ARTICLE pubs.acs.org/JPCB
Limiting Activity Coefficients and GasLiquid Partition Coefficients of Various Solutes in Piperidinium Ionic Liquids: Measurements and LSER Calculations Kamil Paduszynski and Urszula Domanska* Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
bS Supporting Information ABSTRACT: This paper is a continuation of our systematic investigations on piperidinium ionic liquids and presents new data on activity coefficients at infinite dilution for 43 solutes: linear and branched alkanes, cycloalkanes, alkenes, alkynes, benzene, alkylbenzenes, alcohols, water, thiophene, tetrahyrdofuran (THF), methyl tert-butyl ether (MTBE), linear ethers, acetone, and linear ketones in the ionic liquid 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide, [BMPIP][NTf2]. The data were determined by gasliquid chromatography (GLC) at temperatures from 308.15 to 358.15 K. These values were compared to those previously published for the bis(trifluoromethylsulfonyl)imide-based ionic liquids. The partial molar excess enthalpies ΔH1E,¥ and entropies ΔS1E,¥ at infinite dilution were calculated from the experimental γ13¥ values obtained over the temperature range. The values of the selectivities for different separation problems were calculated from γ13¥ and compared to literature values for N-methyl-2-pyrrolidinone (NMP), sulfolane, and additional ionic liquids. Experimental limiting activity coefficients were used to calculate gas-IL partition coefficients of solutes, KL. The modeling with specific linear solvation energy relationship (LSER) equations was performed for data obtained in this work and those reported earlier for 1-butyl-1-methylpiperidinium thiocyanate, [BMPIP][SCN].
’ INTRODUCTION Since the beginning of the 1990s, ionic liquids (ILs) have become to be one of the most studied classes of chemicals. They are organic/inorganic salts melting at relatively low temperatures due to asymmetry and charge dispersion of their organic and inorganic ions. Because of their unique physical and chemical properties such as extremely low volatility, high thermal, chemical, and electrochemical stability, and large range of temperature at which they form liquid phase, they are considered as valid potential “green” replacements for conventional organic solvents in many areas of academic and industrial chemistry.17 The most important feature of ionic liquids is their “tuneability” by means of modification of the structure of the cation and anion allows to select particular IL for specific application. It is easy to change the length of alkyl substituent to obtain ILs differing in melting temperature or solubility. In our group we study an influence of ILs structure on limiting activity coefficients of various solutes in ILs, solidliquid phase (SLE), liquidliquid phase (LLE), and vaporliquid phase (VLE) equilibria in binary and ternary systems as well as volumetric and transport properties of mixtures containing ILs and common organic solvents (hydrocarbons, alcohols, ethers, and ketones) or water.812 Knowledge of all of these properties is one of the crucial points if one considers ILs as solvents in technological processes.13 r 2011 American Chemical Society
Recently we have focused on ILs based on 1-alkyl-1-methylpiperidinium cation combined with different anions.8,10,11 We measured and reported SLE and LLE for binary systems with piperidinium ionic liquids (namely, 1-ethyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide, [EMPIP][NTf 2 ],10 1-propyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide, [PMPIP][NTf2],11 and 1-butyl-1-methylpiperidinium thiocyanate, [BMPIP][SCN]10) and 1-alkanols, hydrocarbons, or water. Our results indicate significant differences in the solubility of piperidinium-based IL differing in anion in alcohols and water. In particular, complete miscibility of [BMPIP][SCN] with water was observed whereas upper critical solution temperatures (UCST) type of phase behavior was obtained for ([EMIM][NTf2] þ water) system. For ([EMIM][NTf2], or [PMPIP][NTf2] þ 1-alcohol) binary systems we observed UCST as well and the same type of phase envelope was obtained from equation-of-state calculations of LLE for systems with [PMPIP][NTf2] performed by using non-random hydrogen bond (NRHB) and PC-SAFT models.11 The values of UCST increases as the length of carbon chain of alcohol increases and decreases with an increase of the length of alkyl substituent in piperidinium cation. The same effect was Received: March 2, 2011 Revised: April 29, 2011 Published: June 02, 2011 8207
