Three-layer Hydrates Research Articles

Molecular dynamics simulation of NH4-montmorillonite interlayer hydration: Structure, energetics, and dynamics

The interaction of ammonium ions (NH4+) with clay is closely associated with many geochemistry, engineering, and environment issues. Molecular dynamics (MD) simulations were performed to investigate to the structure, energetics, and dynamics properties of ammonium-montmorillonite (NH4-Mt) at different hydration levels. The simulation results show that the layer spacing, immersion and hydration energy, atomic density, coordination number, self-diffusion coefficient and residence time of NH4-Mt vary with the water contents. The one-layer hydrate is the most energetically favorable at the water content of 5 H2O/uc but up to three-layer hydrate of NH4-Mt can potentially be formed at the high water content (13 H2O/uc). As the water content increases, some NH4+ ions which adsorbed in inner-sphere (IS) complexes at low hydration level can change to outer-sphere (OS) complexes, and further to random ions in a diffuse aqueous layer, with the interaction between NH4+ ions and basal surfaces becoming weaker. The H-bonding of N-Hn···Os, donated by hydrogen atoms of NH4+ to surface oxygen atoms, contributes to the adsorption of NH4+ ions besides the electrostatic attraction. NH4+ ions in inner-sphere complexes can reside above the hexagonal cavities longer and have more stable hydration shell at the low hydration level.

Study on the differences of Na- and Ca-montmorillonites in crystalline swelling regime through molecular dynamics simulation

The swelling of Na- and Ca-montmorillonites (MMTs) has been studied through energy optimization and molecular dynamics simulations with the Universal Force Field (UFF) in this work. It was performed to reveal the interlayer spacing and water density in the interlayer as the function of water molecules increasing from 0 to 96, and the structural and diffusive properties of interlayer species when formed one-, two-, three-layer hydrates in interlayer. The results presented the swelling pattern of the MMTs, and the process of that water molecules formed layers in MMT interlayers. It has been found that Ca-MMT had a stronger hydration swelling than Na-MMT in the crystalline swelling regime, which might be due to that Ca2+ had a larger hydrated radius than Na+. Water molecules in the interlayers of Na-MMT had a larger self-diffusion coefficient than those of Ca-MMT at the same water content. Moreover, Na+ had a higher self-diffusion coefficient than Ca2+. It was demonstrated that the difference at the swelling of the two MMTs was attributed to that Ca2+ had a stronger hydration than Na+ in the MMT interlayers.

Molecular dynamics simulations of interlayer structure and mobility in hydrated Li-, Na- and K-montmorillonite clays

Molecular dynamics (MD) simulations were carried out to study the hydrate structure and diffusion of water and cations in the clay interlayer region. The goal of this work was to examine the mechanisms by which the clay counterions influence the swelling and dispersion of shale. Three series of MD simulations were performed to study Wyoming montmorillonites containing either Na+, K+ or Li+ counterions and 100, 200 or 300 mg of adsorbed water per gram of clay. Here, the TIP4P interaction model for liquid water is used in the MD simulations. The TIP4P results are compared with simulations using the MCY interaction model, particularly for the K-clay hydrates. For the one- and two-layer K-hydrates, the counterion speciation and water structure observed are in agreement with the MCY model. However, we observe that the two-layer hydrate persists up to high water contents and that – contrary to the MCY model – the hypothetical three-layer hydrate is unstable, in agreement with experimental observations. If one notes that the one- and two-layer hydrates are only saturated at 56 and 96 water molecules, respectively, then the observed equilibrium layer spacings are in excellent agreement with experiment. Integration of the ion density profiles shows that the surface complexes represent 4.4 out of 6 ions for the K-hydrate and only 2 out of 6 ions for Na-hydrate. This means that K+ ions adsorb more strongly and screen the negatively charged surface more effectively than Na+ ions. Self-diffusion coefficients were calculated from mean-square displacement analysis of ions and water molecules. For the Na+ and Li+ cases, satisfactory agreement with experimental quasi-elastic neutron scattering diffusion coefficients was found. No experimental values are available for the K+ case, and therefore predictions are made in this report. Diffusion coefficients for water in K-smectite increase from 10−10 to 10−9 m2 s−1 with water contents increasing from 100 to 300 mg g−1 of clay. We find that water diffusion rates in K-smectites are roughly twice as small as in Na- and Li-clays with the same water content. It appears that the microscopic diffusion coefficients calculated for interlayer diffusion of Na+ and K+ ions are two orders of magnitude smaller than the macroscopic values calculated from experimental work on hindered motion of water and ions through a clay membrane. This implies that the experimental clay film has a tortuous structure, with clay platelets aligned orthogonally to the main direction of mass transport, and a tortuosity factor of . The results show that the MD simulation technique is a valuable tool to study transport properties of water and ions in hydrated smectite clays.

