Monomeric silicic acid oligomerization

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. 

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... 

In the route used in this work, a monomeric or oligomeric silicate is protonated, the resulting silicic acid is esterified, and the alkyl silicate is organodealkoxylated. In each step, the siloxane framework is fully or substantially preserved. For the case starting with a metal disilicate the reactions can be represented as ... 

The kinetics of the silicic acid condensation was investigated using the molybdate method introduced by Alexander [14]. Upon addition of a molybdate solution to a silicate solution, monomeric and dimeric silicic acid react to form the yellow molybdosilicate [SiMoi204o] [5]. The change in the absorption of this solution reflects the decrease in monomeric and dimeric silicic acid due to the condensation reaction. Because of the reaction equilibrium between silicic acid and the molybdate as well as between monomeric, dimeric, and oligomeric silicic acids, it is important to adhere to a well-defmed, constant, and reproducible time protocol for the formation of the molybdosilicate. 

Alkyl silicates may be regarded as esters of the hypothetical silicic acid, which may exist in monomeric or oligomeric form. They are also referred to as silicate esters or... 

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