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The Journal of Physical Chemistry B observed for mixtures of 1-alkyl-3-methylimidazolium ILs based on [NTf2] anion with alcohols14 and 1-alkyl-3-methylimidazolium thiocyanates with hydrocarbons.1517 The difference in solubility of [BMPIP][SCN] in aliphatic and aromatic hydrocarbons suggests high selectivity for different separation problems using this particular IL. It was confirmed by gasliquid chromatography (GLC) measurements of limiting activity coefficients of various organic solutes and water in [BMPIP] [SCN].9 Lower selectivity was obtained for [PMPIP][NTf2].8 However its value is still higher than those for common molecular entrainers such as N-methyl-2-pyrrolidone (NMP) or sulfolane. This paper is a continuation of our systematic investigations on piperidinium ionic liquids. For this study, activity coefficients at infinite dilution for 43 solutes: linear and branched alkanes, cycloalkanes, alkenes, alkynes, benzene, alkylbenzenes, alcohols, water, thiophene, tetrahyrdofuran (THF), methyl tert-butyl ether (MTBE), linear ethers, acetone, and linear ketones in the ionic liquid 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl) imide, [BMPIP][NTf2], were determined by GLC at temperatures from 308.15 to 358.15 K. This work will also give information about the influence of the cation on the selectivity of ionic liquids based on bis(trifluoromethylsulfonyl)imide anion. Experimental limiting activity coefficients were used to calculate gas-IL partition coefficients of solutes. Then a mathematical treatment in terms of different specific linear solvation energy relationship (LSER) equations1823 was performed for data presented in this contribution and those reported by Domanska et al. for [BMPIP][SCN].9
’ EXPERIMENTAL PROCEDURES Materials. The ionic liquid 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([BMPIP][NTf2]; CAS: 62358002-9; purity: >0.99 w/w) was supplied by IoLiTec (Ionic Liquids Technologies GmbH, Denzlingen, Germany). The chemical formula of [BMPIP][NTf2] is presented in the scheme below:
The ionic liquid was further purified by subjecting the liquid to a very low pressure of about 5 103 Pa at the temperature T = 340 K for approximately 12 h. This procedure removed any volatile chemicals from the ionic liquid. The analysis made by HPLC did not show any organic impurities. The water content was analyzed by the Karl Fischer titration technique (method TitroLine KF). The sample of IL was dissolved in methanol and titrated with steps of 2.5 μL. The results obtained have shown the water content to be less than 250 ppm. The solutes, purchased from Aldrich and Fluka, had purities >0.99 mass fraction and were used without further purification because the GLC separated any impurities on the column. Chromosorb W HP 80/100 mesh was used as the solid support and was supplied by SUPELCO. This material was also purified by the same method used for the ionic liquid. The mass of the IL and Chromosorb was checked before conditioning and after loading. The same masses obtained before and after loading confirm that the IL and Chromosorb were clean, without water and volatile chemicals.
ARTICLE
GasLiquid Chromatography. The experiments were carried out with Perkin-Elmer Clarus 500 gas chromatograph equipped with TCD detector. The column preparation and the packing method used in this work have been described in detail elsewhere.24 Coating the solid support material with the ionic liquid was performed by dispersing a certain portion (ca. 2.5 g) of Chromosorb into a solution of the ionic liquid in methanol followed by evaporation of the solvent using a rotary evaporator. The masses of the stationary phase and of the solid support were weighed with a precision (104 g giving in result uncertainty of the IL amount in the column on the level 2 107 mol. The solvent column packing varied from 0.456 to 0.551 w/w of the ionic liquid, large enough to prevent any residual adsorption of solute onto the column packing, as we observed in recently published work. The procedure of measurements was described in our previous work.10 The outlet pressure Po was kept at atmospheric pressure. The pressure drop on the column varied in the range of 3585 kPa depending on the flow rate of the carrier gas. The pressure drop was measured by a pressure transducer implemented in the gas chromatograph with an uncertainty of (0.1 kPa. The atmospheric pressure was measured using a digital barometer with an uncertainty of (0.1 hPa. As the carrier gas, we used helium. The flow rate was determined using an Agilent Precision Gas Flow Meter which was placed at the outlet after the detector with an uncertainty of (0.1 mL min1). The flow rate was set for a series of runs and was allowed to stabilize for at least 15 min before any determinations were made. The flow rates were corrected for water vapor pressure. Solute injections ranged from 0.01 to 0.3 μL and were considered to be at infinite dilution on the column. Experiments were carried out at different temperatures in the range from 308.15 to 358.15 K. The