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)

A model for the surface structure of rutile is proposed, based on infra-red studies of a crystalline rutile sample prepared by the combustion of Ti(iso-Pro),. It is suggested that the exterior surfaces of the rutile crystals correspond to three low index crystal planes-namely the (loo), ( 101) and (110). Of these the first two are capable of adsorbing molecular water as a ligand coordinated to Ti4+ suiface ions, whereas the (110) crystal face adsorbs water dissociatively leading to the presence of equal quantities of two types of OH-ions. One of these types is associated with a surface Ti4+ ion which is five coordinate with respect to lattice oxide ions whereas the other type is bound to a surface Ti4+ ion which is only four fold oxide ion coordinate. It is possible to rationalize the observed thermal dehydroxylation and dehydration properties of the oxide and also account for its pyridine adsorption properties.

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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

Methyl chlorosilane vapours react with the surface hydroxyls of Aerosil silicas at temperatures above about 250°C. Infra-red spectroscopic studies on deuterium exchanged samples show that only the external hydroxyl-groups are affected. The trimethyl-and dimethyl-substituted chlorosilmes react selectively but completely with the isolated or single surface hydroxyls, whereas the monomethyl and tetrachloro compounds also react with some of the interacting hydrogen-bonded surface groups. Analyses of the solid reaction products from the dimethyl and trimethyl chlorosilane+silica reactions suggest that these compounds react largely 1 : 1 with the single surface hydroxyls although 1 : 2 bridging reactions occur on preheating the solid to temperatures of the order of 500°C. By comparing these results with those reported earlier it is suggested that the selectivity of reactions of this type is controlled largely by the stereochemistry of the activated transition-state structure of the organic molecule.