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 and structure of the surfactant-polymer mixture of sodium dodecyl sulphate (SDS) and hexaethylene glycol monododecyl ether (C12E6), and the cationic polymer poly-(dimethyldialyl ammonium chloride) (dmdaac), at the air-water interface. In particular, the effects of surfactant and polymer concentration are investigated. The addition of the cationic polymer results in an interface more rich in SDS. The variation of surfactant composition with polymer and surfactant concentration shows a complex behaviour, which is related indirectly to the bulk solution phase behaviour. The amount of polymer at the interface increases with increasing polymer concentration, but decreases with increasing surfactant concentration at a fixed solution polymer concentration. At a polymer concentration similar to 100 ppm there is a marked change in the structure of the adsorbed polymer layer: the thickness of the absorbed layer increases from similar to 20 Angstrom to similar to 30 Angstrom. In contrast the thickness of the absorbed layer, when only the surfactant is visible at the interface, shows no significant change with increasing polymer concentration.