Neutron diffraction measurements on high pressure supercritical CO2

Abstract

Neutron diffraction measurements are reported of carbon dioxide in dense 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, obtained after subtraction of the intramolecular interference terms, are transformed to r space yielding the neutron weighted pair distribution functions. Throughout the supercritical region investigated, the structure of the fluid depends essentially upon the density. The experimental results are compared also with a model system of fluid CO₂ in the absence of orientational correlations as well as with existing molecular dynamics simulations. It is found that the orientational correlations persist in the supercritical region investigated and that the data are consistent with a predominance of T shaped local configurations at higher densities.