Neutron reflection study on surfactant systems at different interfaces

The interfacial behaviour of several surfactant mixtures containing zwitterionic surfactants has been studied employing surface tension measurements and neutron reflection experiments. The mixing of surfactants at air–liquid interface could not be well understood using the pesudophase approximation and the regular solution model. By introducing the expansion of excess free energy to analysis the surface tension and neutron reflection data simultaneously, it is found that the mixing deviates strongly from ideal and the excess free energy is asymmetric. It is observed that the presence of ionic surfactant at solid–liquid interface enhanced the adsorption of zwitterionic surfactants. The configuration of the surfactant molecule in the inner layer adjacent to the surface is invoked to explain the underlying mechanism. Maxima in the adsorption just above the CMC occurred at all three compositions studied for the mixed system SDS with dodecyl dimethylammonium propanesulfonate (DSB). The neutron reflection experiments suggest that the phenomenon may result in part from the mixture and in part from ion segregation in the micelles. Structural studies on a series of dodecyl dimethylammonium acetate with different inter–charge spacer revealed that the configuration of the surfactants changes dramatically as the headgroup gets larger. Combined with the higher pKa value, it is highly possible that the carboxybetaine with longer spacer is adsorbed in its undissociated form, and the headgroup is oriented towards the air. This configuration might explain why the saturated monolayer structure is favoured over the micellization.