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

In the case of pyrogenic silicas, the concentration in surface hydroxy l groups measured by Si-29 nuclear magnetic resonance (NMR) spectrosco py is overestimated (17 OH/nm(2) against 4 OH/nm(2) measured by thermo gravimetry) if the NMR spectrum is assumed to be composed of only thre e 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). Rama n studies of this pyrogenic silica explains why this decomposition in three components only is not adapted. Indeed, the spectrum of the samp le, without any thermal or chemical pretreatment, exhibits a symmetric al peak at 607 cm(-1), which is assigned to a symmetrical breathing mo de of cyclic trisiloxane rings. These three-membered (SiO-) rings cont ain 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 loc ated at -106 ppm. This population of silicons can be evaluated from th e evolution of the Raman spectrum with thermal pretreatment of the sam ple. 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.