Activity coefficients at infinite dilution and physicochemical properties for organic solutes and water in the ionic liquid 4-(2-methoxyethyl)-4-methylmorpholinium bis(trifluoromethylsulfonyl)-amide

J. Chem. Thermodynamics 47 (2012) 382–388

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Activity coefficients at infinite dilution and physicochemical properties for organic solutes and water in the ionic liquid 4-(2-methoxyethyl)4-methylmorpholinium bis(trifluoromethylsulfonyl)-amide Andrzej Marciniak ⇑, Michał Wlazło Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 18 October 2011 Received in revised form 16 November 2011 Accepted 17 November 2011 Available online 28 November 2011 Keywords: Activity coefficient at infinite dilution Ionic liquid 4-(2-Methoxyethyl)-4-methylmorpho linium bis(trifluoromethylsulfonyl)-amide [COC2mMOR][NTf2] Selectivity Extraction LFER

a b s t r a c t The activity coefficients at infinite dilution, c1 13 and gas–liquid partition coefficients, KL for 62 solutes: alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, thiophene, ethers, ketones, esters, 1-nitropropane, butanal, acetonitrile, acetic acid and water in the ionic liquid 4-(2-methoxyethyl)-4-methylmorpholinium bis(trifluoromethylsulfonyl)-amide were determined by gas–liquid chromatography at the temperatures from (318.15 to 368.15) K. The partial molar excess Gibbs free energies E;1 DGE;1 and entropies DSE;1 at infinite dilution were calculated from the experimental c1 1 , enthalpies DH 1 1 13 values obtained over the temperature range. The selectivities for selected compounds which form azeo1 tropic mixtures were calculated from the c13 and compared to the literature values for other ionic liquids based on bis(trifluoromethylsulfonyl)-amide anion. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Ionic liquids (ILs) have become one of the most studied classes of compounds in the last 20 years. They are salts melting at relatively low temperatures due to asymmetry in structure. Because of their unique physicochemical properties such as extremely low volatility, high thermal and chemical stability and wide liquid range, they are considered as good potential ‘‘green’’ replacements for conventional volatile and often toxic organic solvents in many areas of academic and industrial chemistry. Ionic liquids are ‘‘designer solvents’’ – almost infinite combination of anions and cations allows tailoring solvent properties, therefore the knowledge about influence of cation and anion structure on physicochemical properties of ionic liquids is very important. This work is a continuation of systematic study on ionic liquids and presents new data on activity coefficients at infinite dilution, c1 13 and gas–liquid partition coefficients, KL for 62 solutes: alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, thiophene, ethers, ketones, esters, 1-nitropropane, butanal, acetonitrile, acetic acid and water in the ionic liquid 4-(2-methoxyethyl)-4methylmorpholinium bis(trifluoromethylsulfonyl)-amide, [COC2m-

⇑ Corresponding author. Tel.: +48 22 234 5816; fax: +48 22 628 2741. E-mail address: [email protected] (A. Marciniak). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.11.021

MOR][NTf2], determined by gas–liquid chromatography at the temperatures from (318.15 to 368.15) K. The partial molar excess Gibbs E;1 free energies DGE;1 and entropies DSE;1 at infinite 1 , enthalpies DH1 1 dilution were calculated from the experimental c1 13 values obtained over the temperature range. From the activity coefficients at infinite dilution selectivity 1 1 1 1 (S1 ij ¼ ci =cj ) and capacity (kj ¼ 1=cj ) can be directly calculated for different separation problems. Our previous studies concern on separation of aromatic hydrocarbons from aromatic/aliphatic mixtures and separation of compounds which form azeotropic mixtures. It was found that a large number of ionic liquids have better selectivity and capacity in extraction of aromatic hydrocarbons from aromatic/aliphatic mixture than typical solvents such as sulfolane and NMP [1]. Previously studied ionic liquid [N-C3OHPY][NTf2] reveals good properties as extractant especially for separation of azeotropic mixtures containing a hydrocarbon and a polar solvent [2]. 2. Experimental method 2.1. Materials The ionic liquid [COC2mMOR][NTf2] had a purity of >0.995 mass fraction and was supplied by Merck. The ionic liquid was further purified by subjecting the liquid to a very low pressure of about

