Surface Light-Scattering Studies of Surfactant Solutions

Thermally excited capillary waves on solutions of cetyltrimethylammonium bromide and sodium dodecyl sulfate have been studied by light scattering. On solutions of less than 1/10 of the critical micelle concentration, the wave propagation was found to depart rather markedly from simple expectations for wave frequencies above some 3×10 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 surface waves, appear to be entirely consistent with predictions based on a reduction in the dilatational wave damping. The surface viscoelasticity determined directly from the light scattering data supported this conclusion. The principal novel result is that the apparent dilatational surface viscosity thus determined was negative. This is interpreted as an effective property, indicating that some process (or processes) was reducing the damping of the surface dilatational modes. Both aspects 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 viscosity was found to be nonzero, in contrast to the usual assumption that it is negligible for soluble surfactants. Certain aspects of the surface viscoelasticity suggest that the surface amphiphile film undergoes a phase transition at a rather low surface excess adsorption