Recent studies have shown a rich phase behavior associated with freezing and melting of host phases in porous materials. Depending on the adsorbate and the confining solid matrix, the transition temperature may be lowered or raised relative to the bulk value, and new surface-driven phases may intervene between the liquid and solid phases in the pore. We report experimental measurements and molecular simulation results of the freezing and melting behavior of carbon tetrachloride and krypton confined within pores of cylindrical geometry, specifically carbon nanotubes and silica MCM-41 of different pore sizes. Dielectric relaxation spectroscopy was used to determine the experimental melting points of confined carbon tetrachloride, and molecular simulations were performed using grand canonical and parallel tempering Monte Carlo techniques.

The transition temperatures and the structure of the confined phases are determined for pore sizes up to 5 nm. Our results for the largest carbon nanotube show that the adsorbate layers near the pore walls freeze at temperatures higher than the bulk freezing point, whereas the adsorbate in the inner regions of the pore experience a depression in the freezing temperature when compared to the bulk value. In contrast, only one transition temperature well above the bulk freezing point was obtained for carbon tetrachloride within a carbon nanotube with a diameter of 2.9 nm. The simulation results are in good agreement with the experimental measurements, and both suggest the presence of several inhomogeneous confined phases which are found to be stable over extended temperature ranges.

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1. Confinement effects by means of dielectric methods