Polymeric silicates

However, the free acid quickly starts to condense with itself, accompanied by the elimination of water to form dimers, trimers and eventually polymeric silicic acid. The polymer continues to grow, initially forming polymer aggregates and then polymer spheres, a few Angstroms in diameter. These polymeric spheres are termed the primary particles of silica gel and must not to be confused with the macro-particles of silica gel that are packed into the LC column. 

The form of silica in the matrix is at present unknown. In the freshly prepared cement there are appreciable amounts of silicic acid present which decline as the cement ages (Crisp, Lewis Wilson, 1976d). In the set cement silica could be present as a polymeric silicic acid, a siliceous gel or even a hydrated silicate gel, such as the tobermorite gel present in Portland cements (Taylor, 1966). 

Minerals and mineral series with the same basic chemical units, such as the silicate polymerized ions, and very similar crystal structures are related and referred to collectively as mineral groups. The amphiboles are a group composed of several mineral series, two of which were cited in the preceding examples. The several series that make up the amphibole group reflect the changes in the size and location of cations associated with the polymerized silicate chains. Because several amphibole species occur in fibrous fonn, we discuss this group in much greater detail, and include an idealized crystal structure. 

Inorganic Polymeric Silicates as a Function of Common Geometry... 

Tinker D., Lesher C.E., Baxter G.M., Uchida T., and Wang Y. (2004) High-pressure vis-cometry of polymerized silicate melts and limitations of the Eyring equation. Am. Mineral. 89, 1701-1708. 

Th-oxyhydroxide species readily dissolve upon dilution below the solubility limit, it is not veiy likely that such actinide(IV) colloids play a role away from the source in the far field of a repository. In the near field of a repository, however, they may be predominant species controlling the solubility of tetravalent actinide species such as U(IV) and Pu(IV) and thus the source term. Unusual stability at high ionic strength has been also reported for amorphous SiOz colloids (Iler 1979 Healy 1994) which also cannot be explained solely by electrostatic repulsion. Formation of oligomeric or polymeric silicate species at the colloid-water interface are thought to exert additional steric stabilization by preventing close approach of those particles. 

Zeolitic structures with pore sizes of 2000 to 10000 pm are known as mesoporous solids, and can be formed by a method known as liquid crystal templating (LCT). The combination of a suitable cationic surfactant together with silicate anions form arrays of rod-like surfactant micelles (Figure 3.7) surrounded by a polymeric siliceous framework. On calcination the mesoporous structure is formed. 

The hydrolysis products from the hypothetical Si4+ ion are the many and varied polymeric silicate ions which exist as chains, double chains, and sheets, culminating in the insoluble neutral three-dimensional arrays with the formula SiC>2. 

In principle, all lamellar minerals may be used as barrier pigments, e.g., micaceous iron oxide [5.167]-[5.169], layer silicates (mica), linear polymeric silicates (wollas-tonite), and talc [5.170], However, untreated mica and talc are not very suitable because they are highly permeable to water [5.57]. The surface can be modified with, for example, silanes or titanates, to reduce water permeability and improve adhesion... 

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. 

Evidently, the particulate silica used in Alexander s and Baumann s experiments behaved like polymeric silicic acid. This opinion is supported by the fact that stable solutions of polymeric silicic acids have practically no polymers in the intermediate size range between oligomers and high polymers (19). Correspondingly, in the presence of colloidal... 

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. 

This interpretation poses an interesting question if vitreous silica behaves like polymeric silicic acid or silica gel in aqueous suspension, how do the crystalline modifications of silica interact with aqueous solutions ... 

Owing to this activation threshold, the first precipitation product from aqueous solutions of silicic acids will be an amorphous silica of some degree of hydration, while at room temperature the growth of vitreous and crystalline forms of silica from the precipitate (and thus the approach toward the absolute equilibrium) will proceed extremely slowly. With this understanding the data in Figure 1 are said to represent, an equilibrium—i.e., the reversible equilibrium between silicic acids in aqueous solution and metastable hydrated silica or polymeric silicic acid as precipitate. 

Figure 9 gives some results obtained with vitreous silica. Three runs were made with 5, 15, and 45 sq. meters of total surface area in suspension. Apparently, the data indicate again the approach toward the two-phase equilibrium for polymeric silicic acid—i.e., independent of the total amount of solid material, the equilibrium concentration of about 110 //grams Si02/ml., as indicated in Figure 1, is approached in all three cases. Only the rate of dissolution is different, being determined by the size of the exposed surface area. 

Certain forms of silica and many polymeric silicates do not react with ammonium molybdate. These complex silicates may be decomposed to simple molybdate-reactive silica by high temperature digestion or fusion with sodium bicarbonate. 

As the melt becomes more siliceous, the equilibria for Ni2+ shift strongly in favour of the crystallizing olivine phase due to decreased CFSE s attained in more highly polymerized silicate liquids. Apparently, the proportion of tetrahedral sites in the melt rises with increasing K20 + Si02 components. 

DnR silicates in solution (4). Absolute amounts of silica present in the form of the various silicates are mentioned, together with (in brackets) their relative amounts, i.e. as if no polymeric silicates were present. These polymeric silicates, i.e., silicates consisting of more than 10 Si atoms, cannot be characterized by the chemical trapping method since the silyl esters are not volatile enough to be detected by GLC/flame ionization detection (FID) (14). Moreover, Si-NMR spectroscopy studies have never been successful in positively identifying higher molecular weight silicates than Si] ones (9). 

Figure 2 depicts the compositions of the different solutions. For the polymeric species the absolute amounts are shown for the other, smaller silicates the relative amounts. It is evident that, especially at low OH/Si ratios (i.e., < 0.5, which is a normal value for Si-rich zeolite synthesis mixtures) the larger part of the silicate species present in solution consists of uncharacterized, polymeric silicates. The values obtained in the absence of DMSO (lower part of Table III and Figures 2a and 2b are in good agreement with literature findings (14). 

First, although the use of bulky organic bases clearly shifts the silicate equilibrium to the DnR species, there may be a large amount (up to more than 90%) of polymeric species present in silicate solutions. This is true especially at low OH/Si ratios (<0.5) or high Si concentrations (>2), i.e., normal values for a zeolite synthesis composition. This range of polymeric silicates cannot at present be characterized satisfactorily, and the presence of zeolite precursor species other than DnR silicates in this range cannot be excluded. 

Silica Polymer—Metal Ion Interactions in Solution. The reaction of metal ions with polymeric silicate species in solution may be viewed as an ion-exchange process. Consequendy, it might be expected that silicate species acting as ligands would exhibit a range of reactivities toward cations in solution (59). Silica gel forms complexes with multivalent metal ions in a manner that indicates a correlation between the ligand properties of the surface Si—OH groups and metal ion hydrolysis (60,61). For Cu ", Fe ", Cd ", and Pb ", ... 

Alkali silicates are used as components, rather than reactants, in many appHcations. In many cases they only contribute partially to overall performance. Utility factors are generally not as easy to identify. Their benefit usually depends on the surface and solution chemical properties of the wide range of highly hydrophilic polymeric silicate ions deHverable from soluble silicate products or their proprietary modifications. In most cases, however, one or two of the many possible influences of these complex anions clearly express themselves in final product performance at a level sufficient to justify their use (102). Estimates of the 1995 U.S. consumption of sodium sihcates are shown in Table 6. 

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