Hysteresis Phenomenon Coupled with Capillary Phase Transition of N ₂ /CH ₄ 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 N 2 /CH 4 mixtures in cylindrical carbon nanopores. While pure CH 4 and N 2 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 N 2 concentration increases, the capillary phase-transition pathway shifts from CH 4 -dominated condensation to co-condensation and co-evaporation of both components. Consequently, the apparent hysteresis critical pore diameter ( D hc ) exhibits a nonmonotonic dependence on composition, whereas the apparent hysteresis critical temperature ( T hc ) decreases monotonically with increasing N 2 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.