Surface and colloidal properties of hydrosilane-modified Stöber silica

Triethoxysilane (TES), HSi(OEt)3, was used to functionalize Stöber silica and the resulting particles were characterized in acetone and water by light scattering, disk centrifugal photosedimentometry, microelectrophoresis, transmission electron microscopy, and 29Si CP-MAS and 1H MAS NMR spectroscopy. The thickness of the polymerized TES layer on the silica surface was found to be dependent upon the concentration of the TES solution used. Thus, 38 mM TES (polyTES38) and 75 mM TES (polyTES75) led to bound polyTES layers that were 7.6 nm and 8.2 nm thick respectively. Using an initial concentration of 150 mM TES, polyTES150 coagulated in acetone as reflected by broad particle size distributions. In the range of pH 5.0 to 9.0 the polyTES38 particle diameter increased from 191–200 nm, indicating that an expandable polyTES layer existed in the interfacial region. 1H MAS NMR spectra of polyTES38 showed narrowed 1H line widths, suggesting the presence of a mobile species on the silica surface. The critical coagulation concentrations cc determined for SiOH and polyTES38 using CaI2 were 10 mM and 11.5 mM respectively. Under aqueous conditions at low pH the polyTES layer collapsed due to the poor solvency of the polyTES chains, whereas at pH > 7 the polyTES segments were expanded due to intra/interchain electrostatic repulsion between the negatively charged polyTES segments and with the silica surface.