A new theory of crystallization shows that the solvent, not the solute, controls crystal formation. This two-step process improves crystal growth predictions and has practical applications in fields such as medicine and technology. Remember the old-school chemistry experiment where salt crystals form from a salt water solution or sugar crystals turn into sugar from sugar water? It turns out that your understanding of how crystals form in these solutions may be wrong.

The new theory “clarifies” the crystallization process and shows that the crystallizing material is the dominant component in the solution, that is, the solvent, not the solute. The theory could have implications for everything from drug development to understanding climate change.

“Crystals are ubiquitous; we use them everywhere from technology to medicine—but we don’t have a true understanding of the crystallization process,” says James Martin, a professor of chemistry at North Carolina State University and author of the paper. Subject what the theory describes.

Back to high school chemistry

“The prevailing view about dissolution and precipitation is that they are essentially opposites, but they are not. They are actually completely different processes,” says Martin.

“In the example of a school chemistry experiment about the separation of a precipitate from a solution: when I dissolve salt (solute) in water (solvent), water is dominant. It dissolves the salt, essentially breaking it apart,” says Martin. “If I wanted to grow a salt crystal from that solution, I’d say that’s the dominant phase. “At this point there must be salt, which is the solvent and forms the crystal.”

Thermodynamic phase diagrams that describe concentration- and temperature-dependent transition points in solutions can be used to explain a new theory called transition zone theory.

Theory suggests that crystallization occurs in two stages: First, an intermediate product is formed in the form of a melt. This intermediate can then organize into a crystal structure.

“To obtain crystals from a solution, you need to quickly separate the solvent and solute,” Martin says. “When we talk about ‘melting’ here, we are talking about the pure solvent stage before crystals form. The difference here is that my theory suggests that you can get better and faster crystal growth by shifting your solution towards conditions that emphasize the solvent; In other words, the solvent, not the impurity, controls the rate of crystal growth.”

Application and practical implications

Martin applied his theory to a number of different solutions, concentrations, and temperature conditions and found that it accurately described the rate and size of crystal formation.

“The main problem with previous explanations of crystallization was the idea that crystals grow by the diffusion of independent solute particles and then joining the growing crystal boundary,” says Martin. “Instead, cooperative solvent communities need to be understood to describe crystal growth.”

An important aspect of the new theory, according to Martin, is its focus on understanding how dissolved impurities disrupt this cooperative solvent community.

“By understanding the interaction between temperature and concentration, we can accurately predict how fast and how large crystals will grow from solution.”

Martin believes phase diagrams may have important applications not only for crystal formation but also for preventing crystal formation, such as preventing the growth of kidney stones.

“Crystals are at the heart of technology; they are everywhere and impact our daily lives,” says Martin. “This theory gives researchers simple tools to understand the ‘magic’ of crystal growth and make better predictions. It’s an example of how basic science provides the foundation for solving any real-world problem.”