Co-hydrolysis products of tetraethoxysilane and methyltriethoxysilane in the presence of tetramethylammonium ions

Tetracethoxysilane (TEOS) and methyltriethoxysilane (MTEOS) have been co-hydrolyzed in methanolic solutions containing tetramethylammonium ions that only affect polymerization of silicate species (hydrolysis products of TEOS) to form the Si₈O ₂₀ ^8– cubic octameric silicate species. The effects of water content and TEOS-to-MTEOS molar ratio on the distribution of species formed in the solutions have been investigated with the trimethylsilylation technique and ²⁹Si n.m.r. spectroscopy. Formation of Si₈O ₂₀ ^8– and the cubic octameric species consisting of both Si(O^–)₄ and CH₃Si(O^–)₃ units, (CH₃)_nSi₈O _20–n ^(8–n)– (n=1–5), is found in the solutions. The increase of water content in the solutions solely results in increasing yield of Si₈O ₂₀ ^8– in spite of the presence of hydrolysis products of MTEOS together with those of TEOS, suggesting that water in the solutions plays an important role in the formation of Si₈O ₂₀ ^8– with the aid of tetramethylammonium ions. The TEOS-to-MTEOS molar ratio varies the distribution that is kept under control by the water content, increasing yields of (CH₃)_nSi₈O _20–n ^(8–n)– (n=1, 2). It is found that the water content and TEOS-to-MTEOS molar ratio determine the reaction conditions effective for the formation of CH₃Si(O^–)₃ unit-containing cubic octameric species.