Electrostatic-field-modulated gas dynamics in charged nanotubes.

Abstract

<h4>Context</h4>The behaviour of gas molecules in nanopore materials is essential for research in gas separation, catalytic reactions, and nanofluidic transport. However, controlling gas dynamics at the nanoscale, especially in charged nanopores, remains a significant challenge. Through molecular dynamics simulations, this study reveals that external electric fields can synergise with the surface properties of charged nanotubes to regulate the distribution and aggregation behaviour of gases inside the nanotubes. Increasing the wall charge density of the nanopore leads to gas aggregation and the formation of nanobubbles at the tube centre, thereby obstructing water flow. In contrast, applying an axial external electric field weakens the stability of nitrogen bubbles, causing collapse and reopening the liquid transport channel. The formation and distribution of ordered water layers on nanopore walls is a key influencing factor on the dynamic behaviour of gases within nanotubes. This interfacial structure enables regulating gas adsorption and desorption, as well as liquid flow blockage or activation, by the combined modulation of surface charge and external electric fields. These insights provide a reference for designing electrically controlled nanofluidic valves in electrically charged nanopore materials, a technology with strong relevance to applications such as gas storage, sensing, and integrated nanofluidic systems.<h4>Methods</h4>Classical molecular dynamics simulations were performed using GROMACS 2021.5 in NVT ensemble at 300 K for the water-N₂ system in an uncapped (40,40) carbon nanotube. The nanotube wall was implemented by alternating ± q charges on diagonally adjacent sites of neighbouring hexagons (q = 0-1.4 e/atom in 0.2 e/atom increments), resulting in a net neutral nanotube. The GROMOS43a1 force field was employed for system interactions, and water molecules were modelled using the SPC/E model. Electrostatic interactions were handled using the Particle-Mesh Ewald (PME) method with a real-space cutoff of 10 Å; van der Waals forces were similarly truncated at 10 Å. A uniform static axial electric field in the + z direction was simultaneously applied, with magnitudes varying from 0.00 V/Å to 0.10 V/Å at an interval of 0.01 V/Å. Data, including density distribution, water flux, and water molecule orientation/hydrogen bond counts, were extracted via Python/C +  + scripts. Model Structure and trajectories were visualised using VMD 1.9.3 software.