SOLID-LIQUID EQUILIBRIUM OF DIPOLAR HETERONUCLEAR HARD DUMBBELLS IN AGENERALIZED VAN-DER-WAALS THEORY - APPLICATION TO METHYL-CHLORIDE

The solid-liquid equilibrium of hard dumbbells with embedded point dip oles is calculated using a generalized van der Waals theory to account for long range attractive forces. Molecular parameters are chosen to model a methyl chloride molecule. The solid free energy is calculated using the cell theory of Lennard-Jones and Devonshire with the dipolar contribution estimated by static lattice sums. Thermodynamic perturba tion theory is used to add dipolar effects to a hard dumbbell fluid eq uation of state. The resulting phase equilibria show that the dipole d oes have a significant effect in determining the stable solid structur e on freezing. In particular, the dipole moment stabilizes a non-close -packed orthorhombic structure, similar to the known solid structure o f methyl chloride. An increase in the ratio of triple point temperatur e to critical point temperature is also observed as the dipole moment is increased, as is a decrease in the density change on freezing. At h igh pressure and temperature a solid-solid-liquid triple point is foun d, above which the system freezes into the close-packed structure. (C) 1998 American Institute of Physics. [S0021-9606(98)52040-5].