SURFACE AND COLLOIDAL PROPERTIES OF HYDROSILANE-MODIFIED STOBER SILICA

Triethoxysilane (TES), HSi(OEt)(3), was used to functionalize Stober s ilica and the resulting particles were characterized in acetone and wa ter by light scattering, disk centrifugal photosedimentometry, microel ectrophoresis, transmission electron microscopy, and Si-29 CP-MAS and H-1 MAS NMR spectroscopy. The thickness of the polymerized TES layer o n 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 (p olyTES75) led to bound polyTES layers that were 7.6 nm and 8.2 nm thic k respectively. Using an initial concentration of 150 mM TES, polyTES1 50 coagulated in acetone as reflected by broad particle size distribut ions. In the range of pH 5.0 to 9.0 the polyTES38 particle diameter in creased from 191-200 nm, indicating that an expandable polyTES layer e xisted in the interfacial region. H-1 MAS NMR spectra of polyTES38 sho wed narrowed 1H line widths, suggesting the presence of a mobile speci es on the silica surface. The critical coagulation concentrations c(c) determined for SIGH and polyTES38 using CaI2, were 10 mM and 11.5 mM: respectively. Under aqueous conditions at low pH the polyTES layer co llapsed due to the poor solvency of the polyTES chains, whereas at pH> 7 the polyTES segments were expanded due to intra/interchain electrost atic repulsion between the negatively charged polyTES segments and wit h the silica surface. (C) 1998 Elsevier Science B.V.