Molecular modeling of ordered mesoporous silica for understanding of capillary condensation

We construct accurate atomistic silica pore models mimicking ordered mesoporous silica materials such as MCM-41 and SBA-15, which have atomic-level surface roughness and satisfy the electron density profile (EDP) of the ordered mesoporous silica materials. We perform argon adsorption simulations for two different MCM-41 samples at 75, 80, and 87.3 K, and the results are compared with the experimental counterparts. We demonstrate that our atomistic silica pore models successfully reproduce the experimental argon adsorption isotherms, and the calculated thermodynamic equilibrium transition pressures coincide with the experimental desorption branches at all temperatures investigated, which suggests that the experimental capillary evaporation for the openended cylindrical pores is the thermodynamic equilibrium phase transition. Moreover, we calculate a work required for the vapor-liquid phase transition by integrating the sigmoidal argon adsorption isotherm obtained through the Gauge Cell method. The work profile demonstrates that there exists an energy barrier between vapor-like state and liquid-like state, and the energy barrier becomes lower with increasing pressure. Furthermore, we determine the critical energy barrier Wc* = 0.154 by comparing the simulation and experimental results, and reveal that Wc* is a universal value independent of adsorption temperature and mesopore size and enables us to predict the experimental capillary condensation pressure at all conditions investigated.