Mixed surfactants at the air–water interface

Abstract

The extensive use of surfactants in a wide range of household, technological and commercial applications usually involves mixtures of surfactants. In such formulations different types of surfactants are blended to optimise different aspects of performance, and to produce greater flexibility in processing and formulation. Hence understanding the nature of surfactant mixing in solution and at interfaces is of paramount importance. Until recently our understanding of surfactant mixing has been relatively limited. It has been based predominantly on theoretical thermodynamic treatments such as the pseudo-phase approximation and experimental data from techniques such as surface tension. In recent years new experimental approaches, such as neutron scattering, have transformed our understanding of surfactant mixing and have challenged many aspects of the current theoretical treatments. In this article we will focus on surfactant mixing at the air–water interface. The thermodynamic treatments, based on the pseudo-phase approximation, will be introduced and reviewed. The use of neutron reflectivity, in combination with isotopic substitution, to directly determine the surface composition and surface structure in mixed surfactants adsorbed at the air–water interface will be described. Recent data for different binary, ternary and multi-component mixtures will be presented, and discussed in the context of how they have contributed to an improved understanding of surfactant mixing, and how they confirm or challenge different aspects of the current theoretical treatments.