COMPARATIVE-STUDY OF THE SURFACE HYDROXYL-GROUPS OF FUMED AND PRECIPITATED SILICAS .2. CHARACTERIZATION BY INFRARED-SPECTROSCOPY OF THE INTERACTIONS WITH WATER

The infrared spectra of a fumed silica (A) and of silicas prepared in liquid phases, either aqueous (G, P) or alcoholic (S), have been compared. Spectra obtained by diffuse reflectance of silica powders and by transmission through self-supported silica disks are in qualitative agreement. At least two structures of physisorbed water are observed before cluster formation on precipitated silicas: (i) the most stable adsorption probably takes place on inner surfaces; (ii) water absorbing at 5315 cm-1 is less stable, on outer surfaces. The surface structure of a divided silica strongly depends on its manufacturing process. Most of the silanols of the fumed silica are randomly distributed on outer surfaces. Non-nearest-neighbor isolated silanols have little affinity for water. Inner surfaces are much more expanded for silicas prepared in a liquid phase, mainly for sample S, prepared from ethyl silicate. Water is able to probe inner surfaces inaccessible to more bulky molecules such as argon, nitrogen, or fert-butyl alcohol. Silanol distribution is not observed in the same way through various spectral regions. The vOH fundamental band gives much more weight to bound silanols than the 2vOH overtone and the vOH + ƍSiOH combination do. In the j/OH region, the absorption below 3400 cm-1 is primarily assigned to surface silanols strongly perturbed by water and not to water itself. In contrast, the absorption below 7150 cm-1 in the 2i>OH region is mainly due to weakly perturbed water and not to silanols. The absorption of the silicic lattice around 1100 cm-1 indicates a peculiar morphology for the silica gel G. © 1990, American Chemical Society. All rights reserved.