ADSORPTION-KINETICS OF IONIC SURFACTANTS WITH DETAILED ACCOUNT FOR THE ELECTROSTATIC INTERACTIONS .1. NO ADDED ELECTROLYTE

The problem of diffusion-controlled adsorption from a nonmicellar solu tion of an ionic surfactant in the absence of added electrolyte is sol ved analytically for the case of small deviations from equilibrium. Fo r that purpose the electro-diffusion equations of the transport of sur factant ions and counterions are combined with the Poisson-Boltzmann e quation for the electrical held, The resulting set of equations is lin earized and Laplace transform is applied. Analytical expression for th e Laplace image of the adsorption is obtained in terms of elementary f unctions. Simple formulae for the short-time and long-time asymptotics of adsorption and surface tension relaxation are derived. To illustra te the effect of the electrostatic Interactions we calculated the theo retical dependence of the characteristic relaxation time on the bulk s urfactant concentration and surface potential for aqueous surfactant s olutions in contact with various non-aqueous phases (air, heptane, dec ane, petroleum ether) and two surfactants: SDS and DTAB. The general t rend is that the electrostatic effects decelerate the process of adsor ption, as it could be expected. The derived exact analytical expressio ns quantifying these effects can be directly applied for the interpret ation of experimental data for the kinetics of ionic surfactant adsorp tion. The reliability of our approach is verified through a comparison with other available theories. (C) 1997 Academic Press.