ADSORPTION HYSTERESIS IN POROUS SOLIDS

Hysteresis has been observed in adsorption isotherms for a number of g as-solid systems and, generally, is attributed to adsorption in mesopo rous materials with capillary condensation. This behavior is classifie d as Type IV or Type V in the IUPAC classification scheme. Here, latti ce theory is used to predict adsorption behavior in pores. The Ono-Kon do theory is used with appropriate boundary conditions for fluid adsor ption in infinite and semi-finite slit-like pores. It is shown that th ere can be phase transitions in the adsorbed phase which lead to hyste resis in kinetically controlled experiments. However, hysteresis in eq uilibrium behavior is exhibited only in pores of finite length. For fi nite-length pores, the interface geometry is predicted to be different during the processes of adsorption and desorption and this difference in interface shape leads to hysteresis. This simple molecular model i s able to predict the change in the interface geometry without invokin g the Kelvin equation or the macroscopic concept of surface tension. ( C) 1998 Academic Press.