Disentanglement of hydrophobic and electrostatic contributions to the film pressures of ionic surfactants

Abstract

The theory of electrolyte solution surfaces is extended to treat ionic surfactants spread on hydrocarbon–water interfaces. The object is to study the interplay between hydrophobic and electrostatic interactions responsible for micelle and membrane formation. Considerable attention is given to developing the theory in a reliable way so that hydrophobic effects can be reliably inferred by comparison with measured film pressures of ionic surfactants. The theory so constructed turns out to be remarkably sensitive to molecular-scale features of the interfacial region, particularly to the nature of water penetration behind the ionic head groups. The theory which assumes negligible water penetration provides a reasonable qualitative, but quantitatively deficient, description of the available experimental data for film pressures of sodium decylsulphate and sodium dodecylsulphate. After the cause of this behaviour is traced to the degree of water penetration permitted in the treatment, we adopt a heuristic modification of the result which compensates for the assumed neglect of water penetration. The resulting formulae provide an excellent quantitative description of the experimental film-pressure data. However, the heuristic nature of the result indicates that a considerably deeper molecular understanding of the short-ranged structure of these interfaces will be required before a conclusive picture can be presented.