Polyelectrolyte and surfactant mixed solutions. Behavior at surfaces and in thin films

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 and neutron reflectivity, thin film balance, surface and bulk rheology. 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 (as seen from viscosity measurements). Despite appreciable differences between surface tension behaviour, the adsorbed layer is very similar for all the polymers: their thickness is small and the polymer chains are stretched along the surface. The surface tension behaviour of these polymers with non-ionic surfactants is also 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 far from clear.