Molecular dynamics simulations were performed in order to understand the extent to which the presence of a few oxygenated active sites, modeled as carbonyls, affected the transport properties of confined water. The model for the carbon nanotube was not intended to be realistic, but served to aid 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 Fickian type for sufficiently large hydration levels, became faster as the number of confined water molecules increased, reached a maximum, and slowed as water filled the carbon nanotubes. This was explained in terms of two collective motions, deduced from the analysis of sequences of simulation snapshots. The mechanisms were termed ‘cluster-breakage’ and ‘cluster-libration’. It was observed that the cluster-breakage mechanism produced longer displacements of the confined water molecules than did cluster-libration, but deactivated as the water filled the carbon nanotube. The results were particularly important; showing that, at low hydration levels, the presence of only 8 carbonyl groups could prevent the diffusion of water through (8,8) carbon nanotubes and that 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.

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