Oxygen-17 nuclear magnetic resonance spectroscopic studies of aqueous alkaline silicate solutions

Abstract

High-field (11.75 T) oxygen-17 n.m.r. spectra have been obtained for a variety of aqueous silicate solutions isotopically enriched in the oxygen-17 nuclide. Oxygen-17 n.m.r. linewidths of the silicate anions are broad (Δv_½ca. 3 to 20 p.p.m.) and the chemical shift range is relatively small (about 50 p.p.m.), often resulting in considerable spectral overlaps. Assignment of the observed resonances to known silicate structures is nevertheless possible in some cases by using suitable ‘model’ solutions containing silicate structures which have been previously characterized by ²⁹Si n.m.r. spectroscopy and by making use of spectral simulations. In this manner, two overlapping spectral regions may be distinguished: a 35–55 p.p.m. range (from water) typical of non-bridging, or ‘terminal’ oxygen groups (Si–O^– or Si–O–H, or both), and a 45–85 p.p.m. range, assigned to bridging groups (Si–O–Si). Contrary to recent Raman spectroscopic studies, ¹⁷O n.m.r. spectroscopy shows no evidence for any silicate species other than the monomeric silicate anion in very dilute solution (0.01 mol dm^–3 in SiO₂). Preliminary kinetic investigations indicate that in tetramethylammonium silicate solutions very different bulk water exchange rates exist for the two types of oxygen site present, with that for the non-bridging oxygen being much greater than that of the bridging oxygen. Concentrated alkali-metal silicate solutions yield complex ¹⁷O n.m.r. spectra with many overlapping signals.