Comparison of the retention characteristics of different pseudostationary phases for microemulsion and micellar electrokinetic chromatography of betamethasone and derivatives

Five electrokinetic chromatography systems were compared concerning retention behavior and lipophilicity. Comparison was based on capacity (retention) factors of some steroidal drugs, and on log P(OW) values derived by the aid of reference substances. In all systems the aqueous buffer consisted of phosphate (20 mM, pH 7.5). Two systems had micelles, three systems microdroplets as negatively charged pseudostationary phases. The micelles were formed by sodium dodecyl sulfate (SDS) and sodium cholate, respectively. One microemulsion consisted (as usual) from octane as oil, butanol as cosurfactant and SDS as charged tenside. Two microemulsions were made from biosurfactants (phosphatidylcholine, isopropylmyristate) to better simulate biopartitioning of the drugs. Even for noncharged analytes a change in migration sequence and thus in log P(OW) was observed for the systems consisting of the biosurfactants, compared to the others. For the former systems, log P(OW) derived from the capacity factors agree for all analytes with those obtained from calculation by computer software based on the structure of the drugs, and with experimental data directly obtained from octanol/water partitioning.