Freezing/melting behaviour within carbon nanotubes

We report a study of the freezing and melting of fluids confined within multi-walled carbon nanotubes with an internal diameter of 5 nm, using experimental measurements and molecular simulations. Dielectric relaxation spectroscopy was used to determine the experimental melting points and relaxation times of nitrobenzene and carbon tetrachloride within carbon nanotubes, and parallel tempering Monte Carlo simulations in the grand canonical ensemble were performed for confined carbon tetrachloride. The simulations show that the adsorbate forms concentric layers that solidify into quasi-two-dimensional hexagonal crystals with defects; highly defective microcrystalline regions are formed in the inner layers, owing to the strong geometrical constraints. Our simulations show no formation of common three-dimensional crystalline structures (fcc, hcp, bcc, sc or icosahedral) in confinement. The results suggest the presence of inhomogeneous phases (i.e., combinations of crystalline and liquid regions) within the pore over extended temperature ranges. Our results indicate that the outer layers of adsorbate solidify at temperatures slightly higher than the bulk freezing point, whereas the inner layers freeze at lower temperatures. The simulation results are in good agreement with the experimental measurements.