日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

In ordered mesoporous silica materials such as MCM-41 and SBA-15, the vapor-liquid phase transition, named capillary condensation, occurs at a pressure lower than the saturated vapor pressure in the bulk system. Understanding of the mechanism of capillary condensation is important for the characterization of mesoporous materials; however, despite a long history of research on capillary condensation, it is still difficult to completely describe it using the proposed thermodynamic theories to date, because capillary condensation inherently includes an activated process. In the present study, with the aid of the molecular simulations, we evaluate the free energy barrier for the capillary condensation process and the dimensionless rate constants of capillary condensation. Consequently, we reveal that the rate constant drastically increases with increasing pressure, and capillary condensation occurs at the pressure where the rate constant reaches a critical value (critical rate constant). The critical rate constant determined through the comparison between the simulation results and experimental counterparts allows us to reasonably estimate the capillary condensation pressures over a wide range of temperatures and pore sizes.