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5  10–3 Pa at temperature of about 363 K for approximately 5 h. This procedure removed any volatile chemicals and water from the ionic liquid. The water content was analyzed by Karl-Fischer titration technique (method TitroLine KF). The sample of IL was dissolved in methanol and titrated with steps of 2.5 lL. The results obtained have shown the water content to be less than 200 ppm. The solutes, purchased from Aldrich and Fluka, were used without further purification because the GLC technique separated any impurities on the column. The list of materials with purities are presented in Table 1S. Structure of investigated IL is presented below:

were determined. The flow rates were corrected for water vapor pressure. Solute injections ranged from (0.01 to 0.2) ll and were considered to be at infinite dilution on the column. Experiments were carried out at different temperatures (in steps of 10 K) between (318.15 and 368.15) K. The temperature of the column was maintained constant to within ±0.02 K. At a

TABLE 1 The experimental activity coefficients at infinite dilution c1 13 for the solutes in ionic liquid [COC2mMOR][NTf2] at different temperaturesa. Solute

T/K 318.15

O N+

O

O F

O

N S

S O

F

F

O

F

F F

2.2. Apparatus and experimental procedure The experiments were performed using a PerkinElmer Clarus 500 gas chromatograph equipped with a thermal conductivity detector (TCD). The data were collected and processed using TotalChrom Workstation software. The column preparation and the packing method used in this work, has been described previously [3]. Glass columns of length 1 m and 4 mm internal diameter were used. Chromosorb W/AW-DCMS 120/140 mesh was used as the solid support and was supplied by Sigma–Aldrich. Coating the solid support material with the ionic liquid was performed by dispersing a certain portion of Chromosorb in a solution of the ionic liquid in methanol followed by evaporation of the solvent using a rotating evaporator. The masses of the stationary phase and of the solid support were weighed with a precision ±0.0001 g. The solvent column packing varied from (50.2 to 55.0) mass fraction of the ionic liquid, large enough to prevent any residual adsorption of solute onto the column packing. The uncertainty in the moles of the IL packed on the support was about 3  10–7 mol. Care was taken to ensure that the methanol had completely evaporated from the IL coated solid before making up the column. Before experiment each column was conditioned by blowing carrier gas at flow rate (about 1.5  10–6 m3  s–1) at the high temperature (373.15 K) through about 8 h. The second column was used to check reproducibility of results at different packing. On the second column measurements were performed at two temperatures (338.15 and 358.15) K. Results from these two different columns were repeatable with errors less than 0.35%. The pressure drop (pi  po) was varied between (20 and 60) kPa depending on flow rate of carrier gas. The inlet pressure pi was measured by a pressure gauge installed on the gas chromatograph with an uncertainty of ±0.1 kPa. The outlet pressure po was measured using Agilent Precision Gas Flow Meter with an uncertainty of ±0.07 kPa. The carrier gas was helium. The flow rate of carrier gas was determined using Agilent Precision Gas Flow Meter which was placed at the outlet after the detector with an uncertainty of ±0.1  10– 6 m3  min–1. The flow rate was set for a series of runs and was allowed to stabilize for at least 15 min before the retention times

