Silica crystalline, surface groups

Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice 159). Ar. Weiss and Al. Weiss 160) discovered an unstable fibrous modification with the SiSj structure. Recently, a few high-pressure modifications have been synthesized keatite 161), coesite 162), and stishovite 16S). The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nbrdlingen (Bavaria). 

In principle, there is no difference in the surface groups on quartz and on amorphous silica. The most important question discussed in the literature is whether the structure of crystalline quartz is represented in its surface, too. Many investigators (282-287) reported that there is a disturbed layer of amorphous character present on the quartz surface. It is more readily dissolved by water or by hydrofluoric acid. Holt and King (288) claimed that only a monomolecular layer of silicic acid was adsorbed on quartz surfaces. 

The surface groups S with the highest free energy should be most efficient in the processes of chemical modification. In this case, one can expect the formation of a great variety of modification products (SR groups). For the compounds with the directional chemical bonds between the neighboring atoms in the main crystalline and amorphous silica modifications, these are the products of bond rupture and/or its rearrangement. 

In addition to zeolites and MMS which are crystalline, ordered nanoporous materials and amorphous adsorbents, such as silica and active carbon, have been used in gas cleaning. The silica gel surface is generally terminated with OH groups bonded with a silicon atom, SiOH units, that is, silanols (see Figure 2.30). The concentration of OH groups at the surface is... 

Figure 4-5. (a) Schematic example of isolated and terminal hydroxyl groups, (b) H-bonded hydroxyl chain example ofsilanol condensation with elimination of water andformation of isolated hydroxyls on the silica film surfaces. (Reprinted from Journal of Non-Crystalline Solids, 316, P. Innocenzi, Infrared spectroscopy of solr-gel derived silica-based films a spectra-microstructure overview , p. 318, Copyright (2003), with permission from Elsevier.)... 

Packing Density of Silanol Groups on the Surface of Crystalline and Amorphous Silica... 

Stober (173) found also a close relation of the adsorption sites for ammonia and the number of surface silanol groups. Fused silica and crystalline quartz behaved in a similar manner. About the same concentration of adsorption sites was found in the SOj adsorption. 

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