Silica gels aging

Einarsrud M-A, Kirkedelen MB, Nilsen E, Mortensen K, Samseth J (1998) Structural development of silica gels aged in TEOS. J of Non-Cryst Solids 231 10-16... 

Figure 7. SAXS curves of silica gels aged for various times (a) 1 week, (b) 2.5 months, (c) 5 months. Conc.(Si02) = 0.73 M (4 wt%), pH = 4.0.

Fig. 5.28 Drying of silica gels aged in monomer solution aging times are indicated for xerogels gray lines indicate levels of volumetric shrinkage (adapted from Einarsrud (1998)).

Scherer, G. W., Haereid, S., Nilsen, E., Einarsrud, M.-A., 1996. Shrinkage of silica gels aged in TEOS. J. Non-Cryst. Solids 202 42-52. 

Pore-size distributions of xerogels made from silica gels aged at indicated temperatures. From Yamane and Okano [25]. 

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. 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

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. 

The equation above suggests that one approach would be to use a pore liquid that has a low surface tension. Indeed, two-step acid-base or acid-acid catalyzed silica gels have been made, aged in ethanol or water, washed with various aprotic solvents, and finally evaporatively dried at 323 K for 48 hours and then at 383 K for 48 hours (43). The aprotic solvents used and their corresponding surface tension in N/m at room temperature (shown in... 

Figure 2. Spectral output of free running coumarin 153 in ethanol (left) and in aged silica gel (right.)...

When the gel is kept in contact with the pore-filling liquid, its structure and properties keep changing as a function of time. This process is called aging. During the aging period, four processes affect the porous structure and surface area of the silica gel. These are polycondensation, syneresis, coarsening and phase transformation.6,12,13... 

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