Composition of mixed surfactant–polymer layers adsorbed at the air/water interface as determined by specular neutron reflection

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Abstract
Specular neutron reflection has been used to evaluate the adsorption of the anionic surfactant, sodium dodecyl sulfate, SDS, and of the mixed anionic–non-ionic surfactants of SDS and hexaethylene monododecyl ether, C12E6, in the presence of cationic polymers (dimethyldialylammonium chloride–acrylamide copolymers) at the air/water interface. The surface composition of the mixed surfactants SDS–C12E6 was measured over a wide concentration range, from well below to above the critical micellar concentration (c.m.c.) of the mixture, for low polymer concentrations and different polymer charge densities. In the presence of polymer there is an enhanced adsorption of SDS at the interface below the c.m.c. The SDS adsorption decreases with reducing charge density on the polymer, and increases with increasing polymer concentration. For SDS alone, in the presence of the cationic polymer, the adsorbed amount of the surfactant in constant up to a concentration of 10–3 mol l–1, and then increases at higher surfactant concentrations. Estimates of the amount of polymer at the interface show a marked adsorption of the polymer in the presence of surfactant, which decreases with increasing surfactant concentration. The change in polymer adsorption as the surfactant concentration is increased is much less marked for SDS than for the SDS–C12E6 mixture.