328.15

338.15

348.15

358.15

Pentane 25.0 22.5 20.6 19.0 17.4 Hexane 36.1 32.6 29.8 27.2 25.0 3-Methylpentane 32.3 29.0 26.3 23.9 21.9 2,2-Dimethylbutane 32.4 28.4 25.7 23.2 21.2 Heptane 54.4 48.7 43.8 39.7 36.2 Octane 82.1 71.6 64.6 58.4 51.7 2,2,4-Trimethylpentane 55.9 49.5 45.0 40.8 37.2 Nonane 125 108 95.8 85.6 76.3 Decane 192 165 145 126 111 Cyclopentane 12.2 11.2 10.4 9.68 9.10 Cyclohexane 18.8 17.0 15.6 14.4 13.3 Methylcyclohexane 27.6 24.8 22.8 20.9 19.2 Cycloheptane 26.2 23.6 21.5 19.7 18.1 Cyclooctane 36.6 32.6 29.6 26.7 24.2 Pent-1-ene 11.0 10.3 9.79 9.25 8.83 Hex-1-ene 16.5 15.3 14.3 13.6 12.9 Cyclohexene 9.14 8.62 8.21 7.84 7.45 Hept-1ene 24.9 23.0 21.7 20.3 19.0 Oct-1-ene 38.6 34.9 32.5 30.1 27.5 Dec-1-ene 87.8 78.8 72.1 65.7 59.4 Hex-1-yne 4.88 4.80 4.75 4.70 4.65 Hept-1yne 7.44 7.24 7.09 6.94 6.79 Oct-1-yne 11.4 11.0 10.6 10.3 9.93 Benzene 1.33 1.32 1.32 1.32 1.31 Toluene 2.01 2.01 2.00 1.99 1.98 Ethylbenzene 3.27 3.21 3.16 3.10 3.04 o-Xylene 2.75 2.71 2.68 2.65 2.61 m-Xylene 3.05 3.02 2.99 2.97 2.94 p-Xylene 3.09 3.06 3.03 3.00 2.98 Styrene 1.77 1.77 1.76 1.76 1.76 a-Methylstyrene 2.74 2.76 2.78 2.79 Thiophene 1.09 1.08 1.08 1.08 1.08 Pyridine 0.640 0.644 0.648 0.651 0.654 Methanol 1.28 1.18 1.10 1.02 0.950 Ethanol 1.77 1.61 1.49 1.36 1.26 Propan-1-ol 2.39 2.14 1.96 1.79 1.63 Propan-2-ol 2.17 1.95 1.79 1.63 1.50 Butan-1-ol 3.31 2.94 2.66 2.40 2.18 Butan-2-ol 2.76 2.47 2.26 2.06 1.90 2-Methyl-propan-1-ol 3.19 2.81 2.51 2.25 2.04 tert-Butanol 2.22 2.03 1.88 1.75 1.62 Water 2.49 2.17 1.94 1.73 1.55 Acetic acid 0.451 0.456 0.459 0.465 Methyl acetate 0.684 0.700 0.714 0.729 0.740 Methyl propanoate 0.949 0.966 0.982 0.998 1.01 Methyl butanoate 1.42 1.44 1.45 1.45 1.46 Ethyl acetate 0.984 1.00 1.02 1.03 1.04 Tetrahydrofuran 0.858 0.867 0.873 0.882 0.888 1,4-Dioxane 0.570 0.596 0.615 0.638 0.653 tert-Butyl methyl ether 3.53 3.55 3.57 3.58 3.59 tert-Butyl ethyl ether 9.50 9.21 8.97 8.75 8.49 tert-Amyl methyl ether 5.55 5.50 5.43 5.37 5.32 Diethyl ether 3.49 3.46 3.44 3.41 3.39 Di-n-propyl ether 10.7 10.3 9.92 9.56 9.20 Di-iso-propyl ether 10.1 9.80 9.55 9.26 8.98 Di-n-butyl ether 26.8 24.9 23.3 21.8 20.4 Acetone 0.430 0.442 0.451 0.461 0.470 Pentan-2-one 0.880 0.890 0.900 0.910 0.918 Pentan-3-one 0.895 0.912 0.927 0.941 0.952 Butanal 0.871 0.884 0.891 0.902 0.909 Acetonitrile 0.423 0.426 0.428 0.431 0.433 1-Nitropropane 0.791 0.787 0.783 0.780 a

Standard uncertainties u are u(c1 13 ) < 3%, u(T) = 0.02 K.

368.15 16.3 23.1 20.2 19.5 33.1 47.3 34.1 68.7 98.2 8.54 12.3 17.7 16.6 22.1 8.35 12.1 7.08 18.1 25.9 54.9 4.61 6.65 9.67 1.31 1.97 2.99 2.58 2.92 2.96 1.76 2.81 1.08 0.658 0.897 1.17 1.52 1.40 2.02 1.78 1.89 1.52 1.41 0.468 0.754 1.03 1.48 1.06 0.895 0.674 3.60 8.30 5.28 3.37 8.97 8.75 19.2 0.480 0.925 0.967 0.918 0.436 0.778

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TABLE 2 The experimental gas–liquid partition coefficients KL for the solutes in ionic liquid [COC2mMOR][NTf2] at different temperatures. Solute

T/K 318.15

Pentane Hexane 3-Methylpentane 2,2-Dimethylbutane Heptane Octane 2,2,4-Trimethylpentane Nonane Decane Cyclopentane Cyclohexane Methylcyclohexane Cycloheptane Cyclooctane Pent-1-ene Hex-1-ene Cyclohexene Hept-1ene Oct-1-ene Dec-1-ene Hex-1-yne Hept-1yne Oct-1-yne Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Styrene a-Methylstyrene Thiophene Pyridine Methanol Ethanol Propan-1-ol Propan-2-ol Butan-1-ol Butan-2-ol 2-Methyl-propan-1-ol tert-Butanol Water Acetic acid Methyl acetate Methyl propanoate Methyl butanoate Ethyl acetate Tetrahydrofuran 1,4-Dioxane tert-Butyl methyl ether tert-Butyl ethyl ether tert-Amyl methyl ether Diethyl ether Di-n-propyl ether Di-iso-propyl ether Di-n-butyl ether Acetone Pentan-2-one Pentan-3-one Butanal Acetonitrile 1-Nitropropane

