SURFACE MODIFICATION OF COLLOIDAL SILICA

Monodisperse colloidal silica particles with diameters in the range 50-700 nm were prepared by hydrolysis of tetraethyl orthosilicate in water, ammonia, and ethanol. The 58-nm particles are either non-porous or only slightly porous, and they contain 6.0 mequiv/g of silanol and 1.03 mequiv/g of ethoxy groups. Assuming 6.3-10.0 µmol/g of surface ethoxy and silanol groups, the interior of the bulk silica must contain 6.3-6.6 mequiv/g of (ethoxy + silanol) groups. The surfaces of the 58-nm particles were modified by reactions with (3-mercaptopropyl)trimethoxysilane, (3-aminopropyl)trimethoxysilane, and octadecyltrimethoxysilane. The surface modifications with mercaptopropyl and aminopropyl groups can be done in dimethylformamide. A more convenient, one-pot synthesis for preparation and surface modification of colloidal silica proceeds by reaction of the trimethoxysilane with colloidal silica in the original mixture of water, ammonia, and ethanol. With (mercaptopropyl)silica, this one-pot synthesis requires steric stabilization, which is accomplished by addition of an ABA block copolymer of polyethylene oxide) and poly(propylene oxide) to the reaction mixture. Octadecylsilica precipitates during the synthesis, providing an easy method to purify and transfer the octadecylsilica to a nonpolar solvent. 13C and 29Si NMR, BET surface area analysis, pycnometry, thermogravimetric analysis, mass spectrometry, and elemental analysis have been used to characterize the unmodified and modified colloidal silicas. © 1990, American Chemical Society. All rights reserved.