Prediction of Partition Coefficients of Organic Compounds in Ionic

Feb 23, 2010 - The 21 group-parameters can be used to predict the partition coefficients ... Fangyong Yan , Michael Lartey , Kuldeep Jariwala , Sage B...
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Ind. Eng. Chem. Res. 2010, 49, 3883–3892

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Prediction of Partition Coefficients of Organic Compounds in Ionic Liquids: Use of a Linear Solvation Energy Relationship with Parameters Calculated through a Group Contribution Method Anne-Laure Revelli, Fabrice Mutelet,* and Jean-Noe¨l Jaubert Laboratoire de Thermodynamique des Milieux Polyphase´s, Nancy-UniVersite´, 1 rue GrandVille, BP 20451 54001 Nancy, France

A group contribution method is proposed to determine linear solvation energy relationship parameters (GCLSER) in view of estimating the gas-to-ionic liquid partition coefficients and water-to-ionic liquid partition coefficients. Large sets of partition coefficients were analyzed using the Abraham solvation model to determine the contributions of 21 groups: 12 groups characterizing the cations and 9 groups for the anions. The derived equations correlate the experimental gas-to-ionic liquid and water-to-ionic liquid partition coefficient data to within 0.15 and 0.17 log units, respectively. The 21 group-parameters can be used to predict the partition coefficients of solutes in alkyl or functionalized ionic liquids with a good accuracy. Introduction Ionic liquids (ILs) have been widely promoted as interesting substitutes for traditional industrial solvents such as volatile organic compounds. Much of the interest in ionic liquids is based on their physicochemical properties: thermal stability, low vapor pressure, and high ionic conductivity. Ionic liquids are composed of an asymmetric, bulky organic cation and a weakly coordinating organic or inorganic anion. Presently, more than 1000 different ILs are commercially available, but it has been shown that there are >1014 possible combinations.1 The type of anion and the R groups in the different cations (see Figure 1) can be used to adjust the properties of the ILs. Therefore, the possibility arises to functionalize ILs for specific applications by tuning the relevant solvent properties in a stepwise manner. With the great variety of possible combinations enabling the fine-tuning of their chemical properties, ILs have already become recognized by the chemical industry as new, target-oriented reaction media. Currently, ILs are emerging as alternative green solvents, for example, as alternative reaction media for synthesis, catalysis, and biocatalysis, but also as electrolytes, lubricants, or modifiers of mobile and stationary phases in the separation sciences.2-6 Numerous works have been devoted to a large range of applications of ILs, but the basic understanding and study of their structure-property relationships has been neglected. Few works have systematically studied the relationships between the structures of ILs and their fundamental properties.7-15 To better understand the nature of ionic liquids, knowledge of their physical properties is required. At present, however, data for many physicochemical properties of ionic liquids are lacking or are too unreliable to allow structure-property relationship studies. Gardas and Coutinho proposed a quantitative structureproperty relationship (QSPR) correlation for the estimation of surface tension of ionic liquids at 298.15 K using only information on the molecular volumes.16 Eike et al. demonstrated that the QSPR method can be used to predict values of * To whom the correspondence should be addressed. E-mail: [email protected]. Tel.: +33 3 83 17 51 31. Fax: +33 3 83 17 53 95.

activity coefficients at infinite dilution in different IL solvents.17 A quantitative structure-activity relationship (QSAR) based on a hybrid molecular QSAR model has been applied to the modeling of the aquatic ecotoxicity of ionic liquids.18 Thermodynamic properties of alkylimidazolium-based ionic liquids are relatively well described in the literature.19-25 Recently, functionalized ionic liquids such as ether- or cyanofunctionalized ionic liquids were studied by gas chromatography.26,27 A systematic study of interaction between organic compounds and ionic liquids has been done using a solvation model. In the early 1990s, Abraham et al.28-31 developed a linear solvation energy relationship (LSER) to quantify intermolecular solute-ionic liquid interactions. This method allows for the correlation of thermodynamic properties of phase-transfer processes such as retention volume and partition coefficients. The most recent representation of the LSER model is given by the equations: log KL ) c + eE + sS + aA + bB + lL

(1)

log P ) c′ + e′E + s′S + a′A + b′B + VV

(2)

The capital letters represent the solute properties, and the lowercase letters represent the complementary properties of the ionic liquids. The solute descriptors are the excess molar refraction E; the dipolarity/polarizability S; the hydrogenbond acidity and basicity A and B, respectively; and the

Figure 1. Cations of six families of ionic liquids.

10.1021/ie901776z  2010 American Chemical Society Published on Web 02/23/2010

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Table 1. List of Ionic Liquids Used for the Correlations No.

Imidazolium-Based Ionic Liquids

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

1-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide 1-methyl-3-hexylimidazolium bis(trifluoromethylsulfonyl)imide 1-methyl-3-octylimidazolium tetrafluoroborate 1-methyl-3-butylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium tetrafluoroborate 1-methyl-3-ethylimidazoliumethylsulfate 1-methyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide 1,2-dimethyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide 1-methyl-3-hexylimidazolium tetrafluoroborate 1-methyl-3-ethylimidazolium tetrafluoroborate 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide 1-methyl-3-hexylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium octylsulfate 1,2-dimethyl-3-propylimidazolium tetrafuoroborate 1-methyl-3-ethylimidazolium thyocianate 1-methyl-3-butylimidazolium trifluoromethylsulfonate 1-methyl-3-ethylimidazolium trifluoromethylsulfonate 1-methyl-3-ethylimidazolium trifluoroacetate 1-methyl-3-hexylimidazolium trifluoromethylsulfonate 1-methyl-3-octylimidazolium hexafluorophosphate 1-ethyl-3-methylimidazolium octylsulfate 1-methyl-3-butylimidazolium trifluoroacetate 1-ethanol-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-(methylethylether)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1,3- dimethoxyimidazolium bis(trifluoromethylsulfonyl)imide 1-methyl-3-ethylimidazolium dicyanamide 1-butyl-3-methylimidazolium thiocyanate

