Molecular dynamics simulations of water and sodium diffusion in smectite interlayer nanopores as a function of pore size and temperature

The diffusion coefficients (D) of water and solutes in nanoporous Na-smectite clay barriers have been widely studied because of their importance in high-level radioactive waste (HLRW) management and in the isolation of contaminated sites. However, few measurements have been carried out at the high temperatures that are expected to occur in HLRW repositories. We address this knowledge gap by using molecular dynamics (MD) simulations to predict the temperature dependence of diffusion in clay interlayer nanopores, expressed as a pore scale activation energy of diffusion (E_a). Our sensitivity analysis shows that accurate prediction of pore scale D and E_a values requires careful consideration of the influence of pore size, simulation cell size, and clay structure flexibility on MD simulation results. We find that predicted D values in clay interlayer nanopores are insensitive to the size of the simulation cell (contrary to the behavior observed in simulation of bulk liquid water) but sensitive to the vibrational motions of clay atoms (particularly in the smallest pores investigated here, the one-, two-, and three-layer hydrates). Our predicted D and E_a values are consistent with experimental data. They reveal, for both water and Na⁺, that E_a increases by ∼6 kJ mol^-1 with increasing confinement, when going from bulk liquid water to the one-layer hydrate of Na-montmorillonite.