We report a Monte Carlo and molecular dynamics simulations study of carbon dioxide in hydrated sodium montmorillonite, including thermodynamical, structural and dynamical properties. In order to simulate the behaviour of a clay caprock in contact with a CO 2 reservoir, we consider clays in equilibrium with H 2 O−CO 2 mixtures under conditions close to relevant ones for geological storage, namely a temperature T =348 K, and pressures P=25 and 125 bar, and under which two bulk phases coexist: H 2 O-rich liquid on the one hand and CO 2-rich gas (P=25 bar) or supercritical fluid (P=125 bar) on the other hand. We first use grand-canonical MC simulations to determine the number of stable states in clay, their composition and the corresponding equilibrium interlayer distances. The vertical, horizontal and radial distribution functions of the confined mixture, subsequently obtained using molecular dynamics, reveal some structural feature induced by the presence of CO 2. Finally, the simulations indicate that carbon dioxide considerably influences the diffusion of mobile species in clays. We discuss these results by comparing them with those obtained for the bulk mixtures, as well as for Namontmorillonite in equilibrium with a pure water reservoir water at the same temperature and pressure.

We report a molecular dynamics study of the structure and dynamics of water at a clay surface. The negative charge of the surface and the presence of surface oxygen atoms perturbs water over two to three molecular layers, while the nature of the counterions (Na(+)or Cs(+)) has only a small effect. In the first molecular layer, approximately half of the water molecules are H-bonded to the surface. We also analyze the H-bond network between surface water molecules. The diffusion of water molecules along the surface is slowed down compared to the bulk case. As far as the orientational order and dynamics of the water dipole are concerned, only the component normal to the clay surface is perturbed. We investigate the surface H-bond formation and dissociation dynamics and their coupling to the release of molecules from the first molecular layer. We introduce a simple kinetic model in the spirit of Luzar and Chandler [Nature, 1996, 379, 55] to allow for a comparison with bulk water dynamics. This model semi-quantitatively reproduces the molecular simulation results and suggests that H-bond formation is faster with the surface than in the bulk, while H-bond dissociation is slower.

Models of swelling clays are studied by computer simulations ͑Monte Carlo and molecular dynamics͒. We focus on the comparison of structural and dynamic properties of two montmorillonites with different kinds of counterions Na ϩ and Cs ϩ . The calculated values are compared with available experimental quantities such as interlayer spacing as a function of water content and diffusion coefficients of both water molecules and counterions in the monohydrated state. The results are consistent with experimental values and previous simulations. For the dynamics, the short time behavior of water as observed with quasielastic neutron scattering is in agreement with simulated one. For the ions, the experimental values are related to macroscopic long time motions and are much smaller than the short time values calculated from MD. Thus, the present study provides a detailed insight in the microscopic dynamics of ions related to the structure of the clay: it is shown that Cs ϩ diffuse faster than Na ϩ and that the arrangement of clay surfaces plays a significant role in the choice of the sites occupied by the cations as well as in their mobility.

Within the wider context of water behavior in soils, and with a particular emphasis on clays surrounding underground radioactive waste packages, we present here the translational dynamics of water in clays in low hydrated states as studied by coupling molecular dynamics (MD) simulations and quasielastic neutron scattering experiments by neutron spin echo (NSE). A natural montmorillonite clay of interest is modeled by a synthetic clay which allows us to understand the determining parameters from MD simulations by comparison with the experimental values. We focus on temperatures between 300 and 350 K, i.e., the range relevant to the highlighted application. The activation energy Ea experimentally determined is 6.6 kJ/mol higher than that for bulk water. Simulations are in good agreement with experiments for the relevant set of conditions, and they give more insight into the origin of the observed dynamics.

To simulate the diffusion of a tracer Cs + in very compacted bentonites, the interlayer spaces of two bihydrated Na-montmorillonites containing a small quantity of cesium are studied by computer simulations (Monte Carlo and molecular dynamics). The calculated structural properties show that water and sodium cations behave in the same way as in pure homoionic Na-montmorillonites. The study of the dynamical properties shows that simulated self-diffusion coefficients of water are in agreement with short time coefficients measured by neutron scattering. For the ions, the experimental values are related to macroscopic long-time motion and are not directly comparable to simulations. However, the introduction of simulated values in macroscopic models gives results in agreement with tracer experiments.

Diffusion of water in montmorillonite clays at low hydration has been studied on the microscopic scale by two quasi-elastic neutron scattering techniques, neutron spin-echo (NSE) and time-of-flight (TOF), and by classical microscopic simulation. Experiment and simulation are compared both directly on the level of intermediate scattering functions, I(Q, t), and indirectly on the level of relaxation times after a model of atomic motion is applied. Regarding the dynamics of water in Na- and Cs-monohydrated montmorillonite samples, the simulation and NSE results show a very good agreement, both indicating diffusion coefficients of the order of (1-3) x 10(-10) m(2) s(-1). The TOF technique significantly underestimates water relaxation times (therefore overestimates water dynamics), by a factor of up to 3 and 7 in the two systems, respectively, primarily due to insufficiently long correlation times being probed. In the case of the Na-bihydrated system, the TOF results are in closer agreement with the other two techniques (the techniques differ by a factor of 2-3 at most), giving diffusion coefficients of (5-10) x 10(-10) m(2) s(-1). Attention has been also paid to the elastic incoherent structure factor, EISF(Q). Simulation has played a key role in understanding the various contributions to EISF(Q) in clay systems and in clearly distinguishing the signatures of "apparent" and true confinement. Indirectly, simulation highlights the difficulty in interpreting the EISF(Q) signal from powder clay samples used in experiments.

We report a molecular simulation study of hydrodynamics in clay nanopores, with pore widths ranging from 3 to 10 nm. Understanding mass transfer through clay nanopores is necessary in many contexts such as groundwater hydrology, petroleum and gas reservoir engineering, as well as carbon dioxide sequestration or geological disposal of radioactive waste.Grand-canonical Monte-Carlo simulations first allow to determine the water content in the pores. We then analyze the structure and diffusion of confined water using equilibrium Molecular Dynamics. Finally, Non-Equilibrium MD allow to analyze the hydrodynamic behaviour of the confined fluid and assess the relevance of continuum hydrodynamics to describe the flow under a pressure gradient. The Navier-Stokes equation, using the density and viscosity of the bulk fluid, provides a reasonable description of the flow provided that the pore width is larger than 4 nm and that a slip boundary condition is used. We determine a slip length of 2.

We report the diffusion coefficient and viscosity of popular rigid water models: Two nonpolarizable ones (SPC/E with 3 sites, and TIP4P/2005 with 4 sites) and a polarizable one (DangChang, 4 sites). We exploit the dependence of the diffusion coefficient on the system size [Yeh and Hummer, J. Phys. Chem. B 108, 15873 (2004)] to obtain the size-independent value. This also provides an estimate of the viscosity of all water models, which we compare to the Green-Kubo result. In all cases, a good agreement is found. The TIP4P/2005 model is in better agreement with the experimental data for both diffusion and viscosity. The SPC/E and Dang-Chang water overestimate the diffusion coefficient and underestimate the viscosity.