Preparation of low-density xerogels from mixtures of TEOS with substituted alkoxysilanes. I. 17O NMR study of the hydrolysis–condensation process

Low-density xerogels were synthesised by incorporation of an additive to base catalysed tetraethylorthosilicate (TEOS) alcogels directly during the preparation of the sol. The nucleation mechanism by the additive was established by experiments during sol–gel transition. 17O NMR spectroscopy on TEOS–ethanol–water, 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS)–ethanol–water and EDAS–TEOS–ethanol–water solutions shows that the hydrolysis–condensation of EDAS is much faster than that of TEOS. Consequently it can be assumed that EDAS forms nuclei, onto which TEOS condenses later to form the silica particles.