temperature of the column was maintained constant to within (0.02 K. The temperature of the column was controlled in the ovens of the gas chromatograph with an additional electronic thermometer P 550 (Dostmann electronic GmbH). At a given temperature, each experiment was repeated 2 to 4 times to check the reproducibility. Retention times were generally reproducible within 0.001 to 0.01 min. At each temperature, values of the dead time tG identical to the retention time of a nonretainable component were measured. While our chromatograph was equipped with a TCD detector, air was used as a nonretainable component. The estimated overall error in limiting activity coefficient 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.25
’ THEORETICAL BACKGROUND Limiting Activity Coefficients. In this paper we used the equation proposed by Everett26 and Cruickshank27 to calculate limiting activity coefficients from retention data:
ln γ¥13 ¼ ln
n3 RT P ðB11 V1 Þ Po J2 3 ð2B12 V1¥ Þ þ 1 2 0 RT RT P1 J3 Uo tR ð1Þ
where subscripts 1, 2, and 3 correspond to solute, carrier gas, and solvent (in this case [BMPIP][NTf2] ionic liquid), respectively. 8208
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Table 1. Experimental Activity Coefficients at Infinite Dilution (γ¥ 13), Partial Molar Excess Enthalpies at Infinite Dilution E,¥ (ΔHE,¥ 1 ), and Partial Molar Excess Entropies at Infinite Dilution (ΔS1 ) for Different Solutes in Ionic Liquid [BMPIP][NTf2] T/K solute
348.15
358.15
1 ΔHE,¥ 1 /kJ mol
1 1 ΔSE,¥ K 1 /J mol
308.15
318.15
328.15
338.15
9.54
8.5
7.7
7.34
7.09
6.75
6.14
1.45
11.4 10.2
10.3 9.17
9.81 8.76
9.39 8.39
9.05 8.03
6.36 6.43
0.31 0.83
Alkanes n-pentane n-hexane 3-methylpentane
12.9 11.5
2,2-dimethylbutane
10.7
6.11
0.40
n-heptane
18.2
15.9
9.56
14.4
8.59
13.5
8.25
12.7
7.97
12
7.58
7.45
0.29
n-octane
25.5
22
19.8
18.5
17.4
16.2
8.02
0.62
2,2,4-trimethylpentane
17.5
15.6
14.1
13.1
12.6
11.9
6.98
0.94
n-nonane
35.6
30.6
26.8
25
23.3
21.8
8.82
0.80
n-decane
49.9
42.6
37.2
34.4
31.4
29.4
9.60
1.10
6.01 6.77
5.43 4.97
Cycloalkanes cyclopentane cyclohexane
5.59 7.96
5.02 7.04
4.6 6.42
4.35 6.06
4.12 5.64
4.04 5.51
methylcyclohexane
10.6
9.39
8.51
8.03
7.5
7.23
6.99
3.27
cycloheptane
10.6
9.36
8.45
7.94
7.31
7.1
7.41
4.64
cyclooctane
13.9
9.4
9.03
7.86
3.84 0.24
12.1
11
10.3 Alkenes
1-pentene
4.97
4.57
4.28
4.2
4.06
3.97
3.97
1-hexene
6.96
6.29
5.89
5.64
5.35
5.24
5.15
0.79
cyclohexene 1-heptene
4.49 9.58
4.12 8.74
3.85 8.05
3.7 7.76
3.53 7.39
3.35 7.26
5.18 5.10
4.45 2.04
9.76
5.84
2.49
1-octene
13.5
12.1
11.2
10.7
10.1
Alkynes 1-hexyne
2.49
2.36
2.31
2.32
2.29
2.35
1.03
4.02
1-heptyne
3.43
3.27
3.14
3.14
3.1
3.11
1.74
4.42
1-octyne
4.77
4.49
4.28
4.24
4.2
4.14
2.45
4.86
benzene
0.788
0.793
0.805
0.814
0.82
0.847
1.23
1.95
toluene
1.07
1.07
1.10
1.11
1.13
1.16
1.50
5.35
ethylbenzene o-xylene
1.58
1.60 1.36
1.60 1.36
1.60 1.37
1.62 1.40
1.64 1.41
0.58 0.95
5.69 5.49
Aromatics
m-xylene
1.48
1.48
1.5
1.53
1.56
1.30
7.27
p-xylene
1.48
1.49
1.51
1.54
1.56
1.30
7.32
20.53
Alcohols methanol
1.53
1.42
1.29
1.20
1.11
1.02
7.44
ethanol
1.86
1.71
1.57
1.43
1.31
1.22
7.87
20.30
1-propanol
2.22
2.03
1.85
1.69
1.55
1.44
8.04
19.41
1-butanol 1-pentanol
2.47
2.25
2.03 2.33
1.85 2.17
1.72 2.01
8.72 7.43
19.88 14.94
acetone
0.465
0.475
0.480
0.480
0.490
1.09
2.90
2-pentanone
0.707
0.726
0.733
0.745
0.756
1.52
1.92
3-pentanone 2-hexanone
0.691
0.714
0.723 0.923
0.748 0.932
0.760 0.948
2.25 1.34
4.00 3.29
0.952
0.959
0.985
1.71
4.61
0.685 2.03
0.698 2.07
0.61 0.13
1.35 6.35
Ketones
3-hexanone
Ethers tetrahydrofuran methyl tert-butyl ether
0.67 2.04
0.68 2.04
0.676 2.05
0.682 2.04 8209
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Table 1. Continued T/K solute diethyl ether di-n-propyl ether di-n-butyl ether
1 ΔHE,¥ 1 /kJ mol
1 1 ΔSE,¥ K 1 /J mol
308.15
318.15
328.15
338.15
348.15
358.15
2.14 4.95
2.14 4.84
2.01 4.47
2.04 4.48
2.01 4.38
2.04 4.30
1.11 2.65
2.68 4.65
9.75
8.82
8.5
8.21
7.89
4.57
4.35
10.0
Other Solutes thiophene
0.719
0.724
0.726
0.733
0.738
0.753
0.77
0.27
water
4.79
4.08
3.60
3.15
2.75
2.41
12.46
27.39
Other quantities occurring in eq 1 are as follows: n3, the amount (number of moles) of solvent on the column packing; R, gas constant; T, the column temperature; Uo, column outlet flow rate; tR0 , corrected retention time of a solute defined as the retention time minus the retention time of a completely unretained solute; P1*, the vapor pressure of pure solute; B1i (i = 1,2), the second ¥ virial coefficients; V*, 1 the molar volume of pure solute; V1 , the partial molar volume of solute at infinite dilution in solvent; Po, the outlet pressure, J23, pressure-correction term (James-Martin coefficient; J32 = 1/J23). All of the temperature-dependent quantities are at the column temperature T. Vapor pressures, liquid molar volumes (we assumed that *) and second