Ionic transport in electrostatic Janus Membranes. An explicit solvent molecular dynamic simulation

TL;DR

This study uses explicit solvent molecular dynamics simulations to explore ionic transport mechanisms in Janus membranes, revealing new insights into water and ion behavior that influence rectification efficiency.

Contribution

It provides the first detailed molecular-level understanding of ionic transport in Janus nanopores, highlighting previously unknown water re-orientation and ion segregation effects.

Findings

Water molecules exhibit local re-orientation within pores.

Ionic species segregate in ways not predicted by continuum models.

Electric leakage at pore entrance influences ionic current rectification.

Abstract

Janus --or two-sided, charged membranes offer promise as ionic current rectifiers. In such systems, pores consisting of two regions of opposite charge can be used to generate a current from a gradient in salinity. The efficiency of \textcolor{black}{nanoscale} Janus pores increases dramatically as their diameter becomes smaller. However, little is known about the underlying transport processes\textcolor{black}{, particularly under experimentally accessible conditions}. In this work, we examine the molecular basis for rectification in Janus nanopores using an applied electric field. Molecular simulations with explicit water and ions are used to examine the structure and dynamics of all molecular species in aqueous electrolyte solutions. \textcolor{black}{For several macroscopic observables, the results of such simulations are consistent with experimental observations on asymmetric membranes. Our analysis reveals a number of previously unknown features, including a pronounced local re-orientation of water molecules in the pores, and a segregation of ionic species that has not been anticipated by previously reported continuum analyses of Janus pores. Using these insights, a model is proposed for ionic current rectification in which electric leakage at pore entrance controls net transport.