The thermodynamic properties of liquids or compressed gases of light
atoms and molecules show deviations from classical predictions. These
light molecules does not obey the corresponding states principle. The
observed deviations are caused by so-called quantum effects.
The quantum effects are due to the two main reasons. First, the dis-
continuity of the energy levels at very high densities, when the motions
of particles are restricted (by the boundary conditions), and second, as a
result of the wave function symmetry effects, which occurs at very low tem-
perature (when only the number of particles allowed to share a quantum
state becomes important).
A quantum correction proposed by Deiters is based on the as-
sumption that each molecule is restricted to a cubic cell with a size de-
pending on the free volume. This model is a straightforward and easy
method for prediction of thermodynamic properties of quantum fluids at
high densities. However, the cell model is not suitable at low densities
or high temperatures, and the cubic form of the cell (which is applied for
mathematical convenience) is not a realistic assumption.
In the present work the quantum effects are considered by means of
a more realistic “spherical cell model”. This quantum correction can be
applied to any van der Waals type equation of state. A correction function
has been developed to overcome the weaknesses of the cell model at low
density or high temperature limit. The corrections are applied to the
Deiters equation of state, and the phase equilibria for pure
quantum fluids and binary mixtures are calculated. Calculations have been
made for the pure fluids hydrogen, neon, methane, and nitrogen as well as, for the binary mixtures neon–argon and neon–krypton. The results
of the spherical cell model for all cases show significant improvement in
comparison with the cubic cell model results.
Prediction of the thermophysical properties of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton by Monte Carlo simulation using ab initio potentials. Afshin Eskandari Nasrabad, Rozita Laghaei, Ulrich K. Deiters, J. Chem. Phys. 2004; 121, 6423.
Calculation of Phase Equilibria of Quantum Gases at High Pressures, Rozita Laghaei Universität zu Köln, Mathematische-Naturwissenschaftliche Fakultät, 2003.
