Adsorption transition and capillary condensation in mesoporous materials: experimental and theoretical perspectives...

Capillary condensation and evaporation in mesoporous materials are controlled by complex interfacial and thermodynamic mechanisms. While conventional adsorption models have provided valuable insights into these processes, they may not fully capture all features under confined conditions. We employ the Zeta adsorption model as a complementary approach to existing theories, enabling quantitative prediction of adsorption regimes, elucidation of transition mechanisms, and determination of critical pore-filling pressures in porous materials.

Adsorption experiments and molecular dynamics (MD) simulations were performed to investigate phase transitions. The isotherms were used to validate hysteresis behavior and estimate specific surface areas. At the same time, MD simulations enabled direct observation of fluid configurations, interfacial dynamics, and metastable states during adsorption and desorption. The Zeta adsorption model was applied to distinguish adsorption regimes and predict phase transition points across varying pore sizes.

Evaporation in mesopores involves two distinct pathways: a continuous, meniscus-controlled process and a discontinuous, nucleation-driven transition involving spontaneous bubble formation. In a small pore (∼3.3 nm), meniscus recession dominates even under metastable conditions due to reduced nucleation barriers. However, the larger pore (∼7.2 nm) exhibits cavitation-driven desorption. This size-dependent transition reflects the interplay between film instability and cavitation kinetics, governed by an underlying free-energy barrier. A novel application of the Zeta adsorption model enables quantitative prediction of critical phase transition pressures, and supplements current adsorption models by bridging the adsorption behavior in porous and non-porous systems. These results offer a vital insight in term of the microscopic mechanisms of capillary condensation and evaporation, deepening our understanding of adsorption thermodynamics in confined systems.