Enhanced Ion Rejection in Carbon Nanotubes by a Lateral Electric Field

Abstract

Reverse osmosis membranes hold great promise for dealing with global water scarcity. However, the trade-off between ion selectivity and water permeability is a serious obstacle to desalination. Herein, we introduce an effective strategy to enhance the desalination performance of the membrane. A series of molecular dynamics simulations manifest that an additional lateral electric field significantly promotes ion rejection in carbon nanotubes (CNTs) under the drive of longitudinal pressure. Specifically, with the increase in the electric field, the ion flux shows a deep linear decay, while the water flux decreases only slightly, resulting in a linear increase in ion rejection. The energy barriers of ions around the CNT inlet are obtained by calculating the potentials of mean force to explain enhanced ion rejection. The lateral electric field uniformly raises the energy barriers of ions by pushing them away from the CNT inlet, corresponding to the enhanced ion velocity in the field direction. Furthermore, with the increase in CNT diameter, there is a significant increase in the flux of both ions and water; however, the lateral electric field can also obviously enhance the ion rejection in wider CNTs. Consequently, the enhancement of ion rejection by lateral electric fields should be universal for different CNT diameters, which opens a new avenue for selective permeation and may have broad implications for desalination devices with large pore sizes.

Multiscale simulations of nanofluidics: Recent progress and perspective

“…In both experiments and simulations, the nanochannel-based membrane is widely investigated for the application of water desalination. 32,33,37,38,[213][214][215][216][217][218][219][220][221][222][223] Membrane scientists have been interested in nanochannels, and many highly efficient and specific nanochannels continue to be designed. It has been a continuous study in which computer simulation provides a range of approaches to membrane characterization with simple equipment and high accuracy.…”

“…The diameter of nanotubes is regarded as an important factor in the performance of desalination. 223,239,240 It is well-known that ions exist in solution are in the form of hydrated ions. The energy barrier for ions passing through the membrane can be derived from molecular simulations.…”

“…Nanochannels and sub‐nanochannels are unique in ionic specificity 211,212 and ultrafast water transport, 169,186 which make them ideal candidates for ion separation and water desalination. In both experiments and simulations, the nanochannel‐based membrane is widely investigated for the application of water desalination 32,33,37,38,213–223 . Membrane scientists have been interested in nanochannels, and many highly efficient and specific nanochannels continue to be designed.…”

“…Since CNTs provide a low energy solution for water treatment, they are considered as efficient materials in membrane industry. The diameter of nanotubes is regarded as an important factor in the performance of desalination 223,239,240 . It is well‐known that ions exist in solution are in the form of hydrated ions.…”

“…(a) A simulation snapshot of a seawater desalination system based on nanotube. Reprinted with permission from Ref [223]…”