Confinement Effects on Water Clusters Inside Carbon Nanotubes

The effects of confinement on water clusters inside nonmetallic carbon nanotubes with radii ranging between 4 and 7.5 angstrom have been computationally investigated by means of global optimization and finite temperature simulations. The water-water interaction is described by the TIP4P rigid body potential, and a Lennard-Jones potential is used for the water-carbon interaction. Water clusters containing up to 20 molecules are found to form ID chainlike configurations for the narrow (7, 5) nanotube and 2D ladderlike structures in the (7, 6) tube. In wider tubes, 3D configurations are then formed showing helical motifs, ringlike or closed cage structures, before the most stable structure on flat graphene is eventually found. The same results are obtained by replacing the fully atomistic water-nanotube potential by its continuous approximation [Breton, J.; Gonzalez-Platas, J.; Giradet, C. J. Chem. Phys. 1994, 101, 3334], indicating a negligible effect of corrugation. The effects of additional nanotubes were also considered with the adsorption energies being found to converge rather quickly already for the triple-wall tube. Parallel tempering Monte Carlo simulations of the water octamer reveal a counterintuitive decrease in the melting point relative to the free-standing case. Molecular dynamics simulations show that melting is concomitant with some axial diffusion of the water molecules, and with radial diffusion perpendicular to the tube axis remaining limited. In accordance with previous studies concerned with bulk water, the weakening of the cluster thermal stability is interpreted as being caused by the hydrophobic character of the carbon-water interaction.