Molecular Dynamics Simulations of NaCl Permeation in Bihydrated Montmorillonite Interlayer Nanopores

Abstract

We investigated NaCl diffusion between external bulk solution and bihydrated montmorillonite interlayers using atomistic molecular dynamics (MD) simulations. Two different bulk concentrations, 0.55 and 1.67 M, were tested. In both cases, the simulations showed that excess salt entered the interlayers. Barriers for Cl − entering the interlayer were calculated from the potential of mean force using adaptive biasing forces, and full consistency with the MD results was found. In part the barrier stems from hydration free energy cost for the ions moving from bulk water to clay interlayers. This cost was calculated using free-energy perturbation. We compared the equilibrium interlayer anion concentrations to ion-equilibrium thermodynamics, the general Donnan theory that includes excess free energy contributions in addition to the electrostatic Donnan potential. Numerically consistent predictions of the interlayer Cl − concentration were obtained between MD and Donnan equation when the hydration energy cost was incorporated as the excess free energy. This demonstrates the validity of using the approximation of a Donnan potential for the interlayer electrostatics. The MD density profiles show that cations and anions in the clay interlayers share the same physical space in the center of the two water layers.

Hydrophobic Solvation of Gases (CO2, CH4, H2, Noble Gases) in Clay Interlayer Nanopores

“… a Water is conventionally treated as a reference nonadsorbed species. b Interlayer Cl – concentration was not determined as these ions are almost entirely excluded from the nanopores …”

“… b Interlayer Cl – concentration was not determined as these ions are almost entirely excluded from the nanopores …”

“…Water molecules were kept rigid using the SHAKE alogorithm . The clay structure was kept rigid and immobile, as in most studies of similar systems, ,, with the exception of clay H atoms. Electrostatic and dispersion interactions were computed in real space up to a distance of 15 Å; long-range electrostatic interactions were evaluated in reciprocal space using the particle–particle particle mesh (PPPM) method with an accuracy of 99.99%.…”

Molecular dynamics simulations of cesium adsorption on illite nanoparticles

“…Adsorption on clay edge surfaces remains almost entirely unexamined, in part because the CLAYFF model was not parameterized to simulate clay edge surfaces. The few existing MD simulations of clay edges focused on smectite [78,43,67,99] or pyrophyllite [59,66] rather than illite. Only three of these studies used fully flexible clay structures [59,66,67].…”

Swelling Pressure in Systems with Na-Montmorillonite and Neutral Surfaces: A Molecular Dynamics Study

“…46 The structural charge of the montmorillonite layer was −0.75e per O 20 (OH) 4 formula unit and evenly distributed over all octahedrally coordinated Al atoms as in our previous study. 47 The mineral layers were terminated with −OH and −OH 2 groups at the interface with the explicit bulk water solution. The LJ parameters for these groups were the same as for the corresponding elements in CLAYFF, while the partial charges were adjusted based on Mulliken populations from DFT calculations.…”