived from the temperature dependence of the γ∞ i values. Values of ∞ selectivity s∞ ij and capacity kj at T = 323.15 K for the IL [C2OHmim][C4F9SO3] were calculated for both cyclohexane/ benzene and benzene/methanol binary systems. The two sets of mixtures were chosen because they are usually difficult to separate from one another due to their azeotropic behavior. ∞ Values of selectivity s∞ ij and capacity kj calculated from this work were compared with the published literature data for other [C2OHmim] based ILs.

solid substrate for the ionic liquids in the GC column. The sample description for the chemicals was provided in Table S1. The structures of the investigated ILs are presented in Figure 1.

Figure 1. Chemical structure of 1-(2-hydroxyethyl)-3-methylimidazolium nonafluorobutane-1-sulfonate [C2OHmim][C4F9SO3].

2.2. Experimental Procedure. The detailed experimental procedure used in this work can be found in previous publications.2,3 The GC column (stainless steel) with a length of 2 m and an inner diameter of 2 mm was used. Methanol was used as a solvent to coat IL onto the solid support 101 AW (80/100 mesh) by a rotary evaporator to ensure the uniform spreading of the IL onto the surface of the support. The solid support was weighed before and after the coating process. The masses of the stationary phase and of the solid support were weighed with a precision of ±0.0001 g. The solvent column packing varied from 45.0 to 50.0 mass fraction of the ionic liquid, which was large enough to prevent any potential solute residual adsorption on the column packing. The uncertainty in the moles of the IL packed on the solid support is about ±3 × 10−7 mol. Prior to the experiments, the column was conditioned by passing carrier gas at a high flow rate (about 2 cm3 s−1) and at high temperature (about 373 K) over a period of 8 h. The second column was used to check the reproducibility of results at a different column packing level and performed at two different temperatures, 323 and 353 K. Results from these two different columns were reproducible with errors less than 0.5%. Experiments were carried out on a GC-7900 gas chromatograph equipped with a heat-traced on-column injector and a flame ionization detector. The flow rate of the carrier gas was determined using a GL-102B Digital bubble/liquid flow meter with an uncertainty of ±0.1 cm3·min−1, which was positioned at the outlet of the column. The flow rate of the carrier gas was adjusted to obtain an adequate retention time. The outlet pressure Po was kept at atmospheric pressure. Depending on the flow rate of the carrier gas, the pressure drop (Pi − Po) varied between 45 and 180 kPa. The pressure drop was determined by a pressure transducer employed in the GC with an uncertainty of ±0.1 kPa. A membrane manometer with an uncertainty of ±0.2 kPa was used to measure the atmospheric pressure. Solute injection volumes ranged from 0.2 to 0.5 μL were considered to be at infinite dilution on the column. No significant differences in retention times tr were indicated by injecting individual pure components or their mixtures. Experiments were conducted in the temperature range 313− 364 K. At a given temperature, each experiment was repeated at least twice to exam the reproducibility. The differences in the retention times of the two measurements were in general within 0.01−0.03 min. Depending on the individual solute, absolute values of retention times varied between 0.5 and 30 min. At each temperature, values were measured for the dead time tG identical to the retention time of the nonretainable component. During all experiments, the injector and detector temperature were kept at 473 and 523 K, respectively. The temperature of

2. EXPERIMENTAL SECTION 2.1. Materials. The ionic liquid 1-(2-hydroxyethyl)-3methylimidazolium nonafluoro-1-butanesulfonate ([C2OHmim][C4F9SO3]) was synthesized from 1-(2-hydroxyethyl)-3-methylimidazolium chloride ([C2OHmim][Cl]) and potassium nonafluoro-1-butanesulfonate (KC4F9SO3) according to a literature procedure.6 The synthesized ionic liquid 1-(2hydroxyethyl)-3-methylimidazolium nonafluoro-1-butanesulfonate was characterized by NMR spectroscopy. 1H and 13C NMR spectra were recorded on a Bruker Advance III 400 MHz NMR spectrometer with TMS as the internal standard at room temperature. The experimental peak integrals are reported as follows for 1-(2-hydroxyethyl)-3-methylimidazolium nonafluoro-1-butanesulfonate: 1H NMR (400 MHz, d6-DMSO, δ ppm): 3.736−3.772 (t, 2H), 3.872 (s, 3H), 4.212−4.237 (t, 2H), 5.077 (s, 1H), 7.633 (s, 1H), 7.676 (s, 1H), 9.013 (s, 1H); 13 C NMR (100 MHz, d6-DMSO, δ ppm): 35.737, 51.920, 59.628, 122.840, 123.511, 137.093. A silver nitrate test was performed on the synthesized IL and indicated that less than 100 ppm of chloride ion impurity appeared in the synthesized IL. The ionic liquids were purified and dried under a high vacuum at 363 K for 24 h to remove organic solvents and water. The water content was determined by Karl−Fischer titration, and was found to be less than 400 ppm. The purity of the synthesized ionic liquid was >99%. All of the solutes purchased from Sigma-Aldrich were of analytical grade. The solutes were used without further purification. 101 AW (80/100 mesh, the inert and white diatomite) purchased from Shanghai Reagent Co. was used as a B



