Summary
Highlights
Polymorphism refers to a substance's ability to exist in more than one crystal structure, leading to varying physical and chemical properties. Crystal habit describes the external shape or appearance of a crystal, influenced by growth conditions but not its internal structure.
Crystallization is the process during which polymorphism and crystal habits can emerge. While habits relate to external appearance, the internal structure of crystals from the same compound generally remains consistent, detectable by X-ray diffraction (XRD).
Polymorphs, distinct forms with different internal structures, can have varying physical and chemical properties such as solubility, stability, and biological interaction, particularly important in pharmaceuticals.
Polymorphism happens when molecules of the same compound arrange themselves in different ways within a crystal lattice. This can involve different packing arrangements or changes in molecular orientation or conformation.
Polymorphism is detected using XRD, where each unique crystal structure produces a slightly different X-ray scattering pattern. These patterns act as 'fingerprints' to identify different polymorphs based on molecular packing or orientation.
Powder X-ray Diffraction (PXRD) is the gold standard for phase identification due to its reliability in distinguishing different crystalline phases. Single Crystal XRD offers detailed insight into a crystal's 3D atomic structure but requires a well-formed single crystal, which can be challenging to obtain.
Amorphous substances like amorphous bovine insulin show a broad, diffuse hump or halo in XRD patterns due to random molecular arrangement. Crystalline substances like crystalline Levodopa exhibit sharp, distinct peaks characteristic of their ordered crystal lattice.
DSC measures how substances behave when heated or cooled, identifying phase transitions like melting or solidification and interactions with other ingredients. However, it only indicates that a change happened, not exactly what or why.
TGA measures weight loss of a substance as it's heated, indicating the amount of water or solvents attached. This helps understand the composition of hydrates or solvates, though other ingredients with water can interfere with accurate measurements.
Polymorphs possess different physical and chemical properties, including melting point, solubility, and habits. Habits describe the external shape, with common types being acicular, prismatic, pyramidal, tubular, equant, and columnar.