The mutual effects of two crucial features of carbon nanotubes (surface and confinement) upon the temperature-dependent water diffusion were studied via molecular dynamics simulations. A two-stage diffusion mechanism was detected in the carbon nanotubes of diameter smaller than 12.2, which became obscure as the temperature increased. This peculiar phenomenon could be attributed to the cooperation of the small confinement and the periodic surface. The diffusion coefficient of the confined water exhibited a non-monotonic dependence upon the confinement size and an unexpected increase inside the large carbon nanotubes (compared to that of bulk water). These anomalous behaviors could be attributed to the competition of the smooth surface and the small confinement. Considering the mutual effects, an empirical formula was proposed on the basis of two groups of numerical examples, whose results indicated that the confinement effect would dominate the surface effect until the carbon nanotube diameter increased up to ∼16, whereas thereafter the surface effect became dominant and finally both of them gradually vanished.

Water Diffusion inside Carbon Nanotubes: Mutual Effects of Surface and Confinement. Zheng, Y.G., Ye, H.F., Zhang, Z.Q., Zhang, H.W.: Physical Chemistry Chemical Physics, 2012, 14[2], 964-71