Forces in foam films containing polyelectrolyte and surfactant

Foam films of semi-dilute anionic polyelectrolyte solutions containing addi tionally the cationic surfactant dodecyltrimethylammonium bromide are inves tigated. The polyelectrolytes used are polystyrene sulfonate (PSS) and acry lamidesulfonate (AAS) as examples of strongly and weakly charged polyions, respectively. The disjoining pressure isotherm results in film stratificati on well below the critical micelle concentration (CMC) and the critical agg regate concentration (CAC). These pressure oscillations are related to a ne twork of polyelectrolyte chains in the film bulk, and their period to the m esh size. The network can be described by the theory for semi-dilute polyel ectrolyte solutions, which is different for the two polyelectrolytes becaus e of different charge densities along the chain. The surfactant has no obse rvable influence on the network. To obtain information about the him interface surface tension measurements at the water-air interface are carried out. In comparison with the pure pol yelectrolyte and the surfactant, respectively, the surface tension is much reduced for the mixed systems as a result of the formation of complexes bet ween the surfactant and the polyelectrolyte. The two polymers show differen t behaviour in saturation of the surface coverage, which leads us to the co nclusion that the complexes are different, although the behaviour of the fi lm bulk is not influenced by changes in the interface. (C) 1999 Elsevier Sc ience B.V. All rights reserved.