DIFFUSE LAYER ELECTROSTATIC POTENTIAL AND STABILITY OF THIN AQUEOUS FILMS CONTAINING A NONIONIC SURFACTANT

The stability and equilibrium thicknesses of free thin films produced from aqueous solutions of C12(EO)5 surfactant were studied in a wide range of concentrations and pH values. The films were found to be stabilized by repulsive electrostatic disjoining pressure, and the psi-0 potential of the diffuse layer of the film was estimated from DLVO theory. At the natural pH, ca. 5.7, it was found to have a value of ca. 42 mV at the lower surfactant concentrations and decrease at concentrations above 10(-5) M. The decrease is noticeable already below the critical micelle concentration. Also the equilibrium film thickness was found to be pH dependent; the results showed a pH (iep) almost-equal-to 3 and negative surface charge at pH > 3. It was concluded that the dependence of psi-0 potential on surfactant concentration was due to the displacement of ions (H+ and OH-) from the nonionic surfactant adsorption layer at high surface coverage, where structural changes are expected to occur.