Anomalous water transport in narrow-diameter carbon nanotubes

Carbon nanotubes (CNTs) mimicking the structure of aquaporins support fast water transport, making them strong candidates for building next-generation highperformance membranes for water treatment. The diffusion and transport behavior of water through CNTs or nanoporous graphene can be fundamentally different from those of bulk water through a macroscopic tube. To date, the nanotube-length-dependent physical transport behavior of water is still largely unexplored. Herein, on the basis of molecular dynamics simulations, we show that the flow rate of water through 0.83-nm-diameter (6,6) and 0.96-nm-diameter (7,7) CNTs exhibits anomalous transport behavior, whereby the flow rate increases markedly first and then either slowly decreases or changes slightly as the CNT length l increases. The critical range of l for the flow-rate transition is 0.37 to 0.5 nm. This anomalous water transport behavior is attributed to the l-dependent mechanical stability of the transient hydrogen-bonding chain that connects water molecules inside and outside the CNTs and bypasses the CNT orifice. The results unveil a microscopic mechanism governing water transport through sub-nanometer tubes, which has important implications for nanofluidic manipulation.

Automated summary

This study reveals the anomalous transport behavior of water through carbon nanotubes (CNTs) based on molecular dynamics simulations. The flow rate of water through CNTs with specific diameters exhibits a marked increase followed by a decrease or slight change as the CNT length increases. This behavior is attributed to the mechanical stability of the transient hydrogen-bonding chain connecting water molecules inside and outside the CNTs.