Rectification Correlation between Water and Ions through Asymmetric Graphene Channels.

Rectification phenomena occurring in asymmetric channels are essential for the design of novel nanofluidic devices such as nanodiodes. Previous studies mostly focus on ion current rectification, while its correlations with water dynamics are rarely explored. In this work, we analyze the transport of water and ions through asymmetric graphene channels under the drive of electric fields using molecular dynamics simulations. A key observation is that the water flux also exists in the rectification phenomenon that follows the ion flux behaviors because of their dynamical coupling relation in electric fields, and both their rectification ratios exhibit maximum behaviors with the change of the channel opening ratio. This is because the ion dehydration is highly asymmetric for small opening ratios. In addition, the cations and anions have distinct rectification ratios that are strongly dependent on the field strength, where the values for anions can even be 1-2 orders larger. This can be attributed to their different hydration shell and dehydration processes in the graphene channel. The translocation time of ions displays a power law relation with the field strength, in agreement with the prediction by Langevin dynamics. Due to the exclude-volume effect, the occupancy of water and ions shows a clear competition and thus changes in an opposite trend with the field strength. Our results demonstrate the rectification correlations between water and ions, and tuning the geometry of graphene channels provides a simple and robust new route to achieve high rectification ratios.