Understanding congruent melting in binary solids: Molecular models of benzene-hexafluorobenzene mixtures

We present a Monte Carlo computer simulation study of solid-fluid and solid-solid equilibrium in molecular models representative of binary mixtures of benzene and hexafluorobenzene. The models considered treat the molecular shape with hard sphere interaction site potentials and the electrostatic interactions via quadrupole-quadrupole potentials. Monte Carlo simulations are used to calculate the free energy of the pure solids and the 1:1 solid compound that appear in the phase diagram, and also the solution thermodynamics in the fluid phase. In common with the real fluid phase mixtures of benzene and hexafluorobenzene the fluid phase of the model is close to an ideal mixture. The model also exhibits solid-fluid and solid-solid equilibrium similar to that seen experimentally.The results indicate that enhanced stability of the benzene-hexafluorobenzene 1:1 solid compound can be understood in terms of the cooperative effects of electrostatic interactions and the packing of the anisotropic repulsive cores of the molecules. (C) 2003 American Institute of Physics.