25 26 27

Ammonium-Based Ionic Liquids 28 29 30

trimethylbutylammonium bis(trifluoromethylsulfonyl)imide trioctylmethylammonium bis(trifluoromethylsulfonyl)imide trimethylhexylammonium bis(trifluoromethylsulfonyl)imide

31 32 33

4-methyl-N-butylpyridinium bis(trifluoromethylsulfonyl)imide N-ethylpyridinium bis(trifluoromethylsulfonyl)imide 4-methyl-N-butylpyridinium tetrafluoroborate

Pyridinium-Based Ionic Liquids

and water-to-ionic liquid, P, partition coefficients.35,36 Sprunger et al.37-40 modified Abraham’s solvation model by rewriting each of the six solvent equation coefficients as a summation of the respective cation and anion contributions: log KL ) ccation + canion + (ecation + eanion)E + (scation + sanion)S + (acation + aanion)A + (bcation + banion)B + (lcation + lanion)L (3) log P ) c′cation + c′anion + (e′cation + e′anion)E + (s′cation + s′anion)S + (a′cation + a′anion)A + (b′cation + b′anion)B + (Vcation + Vanion)V (4) Sprunger et al. calculated equation coefficients for 10 cations and 8 anions using a database that contained 976 experimental log KL values and 955 experimental log P values.40 No loss in predictive accuracy was observed upon separation of the equation coefficients into individual cationspecific and anion-specific values. In Sprunger et al.’s approach, the major advantage of splitting the equation coefficients into individual cation-specific and anion-specific contributions is that one can make predictions for more ILs. Most of the cations are alkylimidazolium-based. The use of this model is somewhat limited because it cannot be extrapolated to alkylimidazolium based ionic liquids that were not initially defined by the method (e.g., those with long alkyl chains). In our development, the cation with its alkyl chains is split into different contributions: CH3, CH2, N, CHcyclic, etc. This approach allows the model to be predictive. The aim of this work is to develop a group contribution method allowing for the estimation of the log KL and log P values of organic compounds in ionic liquids at 298 K. Using the LSER model proposed by Abraham, the group contribution method expresses the LSER coefficients ci, ei, si, ai, bi, and li of eq 5 or ci′, ei′, si′, ai′, bi′, and Vi of eq 6

Pyrolidinium-Based Ionic Liquids 34 35 36 37

1-butyl-3-methylpyrolidinium trifluoromethanesulfonate 1-butyl-3-methylpyrolidinium bis(trifluoromethylsulfonyl)imide 1-hexyl-3-methylpyrolidinium bis(trifluoromethylsulfonyl)imide 1-octyl-3-methylpyrolidinium bis(trifluoromethylsulfonyl)imide

21

log KL )

∑

21

nici +

i

∑

21

nieiE +

i

∑ i

i i

i

21

21

i i

i i

i

(5)

i

trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide 21

Sulfonium-Based Ionic Liquids 39

∑naA +

∑nbB + ∑nlL

Phosphonium-Based Ionic Liquids 38

21

nisiS +

triethylsulfonium bis(trifluoromethylsulfonyl)imide

gas-liquid partition coefficient on n-hexadecane at 298 K L. V is the McGowan volume in units of cm3 · mol-1/100. These solute descriptors of about 4000 solutes were measured experimentally or calculated by a group contribution method.28-34 The coefficients c, e, s, a, b, and l or c′, e′, s′, a′, b′, and V are not simply fitting coefficients, but they reflect complementary properties of the solvent phase. The c term is the model constant. The system constants are identified as the opposing contributions of cavity formation and dispersion interactions, l (or V); the contribution from interactions with lone pair electrons, e; the contribution from dipole-type interactions, s; the contribution from the hydrogen-bond basicity of the stationary phase (because a basic phase will interact with an acidic solute), a; and the contribution from the hydrogen-bond acidity of the stationary phase, b. Acree and co-workers reported mathematical correlations based on Abraham’s solvation model for the gas-to-ionic liquid, KL,

log P )

∑

21

nici′ +

i

∑ i

21

niei′E +

∑

21

nisi′S +

i

∑ n a′A + i i

i

21

21

∑ n b′B + ∑ n V V i i

i

i i

(6)

i

where ni is the number of group i present in the ionic liquid. Methodology The experimental data used to calculate Abraham’s model ion-specific equation coefficients were taken from the collection of Sprunger and co-workers40 and were updated with recent data.41-48 A total of 1450 gas-liquid partition coefficients and 1410 water-liquid partition coefficients were used for the calculation. Solutes were mainly n-alkanes, cycloalkanes, alkenes, alkynes, aromatics, alcohols, ethers, aldehydes, ketones, and chloroalkanes. The E scale varies from 0 to 1.5, the S scale from 0 to 1.72, the A scale from 0 to 1.04, the B scale from 0 to 1.28, the L scale from -1.200 to 7.833, and the V scale from 0.109 to 1.799. The list of

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Table 2. Description of the 21 Groups Used for the Estimation of log KL and log P

Table 3. Values of Group Parameters of the GC-LSER Model for the Calculation of the Gas-to-Ionic Liquid Partition Coefficients Using Eq 5 group

nga

ci

ei

si

ai

bi

li

1.152 (0.088) -0.124 (0.012) -0.461 (0.123) 0.064 (0.057) 1.584 (0.130) 0.281 (0.074) 0.381 (0.027) -0.755 (0.152) -0.521 (0.089) -1.615 (0.281) -0.386 (0.287) 0

1.031 (0.136) -0.082 (0.017) -1.235 (0.182) -0.100 (0.103) 1.357 (0.217) 0.161 (0.069) 0.371 (0.046) -0.632 (0.198) -0.315 (0.143) -1.369 (0.431) -0.549 (0.429) 0

-0.191 (0.133) 0.038 (0.018) -0.377 (0.148) 0.266 (0.077) 0.342 (0.185) 0.160 (0.074) 0.044 (0.032) 0.438 (0.183) 0.454 (0.129) 1.302 (0.420) 1.048 (0.340) 0

0.129 (0.014) 0.012 (0.002) -0.440 (0.010) -0.059 (0.013) 0 0.110 (0.010) 0.118 (0.006) -0.117 (0.023) 0.040 (0.015) 0.074 (0.047) 0.265 (0.058 0

