Enhancement of Water Permeation across a Nanochannel by the Structure outside the Channel

We used molecular dynamics simulation to study the effect of the external structure on water permeation across a single-walled nanochannel. In contrast with the macroscopic scenario, the outside structure greatly affects the water transport across the nanochannel. Remarkably, the ratio of maximal to minimal flux reached a value of about two for different outside structures. These findings are expected to be helpful in design of high-flux nanochannels and provide an insight into the contribution of the lipid membrane to water permeation across biological water channels.

Figure 1

Snapshots of the simulation systems. The dark green structures are the nanotube and the graphite sheet; the red and white pillars are representative of oxygen and hydrogen atoms in water molecules. (a) Side-view of the type 1 system. The red solid line denotes the position of the middle of the single-walled carbon nanotube. (b) Type 2 system. Two symmetrical sheets with a distance $\xi$; there is a vacuum space between two sheets.

Figure 2

Water flow, flux, and the depth of the potential barrier $\Delta P_{\mathrm{WS}}$ (a) and Flip frequency $f_{\mathrm{flip}}$ (b) for the system of type 1 with $\zeta =0$ and different systems of type 2, respectively. For simplicity, the points at $\xi =0$ correspond to the type 1 system with $\zeta =0$. In (a), the flow, flux and $\Delta P_{\mathrm{WS}}$ of type 3 (in magenta) and type 4 (in cyan) are shown as half-filled circle, square, and star. In (b), the $f_{\mathrm{flip}}$ is in magenta and cyan, respectively.

Figure 3

Interactions $P_{\mathrm{WS}}$ between water inside the SWNT and water outside the channel in the membrane region, carbon nanotube, and carbon sheets in the membrane region, in a type 1 system with $\zeta =0\AA$ (magenta); type 2 systems with $\xi =3.6\AA$ (cyan), 5.1 Å (blue), 6.1 Å (red) and 10.8 Å (black); type 4 system (green), respectively, (A). Relationship of the net flux with the depth of the barrier $\Delta P_{\mathrm{WS}}$ in the potential profiles (B).