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 (C TAC), have been determined by the current-monitoring method. In the ab sence of surfactants, electroosmotic flow is generally anode to cathod e for a negatively charged capillary surface. With a decrease of the c apillary diameter, the energy consumed per flow volume decreases sligh tly. With surfactants present, electrokinetic behavior is complex. For example, due to the electrophoretic effect, the overall flow of SDS s olutions with concentration above its critical micelle concentration ( CMC), 8.4 mM, was reversed (cathode toward anode, nu = -0.03 cm/s). Ho wever, with the concentration of SDS below its CMC, normal anodic to c athodic elect roosmotic flow (nu = 0.06 cm/s) was observed. In the cas e of a positively charged surfactant CTAC, reverse cathode to anode el ectrokinetic flows (nu = -0.09 cm/s) of CTAC solutions also were obser ved with the concentration of CTAC above or below its CMC (1.0 mM). Ad sorption 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.