Freezing and thawing of montmorillonite — A time-resolved synchrotron X-ray diffraction study

The evolution of phases over time during freezing and thawing of unconfined Na- and Ca-montmorillonites (Wyoming, MX-80) was studied with time-resolved synchrotron X-ray diffraction. The clay samples were: (i) powder equilibrated to ambient atmosphere and (ii) pastes of 30 mass% montmorillonite in pure water. The phases were characterised in-situ using a stream of nitrogen gas for temperature control. The behaviour of montmorillonite during freezing and thawing is important in final repositories for spent nuclear fuel that are using bentonite as a buffer material. The Na-montmorillonite equilibrated to ambient atmosphere (one-layer hydrate) was unaffected by freezing down to − 50 °C. The Ca-montmorillonite equilibrated to ambient atmosphere (two-layer hydrate) showed a minor decrease in basal spacing (0.11 Å) by freezing down to − 50 °C. The magnitude of the decrease in basal spacing was high compared to the thermal contraction of the similar minerals muscovite and pyrophyllite and some dehydration of the clay was likely to be involved. Wet Na-montmorillonite in pure water was highly affected by freezing causing the osmotic phase to collapse during ice formation to 19 Å (three-layer hydrate) and later to a mixture of two and three-layer hydrates (− 15 °C) and at lower temperatures to two-layer hydrate (16 Å, − 50 °C). The Ca-montmorillonite in pure water was present as a 19 Å three-layer hydrate at + 20 °C and expanded upon cooling, producing two partly overlapping 001 reflections corresponding to three and four-layer hydrates prior to the ice formation. A mean d-value of the 002 peaks of the four-layer hydrate was determined to be 10.8 Å, which corresponded to a basal spacing of 21.6 Å. To our knowledge this is the first time a distinct four-layer-water hydrate is reported for Ca-montmorillonite in pure water. After the ice formation started, the montmorillonite was dehydrated to three-layer hydrate and at − 15 °C to a mixture of two and three-layer hydrates. At − 50 °C only two-layer hydrate was present. The ice formation and the dehydration of the montmorillonite occurred simultaneously. The effects of freezing on the montmorillonite were shown to be reversible during the thawing. The two dimensional diffraction rings gave information on the ice texture. The highly dispersed Na-montmorillonite (high surface area) in pure water facilitated the nucleation of the ice crystals and gave rise to uniformly sized crystals, while the Ca-montmorillonite (not dispersed, lower surface area) gave rise to non-uniformly sized ice crystals.

Hydration numbers of sea water basic ions found by the pulse method of proton NMR

Hydration numbers of basic ions in sea water (SW) are found from a proton relaxation rate of 1/T 1 in SW and model solutions. A possibility for the formation of three-layer hydrate shells around main SW cations and two-layer shells around anions is ascertained. About half of water molecules is in influence zones of ions in sea water that correlates with a double shift in the baric dependence maximum of T 1 to a smaller pressure in SW relative to pure water.

THE STATE OF HYDRATION OF Ca-EXCHANGED MONTMORILLONITE AT A DEPTH OF 2 2.7 KILOMETERS

Monte Carlo simulations in NP zz T and μVT ensembles of the hydration of Ca-saturated Wyoming-type montmorillonite show that one-layer Ca-exchanged montmorillonite hydrate with a d 001 of 12.11 A is stable at 353 K, 300 bar, and −7.21 kcal/mol potential. Two- and three-layer hydrates do not appear stable. At 353 K and 625 bar, the one-layer hydrate is nearly stable. In the interlayer space, molecules of H 2 O are clustered on the interlayer midplane, alternatively oriented with their protons toward the siloxane surfaces on both sides and on the midplane. The Ca 2+ cations are solvated, in outer-sphere coordination, and located 2.77 A from the H 2 O molecules. In sedimentary basins under normal geotherms, one-layer Ca-exchanged montmorillonite is the single hydrate stable at 2 km depth. In overcompacted sediments at 2.7 km depth, the hydrate could be unstable.

