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

Abstract

We report new molecular dynamics results elucidating the structure of the electrical double layer (EDL) on smectite surfaces contacting mixed NaCl-CaCl 2 electrolyte solutions in the range of concentrations relevant to pore waters in geologic repositories for CO 2 or high-level radioactive waste (0.34 to 1.83 mol c dm -3 ). Our results confirm the existence of three distinct ion adsorption planes (0-, β-, and d-planes), often assumed in EDL models, but with two important qualifications: (1) the location of the β-and dplanes are independent of ionic strength or ion type and (2) "indifferent electrolyte" ions can occupy all three planes. Charge inversion occurred in the diffuse ion swarm because of the affinity of the clay surface for CaCl + ion pairs. Therefore, at concentrations ≥ 0.34 mol c dm -3 , properties arising from long-range electrostatics at interfaces (electrophoresis, electro-osmosis, co-ion exclusion, colloidal aggregation) will not be correctly predicted by most EDL models. Co-ion exclusion, typically neglected by surface speciation models, balanced a large part of the clay mineral structural charge in the more concentrated solutions. Water molecules and ions diffused relatively rapidly even in the first statistical water monolayer, contradicting reports of rigid "ice-like" structures for water on clay mineral surfaces.

Progress toward bridging from atomistic to continuum modeling to predict nuclear waste glass dissolution.

“…If we identify the location of the silica surface with the Gibbs surface of water obtained from  Ow (z,r) in Figure 3-2, the surfaces of the SiO 2 slab are located at z = 64.3  0.1 and 125.7  0.1 Å (pore length 61.4 Å) and simulated pore radii are r = 4.39 and 9.37 Å. Plots of  Ow and  Hw vs. distance d (measured from Gibbs surface of water O atoms towards the aqueous phase) show density layering up to about three statistical water monolayers (~ 9 Å) from the silica surface ( Figure 3-3), consistent with the behavior of water on a range of solid surfaces (Toney et al, 1995;Schlegel et al, 2002;Bourg and Sposito, 2011). Average water O and H density distributions on the flat external surfaces and concave pore walls of our simulated silica structures (Figure 3-3a, b) are essentially identical beyond the first statistical water monolayer (d > ~ 3 Å).…”

Molecular Dynamic Simulations of Montmorillonite–Organic Interactions under Varying Salinity: An Insight into Enhanced Oil Recovery

“…The results agree with several recent findings, whereby a Stern layer of fixed cations charge balance the clay, and the simulations refute the double layer expansion hypothesis of lowsalinity EOR in this instance. 35,36 DLVO theory states that for a sufficiently highly charged clay-like surface, a Stern layer of immobile cations will be adsorbed to the clay, and act as a buffer to cancel the charge of the clay. The oscillatory nature of the electric field presented in Figure 5 can be described in this manner, whereby an immobile layer of cations charge balanced the clay, followed sequentially by a mobile layer of anions and cations.…”

Molecular Dynamics Simulations of Water Structure and Diffusion in Silica Nanopores

“…Studies of water in hydrophilic nanopores carried out with a range of techniques (thermoporometry 1,24 , capillary imbibition 25 , surface force apparatus 22,26 , infrared and Raman spectroscopy 4,17,18,21,27 , NMR spectroscopy 28 , X-ray and neutron diffraction 6,[29][30][31] , quasi-elastic neutron 4 scattering 2,32 , molecular dynamics simulations 26,[33][34][35][36] ) show that this failure occurs in pores narrower than ~20 nm and can be classified into two regimes. The first regime occurs in pores that are about 2 to 20 nm wide and results from "surface water" [water with structure and dynamics distinct from those of bulk liquid water, found within up to three statistical monolayers (~0.9 nm) from hydrophilic surfaces 34,35 ] constituting a nonnegligible part of the pore water 25,27,34 . In this regime, the average behavior of pore water is sometimes represented with a "core-shell" model, on which pore water is conceptually divided into a core of "free" water with bulk-liquid-like properties and a shell of "surface" water with distinct properties 25,27 .…”

“…Molecular dynamics (MD) and Monte Carlo (MC) simulations are well suited for revealing the structure and dynamics of water in nanopores [34][35][36] . However, of the MD and MC simulation studies that have probed water in silica nanopores 33,[37][38][39][40][41][42][43][44][45] , only four studies used an amorphous SiO 2 structure, silanol surface functional groups, and water-filled cylindrical nanopores 37,38,40,43 .…”