A review article is presented of the research results obtained by the author on the properties of amorphous silica surface. It has been shown that in any description of the surface silica the hydroxylation of the surface is of critical importance. An analysis was made of the processes of dehydration (the removal of physically adsorbed water), dehydroxylation (the removal of silanol groups from the silica surface), and rehydroxylation (the restoration of the hydroxyl covering). For each of these processes a probable mechanism is suggested. The results of experimental and theoretical studies permitted to construct the original model (Zhuravlev model-1 and model-2) for describing the surface chemistry of amorphous silica. The main advantage of this physico-chemical model lies in the possibility to determine the concentration and the distribution of different types of silanol and siloxane groups and to characterize the energetic heterogeneity of the silica surface as a function of the pretreatment temperature of SiO 2 samples. The model makes it possible to determine the kind of the chemisorption of water (rapid, weakly activated or slow, strongly activated) under the restoration of the hydroxyl covering and also to assess of OH groups inside the SiO 2 skeleton. The magnitude of the silanol number, that is, the number of OH groups per unit surface area, h OH , when the surface is hydroxylated to the maximum degree, is considered to be a physico-chemical constant. This constant has a numerical value: h OH,AVER =4.6 (least-squares method) and h OH,AVER = 4.9 OH nm − 2 (arithmetical mean) and is known in literature as the Kiselev-Zhuravlev constant. It has been established that adsorption and other surface properties per unit surface area of silica are identical (except for very fine pores). On the basis of data published in the literature, this model has been found to be useful in solving various applied and theoretical problems in the field of adsorption, catalysis, chromatography, chemical modification, etc. It has been shown that the Brunauer -EmmettTeller (BET) method is the correct method and gives the opportunity to measure the real physical magnitude of the specific surface area, S Kr (by using low temperature adsorption of krypton), for silicas and other oxide dispersed solids.

The present work studies a surface concentration of hydroxyl groups for a large number of amorphous silicas, viz., silica gels, aerosilogels, and porous glasses, differing in production conditions, surface area, and pore size distribution. It is shown that the surface density of OH groups (the silanol number) is a physicochemical constant for a fully hydroxylated surface and that the density as a function of temperature of vacuum treatment does not depend in a significant way on the type of silica.

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The hydroxyl groups of silica (aerosils and silica gels) are divided into surface and bulk groups. The surface OH groups are rapidly substituted by OD, making it possible to determine the surface concentration of hydroxyl groups by exchange between the samples and D20 vapour at ordinary temperatures, as well as the total amount of OH groups in the sample by exchange between D2O vapour and the entire quantity of water evolved when the sample is heated to 1200°C. The isotopic composition of the vapour after deuterium exchange was determined by mass spectrometry. Deuterium exchange was also studied by an infra-red spectroscopy. The ratio between the quantities of surface and bulk OH groups depends on the pre-treatment of the sample and on the size of the silica globules. The concentration of OH groups on the surface of maximally hydroxylated samples is practically independent of their history and dispersity. Surface OH groups may be subdivided in turn into free and bound groups. The main role in specific adsorption (when hydrogen and similar bonds are formed with the adsorbate molecules) and in the chemical reaction with trimethylchlorosilane must be credited to the free hydroxyl groups of the silica surface.* the measurement by I. Yu. Babkin.

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