Epitaxial growth of polymorphic systems: The case of sulfathiazole

The creation of composite crystals formed by the epitaxial interaction between differing polymorphs of sulfathiazole (forms II and IV) has been reported through the experimental growth from ethanol. The intermolecular interaction between the crystal phases was calculated by application of the differential evolution global optimisation algorithm. This indicates that the interaction between form IV and form II is greater than that between form IV and itself, but less than that between form II and itself. Thus from the initial nucleation of form IV the creation of form II through mismatching of the growth units would be favoured leading the growth of form II. This mechanism explains the structure of the observed crystals with an inner layer of form IV surrounded by a shell of form II.