SURFACE LIGHT-SCATTERING-STUDIES OF SURFACTANT SOLUTIONS

Thermally excited capillary waves on solutions of cetyltrimethylammoni um bromide and sodium dodecyl sulfate have been studied by light scatt ering. On solutions of less than 1/10 of the critical micelle concentr ation, the wave propagation was found to depart rather markedly from s imple expectations for wave frequencies above some 3 x 10(5) s(-1). At lower wave frequencies and higher concentrations, the capillary wave behavior was closer to that expected. The discrepancies, which include the occurrence of manifest mixing of the capillary and dilatational s urface waves, appear to be entirely consistent with predictions based on a reduction in the dilatational wave damping. The surface viscoelas ticity determined directly from the light scattering data supported th is conclusion. The principal novel result is that the apparent dilatat ional surface viscosity thus determined was negative. This is interpre ted as an effective property, indicating that some process (or process es) was reducing the damping of the surface dilatational modes. Both a spects of the results indicate the action of some influence acting to decrease the stability of the dilatational waves. Possible mechanisms are briefly discussed. Additionally, the transverse shear surface visc osity was found to be nonzero, in contrast to the usual assumption tha t it is negligible for soluble surfactants. Certain aspects of the sur face viscoelasticity suggest that the surface amphiphile film undergoe s a phase transition at a rather low surface excess adsorption.