Influence of system size on the properties of a fluid adsorbed in a nanopore: Physical manifestations and methodological consequences

We consider the theoretical description of a fluid adsorbed in a nanopore. Hysteresis and discontinuities in the isotherms in general hampers the determination of equilibrium thermodynamic properties, even in computer simulations. A proposed way around this has been to consider both a reservoir of small size and a pore of small extent in order to restrict the fluctuations of density and approach a classical van der Waals loop. We assess this suggestion by thoroughly studying through density functional theory and Monte Carlo simulations the influence of system size on the equilibrium configurations of the adsorbed fluid and on the resulting isotherms. We stress the importance of pore-symmetry-breaking states that even for modest pore sizes lead to discontinuous isotherms and we discuss the physical relevance of these states and the methodological consequences for computing thermodynamic quantities.