Evaluation of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory with disjoining-pressure and film-conductance measurements of surfactant stabilized free foam films.

摘要

We develop a unique film holder combining the thin-film balance with AC impedance spectroscopy to measure simultaneously disjoining pressure, film conductance, and film thickness. 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 over a wide range of solution conditions by comparing the effective surface-charge density values fit to disjoining-pressure isotherms, estimated from surface tension, and inferred from film conductance. To reconcile the different values of effective surface charge density calculated from surface tension and film conductance, we propose that the surfactants adsorbed at the film interface are partially complexed with counterions. We develop a mass-action model, incorporating ion-binding, to predict the effective surface-charge density at the diffuse double layer. Our ion-binding model predicts that the degree of ion binding increases with increasing ionic strength as shown by the fits to the film-conductance data. However, we find it is not possible to describe the trends in the disjoining-pressure data because the DLVO model fits indicate that the effective surface-charge density increases, or ion binding decreases, with increasing ionic strength. Additional film-conductance measurements of Newton-black films indicate these surfactant ions and counterions retain 15% to 24% of their bulk mobility.