Mixed monolayers of polyelectrolytes and surfactants at the air–water interface

Dilute mixed solutions of non surface active anionic polymers (polyacrylamide and polystyrene sulfonate) and various surfactants have been studied with several methods: surface tension, ellipsometry, 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). A simple adsorption model, in which it is assumed that both polymer and surfactant counterions are expelled away from the surface, is in good agreement with the observations on polyacrylamide sulfonate. The behaviour of the more hydrophobic polystyrene sulfonate is very different and still not understood. The behaviour of this polymer with nonionic surfactants is also different. The connection of foam stability with surface and bulk complexation is discussed.