Octanol–water partition coefficients of imidazolium-based ionic liquids

Abstract

Ionic liquids (ILs) are low melting organic salts that are being vigorously investigated as possible replacements for volatile organic solvents. While they cannot contribute to air pollution due to their negligible vapor pressure, they do have significant solubility in water. As a result, this is the most likely medium through which ILs will enter the environment. Therefore, it is important to understand how ILs will influence aquatic ecosystems. A simple thermodynamic measurement that has been extremely useful in estimating effects of chemical pollutants on aquatic environments is the octanol–water partition coefficient (K_OW). It is an extremely important quantity because it describes the hydrophobicity or hydrophilicity of a compound and has been correlated with bioaccumulation and toxicity in fish, as well as sorption to soils and sediments. Here we present measurements of the K_OW of twelve imidazolium-based ILs at room temperature, using the slow-stirring method. For the butylmethylimidazolium cation, K_OW values range from 0.003 to 11.1, depending on the choice of anion. In addition, we find that the K_OW values increase with increasing alkyl chain length on the cation and that replacing the acidic hydrogen on the carbon between the two nitrogens in the imidazolium ring with a methyl group has negligible effect on the K_OW. However, all of the K_OW values measured, even for the most “hydrophobic” imidazolium-based ILs, are less than 15 so these ILs will not accumulate or concentrate in the environment.