Molecular Analysis of the Disjoining Pressure Concept in Two-Phase Porous Systems

A molecular analysis of two-phase distributions within slit-shaped and cylindrical pores with homogeneous walls has been conducted across a wide range of pressures and temperatures, where both liquid and vapor phases coexist. The pore walls are assumed to be nondeformable, acting as an external field for the fluid. The equilibrium states of both phases—“liquid in pore” and “vapor in pore”—satisfy the equality of their chemical potentials with that of the bulk phase and with each other. Molecular distributions were calculated using the lattice gas model (LGM) with a simplified nearest-neighbor pairwise interaction potential. The distribution of molecular density within the inhomogeneous field of the pore walls is associated with two types of pressure: isothermal pressure, linked to the system’s chemical potential, and internal mechanical pressure (or expansion pressure in LGM terminology). The difference between each of these pressures and the corresponding pressure in the bulk phase defines the disjoining pressure. This parameter is widely employed in the thermodynamic interpretation of thin film properties as a function of film thickness. The theory accounts for the effects of the adsorbent–adsorbate interaction potential and its range, deviations in pore geometry from the ideal slit-shaped form, and temperature on the values of disjoining pressure. The molecular theory based on the LGM framework provides a thermodynamic analysis of disjoining pressure as a characteristic of confined systems, distinguishing them from macroscopic systems. It is demonstrated that the concept of disjoining pressure in two-phase systems can be extended to all fluid densities, ranging from vapor to liquid, and to any pore geometry.