On the double phase-transition of fluids adsorbed in disordered porous media

The phase transition of a fluid adsorbed in a heterogeneous system is studied within a mean-field approach to the lattice gas model with quenched impurities. Despite the different origin of the heterogeneity (spatial variation of binding energy or fluid coordination numbers), the fluid can undergo two phase transitions if the hosting system is sufficiently heterogeneous. It is clearly shown that such a coexistence of two liquids with different coordination numbers results from the successive condensations in distinct spatial regions of the host. We have found precise conditions at which one or two phase transitions occur. Alternative scenarios involving the pore blocking and matrix reaction to the fluid adsorption are briefly discussed.