FOURIER-TRANSFORM INFRARED AND RAMAN-SPECTROSCOPIC STUDY OF SILICA SURFACES

Infrared and Raman spectra of silica powders differ for fumed and precipitated samples. Site models of the surfaces of fumed silica are proposed. These models involve surfaces resembling faces {111} of beta-cristobalite, edges and steps. The site SiOH...O-6(H)SiOaH, located at steps between planes {111}, is related to the absorptions at 3500, 3715, and 3742 cm(-1), for the hydroxyl groups from left to right, respectively. Dehydroxylation into SiOSiOH is easy because of the comparatively strong hydrogen bond; this siloxane bridge is a part of two five-fold (-SiO-)(5) rings and does not contribute to the Raman band at 607 cm(-1) (D-2). The single silanols that are closer than about 0.4 nm on faces {111}-and thus are weakly interacting-condense around 400 degrees C into three-fold (-SiO-)(3) rings, the breathing mode of which gives rise to the D-2 Raman band. Most of these siloxane bridges need a high relative humidity to be rehydroxylated. Vicinal single silanols on edges could also condense above 500 degrees C into very reactive (-SiO-)(2) rings.