The hydrophobic interior of carbon nanotubes, which was reminiscent of ion channels in cellular membranes, has inspired scientific research directed towards the production of, for example, membranes for water desalination, drug-delivery devices, and nanosyringes. To develop these technologies it was crucial to understand and predict the equilibrium and transport properties of confined water. A series of molecular dynamics simulation results was presented here in order to understand the extent to which the presence of a few oxygenated active sites, modelled as carbonyls, affects the transport properties of confined water. The model for the carbon nanotube was not intended to be realistic. Its only purpose was to permit an understanding of the effect of a few oxygenated sites upon the transport properties of water confined in a narrow cylindrical pore, which was otherwise hydrophobic. At low hydration levels, little if any water diffusion was found. The diffusion, which appeared to be of the Fickian type for sufficiently large hydration levels, became faster as the number of confined water molecules increases, reaches a maximum, and slows as water fills the carbon nanotubes. The findings were explained on the basis of two collective motion mechanisms observed from the analysis of sequences of simulation snapshots. The two mechanisms were termed cluster-breakage and cluster-libration. It was observed that the cluster-breakage mechanism produced longer displacements for the confined water molecules than the cluster-libration one, but deactivated as water filled the carbon nanotube. From a practical point of view, the results were particularly important for two reasons: (1) at low hydration levels the presence of only eight carbonyl groups could prevent the diffusion of water through (8,8) carbon nanotubes; and (2) the extremely fast self-diffusion coefficients observed for water within narrow carbon nanotubes were significantly decreased in the presence of only a few oxygenated active sites. These results were relevant, for example, for the design of water-desalination membranes.

Water Self-Diffusion through Narrow Oxygenated Carbon Nanotubes. Striolo, A.: Nanotechnology, 2007, 18[47], 475704