ORNSTEIN-ZERNIKE EQUATIONS AND SIMULATION RESULTS FOR HARD-SPHERE FLUIDS ADSORBED IN POROUS-MEDIA

In this paper we solve the replica Ornstein-Zernike (ROZ) equations in the hypernetted-chain (HNC), Percus-Yevick (PY), and reference Percus -Yevick (RPY) approximations for partly quenched systems. The ROZ equa tions, which apply to the general class of partly quenched systems, ar e here applied to a class of models for porous media. These models inv olve two species of particles: an annealed or equilibrated species, wh ich is used to model the fluid phase, and a quenched or frozen species , whose excluded-volume interactions constitute the matrix in which th e fluid is adsorbed. We study two models for the quenched species of p articles: a hard-sphere matrix, for which the fluid-fluid, matrix-matr ix, axid matrix-fluid sphere diameters sigma11, sigma00, and sigma01 a re additive, and a matrix of randomly overlapping particles (which sti ll interact with the fluid particle as hard spheres) that gives a ''ra ndom'' matrix with interconnected pore structure. For the random-matri x case we study a ratio sigma01/sigma11 of 2.5, which is a demanding o ne for the theories. The HNC and RPY results represent significant imp rovements over the PY result when compared with the Monte Carlo simula tions we have generated for this study, with the HNC result yielding t he best results overall among those studied. A phenomenological percol ating-fluid approximation is also found to be of comparable accuracy t o the HNC results over a significant range of matrix and fluid densiti es. In the hard-sphere matrix case, the RPY is the best of the theorie s that we have considered.