The state of water in rhyolitic glasses: A deuterium NMR study

Abstract

The first application of ^2H NMR to the study of water in glasses is reported. Naturally occurring volcanic glass (Los Posos rhyolite) was autoclaved with D_2O to yield samples with total “D_2O” contents ranging from 1.6–4.8 wt%. Infrared results show that, in concordance with results from proton-containing glasses, the ratio of molecular D_2O to OD groups increases with increasing total “D_2O” content. Variable-temperature deuterium NMR spectra obtained at 11.7 T, using composite-pulse quadrupolar-echo pulse sequences, indicate the presence of two species with differing motional behavior. The species that is rigid over the entire temperature range investigated (173–393 K) is assigned to OD groups, and yields a powder pattern governed by a distribution of nuclear quadrupole coupling constants (NQCCs) and with a peak-to-peak splitting of ca. 160 kHz. The species that undergoes thermally activated motional processes in this temperature range is assigned to molecular D_2O. At room temperature, this species gives rise to a powder pattern characterized by a reduced quadrupole coupling constant and an asymmetry parameter (η) close to one, indicating that the D_2O molecules undergo twofold rotations about the bisector axis on a lime scale faster than ca. 10^(−5) s. Separation of both spectral components on the basis of their different spin-lattice relaxation times yields reasonable agreement with the D_2O/OD ratio determined from infrared spectroscopy. Lineshape changes at elevated temperatures suggest the activation of additional motional processes; however, even at 375 K only a minor fraction of isotropically mobile species is observed, indicating that “pools” of liquid-water do not exist in significant amounts in these glasses. Low-temperature studies indicate a gradual freezing-out of D_2O motion on the NMR time scale (ca. 10^(−5) s), resulting in a broad lineshape at 173 K governed by a distribution of NQCCs. The well-established correlation between the NQCC and the O-D ··· O distance, a measure of the hydrogen bonding strength, can be used to simulate the low-temperature ^2H NMR spectrum from an assumed distribution of O-D ··· O bond distances. The resultant spectrum, based on a distribution function obtained from independent IR data, is characterized by a mean NQCC of 223 kHz and is in good agreement with the experimental lineshape.