Molecular Cages in Supercritical Fluids at High Pressures

By considering Frenkel's point of view regarding the concept of relaxation time, every molecule of a fluid possesses an effective space. We name this permissible and accessible space of each molecule of fluid as the molecular cage (MC). All motions of the molecule, including translational, rotational, and vibrational motions, take place within this space. In this paper, by using the new concept of the thermodynamic dimension (DT) and the generalized Debye model for the isochoric heat capacity (CV) of matter in different states, we want to indicate at high pressures the MCs confine the internal molecular vibrations. By calculation of CV of supercritical fluid (SCF), the restriction of molecular vibrations is detected. To investigate the effect of molecular cages, we have studied CV of diatomic SCFs including nitrogen, oxygen, and carbon monoxide.