Anomalous damping of capillary waves with surfactant solutions

We have studied the dilational viscoelasticity of the surface of various su rfactant solutions by using an excited capillary waves technique. We find t hat the dilational viscosity is negative in some cases, depending on the su rfactant chain length, ionic strength, pH, bulk concentration and frequency of the waves. We have investigated the linearity of the response of the mo nolayer, and the possible influence of surface modes coupling. A correlatio n with phase behavior in the monolayer is discussed.