Demixing and Surface-induced Population Inversion of a Binary Gaussian Mixture in Nanopores

We investigate the demixing and the surface-induced population inversion of a binary Gaussian mixture, which exhibits the fluid-fluid demixing transition of the bulk system, in a nanopore. The result shows that the population inversion is the typical shift of the first-order fluid-fluid demixing transition due to the confinement effect. The population inversion line is affected by the cross-interaction between unlike species sigma 12, the mole fraction of species x(2), and the slit width H. At a low cross-interaction between unlike species, the population inversion line for x(2) < x(c) is shifted toward a lower density with decreasing the slit width, where xc is the critical consolute point at the bulk phase, while the population inversion is not observed for x(2) > x(c). At a high cross-interaction, for x(2) < x(c), the population inversion line is shifted toward a higher density with decreasing slit width. In this case, the population inversion line is found at a higher density compared with the bulk mixing transition line because species 1 in a nanopore is favored. However, for x(2) > x(c), it is shifted toward a lower density.