Silica gels from aqueous silicate solutions

Silica Gels from Aqueous Silicate Solutions... 

Chapter 45 Silica Gels from Aqueous Silicate Solutions Combined Si NMR and Small-Angle... 

P.W.J.G. Wijnen, A spectroscopic study of silica gel formation from aqueous silicate solutions, Ph.D. Thesis, TU Eindhoven, 1990. 

Porous silica is one of the different forms of amorphous silica. It can be prepared by acidification of basic aqueous silicate solutions, and when reaction conditions are properly adjusted, porous silica gels are obtained [150], If water is evaporated from the pores of silica hydrogels prepared in this fashion, porous xerogels are obtained [153],... 

The absorption spectrum of nitrobenzene molecules adsorbed on silica gel from the vapor phase has been recorded by Okuda (55) (Fig. 7), and that from a cyclohexane solution in a slurry of silicic acid by Robin and Trueblood (46). The first author has found a shift from 240 in the gas to 260, and the second authors from 253 in cyclohexane to 270 mp, which is equivalent to about about 3000 cm-. Such a large displacement does not necessarily indicate a chemisorption, since the position of the 260 mju. band of adsorbed nitrobenzene is between those of its aqueous and ethanol solutions. However, Fig. 7 shows that after desorption the most firmly held molecules display a broad absorption band with a maximum at 300 m/a, which might indicate a kind of chemisorption. 

In a study [6] of the dissolution of amorphous silica gels in aqueous alkali metal hydroxides, the rate of dissolution was found to depend on the cation used in the dissolution reaction. A maximum in dissolution rate was found for potassium hydroxide solutions, whereas both intrinsically smaller and larger cations (lithium-sodium and rubidium-cesium) showed slower dissolution rates, as can be concluded from the concentration of dissolved silicate species (normalized peak areas) as a function of alkali metal cation (Figure 45.2). This result is contradictory to the expectation that a monotonic increase or decrease in dissolution rate is to be observed for the different cations used. One major effect that occurs at the high pH values of this study is that the majority of silanol... 

Figure 18-13a. Typical SAXS-curve of an aqueous silicate solution after gelation has occurred, b SAXS-curve of a silica gel prepared at pH = 3.9 from potassium waterglass. The gel has been aged for one year at room temperature. The dotted line presents a nonlinear least squares fit of the fractal region in the curve, yielding D = 2.0. The dashed line presents the Porod-law.

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