In situ neutron diffraction analysis of the influence of geometric confinement on crystalline swelling of montmorillonite

The swelling properties of a bentonite MX-80 and a Na montmorillonite obtained from MX-80 purification were analyzed as a function of relative humidity in free and constrained conditions. Gravimetric and XRD techniques were used to study the hydration of powder and compacted pellets in non confined conditions whereas hydration under constrained conditions was investigated in situ by neutron diffraction using a cell specifically designed for the present study. MX-80 bentonite and purified montmorillonite were compacted to an apparent density of 1.7 and equilibrated at various relative humidities using P2O5 and solutions saturated with different salts allowing to work in a wide range of relative humidity between 0 and 0.98.At low relative humidity, the hydration of montmorillonite is similar in free and constrained conditions. For relative pressures higher than 50%, swelling in constrained pellets deviates from that observed for free pellets. Reorientation phenomena of clay layers were also observed for bentonite and montmorillonite starting at 65% RH. At 98% RH, two well-defined basal spacings were obtained in the case of confined bentonite. The major peak corresponds to three-layer hydrates (18.6 Å) and the second one to two-layer hydrates (16 Å). In contrast, only the latter peak is observed when hydration is realized in free geometrical conditions. In the case of Na montmorillonite, for the same relative humidity, the confinement cell did not resist swelling pressure. This was explained by the fact that, for the 1.7 density, porosity was too small to allow the formation of the three layer hydrates to compensate for water swelling pressure.

Stability of Na-, K-, and Ca-Montmorillonite at High Temperatures and Pressures: A Monte Carlo Simulation

Monte Carlo grand canonical molecular simulations on the hydration of Na-, K-, and Ca-montmorillonite show that between 333 and 533 K and 300-1300 bar Na-montmorillonite forms stable one-layer hydrates of d(001) spacings 12.64-12.38 Angstroms, K-montmorillonite of 12.78-12.59 Angstroms, and Ca-montmorillonite of 12.48-12.32 Angstroms. A two-layer hydrate of 14.80 Angstroms occurs for Na-montmorillonite at 533 K and 1300 bar, for K-montmorillonite of 15.32 Angstroms at 533 K and 1300 bar and of 14.74 Angstroms at 533 K and 2000 bar, and for Ca-montmorillonite of 13.83 Angstroms at 473 K and 1000 bar. Three-layer hydrates may possibly form within these same ranges. Outside of them, one-layer hydrates simulate as the only stable hydrates. In sedimentary basins, the two-layer hydrate of Ca-montmorillonite will locate at 6.7 km depth and those of Na- and K-montmorillonite at 8.7 km depth; above and below these depths, the one-layer hydrates are the stable phases.

Influence of Smectite Hydration and Swelling on Atrazine Sorption Behavior

Smectites, clay minerals commonly found in soils and sediments, vary widely in their ability to adsorb organic chemicals. Recent research has demonstrated the importance of surface charge density and properties of exchangeable cations in controlling the affinity of smectites for organic molecules. In this study, we induced hysteresis in the crystalline swelling of smectites to test the hypothesis that the extent of crystalline swelling (or interlayer hydration status) has a large influence on the ability of smectites to adsorb atrazine from aqueous systems. Air-dried K-saturated Panther Creek (PC) smectite swelled less (d(001) = 1.38 nm) than never-dried K-PC (d(001) = 1.7 nm) when rehydrated in 20 mM KCl. Correspondingly, the air-dried-rehydrated K-PC had an order of magnitude greater affinity for atrazine relative to the never-dried K-PC. Both air-dried-rehydrated and never-dried Ca-PC expanded to approximately 2.0 nm in 10 mM CaCl2 and both samples had similar affinities for atrazine that were slightly lower than that of never-dried K-PC. The importance of interlayer hydration status in controlling sorption affinity was confirmed by molecular modeling, which revealed much greater interaction between interlayer water molecules and atrazine in a three-layer hydrate relative to a one-layer hydrate. The entropy change on moving atrazine from a fully hydrated state in the bulk solution to a partially hydrated state in the smectite interlayers is believed to be a major factor influencing sorption affinity. In an application test, choice of background solution (20 mM KCl versus 10 mM CaCl2) and air-drying treatments significantly affected atrazine sorption affinities for three-smectitic soils; however, the trends were not consistent with those observed for the reference smectite. Further, extending the initial rehydration time from 24 to 240 h (prior to adding atrazine) significantly decreased the soil's sorption affinity for atrazine. We conclude that interlayer hydration status has a large influence on the affinity of smectites for atrazine and that air-drying treatments have the potential to modify the sorption affinity of smectitic soils for organic molecules such as atrazine.

