Adsorption of a divalent cationic surfactant onto a silica surface

This study is concerned with the adsorption of a divalent cationic surfacta nt onto an oppositely charged silica surface. The amount adsorbed and thick ness of the adsorbed layer have been determined by means of ellipsometry. T hese data indicate that small micelles, most likely deformed from spherical shape, are formed on the surface at surface concentrations above that requ ired to neutralize the surface. This conjecture is in agreement with the bu lk behavior of the studied surfactant, which is dominated by the presence o f small micelles up to very high concentrations. We have developed a thermo dynamic model for the adsorption process. Central to this model are the bul k micellar aggregation numbers, which have been determined by the time reso lved fluorescence quenching (TRFQ) technique. The model contains three cruc ial 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 su rface 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 b etween the surface and the charged micelle. (C) 1999 Elsevier Science B.V. All rights reserved.