ABNORMAL TEMPERATURE-DEPENDENT VARIATION IN SOUND-VELOCITY FOR THE MOLECULAR LIQUIDS BENZENE AND HEXAFLUOROBENZENE

In this paper we give the results of sound-velocity measurements by ac curate Brillouin techniques in pure liquid benzene and hexafluorobenze ne in the temperature range 20-60-degrees-C. For both fluids, we find a clear change in the slope of the variation of the sound velocity wit h temperature. A change in the Landau-Placzek ratio was also detected. These experimental facts seem to confirm that a molecular-orientation ordering change occurs in this temperature range for these fluids. We see some similarity in these velocity changes with previous results f ound for phase changes in fluids. We believe that when the dominant mo lecular orientation changes in a local order scale, it creates a proce ss strong enough to perturb the hydrodynamic behavior of the fluid, wh ich can then be detected by spontaneous Brillouin light scattering.