The adsorption and self-assembly of surfactant mixtures: How the detailed evaluation of adsorption properties provides access to the bulk behaviour

The nature of surfactant mixing at interfaces and in bulk solution is key to understanding and optimising the diverse industrial, technological, biological and domestic applications of surfactants. The use of neutron reflectivity, NR, and small angle neutron scattering, SANS, in combination with isotopic substitution, has transformed the ability to quantify and understand the nature of surfactant mixing at the air-water interface and in self-assembled aggregates or micelles in solution. The accuracy and scope of the compositional data from NR, the application of recent developments in the pseudo phase approximation, PPA, and the availability of complementary critical micelle concentration, cmc, and micelle composition data, enables a detailed thermodynamical quantification of the mixing properties to be made. The NR data in particular, and the SANS data to a lesser extent, provides constraints on the thermodynamical analysis which reveals important properties and trends about the bulk phase which are not available from the analysis of data such as the variation in the cmc alone. The importance and impact of this approach is illustrated with an overview of a range of mixed surfactant examples from the recent literature, and which encompass mixtures with different degrees of departure from ideality.

Automated summary

Understanding and optimizing the mixing properties of surfactants at interfaces and in bulk solution is crucial for their diverse applications in industry, technology, biology, and households. The use of neutron reflectivity (NR) and small angle neutron scattering (SANS) with isotopic substitution has enabled quantitative analysis and understanding of surfactant mixing at the air-water interface and in self-assembled aggregates or micelles in solution. This approach provides important insights into the thermodynamic properties and trends of mixed surfactants, which are not readily obtainable from analyzing data solely based on the variation in critical micelle concentration (cmc) alone.