Dynamic surface tension and kinetics of surfactant adsorption at fluid interfaces

Surfactant kinetics at fluid-fluid interfaces is studied within a free energy formalism, which provides a general method for calculating dynamic surface tension. For non-ionic surfactants the results coincide with previous models. Common non-ionic surfactants are shown to undergo diffusion-limited adsorption, in agreement with experiments. On the other hand, ionic surfactants such as SDS without added salt behave quite differently. Strong electrolytes lead to kinetically limited adsorption, slower dynamics and intermediate plateaux in the kinetic surface tension as was observed in experiments. Upon addition of salt the electrostatic interactions are screened and the adsorption becomes more similar to the non-ionic case. We also present results for the adsorption of a multi-component surfactant solution. We examine the case when one species has a fast diffusion in the bulk while the second species has slower dynamics but a stronger affinity to the surface. At later times the competition results in the desorption of the faster species due to the adsorption of the surfactant with higher surface affinity.