Polyelectrolyte-surfactant complexes at the air-water interface: Influence of the polymer backbone rigidity

Dilute mixed solutions of non-surface active anionic polymers (polyacrylamide and polystyrene sulfonate, xanthan) and various surfactants have been studied with several methods: surface tension, ellipsometry, X-ray reflectivity, surface rheology and thin film balance. A strong synergistic lowering of the surface tension is found with cationic surfactants in the concentration range where no appreciable complexation of surfactant and polymer occurs in the bulk solution. Despite appreciable differences between surface tension behaviour, the adsorbed layers are very similar for all the polymers: their thickness is small and the polymer chains are stretched along the surface. The behaviour of the linear surface elasticity is also similar, although the non-linear behaviour appears different. When the polymers are confined in thin films, the forces between surfaces are similar and independent of surfactant nature: oscillatory forces are measured, which reflect the existence of a polymer network with a well defined mesh size. The connection of foam stability with surface and bulk complexation is not easy to establish and demands a proper account of the long adsorption kinetics found with these solutions.