Water transport in double-walled carbon nanotubes

Water confined in nanoscale environments exhibits rich phase behaviors. Recently, double-walled carbon nanotubes (DWCNTs) have been found to be able to generate ice nanotubes with large sizes, but their dynamics are still not explored. In this work, upon using molecular dynamics simulations we systematically investigate the water transport in DWCNTs, focusing on the effect of CNT length and temperature. At low temperatures, with the increase in CNT length, the water flow decreases linearly first and then has a sudden reduction to zero because of the liquid-ice phase transition; while at high temperatures the water flow decreases linearly without freezing. Obviously, the critical CNT length for the formation of ice increases at higher temperatures, because longer CNTs facilitate the ordered water structures. Furthermore, for a short CNT the water flow exhibits a perfect linear increase with the temperature; while for long CNTs, the water flow is zero at low temperatures because of the ice phase, and then increases suddenly. Apparently, the critical melting temperature shifts to higher values for longer CNTs. The translocation time has an overall opposite behavior compared to the water flow, and the water occupancy and dipole orientation are also sensitive to the CNT length and temperature. Our results reveal the relationship between water structures and dynamics within DWCNTs, and should have great implications for the design of novel nanofluidic devices.