A Three-Phase Distribution Method for Quantifying the Intermolecular Interactions

The octanol/water partition coefficient, log P, is a standard descriptor of hydrophobicity; however, it compresses the hydrogen bonding, dispersion, and orientation forces onto a single axis and fails for per- and polyfluoroalkyl substances (PFAS), where the orientation forces dominate and a stable octanol/water equilibrium rarely forms─hence, PFAS have long been mischaracterized as "extremely hydrophobic" through log P values. Here, we propose the three-phase distribution method as a new experimental tool that extends log P by partitioning a neutral solute among water, cyclohexane, and perfluorononane─three mutually immiscible phases, each selectively dominated by one of those interactions─hence, each solute is mapped onto a ternary diagram by the percentage shares of three forces. The present paper is a theoretical proposal of this new experimental method, with first-principles COSMO-RS calculations as a consistency check. The results confirm that the method subsumes log P, extends the analysis to PFAS that lie outside the scope of conventional log P, and resolves compounds that are indistinguishable on the log P axis by decomposing their intermolecular-force contributions─capabilities that establish the three-phase distribution method as a general-purpose analytical tool applicable across diverse fields of physical chemistry.