Characterization of Room-Temperature Ionic Liquids by the Abraham Model with Cation-Specific and Anion-Specific Equation Coefficients Laura Sprunger, ² Michael Clark, ‡ William E. Acree Jr.,* ,² and Michael H. Abraham § Department of Chemistry, University of North Texas, P.O. Box 305070, Denton, Texas 76203-5070, Department of Research and Statistical Support, University of North Texas, P.O. Box 305398, Denton, Texas 76203-5398, and Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K. Received January 31, 2007 Gas-to-RTIL (room-temperature ionic liquid) partition coefficients have been compiled for 592 different solute-RTIL combinations. These partition coefficients were converted into water-to-RTIL partition coefficients using the corresponding gas-to-water partition coefficients. Both sets of partition coefficients were analyzed using the Abraham solvation parameter model with cation-specific and anion-specific equation coefficients. The derived equations correlated the experimental gas-to-RTIL and water-to-RTIL partition coefficient data to within 0.10 and 0.14 log units, respectively. The 8 sets of calculated cation-specific equation coefficients and 4 sets of calculated anion-specific equation coefficients can be combined to yield expressions capable of predicting the partition coefficients of solutes in 32 different RTILs. INTRODUCTION Room-temperature ionic liquids (RTILs) have become an increasingly popular solvent choice in the past decade as a reaction media for synthesis of potential new pharmaceutical drug molecules, biomolecules, and polymers, 1-3 as reservoirs for the controlled release of drug molecules in pharmaceutical formulations, 4 and as an extraction solvent for the removal of sulfur compounds 5 and organic contaminants from petro- leum crude oils and soil samples, 6 respectively. Immiscibility of ionic liquids with supercritical carbon dioxide, linear hydrocarbons, and several acyclic organic solvents makes RTILs ideally suited for synthetic preparations involving biphasic catalysis. 7 Most (if not all) of these synthetic methods have been performed in RTILs. Much higher product yields and shorter reaction times were reported for RTILs than for the more conventional organic solvents. Currently synthetic procedures are known for making more than 200 different RTILs. 8,9 Considerable time and effort is devoted to developing ionic liquids having specific solvent characteristics. From a theoretical standpoint it should be possible to control and modify the physical properties (viscosity, density, conductivity) and solubilibizing properties (hydrophobicity, hydrophilicity, hydrogen-bonding character) of RTILs by employing different cation-anion combinations. Our understanding of RTILs has not reached the point where this goal is realized. Several recent papers have reported mathematical expres- sions for correlating and estimating infinite dilution activity coefficients and partition coefficients in select RTILs. Eike et al. 10 proposed quantitative structure-property relationship (QSAR) correlations for the published infinite dilution activity of organic solutes in 4-methyl-N-butylpyridinium tetrafluoroborate ([BMPy] + [BF 4 ] - ), 1-methyl-3-ethylimida- zolium bis(trifluoromethylsulfonyl)imide ([MEIm] + [(Tf) 2 N] - ), and 1,2-dimethyl-3-ethylimidazolium bis(trifluoromethylsul- fonyl)imide ([M 2 EIm] + [(Tf) 2 N] - ) at 298 K that had squared correlation coefficients ranging from R 2 ) 0.952 to R 2 ) 0.975. Ta ¨mm and Burke 11 later reanalyzed the experimental data and reported three-parameter correlations based on molecular descriptors obtained using CODESSA PRO soft- ware. The authors’ three-parameter correlations described the published experimental data to within standard errors from SE ) 0.30 to SE ) 0.35 log units. Acree, Abraham, and co-workers reported mathematical correlations based on the general Abraham solvation parameter model for the gas-to- solvent, K, and water-to-solvent partition coefficients, P, for ten different anhydrous RTILs 12-14 and for two practical water-to-RTIL partition systems. 15 The ten anhydrous water- to-RTIL correlations describe “hypothetical” partitions, and the partition coefficient is calculated as a molar solubility ratio for the solute dissolved in both neat solvents. Practical partitions, on the other hand, represent true equilibrium solute partitioning between the water-saturated organic phase and an aqueous phase that is saturated with the organic solvent. Correlations derived from the Abraham model described the experimental log K and log P data for 10 RTILs to within average standard deviations of (0.086 log units and (0.129 log units, respectively. The quoted values represent the average value of the standard deviations of the individual log K and log P correlations of the 10 RTILs. The drawback in using QSAR and linear free energy relationship (LFER) models is that a separate equation must be derived for each RTIL solvent studied. There is no transfer of knowledge from one ionic liquid to the next. To address this problem we have devised a method for separating the five individual solvent coefficients in the Abraham model into cation-specific and anion-specific values that would * Corresponding author e-mail: acree@unt.edu. ² Department of Chemistry, University of North Texas. ‡ Department of Research and Statistical Support, University of North Texas. § University College London. 1123 J. Chem. Inf. Model. 2007, 47, 1123-1129 10.1021/ci7000428 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/26/2007