0 -0.094 (0.115) 1.110 (0.116) 2.864 (0.219) 2.344 (0.221) 2.168 (0.218) 1.563 (0.192) 3.085 (0.271) 2.009 (0.197)

0 -0.461 (0.112) -0.255 (0.104) -0.694 (0.183) -1.007 (0.190) -0.225 (0.220) -0.253 (0.157) 0 -0.252 (0.154)

0 -0.077 (0.014) -0.054 (0.010) -0.115 (0.019) 0.076 (0.024) -0.056 (0.032) -0.021(0.020) -0.023 (0.025) -0.116 (0.022)

Cation CH3-CH2-O-O-N-OH CH2cyclic CHcyclic Ccyclic Ncyclic+ Nam+ S+ P+

3419 6614 156 98 49 244 3822 150 2457 140 32 36

-0.388 (0.050) 0.050 (0.006) 0 -0.060 (0.054) -0.713 (0.045) -0.075 (0.031) -0.105 (0.020) 0.217 (0.078) 0.247 (0.055) 0.687 (0.172) 0.390 (0.202) 0

0 0 0.125 (0.019) 0 0 -0.077 (0.054) 0 0.484 (0.102) 0 0 0 0

(Tf)2NPF6BF4EtSO4OcSO4SCNCF3SO3AcF3(CN)2N-

833 108 225 53 58 59 105 30 50

0 -0.048 (0.036) -0.199 (0.034) -0.131 (0.079) 0.349 (0.093) -0.683(0.111) -0.263 (0.061) -0.299 (0.078) -0.319 (0.086)

0 -0.185 (0.078) 0.075 (0.072) 0 -0.1309 (0.111) 0 0.180 (0.142) 0 0.302 (0.135)

Anion

a

0 0.611 (0.091) 0.323 (0.091) 0.275 (0.133) -0.113 (0.138) 1.349 (0.138) 0.302 (0.168) 0.422 (0.165) 0.685 (0.134)

Number of occurrences of each of the 21 groups in the log KL data set.

ionic liquids used for the correlations is given in Table 1. The data set consisted of 27 imidazolium-based ionic liquids, as well as 3 ammonium-, 3 pyridinium-, and 4 pyrolidinium-

based ionic liquids. We also included sulfonium and phosphonium ionic liquids, although only one set of KL (or P) data could be found for these families.

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Table 4. Values of Group Parameters of the GC-LSER Model for the Calculation of the Water-to-Ionic Liquid Partition Coefficients Using Eq 6 group

nga

ci′

ei′

si′

ai′

bi′

Vi

CH3-CH2-O-O-N-OH CH2cyclic CHcyclic Ccyclic Ncyclic+ Nam+ S+ P+

3188 5809 45 90 45 240 3510 147 2253 134 31 36

-0.253 (0.074) 0.037 (0.009) 0.158 (0.180) -0.172 (0.089) -0.677 (0.182) -0.052 (0.044) -0.085 (0.030) 0.219 (0.113) 0.300 (0.082) 0.740 (0.250) 0.596 (0.289) 0

0.187 (0.029) -0.035 (0.008) -0.730 (0.163) -0.189 (0.085) 0 -0.246 (0.065) 0 0 0 -0.418 (0.136) -1.921 (0.426) 0

Cation 0.325 (0.087) -0.045 (0.013) -0.496 (0.175) 0.267 (0.089) 0.751 (0.165) 0.381 (0.093) 0.151 (0.031) -0.165 (0.126) -0.395 (0.083) -0.731 (0.294) 1.963 (0.601) 0

0 -0.046 (0.009) -1.091 (0.231) 0.108 (0.116) 0.578 (0.229) -0.079 (0.089) -0.202 (0.055) -0.403 (0.155) -0.319 (0.089) -1.136 (0.159) -1.440 (0.368) 0

-1.13 (0.067) -0.028 (0.012) -0.464 (0.171) 0.098 (0.090) -0.641 (0.159) -0.683 (0.095) -0.592 (0.031) 0.775 (0.132) 0 0.496 (0.243) -1.833 (0.435) 0

0.668 (0.065) 0.049 (0.008) -1.384 (0.150) -0.089 (0.071) 0.375 (0.150) 0.515 (0.041) 0.572 (0.028) -0.487 (0.101) 0.110 (0.074) 0.176 (0.217) 1.061 (0.252) 0

(Tf)2NPF6BF4EtSO4OcSO4SCNCF3SO3AcF3(CN)2N-

735 104 222 53 56 58 104 30 48

0 -0.063 (0.056) -0.156 (0.050) 0 0 -0.427 (0.157) -0.139 (0.090) -0.142 (0.123) -0.200 (0.132)

0 -0.119 (0.096) 0.086 (0.086) -0.352 (0.185) -0.080 (0.088) -1.419 (0.316) 0 -1.456 (0.418) 0

Anion 0 0.493 (0.108) 0.250 (0.107) 0.314 (0.209) 0 2.908 (0.419) 0.329 (0.110) 1.878 (0.498) 0.679 (0.135)

0 0 1.210 (0.122) 2.814 (0.258) 2.528 (0.221) 2.129 (0.313) 1.318 (0.213) 2.043 (0.434) 2.152 (0.217)

0 -0.452 (0.114) -0.295 (0.114) -0.877 (0.252) -1.039 (0.144) -1.187 (0.352) -0.153 (0.146) 0 -0.408 (0.158)

0 -0.222 (0.065) -0.216 (0.049) -0.397 (0.039) 0.595 (0.050) -0.347 (0.135) -0.183 (0.084) -0.163 (0.119) -0.433 (0.113)

a

Number of occurrences of each of the 21 groups in the log P data set. Table 5. Distribution of Residual Differences between the Experimental and Calculated Gas-to-Ionic Liquid Partition Coefficients residual

N

%

1

1098 339 12 1

75.7 23.4 0.9 0.0

is the molar volume of the solvent. The log P values for partition from water to the ionic liquid were calculated with the equation log P ) log KL - log KW

Figure 2. Plot of experimental log KL data versus calculated values based on eq 5.

