Thermodynamics of Fluids in Disordered Porous Materials

Abstract

When a fluid is adsorbed in micro or meso-porous materials such as aerogels or porous glasses, its thermodynamic behavior is considerably altered. The changes result from the combined effect of geometric confinement, randomness of the matrix, and connectivity of the pore network, whose understanding requires a good theoretical description of the matrix microstructure and its interaction with the fluid particles. We present a liquid-state formalism in which the fluid-matrix system is modeled as a ‘quenched-annealed’ binary mixture formed by the matrix particles (quenched species) and the fluid particles (annealed species); the averaging over quenched disorder is handled via the replica method. Model calculations for the phase diagram of the confined fluid within the mean spherical approximation show that for sufficiently attractive matrix-fluid interaction, the liquid-gas transition has a higher critical density than that of the bulk fluid and is accompanied by an additional “pre-condensation” transition. In addition, the results obtained within various standard approximation schemes suggest that the behavior of the fluid at its liquid-gas critical point is the same as that of the random field Ising model (RFIM).

Key Words:

• Thermodynamics, porous materials
• quenched-annealed systems