Ionic Transport in Electrostatic Janus Membranes. An Explicit Solvent Molecular Dynamic Simulation

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 nanoscale Janus pores increases dramatically as their diameter becomes smaller. However, little is known about the underlying transport processes, 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. 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 reorientation of water molecules in the pores, and a segregation of ionic species that had 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 the pore entrance controls net transport.

Ionic Current Rectification Induced by Charge Polarity in Janus Graphene Channels

“…Although there are some reports on the rectification performance of conical or other asymmetric Janus membranes, − to the best of our knowledge, the ICR phenomenon in symmetric Janus channels has not been reported yet. For this reason, herein we explore the ICR phenomenon induced by the charge polarity in a geometrically symmetric graphene slit using molecular dynamics simulations.…”

“…For conical nanochannels, modulating the functionalization time to change the ratio of coatings inside the channel achieves rectification maximization, which indicates the difference in the total current contribution of cations and anions. Janus membranes with a nonuniform cross-section provide differential ion reservoirs, and the “leakage” electric field eventually causes an ICP and ion transport asymmetry . The spin-coated graphene oxide layer on the inner surface of the channel enhances cation transport but inhibits anion transport , which effectively improves the rectification effect as well as ion selectivity .…”

“…Electrolyte concentration and pH , deeply modulate ion enrichment and depletion inside the channel, which in turn impacts the rectification performance of devices or even leads to a reversal of rectification direction . Increasing the solution concentration decreases the Debye length , and alters the range of ion electrostatic shielding as well as the extent to which ions interact with the channel surface. However, excessive electrolyte concentration leads to a substantial accumulation of ions inside the channel, forming solid multi-ion clusters or even inducing a Coulomb blockade effect that impedes ion conduction .…”

Current rectification by nanoparticles in bipolar nanopores

“…While important theoretical and experimental advances have led to a better understanding of the fundamental physics at work in bipolar nanopores, the coupled roles of nanopore geometry and the addition of small concentrations of charged nanoparticles on ion transport and electric current rectification have not been considered before. In previous work 12 , we calculated and analyzed the cation and anion concentration profiles and current-voltage relationship of dilute electrolytes in bipolar nanopores at rest and under applied biases of +2 V and −2 V, respectively. In this work we discuss the effect of adding small quantities of nanoparticles on the concentration profiles and current rectification properties of bipolar nanopores.…”

Intrinsic Interface Adsorption Drives Selectivity in Atomically Smooth Nanofluidic Channels

“…In this letter, we show how interfacial atomic structure affects the directed transport of an electrolyte solution in nanochannels made of atomically flat graphite (GR) and hexagonal boron nitride (BN) walls using molecular dynamics simulations unified with a contemporary perspective on hydrodynamics. These simple systems have been studied extensively because of their intriguing transport properties, such as anomalously high permeabilities in GR, ,− and the potential to incorporate selectivity for desalination or blue energy applications. − By computing the spatially resolved volumetric, charge, and species transport coefficients from equilibrium correlations − we elucidate the importance of molecular interactions on nanofluidic device functionality. While from a continuum perspective, driving the solution with a pressure gradient should result in salt filtration or electric current only when the confining walls have a net charge, we discover that the intrinsic interfacial adsorption of ions can lead to streaming electrical currents and a novel, emergent desalination mechanism.…”