Estimation of critical micelle concentrations of bile acids by reversed-phase high performance liquid chromatography

The critical micelle concentration (CMC) for bile salts or other surfactants is defined as that solute concentration at which appreciable changes in such phenomena as light scattering, surface tension, or solubilization of other organic molecules occur, these changes indicating appearance of surfactant aggregates. The CMC thus reflects hydrophobic interactions of the surfactant with itself. The self-association of hydrophobic molecules resembles the partition of a solute into the lipophilic phase in reversed-phase high performance liquid chromatography (RPLC): Both processes can be considered as transfers of a molecule from an aqueous to a lipophilic medium. The critical micelle concentration of a particular bile salt, being a measure of its hydrophobic self-association, should therefore be correlated with its Chromatographic mobility since they are fundamentally related phenomena. Experimentally, significant correlations between these quantities are obtained, both for bile salts andn-alky1 sulfonates, and only microgram amounts of sample are required for RPLC measurements. Among three homologous series of bile salt surfactants, CMC values predicted from RPLC measurements agree, within a standard error of 7%, with CMC values determined directly. This suggests the applicability of reversed-phase liquid chromatography to the micro-scale determination of critical micelle concentrations of bile salts,n-alkyl sulfonates, and other homologous series of surfactants.