Theoretical description of surface elasticity of ionic surfactants

We propose a new theoretical description of surface elasticity of ionic surfactant solutions. Assuming small periodic disturbances of the solution surface we show that in the range of frequencies available for most experimental techniques used to study viscoelastic properties of interfaces, the analytical expression for the surface dilatational elasticity can be obtained. Our analytical approach is based on the separate solution of the diffusion equation in two regions, inside and outside the electric double layer formed at the interface due to adsorption of ionic surfactant. Then, two solutions are matched together. The obtained results depend strongly on the theoretical model used to describe adsorption of ionic surfactant at the interface. We also present a comparison between theoretical predictions and preliminary experimental results obtained for both anionic and cationic surfactants using the oscillating bubble technique.