Promoting desalination performance of carbon nanotubes through cationic and anionic surface modifications

Carbon nanotubes (CNTs) hold great promise for the design of novel desalination devices. However, the per-formance of pristine CNTs is very limited because of the trade-off between ion selectivity and water permeability, and thus the surface modification could be an effective strategy to promote the performance. Although great effort has been made on the CNT modifications in both experiments and simulations, most of the previous studies focus on the CNT entrance or exterior modification, while the interior modification remains poorly understood. In this work, a series of molecular dynamics simulations demonstrate that the modification of CNT interior with charged residues of either COO- or NH3+ can significantly promote the desalination performance. Specifically, even under a high pressure, the ion rejections can attain a high level of around 90 % that can reach drinkable standards at certain charge densities. The two systems exhibit dynamical bifurcations because of the discrepancy in the ion-residue interaction, where the water flux and ion rejection for NH3+ system are always greater. Consequently, the inner surface modification of CNTs could open a new avenue to break the trade-off between ion selectivity and water permeability, which should have great implications for the design of novel desalination devices.