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Ion Transport in Simple Nanopores

Equilibrium and non-equilibrium molecular dynamics (EMD and NEMD) simulations are reported for the study of ion transport in an infinite long cylindrical nanopore. Results are compared for 3 models of electrolytes including the restricted primitive model (RPM), the solvent primitive model (SPM), and the extended simple point charge model (SPC/E). In EMD simulations, the mean square displacements are used to yield diffusion coefficients. Conductivity can be obtained through the Nernst–Einstein relation. Current and conductivity are calculated directly in NEMD simulations in which an external field is present along the pore axis. The effects of confinement on the ion transport are studied for the 3 model electrolytes. Comparing the EMD results and the NEMD results show that the Nernst–Einstein relation fails for the 3 models of electrolytes in very narrow nanopores. In addition to direct current NEMD simulations, alternate current (AC) NEMD simulations are performed to investigate the frequency dependence of ion transport. Towards high frequencies, a pore-size independent behavior is observed with vanishing conductivity and a phase lag approaching 90°. The effect of confinement is more evident at low frequencies and an electrical capacitor like behavior is observed in the narrowest pores, as indicated by the conductivity, the phase lag and the Cole–Cole plot. The narrowest pores show a combined reactance–resistance–capacitance (LRC) character and a maximum conductivity can be seen at the resonance frequency.