Effect of the counterions on the surface properties of surfactant solutions: Kinetics of the surface tension and surface potential

The kinetics of adsorption from anionic surfactant solutions, containing counterions of different valence, is investigated by measuring the surface tension as a function of time and the surface potential. The surfactant used is sodium dodecyl polyoxiethylene-2 sulfate; the electrolytes are chlorides of sodium, calcium and aluminum at a constant total ionic strength of 0.024 M. A strong effect of the counterions on the adsorption kinetics is found. The increasing counterion valence leads to a faster relaxation of the surface tension toward its equilibrium value. A theoretical model for diffusion controlled adsorption of charged surface active ions in the presence of counterions is proposed. Asymptotic equations for the surface tension as a function of time are derived accounting for the contribution of micelles present in the solution. The theoretical predictions agree quantitatively with the experimental data. The calculated model parameters, characteristic diffusion and micellization time-constants are of reasonable magnitude. The results from surface tension experiments are in accordance with the surface potential measurements, which give an increase of the surfactant adsorption in the presence of multivalent ions.