Sorption in porous crystals: Equilibria and their interpretation

In porous rigid zeolite crystals intracrystalline pore volumes are sub-divided into regular channels and cavities of molecular dimensions only, and the total pore volumes can be determined. A consequence is that both absolute and Gibbs excess sorptions can be measured, examples of which have been considered. The Clapeyron-Clausius equation is derived using solution thermodynamics so that isosteric heats should strictly be derived from absolute sorption isotherms. Consequences of using the equation to give corresponding heats for Gibbs excess sorption isotherms are considered, and the two heats compared for several systems. With the minimum assumption that mean hydrostatic stress intensity and mean concentration of intracrystalline sorbate can be related by a virial equation, the corresponding virial sorption isotherm correlates sorption measurements and gives equilibrium constants and thermodynamic data, which have been exemplified for particular systems. Model isotherms can also have limited applicability and can give information at a molecular level. Model isotherms in the presence of interaction between adjacent pairs of guest molecules have been considered with particular reference to the behaviour of binary mixtures.