The time evolution of the sol-gel process: Si-29 NMR study of the hydrolysis and condensation reactions of tetraethoxysilane

The structure and properties of the glass products prepared by the sol-gel process are strongly dependent on the experimental conditions of their prep aration. The time dependence of the structural intermediate formation obtai ned during hydrolysis and condensation reactions in tetraethoxysilane, etha nol, H2O, HCl (molar ratio 1:3:1.5:0.01) sol-gel reaction mixture was inves tigated with Si-29 nuclear magnetic resonance spectroscopy at -75 degrees C . It is clear that the substitution of the Si ethoxy group (OCH2CH3) by a h ydroxyl (OH) shifts the resonance lines downfield. The dimer peaks are loca ted about 9-10 ppm upfield from the corresponding monomers, and Si located in the middle of the trimers appeared at the chemical shift of 18-19 ppm fr om the corresponding monomers. The peaks of cyclic trimers, cyclic tetramer s and the other higher oligomers are relatively broad and weak and exhibit a multiplet structure. The time dependence of the concentrations of the ind ividual types of silicon in the reaction mixture was evaluated from the int egral intensity of the corresponding Si atoms from the Si-29 NMR spectra. T he results obtained represent an input data to the various kinetic models o f the complicated hydrolysis and condensation reactions.