Structure and dynamics of an electrolyte confined in charged nanopores

Molecular Dynamics simulations are used to investigate the structure and dynamics of an aqueous electrolyte (NaCl) confined within a nanomembrane, which consists of a nanopore with a diameter 3 nm having a negatively charged surface. Both nanomembranes with a diffuse charge and with local charges are considered (in both cases, two surface charge densities are considered, -0.9 e/nm ² and -1.8 e/nm²). For all nanomembranes, significant layering of water and ions in the vicinity of the nanomembrane surface is observed. While the distribution of water and chloride ions is nearly insensitive to the nanomembrane charge and type, the arrangement of sodium cations within the nanomembrane depends on the system being considered. The water and ion density profiles in the nanomembranes are compared with the predictions of a modified Poisson-Boltzmann equation in which charge image, solvation effects, and dispersion interactions with the surface are taken into account [Huang et al. Langmuir, 2008, 24, 1442]. The self-diffusion coefficient for a given species is smaller than its bulk counterpart and is at most 75% of the bulk value. While the self-diffusion coefficients for water and sodium cations decrease with decreasing the overall negative charge of the nanomembrane, the self-diffusion coefficient for the chloride anions is nearly independent of the nanomembrane type and charge. We also estimate the dynamics of the confined aqueous electrolyte by calculating time correlation functions which allow estimating solvation, ion pairing, and residence times.