Thermodynamic studies of binary mixtures involving aromatic fluorocarbons
Solid-liquid phase diagrams have been determined for binary systems of hexafluorobenzene + naphthalene-type compounds, and indicate strong 1:1 congruently melting point complexes. Hexafluorobenzene + cis- and + trans-deca1in were found to give simple eutectic phase diagrams. Excess enthalpies, excess volumes and excess Gibbs functions have been measured for the same hexaf1uorobenzene + naphthalene-type compound mixtures and are large and negative, which is characteristic of systems where specific interactions take place. This contrasts with the large positive excess functions found with hexafluorobenzene + decalin systems, where only dispersion forces are assumed present. The excess Gibbs function for hexafluorobenzene + trans- and cis-decalin have been determined theoretically from freezing point data as well as directly from vapour pressure measurements. A batch calorimeter, besides being used for excess enthalpy measurememts, has been employed in determining heats of solution, which lead to a value for the enthalpy change for the process, solid + solid ? complex. The possibility of charge-transfer interactions occuring in hexafluorobenzene + naphthalene-type compound systems has been discussed in terms of HOMO/LUMO overlap considerations and is supported by the observation that pentafluorocyanobenzene forms stronger (higher melting point) complexes with 1- and 2-methylnaphthalene, than hexafluorobenzene does.