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

Benjamin Rotenberg (1, 2) , Virginie Marry (1) , Rodolphe Vuilleumier (3) , Natalie Malikova (4) , Christian M. Simon (1) , Pierre Turq (1)

1 LI2C - Liquides Ioniques et Interfaces Chargées
2 ANDRA - Agence Nationale pour la Gestion des Déchets Radioactifs
3 LPTMC - Laboratoire de Physique Théorique de la Matière Condensée
4 LLB - UMR 12 - Laboratoire Léon Brillouin

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 (∆F ∼ 9k B T). Calculations of the diffusion tensor in the interlayer and in the micropore complete the study of the exchange dynamics.

Mots clés: clay; ionic exchange; anion exclusion; edge; kinetics; molecular dynamics; potential of mean force; diffusion tensor

Domaines: Chimie-Physique [physics.chem-ph]

Benjamin Rotenberg, Virginie Marry, Rodolphe Vuilleumier, Natalie Malikova, Christian M. Simon, et al.. Water and ions in clays : Unraveling the interlayer/micropore exchange using molecular dynamics. Geochimica et Cosmochimica Acta, 2007, 71, pp.5089-5101. ⟨10.1016/j.gca.2007.08.018⟩. ⟨hal-00369627⟩