Hydrophobic forces in the foam films stabilized by sodium dodecyl sulfate: effect of electrolyte.

Further studies of the hydrophobic force in foam films were carried out, including the effect of added inorganic electrolyte. We used a thin film balance of Scheludko-Exerowa type to obtain the disjoining pressure isotherms of the foam films stabilized by 10(-4) M sodium dodecyl sulfate in varying concentrations of sodium chloride. The results were compared with the disjoining pressure isotherms predicted from the extended Derjaguin-Landau-Verwey-Overbeek theory, which considers contributions from hydrophobic force in addition to those from double layer and van der Waals dispersion forces. The double layer forces were calculated from the surface potentials (psi s) obtained using the Gibbs adsorption equation and corrected for the counterion binding effect, while the dispersion forces were calculated using the Hamaker constant (A232) of 3.7 x 10(-20) J. The hydrophobic forces were calculated from the equilibrium film thickness as described previously. The predicted disjoining pressure isotherms were in good agreement with the experimental ones. It was found that the hydrophobic force is dampened substantially by the added electrolyte.