THE EFFECT OF TEOG ON THE HYDROLYSIS CONDENSATION MECHANISM OF A 2-STEP SOL-GEL PROCESS OF TEOS

The effect of the addition of tetraethoxygermane (TEOG) in the basic step of a two-step silica sol-gel process on the hydrolysis-condensation mechanism of tetraethoxysilane (TEOS) was investigated. Gelation time versus hydrolysis time curves were recorded for mixtures of TEOS, ethanol and water to which TEOG was added. It is found that an ethoxy group of TEOG reacts with silanols under ethanol formation. The rate of this reaction is almost independent of the base concentration. Because unhydrolysed TEOG has four reactive groups, it is efficient in cross-linking chains, which implies that in general its addition results in shorter gelation times. This effect becomes more apparent for mixtures with low water content. At high TEOG:silanol ratios, however, an increase in the gelation time is found to be due to the capturing of silanols by TEOG. The cross-linking effect of TEOG becomes weaker if it is partly hydrolysed before addition to the TEOS mixture.