Electrokinetic transport of ions and water through double-walled carbon nanotubes under axial static electric fields

One of the most challenging issues in nanofluidic systems is the development of highly efficient desalination membranes that can simultaneously achieve high water permeability and ion selectivity. In this work, we demonstrate, through all-atom molecular dynamics (MD) simulations, that double-walled carbon nanotubes (DWCNTs) under axial static electric fields provide a novel and effective strategy to address this long-standing limitation. The results show that transport behavior is governed not only by inner CNT diameter but also by the interplay between interwall confinement, hydration effects, water ordering, and electric-field-induced molecular alignment. Interestingly, the (6,6) CNT exhibits the highest water permeability under strong electric fields despite having the smallest inner size, owing to the contribution of ordered water transport and the interwall transport pathway. In contrast, intermediate CNTs, particularly (8,8) and (9,9), demonstrate enhanced ion rejection by restricting ion migration through both the inner channel and interwall region. Increasing electric-field strength substantially decreases ion and water translocation times, confirming accelerated field-driven transport. Furthermore, occupancy, hydrogen-bond, and dipole-orientation analyses reveal the molecular mechanisms governing the balance between confinement-induced ordering and electrokinetic transport. These findings establish a design principle for electrically tunable DWCNT membranes and provide new insights into the development of next-generation nanofluidic desalination systems.