Hysteresis Phenomenon Coupled with Capillary Phase Transition of N2/CH4 Binary Mixtures in Cylindrical Nanopores.

Capillary condensation/evaporation-induced hysteresis is a fundamental characteristic of confined fluid-phase transitions. Although extensively studied for pure fluids, the hysteresis behavior of confined fluid mixtures remains poorly understood. Here, grand canonical Monte Carlo-molecular dynamics (GCMD) simulations are employed to investigate adsorption-desorption hysteresis of N2/CH4 mixtures in cylindrical carbon nanopores. While pure CH4 and N2 exhibit the expected pore-size- and temperature-dependent hysteresis behavior, binary mixtures show strongly composition-dependent phase transition characteristics. Competitive adsorption significantly modifies the hysteresis behavior and can either suppress or enhance the metastable capillary phase transitions. As the N2 concentration increases, the capillary phase-transition pathway shifts from CH4-dominated condensation to co-condensation and co-evaporation of both components. Consequently, the apparent hysteresis critical pore diameter (Dhc) exhibits a nonmonotonic dependence on composition, whereas the apparent hysteresis critical temperature (Thc) decreases monotonically with increasing N2 content. These findings demonstrate that adsorption-desorption hysteresis in confined fluid mixtures cannot be inferred from pure-fluid behavior alone but instead emerges as a collective phenomenon governed by competitive adsorption and composition redistribution, which fundamentally alters both the capillary phase-transition pathway and the apparent hysteresis criticality.