Water and ions in clays: Unraveling the interlayer/micropore exchange using molecular dynamics

We present the first microscopic description of the exchange of water and ions between clay interlayers and microporosity. A force field based on ab-initio calculations is developed and used in classical molecular dynamics simulations. The latter allow to compute the potential of mean force for the interlayer/micropore exchange for water, Na+ and Cs+ cations and Cl- anions. For the simulated water content (water bilayer, with interlayer spacing 15.4 A) and salt concentration in the micropore (0.52 mol dm(-3)) the exchange is found to be almost not activated for water and cations, whereas the entrance of an anion into the interlayer is strongly unfavorable (Delta F similar to 9k(B)T). Calculations of the diffusion tensor in the interlayer and in the micropore complete the study of the exchange dynamics. (c) 2007 Elsevier Ltd. All rights reserved.