virial coefficients of pure solutes (B11) V¥ 1 ≈ V1 were taken from DIPPR correlations,28 whereas the cross second virial coefficients (B12) were estimated from Tsonopoulus correlation29 coupled with HudsonMcCoubrey mixing rules.30,31 Calculated values of vapor pressures, molar volumes and second virial coefficients for solutes are tabulated in Table S1 in the Supporting Information. Limiting activity coefficients are directly related with excess thermodynamic functions at infinite dilution: excess chemical E,¥ potential (ΔμE,¥ 1 ), excess partial entropy of mixing (ΔS1 ), and E,¥ excess partial molar enthalpy of mixing (ΔH1 ): ¥ ΔμE, ΔSE, ¥ ΔH1E, ¥ 1 ð2Þ ¼ 1 þ RT R RT Assuming a linear dependence of ln γ¥ 13 on reciprocal temperature one can easily calculate enthalpic and entropic contributions respectively from the slope and the intercept obtained from experimental data correlation. GasLiquid Partition Coefficients. During the gas-chromatographic process, one can distinguish three main processes: partitioning (absorption) between the gas and the liquid phase, the adsorption at the solidliquid interface, and adsorption at the gasliquid interface.32 Thus, in terms of the general retention model, effects related to all of these phenomena would have to be explicitly considered. However, as we mentioned in the Experimental Procedures section, when high liquid phase loadings are used, the adsorption effects may be neglected. Then, net retention volume (VN) is approximately proportional to stationary phase volume (VL) according to equation
ln γ¥13 ¼
VN ¼ KL VL
where F3 and M3 stand for the IL density and molar mass, respectively. The KL coefficients of different solutes are used to characterize the investigated ionic liquid in terms of linear solvation energy relationship. LSER Calculations. In order to quantify intermolecular soluteionic liquid interactions, Abraham and co-workers proposed and developed a linear solvation energy relationship (LSER) allowing for the correlation of various thermodynamic properties.18 The most recent representation of the LSER model for gasliquid partition coefficient (KL) is given by the equation log KL ¼ c þ eE þ sS þ aA þ bB þ lL
In eq 5 the capital letters correspond to descriptors representing the solute properties, and the lowercase letters represent the respective complementary properties of the IL. The descriptors will describe the general solutesolvent interactions. A and B are measures of the solute hydrogen-bond acidity and basicity, respectively. E is the solute excess molar refraction and S is the solute dipolarity/polarizability descriptor. Finally, L is the logarithm of the gas-to-hexadecane partition coefficient at T = 298.15 K. The coefficients c, e, s, a, b, and l are obtained from linear regression of experimental gasliquid partition coefficients. However, it is worth pointing out that they are not simply fitting coefficients. They reflect complementary properties of the solvent phase. The c term is the model constant which is identified as the opposing contributions of different effects: cavity formation and dispersion interactions (l), interactions with lone pair electrons (e), dipole-type interactions (s), the hydrogen-bond basicity of the stationary phase (a), and hydrogen-bond acidity of the stationary phase (b). In order to improve predictive applicability of LSER for ionic liquids, several group contribution idea-based approaches for the coefficients c, e, s, a, b, and l were developed.1923 In particular each IL can be characterized by its own set parameters. This methodology will be called ionic-liquid-specific LSER (IL-LSER). Sprunger and co-workers19,20 and Grubbs et al.21 proposed the extension of the IL-LSER by separation of equation coefficients into individual cation-specific and anion-specific contributions log KL ¼ ccation þ canion þ ðccation þ canion ÞE þ ðscation þ sanion ÞS
þ ðacation þ aanion ÞA þ ðbcation þ banion ÞB þ ðlcation þ lanion ÞL
ð3Þ
where the constant of proportionality KL is gasliquid partition coefficient, calculated from limiting activity coefficients by using the following equation:22 F RT KL ¼ ¥ 3 ð4Þ γ13 P1 M3
ð5Þ
ð6Þ We will abbreviate this approach by I-LSER (Ion-specific LSER). To determine the cation- and anion-specific contributions, authors used a database containing approximately 1800 numerical values of gas-IL partition coefficients (ranging from 30 to 350 values per individual ion) determined from experimental 8210
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Figure 1. Experimental limiting activity coefficients for different families of solutes in [BMPIP][NTf2] ionic liquid at 338.15 K: O, n-alkanes; 0, cycloalkanes; Δ, 1-alkenes; ), 1-alkynes; b, benzene and alkylbenzenes; 9, 1-alcohols; 2, ketones; (, dialkyl ethers. Solid lines drawn to guide the eye.
