23Na MAS NMR spectroscopy of the smectite mineral hectorite acquired at temperature from −120 °C to 40 °C shows the presence of complex dynamical processes in the interlayer galleries that depend significantly on their hydration state.

The intercalation of H2O, CO2, and other fluid species in expandable clay minerals (smectites) may play a significant role in controlling the behavior of these species in geological carbon sequestration and enhanced petroleum production and has been the subject of intensive study in recent years. This paper reports the results of a computational study of the effects of the properties of the charge-balancing, exchangeable cations on H2O and CO2 intercalation in the smectite mineral, hectorite, in equilibrium with an H2O-saturated supercritical CO2 fluid under reservoir conditions using grand canonical molecular dynamics methods. The results show that the intercalation behavior is greatly different for the cations with relatively low hydration energies and high affinities for CO2 (here Cs+) than for cations with higher hydration energies (here Ca2+). With Cs+, CO2 intercalation occurs in a 1-layer structure and does not require H2O intercalation, whereas with Ca2+, the presence of a sub-monolayer of H2O is required for CO2 intercalation. The computational results provide a detailed structural, dynamical, and energetic insight into the differences in the intercalation behavior and are in excellent agreement with in situ experimental X-ray diffraction, infrared, quartz crystal microbalance, and nuclear magnetic resonance results for smectite materials obtained under reservoir conditions.

Classical molecular dynamics simulations were performed for the smectite clay hectorite with charge-balancing Cs+ cations using a newly developed structural model with a disordered distribution of Li/Mg substitutions in the octahedral sheet and the fully flexible CLAYFF force field. Calculations for systems with interlayer galleries containing 0-19 H2O/Cs+ suggest that the monolayer hydrate is the only stable state at all relative humidities at ambient pressure and temperature, in agreement with experimental results and previous molecular calculations. The basal spacing of this structure is also in good agreement with experimental values. In contrast to previous molecular modeling results, however, the new simulations show that interlayer Cs+ occurs on 2 different inner sphere adsorption sites: above the center of ditrigonal cavities and above Si tetrahedra. Unlike previous simulations, which employed a rigid clay model and fixed orientations of the structural -OH groups, the present results are obtained for an unconstrained clay substrate structure, where the structural -OH groups are able to assume various orientations, including being nearly parallel to the clay layers. This flexibility allows the Cs+ ions to approach the surface more closely above the centers of the hexagonal rings. In this structural arrangement, Cs+ ions are not hydrated by the H2O molecules which share the same interlayer plane, but rather by the H2O molecules coordinated to the opposite surface. In contrast, on the external basal surface, a significant fraction of H2O molecules are adsorbed above the centers of ditrigonal cavities adjacent to adsorbed Cs+ ions. For these H2O molecules, both HHZO atoms coordinate and H-bond to O-b surface oxygen atoms. The mean residence times for the Cs+-H2O, Cs+-O-b, and H2O-O-b coordination pairs show that Cs+ ions are more strongly coordinated with O-b atoms than H2O molecules. This result is the opposite of the behavior in Ca-hectorite, due to the much smaller hydration energy of Cs+ compared to that of Ca2+.

Variable-temperature X-ray diffraction and H-2 NMR spectroscopy of the smectite mineral, hectorite, containing interlayer Na+, K+, Cs+, Mg2+,Ca2+ Sr2+, and Pb2+ equilibrated at 43% relative humidity (RH) and mixed with (H2O)-H-2 to form a paste provide a comprehensive picture of the structural environments and dynamics of interlayer (H2O)-H-2 and the relationships of these properties to interlayer hydration state, the hydration energy and polarizability of the cation, temperature, and the formation of ice-lh in the interparticle pores. The variation in basal spacing shown by the XRD data correlates well with the H-2 NMR behavior, and the XRD data show show for the first time in hectorites that crystallizationof interpraticle ice-lh causes a decrease in the interlayer spacing, likely due to removal of interlayer (H2O)-H-2. The variation of the H-2 NMR behavior of all the samples with decreasing temperature reflects decreasing frequencies of motion for the rotation of the (H2O)-H-2 molecules around their dipoles, reorientation of the (H2O)-H-2 molecules, and exchange of the (H2O)-H-2 molecules between interlayer sites coordinated to and not coordinated to the cations.

