Phase behavior of confined fluids adsorbed in na-nopores differs significantly from their bulk coun-terparts and depends on the chemical and structural properties of the confining structures. In general, phase transitions in nanoconfined fluids are reflect-ed in stepwise adsorption isotherms with a pro-nounced hysteresis. Here, we show experimental evidence and in silico interpretation of the reversi-ble stepwise adsorption isotherm which is observed when methane is adsorbed in the rigid, crystalline metal-organic framework IRMOF-1 (MOF-5). In a very narrow range of pressures, the adsorbed fluid undergoes a structural and highly cooperative re-construction and transition between low-density and high-density nanophases, as a result of the competition between the fluid-framework and flu-id-fluid interactions. This mechanism evolves with temperature: below 110 K a reversible stepwise iso-therm is observed, which is a result of the bimodal distribution of the coexisting nanophases. This temperature may be considered as a critical temper-ature of methane confined to nanopores of IRMOF-1. Above 110 K, as the entropy contribution in-creases, the isotherm shape transforms to a common continuous S-shaped form that is characteristic to a gradual densification of the adsorbed phase as the pressure increases.
Details on simulation methodology used in this work, additional simulation results of adsorption isotherms, adsorption energy profiles, free energy maps, minimum energy surface, adsorption density maps, and details on synthesis and characterization of IRMOF-1 and gas adsorption measurements