Electrical Double Layer and London — van der Waals Forces in Soap Films Studied by Laser Light Scattering

Abstract

AbstractElectrical double layer and London — van der Waals forces in soap films are studied as a function of thickness by means of time‐averaged light scattering and Photon Correlation Spectroscopy. The free films are drawn from a solution containing 10−2 mol/dm3 of the cationic surfactant dodecyltrimethylammonium bromide. Thermally excited surface waves on the interfaces of a film give rise to a measurable diffuse reflection, being a combination of squeezing mode and bending mode scattering. Because of the large difference in the characteristic time scales the dynamics of the squeezing and bending modes can be studied separately. The relaxation of the bending mode was measured using Photon Correlation Spectroscopy yielding the surface tension of the soap film. Up to frequencies of about 4 MHz this tension appears to be equal to the bulk value of the surface tension. From thickness and angle dependent time‐averaged light scattering experiments we obtained V″, being the second derivative of the free energy of film formation with respect to film thickness. The experimental data are interpreted by the DLVO theory using the Lifshitz theory for the London — van der Waals part of V″. We found an exponential thickness dependence of the electrical double layer contribution to V″. The experimental decay length of the latter contribution agrees very good with the theoretical value of the Debye length. For the surface potential a value of 95 mV was found.