328.15

338.15

2.77 2.35 2.01 5.61 4.49 3.66 5.18 4.23 3.50 3.32 2.86 2.44 10.9 8.30 6.50 20.6 15.3 11.4 10.1 7.92 6.22 38.3 27.2 19.5 71.1 48.1 33.1 8.58 6.97 5.74 16.1 12.7 10.0 21.6 16.6 12.8 44.8 33.2 25.0 109 76.9 55.4 5.20 4.30 3.55 10.4 8.14 6.44 35.7 26.7 20.4 19.7 14.7 11.1 36.6 26.4 19.1 123 81.5 54.9 46.2 33.5 24.8 85.8 59.8 42.7 157 105 72.3 229 162 117 452 305 212 741 484 325 1227 783 516 899 579 384 854 550 367 2024 1265 819 1751 1100 327 228 163 1838 1195 802 160 116 85.5 221 155 110 407 274 189 227 156 110 798 513 339 399 265 180 538 352 237 224 151 105 373 268 194 2132 1393 207 145 104 339 229 160 544 357 241 301 204 143 217 154 112 1218 787 530 36.5 26.8 20.1 24.8 18.2 13.7 68.0 48.0 34.9 18.6 14.2 11.1 41.2 29.7 21.8 20.4 15.1 11.4 128 85.5 58.7 316 223 162 824 545 372 790 523 357 299 208 149 783 549 396 1820 1200

348.15 358.15 368.15 1.73 3.04 2.96 2.13 5.18 8.66 5.01 14.3 23.8 4.77 8.13 10.2 19.3 41.2 3.01 5.17 15.9 8.63 14.4 38.5 18.8 31.3 51.2 86.8 151 226 352 263 251 548 713 120 555 65.1 82.0 135 80.5 235 128 167 75.5 145 939 77.0 114 169 103 83.8 364 15.4 10.5 26.0 8.85 16.5 8.82 41.5 120 261 250 110 292 817

1.53 2.57 2.52 1.87 4.20 6.93 4.12 10.9 17.6 4.02 6.69 8.26 15.2 31.5 2.57 4.24 12.7 6.89 11.2 28.0 14.6 23.5 37.3 65.9 110 161 246 185 177 377 476 90.1 393 50.3 61.8 99.7 60.0 166 92.9 120 56.4 110 647 58.4 84.3 121 76.0 63.8 259 12.1 8.33 19.8 7.18 12.8 6.99 30.3 91.3 188 180 82.9 219 571

1.34 2.20 2.18 1.65 3.47 5.49 3.43 8.45 13.3 3.45 5.61 6.82 12.3 24.4 2.25 3.57 10.3 5.53 8.73 20.5 11.4 18.0 27.6 51.0 82.3 117 177 133 128 267 326 69.2 285 39.4 47.5 74.1 45.4 120 68.6 87.7 43.0 85.2 458 45.0 63.2 88.4 57.4 49.6 187 9.67 6.66 15.4 5.91 10.0 5.62 22.6 70.5 139 132 63.6 168 408

given temperature, each experiment was repeated 2–3 times to check the reproducibility. Retention times were generally reproducible within (0.001 to 0.01) min depending on the temperature and the individual solute. At each temperature values of the dead time tG identical to the retention time of a non-retainable component were measured. While our GC was equipped with a TCD

TABLE 3 enthalpies DHE;1 and Limiting partial molar excess Gibbs free energies, DGE;1 1 1 entropies T ref DSE;1 for the solutes in ionic liquid [COC2mMOR][NTf2] at the reference 1 temperature Tref = 328.15 K. Solute

DGE;1 1 / (kJ  mol–1)

DHE;1 1 / (kJ  mol–1)

T ref DS1E;1 / (kJ  mol–1)

