A priori prediction of the octanol–water partition coefficient (Kow) of ionic liquids

• The octanol–water partition coefficient (Kow) of ionic liquids (ILs) is predicted from first principles.
• The Pitzer–Debye–Hückel model is used for long-range electrostatic interactions.
• The COSMO-SAC model accounts for short-range molecular surface interactions.
• The variation of Kow with molecular structures is dominated by the short-range attractive interactions.
• The proposed methods can be used to screen for environmentally benign ILs.

The octanol–water partition coefficient (Kow) of ionic liquids (ILs) is an important indicator for its toxicity and environment impact. In this work, the Kow is determined from the ratio of infinite dilution activity coefficient of IL in the water-rich and octanol-rich phases. In particular, the Pitzer-Debye−Hückel (PDH) model combined with the predictive COSMO-SAC model is used for calculating the activity coefficients. A root-mean square deviation of 0.75 is achieved for log Kow, or a factor of 4 in Kow, for a total of 67 ILs at ambient condition. The long-range coulomb interactions (from the DH model) contribute an almost constant value of −1.35 to log Kow, regardless of the type of IL. The change of log Kow with the molecular structure of IL is found to be dominated by the short-range attractive interactions between the IL and the solvents, including the hydrogen bonding and nonhydrogen bonding surface interactions. The combination of PDH and COSMO-SAC models provides not only the quantitative predictions of Kow of ILs and but also physical insights to the relations between Kow and the molecular structure of ILs.