NEUTRON-DIFFRACTION MEASUREMENTS ON HIGH-PRESSURE SUPERCRITICAL CO2

Neutron diffraction measurements are reported of carbon dioxide in den se supercritical states distributed along three isotherms (313 K, 373 K and 473 K) and at various densities ranging from 8.7 molecules nm(-3 ) to 14.0 molecules nm(-3). The intermolecular structure factors, obta ined after subtraction of the intramolecular interference terms, are t ransformed to r space yielding the neutron weighted pair distribution functions. Throughout the supercritical region investigated, the struc ture of the fluid depends essentially upon the density. The experiment al results are compared also with a model system of fluid CO2 in the a bsence of orientational correlations as well as with existing molecula r dynamics simulations. It is found that the orientational correlation s persist in the supercritical region investigated and that the data a re consistent with a predominance of T shaped local configurations at higher densities.