THE GAS, LIQUID, AND SOLID-PHASES OF DIMERIZING HARD-SPHERES AND HARD-SPHERE DUMBBELLS

The complete phase diagram of a model associating molecule is determined, including the gas, liquid, and solid phases, the regions of coexistence between these three phases, and the location of the critical and triple points. The model molecule is a hard sphere with two very different attractive interactions, one a short ranged and directional attraction and the other a mean field. The first interaction only forms dimers as a molecule can only interact in this way with one other molecule. This saturable attraction mimics hydrogen and chemical bonding. The second interaction is an approximation for the dispersion forces between molecules. Thermodynamic functions for the liquid and gas phases of this model molecule are obtained from an existing theory for associating fluids but a new theory is developed for the solid phase. This is believed to be the first microscopic theory of a model associating molecule in the solid phase. In the low temperature limit no monomers are present; the system is then a fluid or solid of hard-sphere dumbbells. Simulation data are available in this limit and it is shown that in both the fluid and solid phases the theoretical predictions are close to those of simulation. The pressure equation of state for dumbbells is the most accurate theory available for the solid phase. An approximation for the free energy of a solid mixture of spheres and dumbbells is also presented. © 1995 American Institute of Physics.