We present an X-ray diffraction and multi-nuclear (H-2 and Ca-43) NMR study of Ca-exchanged hectorite (a smectite clay) that provides important new insight into molecular behavior at the smectite-H2O interface. Variable-temperature Ca-43 MAS NMR. and controlled humidity XRD indicate that Ca2+ occurs as proximity-restricted outer-sphere hydration complexes between -120 and +25 degrees C in a two-layer hydrate and at T <= -50 degrees C in a 2:1 water/solid paste. Changes in the Ca-43 NMR peak width and position with temperature are more consistent with diffusion-related processes than with dynamics involving metal-surface interactions such as site exchange. The H-2 NMR signal between -50 and +25 degrees C for a two-layer hydrate of Ca-hectorite is similar to that of Na- and other alkali metal hectorites and represents (H2O)-H-2 molecules experiencing anisotropic motion describable using the H-2 C-2/C-3 jump model we proposed previously. H-2 T-1 relaxation results for Ca- and Na-hectorite are well fit with a fast-exchange limit, rotational diffusion model for (H2O)-H-2 dynamics, yielding GHz-scale rotational reorientation rates compatible with the C-3 component of the C-2/C-3 hopping model. The apparent activation energy for (H2O)-H-2 rotational diffusion in the two-layer hydrate is greater for Ca-hectorite than Na-hectorite (25.1 vs. 21.1 kJ/mol), consistent with the greater affinity of Ca2+ for H2O. The results support the general principle that the dynamic mechanisms of proximity-restricted H2O are only weakly influenced by the cation in alkali metal and alkaline earth metal smectites and provide critical evidence that the NMR resonances of charge-balancing cations in smectites become increasingly influenced by diffusion-like dynamic processes at low temperatures as the charge density of the unhydrated cation increases.

12 We present an X-ray diffraction and multi-nuclear (H and Ca) NMR study of Ca-exchanged 13 hectorite (a smectite clay) that provides important new insight into molecular behavior at the 14 smectite-H2O interface. Variable temperature Ca MAS NMR and controlled humidity XRD 15 indicate that Ca occurs as proximity-restricted outer-sphere hydration complexes between -120 16 °C and +25 °C in a two-layer hydrate and at T ≤ -50 °C in a 2:1 water/solid paste. Changes in 17 the Ca NMR peak width and position with temperature are more consistent with diffusion18

Amorphous calcium carbonate (ACC) is a metastable precursor to crystalline CaCO3 phases that precipitates by aggregation of ion pairs and prenucleation clusters.(1, 2) We use 43Ca solid-state NMR spectroscopy to probe the local structure and transformation of ACC synthesized from seawater-like solutions with and without Mg2+ and computational molecular dynamics (MD) simulations to provide more detailed molecular-scale understanding of the ACC structure. The 43Ca NMR spectra of ACC collected immediately after synthesis consist of broad, featureless resonances with Gaussian line shapes (FWHH = 27.6 ± 1 ppm) that do not depend on Mg2+ or H2O content. A correlation between 43Ca isotropic chemical shifts and mean Ca–O bond distances for crystalline hydrous and anhydrous calcium carbonate phases indicates indistinguishable maximum mean Ca–O bond lengths of ∼2.45 A for all our samples. This value is near the upper end of the published Ca–O bond distance range for biogenic and synthetic ACCs obtained by Ca-X-ray ...

Molecular-scale dynamic processes involving ions and water at smectite-water interfaces play crucial roles in issues such as contaminant transport, reactivity of geochemical systems, and carbon sequestration, yet little is known about the specific manner in which interfacial ion and water dynamics influence one another, particularly at temperatures below 0 degrees C. In this work, we discuss the effects of the charge-balancing cation on the structure and dynamics of alkali metals and H2O at the mineral-water interfaces of alkali metal-smectite pastes over a broad range of temperatures. New variable-temperature H-2 and Na-23 NMR spectroscopic data for a Na-hectorite paste presented here in combination with the results from our previous studies of Cs- and K-hectorite pastes reveal a common anisotropic mechanism of H-2 motion for (H2O)-H-2 restricted by proximity to a surface or cation between -50 and -20 degrees C. This motion is well modeled by combined fast-limit C-2 librations about the (H2O)-H-2 molecular dipole moment and fast-limit octahedral-type jumps of (H2O)-H-2 molecules about the C-3 symmetry axis of a slightly compressed metal-H2O complex. At higher temperatures, (H2O)-H-2 dynamics are dominated by diffusion and/or chemical exchange of deuterons and differ for Na- and K-exchanged samples. Comparing our collective K-39, Cs-133, and Na-23 VT NMR results shows that Na+ has less affinity for the smectite surface in pastes than K+ or Cs+, that the influence of (H2O)-H-2 and (H2O)-H-2 content on alkali metal motion decreases from Na+ to K+ to Cs+, and that slow-to-intermediate (rate < 10(4)-10(5) Hz) two-site exchange is a significant dynamic process above -80 degrees C only for Cs+.