Effect of nanotube diameter on the transport of water molecules in electric fields

Abstract

Electric fields control the water dipole orientation and result in a rotation-translation coupling transport of water through narrow carbon nanotubes (CNTs); however, it is not clear if this phenomenon still holds when the confined water is close to bulk structures for increasing the CNT diameter. Herein, we confirm that this transport phenomenon still exists for larger CNTs with bulk-like water structures. Using molecular dynamics simulations, we investigate the transport of water molecules through CNTs with different diameters in electric fields. The water flow decreases as a whole with the increase of field strength, while the water flux changes oppositely with a two-stage behavior, leading to the increase of unidirectional transport efficiency ƞ. Similar to the experimental wetting phenomenon of CNTs, our simulated water occupancy displays a linear increase with the field strength. Moreover, we find a simple equation of τ = σ*N/flow that can accurately predict the water translocation time τ according to the water occupancy N and flow, where σ is a fixed coefficient for each CNT diameter and numerically close to the ambient temperature of 300 K. The water dipole angels can be highly regulated by the electric field, as the values change from nearly 900 with random orientation to below 300 that along the field direction, which can be further elucidated by the unique dipole distributions. The water dipole orientation exists a two-stage change that accounts for the two-stage water flux behavior. These results enrich our knowledge of the transport behavior of water molecules under external fields, and are helpful for the design of stable and high-efficient nanofluidic devices.