Monomeric silicate

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. 

A powerful technique for investigating these different species is 29Si-NMR (see Fig. 8.1). Different lines can be observed in the 29Si-NMR spectrum of a silicate solution corresponding to the differently positioned 29Si nuclei in the (poly)sili-cate ions. The highest values for the chemical shift (8 = -71.5 ppm with respect to tetra methyl silane) are found for the monomeric silicate units while the resonances of fully condensed silicon atoms (i.e. Si-(0-Si)4 tetrasiloxy silane) are to be found at the lowest values for the chemical shift (8 = -110 ppm). 

In the second step the temperature of the synthesis mixture is raised to the crystallisation temperature (generally between 100 and 200°C). During this process continued dissociation of silicate and aluminate oligomers occurs, resulting in an increasing concentration of monomeric silicate and aluminate species in the solution. 

Some other models for the structure of C-S-H gel that have been proposed are incompatible with the evidence. A proposal identifying it with natural tobermorite, based on IR and extraction results (S37), appears to ignore both composition and degree of crystallinity. Another, assuming it to be closely related to the CH structure, with incorporation of monomeric silicate ions (G38), is inconsistent with the observed silicate anion structure. As noted earlier, one assuming three-dimensional anionic clusters (C23) is inconsistent with the Si NMR evidence, and with the overwhelming proportion of the other evidence on silicate anion structure. 

There are a number of papers which report the use of Si NMR in studies of the structures of various soluble silicates and alkali silicate glasses. (93, 97-103) Early workers concluded that in soluble sodium metasilicate solutions only simple monomeric silicate structures are present. (104) The presence of polymeric structures was subsequently identified using Raman spectroscopy (105-107) and trimethylsilylation experiments. (108) Marsmann (97) first reported the existence of dimer, linear trimer, tricyclic, and polymeric structures based on Si NMR... 

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.

The most important property of silicate solution is its self-polymerization, i.e., monomeric silicate —> low-polymerization silicate —> highly polymerized silicate. Silicate solution without the presence of salts can form a silica sol within a pH range of 7 10, while silicate solution with the presence of salts can form a silica gel in the same pH range (e.g., 7 10) as shown in Figure 5.7. 

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. 

Strongly Alkaline, Confined Environments, Because Si is most soluble in the form of silicate anions, it is not surprising that smectites form most rapidly in dilute alkaline solutions of sodium silicate and magnesium chloride. The layer silicate structure evolves by the precipitation of a planar Mg(OH)2 (brucite) sheet, on which monomeric silicate ions condense. Silica polymers are unable to reorganize into layered structures. It seems that the metal hydroxide sheet must be layered to begin with, a fact that necessitates 6-coordination of the metal ion that is to form the octahedral sheet of the 2 1 layer. Smectites can be formed from hydroxide sheets of the following cations (listed in order of increasing radius) ... 

In contrast with these observations concerning the independence of fractal dimensionality from monomeric silicate concentration, the remarkable observation was made that the presence of oligomeric silicate anions... 

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