In situ ATR-FT-IR kinetic studies of molecular transport and surface binding in thin sol-gel films: Reactions of chlorosilane reagents in porous silica materials

ATR-FT-IR spectroscopy was employed to study the kinetics of transport and binding within thin silica sol-gel films. Studies of transport of several nonbinding probe molecules n-heptane, toluene, and 2-propanol, showed that slow diffusion occurs within the micropores of the sol-gel films which could be modeled as a single-exponential accumulation in agreement with numerical models for diffusion in constricted pores. The rate of transport into the film was found to decrease for molecules that interact strongly with the silica surface, which is consistent with adsorption inhibiting the transport of molecules through the pores. In situ spectroscopic studies of surface reactions with diphenylchlorosilane (DP-SICl) reveal that DPSiCl reacts quickly with surface water to form diphenylhydroxysilane (DPSiOH), the reactive species detected within the film. Analysis of the time-dependent infrared spectra reveals both transport and surface-binding steps in the reaction kinetics. From the magnitudes of the rate constants and the corresponding pure component spectra, it is determined that the surface- binding component is responsible for accumulation of most of the silane at the silica surface. Ex situ spectro scopic studies confirm that Si-O-Si bond formation occurs at room temperature in these sol-gel films. Studies of chlorosilane reactions at silica surfaces pretreated with Methylamine were conducted to investigate the influence of amines on this chemistry; it was determined that the amine enhances the transport of more reagent molecules to the silica surface while the intrinsic rate of the binding reaction is not significantly changed.