Equilibrium and out-of-equilibrium (hysteretic) behavior of fluids in disordered porous materials: Theoretical predictions

We study the relation between out-of-equilibrium (hysteretic) and equilibrium behavior in the capillary condensation of fluids in disordered mesoporous solids. Using mean-field density functional theory, we show that a simple lattice-gas model can reproduce the major experimental observations and that the classical van der Waals picture of metastability fails due the appearance of many metastable states. We find that (i) a true equilibrium capillary phase transition may occur when the perturbation induced by the solid is sufficiently small; (ii) hysteresis does not necessarily imply the existence of this phase transition; (iii) the disappearance of the hysteresis loop is not associated with capillary criticality; and (iv) thermodynamic consistency is violated along the adsorption–desorption isotherms.