Effects of quantum correction on the density profile, adsorption, and phase behavior of confined hydrogen and deuterium fluids in small systems: A DFT study

Abstract

The density functional theory based on the effective Wigner-Kirkwood potential is employed to investigate the effects of quantum correction on the structure, adsorption, and phase behavior of hydrogen and deuterium fluids confined in nano-spherical pores. Results show that quantum correction in the classical method for hydrogen and deuterium fluids influences all their thermodynamic properties when confined in nano-pores. Moreover, they indicate that the repulsion between quantum-mechanical molecules and intermolecular energy effects play important roles in the structure and thermodynamic properties of confined fluids before they come into contact at low temperatures or densities. The wall pressure of the fluid at the wall of the pore increases but the height of the profile in the center of the pore decreases with increased quantum effect, or the de Boer's quantumness parameter Λ. At low temperatures and low density limits, the adsorption and structure of the fluid in the pore are determined by the intermolecular energy effect; this will be reversed for high density limits in which the entropy effect is the dominant factor. It is shown in this study that, at low bulk density limits, the rising trend of adsorption takes a more steep slope in the classical fluid than in the deuterium one and that it is steeper in the deuterium fluid than it is in the hydrogen one; this behavior is reversed for high bulk density limits. Finally, it is shown that the critical temperature decreases with increasing Λ. Also, the locus of the phase transition is shown to shift toward higher values of bulk density as a result of increasing values of quantum effect, Λ.