Molecular Dynamics Simulations of Electrical Conductivity of NaCl Solutions at High Temperatures and Pressures

Abstract

Electrical conductivity measurements of subsurface geochemical systems are used to detect the presence of aqueous fluids that drive chemical reactions in the Earth’s crust and mantle. Experiments on NaCl solutions show that their electrical conductivities (σ) have a non-monotonic dependence on pressure and temperature. In this paper, we study this important property based on an atomic-scale simulation approach. We perform molecular dynamics (MD) simulations of 1.05 mol/kg NaCl solutions along 473 K, 673 and 1073 K isotherms at pressures from 0.1 to 5 GPa. Two different interaction models are used for our MD simulations: ReaxFF, a many-body dissociative force field, and SPC/E, a two-body rigid force field. The simulations suggest that the non-monotonic behavior of the electrical conductivity is caused by a complex interplay between ion self-diffusion and ion pairing. Both models differ in their predictions. Electrical conductivity in the ReaxFF simulations is influenced by both ion self-diffusion and ion pairing at all the studied conditions, whereas the conductivity from the SPC/E model is completely diffusion-driven at low temperatures, with ion pairing effects observed at higher temperatures. We find that the absolute values of σ obtained from MD simulations are largely consistent with the experimental data up to about 1 GPa, but the surprisingly large increase of σ with temperature at higher pressures reported recently could not be reproduced.