Figure 2. Limiting activity coefficients for n-alkanes and cycloalkanes in [BMPIP][NTf2]: O, n-pentane; 0, n-hexane; Δ, n-heptane; ), n-octane; right pointing triangle, n-nonane; left pointing triangle, n-decane; b, cyclopentane; 9, cyclohexane; 2, cycloheptane; (, cyclooctane. Solid lines designated by linear regression.
activity coefficients at infinite dilution, as well as from gas chromatographic retention factors and solubility data for solutes dissolved in ionic liquid. Ion-specific equation coefficients were calculated for 19 different cations and 12 different anions. The range of 12.5 log units of KL was covered with standard deviation of about 0.1 log unit and proved that no loss in predictive accuracy was observed upon separation of the equation coefficients values in comparison with IL-LSER. Application of I-LSER model is somewhat limited because it cannot be extrapolated to ionic liquids that were not initially defined by the method (for example, those with long alkyl chains). To overcome this problem, Ravelli et al. proposed and analyzed the extension of the model by further splitting the cation-specific equation coefficients into functional group values (FG-LSER).22 In opposite to previous approach, they considered a cation as composed of different smaller functional groups. In particular, they distinguished 21 groups: 12 functional groups characterizing cations and 9 assigned to individual anions. In terms of FG-LSER model
LSER covers about 14 log units with standard deviation 0.15 log units. Very recently, Grubbs extended I-LSER and the Ravelli’s FGLSER approaches.23 Finally, the 2349 experimental log KL values were analyzed in terms of eq 6 and eq 7. The coefficients were obtained from experimental data measured at T = 323 K. The current version of the I-LSER covers 25 cation-specific and 14 anion-specific coefficients. In turn, the number of cation functional groups and anions in FG-LSER is extended to 28 (including 14 cation groups and 14 anions). In this contribution we used different LSER equations to regress the obtained experimental gas-IL partition coefficients at T = 323 K for [BMPIP][NTf2] ionic liquid. Then, we performed the test of predictive applicability of the current versions of I-LSER and FG-LSER by prediction of log KL values of [BMPIP][NTf2] and 1-butyl-1-methylpiperidinium thiocyanate, [BMPIP][SCN].
log KL ¼ ð
cation
∑i ni ci þcanionÞ þ ð ∑i ni ei þ eanion ÞE
þð
cation
þð
cation
∑i ni si þ sanion ÞS þ ð ∑i ni ai þ aanion ÞA
cation
’ RESULTS AND DISCUSSION
cation
cation
∑i ni bi þ banion ÞB þ ð ∑i ni li þ lanionÞE
ð7Þ
where ni denotes the number of occurrences of group i in a cation and ci, ei, si, ai, bi, and li are contributions of group i to total value of coefficient c, e, s, a, b, and l, respectively. To obtain the functional group contributions the authors regressed 1450 gasIL partition coefficients at T = 298.15 K for different solutes (n-alkanes, cycloalkanes, alkenes, alkynes, aromatics, alcohols, ethers, aldehydes, ketones, and chloroalkanes) in 39 ionic liquids (based on imidazolium, pyridinium, ammonium, pyrrolidinium, phoshonium, and sulfonium-based cations and 9 anions: bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, tetrafluoroborate, ethylsulfate, octylsulfate, thiocyanate, trifluoromethylsulfonate, trifluoroacetate, and dicyanamide). This version of
Limiting Activity Coefficients Measurements. Table 1 lists the average γ¥ 13 values for the varying amounts of solvent on the column packing within the temperature range from 308.15 to 358.15 K, the average partial molar excess enthalpies at infinite dilution and average partial molar excess entropies of mixing at infinite dilution determined by linear regression the experimental data using eq 2. Exemplary calculations of limiting activity coefficients based on eq 1 and GLC data are presented in Table S2 in the Supporting Information. Figures 14 show the temperature dependence of γ¥ 13 for n-alkanes and cycloalkanes, alkenes, alkynes and aromatics, alcohols, ketones, and water, respectively. In most of the group of compounds investigated γ¥ 13 increases with an increase of the solute alkyl chain. This is also a typical behavior for other measured ionic liquids based on different cations as well as other piperidinium ionic liquids with different anion, e.g. [BMPIP][SCN].9 The interaction between the solute and the IL decreases with an increase of solute alkyl chain. For nalkanes, cycloalkanes, alkenes, alkynes, alcohols, water, and ethers (with exception of THF and MTBE), values of γ¥ 13 decrease with an increasing temperature. For the rest of 8211
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Figure 3. Limiting activity coefficients for alkenes, alkynes and aromatic hydrocarbons in [BMPIP][NTf2]: O, 1-pentene; 0, 1-hexene; Δ, 1-heptene; ), 1-octene; 9, 1-hexyne; 2, 1-heptyne; (, 1-octyne; /, benzene; , toluene; f, ethylbenzene. Solid lines designated by linear regression.
