Confinement of matter on the nanometre scale can induce phase transitions not seen in bulk systems. In the case of water, so-called drying transitions occur on this scale as a result of strong hydrogen-bonding between water molecules, which can cause the liquid to recede from nonpolar surfaces to form a vapour layer separating the bulk phase from the surface. Here we report molecular dynamics simulations showing spontaneous and continuous filling of a nonpolar carbon nanotube with a one-dimensionally ordered chain of water molecules. Although the molecules forming the chain are in chemical and thermal equilibrium with the surrounding bath, we observe pulse-like transmission of water through the nanotube. These transmission bursts result from the tight hydrogen-bonding network inside the tube, which ensures that density fluctuations in the surrounding bath lead to concerted and rapid motion along the tube axis. We also find that a minute reduction in the attraction between the tube wall and water dramatically affects pore hydration, leading to sharp, two-state transitions between empty and filled states on a nanosecond timescale. These observations suggest that carbon nanotubes, with their rigid nonpolar structures, might be exploited as unique molecular channels for water and protons, with the channel occupancy and conductivity tunable by changes in the local channel polarity and solvent conditions.

We study the system-size dependence of translational diffusion coefficients and viscosities in molecular dynamics simulations under periodic boundary conditions. Simulations of water under ambient conditions and a Lennard-Jones (LJ) fluid show that the diffusion coefficients increase strongly as the system size increases. We test a simple analytic correction for the system-size effects that is based on hydrodynamic arguments. This correction scales as N -1/3, where N is the number of particles. For a cubic simulation box of length L, the diffusion coefficient corrected for system-size effects is D 0 = D PBC + 2.837297k B T/(6πηL), where D PBC is the diffusion coefficient calculated in the simulation, k B the Boltzmann constant, T the absolute temperature, and η the shear viscosity of the solvent. For water, LJ fluids, and hard-sphere fluids, this correction quantitatively accounts for the system-size dependence of the calculated self-diffusion coefficients. In contrast to diffusion coefficients, the shear viscosities of water and the LJ fluid show no significant system-size dependences.

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