Structural investigations of liquid binary mixtures: Neutron diffraction and molecular dynamics studies of benzene, hexafluorobenzene, and 1,3,5-trifluorobenzene

The local structure in benzene-1,3,5-trifluorobenzene (BT), hexafluorobenze ne-1,3,5-trifluorobenzene (HT), and benzene- hexafluorobenzene CBH) equimol ar mixtures has been investigated by neutron diffraction experiment and mol ecular dynamics (MD) simulation. Experimentally it is found that the local order is very slightly affected in the temperature range investigated (298- 346 K). The comparison between the present experimental data and those dedu ced from the assumption of ideality in using structure data on pure compone nts revealed a variety of behaviors for the three mixtures investigated. Wh en small differences are observed for the BT and FT mixtures, more signific ant deviations appear for the BK mixture. In this context, MD simulations i ndicate that in BT and FT mixtures each component tends to preserve its loc al structure (i.e., the one existing in the corresponding pure liquid), whi le between unlike molecules, C6H6 (or C6F6) and 1,3,5-C6H3F3, the local str ucture appears as governed by the 1,3,5-C6H3F3 molecules. In contrast, for the BH mixture a strong structural enhancement upon mixing occurs: well def ined heterodimers, in a quasistacked configuration (intermolecular distance similar to 3.7 Angstrom), are found. These heterodimers are also responsib le of orientational correlations at larger distances (in the intermediate r ange, about 7-11 Angstrom). Finally, the role of the competition between re pulsive, dispersive, and electrostatic forces in the above mixtures is disc ussed in detail.