Adsorption kinetics and structural arrangements of cationic surfactants onsilica surfaces

The adsorption of cetyltrimethylammonium bromide (CTAB) to the silica-aqueo us interface! has been studied using optical reflectometry. The effect of p H, salt, and surface preparation on the surface excess and adsorption kinet ics has been studied. The adsorption kinetics have been measured and compar ed to the theoretical diffusion limited flux of surfactant, the quotient be ing the "sticking ratio". Analysis of the sticking ratio as a function of C TAB concentration reveals that the adsorption process is cooperative above the critical micelle concentration (CMC). At the critical surface aggregati on 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 withi n minutes. These results indicate that, above the CMC, micelles adsorb dire ctly to the silica surface, and this is reflected in the structure of the s urface layer. At the CSAC the equilibrium surface structure is;analogous to adsorbed micelles, but as only monomers are present in solution, the adsor ption proceeds to equilibrium slowly.