International audienceClassical 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 H(H2O) atoms coordinate and H-bond to Ob surface oxygen atoms. The mean residence times for the Cs+ - H2O, Cs+ - Ob, and H2O - Ob coordination pairs show that Cs+ ions are more strongly coordinated with Ob 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+

Layered aluminosilicates play a dominant role in the mechanical and gas storage properties of the subsurface, are used in diverse industrial applications, and serve as model materials for understanding solvent-ion-support systems. Although expansion in the presence of HO is well-known to be systematically correlated with the hydration free energy of the interlayer cation, particularly in environments dominated by nonpolar solvents (i.e., CO), uptake into the interlayer is not well-understood. Using novel high-pressure capabilities, we investigated the interaction of dry supercritical CO with Na-, NH-, and Cs-saturated montmorillonite, comparing results with predictions from molecular dynamics simulations. Despite the known trend in HO and that cation solvation energies in CO suggest a stronger interaction with Na, both the NH- and Cs-clays readily absorbed CO and expanded, while the Na-clay did not. The apparent inertness of the Na-clay was not due to kinetics, as experiments seeking a stable expanded state showed that none exists. Molecular dynamics simulations revealed a large endothermicity to CO intercalation in the Na-clay but little or no energy barrier for the NH- and Cs-clays. Indeed, the combination of experiment and theory clearly demonstrate that CO intercalation of Na-montmorillonite clays is prohibited in the absence of HO. Consequently, we have shown for the first time that in the presence of a low dielectric constant, gas swelling depends more on the strength of the interaction between the interlayer cation and aluminosilicate sheets and less on that with solvent. The finding suggests a distinct regime in layered aluminosilicate swelling behavior triggered by low solvent polarizability, with important implications in geomechanics, storage, and retention of volatile gases, and across industrial uses in gelling, decoloring, heterogeneous catalysis, and semipermeable reactive barriers.

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International audienceGrand Canonical Molecular Dynamics (GCMD) simulations were performed to investigate the intercalation of CO2 and H2O molecules in the interlayers of the smectite clay, Na-hectorite, at temperatures and pressures relevant to petroleum reservoir and geological carbon sequestration conditions and in equilibrium with H2O-saturated CO2. The computed adsorption isotherms indicate that CO2 molecules enter the interlayer space of Na-hectorite only when it is hydrated with approximately three H2O molecules per unit cell. The computed immersion energies show that the bilayer hydrate structure (2WL) contains less CO2 than the monolayer structure (1WL) but that the 2WL hydrate is the most thermodynamically stable state, consistent with experimental results for a similar Na-montmorillonite smectite. Under all T and P conditions examined (323–368 K and 90–150 bar), the CO2 molecules are adsorbed at the midplane of clay interlayers for the 1WL structure and closer to one of the basal surfaces for the 2WL structure. Interlayer CO2 molecules are dynamically less restricted in the 2WL structures. The CO2 molecules are preferentially located near basal surface oxygen atoms and H2O molecules rather than in coordination with Na+ ions. Accounting for the orientation and flexibility of the structural −OH groups of the clay layer has a significant effect on the details of the computed structure and dynamics of H2O and CO2 molecules but does not affect the overall trends with changing basal spacing or the principal structural and dynamical conclusions. Temperature and pressure in the ranges examined have little effect on the principal structural and energetic conclusions, but the rates of dynamical processes increase with increasing temperature, as expected

