Interfacial charge and specific adsorption of ions in equilibrium films: Foam film stabilized by nonionic surfactant

In order to clarify the effect of adsorption of nonionic surfactant on the charge and specific adsorption of ions at water/air interface, a modified Poisson-Boltzmann equation was applied to equilibrium thickness of foam films stabilized by nonionic surfactant. Equilibrium thickness of foam films stabilized by decyl methyl sulfoxide in the absence of salt was measured as a function of surfactant concentration at constant disjoining pressure. Specific adsorption of anions, responsible for the negative surface charge, is enhanced by the adsorption of decyl methyl sulfoxide at low surfactant concentration. Strong interaction takes place between surfaces in a pentaethyleneglycol monododecyl ether film at low pH below 3.5. Isoelectric point for the surfactant-loaded water/air interface was found to be 3.05 and smaller than 4.4–4.5 for surfactant-free water/air interface.