Evaluation of DLVO theory with disjoining-pressure and film-conductance measurements of common-black films stabilized with sodium dodecyl sulfate

Abstract

We develop a unique film holder combining a thin-film balance with AC impedance spectroscopy to measure disjoining pressure, film conductance, and film thickness simultaneously. Foam films stabilized by sodium dodecyl sulfate (SDS) are investigated with and without added sodium chloride (NaCl) electrolyte. Classical colloidal theory, Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, is tested rigorously over a wide range of solution conditions by comparing the surface charge densities fit to disjoining-pressure isotherms with those estimated independently from film-conductance, and surface-tension data. Film-conductance measurements strongly suggest that the adsorbed anionic surfactant is partially complexed with counterions. Therefore, to reconcile the different values of charge densities calculated from surface tension and film conductance with those from disjoining pressure, we propose a simple ion-binding electrostatic model. The ion-complexation framework predicts increased ion complexing with increasing solution ionic strength, in agreement with surface-tension and film-conductance data. Unfortunately, it is not possible to describe similarly the trends of the measured disjoining-pressure isotherms because the diffuse-layer charge density increases, or equivalently, the ion complexation decreases with increasing ionic strength. Accordingly, the ion-binding extension of classical DLVO theory does not permit agreement between theory and independent experimental data from surface tension, disjoining pressure, and film conductance. (C) 2003 Elsevier Science (USA). All rights reserved