Blockage of Water Flow in Carbon Nanotubes by Ions Due to Interactions between Cations and Aromatic Rings

Combining classical molecular dynamics simulations and density functional theory calculations, we find that cations block water flow through narrow (6,6)-type carbon nanotubes (CNTs) because of interactions between cations and aromatic rings in CNTs. In wide CNTs, these interactions trap the cations in the interior of the CNT, inducing unexpected open or closed state switching of ion transfer under a strong electric field, which is consistent with experiments. These findings will help to develop new methods to facilitate water and ion transport across CNTs.

Figure 1

(a) DFT adsorption energies ($\mathrm{\Delta }E$) between ${\mathrm{Na}}^{+}$ and CNT (black squares); DFT potential (blue line) fitted from those adsorption energies; classical charmm L-J potential (red line); total potential (green line) by incorporating the DFT potential into the classical CHARMM L-J potential. $z$ is the distance between ${\mathrm{Na}}^{+}$ and the center of the CNT along the tube’s $z$ axis. (b) Residual Hirshfeld charge [63] distribution of ${\mathrm{Na}}^{+}$ adsorbed in the center of a (6,6)-type CNT. (c) Snapshots of the modeling system, comprising a filter membrane (four CNTs) and two reservoirs with cation–$\pi$ interactions. (d) Cumulative water flow through the CNT membrane with (red) and without (green) cation–$\pi$ interactions. Na, Cl, O, and H atoms are shown as blue, green, red, and white spheres, respectively, and water molecules outside the tubes are shown as red lines. The time range of the binding process (0.4 to 2.5 ns) is highlighted by the shaded pink region.

Figure 2

(a) ${\mathrm{Na}}^{+}$ position and structure of the water shell in the binding process. (b) Number of ${\mathrm{Na}}^{+}$ ions bound at the entrance of the CNTs ($N_b$, black line), and the center of mass of ${\mathrm{Na}}^{+}$ along the $z$ direction ($z_c$, blue line) as a function of time. The time range of the binding process is highlighted by the blue shaded region.

Figure 3

(a) Cumulative water flow through the CNT driven by pressure as a function of simulation time in the system with (6,6)-type CNT tubes. Water flow for CNTs functionalized with $-{\mathrm{CH}}_2{\mathrm{CH}}_2-$ groups (blue line), the application of electric fields of 0.05 and $0.20\mathrm{V}{\mathrm{nm}}^{-1}$ (green and red lines, respectively), and pure water (black dashed line) are shown. (b) Distribution probabilities of ${\mathrm{Na}}^{+}$ in the NaCl solution along the $z$ direction. The blue shaded region denotes the location of the CNT. For comparison, an additional system without applying an electric field or functionalizing the ends of the CNTs (black solid line) is also shown.

Figure 4

Open or closed state switching of ion transfer in a (8,8)-type CNT by ${\mathrm{Na}}^{+}$. (a) Snapshots of the modeling system, comprising an (8,8)-type CNT and two solution reservoirs with cation–$\pi$ interactions. The trapped ${\mathrm{Na}}^{+}$ in the CNT channel is indicated by the arrow. (b) Number of ${\mathrm{Na}}^{+}$ ions in the CNT as a function of time under various electric fields. (c) ${\mathrm{Na}}^{+}$ trapping probability $P$ and the number of transport ${\mathrm{Na}}^{+}$ $N_t$ through the CNT in the 20-ns simulation trajectories with respect to electric field $E$.