Silicate oligomer species

STRPCTPRE of the SILICATE OLIGOMERS The experimentally identified species (9.10.11) are comprised of a rather small number of silicon tetrahedra (t < 12). They are for instance linear (t = 1 to 4). cyclic (t = 3 and 4), bicyclic oligomers (t = 6,8 and 12) consisting of 2 identical rings. There are, in addition, more complex species related to the cyclic trimer and tetramer and other polycyclic forms ( t = 7 and 8). 

The model presented here alloas. in principle, the determination of the concentration of any oligomer species in an alkaline silicate solution provided that the monomeric silica concentration and the pH are knoon. Due to their greater complexity and experimental difficulty. the investigation of aluminosilicate solutions corresponds only to an extension of this model and it Has dealt with in a simplified say. 

Because the structure of silicates oligomers tend to be as condensed as possible, structures with long linear chains and large cyclic rings are less plausible. On the basis of this statement, structures containing three-membered rings and threefold substituted Si sites appear the most likely candidates. Therefore, the most probable structure of the analysed species is 7e. In combination with two other samples... 

Table 3.9. Polymerization of Silicic Acid at pH 1-5—Oligomer Species...

Every species ranging from small ones with three Si to larger species with 31 Si are observed. The distribution of the silicate species is shown in Figure 2. There are mostly species up to 12 Si with a majority of pentamers and tetramers, whereas the signal intensities for the larger species are relatively weak. This is consistent with the NMR evidence for silicate speciation of oligomers with Si number up to 12 atoms [6],... 

Comparison with the results of Me Cormick et al. (10). The five sodium silicate solutions studied by these authors have the same silica concentration (about 1.65 mol.l-1) and various pH values (13.67 1 3. 30 1 2. 60 11.86 and 11.54 according to our own measurements). They identified the same oligomers as did Harris, and proposed an additional 12 silicon-tetrahedra hexagonal prismatic species. 

The values given in Table IT and referring to the potassium silicate solution already considered before (paragraph entitled "Comparison with the experimental data obtained by Harris at al. ") show a good agreement between experimental and calculated concentrations for the main oligomers. Satisfactory agreement was found as well for the other species not mentioned in Table IT. 

Analogously to the calculation principle used for silicate species, the activity of an aluminosilicate oligomer can be expressed by... 

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. 

A direct consequence of this is that, for a given conformation, the most stable oligomer will be the one containing the highest number of aluminum atoms, provided it is consistent with Loewenstein s rule (31). This trend becomes weaker as the pH decreases or the overall Si/Al ratio in solution increases, in agreement with the usual observations made in zeolite synthesis (32). These correlations are shown in table V referring to the cyclic tetramers. The numbers in brackets represent the concentration ratio between the species considered and the purely silicic form. 

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