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 introd uction 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 syst em, The resulting theory is analyzed and leads to the estimation of th e electrostatic interactions at the mean field level, A whole range of physical observables is predicted including the electrostatic compone nt of the disjoining pressure isotherm and the Gibbs adsorption isothe rm of the system, We compare the theory with accurate disjoining press ure measurements (using X-ray specular reflectivity) carried out on sy stems composed of sodium dodecyl sulphate (SDS) with two different mon ovalent electrolytes on a wide range of salinity. The theory appears t o account particularly well for the experimental data. A comparison wi th the DLVO theory is given. Finally, we discuss the limitations of th e theory and its modification due to the inclusion of dispersion force s. (C) 1997 Academic Press.