Abstract
Most analytic theories describing electrostatically driven ion transport through water-filled nanopores assume that the corresponding permeation barriers are bias-independent. While this assumption may hold for sufficiently wide pores under infinitely small bias, transport through subnanometer pores under finite bias is difficult to interpret analytically. Given recent advances in subnanometer pore fabrication and the rapid progress in detailed computer simulations, it is important to identify and understand the specific field-induced phenomena arising during ion transport. Here we consider an atomistic model of electrostatically driven ion permeation through subnanoporous C2N membranes. We analyze probability distributions of ionic escape trajectories and show that the optimal escape path switches between two different configurations depending on the bias magnitude. We identify two distinct mechanisms contributing to field-induced changes in transport-opposing barriers: a weak one arising from field-induced ion dehydration and a strong one due to the field-induced asymmetry of the hydration shells. The simulated current–voltage characteristics are compared with the solution of the 1D Nernst–Planck model. Finally, we show that the deviation of simulated currents from analytic estimates for large fields is consistent with the field-induced barriers and the observed changes in the optimal ion escape path.
Highlights
Ionic permeation through nanopores in atomically thin membranes has attracted considerable and ever-growing attention in the past decade, for reasons that are both fundamental and practical in nature.[1,2] There is a wide range of applications including fuel cells,[3] water desalination,[4−8] DNA sequencing,[9−14] and “blue energy” harvesting.[15]
The statistically significant structure of these trajectories can be obtained from the prehistory probability distribution (PPD).[39−41] The underlying concept is that the probability of observing an ion escaping near the boundary xf of the attractors of two metastable states is small, due to the relatively high transition barrier separating them
The PPD can be found by collecting all the trajectories that move the system to the state xf from a given basin of attraction, setting the final time for all trajectories to a fixed value tf and building the distribution of all such trajectories. This concept has been shown to be useful for analysis of the dynamics of comparatively rare fluctuational escape events in systems that are locally far from thermal equilibrium.[40,43]
Summary
Ionic permeation through nanopores in atomically thin membranes has attracted considerable and ever-growing attention in the past decade, for reasons that are both fundamental and practical in nature.[1,2] There is a wide range of applications including fuel cells,[3] water desalination,[4−8] DNA sequencing,[9−14] and “blue energy” harvesting.[15]. This work focuses on the effect of field-induced asymmetry of the hydration shells during cation permeation through cationselective pores, the fundamental mechanisms responsible for the permeation barriers are expected to be the same for anions permeating anion-selective pores, suggesting qualitative applicability of our results regardless of ion charge. Our statistical analysis of these trajectories and the corresponding distributions of ions and water molecules in the system as a function of the applied electric field will be presented and discussed
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