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The surface chemistry of amorphous silica. Zhuravlev model

Publisher Summary The properties of a surface are influenced by the surface groups and the knowledge of their existence and of their chemistry are important for many technological processes This chapter examines the surface compounds on carbon—microcrystalline carbon, graphite, and diamond—on silica, on titania, and on alumina and silica–alumina Most important and best known among the surface compounds of carbon are those with oxygen and with sulfur Thus, two kinds of surface oxides are known Basic surface oxides are formed always when a carbon surface is freed from all surface compounds by heating in a vacuum or in an inert atmosphere and comes into contact with oxygen only after cooling to low temperatures Acidic surface oxides are formed when carbon is treated with oxygen a temperatures near its ignition point Unambiguous identification of carboxyl groups has been achieved by two reactions of the acyl chlorides: Friedel–Crafts reaction and Schmidt rearrangement

Chemical Identification of Surface Groups

The synthesis of glasses from gels has recently aroused considerable interest both from the theoretical and practical standpoint. The conditions of gel formation which determine the structure of the wet gel are first examined. During the drying process, which is a decisive step in the obtention of monolithic gels, an essential role is played by capillary forces. It is shown that, among the various methods of obtaining monolithic gels, that based on the hypercritical evacuation of the solvent is the most reliable.

Synthesis of glasses from gels: the problem of monolithic gels

Synthesis and biomedical applications of aerogels: Possibilities and challenges

The stability of amorphous silica hydrosols has been studied in the presence of alkali metal cations at various pH's. Results show that silica is very stable within appropriate pH ranges, notwithstanding the presence of very high electrolyte concentrations (sometimes reaching saturation). Moreover, coagulation is often reversible. Although all the alkaline cations are able to coagulate silica between pH's about 7 and 11, they can be classified into two groups according to their behavior in more alkaline media: above pH 11, Li + and Na + cations bring coagulation whereas K + , Rb + , and Cs + remain ineffective. It is suggested that the attraction forces responsible for coagulation are related to acid-base interparticle bonding of two types (salt bonding by acidic cations and silanol-dissociated silanol bonds) rather than to London-van der Waals forces.

P. C. Carman

Papers

Constitution of colloidal silica