The diffusion mechanism and coefficient of water confined in carbon nanotubes with diameters ranging from 8 to 54Å were studied using molecular dynamics simulations. It was found that the motion of water molecules, inside nanotubes with a diameter smaller than 12.2Å, involved a two-stage diffusion mechanism. Initially, the water diffusion exhibited a long-time super- or sub-diffusion mechanism, and then changed to a single-file type inside the (6,6) nanotube and shifted to Fickian type inside larger nanotubes. In nanotubes with a diameter greater than 12.2Å, diffusion of the confined water occurred via the Fickian mechanism; which was identical to that of bulk water. The simulation results further revealed that the diffusion coefficient of the confined water was non-monotonically dependent upon the diameter. This was attributed to the double-edged influence of surface and size effects.

Nanoconfinement Induced Anomalous Water Diffusion inside Carbon Nanotubes. H.Ye, H.Zhang, Y.Zheng, Z.Zhang: Microfluidics and Nanofluidics, 2011, 10[6], 1359-64