Terahertz waves promote the desalination performance in double-walled carbon nanotubes

Abstract

Improving ion rejection often occurs at the expense of water permeability, making the simultaneous optimization of both properties particularly challenging. Generally, small pore sizes can effectively hinder ion transport and thus enhance rejection, but at the same time they also restrict water passage, leading to reduced permeability. To address this challenge, we employ molecular dynamics (MD) simulations to investigate ion rejection and water transport through terahertz (THz)-assisted double-walled carbon nanotubes (DWCNTs). We find that applying THz waves to DWCNTs can simultaneously enhance both water permeability and ion rejection. Specifically, as the field strength increases, in simulations, desalination performance improves, approaching 100 % at A = 2 V/nm for all types of DWCNTs, attributed to a combination of resonance and freezing effects. Additionally, across all DWCNTs, maximum ion rejection occurs at f = 10 THz, where confined ion transport is observed while high water flux is preserved. Furthermore, as the inner CNT radius increases, the ion rejection reaches a peak for (8,8) and (9,9) CNTs, owing to their distinct size exclusion characteristics. As a result, our findings not only uncover the underlying mechanisms of THz-assisted transport in DWCNTs but also open up new possibilities for the development of advanced membrane-based desalination systems.