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.