THIN-FILM FORCES IN FOAM FILMS CONTAINING ANIONIC POLYELECTROLYTE ANDCHARGED SURFACTANTS

Foam-film disjoining pressure and surface tension isotherms are measur ed for solutions containing mixtures of charged surfactants and an ani onic random-block copolymer, acrylamide-acrylamidesulfonate. When comb ined with an anionic surfactant, dioctyl sulfosuccinate (AOT), the pol yelectrolyte shows no indication of direct surface interactions and th e surfactant behaves as a simple electrolyte. However, when copolymer is added to cationic dodecyltrimethylammonium bromide (C(12)TAB) solut ions, strong polymer-surfactant interactions create an enhanced adsorp tion of both components to the air-water interface. The resulting poly mer-surfactant surface layers subsequently produce long-range thin-fil m interactions that have not been seen before. For the C(12)TAB-polyme r solutions, film stratification is observed 3 orders of magnitude bel ow the critical micelle concentration and nearly two orders of magnitu de below the critical aggregation concentration (cac). This new type o f film stratification is consistent with recent Monte Carlo simulation s of polyelectrolyte ''bridging'' in thin films. In addition, when the C(12)TAB concentration is increased to the cac, dense polymer-surfact ant surface layers overlap inside the film and produce a gel-like netw ork. The structure and properties of this network are strongly influen ced by the rate of film formation and the long-range forces produced a re crucial to the film's stability.