X-ray diffraction of silica gels made by sol-gel method under different conditions

Silica gels made from tetra-ethyl-orthosilicate (TEOS) under the basic condition and one made by two-step hydrolysis were studied by X-ray radial distribution function and correlation function analyses, and Si-29 NMR. X-ray scattering intensity, the medium range feature of the radial distribution function (RDF) curve and correlation function curve of silica gels was different from those of melt-derived silica. No difference in X-ray data was noted among the gels. The X-ray RDF data of the silica gels prepared under the basic condition and by the two-step by hydrolysis method were explained by using a medium-range-order (MRO) structure model consisting of fourfold siloxane ring units instead of sixfold siloxane rings which prevail in the melt-derived silica glass. It was estimated that the primary particles of the acid-catalyzed silica gels built up by fourfold rings were around 1.0 nm in size, and those of the base-catalyzed silica gels were a little larger. 40-50% of silicon atoms in the primary particle in the acid-catalyzed gels, and 60-70% of silicon in the base-catalyzed gels were bonded to other particles through forming Si-O-Si bonds. The remaining silicon atoms were terminated by forming Si-OH bonds which were considered to stabilize the fourfold siloxane ring unit in the gels. (C) 1998 Elsevier Science B.V. All rights reserved.