The incorporation of Ca 2+ into smectite minerals is well known to have a significant effect on the swelling behavior and mechanical properties of this environmentally and technologically important group of materials. Relative to common alkali cations such as Na + , K + , and Cs + , Ca 2+ has a larger charge/ionic radius ratio, and thus interacts very differently with interlayer water molecules and the oxygens of the clay basal surface. Recent 2 H and 43 Ca NMR studies of the smectite mineral, hectorite, show that the molecular scale interlayer dynamics is quite different with Ca 2+ than with alkali cations. Classical molecular dynamics (MD) simulations presented here use a newly developed hectorite model with a disordered distribution of Li + /Mg 2+ substitutions in the octahedral sheet and provide new insight into the origin of the effects of Ca 2+ on the structure, dynamics and energetics of smectite interlayers. The computed basal spacings and thermodynamic properties suggest the potential for formation of stable monolayer hydrates that have partial and complete water contents, a bilayer hydrate, and possible expansion to higher hydration states. The system hydration energies are comparable to those previously calculated for Ca-montmorillonite 21,53 and are more negative than for Cs-and Nahectorite, due to the higher hydration energy of Ca 2+. The coordination environments of Ca 2+ change significantly with increasing interlayer hydration, with the extent of coordination to basal oxygens decreasing as the number of interlayer molecules increases. On external (001) surfaces, the H2O molecules closest to the surface are adsorbed at the centers of ditrigonal cavities and bridge Ca 2+ to the surface. The Ca 2+ ions on the external surface are all in outer sphere coordination with the basal oxygens of the surface, and the proximity-restricted region with a significant number of Ca 2+ is approximately 6 Å thick. Quantification of these interactions provides a basis for understanding intercalation of Ca 2+ by organic species and smectite minerals.

The results from novel in situ high-pressure nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and X-ray diffraction (XRD) investigation of the interaction of the smectite hectorite with variably wet supercritical methane (scCH4) at 90 bar and 323 K (hydrostatic conditions equivalent to ∼1 km depth) show that CH4 occurs in the clay interlayers, in pores external to the individual clay particles, and as bulk fluid. The occupancy of each environment depends on the relative humidity (RH) of the CH4-rich fluid and the hydration energy and size of the charge-balancing cation. As RH increases, the fraction of interlayer and interparticle CH4 decreases, although with Cs+, addition of a small amount of H2O initially increases CH4 uptake. Maximum interlayer CH4 adsorption occurs when the mean basal spacing just permits methane intercalation (∼11.5 Å) and never below this basal spacing. It is also higher with divalent cations than with monovalent cations. The data show that CH4 adsorption occurs predominantly via a weak dispersion interaction with the clay and that its intercalation occurs via a passive space-filling hydrophobic mechanism. The results suggest that, under reservoir conditions, smectite interlayers may provide a reservoir for CH4 under low-water conditions.

Variable-temperature X-ray diffraction and 2 H NMR spectroscopy of the smectite mineral, hectorite, containing interlayer Na + , K + , Cs + , Mg 2+ , Ca 2+ , Sr 2+ , and Pb 2+ equilibrated at 43% relative humidity (RH) and mixed with 2 H 2 O to form a paste provide a comprehensive picture of the structural environments and dynamics of interlayer 2 H 2 O and the relationships of these properties to interlayer hydration state, the hydration energy and polarizability of the cation, temperature, and the formation of ice-1h in the interparticle pores. The variation in basal spacing shown by the XRD data correlates well with the 2 H NMR behavior, and the XRD data show for the first time in hectorites that crystallization of interparticle ice-1h causes a decrease in the interlayer spacing, likely due to removal of interlayer 2 H 2 O. The variation of the 2 H NMR behavior of all the samples with decreasing temperature reflects decreasing frequencies of motion for the rotation of the 2 H 2 O molecules around their dipoles, reorientation of the 2 H 2 O molecules, and exchange of the 2 H 2 O molecules between interlayer sites coordinated to and not coordinated to the cations.

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 (GCMD) 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 Ca 2+ ). With Cs + , CO2 intercalation occurs in a 1-layer structure and does not require H2O intercalation, whereas with Ca 2+ the presence of a sub-monolayer of H2O is required for CO2 intercalation. The computational results provide detailed structural, dynamical and energetic insight into the differences in intercalation behavior and are in excellent agreement with in situ experimental XRD, IR, quartz crystal microbalance, and NMR results for smectite materials obtained under reservoir conditions.