STUDY OF THE HYDROLYSIS OF TEOS-TMOS MIXTURES UNDER ULTRASOUND STIMULATION

Pure and mixed tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS) were hydrolyzed at 35 degrees C, using oxalic acid as a catalyst and u ltrasound stimulation. The hydrolysis reaction was carried out in a sp ecially designed device, in which a heat flow steady state, between th e ultrasound source and an external thermostatic bath, was maintained, in the absence of reactions. The exothermic hydrolysis causes a time dependent thermal peak. An induction time is apparent in pure TEOS bef ore the hydrolysis peaks starts, which has been explained by the initi al immiscibility gap of the TEOS-water system. The induction time was found to be approximately of the same magnitude as in the HCl catalyze d hydrolysis, in spite of the uncertainty accompanying the peak defini tion. No induction period is apparent in pure TMOS, so that the hydrol ysis starts with its maximum rate. Two independent thermal peaks in th e mixed TMOS-TEOS samples were found, both associated to the respectiv e hydrolyses of the pure component. The induction time for the TEOS hy drolysis is decreased as more alcohol (and silanol) is produced in the earlier TMOS hydrolysis. This effect is explained by improvement of h omogenization by alcohol.