Adsorption of a divalent cationic surfactant onto a silica surface

Abstract

This study is concerned with the adsorption of a divalent cationic surfactant onto an oppositely charged silica surface. The amount adsorbed and thickness of the adsorbed layer have been determined by means of ellipsometry. These data indicate that small micelles, most likely deformed from spherical shape, are formed on the surface at surface concentrations above that required to neutralize the surface. This conjecture is in agreement with the bulk behavior of the studied surfactant, which is dominated by the presence of small micelles up to very high concentrations. We have developed a thermodynamic model for the adsorption process. Central to this model are the bulk micellar aggregation numbers, which have been determined by the time resolved fluorescence quenching (TRFQ) technique. The model contains three crucial steps: i) the formation of a micelle in the bulk; ii) the adsorption of a single micelle to the surface; and iii) the process of increasing the surface concentration of micelles to a finite amount adsorbed. In developing this model, we have used the Poisson–Boltzmann Cell Model (PBCM), together with a Derjaguin-type approximation in the description of the interaction between the surface and the charged micelle.