Surface hydroxylation and silane grafting on fumed and thermal silica.

The optimization of the surface functionalization of flat thermal silicon oxide by silanes was investigated. The difficulties are the low density of silanols at the surface of thermal silica, the lack of precise knowledge of the actual surface chemistry of thermal silica and of its hydroxylation, and the limited number of possible chemical analyses at flat surfaces of small area. This steered our study toward a comparative investigation of the hydroxylation and silane grafting of thermal silica and the well-known fumed silica. The silane grafting density for fumed silica that had undergone thermal treatments of dehydroxylation was related to the surface density of silanols. The surface density of silane on the flat thermal silica as measured by FTIR-ATR spectroscopy was 1.4 micromol/m2, similar to that of fumed silica dehydroxylated at 1000 degrees C. This moderate value was related to the low silanol density present on such silica surfaces. Several rehydroxylation treatments that proved their efficiency on dehydroxylated fumed silica did not lead to any noticeable improvement on thermal silicon dioxide.