Adsorption Kinetics and Structural Arrangements of Cationic Surfactants on Silica Surfaces

Abstract

The adsorption of cetyltrimethylammonium bromide (CTAB) to the silica-aqueous interface has been studied using optical reflectometry. The effect of pH, salt, and surface preparation on the surface excess and adsorption kinetics has been studied. The adsorption kinetics have been measured and compared to the theoretical diffusion limited flux of surfactant, the quotient being the `sticking ratio'. Analysis of the sticking ratio as a function of CTAB concentration reveals that the adsorption process is cooperative above the critical micelle concentration (CMC). At the critical surface aggregation concentration (CSAC), adsorption proceeds slowly in the absence of salt and takes hours to reach an equilibrium value. At all other concentrations and even at the CSAC when salt is present, the adsorption is complete within minutes. These results indicate that, above the CMC, micelles adsorb directly to the silica surface, and this is reflected in the structure of the surface layer. At the CSAC the equilibrium surface structure is analogous to adsorbed micelles, but as only monomers are present in solution, the adsorption proceeds to equilibrium slowly.