Pentane Hexane 3-Methylpentane 2,2-Dimethylbutane Heptane Octane 2,2,4-Trimethylpentane Nonane Decane Cyclopentane Cyclohexane Methylcyclohexane Cycloheptane Cyclooctane Pent-1-ene Hex-1-ene Cyclohexene Hept-1ene Oct-1-ene Dec-1-ene Hex-1-yne Hept-1yne Oct-1-yne Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Styrene a-Methylstyrene Thiophene Pyridine Methanol Ethanol Propan-1-ol Propan-2-ol Butan-1-ol Butan-2-ol 2-Methyl-propan-1-ol tert-Butanol Water Acetic acid Methyl acetate Methyl propanoate Methyl butanoate Ethyl acetate Tetrahydrofuran 1,4-Dioxane tert-Butyl methyl ether tert-Butyl ethyl ether tert-Amyl methyl ether Diethyl ether Di-n-propyl ether Di-iso-propyl ether Di-n-butyl ether Acetone Pentan-2-one Pentan-3-one Butanal Acetonitrile 1-Nitropropane

8.5 9.5 9.2 9.1 10.6 11.7 10.6 12.8 13.9 6.6 7.7 8.8 8.6 9.5 6.4 7.4 5.9 8.6 9.7 11.9 4.3 5.4 6.5 0.76 1.9 3.2 2.7 3.0 3.1 1.6 2.8 0.21 1.2 0.45 1.3 2.1 1.8 2.9 2.5 2.8 1.9 2.1 2.2 0.97 0.09 0.99 0.01 0.39 1.4 3.5 6.1 4.7 3.4 6.4 6.2 8.8 2.2 0.32 0.25 0.34 2.3 0.64

8.4 8.7 9.1 9.8 9.7 10.7 9.5 11.6 13.0 6.9 8.2 8.6 8.8 9.8 5.3 6.0 4.9 6.2 7.8 9.2 1.1 2.2 3.3 0.25 0.38 1.7 1.2 0.85 0.84 0.06 0.60 0.10 0.54 6.9 8.0 8.8 8.6 9.7 8.6 10.3 7.4 11.1 0.94 1.9 1.6 0.71 1.4 0.82 3.2 0.39 2.6 1.02 0.70 3.5 2.8 6.5 2.1 0.99 1.5 1.00 0.58 0.42

0.12 0.79 0.04 0.66 0.90 0.98 1.11 1.16 0.92 0.32 0.42 0.17 0.19 0.31 1.1 1.5 0.97 2.3 1.9 2.8 3.2 3.2 3.3 0.51 1.5 1.4 1.5 2.2 2.2 1.5 3.4 0.11 0.67 6.5 6.7 6.7 6.8 6.7 6.1 7.5 5.4 9.0 1.2 0.91 1.5 1.7 1.4 0.43 1.8 3.8 3.4 3.6 2.7 2.8 3.4 2.3 0.15 0.67 1.2 0.67 1.8 1.1

detector, air was used as a non-retainable component. The estimated overall error in c1 13 was less than 3%, taking into account the possible errors in determining the column loading, the retention times and solute vapor pressure. The GLC technique was tested for the system hexane in hexadecane at T = 298.15 K and the results compared very favorably with the literature values [4].

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A. Marciniak, M. Wlazło / J. Chem. Thermodynamics 47 (2012) 382–388 TABLE 4 Selectivities, S at T = 328.15 K for selected azeotropic mixtures. Solvent

