The authors have used atomistic molecular dynamics simulations to study the structure and dynamics of water molecules inside an open ended carbon nanotube placed in a bath of water molecules. The size of the nanotube allows only a single file of water molecules inside the nanotube. The water molecules inside the nanotube showed solid like ordering at room temperature, which they quantify by calculating the pair correlation function. It was shown that even for the longest observation times, the mode of diffusion of the water molecules inside the nanotube was Fickian and not sub-diffusive. They also propose a one-dimensional random walk model for the diffusion of the water molecules inside the nanotube. They find good agreement between the mean-square displacements calculated from the random walk model and from molecular dynamics simulations, thereby confirming that the water molecules undergo normal mode diffusion inside the nanotube. They attribute this behavior to strong positional correlations that cause all the water molecules inside the nanotube to move collectively as a single object. The average residence time of the water molecules inside the nanotube was shown to scale quadratically with the nanotube length.

Strong Correlations and Fickian Water Diffusion in Narrow Carbon Nanotubes. Mukherjee, B., Maiti, P.K., Dasgupta, C., Sood, A.K.: Journal of Chemical Physics, 2007, 126[12], 124704