Oxygen Speciation in Hydrous Silicate Glasses: An Oxygen-17 NMR Study

The structural roles of H2O and OH in hydrous Na2Si4O9 (sodium tetrasilcate, NTS) and NaAlSi3O8 (“albite”) glasses have been investigated with 17O, one-dimensional fast magic-angle spinning (MAS), static, and cross-polarization (CP), and two-dimensional triple quantum MAS (3QMAS) NMR techniques. Silica gel and crystalline analcime (NaAlSi2O6·H2O) were also studied as model compounds. Peaks for molecular H2O are clearly identified in the analcime and in the hydrous glasses in one-dimensional, partial saturation experiments. Bridging and nonbridging oxygen sites are clearly resolvable in the 3QMAS spectra, as is a feature related to SiOH in hydrous NTS glass. This feature may also be present in the hydrous albite glass, although, lacking further characterization of other site types (e.g., NaOH, AlOH, Si−OH−Al), this assignment remains tentative. 1H−17O CP experiments with short contact times reveal a peak that is similar in both hydrous glasses and silica gel, again possibly related to SiOH groups.