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 equilibri um behavior in the capillary condensation of fluids in disordered mesoporou s solids. Using mean-field density functional theory, we show that a simple lattice-gas model can reproduce the major experimental observations and th at the classical van der Waals picture of metastability fails due the appea rance of many metastable states. We find that (i) a true equilibrium capill ary phase transition may occur when the perturbation induced by the solid i s sufficiently small; (ii) hysteresis does not necessarily imply the existe nce of this phase transition; (iii) the disappearance of the hysteresis loo p is not associated with capillary criticality; and (iv) thermodynamic cons istency is violated along the adsorption-desorption isotherms.