Surface Tension of an Anionic Surfactant:  Equilibrium, Dynamics, and Analysis for Aerosol-OT

The equilibrium and dynamic surface tension of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT) are studied as a function of concentration and ionic strength controlled by the addition of either the monovalent salt sodium chloride or the divalent salt calcium chloride. These data are compared to a surfactant mass-transfer model with a quasi-equilibrium treatment of the electrostatics. The Davies adsorption isotherm and surface equation of state relate the bulk concentration, surface concentration, and surface tension. At equilibrium, the surface concentration increases with the ionic strength of the electrolyte, so the surface tension reduces more strongly. The data at all ionic strengths are well described by the Davies model. Because the characteristic diffusion time scale increases as the square of the surface concentration, an increasing equilibration time with ionic strength might be anticipated for this molecule. However, the time required for the surface tension relaxation observed in experiment is fairly insensitive to changes in the ionic strength over the range of surfactant concentrations studied for both monovalent and divalent electrolytes at fixed surfactant bulk concentration. When these data are compared to a full integration of the surfactant transport equations, they are found to agree with a diffusion-controlled mass-transfer mechanism. The key issue behind the apparently contradictory behavior of increased adsorption resulting in lower equilibrium surface tensions, while diffusion time scales remain essentially unchanged, is the high surface activity of Aerosol-OT. Even at the most dilute concentrations studied, Aerosol-OT adsorbs close to its maximum packing limit. The surface concentration increases weakly near this value with ionic strength. Therefore, the diffusion time scale also changes weakly. Concomitantly, the equilibrium surface tension changes strongly because it is highly sensitive to small changes in surface concentration near this limit.