Statistical modeling of equilibrium phase transition in confined fluids

Abstract

The phase transition of confined fluids in mesoporous materials deviates from that of bulk fluids due to the former's interactions with the surrounding heterogeneous structure. For example, metal-organic frameworks (MOFs) create a strong heterogeneous field, so adsorbed fluids in MOFs have atypical phase characteristics such as capillary condensation and higher-order phase transitions. These characteristics are modeled by decoupling the host-guest and guest-guest interactions as a many-body problem in the presence of an external nonuniform field. To solve the three-dimensional Ising model, we use mean-field theory to approximate the guest-guest interactions and Mayer's (f)-functions to describe the host-guest interactions in a unit cell. Later, using Hill's theory of nanothermodynamics, we define differential and integral thermodynamic functions to describe confined fluids. These integral properties are then used to understand the phase transition in confined fluids. The investigation reveals a distinct behavior where fluids confined in larger pores undergo a discontinuous (first-order) phase transition, whereas those confined in smaller pores undergo a continuous (higher-order) phase transition. Furthermore, the results indicate that the free-energy barrier for phase transitions is lower in confined fluids than in bulk fluids, which helps explain the lower condensation pressure relative to the bulk saturation pressure. Finally, the integral thermodynamic functions are succinctly presented in the form of a phase diagram, marking an initial step toward a more practical approach for understanding the phase behavior of confined fluids.