Monte Carlo Molecular Simulation of the Hydration of K-Montmorillonite at 353 K and 625 bar

Monte Carlo molecular simulations of the hydration of K-saturated Wyoming-type montmorillonite at constant stress in the NPzzT ensemble and at constant chemical potential in the grand canonical muVT ensemble, under basin-like conditions of 353 K and 625 bar, show a strong tendency of the K+ ions to adhere to the siloxane surface, forming predominant inner-sphere complexes with tetrahedral oxygen atoms and adsorbed water molecules. Simulations in the grand canonical ensemble predict that none of the K-montmorillonite hydrates, the one-, two-, and three-layer hydrates, are stable in this environment of high depth, temperature, and pressure. The most nearly stable configuration corresponds to the one-layer hydrate, characterized by a d001 spacing of 12.75 A, the adsorbed water being 60 molecules/layer or 180.83 mg of H2O/g of clay, an internal energy of -22.73 kcal/mol, an interlayer density of 0.365 g/mL, and a pressure tensor, Pzz, of 1999.9 bar. The interlayer structure consists of two close layers of water molecules 0.50 A from the midplane, with broad shoulders on the sides, the protons oriented toward the midplane and the siloxane surfaces, and the K+ ions close to the clay surfaces and on the interlayer midplane.

Molecular Simulations of Swelling Clay Minerals

We have carried out molecular simulations in the grand-canonical ensemble of water and cations in Wyoming and Arizona montmorillonite clay minerals, with varying relative humidity. Several water models and cations are used to investigate the swelling of these clays. We show how the water content depends on the type of clay, type of cation, the basal spacing, and the relative humidity. Related to the layering of water molecules in the interlayer space, the pressure normal to the clay sheets oscillates as a function of the basal spacing. Minima in corresponding free energy curves indicate the presence of dehydrated states and layered hydrates. The development of these stable states and the corresponding basal spacings are in agreement with experimental data. Density profiles show significantly different interlayer structures depending on the type of clay and models used. We show a relation between formation of two-layer hydrates and the position of the cations. The simulations with the MCY water model underestimate the spacings of two- and three-layer hydrates, whereas our simulations with the TIP4P model produce a better agreement. Therefore, we recommend the TIP4P model for simulating clay minerals. In addition, we report remarkable ordering of cations and water molecules in a one-layer Arizona montmorillonite hydrate.

Do Montmorillonite Surfaces Promote Methane Hydrate Formation? Monte Carlo and Molecular Dynamics Simulations

Recent laboratory experiments on methane hydrate formation in the presence of Na-montmorillonite, a clay mineral found coexistently with hydrates in oceanic sediments, suggest that clay surfaces may facilitate methane hydrate crystallization from aqueous solution. Monte Carlo and molecular dynamics simulations were carried out to determine whether stable methane clathrates could in principle form in the interlayers of hydrated Na-montmorillonite in equilibrium with seawater under ambient conditions. Stable interlayer methane clathrate structures were indeed found to occur under pressures as low as 10 atm and at temperatures as high as 300 K in simulations of a three-layer hydrate of Na-montmorillonite containing 0.5 CH4 per clay mineral unit cell. This result is consistent with a “thermodynamic promotion effect” of clay mineral surfaces on hydrate formation. Visualization of local methane clathrate structure in the 0.5 CH4 per unit cell system revealed that the methane molecule is nested on a hexagonal ring of clay surface oxygens while surrounded by a clathrate-like water structure. The calculated CH4−O coordination number was 20−22, in agreement with previous simulations and with neutron diffraction data on methane hydrate formation in bulk solution. Our MD simulations indicated that the power spectrum of the interlayer hydrate was essentially the same as that for the hydrate in bulk water. Higher methane loading than 0.5 CH4 per unit cell was found to destabilize the hydrate structure similarly to a temperature increase. On the basis of these simulations, methane hydrate is proposed to occur in natural sediments with a portion of a clay mineral surface actively involved in promoting clathrate formation.

