Nonadditive penetrable mixtures in nanopores: surface-induced population inversion

We investigate the surface-induced population inversion of the nonadditive penetrable mixtures which exhibits the fluid-fluid demixing transition of the bulk system due to the confinement effect. The result shows that the population inversions are strongly affected by the extra repulsion between unlike species, the mole fraction of species, the width of nanopores, and the nonadditive walls. The extra repulsion between unlike species in a confined system increases the contact density of both species at the wall and promotes the population inversion in nanopores. The population inversion is the typical shift first-order fluid-fluid demixing transition due to the confinement effect in nanopores. The population inversions are only observed in nanopores with finite widths. The population inversion line is shifted toward a higher fluid density with decreasing width of the nanopores and lies slightly in lower density compared with the coexistence curves of the bulk system. The nonadditive wall for the big particles leads to the population inversion in lower density compared with that of the nonadditive wall for the small particles. The population inversion line is terminated at a lower mole fraction.