A Three-Phase Distribution Method for Quantifying the Intermolecular Interactions.

Area of Science:

• Physical Chemistry
• Environmental Chemistry
• Analytical Chemistry

Background:

• The octanol/water partition coefficient (log P) is a standard hydrophobicity measure but fails for per- and polyfluoroalkyl substances (PFAS).
• PFAS orientation forces dominate, preventing stable octanol/water equilibrium, leading to mischaracterization as extremely hydrophobic via log P.
• Existing methods lack the precision to differentiate compounds with similar log P values.

Purpose of the Study:

• To propose a novel three-phase distribution method for accurately assessing solute hydrophobicity.
• To extend hydrophobicity analysis beyond the limitations of conventional log P, particularly for PFAS.
• To provide a method capable of resolving compounds indistinguishable by traditional log P measurements.

Main Methods:

• Development of a three-phase distribution method partitioning solutes among water, cyclohexane, and perfluorononane.
• Utilizing three mutually immiscible phases, each selectively sensitive to hydrogen bonding, dispersion, and orientation forces.
• Theoretical proposal validated with first-principles COSMO-RS calculations for consistency.

Main Results:

• The proposed method successfully subsumes the traditional log P.
• It extends hydrophobicity analysis to PFAS and other compounds outside the scope of conventional log P.
• The method decomposes intermolecular-force contributions, resolving compounds previously indistinguishable on the log P axis.

Conclusions:

• The three-phase distribution method offers a more comprehensive and accurate assessment of hydrophobicity.
• This technique overcomes the limitations of log P for complex substances like PFAS.
• The method establishes itself as a general-purpose analytical tool applicable across diverse physical chemistry fields.