Carbon Nanotubes as Nano-Pumps: A Molecular Dynamics Investigation

This paper focuses on the use of carbon nanotubes (CNT) for ion separation and encapsulation from a solution containing both positive and negatively charged ions. Metal ion separation from drinking water or during material processing applications can be an important issue. We use molecular dynamics simulations to demonstrate that a pair of carbon nanotubes with patterned positive and negative charges can form the basis of an effective device for the separation or encapsulation of ions. We consider three different charge patterns: i) Electrodes, where all the atoms of a CNT are charged with a finite surface charge density; ii) Alternate axial bands of positive and negative charges on each electrode; and iii) Alternate circumferential rings of positive and negative charges on the electrodes. The charge pattern determines the preferential intake of water and/or ions by a nanotube. As conventional electrodes they adsorb ions, but with an alternate band or ring charge pattern they adsorb the water molecules. Our simulations show that a charged CNT can be used as a nano-pump that provides purified water or ions from an impure solution.