Monomeric silicic acid

Figure 1 gives results obtained by Alexander et al. (I) and Baumann (2) by dissolving fine particles of commercially available vitreous silica powders in aqueous solutions. Similar data obtained in polymerization and depolymerization experiments by Scheel et al. (15) and Friedberg (10) indicate that the curve shown in Figure 1 represents an equilibrium concentration for oligomeric acid. It can be approached from the supersaturated state of monomeric silicic acid as well as from a solution of pure polymeric silicic acid. 

Figure 7.5. Species in equilibrium with amorphous silica. Diagram computed from equilibrium constants (25°C, I = 0.5). The line surrounding the shaded area gives the maximum soluble silica. The mononuclear wall represents the lower concentration limit below which multinuclear silica species are not stable. In natural waters the dissolved silica is present as monomeric silicic acid.

In alkaline solutions, silica exists in the form of silicate ions e.g., Si03 ). In dilute solutions (up to 0.1 mg of Si per ml) between pH 1 and 8, water-soluble monomeric silicic acid is the stable form. In more concentrated solutions of the same acidity, monosilicic acid condenses to disilicic acid and polysilicic acids which can be transformed into colloidal species. 

Silicic acid can be in the form of a monomer or in the form of low molecular weight polymeric units. Monomeric silicic acid Si(OH)4 has not been isolated or obtained in a concentrated solution without considerable polymerization, therefore is not a practical form to use in catalyst slurries to spray dry. Also not very practical is silicic acid formed in a way that does not separate it from the electrolyte products of the forming reaction. Residual electrolytes increase the ionic strength of the solution and result in destabilization followed by premature gelling of the silicic acid. 

Because polymerization of the water-glass solution gives rise to a broad distribution of silicate anions and thus results in poorly resolved Si NMR fines, the application of Si NMR spectroscopy in this type of reaction provides minor information about the different oligomerization steps and the reaction mechanism of monomeric silicic acid. Therefore a different approach to the study of aqueous silicate solutions was apphed. Because of the... 

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