Supercritical carbon dioxide behavior in porous silica aerogel

Analysis of the tails of the small-angle neutron scattering (SANS) intensities relevant to samples formed by porous silica and carbon dioxide at pressures ranging from 0 to 20 MPa and at temperatures of 308 and 353 K confirms that the CO₂ fluid must be treated as a two-phase system. The first of these phases is formed by the fluid closer to the silica wall than a suitable distance and the second by the fluid external to this shell. The sample scattering-length densities and shell thicknesses are determined by the Porod invariants and the oscillations observed in the Porod plots of the SANS intensities. The resulting matter densities of the shell regions (thickness 15-35 Å) are approximately equal, while those of the outer regions increase with pressure and become equal to the bulk CO₂ at the higher pressures only in the low-temperature case.