The role of surfactants in capillary electroosmosis and electrophoresis

Capillary flow behaviors of two different surfactants, negative sodium dodecyl sulfate (SDS) and positive cetyltrimethylammonium chloride (CTAC), have been determined by the current-monitoring method. In the absence of surfactants, electroosmotic flow is generally anode to cathode for a negatively charged capillary surface. With a decrease of the capillary diameter, the energy consumed per flow volume decreases slightly. With surfactants present, electrokinetic behavior is complex. For example, due to the electrophoretic effect, the overall flow of SDS solutions with concentration above its critical micelle concentration (CMC), 8.4 mM, was reversed (cathode toward anode, nu = -0.03 cm/s). However, with the concentration of SDS below its CMC, normal anodic to cathodic elect roosmotic flow (nu = 0.06 cm/s) was observed. In the case of a positively charged surfactant CTAC, reverse cathode to anode electrokinetic flows (nu = -0.09 cm/s) of CTAC solutions also were observed with the concentration of CTAC above or below its CMC (1.0 mM). Adsorption effects of CTAC may be responsible for reversing the net flow in this case, whose electrophoresis should be anode to cathode for a positively charged micelle.