Coating of Silica on Titania Pigment Particles Examined by Electroacoustics and Dielectric Response

The coating of titania with a hydrous silica layer was investigated by electroacoustics, high-frequency dielectric response (DR), IR spectroscopy, and TEM microscopy. The shift in the isoelectric point measured by electroacoustics was the most sensitive indicator of silica coverage of the titania surface. Changes in the shape of the zeta potential pH dependence corresponding to formation of silica multilayers were accompanied by small changes in the IR spectra and TEM thickness. The coatings formed at 90 °C were dense and smooth and extended up to 10 nm from the titania surface. Changes in the dielectric constant of the coated pigment occurred over three domains and corresponded to changes in the nature of the adsorbing silica layer. At low concentrations, the relative permittivity of the coating layer was ∼5 but at higher concentrations it was ∼20, indicating greater hydration of the multilayer.