S

Solvent

S

Solvent/water 2,2,4-Trimethylpentane Nonane Cyclohexane

Pentane Hexane Heptane

10.4 15.0 22.4

Hexane Heptane Octane Nonane

72.3 108 159 239

Pentane Heptane Octane

19.1 41.3 60.7

Nonane Cyclopentane Cyclohexane

Pentane Hexane Heptane

14.0 20.2 30.2

Octane Cyclopentane Cyclohexane

Hexane Heptane

15.2 22.8

Octane Cyclohexane

Pentane Hexane

11.5 16.7

Heptane Octane

Hexane Heptane Octane

11.1 16.6 24.4

Hexane Heptane

22.8 49.8 7.8

Solvent

S

Oct-1-ene Di-n-butyl ether

16.1 11.5

Solvent/acetic acid Decane Cyclohexane Methylcyclohexane Ethylbenzene

366 37.7 55.0 7.1

o-Xylene m-Xylene p-Xylene

6.0 6.7 6.8

Solvent/methanol 91.5 9.5 14.4

Methylcyclohexane Di-n-propyl ether

21.0 8.7

44.5 7.0 10.6

Di-n-propyl ether

33.5 7.9

Methylcyclohexane

11.6

25.0 36.7

Cyclohexane Di-iso-propyl ether

8.7 5.0

Nonane Cyclohexane Methylcyclohexane

36.7 5.8 8.4

Di-n-butyl ether

8.5

16.1 24.0

Solvent/tert-butanol 2,2,4-Trimethylpentane Cyclohexane

24.4 8.4

Methylcyclohexane Di-n-propyl ether

12.2 5.1

Pentane Hexane

50.9 73.8

Heptane Cyclopentane

110 25.3

Cyclohexane Di-iso-propyl ether

38.5 22.2

Hexane

30.2

Heptane

45.1

Cyclohexane

15.7

24.7 36.9

Solvent/benzene 2,2,4-Trimethylpentane Cyclohexane

37.5 12.9

Cyclohexene

6.5

Solvent/ethanol 6.4

Solvent/propan-1-ol

Solvent/propan-2-ol

Solvent/butan-1-ol

Solvent/acetone

Solvent/thiophene

Hexane Heptane

100 100

10

S

[N-C3OHPY]

[emim]

[Et3S]

[pmPIP]

[bmPY]

[bmPIP]

[hmim]

[omim]

[C6OCmim]

FIGURE 1. Selectivity, S1 for alcohols/water and ethers/alcohols separation at T = 318.15 K for selected ionic liquids based on [NTf2]– anion; (—s—) ethanol/ water; (—h—) propan-1-ol/water; (—D—) butan-1-ol/water; (- - -h- - -) di-n-propyl ether/propan-1-ol; (- - -s- - -) di-n-propyl ether/ethanol; (- - -e- - -) di-n-propyl ether/methanol; (- - -D- - -) di-n-butyl ether/butan-1-ol.

1

[P66614]

[N-C3OHPY]

[COC2mMOR]

[emim]

[Et3S]

[pmPIP]

[bmPY]

[bmPIP]

[C6OCmim]

[omim]

0.1

[COC2mMOR]

S 10

1

FIGURE 2. Selectivity, S1 for heptane/alcohols at T = 318.15 K for selected ionic liquids based on [NTf2]– anion; (—s—) heptane/methanol; (—h—) heptane/ethanol; (—D—) heptane/propan-1-ol.

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A. Marciniak, M. Wlazło / J. Chem. Thermodynamics 47 (2012) 382–388

where tR and tG are the retention times for the solute and an unretained gas, respectively, Uo is the column outlet flow rate corrected for the vapor pressure of water given by

100

  p T Uo ¼ U 1  w ; po T f S 10

ð4Þ

where Tf is the temperature at the column outlet, pw is the vapor pressure of water at Tf and U is the flow rate measured with the flow meter. The gas–liquid partition coefficients K L ¼ cL1 =cG1 for a solute partitioning between a carrier gas and the ionic liquid are calculated from the solute retention according to the following equation:


 po J 32 2B12  V 1 V N q3 1  lnðK L Þ ¼ ln ; m3 RT 

[N-C3OHPY]

[COC2mMOR]

[Et3S]

[emim]

[pmPIP]

[bmPY]

[bmPIP]

[hmim]

[C6OCmim]

[omim]

[P66614]

1

FIGURE 3. Selectivity, S1 for alkane/benzene and heptane/thiophene at T = 318.15 K for selected ionic liquids based on [NTf2]– anion; (——) hexane/ benzene; (—j—) heptane/benzene; (—h—) heptane/thiophene.

ð5Þ

where q3 is the density of the solvent (table 6). While the activity coefficients at infinite dilution are determined as a function of temperature, ln c1 13 can be split to its respective enthalpy and entropy components

ln c1 13 ¼

DHE;1 DSE;1 1  1 : RT R

ð6Þ

2.3. Theoretical basis

Assuming that the temperature dependence follows a linear van’t Hoff plot

The equation developed by Everett [5] and Cruickshank et al. [6] was used in this work to calculate the c1 13 of solutes in the ionic liquid.

ln c1 13 ¼ a=ðT=KÞ þ b

ln c1 13 ¼ ln






 p B11  V 1 n3 RT p J 3 ð2B12  V 1 1 Þ  1 þ o2 :  RT V N p1 RT

ð1Þ

The VN denotes the net retention volume of the solute, po the outlet pressure, po J 32 the mean column pressure, n3 the amount of substance of solvent on the column packing, T the column temperature, p1 the saturated vapor pressure of the solute at temperature T, B11 the second virial coefficient of pure solute, V 1 the molar volume of the solute, V 1 1 the partial molar volume of the solute at infinite dilution in the solvent and B12 (where 2 refers to the carrier gas, helium), the mixed second virial coefficient of the solute and the carrier gas. The thermophysical properties required in calculations were calculated using equations and constants taken from the literature [7]. The values of B12 were calculated using Tsonopolous [8] equation. The pressure correction term J 32 is given by

J 32 ¼

2 ðpi =po Þ3  1 : 3 ðpi =po Þ2  1

ð2Þ

The net retention volume of the solute VN, is given by

V N ¼ J 32 U o ðtR  t G Þ;

ð3Þ

the partial molar excess enthalpy be obtained from the slope.

