Accelerating water transport through a charged SWCNT: a molecular dynamics simulation

Abstract

The properties of a nanotube, such as the hydrophobicity and charge of the surface, can significantly affect water transport behavior. However, our knowledge of the effects of charge density, dipole orientation, frequency of flipping, and movement behavior on water flow through carbon nanotubes (CNTs) is far from adequate. This study is aimed at gaining insight into the transport of single-file water molecules in a charged carbon nanotube. It was shown that the water chains inside the charged nanotube exhibit bipolar properties. The water dipoles are parallel to the z-axis, and point toward (D-defect) and away from (L-defect) the center of the nanotube for a negatively charged nanotube and a positively charged one, respectively. Compared with a pristine single-wall carbon nanotube (SWCNT), the charged nanotubes, including both positively charged and negatively charged, favor the water-filling process due to electrostatic interactions. According to the dipole distribution in the nanotube, the water dipole only flips in the middle region because of the bipolar nature of water chains. Additionally, flipping of the entire water chain is inhibited, which allows for the enhanced water flux. A negatively charged single-walled carbon nanotube (N-SWCNT) accelerated water transport by tuning the single-file flow from a "hopping" to a "continuous" mode, thus decreasing the energy barrier. The hydrogen bonds between water molecules inside the nanotube are also strengthened in the negatively charged nanotube, favoring water transport. Any distortion of uniformity will lead to additional energy barriers to water flux. Our results provide a comprehensive view of molecular events underpinning the water transport inside a SWCNT, which may be of assistance in creating innovative designs for water nanochannels.