(8)

where KW is the solute’s gas-phase partition coefficient into water. The 21 groups defined in this study are listed in Table 2. The decomposition into groups of the ionic liquids is very easy, that is, as simple as possible. No substitution effects are considered. No exceptions are defined. In Figure 1 are represented all ionic liquids studied in this work. Five groups are

Figure 3. Plot of experimental log P data versus calculated values based on eq 6.

Partition coefficients KL at 298 K were calculated from the ∞ experimental activity coefficients at infinite dilution, γ1,2 , using the equation KL )

RT γ∞1,2P01Vsolvent

(7)

In eq 7, R is the gas constant, T is the system temperature, P10 is the vapor pressure of the solute at temperature T, and Vsolvent

Figure 4. Mean absolute error observed with the ionic liquids studied in this work.

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Table 6. Prediction of log KL at 298.15 K for Organic Compounds in Three Ionic Liquids Using the GC-LSER Model solute

E

S

A

n-pentane hexane heptane octane nonane decane cyclopentane cyclohexane cycloheptane cyclooctane benzene toluene ethylbenzene m-xylene p-xylene o-xylene pent-1-ene 1-hexene hept-1-ene oct-1-ene pent-1-yne 1-hexyne 1-heptyne oct-1-yne methanol ethanol 1-propanol 1-butanol thiophene water tetrahydrofuran methyl tert-butyl ether

0 0 0 0 0 0 0.263 0.305 0.35 0.413 0.61 0.601 0.613 0.623 0.613 0.663 0.093 0.078 0.092 0.094 0.172 0.166 0.16 0.155 0.278 0.246 0.236 0.224 0.687 0 0.289 0.024

hexane heptane octane nonane cyclohexane benzene toluene ethylbenzene m-xylene p-xylene o-xylene 1-hexene 1-hexyne 1-heptyne 2-butanone 2-pentanone 1,4-dioxane methanol ethanol 1-propanol 2-propanol 2-methyl-1-propanol 1-butanol ether chloroforme dichloromethane tetrachloromethane acetonitrile 1-nitropropane pyridine thiophene

0 0 0 0 0.305 0.61 0.601 0.613 0.623 0.613 0.663 0.078 0.166 0.16 0.166 0.143 0.329 0.278 0.246 0.236 0.212 0.217 0.224 0.041 0.425 0.387 0.458 0.237 0.242 0.631 0.687

1-Hexadecyl-3-methylimidazolium 0 0 0 0 0 0 0 0 0.1 0 0.52 0 0.52 0 0.51 0 0.52 0 0.52 0 0.56 0 0.08 0 0.22 0.1 0.23 0.09 0.7 0 0.68 0 0.75 0 0.44 0.43 0.42 0.37 0.42 0.37 0.36 0.33 0.39 0.37 0.42 0.37 0.25 0 0.49 0.15 0.57 0.1 0.38 0 0.90 0.07 0.95 0 0.84 0 0.57 0

heptane 2,2,4-trimethylpentane octane nonane decane cyclohexane methylcyclohexane cycloheptane benzene toluene ethylbenzene

0 0 0 0 0 0.305 0.244 0.35 0.61 0.601 0.613

B

L

log KL,exp

log KL,pred

0.742 1.198 1.588 1.958 2.346 2.651 1.321 1.694 2.124 2.734 2.828 3.173 3.417 3.515 3.498 3.689 1.054 1.481 1.853 2.204 1.934 2.320 2.646 2.983 3.024 3.197 3.552 3.965 3.036 3.712 2.705 1.974

0.667 1.012 1.328 1.701 2.046 2.391 1.187 1.528 2.041 2.479 2.596 2.974 3.264 3.338 3.336 3.517 0.842 1.198 1.536 1.882 1.724 1.958 2.269 2.598 2.978 3.036 3.407 3.794 2.778 3.986 2.598 1.618

Tetrafluoroborate 0 2.668 0 3.13 0 3.677 0 4.182 0 2.964 0.14 2.768 0.14 3.325 0.15 3.778 0.16 3.839 0.16 3.839 0.16 3.939 0.07 2.572 0.12 2.51 0.1 3 0.51 2.287 0.51 2.755 0.64 2.892 0.47 0.97 0.48 1.485 0.48 2.031 0.56 1.764 0.48 2.413 0.48 2.601 0.45 2.015 0.02 2.48 0.05 2.019 0 2.833 0.32 1.739 0.31 2.894 0.52 3.022 0.15 2.819

2.334 2.797 3.296 3.854 2.628 2.974 3.431 3.810 3.907 3.891 4.037 2.325 2.612 2.947 2.827 3.196 3.311 2.581 2.830 3.231 2.954 3.540 3.751 1.862 2.903 2.466 2.728 2.960 3.500 3.595 2.951

2.136 2.514 2.962 3.375 2.488 2.852 3.307 3.679 3.748 3.747 3.868 2.201 2.570 2.935 2.941 3.305 3.618 2.650 2.916 3.362 3.070 3.647 3.827 2.260 2.823 2.420 2.636 2.657 3.466 3.708 2.952

1-Ethanol-3-methylimidazolium Hexafluorophosphate 0 0 0 3.173 0 0 0 3.106 0 0 0 3.677 0 0 0 4.182 0 0 0 4.686 0.1 0 0 2.964 0.1 0 0 3.323 0.1 0 0 3.704 0.52 0 0.14 2.768 0.52 0 0.14 3.325 0.51 0 0.15 3.778

