Water in Silicate Glasses: Quantitation and Structural Studies by 'H Solid Echo and MAS-NMR Methodst

'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 % H20. For glasses free of paramagnetic metal ions, the absolute water contents can be accurately determined by a solid echo IH NMR technique with pyrophyllite, A12Si,010(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 (C2-4 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 'H chemical shifts on the O-H.-O distance (a measure of the hydrogen bonding strength) leads to average distances of 290 f 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 '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/H20 ratios in these cases.