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), hexafluorobenzene-1,3,5-trifluorobenzene (HT), and benzene- hexafluorobenzene CBH) equimolar mixtures has been investigated by neutron diffraction experiment and molecular 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 deduced from the assumption of ideality in using structure data on pure components revealed a variety of behaviors for the three mixtures investigated. When small differences are observed for the BT and FT mixtures, more significant deviations appear for the BK mixture. In this context, MD simulations indicate that in BT and FT mixtures each component tends to preserve its local structure (i.e., the one existing in the corresponding pure liquid), while between unlike molecules, C6H6 (or C6F6) and 1,3,5-C6H3F3, the local structure appears as governed by the 1,3,5-C6H3F3 molecules. In contrast, for the BH mixture a strong structural enhancement upon mixing occurs: well defined heterodimers, in a quasistacked configuration (intermolecular distance similar to 3.7 Angstrom), are found. These heterodimers are also responsible of orientational correlations at larger distances (in the intermediate range, about 7-11 Angstrom). Finally, the role of the competition between repulsive, dispersive, and electrostatic forces in the above mixtures is discussed in detail.