A structural study of the hexafluorobenzene from liquid to supercritical conditions using neutron diffraction and molecular dynamics

Abstract

This paper is devoted to an investigation of the local order in hexafluorobenzene by neutron diffraction and molecular-dynamics simulations. Experimentally, the fluid has been studied under isobaric (P∼16 MPa) and isothermal (T∼573 K) conditions in a broad density domain ranging from the liquid value (ρ∼1600 kg m−3) down to typical densities on the supercritical domain (ρ∼60 kg m−3). It is found that the translational ordering, which is characterized by two well defined shells of neighboring molecules at ambient conditions, is gradually weakened and extends only to the first shell as the density decreases in the supercritical domain. This behavior is in full agreement with the general trend reported for the positional ordering in our previous investigations for some cyclic molecules. In marked contrast, the short range orientational ordering existing in the first shell of molecule is almost preserved. It is found that parallel and perpendicular configurations of a pair of neighboring molecules coexist, although parallel configurations are found predominant at the shortest intermolecular distances. Finally, the structural evolution in the three fluids, hexafluorobenzene, benzene and 1,3,5-trifluorobenzene are comparatively discussed. It is argued that the orientational ordering in the hexafluorobenzene is closer to that existing in 1,3,5-trifluorobenzene than in liquid benzene. However, no presence of stacked dimers (C6F6)2 have been revealed from both experimental and simulated structural analyze.