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 n onbinding probe molecules n-heptane, toluene, and 2-propanol, showed that s low diffusion occurs within the micropores of the sol-gel films which could be modeled as a single-exponential accumulation in agreement with numerica l models for diffusion in constricted pores. The rate of transport into the film was found to decrease for molecules that interact strongly with the s ilica surface, which is consistent with adsorption inhibiting the transport of molecules through the pores. In situ spectroscopic studies of surface r eactions with diphenylchlorosilane (DP-SICl) reveal that DPSiCl reacts quic kly 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 ki netics. From the magnitudes of the rate constants and the corresponding pur e component spectra, it is determined that the surface- binding component i s 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 reactio ns at silica surfaces pretreated with Methylamine were conducted to investi gate the influence of amines on this chemistry; it was determined that the amine enhances the transport of more reagent molecules to the silica surfac e while the intrinsic rate of the binding reaction is not significantly cha nged.