Crystal growth and morphology control of needle-shaped organic crystals

Understanding the growth of needle-shaped molecular crystals and the factors that influence the needle morphology is not only a fundamental research question but is also important in the design of industrial crystallization processes. Needle-like crystals have poor flow properties, are difficult to filter and to dry and - in the case of pharmaceuticals - difficult to formulate. While the morphology can often be controlled by changing the solvent, some compounds are persistent needle-formers that grow from all solvents as highly anisotropic crystals. In this highlight article we discuss the structural properties that lead to (persistent) needle growth. We also review strategies to modify the morphology such as the use of tailor-made additives, polymeric growth modifiers and cocrystallization. In this highlight fundamental research on the growth of needle-like crystals and strategies to control the aspect ratio are reviewed.