Silanols and Siloxanes on Silica Surface

The above pictures discussed ideal surfaces consisting only of the silicon and oxygen atoms. Meanwhile, an extremely important and characteristic property of the bare surfaces of real silicas is their readiness to react with water molecules when exposed to ambient atmosphere. The interactions result in the formation of numerous =Si-OH groups (silanols) on the surface Kiselev [45] discovered it some 70 years ago. 

To date there has been found only one contrasting exception. Namely, Bakaev and Steele [46] predicted an extreme hydrophobicity for surfaces dominated by bridging O atoms in Si-O-Si moieties. Such a surface, strikingly unreactive toward water, could indeed be obtained [47,48]. It required controlled oxidation of a monoatomic Si film deposited on a Mo (112) template under ultra high vacuum. The obtained ultrathin SiC 2 film was relatively free of defects. No evidence for dissociation of water at the surface was found even at low coverage, water molecules made three-dimensional clusters. Thus, the water-silica interaction was weaker than water-water hydrogen bonding. 

Numerous experimental studies, made mostly with silica gels and other high specific area silicas exposed to water vapor, reported abundances of the silanol groups as high as 6per 100 A2 [33,49]. Zhuravlev [50] believes that the number of OH groups per unit area, when the surface is hydroxylated to the maximal degree, is a physico-chemical constant, equal to (4.6 to 4.9) hydroxyls per 100 A. He obtained this value 

Hydroxylation involves dissociation of the molecules of water. It is no surprise that the fresh fracture surfaces, with their chemically active bond defects, vigorously 

Reproduced from Journal of Physics Condensed Matter, 14(16) 4133 1144, Masini P, Bemasconi M, Ab initio simulations of hydroxylation and dehydroxylation reactions at surfaces amorphous silica and brucite, 2002, with permission from IOP Publishing Ltd. 

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