Size effect in determining the water diffusion rate in carbon nanotubes

Abstract

Water diffusion in nanochannels is the basis of a variety of emerging nanotechnologies. The self and transport diffusions of water in carbon nanotubes (CNTs) having different pore sizes were investigated using equilibrium (EMD) and non-equilibrium molecular dynamics (NEMD) simulations. It is found that the formation of a depletion layer near the carbon wall in which water molecules lost hydrogen bonds accounts for the transforming of the self-diffusion mode. The reduced life time of hydrogen bond networks in larger CNTs stimulates the decorrelation of single water molecule from the center of mass of fluid, leading to the enlarged deviation between the self and transport diffusions. Our simulations reveal that the friction coefficient for water transporting in the CNT is enhanced by more than 60 times when the diameter of the CNT is increased from 8.1 to 33.9 Å, leading to the transforming of the transport diffusion mode from ballistic into the Fickian mode. We show the smoothness of the CNT wall enhances the water flowrate in the CNT by orders of magnitude with respect to the value predicted by the Hagen-Poiseuille equation. However, this enhancement factor decreases from around 10,000 to 10 when the pore size of the CNT increases. We speculate that water transport in different CNTs experience different diffusion modes could be the explanation for the large discrepancies among the measured water flowrates in different studies. Our simulations also show that for the non-Fickian diffusion, the transport diffusivity calculated from EMD simulations is no longer effective in connecting the driving force and the driven flowrate.