Polymorphism of sulfanilamide: (II) Stability hierarchy of alpha-, beta-, and gamma-forms from energy calculations by the atom-atom potential method and from the construction of the p, T phase diagram.

PURPOSE: Sulfanilamide trimorphism was chosen as a model system for comparison between stability hierarchies obtained from lattice-energy calculations with those deduced from the relative locations of the sublimation curves of polymorphs in the sulfanilamide p, T diagram.
METHODS: The atom-atom potential (AAP) method was used for lattice-energy calculations. The p, T diagram was constructed by using crystallographic and thermodynamic data for alpha-, beta-, and gamma-forms, and by assigning the temperatures of the experimentally observed phase transitions to triple points involving the vapour phase.
RESULTS: The hierarchy obtained with the AAP method (E alpha > or = E gamma > > E beta) differs only slightly from that deduced from the positions of the sublimation curves (p gamma > p alpha > p beta) in the p, T diagram at room temperature. No stable phase region was found for form alpha. Thus it is really monotropic.
CONCLUSIONS: Provided enthalpy and volume changes at the transitions are accurate enough, it is possible to draw a p, T diagram that accounts for the stability hierarchy of polymorphs.