Figure 4. Limiting activity coefficients for alcohols, ketones and water in [BMPIP][NTf2]: O, methanol; 0, ethanol; Δ, 1-propanol; ), 1-butanol; left pointing triangle, 1-pentanol; (, acetone; f, 2-pentanone; , 2-hexanone; / water. Solid lines designated by linear regression.
investigated solutes i.e. aromatic compounds, acetone and other ketones this dependence is reversed. The values of γ¥ 13 for alkenes and alkynes are lower than those for alkanes for the same number of carbon atoms. This is easy to explain by an interaction of double, or triple bonds with the polar IL, since the higher value of γ¥ 13 means the weaker mutual interaction between the IL and the solute. Moreover, the cyclic structure of cycloalkanes causes a decrease of the values of γ¥ 13 in comparison to the corresponding n-alkanes. The same effect is usually observed in the phase equilibria measurements. Cycloalkanes are more soluble in ILs than n-alkanes.1517 Moreover, the molar volume of cycloalkanes is smaller than that for linear alkanes. Therefore, the packing effect additionally increases interactions. The packing effect is also observed for normal and branched alkanes, for example n-hexane has slightly higher values of γ¥ 13 than 3-methylpentane (molar volumes 133.2 and 132.2 cm3 mol1 at temperature T = 308.15 K). Aromatic hydrocarbons reveal lower values of γ¥ 13 than alkynes and in the same range as alcohols. Water has a higher value of γ¥ 13
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Figure 5. Comparison of selectivity for n-hexane/benzene separation problem of different bis(trifluoromethylsulfonyl)imide ionic liquids: PIP, 1-alkyl-1-methylpiperidinium (this work and ref 8); IM, 1-alkyl-3methylimidazolium;3640 N, trialkylmethylammonium;41,42 PYR, 1-alkyl-1-methylpyrrolidinium40,43 (values for n = 6,8 were predicted by Mod. UNIFAC43), where alkyl = n-CnH2nþ1.
Figure 6. Selectivity for n-hexane/benzene separation problem versus temperature for [BMPIP][NTf2] and [PMPIP][NTf2] ionic liquids and common molecular entrainers: b, [BMPIP][NTf2] (this work); O, [PMPIP][NTf2];8 0, NMP;44 Δ, sulfolane.45
in [BMPIP][NTf2] than alcohols. The lowest values of γ¥ 13, indicated the stronger interactions between the solvent and solute, which were found in acetone, thiophene, and THF. Therefore, the oxygen atoms in anions and the nitrogen atom in cations of the IL may be able to interact with the solute’s polar groups. The excess enthalpies values are negative for aromatic hydrocarbons, thiophene, MTBE, THF, and all ketones. The negative values of the partial molar excess enthalpies at infinite dilution mean that the interactions of solutesolvent pairs are higher than for solutesolute pairs. This behavior is caused by interaction between polar anion/cation of the IL and polarizable π-electrons in aromatic compounds, as well as with polar carbonyl group in ketones, or oxygen in ethers. The influence of alkyl chain length in IL cation on the values of γ¥ 13 is revealed by comparison of results reported in this paper for [BMPIP][NTf2] with those presented in our previous work for [PMPIP][NTf2].8 We observed that the increase of alkyl chain 8212
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Table 2. Results of Regression and Prediction of LSER Equation Coefficients for the Gas-IL Partition Coefficients of Various Solutes in [BMPIP][NTf2] and [BMPIP][SCN] Coefficients at 323 Ka IL
model
c
e
s
a
b
[BMPIP][NTf2]
IL-LSER
0.463
0.094
2.442
2.106
0.005
(0.042)
[BMPIP][SCN]
(0.068) (0.087) (0.082) (0.089) statistical summary:b N = 43; R2 = 0.995; F = 1476
l
σc
0.611
0.052
(0.013)
I-LSER
0.463
0.094
2.442
2.106
0.005
0.611
0.052
FG-LSER
0.403
0.217
2.431
2.251
0.017
0.610
0.169
IL-LSER
0.984
0.089
(0.099)
0.587
2.629
4.683
0.062
0.536
(0.134)
(0.168)
(0.203)
(0.163)
(0.028)
statistical summary:b N = 32; R2 = 0.989; F = 485 I-LSER
1.024
0.388
2.831
4.522
0.173
0.578
0.122
FG-LSER
0.964
0.669
2.944
4.612
0.091
0.586
0.309
a
Standard uncertainties in the equation coefficients are given in parentheses directly below the respective values; predictions performed on the basis on parameters published in ref 23. b N, the number of experimental data points; R2 the squared correlation coefficient; F the Fisher F-statistic. c Standard deviation, eq 11.
