Colloidal crystal model reveals new factors in controlling polymorph formation

Polymorphs are not mythical, chimeric beasts—they are substances with identical chemical compositions but differing crystal structures that also exhibit different physical and chemical properties. What this means for practical use, is that companies often want to create a certain polymorph—but not the others.

Researchers at Tohoku University took a deep dive into using colloidal crystallization as a model system to figure out how to achieve that fine control over specific polymorph formation. The research is published in the journal Communications Physics.

Zoom in on a crystal at a microscopic level, and you’ll find that what makes it unique is its highly ordered structure. A colloidal crystal has a similar ordered structure, but with the addition of suspended, submicron-sized particles.

These special crystals are regarded as a good model of phase transition, and versatile materials for a wide range of scientific and industrial applications. However, the mechanisms behind polymorph selection during the crystallization process are not fully understood. One of the goals of this study was to reveal novel insights about these mechanisms.

“The ability to control the growth of specific crystal polymorphs is essential in fields like materials science and pharmaceuticals,” says Jun Nozawa (Tohoku University). “Any change to the polymorphs results in changes to product performance and functionality, so being able to confidently select for a specific polymorph is crucial.”

The study used colloidal crystallization as a model system and conducted in situ observations with single-particle resolution to investigate polymorph selection mechanisms. The study employed a method called heteroepitaxial growth using polystyrene colloidal particles. The crystallization process involved nucleation, growth, and dissolution—each influenced by polymorphic transitions.

The final products were found to be governed by polymorph transitions. They found that the probability of a certain polymorph occurring was driven by size and cluster stability. Particle additives were able to effectively control polymorph formation as well.

“These factors we analyzed can be used in principle to help create the desired polymorph, depending on the situation. This opens new pathways for polymorph regulation technologies,” remarks Nozawa.

This research has advanced the understanding of polymorph control, providing insights applicable to material fabrication and drug development. These findings emphasize the significance of cluster dynamics and growth rates beyond thermodynamic stability for polymorphic crystal selection.