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

Abstract

Specular neutron reflection has been used to investigate the composition and structureof the surfactant-polymer mixture of sodium dodecyl sulphate(SDS) and hexaethyleneglycol monododecyl ether (C12E6), and the cationic polymerpoly-(dimethyldialyl ammonium chloride)(dmdaac), at the air-water interface. In particular,the effects of surfactant and polymer concentration are investigated. The addition ofthe cationic polymer results in an interface more rich in SDS. The variation ofsurfactant composition with polymer and surfactant concentration shows a complexbehaviour, which is related indirectly to the bulk solution phase behaviour. Theamount of polymer at the interface increases with increasing polymer concentration,but decreases with increasing surfactant concentration at a fixed solution polymerconcentration. At a polymer concentration ~100 ppm there is a marked change inthe structure of the adsorbed polymer layer: the thickness of the absorbed layerincreases from ~20 Å to ~30 Å.In contrast the thickness of the absorbedlayer, when only the surfactant is visible at the interface, shows no significant changewith increasing polymer concentration.