The early stages of the sol-gel processing of TEOS

The reactivity of tetraethoxysilane (TEOS) used in many sol-gel syntheses can be increased by prehydrolysis. In contrast to one-step prehydrolysis acidic two-step prehydrolysis causes an increase of the reactivity of the low molecular species provided the total water/TEOS ratios are the same in both cases. By choice of the processing conditions of the hydrolysis—shown on variation of water content, hydrolysis steps and heat treatment—the degree of condensation, the ratio of hydrolyzable (-OC2H5) and hydrolyzed groups (-OH) and the structure of the low molecular species (short chains, 3-membered and 4-membered rings) can be influenced systematically. The development of the building units formed in the early reaction stages is ascertained by molybdate method and 29Si NMR measurements.