The surface hydroxylation of silica gels and powders has been studied by determining the stoichiometry of the reactions of SiMezClz, Tic&, and BCI, with the surface hydroxyl groups. The results demonstrate that the fully hydroxylated silica surfaces studied carry two distinct types of surface hydroxyl sites, which are distributed as follows: 1.4 f 0.1 single surface hydroxyls (type A sites), and about 3.2 f 0.1 interacting hydroxyl groups arranged in pairs (1.6 f 0.1 type B sites). On heating the silicas above ambient temperatures, in uucuo, the type B sites are progressively removed. By correlating this result with earlier studies, it is concluded that at temperatures of 500 50", in uucuo, virtually all the type B sites are eliminated. However, the type A site concentration remains constant at evacuation temperatures up to 600 f : 50" at around 1.4 f 0.1 A sites/100 A2.These conclusions suggest that the silica surface corresponds to an array of different crystal planes, some of which contain OH groups at relatively large interhydroxyl spacings and others containing OH groups held in such a way as to promote interhydroxyl hydrogen bonding.(1)
M60 1 QD Hydroxylated silicas react with trimethylaluminium to produce surfaces that give rise to organic radicals on exposure to air or dry oxygen a t room temperature. The g-value of the paramagnetic species is 2.0027 f 0.0003 which suggests that it is a methyl radical. The stability of the radical species on the sample under ambient atmospheric
The characteristics of the residual band at 365Ocm-1 in the infra-red spectrum of deuterated aerosils have been determined. The band corresponds to hydroxyl groups and not to molecular water. It is assigned to interacting hydroxyls sited on neighbouring silicon atoms within the solid lattice of the silica particles. The effects of annealing the solid on the intensity of the residual band are described and a mechanism proposed for the removal of the corresponding species. The rehydration or chemisorption phenomena noted during adsorption and wetting studies performed at room temperatures are confined to the external surfaces of the solid.
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