The Influence of Added Salt on the Adsorption of a Divalent Cationic Surfactant onto Silica. Adsorption Isotherms and Thermodynamic Modeling

We report the adsorption isotherms for a cationic divalent surfactant on negatively charged silica. The adsorption isotherms, which have been obtained by means of null ellipsometry, have been determined both in the absence and in the presence of added salt. In order to rationalize the data, we resort to thermodynamic modeling. Two models are used. In one, the adsorbed surfactant is assumed to form spherical micelles at the silica surface. In the other, the surfactant is adsorbed as a bilayer. Both approaches are treated using the Poisson–Boltzmann equation. Calculations show that the model based on spherical aggregates is capable of producing adsorption isotherms resembling the experimentally determined ones. The resemblance is not quantitative, and possible reasons for the deviations are discussed, most notably the assumption used in the model that the micellar surface charge density is not allowed to adapt to the charge density of the silica surface. The bilayer model fails to describe the experimentally obtained adsorption isotherms.