Phase diagrams of single-component fluids in disordered porous materials: Predictions from integral-equation theory

We present the calculation of phase diagrams for fluids in disordered porous materials using theories based on the replica symmetric Ornstein-Zernike equations. We consider molecular models in which the porous medium is described by quenched disordered confirmations of spheres and the fluid-fluid and matrix intermolecular potentials are the sum of a hard-sphere core and an attractive tail. Such models account for the combined effect of confinement, wetting, and disorder that are expected to be important to describe recent experimental observations. We use the replica method to derive the expressions relating the thermodynamic properties of the fluid inside the porous material to the pair distribution functions within the mean-spherical approximation and the optimized random-phase approximation (ORPA). We also consider higher-order corrections within the optimized cluster theory developed by Andersen and Chandler for bulk fluids. In most cases a vapor-liquid coexistence curve, similar to that observed for the bulk fluid, although displaced and somewhat narrowed, is obtained. The improved ORPA+B-2/EXP approximation also predicts the appearance of a second fluid-fluid phase transition at a lower temperature. (C) 1997 American Institute of Physics.