Hydrolysis, adsorption, and dynamics of silane coupling agents on silica surfaces

The hydrolysis of alkoxysilane coupling agents has been followed using proton NMR. The disappearance of the silane ester and the appearance of the alkoxy group were observed to follow pseudo-first-order kinetics. The apparent rate constant was found to be dependent on the concentration of the silane coupling agent and the pH. The adsorption of the coupling agents onto Cab-O-Sil silica was quantified using FT-IR spectroscopy. It was found that monolayer coverage was obtained with about 0.65 mmol/100 m2. Solid-state deuterium NMR was used to probe the behavior of the coupling agent on the silica. The coupling agents adsorbed on the surface from deuterated aminopropyltriethoxysilane (DAPES) and deuterated aminobutyltriethoxysilane (DABES) showed similar motional characteristics. In contrast, the mobility of condensed polymers from DABES was much less restricted than that from DAPES. The amount of hydrolysis in acetone-water mixtures increased the amount of surface-adsorbed material and lessened surface mobility. Overpolymerization with bismaleimide increased the rigidity of the surface layer of the coupling agent.