Supercritical fluid adsorption fundamentals

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

Adsorption equilibria and adsorption dynamics in supercritical fluids have been reported recently and it will be possible to apply the supercritical fluid to some new adsorptive separation processes. Fundamental informations on adsorption under supercritical conditions are necessary to design such processes. Supercritical chromatography has been used for study on the adsorption equilibria and adsorption dynamics. Adsorption of organics, i.e., benzene, toluene and m xylene, respectively, on MSC under supercritical conditions has already been reported in reference (Chihara, 1995). In the previous study, chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on MSC in supercritical CO2 mixed with benzene, toluene and m-xylene respectively. Moment analysis of the chromatogram was carried out. In the study, the organics used in the form of pulse were the same as organics mixed with supercritical CO2. The dependencies of adsorption equilibrium and micropore difliisivities on the amount adsorbed were obtained. 

The van der Waals equation describes the critical phenomena of vapour to supercritical gas or fluid. Below critical temperature Tc gas which coexists with the liquid phase is called a vapour. Vapor has own saturated vapour pressure Pq. Then we can use the relative pressure P/Pq for description of adsorption. Fundamentally, physical adsorption is valid for vapours [10]. As the molecule-surface interaction of physical adsorption is weak, a sufficient intermolecular interaction corresponding to heat of vapourization is necessary for predominant physical adsorption. Micropore filling is a physical adsorption enhanced by overlapping of the molecule-surface interaction potentials from opposite pore walls and the adsorptive force is the strongest in physical adsorption. Nevertheless, micropore filling is a predominant process only for vapour. 

---