Vibrational and Si-29 NMR spectroscopies of soluble silicate oligomers

Vibrational spectroscopy has been used to correlate the features of silicat e solution spectra with Si-29 NMR spectra, as a function of SiO2:Na2O ratio and SiO2 concentration. Assignments of infrared and Raman component bands have been made for different anion types and are consistent with results re ported for crystalline silicates and silicate glasses. The extent of depoly merization of larger anions has been found to differ, depending on both the degree of dilution and alkalinity. The relative amounts of polymeric (Q(4) ) silica and larger three dimensional (Q(4R)(3) anions are greater for the more siliceous solutions (3.3 and 2.95 ratio) at high concentrations. These large anions continue to be major contributors to the anion distribution e ven at 15 to 50 fold dilution. In the case of more alkaline solutions, espe cially below 2.5 ratio, the relative contribution of the large anions to th e distribution drops rapidly, even with 2 or 3 fold dilution. At low concen trations, in very alkaline solutions, the anion distribution is mainly mono mer, dimer and cyclic trimer. Vibrational spectroscopies show great promise for observing changes in anion distribution on a much shorter time scale t han is possible with Si-29 NMR and can be useful for following reactions in volving silicate solutions.