The structure and composition of surfactant-polymer mixtures of sodium dodecyl sulphate, hexaethylene glycol monododecyl ether and poly-(dimethyldialyl ammonium chloride) adsorbed at the air-water interface

Specular neutron reflection has been used to investigate the composition an d structure of the surfactant-polymer mixture of sodium dodecyl sulphate (S DS) and hexaethylene glycol monododecyl ether (C12E6), and the cationic pol ymer poly-(dimethyldialyl ammonium chloride) (dmdaac), at the air-water int erface. In particular, the effects of surfactant and polymer concentration are investigated. The addition of the cationic polymer results in an interf ace more rich in SDS. The variation of surfactant composition with polymer and surfactant concentration shows a complex behaviour, which is related in directly to the bulk solution phase behaviour. The amount of polymer at the interface increases with increasing polymer concentration, but decreases w ith increasing surfactant concentration at a fixed solution polymer concent ration. At a polymer concentration similar to 100 ppm there is a marked cha nge in the structure of the adsorbed polymer layer: the thickness of the ab sorbed layer increases from similar to 20 Angstrom to similar to 30 Angstro m. In contrast the thickness of the absorbed layer, when only the surfactan t is visible at the interface, shows no significant change with increasing polymer concentration.