Hydrophobic forces in thin aqueous films and their role in film thinning

The thin film pressure balance (TFPB) technique was used to study the kinetics of film thinning in the presence of sodium dodecyl sulfate (SDS) and sodium chloride (NaCl). The experimental results were compared with the predictions of the Reynolds equation. During the initial stages, the film thinning was controlled by the capillary pressure. As the film thickness is reduced, the process is controlled by surface forces. The thickness at which the surface forces begin to control the film thinning process varies with the surfactant and electrolyte concentrations. At low surfactant and electrolyte concentrations, there was an excellent fit between the film thinning experiment and the Reynolds equation corrected for the contributions from the hydrophobic forces to the disjoining pressure. In the present work, the magnitudes of the hydrophobic forces were determined from the equilibrium film thicknesses using the TFPB technique. At low surfactant and high electrolyte concentrations, where it was difficult to determine the equilibrium film thicknesses, contributions from the hydrophobic forces were determined by fitting the film thinning data to the Reynolds equation. The hydrophobic forces determined in the present work decreased with increasing SDS and NaCl concentrations. It may, thus, be the role of surfactant and electrolyte to dampen the hydrophobic force and thereby to retard the film thinning process.