length cause decrease of γ¥ 13 values for all the solutes except water. This trend was also observed in our previous studies for 1-alkyl-3methylimidazolium thiocyanates (alkyl = ethyl, butyl, hexyl).3335 To estimate potential applications of [BMPIP][NTf2] as an entrainer in liquidliquid extraction the selectivity is taken into consideration. In general the selectivity for separation problem solute “i”/solute “j” is defined as S¥ij ¼
γ¥i3 γ¥j3
Therefore we can apply obtained parameters to predict gas-IL partition coefficients for other ionic liquids containing the nska and [BMPIP]þ cation. As we mentioned above, Doma Krolikowska reported limiting activity coefficients of various solutes in [BMPIP][SCN].9 We can use those data and eq 4 to calculate log KL for 33 solutes in [BMPIP][SCN] and then respective LSER parameters
ð8Þ
Figure 5 shows selectivity of [BMPIP][NTf2] for n-hexane/ benzene system compared with selectivity of different [NTf2] anion-based ionic liquids at T = 328.15 K, namely 1-alkyl-3methylimidazolium,3640 trialkylmethylammonium,41,42 and 1-alkyl-1-methylpyrrolidinium.40,43 In turn, Figure 6 presents temperature dependence of S¥ ij in comparison with common molecular entrainers used in chemical engineering applications (NMP44 and sulfolane45). As can be easily noticed, the selectivities of different ionic liquids with the same alkyl chain length have approximately the same values. Moreover, the value decreases significantly as the alkyl chain length increases. A comprehensive review of the influence of cation and anion structure of the ionic liquid on extraction processes based on activity coefficients at infinite dilution was given recently by Marciniak.46 LSER Calculations. The experimental data measured in this work can be used to calculate the Abraham model coefficients: c, e, s, a, b and l characterizing [BMPIP][NTf2] ionic liquid. Here, calculation means multiple linear regression analysis of log KL with accordance to eq 5 (IL-LSER) or prediction (with ion- or functional group-specific coefficients). Density of [BMPIP][NTf2] at temperature T = 323.15 K was taken from literature (F3 = 1356.9 kg m3).47 All the calculations were carried out by using Microsoft Excel or MATLAB (Mathworks, Inc.). It is worth to be noticed that obtained values of IL-LSER coefficients correspond to the coefficients of I-LSER model for [BMPIP]þ cation since the ion-specific coefficients for [NTf2] anion were set to be equal zero. Therefore c½BMPIP½NTf 2 ¼ c½BMPIPþ , e½BMPIP½NTf 2 ¼ e½BMPIPþ , s½BMPIP½NTf 2 ¼ s½BMPIPþ a½BMPIP½NTf 2 ¼ a½BMPIPþ , b½BMPIP½NTf 2 ¼ b½BMPIPþ , l½BMPIP½NTf 2 ¼ l½BMPIPþ
ð9Þ
c½BMPIP½SCN ¼ c½BMPIPþ þ c½SCN ¼ c½BMPIP½NTf 2 þ c½SCN e½BMPIP½SCN ¼ e½BMPIPþ þ e½SCN ¼ e½BMPIP½NTf 2 þ e½SCN s½BMPIP½SCN ¼ s½BMPIPþ þ s½SCN ¼ s½BMPIP½NTf 2 þ s½SCN a½BMPIP½SCN ¼ a½BMPIPþ þ a½SCN ¼ a½BMPIP½NTf 2 þ a½SCN b½BMPIP½SCN ¼ b½BMPIPþ þ b½SCN ¼ b½BMPIP½NTf 2 þ b½SCN l½BMPIP½SCN ¼ l½BMPIPþ þ l½SCN ¼ l½BMPIP½NTf 2 þ l½SCN ð10Þ where the parameters characterizing the [SCN] anion can be found in literature.23 The density of [BMPIP][SCN] was estimated by group contribution method (GCM). Reliability of GCM strongly depends on the size of databank used to determine model parameters. However, the size of database is not the most important criterion. It is crucial for GCM to predict temperature (and/or pressure) dependence of density. GCM proposed by Lazzus48 meets those requirements. The density of [BMPIP][SCN], calculated at T = 323.15 K in accordance with this method is 1009.7 kg m3. Obtained value seems to be reasonable if it is compared it with experimental density of [BMIM][SCN] at this temperature which is equal to 1055.2 kg m3.49 Moreover, density of [BMIM][SCN] at T = 323.15 K calculated from adopted model is 1049.5 kg m3 (the error is about 0.5%). It confirms reliability and accuracy of the Lazzus GCM. The values of log KL for [BMPIP][NTf2] and [BMPIP][SCN] required for the gas-to-IL LSER calculations were obtained by using eq 4. They are listed in Table S3 in the Supporting Information together with solutes’ descriptors and predicted results using IL-LSER and FG-LSER. The values of gas-IL partition coefficients are calculated at T = 323.15 K since at this temperature literature coefficients for I-LSER and FG-LSER have been determined. 8213
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Figure 7. Plot of experimental gas-IL partition coefficients for [BMPIP][NTf2] ionic liquid vs calculated values in accordance with LSER: O, IL-LSER (regression); b, FG-LSER (prediction).
