Surfactant self-assembly at the barite-solution interface

The molecular organisation of a quaternary ammonium surfactant on the barium sulphate (0 0 1) surface in aqueous solution has been studied by atomic force microscopy (AFM). In dilute solution far below the critical micelle concentration (cmc) the surfactant molecules are preferentially adsorbed at defect surface sites, forming tube-like surface aggregates. At surfactant concentrations close to the cmc, the formation of a self-assembled bilayer was observed, suggesting a stronger total interfacial interaction between surfactant and surface than for model substrates such as graphite and mica. ering the surface charge on BaSO4 (0 0 1) through addition and specific adsorption of the potential determining ions Ba2+ and SO42− the adsorbate changes to a more curved structure interrupted by highly repulsive island-like areas of agglomeration of Ba2+ or SO42−. In the case of adsorbed sulphate the effects of negative surface charge appear to lead to clustered micellar or vesicular structures in addition to a flat surfactant layer.