Entrance resistance of water transport into carbon nanotubes: Insights from molecular dynamics simulations

Abstract

Water transport through nanoporous materials has aroused wide interest in membrane desalination, biological medicine and nanofluids, but the underlying mechanism of water filling an empty nanopore has been ignored. We performed non-equilibrium molecular dynamics (NEMD) simulations of water filling empty CNTs with lengths of 200 and 600 Å and diameters ranging from 8.14 to 20.35 Å, at temperatures from 280 to 360 K. It is found water molecules liberated from the bulk phase to enter the narrow CNTs need to break the hydrogen bond (HB) network and overcome an energy barrier, and these two effects together define the entrance resistance. In addition, the strong confinement induces the highly ordered water dipole distribution in the rather small CNTs, making additional contribution to the entrance resistance. In this connection, an efficient method is deduced to measure the entrance resistance. We show that the entrance resistance accounts for more than 94.9% of the total resistance including the entrance resistance and the resistance to mass transport inside the CNT. However, with increasing diameter from 8.14 to 20.35 Å, the water filling flux increases by a factor of 2 due to the dramatically reduced HB breaking effect and energy barrier. Further, when increasing temperature from 280 to 360 K, the filling flux increases by three-fold in the (10,10) CNT, associated with sharply reduced entrance resistance. It is understood that the reduced lifetime of HB at the interface has overcome the effect of enhanced energy barrier, which subsequently accounts for the reduced entrance resistance when the temperature is increased.