Partition coefficient

A partition coefficient is the equilibrium ratio of a substance’s concentrations in two phases, usually two immiscible liquids. Its value depends on the substance, phases, temperature, and concentration convention.

Nernst distribution law: A thermodynamic relation describing how a solute distributes between two immiscible phases at equilibrium. It gives the conditions under which a concentration ratio remains constant.
Octanol-water partition coefficient: The ratio describing a compound’s equilibrium distribution between 1-octanol and water. This standard pair estimates how compounds divide between lipid-like and aqueous environments.
Chemical equilibrium: A state in which forward and reverse processes occur at equal rates, leaving macroscopic quantities constant. Partition coefficients describe phase distributions after transfer reaches equilibrium.
Adsorption coefficient: A parameter describing how much of a substance accumulates on a surface relative to its concentration in a fluid. It concerns surface binding rather than equilibrium distribution between bulk phases.
Liquid-liquid extraction: A separation method that transfers compounds between immiscible liquid phases. Extraction uses unequal partitioning to move a target compound into a chosen solvent.
LogP: The base-10 logarithm of a neutral compound’s octanol-water partition coefficient. Its scale makes hydrophobicity comparisons convenient across many orders of magnitude.
Concentration: The amount of a substance present in a defined volume or mass of mixture. The chosen concentration measure determines the numerical ratio.
Henry's law constant: A proportionality constant relating a dilute solute’s gas-phase abundance to its abundance in a liquid. It describes gas-liquid equilibrium using a specified pressure-concentration convention.
Distribution ratio: The ratio of a substance’s total concentrations in two phases, including all chemical forms. Unlike a partition coefficient, it can include ionized and complexed forms.
LogD: The logarithm of a compound’s distribution ratio between octanol and water at a specified pH. It captures pH-dependent partitioning when compounds ionize.

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