Hydrolysis-condensation reactions in the acid step of a two-step silica sol-gel process, investigated with 29Si NMR at −75°C

Abstract

The relation between the hydrolysis time in the acid step and the gelation time in the basic step of a two-step silica sol-gel process was investigated using different HCl concentrations in tetraethoxysilane (TEOS), ethanol and water mixtures with a molar ratio of 1:4:( a + 0.5), where a was in the range of 0.25 and 4. At values of a < 3 a minimum in gelation time was followed by a maximum and subsequently by a gradual decrease; at ⩾ 3 a strong decrease in gelation time, followed by a nearly constant value, was found as a function of the hydrolysis time. For all the mixtures, the hydrolysis times associated with the minima in gelation time are found to be inversely proportional to the HCl concentration. The hydrolysis-condensation reactions in the acid step at 50°C were observed by 29Si NMR spectroscopy at −75°C using samples drawn from the hydrolysing mixtures and immediately quenched to temperatures below −100°C. At any desired moment in the course of the hydrolysis, an accurate spectrum with a good signal-to-noise ratio can thus be obtained, corresponding with a well-defined hydrolysis time.