Complexation of cationic surfactant and anionic polymer at the air-water interface

Equilibrium surface tension measurements have been carried out on solutions of a non-surface-active anionic polyelectrolyte (poly(acrylamide sulfonate)) mixed with various surfactants. While the surface tension does not change when the polymer is added to nonionic and anionic surfactant solutions, a strong synergistic lowering of the surface tension is found with cationic surfactants. In the latter case, we also find that the surface tension is practically independent of the amount of polymer over the range of concentrations studied. Assuming that the polymer stretches out at the interface to form a neutral complex with the surfactant, this behavior can be explained by the classical Gibbs adsorption equation. Furthermore, ellipsometry measurements on these solutions are consistent with our adsorption model.