Renal stone diseases are a global health issue with little effective therapeutic recourse aside from surgery and shock-wave lithotripsy, primarily because the fundamental chemical mechanisms behind calcium biomineralization are poorly understood. In this work, we show that natural abundance Ca-43 NMR at 21.1 T is an effective means to probe the molecular-level Ca2+ structure in oxalate-based kidney stones. We find that the Ca-43 NMR resonance of an authentic oxalate based kidney stone cannot be explained by a single pure phase of any common Ca2+-bearing stone mineral. Combined with XRD results, our findings suggest an altered calcium oxalate monohydrate-like Ca2+. coordination environment for some fraction of Ca2+ in our sample. The evidence is consistent with existing literature hypothesizing that nonoxalate organic material interacts directly with Ca2+ at stone surfaces and is the primary driver of renal stone aggregation and growth. Our findings show that Ca-43 NMR spectroscopy may provide unique and crucial insight into the fundamental chemistry of kidney stone formation, growth, and the role organic molecules play in these processes.

Seven natural analcime samples with atomic Si/Al ratios from 1.97 to 2.63 were investigated to explore the effects of intermediate range structure and Al for Si substitution up to the fourth nearest neighbor coordination shell on the Si-29 NMR chemical shifts in the framework aluminosilicates. With increasing bulk Si/Al ratio, the Si-29 chemical shifts of all Si(nAl) resonances become more negative (more shielded), consistent with previously reported trends for faujasite and LTA zeolite (Newsam 1985). For our analcimes, the total observed changes in chemical shift for the Si(3Al), Si(2Al), and Si(1Al) sites are similar to 0.5, 0.6, and 1.1 ppm, respectively, demonstrating that the effect of Si/Al ratio is more significant for the Si sites with a smaller number of next-nearest neighbor Al atoms. The mean value of the change in chemical shift per added Al on fourth nearest neighbor sites is 2.8 ppm [2.3 ppm if Si(3Al) is excluded]. This value is similar to the results of recent QM/MM calculations and is somewhat larger than those previously reported for faujasite and LTA framework zeolite (similar to 1.4 and 1.3 ppm). This difference correlates with the overall denser structure of analcime, including smaller cages and shorter Si-fourth neighbor distances. Combining these results with the known changes in Si-29 chemical shifts for framework silicates due to changes in the first coordination shell, tetrahedral polymerization and second neighbor Al for Si substitution for tetrahedrally coordinated Si, we present an empirical relation between the changes in Si-29 chemical shift and interatomic distance between Si and nearby atoms.

There are few effective methods for characterizing the molecular scale structural environments of Ca2+ in hydrated cements, which has limited our ability to understand the structure of, for example, Ca–silicate hydrate (C–S–H). 43Ca nuclear magnetic resonance (NMR) spectroscopy has long been considered too insensitive to provide useful data in this regard, but 43Ca magic angle spinning (MAS) NMR spectra reported here for synthetic tobermorite and jennite with naturally abundant levels of 43Ca demonstrate that this is a viable approach. We show that spectra with useful signal/noise ratios can be obtained in a reasonable acquisition period (∼2 days) using an H0 field strength of 21.1 T, 5 mm rotors spinning at a frequency of 5 kHz, and a double frequency sweep preparatory pulse sequence. Tobermorite and jennite produce relatively broad resonances due to their complex structures and structural disorder, however, the chemical shift differences between six‐coordinate 43Ca in jennite and seven‐coordinate 43Ca in 11 Å tobermorite are large enough that the signals are entirely resolved at this field. These data suggest that signal from ideal tobermorite‐like and jennite‐like sites in cement C–S–H can most likely be distinguished by 43Ca NMR and that this method will be a powerful approach for studying cement‐based ceramic materials in the coming decade.

