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The Surface Chemistry of Amorphous Silica. Zhuravlev Model

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The Surface Chemistry of Amorphous Silica. Zhuravlev Model Colloids and Surfaces A: Physicochemical and Engineering Aspects 173 (2000) 1–38 www.elsevier.nl/locate/colsurfa The surface chemistry of amorphous silica. Zhuravlev model L.T. Zhuravlev Institute of Physical Chemistry, Russian Academy of Sciences, Leninsky Prospect 31, Moscow 117915, Russia Received 14 January 1999; accepted 21 February 2000 Abstract 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 SiO2 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 SiO2 skeleton. a The magnitude of the silanol number, that is, the number of OH groups per unit surface area, OH, when the surface is hydroxylated to the maximum degree, is considered to be a physico-chemical constant. This constant has a a a −2 numerical value: OH,AVER =4.6 (least-squares method) and OH,AVER =4.9 OH nm (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–Emmett– Teller (BET) method is the correct method and gives the opportunity to measure the real physical magnitude of the specific surface area, SKr (by using low temperature adsorption of krypton), for silicas and other oxide dispersed solids. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Amorphous silica; Surface characterization; Silanol and siloxane groups; Internal silanols; Physico-chemical model; Physico-chemical constant; BET method 1. Introduction the problems involved and for practical applica- tions [1]. In this connection an investigation of the Research into the silica-water system is impor- so-called combined, structurally bound water [2– tant both for elucidating the theoretical aspects of 5] in dispersed amorphous silica is of interest. 0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0927-7757(00)00556-2 2 L.T. Zhura6le6 / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 1–38 This term describes OH groups that are bound via Silanol groups are formed on the surface by two the valence bond with Si atoms on the silica main processes [1,36]. First, such groups are surface (hydroxyl coverage), and in some cases formed in the course of silica synthesis, e.g. during with Si atoms inside the particles of silica. the condensation polymerization of Si(OH)4 (Fig. In the 1930s, studies of the condensation pro- 1a). Here, the supersaturated solution of the acid cesses of silicic acids, carried out by Hofmann, becomes converted into its polymeric form, which Endell and Wilm [6], Rideal [7] and Kiselev [8], then changes into spherical colloidal particles con- and slightly later by Carman [9], showed that taining SiOH groups on the surface. Upon hydroxyl (silanol) groups, SiOH, should be drying, the hydrogel yields xerogel, the final present on the surface of silicates and silicas. On product, which retains some or all of the silanol the basis of measurements of the heat of wetting groups on its surface. Secondly, surface OH and a comparison of the adsorption data with the groups can form as a result of rehydroxylation of data from chemical analysis and the correspond- dehydroxylated silica when it is treated with water ing results reported in the literature, Kiselev sug- or aqueous solutions. The surface silicon atoms gested that the water evolved during calcination tend to have a complete tetrahedral configuration, of silica gel, besides physically adsorbed water, is and in an aqueous medium their free valence formed from OH groups that are chemically held becomes saturated with hydroxyl groups (Fig. 1b). on the silica surface. This suggestion led to an The surface properties of amorphous silica, understanding of the dehydroxylation mechanism which is considered to be an oxide adsorbent, in [8]. many cases depend on the presence of silanol Yaroslavsky and Terenin [11–13], by using an groups. At a sufficient concentration these groups infrared spectroscopy method, proved for the first make such a surface hydrophilic. The OH groups time the existence of hydroxyl groups on the silica act as the centers of molecular adsorption during surface (porous glass). This fact was soon confi- their specific interaction with adsorbates capable rmed by Kurbatov and Neuymin [14]. Now nu- of forming a hydrogen bond with the OH groups, merous spectral and chemical data or, more generally, of undergoing donor–acceptor unambiguously confirm the presence of the OH interaction. The removal of the hydroxyl groups groups on such SiO2 surface. from the surface of silica leads to a de- Fig. 1. The formation of silanol groups on the silica surface: (a) Condensation polymerization; (b) Rehydroxylation. L.T. Zhura6le6 / Colloids and Surfaces A: Physicochem. Eng. Aspects 173 (2000) 1–38 3 Various problems related to silica surface char- acteristics are encountered in different areas of science and technology: physics, chemistry and physical chemistry, agriculture, soil science, biol- ogy and medicine, electrical energetics, the oil processing industry, the metallurgical and mining industries, some fields of geology, etc. Traditional building and other materials based on silica, such as cement, concrete, firebrick, silicate glasses, rough and fine ceramics, and enamels, occupy a significant place in human life. Different types of silica are widely used as efficient adsorbents and selective absorbents, active phase carriers in catal- ysis, fillers for polymeric systems, adsorbents and Fig. 2. Types of silanol groups and siloxane bridges on the surface of amorphous silica, and internal OH groups (see text). supports for gas and liquid chromatography, Qn-terminology is used in NMR, where n indicates the number thickeners for dispersion mediums, binding agents of bridging bonds (OSi) tied to the central Si atom: Q4, for molding materials, reinforcing fibres, and so surface siloxanes; Q3, single silanols; Q2, geminal silanols forth. Chemical modification of the surface of (silanediols). dispersion silica has received a large amount of interest; this process allows researchers to regulate crease in the adsorption, and the surface acquires and change adsorption properties and technologi- more and more hydrophobic properties [1,36]. cal characteristics of composite materials. Of late Surface OH groups are subdivided as following the use of SiO2 is on the rise in the manufacture (Fig. 2): (i) isolated free (single silanols), SiOH; of modern high-quality materials (microelectron- (ii) geminal free (geminal silanols or silanediols), ics, optics, fiber optics, liquid crystals, different composites including biocomposites, ordered =Si(OH)2; (iii) vicinal, or bridged, or OH groups bound through the hydrogen bond (H-bonded nanostructured silica materials, monodispersed single silanols, H-bonded geminals, and their H- colloids, etc.). In the past 50 years, many reviews have ap- bonded combinations). On the SiO2 surface there also exist surface siloxane groups or SiOSi peared on the subject of surface chemistry of bridges with oxygen atoms on the surface. At last, silica. And we refer readers to these numerous monographs, reviews, and the most interesting there is structurally bound water inside the silica and significant, in our opinion, articles [1,11– skeleton and very fine ultramicropores, dB1nm 300]. (d is the pore diameter), i.e. internal silanol In view of this, is there need for yet another groups. review in this field? The author of the present The last number of decades saw a rapid growth review article has carried out numerous experi- in those fields of science and technology that deal mental studies on the subject in question, system- with production and utilization of various colloid ized such important characteristics as the and microheterogeneous forms of silica with de- concentration and the distribution of different veloped surfaces, such as sols, gels, and powders. types of silanol groups, established the energetic The properties of a pure silica, as an oxide adsor- heterogeneity of the surface in a wide temperature bent, are determined in the first place by: (i) the range of the pretreatment, and investigated the chemical activity
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