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Table 1. Critical Constants Zc, Tc, Pc, and Vc and Ionization Energies I of the Solutes Used in the Calculation of the Virial Coefficients solute (i)

Zc

Tc (K)

Pc (bar)

Vc (cm3·mol−1)

I (kJ·mol−1)

pentane hexane heptane octane nonane cyclohexane methyl cyclohexane 1-hexene 1-octene 1-decene 1-pentyne 1-hexyne 1-heptyne 1-octyne benzene toluene ethylbenzene o-xylene m-xylene p-xylene methanol ethanol 1-propanol 1,4-dioxane acetonitrile acetone tetrahydrofuran chloroform dichloromethane

0.27 0.266 0.261 0.256 0.255 0.273 0.27 0.267 0.262 0.253 0.289 0.272 0.273 0.267 0.268 0.264 0.263 0.264 0.259 0.259 0.222 0.241 0.254 0.254 0.184 0.233 0.259 0.293 0.265

469.7 507.5 540.3 568.8 594.6 553.5 572.1 504 566.7 616.6 481.2 516.2 547.2 574.2 562.2 591.8 617.15 630.3 617.1 616.2 512.6 513.92 536.8 587 545.5 508.2 540.15 536.4 510

3370 3025 2740 2490 2290 4080 3480 3210 2660 2223 4170 3620 3210 2880 4895 4110 3609 3732 3541 3511 8084 6140 5169 5210 4830 4701 5190 5472 6080

304 370 432 492 555 308 369 350 464 584 277 332 387 442 259 316 374 370 376 378 118 168 219 238 173 209 224 239 185

998.6 977.4 957.1 947.5 938 951.3 930 910.8 909.9 909.1 974.5 960 960 951.3 892.1 851.1 846.1 826.1 826.1 814.1 1047.1 1010.2 986.3 887.1 1176.6 936.4 908.2 1097.2 1092.1

⎛ n RT ⎞ P*(B − V1*) ln γ13∞ = ln⎜ 3 ⎟ − 1 11 * RT ⎝ VNP1 ⎠

the oven was controlled within ±0.1 K. The GLC apparatus was tested for the system hexane in hexadecane as stationary phase at 298 K, and the results were within 2.0% of the literature values.7 To check the stability of the experimental conditions on the likely elution of the stationary phase by the stream of the carrier gas, the measurements of the retention times were repeated systematically on a daily basis for hexane and benzene. No changes of the retention times were observed during the three months of nonstop operations. The uncertainty of γ∞ i values was obtained from the error propagation law. The following measured parameters were considered in the error calculations with their corresponding standard deviations: the flow rate of the carrier gas, ±0.0017 cm3 s−1; the inlet pressure, ±0.1 kPa; the outlet pressure, ±0.2 kPa; the temperature of the oven, ±0.1 K; the adjusted retention time tr′, ±0.6 s; the mass of the stationary phase, ±0.05%. The mass of the stationary phase is the main source of uncertainty in the calculation of the net retention volume. The assessed uncertainty in determining the net retention volume VN is about ±2%. Considering that thermodynamic parameters are also exposed to errors, the resulting uncertainties in the γ∞ i were estimated to be accurate within ±4%.