The investigation of the dynamics of water and organic species confined in minerals or adsorbed at their surface is of significant geochemical, environmental, catalytic, biomedicine, and life's growth interests but is poorly understood on the molecular scale. This work explores the behavior of water molecules and glutamate species adsorbed on and between the double hydroxide layers of hydrotalcite [HT; (Mg2Al)(OH)(6)A(-)center dot nH(2)O, where A(-) is a counteranion which may bear different charges] and compares the results to those for HT containing small inorganic anions. The relative humidity (RH) dependence of the H-1 T-1 relaxation rates for all samples reveals the existence of two separate spin systems with 1/T-1 relaxation rates differing by a factor of approximately 2 x 10(3). The static H-1 spectral line widths allow assigning the fast relaxing protons to the fixed "static" interlayer and adsorbed species-i.e. bound water, bound organic species, and most of the structural hydroxyl groups (-OH)-and the slow ones to the "mobile" species-i.e. free water and solvated organic molecules and some of the structural -OH groups.

(13)C MAS NMR spectroscopy of isotopically enriched samples of the layered double hydroxide hydrotalcite (HT) [(Mg(2)Al)(OH)(6)A(-),nH(2)O, where A(-) is a counteranion that may bear different charges] exchanged with glutamate (Glu) shows an unexpected preferential adsorption of the lower-charged species (Glu(1-)) relative to the higher-charged species (Glu(2-)) by a layered double hydroxide (LDH) compound. At pH 11.0, the Glu(1-)/Glu(2-) ratio is about 0.44, an order of magnitude greater than expected in solution. Previous studies of phosphate and carbonate exchange onto LDH compounds (refs 25 and 26) show a strong preference for the higher-charged anion. The preference for Glu(1-), in which the amine site is protonated, may be due in part to -NH(3)(+) allowing for an energetically more favorable H-bonding network among the anions, the metal hydroxide substrate, and the interlayer and surface water molecules compared to -NH(2). Changes in the pH and the pK(a) of Glu near the HT surface and due to nanoconfinement may also play important roles. These results suggest that the interactions dominating the exchange of amino acids and proteins onto LDH compounds may be quite different from those that control the exchange of small inorganic anions.

This article describes a (39)K nuclear magnetic resonance (NMR) spectroscopic study of K+ displacement at the muscovite/water interface as a function of aqueous phase pH. (39)K NMR spectra and T 2 relaxation data for nanocrystalline muscovite wet with a solid/solution weight ratio of 1 at pH 1, 3, and 5.5 show substantial liquid-like K+ only at pH 1. At pH 3 and 5.5, all K+ appears to be associated with muscovite as inner- or outer-sphere complexes, indicating that H(3)O+ does not displace basal surface K+ beyond the (39)K detection limit under these conditions. In our pH 1 mixture, only approximately 1/3 of the initial basal surface K+ population is located more than 3-4 A from the surface. (29)Si and (27)Al MAS NMR spectra and SEM images show no evidence of dissolution during the (39)K experiments, consistent with the liquid-like (39)K fraction originating from displaced basal surface K+. Assuming no muscovite dissolution or interlayer exchange, the K+/H(3)O+ ratio relevant to the solution/surface exchange equilibrium is controlled by the total amount of K+ on the surface and H(3)O+ in solution (K+(surf)/H(3)O+(aq)). These parameters, in turn, depend on the basal surface area, solution pH, and the solid/solution ratio. The results here are consistent with significant displacement of surface K+ only under conditions where the initial K+(surf)/H(3)O+(aq). ratio is less than approximately 1. Computational molecular models of the muscovite/water interface should account for both K+ and H(3)O+ in the near-surface region.