The mutual effects of two crucial features, surface and confinement, of carbon nanotubes upon temperature-dependent water diffusion were studied using molecular dynamics simulations. A two-stage diffusion mechanism was detected in nanotubes with a diameter smaller than 12.2Å, which became obscure as the temperature increased. This peculiar phenomenon could be attributed to 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 nanotubes (compared to that of bulk water). These anomalous behaviors could be attributed to competition between 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 nanotube diameter reached ∼16Å, whereupon the surface effect became predominant and both finally vanished gradually.

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