SURFACE CONDUCTIVITY AND DISJOINING PRESSURE OF COMMON BLACK FILMS STABILIZED WITH SODIUM DODECYL-SULFATE

We have conducted simultaneous measurements of the disjoining pressure and the film conductance for sodium dodecyl sulfate- (SDS-) stabilize d 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 m obilities and concentrations of the ions within the thin-film region, relative to their bulk values. The disjoining pressure is measured usi ng a modified version of the porous-glass-plate technique. To permit e lectrical 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 s erve as electrodes, and they constrain the current to flow radially th rough the him. To avoid electroosmosis, the film conductance is obtain ed using AC impedance spectroscopy. New data are reported for the disj oining pressure, conductance, and film thickness of 0.002-M SDS foam f ilms with and without added sodium nitrate. Film conductances exhibit a decrease with decreasing film thickness, which suggests a decrease i n the mobility of the film ions relative to their bulk values, even fo r films as thick as 40 mn. Surface-charge densities are estimated from the conductance data and are consistent with available surface tensio n data. However, the surface-charge densities estimated from the disjo ining-pressure data are more than an order of magnitude smaller than t he values estimated from conductance and surface-tension data. Improve ments in both experimental precision and theory are needed to overcome this discrepancy. (C) 1998 Academic Press.