Abstract
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 silican 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.
