Neutron diffraction measurements on CO2 in both undercritical and supecritical states

Neutron diffraction measurements on fluid CO2 in a wide thermodynamic range covering both undercritical and supercritical stares are presented. From these measurements, performed using 7C2 spectrometer at the Orphee reactor, we have derived the experimental neutron weighted radial distribution functions g(r). Moreover, for each investigated thermodynamic state, we performed molecular dynamics simulations from which the various molecular pair distribution functions g(R, omega(1), omega(2)) have been calculated, thus gaining a detailed information on the microscopic structure of the fluid, particularly on the orientational correlations. The main results, valid for all the investigated densities and temperatures, are that: (i) Fluid CO2 shows significant orientational correlations only among first neighbor molecules; such correlations become less pronounced as the temperature increases although they are still quite evident up to 1.56 times the critical temperature and density down to 0.76 times the triple point density. (ii) T-shaped local configurations are found to be favored. (iii) The inclusion of electric quadrupole interactions in the intermolecular pair potential is essential in order to obtain agreement between Molecular Dynamics simulations and experimental g(r). The electric quadrupole interaction is also the origin of the predominance of T-shaped local configurations. (C) 1998 Elsevier Science B.V. All rights reserved.