Contribution to the kinetics of glass formation from solutions

The results of the measurements of hydrolysis reaction rates in the systems TEOS and TMOS with HCl and NH3 as catalysts show that considering only an overall hydrolysis reaction can be misleading. Therefore the single reaction steps of Eq.(1) have to be taken into consideration. From theoretical considerations it becomes probable that the hydrolysis rate increases with an increasing number of hydrolyzed OR groups of the reaction silane due to a decrease in the stabilizing effect of alkyl groups in the transition state. In order to find out the effect on the hydrolysis reaction of this assumption, a computer simulation of the single hydrolysis steps was performed. For computing the actual concentrations of the (RO)nSi(OH)4-n species first order kinetics and a ratio of 1:2:4:8 for the k values of the single steps of reaction (1) was assumed. This behavior shows an "inhibition phase" followed by a region of maximum activity where water is consumed very rapidly to an "acceleration phase" i n the H2O consumption curve. These results however do not indicate whether the reactions of the single species follow first or second order kinetics.