Diffusivity and Conductivity of a Solvent Primitive Model Electrolyte in a Nanopore by Equilibrium and Nonequilibrium Molecular Dynamics Simulations

Abstract

Equilibrium and nonequilibrium molecular dynamics simulations are performed to calculate the diffusion coefficient and electric conductivity of ions in a 0.1 M concentration solution confined in neutral cylindrical pores. The applied model is a solvent primitive model (SPM) in which both ions and solvent molecules are soft core spheres and the polar nature of the solvent is represented implicitly as a background with a given dielectric constant. The simulations are carried out in an isokinetic ensemble, and the system, responsing to an applied electric field, is maintained at constant temperature by a Gaussian thermostat. From equilibrium molecular dynamics, diffusion coefficients of ions and solvent decrease with decreasing pore radius or increasing packing fraction of solvent particles. The conductivity determined by nonequilibrium molecular dynamics shows a similar trend, but the pore-size dependence of conductivity does not have a local maximum as was found in the restricted primitive model in which solvent spheres are absent. Using the Nernst-Einstein relation, the ionic conductivity is also calculated from the equilibrium diffusion coefficient and compared with the conductivity obtained from nonequilibrium simulations. The comparison shows that the Nernst-Einsten relation is not valid only at low solvent packing and in very small pores.

Non-equilibrium molecular dynamics simulation of oxygen ion mobility in yttria stabilized zirconia

“…(7) for strongly interacting ions in the case of YSZ. This discrepancy between NEMD and EMD conductivities was similarly observed and discussed previously for confined electrolytes [22].…”

Monte Carlo study of the electrode|solvent primitive model electrolyte interface

“…Thus the theoretical predictions can be directly verified by the simulation results. The model was used in the investigation of phase equilibrium [11], membranes [12,13], ion channels [14], conductivity and diffusivity in a nanopore [15]. We concentrate on its application to the planar electric double layer.…”

Non-equilibrium molecular dynamics simulation study of the frequency dependent conductivity of a primitive model electrolyte in a nanopore

“…The uniform profiles are due to the low particle density in the RPM model, and for the solvent primitive model more structured density and current profiles were observed [ZO]. However, even in the case of the SPM model, the density and current profiles correlate closely [20], indicating uniform velocity distributions. Artefacts induced by the homogeneous thermostat for the inhomogeneous system would therefore be second order.…”

“…In the NEMD simulations, the accumulated density profiles and current density profiles are uniform and flat with respect to distance from the central axis, due to the low packing density of the RPM. In the previous study of the solvent primitive model (SPM) [20], the current density profiles have peaks near the wall. The current density profiles, however, correlate closely with the number density profiles.…”

“…For very small pores, however, the dc conductivity rapidly decreases to zero because ions travelling in opposite directions cannot get past each other. A later study of dc conductivity variations with pore size was attempted for the solvent primitive model [20]. In this paper, the ac conductivity was studied for the RPM electrolyte confined in cylindrical pores of various sizes.…”