Silicate oligomers concentrations

Observed and Calculated Silicate and Aluminosilicate Oligomer Concentrations in Alkaline Aqueous Solutions... 

The variation of oligomer concentrations as a function of polymerization degree and connectivity is analogous to the variation observed for silicate solutions. Nevertheless some specific features become apparent. 

Several recent NMR studies have alloHed the identification of about tnenty silicate oligomers and quantitative or semi-quantitative estimates of their respective concentrations. In spite of the precautions used by some authors, the measured concentrations and in particular the experimental monomer concentration (used as a basis in our calculations) are probably not alaays very accurate expecial-ly in the case of the less alkaline or the more concentrated (in silica) solutions. The use of an inert silicon-containing internal standard could perhaps be useful. To this lack of accuracy is added the uncertainty of the pH measurement in such strongly alkaline solutions. 

Caullet, P. and Guth, J. L. 1989. Observed and calculated silicate and aluminosilicate oligomer concentrations in alkaline aqueous solutions. 196th National Meeting of the American Chemical Society, Los Angeles, California, September 25-30,1988 in Zeolite Synthesis, Ocelli, M. L. and Robson, H. E. eds, ACS Symposium Series, 398 (6) 83-7. 

In the inhibition period aluminum cations react with the silica oligomers and subcolloidal particles to form isolated patches of alumino-silicates on the surface of the small silica particles. From the review work of Iler (Her 1979), it is known that the reaction of aluminum cations with silicate oligomers is fast compared to the clustering of particles. After the inhibition period, the size of the aggregates slowly increases. Increasing the concentration of aluminum cations in the solution involves the formation of more homogeneous alumino-silicate particles. At a concentration of 0.10 mol% aluminum in the silicate solution, the fractal dimensionality... 

Although the reaction scheme shows a complete hydrolysis before condensation begins, this is likely not correct as stated earlier. The relative rates and extents of these two reactions will particularly depend on the amount of water added and the acidity of the system (10,11). The high functionality of the triethoxysilane endcapped PTMO oligomer should enhance the incorporation of PTMO molecules into the TEOS network. It was also assumed that the reactivities would be the same between silanol groups from silicic acid and endcapped PTMO. Therefore, no preferential condensation was expected and the deciding factors for which type of condensation (self- or co-) took place would be the diffusivities and local concentrations. 

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. 

Table III Experimental ( 1 0) and calculated concentrations of oligomers in a sodium silicate solution (1.65 mol.I-1Si02> ( t number of tetrahedra in oligomers)...

---