Condensation and Evaporation in Slit-Shaped Pores: Effects of Adsorbate Layer Structure and Temperature

Abstract

We have carried out an extensive computer simulation study of the effects of temperature on adsorption and desorption of argon in two slit mesopores; one of which has both ends open to the surroundings, and the other with one end closed, to explore the fundamental reasons for the disappearance of the hysteresis loop as the temperature approaches the critical hysteresis temperature, Tch. Detailed mechanisms are presented for adsorption and desorption. At temperatures below Tch, both adsorption and desorption branches of the isotherm are metastable resulting in a hysteresis loop. As the temperature is increased, waves, due to thermal fluctuations, appear at the boundary between the dense adsorbed phase close to the pore walls and the gas-like phase in the core. For temperatures above Tch, these thermal fluctuations override the formation and subsequent movement of the meniscus (interface); adsorption is entirely due to the densification of the adsorbate, and desorption proceeds by rarefaction of the adsorbate. This mechanism gives rise to reversible isotherms in open ended pores and in the corresponding closed end pores; consequently, the critical hysteresis temperature in open and closed pores is the same; a result that has not been previously noted in the literature.