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 present ed. From these measurements, performed using 7C2 spectrometer at the O rphee reactor, we have derived the experimental neutron weighted radia l distribution functions g(r). Moreover, for each investigated thermod ynamic state, we performed molecular dynamics simulations from which t he various molecular pair distribution functions g(R, omega(1), omega( 2)) have been calculated, thus gaining a detailed information on the m icroscopic structure of the fluid, particularly on the orientational c orrelations. The main results, valid for all the investigated densitie s and temperatures, are that: (i) Fluid CO2 shows significant orientat ional correlations only among first neighbor molecules; such correlati ons 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 loc al configurations are found to be favored. (iii) The inclusion of elec tric quadrupole interactions in the intermolecular pair potential is e ssential in order to obtain agreement between Molecular Dynamics simul ations and experimental g(r). The electric quadrupole interaction is a lso the origin of the predominance of T-shaped local configurations. ( C) 1998 Elsevier Science B.V. All rights reserved.