Surface Conductivity and Disjoining Pressure of Common Black Films Stabilized with Sodium Dodecyl Sulfate

Abstract

We have conducted simultaneous measurements of the disjoining pressure and the film conductance for sodium dodecyl sulfate- (SDS-) stabilized foam films in aqueous electrolyte solutions and over a range of film thicknesses. The disjoining pressure ascertains the repulsive forces within the film while the conductance gauges the possible changes in mobilities and concentrations of the ions within the thin-film region, relative to their bulk values. The disjoining pressure is measured using a modified version of the porous-glass-plate technique. To permit electrical measurements in the same apparatus, a novel film holder was designed in which the film is formed in the annular region between two concentric, porous stainless steel disks. The inner and outer disks serve as electrodes, and they constrain the current to flow radially through the film. To avoid electroosmosis, the film conductance is obtained using AC impedance spectroscopy. New data are reported for the disjoining pressure, conductance, and film thickness of 0.002-M SDS foam films with and without added sodium nitrate. Film conductances exhibit a decrease with decreasing film thickness, which suggests a decrease in the mobility of the film ions relative to their bulk values, even for films as thick as 40 nm. Surface-charge densities are estimated from the conductance data and are consistent with available surface tension data. However, the surface-charge densities estimated from the disjoining-pressure data are more than an order of magnitude smaller than the values estimated from conductance and surface-tension data. Improvements in both experimental precision and theory are needed to overcome this discrepancy.