ABSTRACT
Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . 297
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
Hydration and Dehydration at
Room Temperature . . . . . . . . . . . . . . . . . . . . . . . 297
Silanol Heterogeneity . . . . . . . . . . . . . . . . . . . . . . 299
Thermal Treatments . . . . . . . . . . . . . . . . . . . . . . . 300
Infrared Spectra . . . . . . . . . . . . . . . . . . . . . . . . 300
Raman Spectra between 150 and
1200 cm21 . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
Methyl Grafts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
Network Models . . . . . . . . . . . . . . . . . . . . . . . . . 305
Surface Models . . . . . . . . . . . . . . . . . . . . . . . . . . 305
Surface of a Fumed Silica . . . . . . . . . . . . . . . . . . 307
Surface of a Gel or a Precipitated Silica . . . . . . . 308
Unified Interpretation of the D0-D1 and D2 Raman Bands of Disperse Silicas, Monolithic
Gels, and Fused Silicas . . . . . . . . . . . . . . . . . . . 309
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
Infrared (between 400 and 8000 cm21) and Raman (between 5 and 400 cm21) spectra of silica powders
are interpreted to discuss the differences between fumed, gel and precipitated samples. Site models of
the surfaces of fumed silica are proposed. These models are based on surfaces resembling faces f111g of b-cristobalite, edges and steps. The site SiOH...Ob(H)SiOaH, located at steps between planes f111g, is related to the absorptions at 3500, 3715, and 3742 cm21, for the hydroxyl groups from left to right, respect-
ively. Because of the comparatively strong hydrogen bond involved in this kind of site, a weak heating of
the sample dehydroxyls these silanols into SiOSiOH to give a siloxane bridge which is a part of two fivefold
(-SiO-)5 rings. The models of single silanols are hydroxyl groups placed about 0.4 nm apart on faces f111g. Those silanols are weakly interacting and are condensed around 4008C into three-fold (-SiO-)3 rings the breathing mode of which gives rise to the D2 Roman band at 607 cm
21. Most of the latter rings need
a high relative humidity to be rehydroxylated. Vicinal single silanols on edges could also condense
above 5008C into very reactive (-SiO-)2 rings. A more disordered surface must be considered to explain the heterogeneous distribution of surface groups for gel and precipitated silica samples.
