Quantitation of hydrophobicity in micellar liquid chromatography

Abstract

Micellar liquid chromatography (MLC) is shown to be a promising technique for measuring the hydrophobicity of solutes. The presence of micelles has a profound effect on the chromatographic characteristics of reversed-phase columns. The linear relationships between the logarithm, log k, of the retention factor and such diverse properties as the number of carbon atoms in homologous series, octanol–water partition coefficients and solvatochromic parameters, which are observed in conventional reversed-phase liquid chromatography (RPLC), are not usually valid in MLC. For series of compounds exhibiting a wide range of hydrophobicity, k itself is linearly related to these properties. The reasons for this behavior, and the advantages of MLC over conventional RPLC in the quantitation of hydrophobicity, are discussed. The values of k in mobile phases extrapolated to zero micelle concentration are shown to give a better measure of hydrophobicity than k with 100% water in conventional RPLC. The ability of MLC to characterize bioactive compounds is also discussed. A single MLC retention parameter can describe the bioactivity of phenols, while three conventional structural descriptors are needed to achieve a similar correlation. The correlation between the retention in MLC and the sites of action of diuretics within the nephron provides another interesting example of its mimetic value for physiological systems, where hydrophobic and electrostatic interactions, and kinetic phenomena are important.