Modulation of Ion Distributions and Electroneutrality inside Nanopores by Ion Transport under Electric Fields

Nanopores have broad applications in biosensing, energy conversion, and material separation, with their performance highly dependent on the ion distribution and transport. Here, molecular dynamics simulations were employed to systematically examine the ionic behaviors under varying surface charge densities, applied voltages, and solution concentrations. We focused on the static ion distribution without electric fields, as well as the dynamic distribution and transport of ions under electric fields. Based on the radial ion distributions, whose positional characteristics are determined solely by surface charges, ions inside nanopores can be divided into those in the electric double layers (EDLs) and those in the pore center, which contribute to the surface conductance and bulk conductance, respectively. The ion number in the EDL region is proportional to the surface charge density and solution concentration and inversely proportional to the applied electric fields. The obtained ratio of the surface conductance to bulk conductance presents a trend consistent with the prediction by the Dukhin number. Due to the nanoscale spatial confinement and non-mean-field conditions, simulated results are significantly higher than the theoretical prediction. Inside nanopores, surface charges could not be fully screened by counterions, leading to local electroneutrality breakdown, and the unbalanced charge increases with the charge density and voltage and decreases with the solution concentration. Additionally, the electric field induces ion concentration polarization (ICP), whose characteristics are quantitatively analyzed based on the axial distribution of anions and cations. Through investigation of the dynamic transition of EDLs from static equilibrium to dynamic states under electric fields, this work reveals the coupling mechanisms of the surface charge density, voltage, and concentration on ion behaviors and provides atomic-scale insights for the rational design and optimization of high-performance nanofluidic devices.