The diffusion mechanism and coefficient of water confined in carbon nanotubes of diameter ranging from 8 to 54Å were studied by molecular dynamics simulations. It was found that the motions of water molecules inside the carbon nanotubes of diameter smaller than 12.2Å follow a two-stage diffusion mechanism. Initially, the water diffusion exhibits a long-time super- or sub-diffusion mechanism, and thereafter it transits to the single-file type inside the (6,6) carbon nanotube and shifts to the Fickian type inside the larger carbon nanotubes. As for the carbon nanotubes of diameter larger than 12.2Å, the diffusion of the confined water occurred through the Fickian mechanism, which was identical to that of the bulk water. The simulation results further revealed that the diffusion coefficient of the confined water was non-monotonically dependent upon the diameter, which couldbe ascribed to the double-edged effect of carbon nanotubes, i.e., the surface effect and the size effect.

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

Figure 1

Diffusion of H2O in C nanotube

White circles: armchair, black circles: zig-zag