Density dependence of structure of supercritical carbon dioxide along an isotherm

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Abstract
Neutron diffraction experiments for supercritical CO2 have been carried out over a wide range of Q (0.018≤Q≤30 Å−1) at ρ*=ρ/ρc=1.5, 1.2, 0.77, and 0.34 along an isotherm at 310 K (T*=T/Tc=1.02). The measurement enabled us to obtain quantitatively reliable radial distribution functions of the fluid including both short-ranged structure and long-ranged density fluctuation. The structure factor and radial distribution function showed the structural change from the attraction-predominant gaslike structure to the repulsion-predominant liquidlike one with increasing fluid density. With respect to the long-ranged structure, almost linear Ornstein–Zernike–Debye plots were obtained for S(Q) of the fluids at all densities. A plot of correlation length against reduced density seems to have a maximum at the critical density. A ratio, α(r), of the density fluctuation produced by the correlation within r to that to the infinity presented a new aspect of the density fluctuation. Molecular dynamics simulation has also been performed in order to investigate a molecular basis of the experimental radial distribution functions. The calculated GN(r)’s were in good agreement with experimental ones at each density. In laboratory space, a variety of orientational coordination structures are almost equally found in the fluid at all densities examined, while T-shaped structure is preferentially present in polar-angle space.