Theoretical description of surface elasticity of ionic surfactants

We propose a new theoretical description of surface elasticity of ionic sur factant solutions. Assuming small periodic disturbances of the solution sur face we show that in the range of frequencies available for most experiment al techniques used to study viscoelastic properties of interfaces, the anal ytical 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 forme d at the interface due to adsorption of ionic surfactant. Then, two solutio ns are matched together. The obtained results depend strongly on the theore tical model used to describe adsorption of ionic surfactant at the interfac e. We also present a comparison between theoretical predictions and prelimi nary experimental results obtained for both anionic and cationic surfactant s using the oscillating bubble technique. (C) 2001 Elsevier Science B.V. Al l rights reserved.