Analysis of FII crystals of sulfathiazole: Epitaxial growth of FII on FIV

This work describes the phenomenon of a less stable polymorph wedged as a middle layer in a more stable polymorph of sulfathiazole. Isolation of the pure FII polymorph of sulfathiazole consistently yielded crystals with a distinctive middle layer. Raman spectroscopy and X-ray diffraction have identified this middle layer as another polymorph of sulfathiazole, namely FIV. The solubilities of FII and FIV sulfathiazole are almost identical, with FIV slightly more soluble. It is thought that this causes FIV to nucleate first, followed by the epitaxial growth of FII. A morphological examination of the crystals demonstrated that the (100) face of the FII crystal matches the (101) face of the FIV crystal. It is proposed that the similarity of these faces supports the epitaxial growth of the FII polymorph on the surface of the FIV polymorph.