log P Definitions
According to the International Union of Pure and Applied Chemistry (IUPAC)*:
“Lipophilicity represents the affinity of a molecule or a moiety for a lipophilic environment. It is commonly measured by its distribution in a biphasic system (e.g. partition coefficient in octanol-water)”.
*Pure & Appl. Chem., Vol. 70, No. 5, pp. 1129-1143,1998.
The log P value refers to the lipophilicity of the unionised species. Lipophilicity changes as a function of pH for ionisable compounds. The log D value denotes the lipophilicity at a given pH. The log D and log P values are equal at the pH where the compound is neutral, i.e. completely unionised.