The dependence of the interactions in foam films on surfactant concentration

The interaction free energy in foam films is investigated depending on surfactant concentration, to prove the differences between the adsorption density of the stabilizing surfactant at the film surfaces and the surface of the bulk phase of the film forming solution. A change of the adsorption density at the film surface compared to the surface of the bulk phase has been indicated by earlier measurements of the gas permeation through foam films. The film interaction free energy was determined from measurements of the contact angles between free foam films and the surrounding bulk phase meniscus. Also, the disjoining pressure isotherms of foam films were measured and used to estimate the film interaction free energy. The foam films were stabilized with the anionic surfactant sodium dodecyl sulphate (SDS). At the surface of Newton black films, the adsorption density is higher than at a normal surface. The disjoining pressure isotherms measured at constant salt concentration but at different surfactant concentrations, reflect the influence of the surfactant concentration on the total electrolyte concentration. The disjoining pressure is always larger than that predicted from the Derjaguin-Landau-Verwey-Overbeek theory (DLVO theory). The investigation of common black films gives some indication that the adsorption density at the common black film surfaces is smaller than at the normal solution surface. Deviations of the measured interaction free energy from the values expected from the DLVO theory found in our measurements as well as reported in the literature can be explained by this change of adsorption.