Modulation of water transport in carbon nanotubes by local charges

Abstract

Carbon nanotubes (CNTs) enable extremely fast water transport that make them strong candidates for constructing artificial nanochannels and membranes. However, water transport properties in CNTs with defects along with local charges are rarely studied. In this work, with molecular dynamics simulations, it is found that water transport in CNTs can be greatly modulated by local charges at the CNT middle. The flux modulation factors show an increasing trend with CNT diameters and are slightly influenced by the axial positions of local charges and the CNT lengths. Analysis suggests that water transport is hindered by the confinement-enhanced concentration oscillations and bipolar orientations of water molecules induced by local charges. It is also interesting that the water flux of CNT (6, 6) with high local charges has a reversal behavior due to the overscreening effect. Simultaneously, ions are also impeded due to the water structuring and bipolarizations, which can improve the salt rejection of wide CNTs. In general, introducing local charges on CNT (10, 10) can reject 100% ions while maintaining a high water permeability of ∼90 L/cm2/day/Mpa in the reverse osmosis desalination. This study promotes the understanding of water transport in nanochannels with defects and guides the design of CNTs for high-performance desalination membranes.