Electropumping Phenomenon in Modified Carbon Nanotubes.

Controlling the water transport in a given direction is essential to the design of novel nanofluidic devices, which is still a challenge because of thermal fluctuations on the nanoscale. In this work, we find an interesting electropumping phenomenon for charge-modified carbon nanotubes (CNTs) through a series of molecular dynamics simulations. In electric fields, the flowing counterions on the CNT inner surface provide a direct driving force for water conduction. Specifically, the dynamics of cations and anions exhibit distinct behaviors that lead to thoroughly different water dynamics in positively and negatively charged CNTs. Because of the competition between the increased ion number and ion-CNT interaction, the cation flux displays an interesting maximum behavior with the increase in surface charge density; however, the anion flux rises further at higher charge density because it is less attractive to the surface. Thus, the anion flux is always several times larger than cation flux that induces a higher water flux in positive CNTs with nearly 100% pumping efficiency, which highly exceeds the efficiency of pristine CNTs. With the change in charge density, the translocation time, occupancy number, and radial density profiles for water and ions also demonstrate a nontrivial difference for positive and negative CNTs. Furthermore, the ion flux exhibits an excellent linear relationship with the field strength, leading to the same water flux behavior. For the change in salt concentration, the pumping efficiency for positive CNTs is also nearly 100%. Our results provide significant new insight into the ionic transport through modified CNTs and should be helpful for the design of nanometer water pumps.