Effect of cetyltrimethylammonium chloride on electroosmotic and electrophoretic mobilities in capillary zone electrophoresis

Abstract

The effect of a cationic surfactant, cetyltrimethylammonium chloride (CTAC), on the electroosmotic mobility and the electrophoretic mobility of organic anions in capillary zone electrophoresis was investigated. The electroosmotic mobility showed four stepwise changes, including a reversal, with increasing CTAC concentration. The behaviour, especially the reversal of the electroosmotic mobility, was explained by assuming the formation of hemimicelles on the capillary wall. That is, CTAC first adsorbs individually by electrostatic interactions and then begins to associate into hemimicelles by Van der Waals attraction. The formation of hemimicelles changes the surface charge of the capillary wall from negative to positive and causes the reversal of the electroosmotic mobility. The effective electrophoretic mobilities of organic anions such as benzoic acid analogues were also influenced by the CTAC concentration. It was concluded that the behaviour was due to the interaction with hemimicelles on the capillary wall and also ion association with the monomer of CTAC and the interaction of micelles in bulk solution.