0.553 0.533 0.862 1.228 1.624 0.621 0.723 1.068 2.180 2.442 2.564

0.748 0.714 1.005 1.263 1.520 0.905 1.100 1.274 2.206 2.491 2.696

1-Ethyl-3-methylimidazolium Trifluoromethylsulfonate 0 0 0 2.162 0 0 0 2.668 0 0 0 3.13 0 0 0 3.677 0 0 0 4.182 0 0 0 4.686 0.1 0 0 2.477 0.1 0 0 2.964 0.1 0 0 3.704 0.1 0 0 4.329 0.52 0 0.14 2.768 0.52 0 0.14 3.325 0.51 0 0.15 3.778 0.52 0 0.16 3.839 0.52 0 0.16 3.839 0.56 0 0.16 3.939 0.08 0 0.07 2.047 0.08 0 0.07 2.572 0.08 0 0.07 3.063 0.08 0 0.07 3.568 0.23 0.12 0.12 2.01 0.22 0.1 0.12 2.51 0.23 0.09 0.1 3 0.22 0.09 0.1 3.521 0.44 0.43 0.47 0.97 0.42 0.37 0.48 1.485 0.42 0.37 0.48 2.031 0.42 0.37 0.48 2.601 0.57 0 0.15 2.819 0.45 0.82 0.35 0.26 0.52 0 0.48 2.636 0.21 0 0.59 2.372

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Table 6. Continued solute

E

S

A

B

L

log KL,exp

log KL,pred

m-xylene p-xylene o-xylene 1-hexyne 1-heptyne 2-butanone 2-pentanone 3-pentanone 1,4-dioxane methanol ethanol 1-propanol 2-propanol 2-methyl-1-propanol 1-butanol diisopropyle ether chloroforme dichloromethane tetrachloromethane acetonitrile nitromethane thiophene formaldehyde propionaldehyde butyraldehyde

0.623 0.613 0.663 0.166 0.16 0.166 0.143 0.154 0.329 0.278 0.246 0.236 0.212 0.217 0.224 0 0.425 0.387 0.458 0.237 0.313 0.687 0.22 0.196 0.187

0.52 0.52 0.56 0.23 0.23 0.7 0.68 0.66 0.75 0.44 0.42 0.42 0.36 0.39 0.42 0.19 0.49 0.57 0.38 0.90 0.95 0.57 0.73 0.65 0.65

0 0 0 0.12 0.12 0 0 0 0 0.43 0.37 0.37 0.33 0.37 0.37 0 0.15 0.1 0 0.07 0.06 0 0 0 0

0.16 0.16 0.16 0.1 0.1 0.51 0.51 0.51 0.64 0.47 0.48 0.48 0.56 0.48 0.48 0.41 0.02 0.05 0 0.32 0.31 0.15 0.33 0.45 0.45

3.839 3.839 3.939 2.51 3 2.287 2.755 2.811 2.892 0.97 1.485 2.031 1.764 2.413 2.601 2.482 2.48 2.019 2.833 1.739 1.892 2.819 0.73 1.815 2.27

2.685 2.696 2.909 1.373 1.571 2.875 2.986 2.920 3.556 2.734 2.805 2.978 2.776 3.013 3.210 1.210 2.017 1.923 1.398 3.326 3.639 2.470 2.079 2.558 2.672

2.766 2.767 2.937 1.510 1.761 2.917 3.096 3.058 3.460 2.484 2.540 2.820 2.456 2.922 3.113 1.334 2.292 2.214 1.706 3.294 3.484 2.385 2.064 2.462 2.696

defined to describe the chains R1, R2, R3, and R4 grafted on the cation: CH3, CH2, -O-, -O-Ncyclic, and -OH. These groups allow for the calculation of partition coefficients of alkyl-based ionic liquids as well as functionalized ionic liquids such as ethers and alcohols. The remaining seven groups are CH2cyclic, CHcyclic, Ccyclic, Ncyclic, N+ (ammonium cation), P+ (phosphonium cation), and S+ (sulfonium cation). More precisely, Ncyclic represents two structures:

Nine groups are used for anions: bis(trifluoromethylsulfonyl)imide, (TF)2N-; hexafluorophosphate, PF6-; tetrafluoroborate, BF4-; ethylsulfate, EtSO4-; octylsulfate, OcSO4-; thiocyanate, SCN-; trifluoromethylsulfonate, CF3SO3-; trifluoroacetate, ACF3-; and dicyanamide, (CN)2N-. As an example, consider the decomposition of 1-butyl-3methylimidazolium hexafluorophosphate. In this case, the decomposition of the molecule into elementary groups is as follows: two group 1 (-CH3) + three group 2 (-CH2) + three group 7 (Ccyclic) + two group 9 (Ncyclic) + one group 14 (PF6-). Once the decomposition has been done, the matrices required to determine the LSER parameters of each group are built using the solute descriptors E, S, A, B, L, and V weighted by the number of groups present in each ionic liquids. The values are arranged in a 126 column × 1450 row matrix for log KL and in a 126 column × 1410 row matrix for log P. The group parameters were determined in order to minimize the deviations between calculated and experimental log KL (or log P) data. As proposed by Sprunger and co-workers, we have set the anion-specific equation coefficients of (TF)2N- equal to zero.40 Regression analyses were performed using Minitab software. Results and Discussion GC-LSER Correlation. In this article, we report a correlation of log KL and log P that differs from Sprunger et al.’s LSER in two important regards: first, the database has been expanded to include 39 ionic liquids such as imidazolium, pyridinium, ammonium, pyrrolidinium, sulfonium, and phosphonium. Second, the ionic liquids are decomposed so that the model can be

used as a purely predictive model. Important requirements that need to be set for an LSER to correctly represent the log KL (or log P) data are a high correlation coefficient and a low standard deviation. We first give the correlations for the total data set. Analysis of the 1450 experimental log KL values gave the contribution of each group for the calculation of LSER coefficients ci, ei, si, ai, bi, and li needed in eq 5 with a standard deviation (SD) of 0.155, a squared correlation coefficient (R2) of 0.997, and a Fisher’s F statistic (F) of 4817. The data set of experimental log P values was also used to determine each group contribution for the calculation of LSER coefficients ci′, ei′, si′, ai′, bi′, and Vi of eq 6. Equation 6 is statistically good, with a standard deviation of 0.173 log units for 1410 experimental data and a Fisher’s F statistic of F ) 2756. The contributions of the (′) various groups for the calculation of LSER coefficients c(′) i , ei , (′) (′) (′) si , ai , bi , li, and Vi needed in eqs 5 and 6 are given in Tables 3 and 4, respectively. The phosphonium LSER parameters were set to zero because their estimated values were close to zero and had a high uncertainty. Plots of calculated values of log KL and log P based on eqs 5 and 6, respectively, against the observed values are presented in Figures 2 and 3, respectively. A few experimental measurements are not well represented by the correlations. In these cases, we suspect not the quality of the GC-LSER, but rather the quality of the experimental data. The standard errors in the coefficients are given in parentheses with the respective values. Larger standard errors are observed for groups for which the experimental data are limited. Both correlations were found to be statistically very good and to describe an experimental log KL (log P) database that covers a 12.6 log (9.5 log) unit range to within standard deviations of 0.153 log units (eq 5) (0.173 log units (eq. 6)). The distribution of the residuals is given in Table 5. Residuals were calculated as the difference between the observed and the calculated of the logarithm of the gas-to-ionic liquid partition coefficient. It can be seen that about 76% of the residuals are lower than 0.1. It is good to note that the model is probably somewhat limited in prediction for sulfonium- and phosphonium-based ionic liquids because the data set for log KL is relatively poor in those cases. Therefore, groups 11 and 12 should be used to obtain approximate values of log KL. Examination of the residuals and visual analysis of Figure 4 show two outliers. It can be observed that 1-propyl-2,3-