Simulations of Clay Mineral Swelling and Hydration: Dependence upon Interlayer Ion Size and Charge

The dependence of the crystalline swelling and hydration properties of clay minerals on interlayer ion size and charge was investigated using molecular dynamics computer simulations of Na-, Cs-, and Sr-substituted montmorillonites. For all clays studied, layer spacings measured as a function of water content exhibited plateaus at the one-layer hydrate spacing. Calculated immersion energies exhibited minima for integer-layer hydrates up through the three-layer hydrate, with apparent global minima identified with the one-layer hydrate for Cs−montmorillonite and with the two-layer hydrates for the Na− and Sr−montmorillonites. In addition, for Sr−montmorillonite, layer spacings jumped discontinuously between one-layer and two-layer separations and showed a second plateau at the two-layer hydrate spacing. The immersion energy curve for Sr−montmorillonite showed similar discontinuities. These results provide evidence for a constant water content swelling transition between one-layer and two-layer spacings. This...

Monte Carlo and Molecular Dynamics Simulations of Electrical Double-Layer Structure in Potassium−Montmorillonite Hydrates

Monte Carlo and molecular dynamics simulations of interlayer molecular structure in the one-, two-, and (hypothetical) three-layer hydrates of K-montmorillonite were performed concurrently in order to elucidate counterion speciation and water structure in the electrical double layer of this clay mineral. Calculated layer spacings, interlayer water potential energies, and counterion mobilities were in agreement with available experimental data. In the one-layer hydrate, both outer-sphere and inner-sphere surface complexes of K+ were observed, the latter always near sites of tetrahedral charge substitution, with the counterion species exchanging readily on the simulation time scale (up to 200 ps). In the two- and three-layer hydrates, the surface complexes persisted, but an incipient diffuse layer of counterions also was observed, with all three types of surface species engaging in sluggish exchange. Water molecules in the one-layer hydrate resided at the interlayer midplane, whereas in the two-layer hydrat...

Monte Carlo and Molecular Dynamics Simulations of Interfacial Structure in Lithium-Montmorillonite Hydrates

Molecular structure in the interlayers of lithium-Wyoming montmorillonite with one, two, or three adsorbed water layers was investigated for the first time by concurrent Monte Carlo and molecular dynamics simulation, based on the MCY model of water−water interactions. Calculated layer spacings, as well as interlayer-species self-diffusion coefficients, were in good agreement with available experimental data. Inner-sphere surface complexes of Li+ with tetrahedral charge sites were observed in all hydrates, whereas outer-sphere surface complexes of Li+ with octahedral charge sites, found in the one-layer hydrate, dissociated from the clay mineral basal planes into a diffuse layer in the two- and three-layer hydrates, a signature of the strong interaction between Li+ and water molecules. Interlayer water molecules tended to solvate Li+, although some were entrapped within cavities in the montmorillonite surface. All of the adsorbed Li+ and interlayer water species exchanged on the time scale (0.2 ns) of the ...

Computer Simulation of Interlayer Molecular Structure in Sodium Montmorillonite Hydrates

Molecular structure in the interlayers of sodium Wyoming montmorillonite with one, two, or three water monolayers was investigated by Monte Carlo and molecular dynamics simulation, based on tested water-water, water-counterion, clay-water, and clay-counterion potential unctions. Calculated layer spacings and thermodynamic properties, as well as interlayer water configurations and interlayer-species self-diffusion coefficients, were in good agreement with available experimental data. Inner-sphere surface complexes of Na+ with tetrahedral charge sites persisted in all hydrates, whereas the outer-sphere surface complexes with octahedral charge sites found in the one-layer hydrate dissociated partially into an incipient diffuse layer in the two- and three-layer hydrates. Even in the three-layer hydrate, interlayer water molecules did not replicate their structure or mobility in bulk aqueous solution. Improved diffraction and spectroscopic data on interlayer water are needed to guide future simulation studies.