+

P0 J2 3(2B12 − V1∞) (1)

RT

where n3 is the number of moles of solvent on the column packing; VN is the standardized retention volume of the solute; P*1 is the saturated vapor pressure of the solute at temperature T; V*1 is the molar volume of the solute; T is the column temperature; P0 is the outlet pressure; V∞ 1 is the partial molar volume of the solute at infinite dilution in the solvent (assumed ≈ V*1 ); B11 is the second virial coefficient of the pure solute; and B12 (where 2 denotes the carrier gas, nitrogen) is the cross second virial coefficient for the solute and the carrier gas. The values of B11 and B12 were computed using the McGlashan and Potter10 equation for alkanes and the Tsonopolous11 equation for the rest of the solvents. By means of the Hudson and McCoubrey combining rules,12,13 critical parameters for mixtures were computed from the critical properties of the pure component. The net retention volume VN was computed with the following usual relationship, as shown in eq 2 VN = (J2 3)−1U (tr − tG)

3. THEORETICAL BASIS 8

In this work, the equation developed by Everett and Cruickshank et al.9 was used to calculate the γ∞ i of solutes in the ionic liquids [C2OHmim][C4F9SO3], as shown in eq 1

Tcol ⎡ Po ⎤ ⎢1 − w ⎥ Tf ⎣ Po ⎦

(2)

where tr is the retention time, tG is the dead time, U is the volumetric flow rate of the measured carrier gas by bubble flow meter at the column outlet, Tf is the flowmeter temperature, C



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Table 2. Experimental Activity Coefficients at Infinite Dilution, γ∞ i , for Various Solutes in the Ionic Liquid [C2OHmim][C4F9SO3] at Different Temperatures of 313−353 K and Pressure P0 = 101.3 kPaa

a

solute (i)

303 K

313 K

323 K

333 K

343 K

353 K


9.38 23.53 53.01 98.96 156.76 24.28 38.08 15.74 53.89 122.10 5.99 10.38 16.74 26.23 4.71 7.40 12.12 10.44 11.64 11.92 1.10 1.56 2.22 1.67 0.99 0.93 1.73 2.27 2.14

6.94 17.67 39.68 75.84 131.62 19.15 29.74 12.41 45.18 106.91 5.07 9.24 15.07 23.76 4.27 6.83 11.01 9.70 10.83 10.96 1.02 1.48 2.07 1.64 0.97 0.91 1.70 2.20 1.99

5.46 13.71 30.88 61.19 104.69 15.20 24.27 9.71 36.62 91.55 4.21 7.92 13.36 21.16 3.85 6.21 10.08 8.90 9.86 9.88 0.93 1.38 1.91 1.61 0.93 0.90 1.64 2.11 1.85

4.34 10.61 23.62 47.31 88.02 11.72 19.02 7.62 29.53 79.61 3.68 6.97 11.93 19.15 3.34 5.44 8.82 7.81 8.71 8.78 0.90 1.31 1.81 1.59 0.90 0.88 1.59 2.01 1.69

3.39 8.00 18.03 37.15 70.96 9.39 15.22 6.07 24.29 67.69 3.11 6.03 10.44 16.88 2.95 4.94 7.93 7.04 7.91 8.00 0.87 1.22 1.66 1.56 0.87 0.85 1.52 1.91 1.54

2.75 6.31 13.93 28.44 54.11 7.54 12.33 4.97 19.93 59.25 2.59 5.18 9.29 14.77 2.65 4.44 7.09 6.52 7.20 7.59 0.82 1.14 1.53 1.53 0.84 0.83 1.48 1.84 1.45

Standard uncertainties u are u(T) = 0.1 K, u(p) = 0.2 kPa, and u(γ∞ i ) = 0.04.

Tcol is the column temperature, Po is the pressure at the column outlet, and Pow is the saturation vapor pressure of water at Tf. The factor J23 appearing in eq 1 and eq 2 amends for the influence of the pressure drop alongside the column given by eq 314 J2 3 =

3 2 (Pi /Po) − 1 2 3 (Pi /Po) − 1

where T is the column temperature. The correlation coefficients R2, the coefficients a and b, and the values of γ∞ i at 298.15 K calculated using eq 4 are given in Table 3. The quality of the linear regression was excellent, as the correlation coefficient R2 lies between 0.990 and 0.999. Activity coefficients at infinite dilution are related with excess E,∞ thermodynamic functions at infinite dilution, ΔHE,∞ i , ΔGi , E,∞ and ΔSi :

(3)

ΔGiE, ∞ = RT ln(γ13∞) = ΔHiE, ∞ − T ΔSiE, ∞

where Po and Pi are the outlet and inlet pressures of the GC column, respectively. The values of the vapor pressure were computed using the Antoine equation, and the constants were obtained from the literature.15 Critical data and ionization energies used to calculate Vc,12 were acquired from the literature.15−17 The critical data needed to compute B11 and B12, and ionization energies used in the computation of Tc,12, are given in Table 1. The values of B11, B12, P1*, and V1* used in the calculation of γ∞ 13 are provided in Table S2.

