Characterization of Adsorbed Ionic Surfactants on a Mica Substrate

An AFM study is presented to investigate the morphology of adsorbed cationic surfactants on mica substrates. At low surfactant concentrations and low pH values, discrete aggregates of adsorbed surfactants are found on the surface. As the surfactant concentration is increased, these aggregates become more organized into elongated cylindrical shapes. The continuity of the network of patches increases until a concentration just below the cmc. At this concentration, the patches become continuous “wormlike” admicelles on the surface. Contact angle measurements corroborate the change in the surface properties from hydrophobic at low surfactant concentrations to hydrophilic at concentrations slightly below the cmc. Force versus distance curves also clearly indicate a change in surface morphology. It is clear from our observations that the formation of dense continuous monolayers or bilayers does not occur as postulated earlier. Indeed our observations suggest the formation of discrete surfactant aggregates on the surface which under certain conditions (high pH and high surfactant concentration) become continuous and form wormlike admicelles on the mica surface. This change in surfactant morphology from adsorbed surfactant aggregates to more continuous structures is also responsible for the transition from hydrophilic to hydrophobic surface properties. It is also shown that pH and salt concentration play an important role in this transition.