In situ Infrared Technique for Studying Adsorption onto Particulate Silica Surfaces from Aqueous Solutions

An in situ infrared technique is described which allows the detection of adsorbed surface species on metal oxide particles in an aqueous environment. The technique involves first formulating a “coating” comprised of high-surface-area silica particles and a polymeric binder in a suitable solvent. The resulting coating is applied to the surface of an internal reflection element and mounted in a flow-through attenuated total reflection (ATR) apparatus. The technique is demonstrated with a ZnSe element coated with fumed silica particles in a polyethylene (PE) matrix. Access of the silica surface in the matrix to adsorbates was evaluated by comparing the gas-phase reaction of silanes on silica/PE-coated CsI windows in transmission with silica/PE-coated ZnSe in an ATR evacuable cell. It is shown that the PE weakly perturbs about 25% of the surface hydroxyl groups, and that all surface groups are available for reaction with silanes. The silica/PE is indefinitely stable in an aqueous environment and has advantages of at least 2 orders higher sensitivity and a wider spectral range over studies using oxidized silicon wafers. The usefulness of this technique for studying adsorption on metal oxide surfaces is demonstrated with the reaction of succinic anhydride on an aminosilanized silica surface. This reaction sequence is a common method used to prepare glass surfaces in the attachment of probe oligonucliotides for microarray biochip technology.