Partitioning of Organic Compounds in Octanol-Water Systems

When plots of the log of the octanol-water partition coefficients (K_ow) vs. log of water solubilities (S) of a wide variety of liquid and solid solutes are modified by converting the solid solubilities to the estimated solubilities of the corresponding supercooled liquids, the differences between the correlations for different classes of compounds are significantly reduced. Nevertheless, the experimental points deviate significantly below the “ideal” line that is calculated on the assumption of Raoult's law in the octanol phase and zero effect of dissolved octanol on water solubility; the downward deviations increase systematically with reducing S (or increasing K_ow), with the result that the slope of the correlation line has a significantly lower magnitude than the ideal value of −1. The deviations are caused by liquid solute incompatibility in the (water-saturated) octanol phase (as measured by the activity coefficient) and by the (lesser) effect of dissolved octanol on water solubility, both of which increase systematically with decreasing S.