Transport of water and ions through single-walled armchair carbon nanotubes: A molecular dynamics study

The narrow hydrophobic interior of a carbon nanotube (CNT) poses a barrier to the transport of water and ions, and yet, unexpectedly, numerous experimental and simulation studies have confirmed fast water transport rates comparable to those seen in biological aquaporin channels. These outstanding features of high water permeability and high solute rejection of even dissolved ions that would typically require a lot of energy for separation in commercial processes makes carbon nanotubes an exciting candidate for desalination membranes. Extending ion exclusion beyond simple mechanical sieving by the inclusion of electrostatics via added functionality to the nanotube bears promise to not only reduce the energy requirement of the ion rejection process but to also lend the nanotube an added attribute of perm-selectivity.

Confinement of water and ions in the nanotube leads to unique structure, dynamics, and electrostatic effects, which manifest as a result of discreteness of molecules, ion-ion interactions, and ion-specific interactions at nanoscale confinements that are not accounted for in continuumbased transport equations. Using Molecular dynamics (MD), an attempt has been made to provide a detailed molecular-level understanding of the structure, dynamics, and energetic aspects of the permeation mechanism as functions of CNT pore sizes, external solution concentrations, the number and nature of charges on the CNT wall, and external electric fields. Ion transport and electrolyte rejection rates are calculated from long-timescale MD simulations for the cases studied herein, and these are compared to the predictions of continuum theory. Additionally, ion conduction rates are indirectly calculated as functions of the energy barriers that are obtained by using umbrella sampling and free energy perturbation methods. The feasibility of using the thermodynamically-derived Donnan theory to make realistic predictions of co-ion concentrations in charged CNT-based nanoporous membranes with diameters less than 3 nm is discussed. Furthermore, the deviations from macroscopic ion transport predictions and their possible causes are highlighted.

It is hoped that this work, when taken in conjunction with experimental studies, will not only help to extend the general continuum-based transport equations to cover nanoscale transport phenomena but will also help to improve the MD force-fields to enable predictions with greater accuracy.