Measurement of the relative hydrophobicity of organic compounds without organic solvent. Effects of salt composition and pH on organic acids and nonionic compounds

Partitioning of a variety of nonionic and acidic organic compounds was exam ined in aqueous dextran-poly(ethylene glycol) (PEG) two-phase systems of di fferent ionic composition at pH 2.00 and 7.40. Relative hydrophobicity of t he compounds was estimated in terms of equivalent numbers of methylene unit s. Effect of the ionic composition and pH of aqueous media on the relative hydrophobicity of organic compounds was established and described quantitat ively. The results obtained indicate that the ionic composition affects the relative hydrophobicity of ionized compounds and has no effect on that of nonionic compounds and ionizable compounds in the undissociated state. Ioni zation of a given ionic group seems to bring about a constant change in the relative hydrophobicity of a compound. This result supports the applicabil ity of the additivity principle to the relative hydrophobicity estimates ob tained with the aqueous two-phase partitioning technique. (C) 2001 Wiley-Li ss, Inc. and the American Pharmaceutical Association.