Phase behavior of carbon dioxide confined in silica aerogel in the vicinity of the bulk critical point

The small angle neutron scattering intensities from silica aerogel filled with carbon dioxide at different loading densities of the bulk fluid (0.3-0.65 g/cm³) were measured at fixed volume condition as a function of decreasing temperature, from 35 to 25 °C, to characterize the phase behavior of the confined CO₂ about the critical point of the bulk CO ₂. The data present no evidence of a bulklike phase transition of the confined CO₂ in the explored parameter region. They show that the confined CO₂ may be approximated as a two phase system. The first phase (liquid film) is formed by CO₂ molecules at the silica surface, and the second phase (confined fluid) fills up the rest of the pore volume. The thickness δ of the liquid film as well as the scattering-length densities of the two phases were obtained by analyzing the Porod invariants and the oscillations observed in the Porod plots at each loading density and temperature. The resulting δ values vary in the range 25-45 Å depending on the temperature and loading density. The density of the liquid film is 1.5-2 times larger than that of the confined fluid, which in its turn exceeds the density of bulk CO₂ at similar temperature and pressure. At the lowest temperature (25 °C), the densities of the liquid film and confined fluid respectively approach 1.25 g/cm³ and 0.8 g/cm ³ independently of the loading conditions.