Electrostatic effects during surfactant adsorption

Abstract

The adsorption of surface-active materials is often modeled with empirical semiempirical equations which frequently fail to account for key features such as the effect of chain length and electrostatic interactions either between the adsorbing molecules themselves or between the adsorbents and the adsorbing surface. A mean field theory, which incorporates electrostatic effects, has been used in this study to overcome these deficiencies and predict the adsorbed amount as well as the conformation of surface-active species. For simple surfactants (linear alkyl sulfates) the model predictions for adsorption isotherms exhibit the correct behavior, although the amount adsorbed predicted at saturation is lower than that observed experimentally. The adsorbed molecules are predicted to have very flat conformations under most conditions with significant extension occurring only when the surface density of the adsorbing surface becomes large enough that complete exclusion of the surfactant from the surface is imminent. The potential for local maxima and minima in adsorption isotherms is also demonstrated.