On the Crystallization of Epitaxial Racemic Conglomerates

The resolution of racemic conglomerates by crystallization methods is often prevented by the formation of epitaxial racemic conglomerates. These laminar crystals composed of alternating layers of opposite enantiomers are formed by the interplay between mass transport and surface kinetics involving normal and epitaxial two-dimensional nucleation. Three classes of epitaxial racemic conglomerates (ERC’s) are distinguished from crystallographic symmetry considerations. For the two most commonly occurring classes an analytical model has been developed that describes the “competition” of the two opposite enantiomers in their transport toward the crystal surface and their incorporation in the crystal by 2D nucleation growth. From this model and considering a limiting, stationary case, the conditions for ERC formation as a function of, among others, diffusion boundary layer thickness, supersaturation and epitaxial interfacial energy are derived. The dynamics of the oscillatory process of ERC growth is studied by numerical integration of the analytical expressions, using characteristic parameters for the solution growth of small molecule organic crystals.