The above equation can be presented in the following form ln(γ13∞) =

b t

ΔHiE, ∞ ΔSiE, ∞ − RT R

(6)

where R is the gas constant. In combining eq 4 and eq 6, the limiting partial molar excess enthalpy ΔHE,∞ and entropy i ΔSE,∞ at infinite dilution can be obtained from the slope and i the intercept, ΔHE,∞ = bR and ΔSE,∞ = −aR, respectively. i i ΔGE,∞ was therefore calculated according to eq 5 at a reference i E,∞ E,∞ temperature. The calculated ΔHE,∞ for the i , ΔGi , and ΔSi solutes in the IL [C2OHmim][C4F9SO3] at the reference temperature Tref = 323.15 K are presented in Table 3. These thermodynamic functions provide the fundamental information on the extent of interactions between the solute and the IL.35 The smaller values of ΔHE,∞ were observed for 1,4-dioxane, i acetonitrile, acetone, and tetrahydrofuran, which are probably caused by interactions between cations and/or anions of the IL

4. RESULTS AND DISCUSSION Experimental results of γ∞ i for 29 different organic solutes in the ionic liquid [C2OHmim][C4F9SO3] in the temperature range from 303 to 353 K are presented in Table 2. They were approximated by the linear regression ln γi∞ = a +

(5)

(4) D



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Table 3. Coefficients of a and b of eq 4, Correlation Coefficient R2, γ∞ i at 298.15 K Calculated Using eq 4, Values of the Partial E,∞ E,∞ Molar Excess Enthalpies at Infinite Dilution (ΔHE,∞ ), Entropies (T ΔS i ref i ), and Gibbs Energies (ΔGi ) of Organic Solutes in the Ionic Liquid [C2OHmim][C4F9SO3] at a Reference Temperature of Tref = 323.15 K solute (i)

a

b (K)

R2

γ∞ i

ΔHE,∞ (kJ·mol−1) i

ΔGE,∞ (kJ·mol−1) i

TrefΔSE,∞ (kJ·mol−1) i


−6.399 −6.116 −5.401 −4.079 −2.298 −5.085 −4.251 −5.458 −3.092 −0.344 −4.019 −2.549 −1.343 −0.728 −2.567 −1.656 −1.303 −1.098 −1.015 −0.871 −1.919 −1.791 −1.777 −0.085 −1.208 −0.872 −0.607 −0.736 −2.078

2615 2816 2845 2636 2241 2514 2394 2493 2154 1566 1764 1489 1267 1217 1255 1116 1158 1051 1059 1018 618 681 783.1 181 365 245.1 354 476 865

0.999 0.999 0.999 0.997 0.990 0.998 0.998 0.999 0.997 0.997 0.995 0.993 0.994 0.990 0.992 0.991 0.991 0.990 0.990 0.995 0.994 0.991 0.992 0.993 0.993 0.995 0.990 0.992 0.996

10.72 27.91 62.88 117.00 184.63 28.42 43.75 18.24 62.34 135.42 6.67 11.53 18.29 28.61 5.17 8.06 13.21 11.33 12.64 12.72 1.17 1.64 2.34 1.69 1.02 0.95 1.79 2.36 2.28

21.74 23.41 23.65 21.92 18.63 20.90 19.90 20.73 17.91 13.02 14.67 12.38 10.53 10.12 10.43 9.28 9.63 8.74 8.80 8.46 5.14 5.66 6.51 1.50 3.03 2.04 2.94 3.96 7.19

4.55 6.98 9.14 10.96 12.46 7.24 8.48 6.06 9.60 12.10 3.87 5.53 6.93 8.16 3.54 4.83 6.13 5.79 6.08 6.12 −0.02 0.85 1.74 1.28 0.04 −0.12 1.44 2.13 2.04

