Microscopic Simulations of Interlayer Structure and Dynamics in Bihydrated Heteroionic Montmorillonites

Abstract

To simulate the diffusion of a tracer Cs + in very compacted bentonites, the interlayer spaces of two bihydrated Na-montmorillonites containing a small quantity of cesium are studied by computer simulations (Monte Carlo and molecular dynamics). The calculated structural properties show that water and sodium cations behave in the same way as in pure homoionic Na-montmorillonites. The study of the dynamical properties shows that simulated self-diffusion coefficients of water are in agreement with short time coefficients measured by neutron scattering. For the ions, the experimental values are related to macroscopic long-time motion and are not directly comparable to simulations. However, the introduction of simulated values in macroscopic models gives results in agreement with tracer experiments.

Connecting the Molecular Scale to the Continuum Scale for Diffusion Processes in Smectite-Rich Porous Media

“…In the present study, we carried out new MD simulations of Na-montmorillonite interlayers in stacked lamellae (some of which were doped with trace amounts of Sr 2+ or Cs + ) using (1) water and solute interatomic potentials known to predict diffusion in bulk liquid water accurately (25)(26)(27), (2) the most recent ab initio estimates of clay atom partial charges (28), and (3) MD simulation times 10 to 50 times longer than in most previous studies (22,23,29,30 Our primary interest being diffusion, we used the extended simple point charge (SPC/E) model (40) along with water-solute inter-atomic potentials (25,41,42) known to describe diffusion in bulk liquid water accurately (25)(26)(27). For montmorillonite, we used the estimates of clay atom partial charges and short-range interaction parameters developed by Cygan et al (28).…”

Investigation of Reactive Transport and Coupled THMC Processes in the EBS

“…Table 3.3 summarizes the D values of Na + and water for each hydration state and temperature modeled with fully flexible clay layers (along with the ratio D 0,exp /D 0,s for each species at each temperature). These pore scale diffusion coefficients are not directly equivalent to the "apparent" diffusion coefficient obtained from macroscopic scale diffusion experiments (Marry and Turq, 2003;Kosakowski et al, 2008;Bourg and Sposito, 2010). This is because our MD simulations inherently cannot describe pore-networkscale geometric effects such as tortuosity and pore connectivity.…”

“…Molecular dynamics (MD) simulations are routinely used to gain insight into the molecular scale basis of water and solute diffusion in nanoporous materials, including clay interlayers (Sposito et al, 1999;Marry and Turq, 2003;Malikova et al, 2004aMalikova et al, ,b, 2005Rotenberg et al, 2007a,b;Kosakowski et al, 2008;Marry et al, 2008Marry et al, , 2011Mazo et al, 2008;Tournassat et al, 2009;Sposito, 2010, 2011a,b;Botan et al, 2011;Pitman and van Duin, 2012 ) adsorb on smectite surfaces primarily as outer-sphere surface complexes (Sposito et al, 1999;Bourg and Sposito, 2011a); that ion adsorption on smectite basal surfaces is roughly consistent with the well known triple layer model Bourg and Sposito, 2011b); and that the primary diffusion pathway of cations in compacted bentonite is through smectite interlayer nanopores (Rotenberg et al, 2007a;Bourg and Sposito, 2010). In principle, this type of simulation can readily predict the T-dependence of water and solute diffusion in clay interlayer nanopores.…”

“…For example, previous MD simulation studies of smectite interlayer nanopores used either 4 (Kosakowski et al, 2008), 4.5 (Marry and Turq, 2003;Malikova et al, 2005), or 5 (Bourg and Sposito, 2010) water molecules per unit cell per hydration layer. Our tests with 4, 5, or 6 water molecules per unit cell per hydrate layer (not shown) indicated that the D values of interlayer water and Na + are highly sensitive to the water content of the interlayer: an increase in water content from 4 to 5 or from 5 to 6 water molecules per unit cell and hydration layer caused the D values of interlayer water and Na + to increase by on average 15-20%, decreasing with increasing temperature.…”

“…This is because our MD simulations inherently cannot describe pore-networkscale geometric effects such as tortuosity and pore connectivity. Nevertheless, macroscopic scale diffusion coefficients can in principle be estimated from our predicted pore scale diffusion coefficients if the relevant microstructural parameters are known (Marry et al, 2003;Gonzalez-Sánchez et al, 2009;Bourg and Sposito, 2010;Churakov and Gimmi, 2011). Table 3.2.…”