Gas adsorption is a powerful technique for comprehensively characterizing porous materials. It provides critical information about the surface area and pore size distribution. However, to fully understand this process, one must consider the adsorption behavior of the fluid within the pores, including phase changes and their impact on the adsorption isotherm—key factors in surface and pore analysis.
The pore structure, including its width and shape, plays a crucial role in the adsorption mechanism. For micropores (pore width < 2 nm), the filling process is continuous, while for mesopores (2–50 nm), it involves gas condensation inside the pores, leading to a first-order gas-liquid phase transition. These differences necessitate distinct analytical approaches for accurate pore size distribution.
Previously, we discussed the capillary condensation theory (BJH) for mesoporous materials and microporous models (HK and SF), which are based on macroscopic thermodynamics. These methods, however, cannot unify the analysis of both micropores and mesopores under a single framework. The classic thermodynamic assumptions often fail to account for the unique properties of confined fluids, such as shifts in critical points or phase behavior. As a result, more advanced techniques like density functional theory (DFT) and molecular simulations have emerged.
Non-localized DFT (NLDFT) and Monte Carlo (MC) simulations offer more accurate descriptions of fluid behavior in confined spaces. These methods go beyond traditional thermodynamics by considering molecular-level interactions and equilibrium density distributions. They can capture the detailed structure of fluids near solid surfaces or within complex geometries like slit pores, cylinders, or spheres.
Unlike conventional DFT, which may not accurately represent adsorption in narrow pores, NLDFT and MC simulations provide precise insights into fluid behavior. For example, they reveal that in wedge-shaped mesopores, both gas and liquid phases coexist, with density varying based on the distance from the pore walls. This leads to multilayer adsorption near the walls and a gradual decrease in density toward the center.
These advanced methods also allow for the calculation of key parameters such as adsorption isotherms, heat of adsorption, and neutron scattering patterns. By fitting experimental data and using realistic interaction potentials, they bridge the gap between molecular-level behavior and macroscopic observations. This makes them essential tools for understanding and characterizing complex porous systems.
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