Adsorption of cationic surfactants on silica. Comparison of experiment and theory

The adsorption of alkylpyridinium surfactants on Aerosil OX50 silica has been studied as a function of surfactant concentration, aliphatic chain length, pH, and ionic strength in solution. The adsorption isotherms are complemented by electrophoretic mobility measurements. The experimental results have been compared with calculations based on the SCFA theory for adsorption on a charge-regulating surface. The experimental results and SCFA calculations both show that the isotherms of the cationic surfactant on silica, obtained at different salt concentrations, have a common intersection point (cip). As was shown before by surface charge adaptation experiments and is now confirmed by the theoretical calculations, aliphatic segments have a moderate affinity to the surface due to hydrophobic interactions. These interactions result in a flat orientation of the surfactant molecules in the adsorbed layer. The shape of the adsorption isotherms depends on the salt concentration. The first pseudoplateau observed at low salt concentrations vanishes at high salt concentration. At low salt concentrations, electrostatic repulsions between ''head-on'' adsorbed headgroups and local crowding prevent the aggregation process in the adsorbed layer and inhibit further adsorption in the region of the first pseudoplateau. At high salt concentrations, the adsorption starts later due to the screening of the surface charge by the salt ions, and it increases steeper due to the diminished lateral repulsions between surfactant molecules. The common intersection point in the isotherms measured at different salt concentrations marks the transition from a nat monolayer to a flat bilayer.