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Published April 1988 | public
Journal Article

Water in Silicate Glasses: Quantitation and Structural Studies by ^(1)H Solid Echo and MAS-NMR Methods


^(1)H wideline and magic angle spinning (MAS) NMR results are reported for water in a series of synthetic and naturally occurring silicate glasses containing from 0.04 to 9.4 wt % H_(2)0. For glasses free of paramagnetic metal ions, the absolute water contents can be accurately determined by a solid echo ^(1)H NMR technique with pyrophyllite, A1_(2)Si,0_(10)(OH)_2, as an intensity reference. The MAS-NMR spectra can be interpreted as superpositions of the individual spectra of OH and anisotropically constrained H_(2)0 groups, the latter giving rise to spinning sidebands extending over ca. 100 kHz. Two methods are described to obtain percentages of OH and H_(2)0 groups from the relative intensities of the centerband and the spinning sidebands in these glasses. The MAS-NMR results are consistent with previous IR analyses indicating that low levels of water (<2-4 wt %) are mainly present as OH groups whereas at higher concentrations molecular H_(2)0 species dominate. Simulations of the MAS-NMR spectra based on the individual spectra of compounds in which the hydrogen-bearing species are structurally isolated (OH in tremolite and H_(2)0 in analcite) accurately reproduce the experimental spectra, indicating that the OH or H_(2)0 groups in the glasses are not preferentially clustered. The MAS-NMR centerband line shapes are dominated by a distribution of isotropic chemical shifts. The well-established linear dependence of ^(1)H chemical shifts on the O-H•••O distance (a measure of the hydrogen bonding strength) leads to average distances of 290 ± 1.5 pm in all synthetic glasses except silica, 293 ± 1.5 pm in the volcanic rhyolite glasses, and 298 pm in silica glass. This value does not depend on the total water contents, indicating that the hydrogen-bonding characteristics of OH and H_(2)0 species in the glasses are similar. The ^(1)H wideline NMR procedure above yields underestimates of the total water content for synthetic and volcanic glasses containing ca. 1 wt 5% iron, presumably due to extreme signal broadening by the strong dipolar fields from the electron spins of the paramagnetic ions. These dipolar couplings also affect the line shape of the observable portion of the hydrogen resonance and produce intense spinning sidebands in the MAS-NMR spectra which invalidate determinations of OH/H_(2)0 ratios in these cases.

Additional Information

© 1988 American Chemical Society. Received: September 3, 1987. The NMR experiments were carried out at the Southern California Regional NMR Facility at the California Institute of Technology, supported by NSF grant no. CHE84-40137. E.M.S. acknowledges additional support by NSF grants EAR 84-17434 and EAR 86-18229. We also thank the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Professor A. L. Boettcher (University of California, Los Angeles) for assistance with preparation of some of the glasses and Drs. P. Dobson and S. Newman (Division of Geological and Planetary Sciences, California Institute of Technology) for help with the manometric analyses, which were carried out in the laboratory of Prof. S. Epstein. Thanks are also due to Prof. David Live (Chemistry, Emory University) for stimulating discussions, to Dr. G. J. Fine and the Corning Glass Works for providing the anhydrous starting materials for several of the samples investigated, and to Prof. J. R. Holloway (Chemistry, Arizona State University) for providing a water-containing rhyolite glass isotopically diluted with D_(2)O.

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