Carbon nanotubes showed exceptional physical properties that render them promising candidates as building blocks for nanostructured materials. Many ambitious applications, ranging from gene therapy to membrane separations, require the delivery of fluids, in particular aqueous solutions, through the interior of carbon nanotubes. To foster these and other applications, it was necessary to understand the thermodynamic and transport properties of water confined within long narrow carbon nanotubes. Previous theoretical work considered either short carbon nanotubes or short periods of time. By conducting molecular dynamics simulations in the microcanonical ensemble for water confined in infinitely long carbon nanotubes of diameter 1.08nm, it was shown here that confined water molecules diffused through a fast ballistic motion mechanism for up to 500ps at room temperature. By comparing the results obtained for the diffusion of water to those obtained for the diffusion of a reference Lennard-Jones fluid, it was proved here that long-lasting hydrogen bonds were responsible for the ballistic diffusion of water clusters in narrow carbon nanotubes, as opposed to spatial mismatches between pore - fluid and fluid - fluid attractive interactions which, as shown previously by others, were responsible for the concerted motion of simple fluids in molecular sieves. It was proved here, for the first time that, in spite of the narrow diameter of the carbon nanotubes considered which may suggest the existence of single-file diffusion, when the trajectories of confined water were studied at time-scales exceeded 500ps, a Fickian-type diffusion mechanism prevailed.

The Mechanism of Water Diffusion in Narrow Carbon Nanotubes. Striolo, A.: Nano Letters, 2006, 6[4], 633-9