17.19 16.43 14.51 10.96 6.17 13.66 11.42 14.66 8.31 0.92 10.80 6.85 3.61 1.96 6.90 4.45 3.50 2.95 2.73 2.34 5.16 4.81 4.77 0.23 3.25 2.34 1.63 1.98 5.58

and the polar group in the above-mentioned solutes. The were observed for the linear alkanes, 1higher values of ΔHE,∞ i alkynes, and alkanols, and the values increase with increasing chain length. ΔGE,∞ are positive for nonpolar solutes such as i alkanes, alkenes, alkynes, and aromatic hydrocarbons. The were observed for alcohols, 1,4smaller values of ΔGE,∞ i dioxane, acetonitrile, acetone, tetrahydroform, chloroform, and dichloroform. The negative values were observed for methanol, acetone, and acetonitrile due to the stronger hydrogen bonding are positive formation between the IL and the solutes. TΔSE,∞ i for all investigated solutes, indicating the hydrogen bonds are breaking during the dissolution process. The natural logarithm of the activity coefficients (ln γ∞ i ) in the ionic liquid [C2OHmim][C4F9SO3] as a function of the inverse absolute temperature (1000/T(K−1)) for all investigated solutes is plotted in Figures 2−5. As shown in Figures 2−5, the values of γ∞ i for the series of solutes increase with an increase of chain length. Similar behavior was also observed for other reported methyl imidazolium cation based ionic indicate weaker liquids.33,34,40−45 The higher values of γ∞ i interactions between the solute and the solvent. The values of γ∞ i for both alkenes and cycloalkanes are alike for the same carbon numbers. The cyclic structure of cycloalkanes lowered the value of γ∞ i when compared to the corresponding linear alkane. The values of γ∞ i for alkenes are lower than those for alkanes with the same carbon number, which is caused by the enhanced interactions between the double bond in alkenes and the polar ionic liquid. Alkynes and aromatic hydrocarbons have

Figure 2. Plot of ln(γ∞ i ) for ionic liquid [C2OHmim][C4F9SO3] versus 1/T for the solutes: pentane, hexane, heptane, octane, nonane, cyclohexane, and methyl cyclohexane.

lower values of γ∞ i than those for alkanes, cycloalkanes, and alkenes, indicating stronger interactions between the triple bond in alkynes with the polar ionic liquid and six π-delocalized electrons in aromatics with the polar ionic liquid, respectively. Relatively much smaller values of γ∞ were observed for i alkanols, 1,4-dioxane, acetonitrile, acetone, tetrahydrofuran, chloroform, and dichloromethane, which can be ascribed to the E



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Figure 5. Plot of ln(γ∞ i ) for ionic liquid [C2OHmim][C4F9SO3] versus 1/T for the solutes: methanol, ethanol, 1-propanol, 1,4-dioxane, acetonitrile, acetone, tetrahydrofuran, chloroform, and dichloromethane.

Figure 3. Plot of ln(γ∞ i ) for ionic liquid [C2OHmim][C4F9SO3] versus 1/T for the solutes: benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene.

ionic liquid have lower ln γ∞ i values and smaller slopes in the ∞ plots. Therefore, the ln γ∞ i values and the slope of the ln γi / (1/T) plot follow the order of alcohols < aromatics < alkynes < alkenes < alkanes. This trend is consistent with the reported solubility in previous works of Zhang et al.46,47 The ln γ∞ i values in this work may be used to predict the effectiveness of separating different compounds. The results show that the ionic liquid may be applied for the separation of aromatics and aliphatics, as was reported by the work of Zhang et al.46,47 The chemical nature of the interaction of aromatic molecules with ionic liquids has also been reported.48 The selectivity at infinite dilution for the ionic liquid, which revealed aptness of a solvent for separating mixtures of components i and j (where i and j refers to the solutes to be separated by extraction), can be calculated directly from 7 ∞ experimental γ∞ i and γj values.

sij∞ = γi∞/γj∞

(7)

The capacity, k∞ j , is defined as

34

Figure 4. Plot of ln(γ∞ i ) for ionic liquid [C2OHmim][C4F9SO3] versus 1/T for the solutes: 1-hexene, 1-octene, 1-decene, 1-pentyne, 1hexyne, 1-heptyne, and 1-octyne.

