DISTRIBUTION OF ORGANIC GROUPS IN SILICA-GELS PREPARED FROM ORGANO-ALKOXYSILANES

Organically modified silicas were prepared from mixtures of tetraethyl orthosilane and methyltriethoxysilane (0-40 mol%) or dimethyldiethoxys ilane (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 st ructure of dry gels were inferred from N-2 adsorption, Si-29 MAS-NMR a nd differential thermal analysis. Also the water sorption capacity of the products was measured. The distribution of the organic groups betw een the silica network and the gel surface determines to a large exten t texture and water sorption of resulting gels. As indicated by MAS-NM R, the exothermic peak which arises in differential thermal analysis d ue to combustion of organic groups contains valuable information on th e distribution of organic groups. A broad peak results when methyl gro ups are homogeneously divided over the entire gel network whereas a na rrow 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.