Ion Transport In Nanopores Research Articles

Modulation of ion transport through nanopores in water desalination: a molecular dynamics study

ABSTRACT A good understanding of ion transport mechanisms through nanopores is an important issue for the development of advanced water desalination technologies. We use the molecular dynamics simulation method to systematically investigate the translation dynamics of ions through nanopores in the water desalination process by designing four kinds of nano-membranes based on carbon nanomaterials. Results indicate that circular-shaped pore exhibits better water permeability, nevertheless, the slit pore has a lower resistance due to the larger pore area; nanochannel membranes increase the residence time of ions. Fluorination induces more ordered ionic hydration structures, and enhances Na + -Cl- ion pair association. -OH groups replace partial ionic hydration water molecules and facilitate ions transport into membranes. The -NH3 +, -COO- groups can strongly adsorb the oppositely charged ions, and substantially slow down ion dynamics. Functionalisation within nanochannel interior can further enhance interfacial friction and transport resistance, even causing pore blocking by charged groups. The fluorinated nanochannel membrane demonstrates complete rejection of ions with a water permeability coefficient of 1.88 × 104 L·m−2·h−1·bar−1, breaking the permeability-selectivity trade-off. This study indicates that ion transport in nanopores could be finely modulated to obtain enhanced performance in water desalination.

Ion Transport in pH-Regulated Double-Barreled Nanopores.

Single-molecule detection and characterization with nanopores is a powerful technique that does not require labeling. Multinanopore systems, especially double nanopores, have attracted wide attention and have been applied in many fields. However, theoretical studies of electrokinetic ion transport in nanopores mainly focus on single nanopores. In this paper, for the first time, a theoretical study of pH-regulated double-barreled nanopores is conducted using three-dimensional Poisson-Nernst-Planck equations and Navier-Stokes equations. Four ionic species and the surface chemistry on the walls of the nanopores are included. The results demonstrate that the properties of the bulk salt solution significantly affect nanopore conductivity and ion transport phenomena in nanopores. There are two ion-enriched zones and two ion-depleted zones in double-barreled nanopores. Due to the symmetry of the double-barreled nanopore structure and surface charge density, there is no ionic rectification effect in double-barreled nanopores. The ion selectivity is similar to that of conventional single pH-regulated nanopores.

Ion transport in nanopores with highly overlapping electric double layers.

Investigation of ion transport through nanopores with highly overlapping electric double layers is extremely challenging. This can be attributed to the non-linear Poisson-Boltzmann equation that governs the behavior of the electrical potential distribution as well as other characteristics of ion transport. In this work, we leverage the approach of Schnitzer and Yariv [Phys. Rev. E 87, 054301 (2013)] to reduce the complexity of the governing equation. An asymptotic solution is derived, which shows remarkable correspondence to simulations of the non-approximated equations. This new solution is leveraged to address a number of highly debated issues. We derive the equivalent of the Gouy-Chapman equation for systems with highly overlapping electric double layers. This new relationship between the surface charge density and the surface potential is then utilized to determine the power-law scaling of nanopore conductances as a function of the bulk concentrations. We derive the coefficients of transport for the case of overlapping electric double layers and compare it to the renowned uniform potential model. We show that the uniform potential model is only an approximation for the exact solution for small surface charges. The findings of this work can be leveraged to uncover additional hidden attributes of ion transport through nanopores.

How to speed up ion transport in nanopores

Electrolyte-filled subnanometre pores exhibit exciting physics and play an increasingly important role in science and technology. In supercapacitors, for instance, ultranarrow pores provide excellent capacitive characteristics. However, ions experience difficulties in entering and leaving such pores, which slows down charging and discharging processes. In an earlier work we showed for a simple model that a slow voltage sweep charges ultranarrow pores quicker than an abrupt voltage step. A slowly applied voltage avoids ionic clogging and co-ion trapping—a problem known to occur when the applied potential is varied too quickly—causing sluggish dynamics. Herein, we verify this finding experimentally. Guided by theoretical considerations, we also develop a non-linear voltage sweep and demonstrate, with molecular dynamics simulations, that it can charge a nanopore even faster than the corresponding optimized linear sweep. For discharging we find, with simulations and in experiments, that if we reverse the applied potential and then sweep it to zero, the pores lose their charge much quicker than they do for a short-circuited discharge over their internal resistance. Our findings open up opportunities to greatly accelerate charging and discharging of subnanometre pores without compromising the capacitive characteristics, improving their importance for energy storage, capacitive deionization, and electrochemical heat harvesting.

