Polymorphic and polytypic transformations during crystallization of long-chain compounds

Crystallization processes of long-chain compounds have been studied with FT-IR spectroscopy, which is highly sensitive to structural differences in functional groups. It has been found that several types of structural changes take place during the crystallization of fatty acids. In the solution crystallization of stearic acid, single crystals of the E form transform to the B form. This transformation starts with a heterogeneous nucleation of the B form on the {0 0 1} faces of the single crystal of E, and the B domains spread to the whole region of the single crystal through a solid-state phase transition. Screw dislocations incorporated into a mother phase in the early stages of crystallization provide a field for the emergence of the product B phase and a channel for the E solid-state transition. In addition, the polytypic transformation from a single-layer to a double-layer structure takes place during the crystallization of the E form. A similar polytypic transformation also occurs in petroselinic acid.