ð7Þ DHE;1 1

¼ Ra at infinite dilution can

3. Results and discussion Tables 1 and 2 list the activity coefficients at infinite dilution and the gas–liquid partition coefficients values respectively for investigated ionic liquid in the temperature range from (318.15 to 368.15) K. The values of activity coefficients at infinite dilution increase with an increase of alkyl chain of solute. It means that the interaction between the solute and the IL decreases with an increase of solute alkyl chain. Branched structure of compounds decreases the value of c1 13 , therefore 3-methylpentane and 2,2-dimethylbutane have lower values of c1 13 than hexane, the same is for 2,2,4trimethylpentane and octane; propan-1-ol and propan-2-ol; butanols; di-n-propyl ether and di-iso-propyl ether. This is also a typical behavior for other measured ionic liquids. The cyclic structure of cycloalkanes causes a decrease of the values of c1 13 in comparison to the corresponding linear alkanes. The same effect is usually observed in the (liquid + liquid) equilibria measurements [9,10]. Additionally in branched and cyclic compounds the packing effect play additional role – these types of compounds have lower molar volume than corresponding linear ones. Interactions between polar

TABLE 5 LFER system constants as a function of temperature for ionic liquid [COC2mMOR][NTf2]. T/K

a b

System constantsa

Statisticsb

l

b

a

s

e

c

r2

SD

F

df

318.15

0.527 (0.017)

0.504 (0.072)

2.25 (0.09)

2.53 (0.07)

0.169 (0.066)

0.622 (0.056)

0.992

0.076

1363

54

328.15

0.492 (0.015)

0.451 (0.066)

2.14 (0.08)

2.45 (0.06)

0.173 (0.061)

0.622 (0.051)

0.993

0.070

1478

54

338.15

0.457 (0.015)

0.410 (0.062)

2.03 (0.07)

2.37 (0.06)

0.181 (0.057)

0.622 (0.048)

0.993

0.066

1560

54

348.15

0.426 (0.014)

0.374 (0.058)

1.93 (0.07)

2.29 (0.05)

0.188 (0.054)

0.621 (0.045)

0.993

0.062

1633

54

358.15

0.398 (0.013)

0.341 (0.056)

1.84 (0.07)

2.22 (0.05)

0.191 (0.051)

0.621 (0.043)

0.994

0.059

1666

54

368.15

0.371 (0.013)

0.311 (0.054)

1.75 (0.06)

2.15 (0.05)

0.199 (0.049)

0.618 (0.042)

0.994

0.057

1674

54

Values in parentheses are standard uncertainties of the parameters. r2, the coefficient of determination; SD, the standard error; F, the F statistic; df, the degrees of freedom.

A. Marciniak, M. Wlazło / J. Chem. Thermodynamics 47 (2012) 382–388 TABLE 6 Density, qa as a function of temperature for ionic liquid [COC2mMOR][NTf2].

a b

T/K

q/(g  cm–3)

298.15 308.15 318.15 328.15 338.15 348.15 358.15 368.15

1.50040 1.49091 1.48181 1.47270 1.46361 1.45457 1.44533 1.43619b

Determined using Anton Paar DMA 4500 densimeter. Extrapolated value.

