Epitaxial
2D Nucleation of Metastable Polymorphs: A 2D Version of Ostwald's Rule
of Stages
C. Stoica, P. Tinnemans, H. Meekes, E.
Vlieg, P. J. C. M.
van Hoof and F. M. Kaspersen
Abstract:
We present in-situ observations of the epitaxial nucleation of the
metastable polymorph of a steroid on specific faces of the stable form
in different solutions using optical microscopy. The polar morphology
of the crystals allows for the determination of the specific face where
the epitaxial nucleation takes place. The observations prove that there
is a different barrier for 2D epitaxial nucleation of the metastable
polymorph on the opposite polar faces. In-situ Raman measurements are
used to confirm the structure of the epitaxially growing crystals. The
metastable zone width and the role of the solvent in this process are
discussed. The relatively large lifetime of the metastable polymorph in
ethanol solutions is exploited to determine the solubility curves of
the bulk phases of both polymorphs. The relation between the solubility
of the two polymorphs in different solvents and the polymorphic
transformation rate is discussed.