Control of crystal morphology and polymorphism using a model steroid

Abstract

Although theoretical models for heterogeneous nucleation have been derived and a number of studies regarding the 3D inhibition or promotion of polymorphs using pre-defined templates were reported, the special case of 2D epitaxial nucleation and growth of (meta)stable polymorphs on specific faces of the (meta)stable ones is relatively new and little explored experimentally. Therefore it opens new possibilities of understanding and controlling the factors that influence the nucleation and growth of polymorphs and it is expected to have important consequences for the kinetic phase diagram of polymorphs. The work presented in this thesis is focused mainly on the epitaxial growth and nucleation on specific faces of both polymorphs of a steroid compound. The steroid has two known polymorphs: a monoclinic form and a triclinic form that are monotropically related within a practical temperature range. The two structures are quite similar in layers perpendicular to the [010] direction making it possible for either polymorph to nucleate epitaxially on the (010) faces of the other polymorph.Therefore, engineering the crystal growth (i.e. designing the crystal size and crystal habit) and the polymorph formed is possible by understanding the mechanism of epitaxial nucleation of the polymorphs of a given system that preferentially occurs on specific faces. However, in order to understand the mechanism of epitaxial polymorphic nucleation, one needs also to consider the influence of solvent on the growth habit of the crystals of two polymorphs. The solvent of crystallization can promote or hamper the growth of specific faces needed for epitaxial nucleation and will lower or enhance the barrier for 2D epitaxial polymorphic nucleation.