A SI-29 NMR-STUDY OF THE SILANOL POPULATION AT THE SURFACE OF DERIVATIZED SILICA

The type of silicon atoms at the surface of silicon dioxide samples, modified by dense grafts of three triorganylsiloxy substituents, was studied by 29Si NMR. Signals were enhanced by magic angle spinning (MAS), cross polarization by the surface proton population (CP), broad-band decoupling of the protons, and signal averaging. It was shown that quantitative information could be obtained about the surface silanol population via the analysis of the 29Si NMR spectra provided that the sample had no protons in the silicon dioxide matrix and that precautions were taken in the evaluation of the CP-enhanced signals. Two results were arrived at. (i) The sum of the surface concentration of reacted and unreacted silanols was TqH = 8.45 ± 0.10 Mmol m-l, considered to represent the surface concentration of silanols on the unmodified fully hydrated fume silica, (ii) Of the total surface silanols, 29% are of the geminal type and 71% are on silicon atoms having only one hydroxyl substituent (mono type). © 1990, American Chemical Society. All rights reserved.