ARTICLE
’ CONCLUDING REMARKS The activity coefficients at infinite dilution of 43 solutes in [BMPIP][NTf2] ionic liquid were presented. We found that the investigated ionic liquid shows high selectivity for aliphatic/ aromatic hydrocarbons separation problem. The value of selectivity is comparable with generally used organic solvents such as NMP or sulfolane. Hence [BMPIP][NTf2] can be considered as a “green” replacement for these solvents in liquidliquid extraction processes. Results of LSER calculations indicate that ion-specific approach seems to be more accurate. It is in agreement with general rule governing group contribution methods: accuracy of a group contribution model increases as the number of defined groups increases, in particular, more detailed functional groups taking into account diverse “proximity” effects. Particularly, in the case of the LSER models, it is recommended as the first choice the IL-LSER model, followed by the I-LSER, and finally the FG-LSER group contribution model. Of course, for many ILs the functional group contribution model will be the only option available at the present time because of the lack of experimental data. ’ ASSOCIATED CONTENT
bS
Supporting Information. Vapor pressures, molar volumes, and second virial coefficients of solutes used in calculations of limiting activity coefficients. Exemplary calculations of limiting activity coefficients from GLC measurements. Experimental and predicted gas-IL partition coefficients for [BMPIP] [NTf2] and [BMPIP][SCN] ionic liquids at T = 323.15 K and solute descriptors. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ48-22-628 2741. Phone: þ4822-621 3115. Figure 8. Plot of experimental gas-IL partition coefficients for [BMPIP][SCN] ionic liquid vs calculated values in accordance with LSER: O, IL-LSER (regression); 0, I-LSER (prediction); b, FG-LSER (prediction).
The results of calculations are given in Table 2 and showed in Figures 7 and 8. In the case of regression analysis we can see that the statistics is quite good for [NTf2] and [SCN]-based ionic liquids as evidenced near unity squared correlation coefficients and small standard deviations defined as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uN u exptl 2 u ðlog KL,calcd i log KL, i Þ ð11Þ ti ¼ 1 σ ¼ N 6
∑
The results of predictions are very promising as well. In the case of [BMPIP][NTf2] and [BMPIP][SCN] standard deviations of FG-LSER prediction are 0.169 and 0.309 log units, respectively. I-LSER predictions for [BMPIP][SCN] based on cationspecific parameter taken from this work and anion-specific coefficient taken from ref 23 yields in lower standard deviation of 0.122 log units.
’ ACKNOWLEDGMENT This work has been supported by the European Union in the framework of European Social Fund through the Warsaw University of Technology Development Programme. ’ REFERENCES (1) Wasserscheid, P., Welton, T., Eds.; Ionic liquids in Synthesis; Wiley-VCH: Weinheim, Germany, 2002. (2) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123–150. (3) Quijano, G.; Couvert, A.; Amrane, A. Biores. Technol. 2010, 101, 8923–8930. (4) Moniruzzaman, M.; Nakashima, K.; Kamiya, N.; Goto, M. Biochem. Eng. J. 2010, 48, 295–314. (5) Olivier-Bourbigou, H.; Magna, L.; Morvan, D. Appl. Cat. A: Gen. 2010, 373, 1–56. (6) Sun, P.; Armstrong, D. W. Anal. Chim. Acta 2010, 661, 1–16. (7) Wu, T.-Y.; Su, S.-G.; Gung, S.-T.; Lin, M.-W.; Lin, Y.-C.; Lai, C.A.; Sun, I.-W. Electrochim. Acta 2010, 55, 4475–4482. (8) Doma nska, U.; Paduszy nski, K. J. Chem. Thermodyn. 2010, 42, 1361–1366. (9) Doma nska, U.; Krolikowska, M. J. Chem. Eng. Data 2011, 56, 124–129. 8214
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