Mechanism of Sol-Gel Formation

The alkoxides, Si(OR) such as tetraethyl orthosilicate (TEOS), Si(OC2Hj) or tetra-methyl orthosilicate (TMOS), Si(OCHj) are the well started precursors in the sol-gel formation. Alkoxides are ideal chemical precursors for sol-gel synthesis because they react readily with water, in which the hydrolysis of Si atom takes place to form a hydroxytriethyl orthosilicate or a hydroxytrimethyl orthosilicate.The reaction is called hydrolysis, because a hydroxyl ion becomes attached to the silicon atom as follows  

Complete hydrolysis often requires an excess of water and/or the use of a hydrolysis catalyst such as acetic add or hydrochloric acid  

Sometimes the intermediate species, such as [(O C2Hj)2-Si-(OH)J or [(O C H ) -Si-(OH)] orsiloxane [Si-O-Si], may result as products of partial hydrolysis reactions. 

Small molecules such as water or ethyl alcohol were obtained after the condensation of the alkoxides in the presence of catalyst. A branched polymer was obtained because a fully hydrolyzed monomer Si(OH) is tetrafunctional (can branch or bond in 4 different directions). But, under certain conditions (e.g., low water concentration), incomplete branching will occur because less than 4 groups of the OC Hj or OH groups (ligands) will be capable of condensation. The mechanisms of hydrolysis and condensation, and the factors that control the structure toward linear or branched structures, are the most critical issues of sol-gel science and technology [6-13]. 

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