Mineral- and Ion-Specific Effects at Clay–Water Interfaces: Structure, Diffusion, and Hydrodynamics

Abstract

We use molecular dynamics to investigate how the structure, diffusion and hydrodynamic properties of clay interfaces with aqueous solution depend on the nature of the clay, the nature of the counterions and the salt concentration in the solution. Specifically, we study water-filled nanopores between uncharged (pyrophyllite) and charged (montmorillonite and beidellite, with susbtitutions located in the octahedral and tetrahedral layers, respectively) clays, with sodium or cesium as counterions, in the absence and in the presence of added salt. We discuss how the balance between solvation and attraction of the cations to the surface results in various distributions between innerand outer-sphere complexes and how this influences the dynamics of water near the surface, as well as the hydrodynamic flow in the presence of an external force. In the latter case, the discussion based on mapping the molecular velocity profiles to a continuous description (parabolic Poiseuille flow) shows that the larger effects come from the presence/absence of charge in the mineral, as well as the localization of substitutions within the clay layer. The salt concentration and the nature of the counterions have a comparatively less important impact far from the surface-even though some differences are observed in its close vicinity, which are not properly captured by the continuous description.

Sampling mobility profiles of confined fluids with equilibrium molecular dynamics simulations

“…In practice, one applies one type of perturbation and samples the fluxes of all species as a function of the position with respect to the surfaces; this is then repeated for each type of perturbation. 5,15,[18][19][20][21][22][23][24][25][26] While natural and efficient, the NEMD approach brings the system out of equilibrium and requires appropriate thermostatting strategies, on which the resulting fluxes should not depend. [27][28][29] In addition, the mobilities are only defined in the linear response regime, and several simulations need to be performed to find the good compromise between the validity of the linear response regime (implying small perturbation) and signal-to-noise ratio (larger for large perturbation).…”

A large deviation theory perspective on nanoscale transport phenomena

“…Extensions of this analysis to explicit solvent models has been recently undertaken, 108 and their implications for simulations with explicit nanoconfinement remains to be studied. [151][152][153] V. BEYOND…”

Statistical physics and applied geosciences: some results and perspectives

“…If mechanical stresses due to swelling induce fracturation, the overall societal interest of the storage may be questionable. These successes motivated many SP studies to describe fluid motion at the nanoscale [20,22,25,[28][29][30]. The main goal is to be able to quantify the net effect of slippage and of electrostatic interactions at the pore boundaries, that may be neglected for usual pore sizes.…”