Capillary Waves in Ionic Surfactant Solutions:  Effects of the Electrostatic Adsorption Barrier and Analysis in Terms of a New Dispersion Equation

We have studied the dispersion of capillary waves on the surface of aqueous solutions of the cationic surfactant dodecylammonium chloride (DAC) by using surface quasi-elastic light scattering (SQELS) combined with equilibrium surface tension measurements. To test the role of electrostatics on the dynamical behavior of the adsorbed surfactant monolayer at the air/solution interface, experiments in absence and presence of an inert electrolyte (KCl) have been performed. The equilibrium results have been analyzed in terms of the Frumkin−Davies equation of state. For the salt-free system, the Lucassen−van den Tempel model describes the dilational viscoelasticity results provided that the effects of adsorption barrier are accounted for. In contrast, when inert electrolyte is added, the viscoelastic behavior is anomalous, and negative apparent dilational viscosities are obtained. A new dispersion equation is proposed that accounts for internal coupling between capillary and dilational modes and leads to nonanomalous dilational viscosities.