Water Dynamics in Hectorite Clays: Infuence of Temperature Studied by Coupling Neutron Spin Echo and Molecular Dynamics

Abstract

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.

Molecular Dynamics Simulations of Anion Exclusion in Clay Interlayer Nanopores

“…Mg isomorphic substitution was assigned to the site of substitution and its six surrounding O atoms in accordance with the original CLAYFF model. The treatment of clay mineral negative structural charge used in the present study was derived from quantum mechanical calculations of the electrostatic potential near clay mineral surfaces (Cygan et al, 2004), which predicts interlayer water and ion distributions and dynamics in excellent agreement with experimental results (Bourg and Sposito, 2010;Ferrage et al, 2011;Marry et al, 2011). The present results do not allow an accurate quantification of the height of the energy barrier opposing Cl À entry into the interlayer space of the 2WL hydrate, but strongly suggest that this barrier is greater than that predicted by Hsiao and Hedstrom (2015).…”

Molecular dynamics simulations of the electrical double layer on smectite surfaces contacting concentrated mixed electrolyte (NaCl–CaCl2) solutions

“…In the present study, we used a set of ion-water interatomic potentials that correctly describe the coordination structures and diffusion coefficients of water and ions in ambient liquid water and the static dielectric constant of liquid water [85,[88][89][90]. Our clay-water and clay-ion interaction were successfully tested against experimental data on the structure and dynamics of water and ions in smectite interlayer nanopores [54,130]. The concurrence of our simulation results with experimental data, theoretical calculations, and previous MD simulation studies, described throughout this paper, suggests that our results are quite robust.…”

Investigation of Reactive Transport and Coupled THMC Processes in the EBS

“…Models that account for this "bimodal" distribution of pore widths (Bourg et al, 2006;Ichikawa et al, 2004;Tournassat and Appelo, 2011) provide useful insights into the diffusion of water and cations in the EBS. The power of these models arises from the fact that they can harness the extensive knowledge base that exists on the behavior of water and ions in the crystalline hydrate nanopores, from experimental (Sposito and Prost, 1982;González Sánchez et al, 2009;Ferrage et al, 2011;Marry et al, 2011) and molecular modeling studies (Sposito et al, 1999;Rotenberg et al, 2007a;Kosakowski et al, 2008;Bourg and Sposito, 2010;Churakov and Gimmi, 2011).…”

“…In the case of water diffusion, E a values in water-saturated smectite have been studied by a number of research groups using both macroscopic scale diffusion experiments (Nakazawa et al, 1999;Nakashima, 2000;Suzuki et al, 2004;Gonzalez-Sánchez et al, 2009) and quasi-inelastic neutron scattering (QENS) experiments (Gonzalez-Sánchez et al, 2009;Marry et al, 2011). These studies yielded conflicting results, with two groups (Nakazawa et al, 1999;Nakashima, 2000) reporting E a values of 15 to 19 kJ mol -1 [near the value for bulk liquid water, 17.4 ± 0.2 kJ mol -1 in the temperature range 278-353K (Holz et al, 2000)] and three groups (Suzuki et al, 2004;Gonzalez-Sánchez et al, 2009;Marry et al, 2011) reporting E a values of 19 to 23 kJ mol -1 in overlapping ranges of dry bulk density.…”

“…These studies yielded conflicting results, with two groups (Nakazawa et al, 1999;Nakashima, 2000) reporting E a values of 15 to 19 kJ mol -1 [near the value for bulk liquid water, 17.4 ± 0.2 kJ mol -1 in the temperature range 278-353K (Holz et al, 2000)] and three groups (Suzuki et al, 2004;Gonzalez-Sánchez et al, 2009;Marry et al, 2011) reporting E a values of 19 to 23 kJ mol -1 in overlapping ranges of dry bulk density.…”