IL and polarizable p-electrons in aromatic compounds cause that these types of compounds have the lowest values of c1 13 than other hydrocarbons. Polar compounds, especially lower alcohols, acetic acid, esters, ketones, butanal, acetonitrile, 1-nitropropane and cyclic esters (tetrahydrofuran and 1,4-dioxane), interact strongly with IL because contain oxygen or nitrogen in structure. These polar parts of compounds can strongly interact with polar ionic liquid, therefore the values of c1 13 for these compounds are the lowest. For most compounds the excess enthalpies (table 3) values are positive or close to 0. Negative values of the excess enthalpies for a-methylstyrene, pyridine, acetic acid, esters, tetrahydrofuran, 1,4-dioxane, methyl tert-butyl ether, ketones, butanal, acetonitrile mean that the interactions of solute–solvent pairs are higher than for solute–solute ones. High positive values of excess enthalpies and entropies for alcohols and water indicate on breaking hydrogen bonds. To estimate potential applications of [COC2mMOR][NTf2] as an entrainer in liquid–liquid extraction the selectivity, S at infinite dilution is taken into consideration. Table 4 presents selectivities at T = 328.15 K for selected compounds which form azeotropic mixtures [11]. Due to a large number of azeotropic mixtures only systems with selectivity above 5 are presented. Figures 1–3 present influence of cation structure for selected ionic liquids based on [NTf2]– anion on selectivity for selected azeotropic mixtures. The list of cations with abbreviations is as follow: trihexyl-tetradecyl-phosphonium, [P6,6,6,14]+ [12]; triethyl-sulfonium, [Et3S]+ [13]; 1-ethyl-3-methyl-imidazolium, [emim]+ [14]; 1-hexyl-3-methyl-imidazolium, [hmim]+ [15]; 1-octyl-3-methylimidazolium, [omim]+ [15]; 1-hexyloxymethyl-3-methyl-imidazolium, [C6OCmim]+ [16]; 1-butyl-4-methyl-pyridinium, [bmPY]+ [17]; 1-(3-hydroxypropyl)pyridinium, [N-C3OHPY]+ [2]; 1-propyl1-methylpiperidinium, [pmPIP]+ [18]; 1-butyl-1-methylpiperidinium, [bmPIP]+ [19]. The selectivities for separation of alcohols/water systems are low, below or close to unity, and increase with the increase of aliphatic character of an alcohol, so the selectivities for butan-1-ol/ water are higher than for ethanol/water (figure 1). In general with an increase of the alkyl chain in cation structure the decrease of the selectivity is observed, the selectivities are lower for [omim]+ cation than for [emim]+ one and for [bmPIP]+ than for [pmPIP]+. This effect is observed for other systems, like ethers/alcohols (figure 1), alkanes/alcohols (figure 2), alkanes/aromatic hydrocarbons (figure 3) and was previously reported [1]. For two cations [bmPY]+ and [N-C3OHPY]+ (CH3–C3H6–PY and HO–C3H6–PY) substitution of the methyl group with hydroxyl one causes increase of selectivity for each presented systems, especially for heptane/methanol (above six times). For ethers/alcohols systems the selectivities are higher than for alcohols/water and are higher for lower alcohols. The worst cation for separation of alkanes/alcohols and aliphatic/aromatic hydrocarbons is trihexyl-tetradecyl-phosphonium one due to high

387

aliphatic character. Investigated ionic liquid reveals quite good selectivities especially for mixtures containing a hydrocarbon and a polar solvent, but lower than [N-C3OHPY][NTf2] [2]. It is difficult to say which part of cation (hydroxyl or pyridinium) is responsible for augmentation of selectivity, therefore there is a need to examine ionic liquid [N-C3OHmMOR][NTf2] or [COC2PY][NTf2]. The Abraham solvation parameter model, given by following equation [20]

lg K L ¼ c þ eE þ sS þ aA þ bB þ lL

ð8Þ

enables to estimate the gas–liquid partition coefficients of additional solutes into [COC2mMOR][NTf2]. The independent variables in equation (8) are solute descriptors as described previously [20,21]: E is the solute excess molar refraction, S is the solute dipolarity/polarizability, A and B are the overall or summation solute hydrogen bond acidity and basicity, and L is the logarithm of the gas-hexadecane partition coefficient at T = 298 K. Solute descriptors are available for wide range of compounds. The six regression coefficients (c, e, s, a, b and l) are determined by regression analysis. LFER system constants as a function of temperature with statistics for investigated ionic liquid are presented in table 5. 4. Conclusions Activity coefficients at infinite dilution and the gas–liquid partition coefficients for various solutes in the ionic liquid 4-(2methoxyethyl)-4-methylmorpholinium bis(trifluoromethylsulfonyl)-amide were measured by gas–liquid chromatography at the temperatures from (318.15 to 368.15) K. It was found that the investigated [COC2mMOR][NTf2] ionic liquid reveals good selectivities in liquid–liquid extraction of azeotropic mixtures especially for mixtures containing a hydrocarbon and a polar solvent, but lower than previously studied [N-C3OHPY][NTf2] one. Acknowledgments Funding for this research was provided by the Ministry of Science and Higher Education in the years 2010–2011 from the budgetary means for science (Grant No. 0242/H03/2010/70; Project No. IP2010 024270). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jct.2011.11.021. References [1] [2] [3] [4]

[5] [6] [7] [8] [9] [10] [11]

[12] [13]

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JCT -11-475