Controlling Foam Film Stability Based on Thermodynamics of Mixed Surfactant Adsorbed Films

Abstract

Stability and thickness of the common black films (CBFs) observed in a series of ionic–nonionic mixed surfactant systems were studied based on the phase diagram of adsorbed film deduced thermodynamically from surface-tension data. It was demonstrated that the synergetic attraction between ionic and nonionic surfactants maintained the surface density of ionic surfactant high enough to stabilize CBF even when the bulk ionic surfactant concentration was considerably low. In the absence of the synergetic adsorption, the foam film thickness was determined solely by the electrolyte concentration in the bulk solution, c, which is equal to the ionic surfactant concentration, m1, in the absence of an additional inorganic electrolyte. Exceptionally, the specific interaction between ionic surfactant head groups and their counterions reduced the surface charge density greatly in the Stern layer, leading to a foam film much thinner than the one expected from m1.