Distribution of organic groups in silica gels prepared from organo-alkoxysilanes

Organically modified silicas were prepared from mixtures of tetraethylorthosilane and methyltriethoxysilane (0-40 mol%) or dimethyldiethoxysilane (0-20 mol%) by a two-step sol-gel synthesis procedure involving sol aging at pH = 1.95 and gel formation at pH = 6. The texture and structure of dry gels were inferred from N-2 adsorption, Si-29 MAS-NMR and differential thermal analysis. Also the water sorption capacity of the products was measured. The distribution of the organic groups between the silica network and the gel surface determines to a large extent texture and water sorption of resulting gels. As indicated by MAS-NMR, the exothermic peak which arises in differential thermal analysis due to combustion of organic groups contains valuable information on the distribution of organic groups. A broad peak results when methyl groups are homogeneously divided over the entire gel network whereas a narrow peak indicates preferential localization of organic functions at the gel surface. In the latter case, products with higher pore volume and lower water sorption are obtained.