Recent developments and applications of the thermodynamics of surfactant mixing

In their diverse domestic, industrial and technological applications surfactants are invariably used as mixtures which optimise different aspects of their performance. The neutron scattering techniques of reflectivity and small-angle scattering have recently transformed our ability to probe surfactant mixing at interfaces and in self-assembly. This has in part stimulated developments in the application of different thermodynamic approaches, and in particular the pseudophase approximation, to quantify the mixing behaviour. In this paper, we present some recent applications of developments of the pseudo phase approximation to the surface mixing of nonionic, anionic-nonionic, and cationic - nonionic mixtures. These examples provide new insights into the factors controlling surfactant mixing. They highlight the importance of accounting for asymmetry in the mixing, the advantages of co-refining surface and micelle data, and the importance of determining the mixing properties over a wide concentration and composition range.