Effects of Surface Trapping and Contact Ion Pairing on Ion Transport in Nanopores

Ion transport in highly-confined space is important to various applications, such as biosensing and seawater desalination with nanopores. All-atom molecular dynamics simulations are conducted to investigate the transport of Na$^+$ and Cl$^-$ ions through nanopores with the diameter below 6 nm. It is found that the formation of the contact ion pair plays a critical role in reducing the ion mobility inside a nanopore without surface charges. The mobility for both cations and anions decreases with the reduced pore size because it is easier to form the contact ion pairs inside the neutral nanopore with smaller diameter. Inside a charged nanopores, besides the contact ion pair formation, the surface charges also play a significant role in reducing the counterion mobility through surface trapping. It is uncovered that the mobility of Na$^+$ ions increases first and then decreases with the surface charge density, while Cl$^-$ ions have the opposite trend. A modified first-passage time model is proposed to take into account the ion pair formation and the trapped ions inside a nanopore, which provides a clear picture in describing ion transport through a nanopore.

Drastically Reduced Ion Mobility in a Nanopore Due to Enhanced Pairing and Collisions between Dehydrated Ions.

Ion transport through nanopores is a process of fundamental significance in nature and in engineering practice. Over the past decade, it has been found that the ion conductivity in nanopores could be drastically enhanced, and different mechanisms have been proposed to explain this observation. To date, most reported studies have been carried out with relatively dilute electrolytes, while ion transport in nanopores under high electrolyte concentrations (>1 M) has been rarely explored. Through systematic experimental and atomistic simulation studies with NaCl solutions, here we show that at high electrolyte concentrations, ion mobility in small nanopores could be significantly reduced from the corresponding bulk value. Subsequent molecular dynamics studies indicate that in addition to the low mobility of surface-bound ions in the Stern layer, enhanced pairing and collisions between partially dehydrated ions of opposite charges also make important contributions to the reduced ion mobility. Furthermore, we show that the extent of mobility reduction depends on the association constant between cations and anions in different electrolytes with a more drastic reduction for a larger association constant.

Ionic Conduction in Biological Nanopores Created by Ultrashort9 High-Intensity Pulses.

Ultrashort, high-intensity electric pulses open nanopores in biological cell membranes. Ion transport in nanopore is analyzed using a numerical method that couples the Nernst-Planck equations for ionic concentrations, the Poisson equation for the electric potential, and Navier-Stokes equations for the fluid flow. Roles of the applied bias, pore size, as well as the surface charge lining the membrane are comprehensively examined through I-V characteristics, conductance variations of the pore. Our results show that the surface charge distribution has an impact on the ionic conduction due to mutual electrostatic force interference. In addition, a larger pore would conduct a larger ionic current thus being more conductive on the condition of the same bias applied, which would suggest a bias-dependent expansion of pores.

Simulation of pH-Regulated Electrokinetic Ion Transport in Nanopores with Polyelectrolyte Brushes.

Nanopores with polyelectrolyte brushes in a pHregulated ion channel have been simulated. Electrokinetic ion transport was studied taking account of the effects of pH, presence of multiple ionic species and bulk ionic concentration. A continuum-based model that composed of coupled Poisson-Nernst- Planck equations for the ionic mass transport with electric potential, and Navier-Stokes and Brinkman equations for the fluid flow, were employed. Our results demonstrate that the bulk ionic concentration and pH together can greatly influence the charge of the polyelectrolyte brushes, the distribution of electric potential in the nanopore and also the fluid flow is sensitive to the pH variations.

