Structural study of supercritical carbon dioxide confined in nanoporous silica by in situ neutron diffraction

In situ neutron diffraction measurements of adsorbed carbon dioxide in an ordered mesoporous silica (MCM-41) have been carried out along an isotherm slightly above the critical point (308 K) and at a range of pressures below and above the critical one (30-125 bar). The experiment has been performed with the aid of a novel high-pressure adsorption apparatus and the GEM diffractometer (ISIS, Rutherford Appleton Laboratory, UK). Diffraction measurements of both bulk supercritical and bulk liquid carbon dioxide have also been carried out. The structure factors and the total differential correlation functions of the adsorbed carbon dioxide suggest that the confined carbon dioxide has liquid-like properties. However, some differences are observed when the confined phase is compared to those of bulk liquid and bulk supercritical fluid respectively, mainly arising from orientational correlations between adsorbed molecules. These differences could be attributed either to strong interactions between the silica walls and CO2 molecules or to the confinement of the fluid combined with the relatively large quadrupole moment of carbon dioxide.