Solid-liquid equilibrium for quadrupolar molecules

Theoretical calculations are presented of solid-liquid equilibrium for linear quadrupolar molecules, focusing especially on the ratio of the triple point temperature to the critical temperature. The proposed treatment uses the hard quadrupolar dumbbell model as the reference system, and incorporates the effect of dispersion forces through a mean-field term. The reference system of quadrt/polar hard dumbbells is described using perturbation theory for the liquid and cell theory for the solid. Good agreement with simulation results of quadrupolar hard dumbbells is obtained through the theoretical description. After incorporating attractive dispersion forces, the temperature dependence of solid-liquid and vapour-liquid equilibrium was studied and the triple point determined for two linear quadrupolar model systems. The two models were representative of carbon dioxide and acetylene, respectively. The ratio of the triple to critical point temperatures determined from the theory was about 0.7, in good agreement with experiment.