The density functional theory has been employed to investigate the population inversion, selective adsorption, and demixing of confined mixture fluids in a spherical nanocavity. In the case of hard sphere fluids for which only the entropy effect has the dominant role, the selective adsorption process strongly depends on size ratio, population of the adsorbed component, and pore size. The effects of such parameters as interaction strength, size ratio, and thermodynamic state on population inversion and selective adsorption have been investigated for L-J mixture fluids. The results for L-J asymmetric binary mixture fluids indicate that the mole fraction of large species (molecules with bigger radii) inside the cavity becomes greater with increasing size ratio or with decreasing temperature than does that for the other component despite its lower population in the bulk fluid (i.e., the so-called population inversion phenomenon). Our results indicate that the inversion population density decreases with size ratio, and the mole fraction of the component with the bigger radius in the pore increases with temperature. Thus, by selecting a small spherical cavity under special conditions, it will be possible to give rise to the selective separation of component 2 in spite of its lower concentration in the bulk asymmetric L-J mixture. Finally, we have investigated the phase separation, demixing phenomenon, of an asymmetric L-J mixture inside a spherical cavity. Also we investigated the cases for which the layered demixing phenomenon occurs in the asymmetric L-J fluid in a nanospherical pore as a result of the difference between the entropy and energy effects.