∞ k∞ j = 1/ γj

(8)

(s∞ ij )

(k∞ j )

Values of the selectivity and capacity for the separation of cyclohexane (i)/benzene (j) and benzene (i)/ methanol (j) in the ionic liquid [C2OHmim][C4F9SO3] at infinite dilution at T = 323.15 K are listed in Table 4. The results are compared with reported literature data for other [C2OHmim] based ILs. The selectivity values presented in Table 4 provide important information that the anion in the ILs containing the same cation plays a key role in affecting the separation efficiency in the mixture of cyclohexane−benzene and benzene−methanol components. As shown in Table 4, for the separation of the cyclohexane− ∞ benzene system, the s∞ ij and kj using [C2OHmim][BF4] as the solvent at T = 323.15 K are 53.7 and 0.11, respectively.30 The selectivity is high, indicating that [C2OHmim][BF4] could be an effective solvent for this separation. However, the capacity (0.11) is very low. This implies a low solubility and therefore a low throughput of the separated component. The s∞ ij using [C2OHmim][FAP] as the solvent at T = 323.15 K gives 15.8

much stronger interactions between the polar groups of these compounds and the polar ionic liquid [C2OHmim][C4F9SO3]. For alkanes, 1-alkenes, 1-alkynes, alcohols, alkyl benzenes, chloroform, and dichloromethane, values of γ∞ i decrease with increasing temperature. For the rest of the investigated solutes, 1,4-dioxane, acetonitrile, acetone, and tetrahydrofuran, values of γ∞ i slightly decrease with increasing temperature. It is interesting to note that Figures 3, 4, and 5 feature the overlay effect. The results of Figures 3−5 show that, within each group of compounds including alkanes, alkenes, alkynes, alcohols, and aromatics, ln γ∞ i increases with the molecular mass, and the slope of ln γ∞ i plot over 1/T remained nearly the same. This observation may be explained by the increased interaction of the hydrocarbons with the ionic liquid due to increased mass, but the slope of the temperature dependence of ln γ∞ i in each group of compounds reflects the nature of the compounds. Compounds with stronger interaction with the F



Article

∞ Table 4. Selectivity (s∞ ij ) and Capacity (kj ) for Benzene (i)/Methanol (j) and Cyclohexane (i)/Benzene (j) at Infinite Dilution for [C2OHmim] Based Ionic Liquids at 323.15 K

s∞ ij

k∞ j

ionic liquids

benzene (i)/methanol (j)

cyclohexane (i)/benzene (j)

methanol

benzene

refs

[C2OHmim][FAP] [C2OHmim][NTF2] [C2OHmim][BF4] [C2OHmim][C4F9SO3]

1.61 2.38 9.9 4.1

15.8 11.7 53.6 4.0

1.43 1.12 1.09 1.08

0.88 0.47 0.11 0.26

32 41 30, 31 this work

32 and the k∞ making it a more effective solvent for j is 0.88, ∞ performing this separation. The s∞ ij and the kj calculated from this work, using [C2OHmim][C4F9SO3] as the solvent at T = 323.15 K, are 4.0 and 0.26, respectively. The selectivity is lower than the other reported data but high enough to be acceptable; however, the capacity is most likely too low for effective separation of cyclohexane and benzene by extractive distillation. For the system of benzene−methanol in [C2OHmim]∞ [C4F9SO3] at T = 332.15 K, the s∞ ij and kj calculated from this work are 4.1 and 1.08, respectively. The selectivity for this separation was higher than that reported for [C2OHmim][FAP] (1.61)32 and [C2OHmim][NTF2] (2.38)41 but not as high as that for [C2OHmim][BF4] (9.9)31 in Table 4. The capacity value for this system is 1.08, which is similar to that for [C2OHmim][BF4] (1.09). This makes [C2OHmim][C4F9SO3] a possible solvent for the separation of benzene and methanol by extractive distillation. The relatively high selectivity and capacity for the benzene−methanol system using [C2OHmim][C4F9SO3] as a solvent is in accordance with a relatively smaller γ∞ i for methanol caused by the enhanced interactions between the methanol and the polar IL. More importantly, these data generated through this work may be helpful in understanding the nature of ILs and enriching our knowledge base for expanding and developing thermodynamic models for mixtures containing ILs suitable for the separation process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 516-299-2013. Fax: 516299-3944. ORCID

Cheng Zhang: 0000-0002-5281-5979 Funding

The author gratefully acknowledges the financial support from Long Island University (Post) Faculty Research Grant LIU_2016_36037. Notes

The author declares no competing financial interest.

