Pore Size Dependence of Melting Point for Kr Confined in Crystalline Carbon Pores

摘要

To elucidate the origin of the large depression in the melting point of Kr in the crystalline carbon pores, we examined the pore-size dependence of the melting point for the Kr confined in the hexagonally shaped pores of turbostratic carbon, compared with the cylindrical pores of amorphous silica of comparable size, by means of X-ray diffraction (XRD). For the amorphous silica pores of cylindrical shape with radius R, the melting-point depression of the confined solid Kr showed a usual 1/R dependence. However, for the solid Kr confined to the crystalline carbon pores, a plot of the melting-point depression against 1/R seems to cross the ordinate at a finite positive value. This strongly suggests that a bulk component of the confined solid has a melting point lower than that for a bulk solid. In accord with this observation, simulation studies (Maddox, M. W.; Gubbins, K. E. J. Chem. Phys. 1997, 107, 9659-9667; Kanda, H.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 8529—8535) have suggested that, in the cylindrical pore of graphitic carbon wall, fluid molecules can no longer take up their preferred close packed solid structure at low temperatures but are instead forced into a different, higher entropy solid structure, which is less stable than the bulk solid, due to the geometrical constraints in pore shape. However, the XRD pattern observed clearly indicates that the solid Kr confined in the crystalline carbon pores as well as the amorphous silica pores takes the same face-centered cubic structure as in the bulk solid.