Identification of self- and cross-condensation products in organically modified silica sols by Si-29 and O-17 NMR spectroscopy

High resolution Si-29 and O-17 NMR spectroscopy has been used to identify self- and cross-condensation products in sols prepared from tetraethoxysilane (TEOS) and organically modified triethoxysilanes (RTES, where R is a methyl, ethyl or phenyl group). The sols were prepared by the acid catalyzed hydrolysis of TEOS and RTES with H2O/Si mole ratios of 0.15, 0.3 or 0.5, Resonance assignments were made by comparing the positions and intensities of peaks in the spectra of single and multicomponent systems as a function of reaction time. Both Si-29 and O-17 NMR spectroscopy exhibit resonances which are uniquely attributed to species formed by cross-condensation reactions between TEOS and RTES. Approaches to improve the resolution of O-17 NMR spectra were only moderately successful because the resonance envelopes have substantial contributions from inhomogeneous components.