Monitoring Phase Behavior of Sub- and Supercritical CO2 Confined in Porous Fractal Silica with 85% Porosity

Phase behavior of CO2 confined in porous fractal silica with volume fraction of SiO2 phi(s) = 0.15 was investigated using small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) techniques. The range of fluid densities (0 < (rho(CO2))(bulk) < 0.977 g/cm(3)) and temperatures (T = 22 degrees C, 35 and 60 degrees C) corresponded to gaseous, liquid, near critical and supercritical conditions of the bulk fluid. The results revealed formation of a dense adsorbed phase in small pores with sizes D < 40 angstrom at all temperatures. At low pressure (P < 55 bar, (rho(CO2))(bulk) < 0.2 g/cm(3)) the average fluid density in pores may exceed the density of bulk fluid by a factor up to 6.5 at T = 22 degrees C. This enrichment factor gradually decreases with temperature, however significant fluid densification in small pores still exists at temperature T = 60 degrees C, i.e., far above the liquid gas critical temperature of bulk CO2 (T-C = 31.1 degrees C). Larger pores are only partially filled with liquid-like adsorbed layer which coexists with unadsorbed fluid in the pore core. With increasing pressure, all pores become uniformly filled with the fluid, showing no measurable enrichment or depletion of the porous matrix with CO2.