Quantum effect on adsorption of fluids in slit pores: A DFT study

Abstract

In this paper, the Wigner-Kirkwood quantum theory and the Weeks-Chandler-Andersen (WCA) effective diameter method in Classical DFT are employed to account for the effects of quantum corrections on the Helmholtz free energy of bulk and confined fluids. Initially, experimental data are exploited toward accurate prediction of pressure-volume isotherms and vapor-liquid equilibrium in bulk fluids. In the second part, the effects of quantum correction on the structure and thermodynamic properties of fluids confined in slit pores are investigated. Our results show that quantum effects may come into account by considering the effective size of the particles and also intermolecular interaction which both of them have direct effects on the density distribution of molecules in the slit pore. Furthermore, the excess adsorption of fluids at a fixed bulk density is shown to increase with quantum effect, but this trend is reversed with increasing wall fluid interaction because of the entropy and energy effects. Also, it is found that the excess adsorption and interfacial tension of the fluid against the solid surface of fluids exhibit oscillatory behaviors with maximum and minimum values for half integer and integer values of pore width. This oscillatory behavior disappears at high values of pore width with increasing quantum effect. Another finding of the present study indicates that the interfacial tensions of the fluid against the solid surface for a classical fluid are negative but quantum effect increases it and shifts its sign to positive. Finally, it is shown that the locus of phase transition is shifted toward higher bulk densities as a result of increasing quantum effect.