Enhanced spreading of aqueous films containing ionic surfactants on solid substrates

The spreading behavior of ionic surfactant aqueous systems on organosulfur monolayers with differing compositions and surface energies has been investigated. Aqueous solutions of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and the cationic surfactant didodecyldimethyl ammonium bromide (DDAB) exhibit spreading characteristics that were previously thought to be reserved to nonionic surfactant aqueous systems. These characteristics include rapid spreading over moderately hydrophobic substrates that are not wetted in the absence of surfactant, a maximum in spreading rate at intermediate substrate surface energies, and a maximum in spreading rate with respect to surfactant concentration (well above the respective critical micelle concentrations). Addition of coordinating alkali metal salts increased the spreading rates of AOT/H2O mixtures but reduced the spreading rates of the DDAB/H2O mixtures. Addition of noncoordinating salts did not significantly increase spreading rates of AOT/H2O mixtures but still resulted in reduced spreading rates of the DDAB/H2O mixtures. These results illustrate 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.