Determination of the Phase Region and Two-Phase Equilibrium of the Fluid in Pore Spaces

Vapor–liquid separation of a fluid in slit-like and cylindrical pores with unchanged walls has been studied. Calculations were performed based on the lattice gas model in a quasi-chemical approximation, reflecting the effects of direct correlations of interacting particles. The interaction of particles is described by a pair potential of the Lennard-Jones type. The state of coexisting liquid and vapor in a pore was determined from the condition of thermodynamic equilibrium, including chemical, thermal, and mechanical equilibrium. It was shown for the first time that in small systems bounded by unchangeable solid walls, phases in direct contact with the walls are parts of the system, including the homogeneous region and the corresponding transition region between it and the solid wall. Between the various heterogeneous components of a liquid–vapor system, mechanical equilibrium must be maintained when they are in thermodynamic equilibrium. We studied the effects of the choice of the fluid phase region in pores existing in mechanical equilibrium on the molecular distributions in the pore; the surface tension at the liquid–solid wall, vapor–solid wall, and liquid–vapor boundaries; and the contact angle of the vapor–liquid meniscus at the surface of the pore walls.