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 Ornstei n-Zernike equations. We consider molecular models in which the porous medium is described by quenched disordered confirmations of spheres an d 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 expect ed to be important to describe recent experimental observations. We us e the replica method to derive the expressions relating the thermodyna mic properties of the fluid inside the porous material to the pair dis tribution functions within the mean-spherical approximation and the op timized random-phase approximation (ORPA). We also consider higher-ord er corrections within the optimized cluster theory developed by Anders en and Chandler for bulk fluids. In most cases a vapor-liquid coexiste nce curve, similar to that observed for the bulk fluid, although displ aced and somewhat narrowed, is obtained. The improved ORPA+B-2/EXP app roximation also predicts the appearance of a second fluid-fluid phase transition at a lower temperature. (C) 1997 American Institute of Phys ics.