H2O and cation structure and dynamics in expandable clays:: 2H and 39K NMR investigations of hectorite

Variable temperature K-39 and H-2 nuclear magnetic resonance (VT NMR) spectroscopy of K+-saturated hectorite, a prototypical smectite clay, provides new insight into the relationships between the structural and dynamical behavior of K+ and H2O in confinement and at surfaces. In d = 10 angstrom K-exchanged hectorite, interlayer K+ is rigidly held by the silicate rings, probably in 12-coordinate inner-sphere sites as in muscovite mica. In a 1/1.5 by weight bectorite/water paste, K+ occurs on interlayer and external surface sites that are indistinguishable by K-39 NMR. The K+ environments experience changes in dynamical behavior over the temperature range from -50 to 60 degrees C that are directly related to H2O dynamics. K-39 NMR of the paste sample shows dynamic line narrowing at low temperatures due to modulation of the electric field gradient (EFG) at frequencies of the order of the static line width (approximate to 20 kHz) and two "melting"-type dynamic transitions near -10 degrees C, one for surface and one for confined K+. At and above 0 degrees C, K+ remains closely associated with the clay surfaces and experiences motion at frequencies greater than 200 kHz and less than 10 MHz, as revealed by K-39 T-1 relaxation behavior, nutation behavior, and the K-39 quadrupolar product. These data are consistent with rapid exchange between inner- and outer-sphere K+ sites reported previously for K-montmorillonite based on molecular dynamics simulations. Deuterium NMR shows the presence of two unique H2O environments in the system: one structurally and dynamically consistent with bulk water between particles and one attributable to H2O confined in the interlayer. Confined H2O experiences anisotropic motion between -50 and 0 degrees C via fast rotation (>2 MHz) about a single axis oriented 127.5 +/- 0.5 degrees from the principal axis of the H-2 EFG, potentially due to C2 rotation. This motion does not affect the K-39 EFG sianificantly. Melting of free and confined H2O occurs between - 10 and 0 degrees C and near 0 degrees C, respectively, similar to the melting behavior of K+ and likely reflecting the onset of molecular diffusion. At and above 10 degrees C, all H2O environments experience motion near or in excess of 300 kHz through at least three NMR-indistinguishable mechanisms, including Brownian motion of free water, exchange of free and confined H2O near particle edges, and diffusive motion of H2O that remains confined on the experimental time scale. The correlation between the rates of H-2 and K-39 motion and the observed melting transitions for both spin populations strongly suggest that K-39 melting and dynamics above the melting transition are linked to an increase in the motional freedom of H2O.