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REFERENCES

(1) Chen, J. Y.; Jiang, H. X.; Liu, J. L.; Jia, H. X.; Jiao, Y. H.; Ge, M. L.; Kang, R. X.; Peng, X. M.; Xiong, J. M. Thermodynamics and activity coefficients at infinite dilution for organic compounds and water in the ionic liquid 1-butyl-3-methylimidazolium perchlorate. J. Chem. Thermodyn. 2017, 115, 12−18. (2) Yan, P. F.; Yang, M.; Liu, X. M.; Liu, Q. S.; Tan, Z. C.; WelzBiermann, U. Activity Coefficients at Infinite Dilution of Organic Solutes in 1-Ethyl-3-methylimidazolium Tris(pentafluoroethyl)trifluorophosphate [EMIM][FAP] Using Gas-Liquid Chromatography. J. Chem. Eng. Data 2010, 55, 2444−2450. (3) Yan, P. F.; Yang, M.; Liu, X. M.; Wang, C.; Tan, Z. C.; WelzBiermann, U. Activity coefficients at infinite dilution of organic solutes in the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate [EMIM][TCB] using gas−liquid chromatography. J. Chem. Thermodyn. 2010, 42, 817−822. (4) Yan, P. F.; Yang, M.; Li, C. P.; Liu, X. M.; Tan, Z. C.; WelzBiermann, U. Gas-liquid chromatography measurements of activity coefficients at infinite dilution of hydrocarbons and alkanols in 1-alkyl3-methylimidazolium bis(oxalato)borate. Fluid Phase Equilib. 2010, 298, 287−292. (5) Yan, P. F.; Liu, Q. S.; Yang, M.; Liu, X. M.; Tan, Z. C.; WelzBiermann, U. Activity coefficients at infinite dilution of organic solutes in N-alkylpyridinium bis(trifluoromethylsulfonyl)imide ([CnPY][NTf2], n = 2, 4, 5) using gas−liquid chromatography. J. Chem. Thermodyn. 2010, 42, 1415−1422. (6) Branco, L. C.; Rosa, J. N.; Moura Ramos, J. J.; Afonso, C. A. M. Preparation and Characterization of New Room Temperature Ionic Liquids. Chem. - Eur. J. 2002, 8, 3671−3677. (7) Tiegs, D.; Gmehling, J.; Medina, A.; Soares, M.; Bastos, J.; Alessi, P.; Kikic, I. DECHEMA Chemistry Data Series IX, Part 1, DECHEMA: Frankfurt/Main, 1986, 586. (8) Everett, D. H. Effects of Gas Imperfections on GLC Measurements. Trans. Faraday Soc. 1965, 61, 1637−1645. (9) Cruickshank, A. J. B.; Windsor, M. L.; Young, C. L. The Use of Gas-Liquid Chromatography to Determine Activity Coefficients and

5. CONCLUSION Activity coefficients at infinite dilution (γ∞ i ) for 29 solutes in the ionic liquids 1-(2-hydroxyethyl)-3-methylimidazolium nonafluoro-1-butanesulfonate ([C 2OHmim][C 4F 9SO 3]) were measured in the temperature range from 303 to 353 K using the GLC method. Values of the selectivity and capacity related to the separation of cyclohexane from benzene and the separation of benzene from methanol were calculated from the measured γ∞ i . These values of selectivity and capacity were compared to the reported literature values for other [C2OHmim] based ionic liquids. The results indicate that [C2OHmim][C4F9SO3] is not an ideal solvent for separating cyclohexane and benzene by extraction due to the lower selectivity and capacity when compared to other reported [C2OHmim] based ILs. However, [C2OHmim][C4F9SO3] appears to be a possible solvent for the separation of benzene from methanol by extractive distillation, as values of the selectivity and capacity are reasonably high (s∞ ij = 4.1, k∞ j = 1.08) in comparison to those reported [C2OHmim] based ILs except for [C2OHmim][BF4].

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Sample description table (Table S1) and vapor pressure P1*, molar volume V1*, and virial coefficients B11 and B12 used in the calculation of γ13 at temperatures of 313−364 K (Table S2) (PDF)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00080. G



Article

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Determination of Activity Coefficients at Infinite Dilution of Organic

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