Analogy in capillary condensation mechanism across simple nanoconfinements

In this molecular simulation study, we unveil unprecedentedly analogous features of capillary condensation in nanoconfined fluids across comparable surface substrates and confinement dimensions. Through systematic Monte Carlo simulations in both grand canonical (GCEMC) and meso-canonical (MCEMC) ensembles, we examine the equilibrium vapor-liquid phase transition of argon at its normal boiling point inside graphitic slit and cylindrical nanoconfinements of three distinct configurations: periodic-length, finite-length, and closed mesopores. By analyzing local density distributions and particle number fluctuations, we identify consistent thermodynamic signatures and distinct microscopic mechanisms that control vapor-liquid transitions across these configurations. Specifically, we find that both metastable and unstable condensation begin at the same pressure in periodic and finite pores, while similar evaporation and unstable liquid-phase transitions occur at matched pressures in periodic and closed pores. Most significantly, at equilibrium phase coexistence, the structural properties of liquid-like bridges and adsorbed layers are statistically identical across all configurations, regardless of pore length or end effects. These findings provide a unified microscopic picture of capillary condensation and offer new insights into how confinement configuration shapes phase transitions in nanoporous materials.