Octanol/water partition coefficients of ionic liquids

Abstract

BACKGROUND: Room temperature ionic liquids (ILs) are attractive alternatives to environmentally unfriendly volatile organic solvents. Partitioning is one of the most important and fundamental properties of a chemical, and the octanol/water partition coefficient is widely used to measure the tendency of a chemical to cross biological membranes. However, there is very limited information on the concentration dependence of the partition coefficients of ILs. This study investigated the octanol/water partitioning of 1-butyl-3-methylimidazolium ([bmim]) ILs containing either hexafluorophosphate ([PF6]) or bis[(trifluoromethyl)sulfonyl]amide ([Tf2N]) over a wide range of IL concentrations of three to five orders of magnitude. RESULTS: It was found that the apparent partition coefficients of the ILs increased with increasing IL concentration. A model based on the ionic nature of ILs was proposed to explain this behaviour, and the results showed a good fit with the experimental data. The intrinsic partition coefficients and dissociation constants of the ILs were determined using the equations from the proposed model. The differences in the intrinsic partition parameter values between the two ILs showed a good correlation with other physicochemical properties. CONCLUSIONS: The present study clearly shows that the octanol/water partition coefficients of ILs increase with increasing IL concentration owing to the formation of ion pairs. By using the proposed partition model, it was possible to determine the intrinsic partition coefficients of ILs, and it was found that the apparent partition coefficients of ILs converge to the intrinsic partition coefficients of the ionic species and ion pairs of ILs with decreasing and increasing IL concentration respectively. (C) 2008 Society of Chemical Industry