Microstructural Characterization of Adsorption and Depletion Regimes of Supercritical Fluids in Nanopores

Fluid accommodation in porous media has been studied over a wide range of pressures at three supercritical temperatures by small-angle neutron scattering. A new formalism gives for the first time the mean density and volume of the adsorbed fluid phase formed in the pores from experimental data; thus, excess, absolute, and total adsorption become measurable quantities without the introduction of further assumptions. Results on propane adsorption to a silica aerogel show the formation of a thin adsorption layer of high density at low bulk fluid pressures and densities. In that region, the density of the adsorption layer increases with increasing fluid density while its volume remains approximately constant. Depletion of the fluid from the pore space is found near and above the critical density, which leads to negative values of the excess adsorption. At high fluid densities, the pores are evenly filled with fluid of lower density than the bulk fluid. The total amount of fluid confined in the pore spaces increases with the fluid density below the critical density and remains approximately constant at higher fluid densities. Application of the new model also gives insight into the sorption properties of supercritical carbon dioxide in silica aerogel. The concept presented here has potential to be adopted for the study of numerous other sub- and supercritical fluids and fluid mixtures in a variety of micro- and nanoporous materials