Heat capacity behaviour of pore confined benzene and hexafluorobenzene in NaY zeolite

Abstract

The heat capacities of benzene, hexafluorobenzene and of its 1 : 1 binary mixture adsorbed on NaY zeolite were measured as a function of temperature for 120 K ≤ T ≤ 370 K. They reveal strong loading dependencies and reach extraordinarily high values at intermediate loadings. They are indicative of an energy gap above the ground state or of a state in which soft anharmonic intermolecular modes of the dilute adsorbate are excited. The absence of any phase transitions over a large temperature and coverage range resembles supercritical behaviour, as was predicted in recent theoretical work. At a certain loading, small heat capacity fluctuations reveal subtle structural instabilities which are possibly related to the formation and break-up of aggregates, phenomena which are also expected for matter near the critical point.