Silicic acid polymerization

The fate of silicic acid is of some interest. Silicic acid polymerizes, by condensation, and finally a silica gel is formed (Wilson Mesley, 1968). The insolubilization of silicic acid has been observed to parallel closely the precipitation of phosphate (Wilson Batchelor, 1967b) and is related to an increase of pH within the cement (Kent Wilson, 1969). A low concentration of silicic acid must remain in the matrix. All this is in accord with the known aqueous chemistry of silica. 

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 history and use of the silicomolybdate method for analyzing for soluble silica has been discussed in Chapter 1. It is sufficient here to point out that molybdic acid reacts only with monomeric silica to form the yellow silicomolybdic acid. It is fortunate that the reaction with molybdic acid occurs at pH 1-2, where silicic acid polymerizes least rapidly. Thus polymeric silica must first depolymerize before it can react hence the higher the degree of polymerization, the longer the time required for depolymerization and color development. This is reviewed in detail in Chapter 3. 

It was Carmen (4c) who first clearly stated that silicic acid polymerizes to discrete particles which then aggregate into chains and networks. 

In the case of polysilicic acid obtained by adding a 3.25 ratio sodium silicate solution to a rapidly agitated solution of acid to obtain a final pH of about 2. the relation between X value and the degree of polymerization as the silicic acid polymerizes... 

Experience has shown that alkali-stabilized sols of silicic acid polymerize during a few months at 30 C until the surface area slowly drops to about 500 m g" , corresponding to 5 nm particles, after which there is little change. At 90 C, the particles grow in a few days to around 7-8 nm, after which spontaneous growth is very slow. . . ... 

As discussed in Chapter 3 (see Figure 3.1) silicic acid polymerizes to form discrete particles in the pH range 7-10. No aggregation occurs if the concentration of electrolyte is generally less than 0.1-0.2 N, depending on silica concentration. 

The particulate theory of gel formation was described by Her (2) in 1955. Kiselev and Lygin (229) verified by electron microscope studies that as silicic acid polymerized, particles up to 5-10 nm in size can be formed as a sol and this size retained in the gel structure. 

Solutions of silicic acid thicken slowly and finally form a gel. Since the gel appears outwardly like organic gels, it was generally thought that Si(OH)4 polymerized into siloxane chains (i.e., chains with Si—O—Si bonds) that branched and crosslinked like many organic polymers. However, in his book. Her (10) clearly states that silicic acid polymerizes into discrete particles that in turn aggregate into chains and networks. Polymerization occurs in three stages ... 

Coradin T., Livage J. Effect of some amino acids and peptides on silicic acid polymerization. Colloid. Surf. B Biointerf. 2001 21 329-336... 

It is generally believed that the base-catalyzed condensation mechanism involves penta- or hexacoordinated silicon intermediates or transition states [79,81,90-92]. For silicic acid polymerization, Okkerse [90] proposed a bimolecular intermediate involving one hexacoordinated silicon ... 

The solution is gelatinous and becomes cloudy on standing. This is because the silicic acid polymerizes by splitting off water, to form Si—O—Si bridges, which results in a mixture of molecules of different chain lengths. 

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