Molecular Dynamics Simulations of Ion Drift in Nanochannel Water Flow

Abstract

The present paper employs Molecular Dynamics (MD) simulations to reveal nanoscale ion separation from water/ion flows under an external electric field in Poiseuille-like nanochannels. Ions are drifted to the sidewalls due to the effect of wall-normal applied electric fields while flowing inside the channel. Fresh water is obtained from the channel centerline, while ions are rejected near the walls, similar to the Capacitive DeIonization (CDI) principles. Parameters affecting the separation process, i.e., simulation duration, percentage of the removal, volumetric flow rate, and the length of the nanochannel incorporated, are affected by the electric field magnitude, ion correlations, and channel height. For the range of channels investigated here, an ion removal percentage near 100% is achieved in most cases in less than 20 ns for an electric field magnitude of E = 2.0 V/Å. In the nutshell, the ion drift is found satisfactory in the proposed nanoscale method, and it is exploited in a practical, small-scale system. Theoretical investigation from this work can be projected for systems at larger scales to perform fundamental yet elusive studies on water/ion separation issues at the nanoscale and, one step further, for designing real devices as well. The advantages over existing methods refer to the ease of implementation, low cost, and energy consumption, without the need to confront membrane fouling problems and complex electrode material fabrication employed in CDI.

Electrochemistry under confinement

“…42 Within the last years, MD simulations were used in electrochemical applications mainly to investigate the structure of the electrical double layer in dependence of surface charge, confinement effects, solvent and solute materials. A MD approach was successfully used by Sofos et al 43 to investigate the ion accumulation at the charged walls of nanochannels in water desalination applications. Utilizing similar approaches Vatamanu et al 44 and Feng et al 45 investigated the structure of the EDL close to a graphite electrode in dependence of applied electric fields in water-poor solutions for battery and ionic liquid research.…”

Biomimetic Nanochannels: From Fabrication Principles to Theoretical Insights

“…[ 202 ] Several popular MD program can be utilized in simulation of ionic transport in nanochannels, include LAMMPS, GROMACS, and AMBER, etc. [ 21,202,203 ]…”

“…attempted to reveal the laws of ion separation from water/ion flows in nanochannels assisted by MD simulations. [ 203a ] In their report, periodic box of water molecules was placed between two planar carbonic walls. When applied voltage, water, and Na + /Cl − tended to migrate in different positions, the former would flow in the central region while ions preferred to locate near the walls.…”

A Water/Ion Separation Device: Theoretical and Numerical Investigation

“…Each cell resembles a nanochannel, where water molecules and Na + and Cl − ions flow between two planar, periodic, carbon surfaces, theoretically analogous to Poiseuille flow (see the inset of Figure 1). In a previous work, we have investigated the effect of the electric field strength, E, and the distance between the two wall surfaces, h, on desalination performance in a stand-alone nanochannel [30]. In this work, we consider a parallel network of such nanochannels and extend the investigation over the whole network.…”

“…All these parameters are presented in Table 1. At first, the minimum channel length L x for a cell of height h that could yield satisfying ion separation results has been estimated in a previous work and is given by L x = 320h − 27.46 [30]. For the device dimension on z-direction equal to L z = 1 cm, the number of cells, N, in the array can be calculated, as well as the resulting volumetric flow rate q (per unit length).…”