Characterization of the Cationic Surfactant Induced Reversal of Electroosmotic Flow in Capillary Electrophoresis

Reversal of electroosmotic flow in capillary electrophoresis can be achieved by the addition of cationic surfactants to the electrophoretic buffer. This reversal of flow is caused by the formation of a bilayer or hemimicelle at the walls of the capillary, effectively making the wall charge positive. The bilayer can be formed either by adsorption of monomeric surfactants or by admicelles (surfactant pairs), depending on the ionic strength of the buffer and the surface charge at the capillary walls. At concentrations of cationic surfactant sufficient to form the bilayer (i.e., greater than the critical micelle concentration) the electroosmotic flow velocity is independent of pH above a pH of 4. Adjustment of the ionic strength causes minor variations in the reversed electroosmotic flow rate due to the opposing forces of increased concentration of adsorbed surfactant and decreased double layer thickness. The nature of the anionic counterion in solution has a strong effect on the magnitude of the reversed electroosmotic flow observed, analogous to the effect of the buffer cation's effect on normal electroosmotic flow. This effect is explained using a simple ion exchange model.