Surface Charging Mechanism and Disjoining Pressure of Electrolytic Soap Films

We present a theory developed to describe the diffuse double layer of electrolytic soap films. The hydrophobic nature of the soap molecules is modeled, in an effective field theory for the system, by the introduction of a free energetic potential favoring the presence of the soap molecules at the film surface. This potential can take various forms, depending on how accurately we wish to model the geometry of the system. The resulting theory is analyzed and leads to the estimation of the electrostatic interactions at the mean field level. A whole range of physical observables is predicted including the electrostatic component of the disjoining pressure isotherm and the Gibbs adsorption isotherm of the system. We compare the theory with accurate disjoining pressure measurements (using X-ray specular reflectivity) carried out on systems composed of sodium dodecyl sulphate (SDS) with two different monovalent electrolytes on a wide range of salinity. The theory appears to account particularly well for the experimental data. A comparison with the DLVO theory is given. Finally, we discuss the limitations of the theory and its modification due to the inclusion of dispersion forces. Copyright 1997 Academic Press. Copyright 1997Academic Press