Retention mechanism and implications for selectivity for a group of dihydropyridines in ionic micellar liquid...

The retention behaviour of a group of dihydropyridines in micellar liquid chromatography was studied using sodium dodecyl sulphate and hexadecyltrimethylammonium bromide as surfactants in the mobile phase containing 5% of n-butanol and a C18 column. When the surfactant concentration in the mobile phase is increased, a tendency to change from a three partition equilibria mechanism to direct transfer of solutes from micelles to the stationary phase is observed for both surfactants. This progressive change in the retention mechanism is explained through the large micellar phase-water partition coefficients of these compounds and the increase produced in the fraction of solute molecules in the micellar phase due to the increase in the volume of this phase originating from the increase in surfactant concentration. As a result, the selectivity coefficients show a tendency to match the ratio of the stationary phase to micellar phase partition coefficients of these compounds, constituting further proof of the progressive change in the retention mechanism when the surfactant concentration is increased.