Sorption phase of supercritical CO₂ in silica aerogel: Experiments and mesoscale computer simulations

Adsorption of supercritical CO₂ in nanoporous silica aerogel was investigated by a combination of experiments and molecular-level computer modeling. High-pressure gravimetric and vibrating tube densimetry techniques were used to measure the mean pore fluid density and excess sorption at 35 and 50 °C and pressures of 0-200 bar. Densification of the pore fluid was observed at bulk fluid densities below 0.7 g/cm³. Far above the bulk critical density, near-zero sorption or weak depletion effects were measured, while broad excess sorption maxima form in the vicinity of the bulk critical density. The CO₂ sorption properties are very similar for two aerogels with bulk densities of 0.1 and 0.2 g/cm³, respectively. The spatial distribution of the confined supercritical fluid was analyzed in terms of two nanodispersed phases with sorption- and bulk-phase densities and their volumes by means of the adsorbed phase model (APM), which used data from gravimetric sorption and small-angle neutron scattering experiments. To gain more detailed insight into supercritical fluid sorption, large-scale lattice gas GCMC simulations were utilized and tuned to resemble the experimental excess sorption data. The computed three-dimensional pore fluid density distributions show that the observed maximum of the excess sorption near the critical density originates from large density fluctuations pinned to the pore walls. At this maximum, the size of these fluctuations is comparable to the prevailing pore sizes.