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Table 7. Prediction of log P at 298.15 K for Organic Compounds in Three Ionic Liquids Using the GC-LSER solute

E

S

A

n-pentane hexane heptane octane nonane decane cyclopentane cyclohexane cycloheptane cyclooctane benzene toluene ethylbenzene m-xylene p-xylene o-xylene pent-1-ene 1-hexene hept-1-ene oct-1-ene pent-1-yne 1-hexyne 1-heptyne oct-1-yne methanol ethanol 1-propanol 1-butanol thiophene tetrahydrofuran methyl tert-butyl ether

0 0 0 0 0 0 0.26 0.31 0.35 0.41 0.61 0.6 0.61 0.62 0.61 0.66 0.09 0.08 0.09 0.09 0.17 0.17 0.16 0.16 0.28 0.25 0.24 0.22 0.69 0.29 0.02

hexane heptane octane nonane cyclohexane benzene toluene ethylbenzene m-xylene p-xylene o-xylene 1-hexene 1-hexyne 1-heptyne 2-butanone 2-pentanone 1,4-dioxane methanol ethanol 1-propanol 2-propanol 2-methyl-1-propanol 1-butanol ether chloroforme dichloromethane tetrachloromethane acetonitrile 1-nitropropane pyridine thiophene

0 0 0 0 0.31 0.61 0.6 0.61 0.62 0.61 0.66 0.08 0.17 0.16 0.17 0.14 0.33 0.28 0.25 0.24 0.21 0.22 0.22 0.04 0.43 0.39 0.46 0.24 0.24 0.63 0.69

1-Hexadecyl-3-methylimidazolium 0 0 0 0 0 0 0 0 0.1 0 0.52 0 0.52 0 0.51 0 0.52 0 0.52 0 0.56 0 0.08 0 0.22 0.1 0.23 0.09 0.7 0 0.68 0 0.75 0 0.44 0.43 0.42 0.37 0.42 0.37 0.36 0.33 0.39 0.37 0.42 0.37 0.25 0 0.49 0.15 0.57 0.1 0.38 0 0.90 0.07 0.95 0 0.84 0 0.57 0

heptane 2,2,4-trimethylpentane octane nonane decane cyclohexane methylcyclohexane cycloheptane benzene toluene ethylbenzene m-xylene p-xylene

0 0 0 0 0 0.31 0.24 0.35 0.61 0.6 0.61 0.62 0.61

B

V

log Pexp

log Ppred

2.442 3.018 3.548 4.068 4.496 4.911 2.201 2.594 2.704 3.364 2.198 2.523 2.837 2.905 2.908 2.949 2.284 2.641 3.074 3.614 1.944 2.530 3.086 3.503 -0.716 -0.473 -0.008 0.505 1.996 0.155 0.354

2.288 2.731 3.173 3.616 4.058 4.500 2.097 2.553 3.010 3.474 1.912 2.351 2.749 2.716 2.713 2.754 1.937 2.375 2.822 3.265 1.712 2.145 2.676 3.110 -0.909 -0.533 -0.095 0.343 1.689 0.073 0.119

Tetrafluoroborate 0 0.954 0 1.095 0 1.236 0 1.377 0 0.845 0.14 0.716 0.14 0.857 0.15 0.998 0.16 0.998 0.16 0.998 0.16 0.998 0.07 0.911 0.12 0.868 0.1 1.009 0.51 0.688 0.51 0.829 0.64 0.681 0.47 0.308 0.48 0.449 0.48 0.59 0.56 0.59 0.48 0.731 0.48 0.731 0.45 0.731 0.02 0.617 0.05 0.494 0 0.739 0.32 0.404 0.31 0.706 0.52 0.675 0.15 0.641

4.154 4.757 5.406 6.004 3.528 2.344 2.781 3.230 3.297 3.301 3.297 3.485 2.822 3.387 0.107 0.616 -0.399 -1.159 -0.840 -0.329 -0.526 0.240 0.291 0.692 2.113 1.506 2.918 0.110 1.369 0.155 1.911

3.855 4.389 4.924 5.458 3.412 2.191 2.726 3.213 3.163 3.164 3.156 3.345 2.851 3.487 0.334 0.872 -0.326 -1.205 -0.671 -0.136 -0.479 0.403 0.399 0.840 2.290 1.713 2.968 0.083 1.318 0.192 1.847

1-Ethanol-3-methylimidazolium Hexafluorophosphate 0 0 0 1.095 0 0 0 1.236 0 0 0 1.236 0 0 0 1.377 0 0 0 1.518 0.1 0 0 0.845 0.1 0 0 0.845 0.1 0 0 0.986 0.52 0 0.14 0.716 0.52 0 0.14 0.857 0.51 0 0.15 0.998 0.52 0 0.16 0.998 0.52 0 0.16 0.998

