Reviewed by Lexie CornerOct 3 2024
A recent theory challenges conventional understanding of crystallization. It shows that the dominant element in a solution—the solvent, not the solute—is the material that crystallizes. This finding, published in Matter, overturns traditional views of crystal formation.
Remember those classic high school chemistry experiments where salt crystals form from saltwater, or rock candy forms from sugar water? It turns out that our long-held understanding of these processes might not be entirely accurate.
Crystals are ubiquitous–we use them in everything from technology to medicine–but our actual understanding of the crystallization process has been lacking.
James Martin, Study Author and Professor, Chemistry, North Carolina State University
He added, “The prevailing ideas around dissolving and precipitating are that they’re essentially the reverse of each other, but they aren’t. In reality, they are completely different processes. Using the high school chemistry experiment with getting precipitate out of a solution as an example: when I dissolve salt (the solute) into water (the solvent), the water is dominant. It dissolves the salt by essentially ripping it apart.”
“If I then want to grow a salt crystal from that solution, the dominant phase must become the salt – which is the solvent at that point and is the one that forms the crystal,” he further added.
Thermodynamic phase diagrams, which illustrate how concentration and temperature influence transitions in solutions, can be used to explain this new transition-zone theory.
The theory reveals that crystallization happens in two stages: first, a melt-like pre-growth intermediate form is created. This intermediate phase is then organized into a crystal structure.
Martin noted, “To grow a crystal out of a solution, you have to quickly separate the solvent and solute. When we refer to the ‘melt’ here, we’re talking about the pure phase of the solvent prior to crystal formation. The difference here is that my theory shows you get better, faster crystal growth by moving your solution toward conditions that emphasize the solvent; in other words, the solvent – not the impurity within it – controls the rate of crystal growth.”
Martin tested his theory with various solutions, concentrations, and temperatures, finding that it accurately predicts the rate and size of crystal formation.
“The main issue with previous descriptions of crystallization was the perception that crystals grow by having independent solute particles diffuse to and then attach to a growing crystal interface. Instead, it is necessary to understand cooperative ensembles of the solvent to describe crystal growth,” Martin stated.
According to Martin, the new theory is important because it focuses on understanding how solute impurities disrupt the cooperative ensemble of solvents.
He noted, “By understanding the interplay of temperature and concentration, we can predict exactly how fast and large crystals will grow out of solution.”
He also believes phase diagrams could have broader applications, such as preventing the formation of kidney stones.
Martin concluded, “Crystals underpin technology – they’re all around us and impact our daily lives. This theory gives researchers simple tools to understand the ‘magic’ of crystal growth and make better predictions. It’s an example of how foundational science lays a foundation for solving all kinds of real-world problems.”
The study appeared in Matter and was partially funded by the National Science Foundation.
Journal Reference:
Martin, J. D. (2024) Solutes don’t crystallize! Insights from phase diagrams demystify the “magic” of crystallization. Matter. doi.org/10.1016/j.matt.2024.08.011