DEPENDENCE OF THE EQUILIBRIUM THICKNESS AND DOUBLE-LAYER POTENTIAL OF FOAM FILMS ON THE SURFACTANT CONCENTRATION

The effect of the surfactant concentration cs on the equilibrium thickness hr, of foam films and the potential φ{symbol}0 of the diffuse electric layer is studied in a wide concentration range (including cmc). The cell for investigation of thin liquid films is improved and a good reproducibility of the measured hr is obtained. The possibilities for determination of the capillary pressure Pσ in.the cell for investigation of microscopic thin liquid films are discussed. The values of Pσ can be measured with sufficient accuracy by the capillary-rise method, using a tube, identical with the tube of the cell for thin liquid films. The dependence hr(cs) is studied for nonionic surfactants (decylmethyl sulphoxide, nonyl-phenol 20-glycol ether and dodecyl 11-ethylene oxide) and the ionic sodium dodecyl sulphate. After reaching a steady value (a plateau in the hr(cs) curve) at high concentrations (about cmc) a decrease in the equilibrium thickness of nonionic surfactants is found. The possible reasons for this decrease are discussed. It is shown that this effect probably is due to the electrostatic disjoining pressure. The values of the φ{symbol}o-potential at the plateau of the hr(cs) curves are determined by the DLVO-theory using the hr, values. The so determined φ{symbol}0-values are more correct than the obtained earlier by this method because of the more precise measurement of the equilibrium thickness and the direct experimental determining of the capillary pressure. A qualitative interpreting of the φ{symbol}0(cs) dependence is made on the basis of the surfactant adsorption at the solution/air interface and it is found that for nonionic surfactants the surface charge saturation is very near to the saturation in the adsorption layer, and for the ionic surfactants the adsorption layer saturation is at higher concentration. © 1979 Dr. Dietrich Steinkopff Verlag.