The Colloidal Stability and Surface Chemistry of Stöber Silica

Abstract

<p>Colloidal silica was prepared by the Stöber method to give particles having a narrow size distribution and spherical geometry. The electrokinetic and coagulation behavior of the particles were investigated in water, acetone, and acetone-water mixtures and the experimental data were compared to simple theoretical models. The relative permittivity, er. of the dispersion medium was varied between 21 and 79 by controlling the acetone:water ratio. For εᵣ >33, the particles could not be coagulated using Nal concentrations up to 0.5 M. The critical coagulation concentration (Cᴄ) values could not be predicted by calculations based on using standard DLVO theory. In the absence of salt, the zeta-potentials (ζ-potentials) remained constant at approximately -50 mV when 24.3 < εᵣ < 78.5 but increased to -80 mV for εᵣ < 24.3. A single-site dissociation model predicted that the ζ-potentials should decrease with increasing acetone content. The inability of the theoretical models to predict the colloidal stability behavior of silica in acetone and acetone - water mixtures, especially when εᵣ > 33, was believed to be due to the presence of a silica ger surface layer which acted as a stene barrier at short-range interparticle distances.</p> <p>The colloidal stability and surface properties of Stöber silica modified with triethoxysilane (TES) (HSi(OEt)₃) was studied (polyTES). The polyTES particles were reacted with vinylnaphthalene, styrene, and vinyl-terminated poly(dimethylsiloxanes) using the platinum-catalyzed hydrosilation reaction. These reactions demonstrated that Stöber silica modified with TES can lead to a variety of colloidally stable dispersions in solvents ranging in polarity from water to hexane. It was also shown that bis(1,3-divinyl,1,1,3,3-tetramethyldisiloxane)Pt⁰ could be hydrosilated on the polyTES surface to give catalytically active, surface-bound platinum nanoparticles with an average diameter of 2 nm. The polyTES layer grafted to the silica surface was believed to be critical in controlling the ultimate size of the supported platinum nanoparticles. The surface properties of the polyTES particles were studied and it was shown that the polyTES chains had a high surface density of 2.22 nm⁻² and they expanded when solvated by aqueous alkaline solutions.</p>