Sol-gel transition in the hydrolysis of silicon methoxide

The sol-gel transition in the hydrolysis of silicon methoxide in acidic, basic and neutral pH media has been investigated by means of gas chromatography and small angle X-ray scattering measurements to elucidate the effect of a catalyst on gelling time and the properties of the obtained gel. In an acidic medium, the evolution of methanol immediately after the addition of water was remarkable, and only a trace of silicon methoxide could be detected after 30 min, although the gelling time was as long as 150 h. In an ammonia catalyzed basic medium, the rates of the polycondensation reaction and the coagulation of particles were very large and the gelling occurred after several hours. But about half of the silicon methoxide remained unreacted in the sol sampled after two thirds of the gelling time. In a neutral pH medium, both the hydrolysis and polycondensation reactions proceeded at the intermediate rates of those in the acidic and the basic media.