CHARACTERIZATION OF ADSORBED IONIC SURFACTANTS ON A MICA SUBSTRATE

An AFM study is presented to investigated the morphology of adsorbed c ationic surfactants on mica substrates. At low surfactant concentratio ns and low pH values, discrete aggregates of adsorbed surfactants are found on the surface. As the surfactant concentration is increased, th ese aggregates become more organized into elongated cylindrical shapes . The continuity of the network of patches increases until a concentra tion just below the cmc. At this concentration, the patches become con tinuous ''wormlike'' admicelles on the surface. Contact angle measurem ents corroborate the change in the surface properties from hydrophobic at low surfactant concentrations to hydrophilic at concentrations sli ghtly below the cmc. Force versus distance curves also clearly indicat e a change in surface morphology. It is clear fi om our observations t hat the formation of dense continuous monolayers or bilayers does not occur as postulated earlier. Indeed our observations suggest the forma tion of discrete surfactant aggregates on the surface which under cert ain conditions (high pH and high surfactant concentration) become cont inuous and form wormlike admicelles on the mica surface. This change i n surfactant morphology from adsorbed surfactant aggregates to more co ntinuous structures is also responsible for the transition from hydrop hilic to hydrophobic surface properties. It is also shown that pH and salt concentration play an important role in this transition.