Abstract
This paper reports on a molecular dynamics study of aqueous electrolyte solutions confined in hydrophobic nanopores. We examined for the first time the effect of the size and polarizability of the ions on the structure and dynamics of the confined electrolyte solution by considering the series of sodium halides (NaX with X = F, Cl, Br, and I). We also address the effect of pore size by varying the diameter of the nanochannel. As far as structural properties are concerned, the behavior of the NaF electrolyte solution significantly differs from that of the other sodium halide solutions. Because of their small size, Na and F in NaF are found to be significantly solvated by water. In addition, due to steric and hydrophobic effects [Chandler, D. Nature 2005, 437, 640], Cl, Br, and I tend to be repelled from the regions where the density of water is larger. Ion-specific effects on the dynamics of water and ions are found to be minimized when the electrolyte solution is confined at the nanoscale in comparison to bulk water and the water-air interface. For instance, both the data for water and the ionic species indicate that the ratio of the self-diffusivity for the confined solution to that for the bulk is independent of the nature of the anion F, Cl, Br, and I. Moreover, while the average solvation times for Na in NaF and Na in NaX (X = Cl, Br, I) significantly differ for bulk electrolyte solutions, they turn out to be very similar for the confined solutions. Such a leveling of the dynamical properties of the electrolyte solutions due to confinement is also observed on the pairing of the anions and cations.