ESTIMATION OF HYDROXYL DENSITY AT THE SURFACE OF PYROGENIC SILICAS BY COMPLEMENTARY NMR AND RAMAN EXPERIMENTS

In the case of pyrogenic silicas, the concentration in surface hydroxyl groups measured by Si-29 nuclear magnetic resonance (NMR) spectroscopy is overestimated (17 OH/nm(2) against 4 OH/nm(2) measured by thermogravimetry) if the NMR spectrum is assumed to be composed of only three components corresponding to three types of site: geminal silanols (Q(2) chemical shift at -91ppm), single silanols (Q(3) at -101 ppm), and silicons bonded with four neighbour silicons (Q(4) at -110 ppm). Raman studies of this pyrogenic silica explains why this decomposition in three components only is not adapted. Indeed, the spectrum of the sample, without any thermal or chemical pretreatment, exhibits a symmetrical peak at 607 cm(-1), which is assigned to a symmetrical breathing mode of cyclic trisiloxane rings. These three-membered (SiO-) rings contain silicon atoms for which the average angle of the four SiOSi angles is 142 degrees instead of the usual 147 degrees for other Q(4) atoms. The NMR chemical shift corresponding to this type of atom is then located at -106 ppm. This population of silicons can be evaluated from the evolution of the Raman spectrum with thermal pretreatment of the sample. The NMR spectrum can then be decomposed taking into account this population at -106 ppm; thus, an overestimation of the Q(3) population is avoided and an hydroxyl number of about 4 OH/nm(2) is obtained.