2.513 2.653 2.972 3.378 3.884 1.521 1.973 1.648 1.550 1.792 1.984 2.075 2.106

2.552 2.954 2.955 3.357 3.760 1.953 1.953 2.355 1.496 1.899 2.249 2.220 2.220

1-Ethyl-3-methylimidazolium Trifluoromethylsulfonate 0 0 0 0.813 0 0 0 0.954 0 0 0 1.095 0 0 0 1.236 0 0 0 1.377 0 0 0 1.518 0.1 0 0 0.705 0.1 0 0 0.845 0.1 0 0 0.986 0.1 0 0 1.127 0.52 0 0.14 0.716 0.52 0 0.14 0.857 0.51 0 0.15 0.998 0.52 0 0.16 0.998 0.52 0 0.16 0.998 0.56 0 0.16 0.998 0.08 0 0.07 0.77 0.08 0 0.07 0.911 0.08 0 0.07 1.052 0.08 0 0.07 1.193 0.23 0.12 0.12 0.727 0.22 0.1 0.12 0.868 0.23 0.09 0.1 1.009 0.22 0.09 0.1 1.15 0.44 0.43 0.47 0.308 0.42 0.37 0.48 0.449 0.42 0.37 0.48 0.59 0.42 0.37 0.48 0.731 0.57 0 0.15 0.641 0.52 0 0.48 0.622 0.21 0 0.59 0.872

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Table 7. Continued solute

E

S

A

B

V

log Pexp

log Ppred

o-xylene 1-hexyne 1-heptyne 2-butanone 2-pentanone 3-pentanone 1,4-dioxane methanol ethanol 1-propanol 2-propanol 2-methyl-1-propanol 1-butanol ether diisopropyle ether chloroform dichloromethane tetrachloromethane acetonitrile nitromethane thiophene propionaldehyde butyraldehyde

0.66 0.17 0.16 0.17 0.14 0.15 0.33 0.28 0.25 0.24 0.21 0.22 0.22 0.04 0 0.43 0.39 0.46 0.24 0.31 0.69 0.2 0.19

0.56 0.23 0.23 0.7 0.68 0.66 0.75 0.44 0.42 0.42 0.36 0.39 0.42 0.25 0.19 0.49 0.57 0.38 0.9 0.95 0.57 0.65 0.65

0 0.12 0.12 0 0 0 0 0.43 0.37 0.37 0.33 0.37 0.37 0 0 0.15 0.1 0 0.07 0.06 0 0 0

0.16 0.1 0.1 0.51 0.51 0.51 0.64 0.47 0.48 0.48 0.56 0.48 0.48 0.45 0.41 0.02 0.05 0 0.32 0.31 0.15 0.45 0.45

0.998 0.868 1.009 0.688 0.829 0.829 0.681 0.308 0.449 0.59 0.59 0.731 0.731 0.731 0.731 0.617 0.494 0.739 0.404 0.424 0.641 0.547 0.688

2.249 1.583 2.011 0.155 0.406 0.420 -0.154 -1.006 -0.865 -0.582 -0.704 -0.287 -0.250 0.934 0.160 1.227 0.963 1.588 0.476 0.689 1.430 0.038 0.342

2.265 1.670 2.073 0.121 0.500 0.478 -0.369 -1.424 -1.040 -0.637 -1.000 -0.269 -0.235 -0.027 0.067 1.551 1.209 1.969 0.257 0.417 1.298 -0.095 0.307

dimethylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium octylsulfate present large mean absolute errors (about 0.250). For 1-ethyl-3-methylimidazolium octylsulfate, the gasto-liquid partition coefficients log KL for heptane and hexadecane were calculated from liquid-liquid equilibria from the literature.49 Whereas the calculated value of log KL of heptane is in good agreement with the literature, the predicted value for hexadecane is overestimated (0.45 log unit). This large deviation is probably of the same magnitude as the experimental measure. Indeed, it is well-known that n-alkanes have low solubilities in ionic liquids, particularly those with long alkane chains. Concerning 1-propyl-2,3-dimethylimidazolium tetrafluoroborate, this large deviation seems to indicate that these measurements are spurious. Results obtained from this research group for 1-butyl-3-methylimidazolium tetrafluoroborate already disagreed with different sources coming from the literature. To test the robustness of the GC-LSER correlations, the database was divided into a training set and a test set. Both sets were built randomly by selecting one-half of the database (725 log KL and 705 log P). All groups are represented in each set. Correlations obtained with the training set are summarized in Tables 1S and 2S (Supporting Information). Results obtained with 725 experimental log KL values gave SD ) 0.161, R2 )0.997, F ) 2457. The values of each group needed for the calculation of LSER parameter obtained with the training set were used to estimate log KL of the test set. For the test set, we found a standard deviation of SD ) 0.180 and an average absolute error of 0.122. A similar approach was used for the log P correlation. The results obtained with the training set of log P values gave SD ) 0.172, R2 ) 0.996, and F ) 1623. For the test set, we found a standard deviation of SD ) 0.202 and an average absolute error of 0.151. The results demonstrate that the LSER method coupled with a group contribution method can be applied to represent partition coefficients. Use of the GC-LSER Model to Predict the Properties of ILs Not Included in the Database. An important aspect of the proposed method is its predictive power, which can be appreciated with three ionic liquids not taken into account in our database: 1-ethyl-3-methylimidazolium trifluoromethylsulfonate,50 1-hexadecyl-3-methylimidazolium tetrafluoroborate,51 and 1-ethanol-3-methylimidazolium hexafluorophosphate.13 This training set consists of 130 log KL and 126 log P values. Experimental data and results are summarized in Tables 6 and 7.

