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.
OBJECTIVE
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.
Toscani
S
S
Laboratoire de Chimie Physique et Chimie Minérale, Faculté des Sciences Pharmaceutiques et Biologiques, Paris, France.
Dzyabchenko
A
A
Agafonov
V
V
Dugué
J
J
Céolin
R
R
D003461:Crystallography; D004355:Drug Stability; D008958:Models, Molecular; D011312:Pressure; D013237:Stereoisomerism; D000077145:Sulfanilamide; D013424:Sulfanilamides / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
D013424:Sulfanilamides; D000077145:Sulfanilamide