Chromatographic quantitation of the hydrophobicity of ionic compounds by the use of micellar mobile phases

Abstract

Many biologically active compounds of interest in structure–activity relationships are ionic at physiological pH. However, ionic organic compounds are only weakly or not retained in conventional RPLC which impedes the chromatographic estimation of their hydrophobicity and the development of quantitative retention–activity relationship studies. The use of micellar mobile phases allows the retention of ionic compounds. Hydrophobic and electrostatic forces govern the retention of ionic compounds in micellar liquid chromatography. In this paper three different retention models log k–log P for ionic compounds are tested ( P=partition coefficient). The retention model (log k= a log P+ b α+ c) which includes the hydrophobicity and the molar total charge of compound at a given pH value has proven to be valid for all types of compounds tested, catecholamines, local anesthetics, diuretics and o-phthalaldehyde–N-acetyl- l-cysteine amino acid derivatives.