Abstract
The time evolution of the structural intermediate formation obtained during hydrolysis and condensation reactions in tetramethoxysilane (TMOS), methanol, H2O, HCl (molar ratio 1:5:2: 0.001) sol-gel reaction mixture was investigated by Si-29 NMR spectroscopy at -75 degrees C. It is obvious that the substitution of the Si methoxy group (OCH3) by a hydroxyl (OH) shifts the resonance lines downfield. The dimer peaks are located about 9-10 ppm upfield from the corresponding monomers, and the Si located in the middle of the trimers appeared at the chemical shift of 18-19 ppm upfield from the corresponding monomers. The peaks of trimers, cyclic tetramers and the other higher oligomers are relatively broad and weak, and exhibit a multiplet structure. The time dependence of the concentrations of the individual types of silicon in the reaction mixture were evaluated from the integral intensity of the corresponding Si atoms from the 29Si NMR spectra. The results obtained represent input data to the various kinetic models of the complex hydrolysis and condensation reactions.