ENHANCED SPREADING OF AQUEOUS FILMS CONTAINING IONIC SURFACTANTS ON SOLID SUBSTRATES

The spreading behavior of ionic surfactant aqueous systems on organosu lfur monolayers with differing compositions and surface energies has b een investigated. Aqueous solutions of the anionic surfactant sodium b is(2-ethylhexyl) sulfosuccinate (AOT) and the cationic surfactant dido decyldimethyl ammonium bromide (DDAB) exhibit spreading characteristic s that were previously thought to be reserved to nonionic surfactant a queous systems. These characteristics include rapid spreading over mod erately hydrophobic substrates that are not wetted in the absence of s urfactant, a maximum in spreading rate at intermediate substrate surfa ce energies, and a maximum in spreading rate with respect to surfactan t concentration (well above the respective critical micelle concentrat ions). Addition of coordinating alkali metal salts increased the sprea ding rates of AOT/H2O mixtures but reduced the spreading rates of the DDAB/H2O mixtures. Addition of noncoordinating salts did not significa ntly increase spreading rates of AOT/H2O mixtures but still resulted i n reduced spreading rates of the DDAB/H2O mixtures. These results illu strate the complexity of the surfactant-enhanced spreading process and support an important role for surfactant adsorption near the leading edge of the solid-liquid interface.