Surface Reaction, Solvent Inhomogeneity, and Ion Transport in Electric Double Layers: Predictions from a Classical Density Functional Theory

Electric double layers (EDLs) are formed at the interface of a charged fluid in contact with an electrified surface or an electrode leading to drastic changes in the fluid structure and local thermodynamic properties. Whereas conventional theories of EDLs are often concerned with the local inhomogeneity in terms of ionic distributions near a planar surface, the simplistic model is insufficient to capture many important properties relevant to emerging electrochemical processes for energy storage and conversion. In this talk, we present a few recent applications of the classical density functional theory (CDFT) that account for not only ionic excluded volume effects and electrostatic correlations ignored in conventional theories of EDLs but also electrode geometry and pore size distributions, the local solvent and dielectric inhomogeneity, surface reactions, and hydrodynamic effects. The CDFT calculations will be illustrated with practical examples for unusual ion transport in nanopores, optimization of capacitive energy extraction processes, design of nanostructured electrodes for flow supercapacitors and pseudo-capacitors. The theoretical predictions are validated with experiment results and provide insights for novel electrochemical behavior that are not captured in conventional EDL theories. Acknowledgements: This research is sponsored by the Fluid Interface Reactions, Structures, and Transport (FIRST) Center and the U.S. Department of Energy (DE-FG02-06ER46296). The numerical calculations were performed at the National Energy Research Scientific Computing Center (NERSC).

Microsecond simulations of DNA and ion transport in nanopores with novel ion-ion and ion-nucleotides effective potentials.

We developed a novel scheme based on the grand-canonical Monte Carlo/Brownian dynamics simulations and have extended it to studies of ion currents across three nanopores with the potential for single-stranded DNA (ssDNA) sequencing: solid-state nanopore Si₃N₄, α-hemolysin, and E111N/M113Y/K147N mutant. To describe nucleotide-specific ion dynamics compatible with ssDNA coarse-grained model, we used the inverse Monte Carlo protocol, which maps the relevant ion-nucleotide distribution functions from all-atom molecular dynamics (MD) simulations. Combined with the previously developed simulation platform for Brownian dynamics simulations of ion transport, it allows for microsecond- and millisecond-long simulations of ssDNA dynamics in the nanopore with a conductance computation accuracy that equals or exceeds that of all-atom MD simulations. In spite of the simplifications, the protocol produces results that agree with the results of previous studies on ion conductance across open channels and provide direct correlations with experimentally measured blockade currents and ion conductances that have been estimated from all-atom MD simulations.

Field-responsive ion transport in nanopores

The transport behavior of solvated ions in nanopores of a zeolite Y is field responsive. As an external electric field is applied, the observed change in effective solid-liquid interfacial tension is contradictory to the prediction of classic electrochemistry theory; it considerably increases no matter whether the applied voltage is positive or negative. This may be attributed to the breakdown of solvated ion and double-layer structures in the nanoenvironment.

Effects of anion concentration on ion-transport pressure in nanopores

In an experiment on pressure-driven ion transport in nanopores of a zeolite, the anion concentration is varied in a broad range. As the anion concentration is relatively low, its influence on the ion transport pressure, which reflects the system free energy variation rate, is negligible. When the anion concentration is relatively high, it has a pronounced effect on the ion transport pressure, which should be attributed to the unique surface ion structure in the confining nanoenvironment. The testing data also indicate that the effective interfacial tension is highly dependent on the anion size.

Theoretical Description of the Ion Transport Across Nanopores With Titratable Fixed Charges: Analogies Between Ion Channels and Synthetic Pores

Recently developed nanometer-sized synthetic pores display several properties so far believed to be distinctive features of a large variety of biological wide ion channels. Thus conductance in the pS-nS range, pH-dependent ion selectivity, fluctuations of current between open and closed states, flux inhibition caused by protons or divalent cations, current rectification, and the ability to perform selective macromolecule sizing and counting are found in synthetic and biological channels alike. Despite other differences such as pore size and geometry, the similarities open a new field for exploring specific technological applications via the chemical modification of synthetic pores with biological molecules. This article reviews some of the basic concepts and theories relevant to ion transport in nanopores with titratable charges stressing the analogies between synthetic pores and biological ion channels. The ultimate goal is to show that continuum theories may account for the essential features of these systems. A simple electrodiffusion model and its comparison with experimental results are chosen as a case study.