1-Hexadecyl-3-methylimidazolium Tetrafluoroborate. This ionic liquid is particularly interesting for the evaluation of the GC-LSER model. Indeed, the heaviest alkyl chain grafted in ionic liquids of the database is an octyl chain. First, the mean absolute errors (0.150 log units for log KL and 0.153 log units for log P) observed with this ionic liquid are close to the mean absolute errors of the database (0.107). A significant deviation is observed for n-alkanes increasing with the carbon number. The difference between experimental and calculated log KL values for polar compounds is below 0.1 log unit. These results confirm that the GC-LSER model can be extrapolated to long alkylimidazolium-based ionic liquids. 1-Ethanol-3-methylimidazolium Hexafluorophosphate. The mean absolute errors in log KL and log P for 35 organic compounds are 0.145 and 0.185, respectively. Series of alkanes, aromatics, ketones, and aldehydes are well represented, with absolute errors lower than 0.10 log units. The GC-LSER model has some difficulties in estimating with good accuracy polar compounds such as light alcohols or chloroalkanes. Nevertheless, the parameters of the -OH group were determined with only one ionic liquid in the data set. New measurements of partition coefficients on alcohol-functionalized ionic liquids are necessary to increase the predictive power of this model. 1-Ethyl-3-methylimidazolium Trifluoromethylsulfonate. Thirtyone experimental values for log KL and 30 values for log P were predicted using the GC-LSER approach. The mean absolute errors in log KL and log P are 0.220 and 0.244, respectively. In most cases, the values of log KL are underestimated. A study of the results indicates that light hydrocarbons exhibit larger deviations. This data set comes from inverse gas chromatography. In gas chromatography, it is well-known that light hydrocarbons are susceptible to adsorption. If retention data are not corrected for this adsorption, the partition coefficient can be over- or underestimated. The results obtained in the prediction of log KL (or log P) for organic compounds in these three ionic liquids show that the GC-LSER model can be used with good accuracy. Indeed, the accuracy of the experimental data, about 3-5%, is of the some magnitude as the accuracy of the GC-LSER model. Conclusions A group contribution model coupled with an LSER (GCLSER) for estimating gas-to-ionic liquid and water-to-ionic

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liquid partition coefficients was proposed. The GC-LSER model allows for the prediction, with good accuracy, of the log KL and log P values at 298 K of not only alkyl-based ionic liquids but also functionalized ionic liquids. The parameters of the group contribution methods were determined for imidazolium-, pyridinium-, pyrrolidinium-, phosphonium-, ammonium-, and sulfonium-based ionic liquids containing several different anions. A comparison between the experimental and calculated values showed that the proposed model describes the experimental data available with a mean absolute error of about 0.15 log unit. Much more experimental data will have to be collected in order to increase the predictive power of the model. Supporting Information Available: Correlations obtained with the training set. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Meindersma, G. W.; Galan` San`chez, L. M.; Hansmeier, A. R.; De Haan, A. B. Application of task-specific ionic liquids for intensified separations. Monatsh. Chem. 2007, 138, 1125–1136. (2) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: New York, 2003. (3) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. ReV. 1999, 99, 2071–2083. (4) Brennecke, J. F.; Maginn, E. J. Ionic liquids: Innovative fluids for chemical processing. AIChE J. 2001, 47, 2384–2389. (5) Alonso, L.; Arce, A.; Francisco, M.; Soto, A. Solvent extraction of thiophene from n-alkanes (C7, C12, and C16) using the ionic liquid [C8mim][BF4]. J. Chem. Thermodyn. 2008, 40, 966–972. (6) Roth, M. Partitioning behaviour of organic compounds between ionic liquids and supercritical fluids. J. Chromatogr. A 2009, 1216, 1861–1880. (7) Gardas, R. L.; Coutinho, J. A. P. Extension of the Ye and Shreeve group contribution method for density estimation of ionic liquids in a wide range of temperatures and pressures. Fluid Phase Equilib. 2008, 263, 26– 32. (8) Katritzky, A. R.; Lobanov, V. S.; Karelson, M. QSPR: The correlation and quantitative prediction of chemical and physical properties from structure. Chem. Soc. ReV. 1995, 24, 279–287. (9) Wasserscheid, P.; Keim, W. Ionic liquidssNew ‘solutions’ for transition metal catalysis. Angew. Chem., Int. Ed. 2000, 39, 3773–3789. (10) Eike, D. M.; Brennecke, J. F.; Maginn, E. J. Predicting melting points of quaternary ammonium ionic liquids. Green Chem. 2003, 5, 323– 328. (11) Abraham, M. H.; Acree, W. E., Jr. Comparative analysis of solvation and selectivity in room temperature ionic liquids using the Abraham linear free energy relationship. Green Chem. 2006, 8, 906–915. (12) Trohalaki, S.; Pachter, R.; Drake, G. W.; Hawkins, T. Quantitative structure-property relationships for melting points and densities of ionic liquids. Energy Fuels 2005, 19, 279–284. (13) Revelli, A.-L.; Mutelet, F.; Jaubert, J.-N. Partition coefficients of organic compounds in new imidazolium based ionic liquids using inverse gas chromatography. J. Chromatogr. A 2009, 1216, 4775–4786. (14) Gardas, R. L.; Coutinho, J. A. P. Group contribution methods for the prediction of thermophysical and transport properties of ionic liquids. AIChE J. 2009, 55, 1274–1290. (15) Gardas, R. L.; Coutinho, J. A. P. A group contribution method for viscosity estimation of ionic liquids. Fluid Phase Equilib. 2008, 266, 195– 201. (16) Gardas, R. L.; Coutinho, J. A. P. Applying a QSPR correlation to the prediction of surface tensions of ionic liquids. Fluid Phase Equilib. 2008, 265, 57–65. (17) Eike, D. M.; Brennecke, J. F.; Maginn, E. J. Predicting InfiniteDilution Activity Coefficients of Organic Solutes in Ionic Liquids. Ind. Eng. Chem. Res. 2004, 43, 1039–1048. (18) Irabien, A.; Garea, A.; Luis, P. Hybrid Molecular QSAR Model for Toxicity Estimation: Application to Ionic Liquids. Comput.-Aided Chem. Eng. 2009, 26, 63–67. (19) Krummen, M.; Wasserscheid, P.; Gmehling, J. Measurement of Activity Coefficients at Infinite Dilution in Ionic Liquids Using the Dilutor Technique. J. Chem. Eng. Data 2002, 7, 1411–1417. (20) Letcher, T. M.; Soko, B.; Ramjugernath, D.; Deenadayalu, N.; Nevines, A.; Naicker, P. K. Activity Coefficients at Infinite Dilution of

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ReceiVed for reView November 10, 2009 ReVised manuscript receiVed January 13, 2010 Accepted January 20, 2010 IE901776Z