Thin-film forces in foam films containing anionic polyelectrolyte and charged surfactants

Foam-film disjoining pressure and surface tension isotherms are measured for solutions containing mixtures of charged surfactants and an anionic random-block copolymer, acrylamide-acrylamidesulfonate. When combined with an anionic surfactant, dioctyl sulfosuccinate (AOT), the polyelectrolyte shows no indication of direct surface interactions and the surfactant behaves as a simple electrolyte. However, when copolymer is added to cationic dodecyltrimethylammonium bromide (C(12)TAB) solutions, strong polymer-surfactant interactions create an enhanced adsorption of both components to the air-water interface. The resulting polymer-surfactant surface layers subsequently produce long-range thin-film interactions that have not been seen before. For the C(12)TAB-polymer solutions, film stratification is observed 3 orders of magnitude below the critical micelle concentration and nearly two orders of magnitude below the critical aggregation concentration (cac). This new type of film stratification is consistent with recent Monte Carlo simulations of polyelectrolyte ''bridging'' in thin films. In addition, when the C(12)TAB concentration is increased to the cac, dense polymer-surfactant surface layers overlap inside the film and produce a gel-like network. The structure and properties of this network are strongly influenced by the rate of film formation and the long-range forces produced are crucial to the film's stability.