Aging of Silica Gels

On the basis of studies of the thermal aging of silica gels, Shapiro and Kolthoff (117) agreed with Elkins, Shull, and Roess (118) that the structure of siEca gel could best be visualized as being jnade up of discrete particles. The latter authors estimated, from small angle X-ray scattering, that the average size of the primary particles, assumed to be spherical, in freshly prepared silica gel was around 30-60 A. 

In Section 17.2.1.1, the preparation of silica gels has been discussed in detail. Some of the groimdbreaking work on aging of silica gels ((iv) in Figure 17.1) has... 

Even though silicon is extremely abundant, only one silicon-containing compound appears in the list of top 50 industrial chemicals. That is sodium silicate, Na2 Si03, used for the manufacture of silica gel and glass. Nevertheless, with the advent of the electronic age silicon has become an extremely important substance that is the primary ingredient of most semiconductors. Because these are microscale devices, the quantity of production of silicon remains small compared with that of fertilizers and construction materials. Although relatively small in quantity, the value of silicon products is quite high. 

The qualitative analysis of retention behaviour in liquid chromatography has now become possible. Quantitative retention-prediction is, however, still difficult the prediction of retention time and the optimization of separation conditions based on physicochemical properties have not yet been completely successful. One reason is the lack of an ideal stationary phase material. The stationary phase material has to be stable as part of an instrument, and this is very difficult to achieve in normal-phase liquid chromatography because the moisture in organic solvents ages the silica gel. 

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. 

In the presentation, the effect of synthesis and aging conditions on the pore and chemical structures of silica gels will be introduced and then the relationship between pore and chemical structures and dehumidification behavior will be discussed. 

SCHEME 23 Coalescence of silica gel through Ostwald ripening during alkaline aging, resulting in loss of surface area, expanding pore size (initially), and a change from convex to concave pore surface. 

Estella, J., Echerverrfa, J.C., Laguna, M., and Garrido, J.J., Effects of aging and drying conditions on the structural and textural properties of silica gels, Microporous andMesoporous Materials, 102, 274-282, 2007. 

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