A new crystallization theory shows that the solvent, not the solute, controls crystal formation. This two-step process improves predictions of crystal growth and has practical applications in areas like medicine and technology.
Do you remember that old high school chemistry experiment where salt crystals would form from a saltwater solution—or the one where sugar crystals grew into rock candy from sugar water? It turns out that your understanding of how crystals formed in those solutions might be wrong.
A new theory “demystifies” the crystallization process and shows that the material that crystallizes is the dominant component within a solution – which 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 in everything from technology to medicine – but our actual understanding of the crystallization process has been lacking,” says James Martin, professor of chemistry at North Carolina State University and author of a paper in Matter that outlines the theory.
High School Chemistry Revisited
“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,” Martin says.
“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,” Martin says. “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.”
Thermodynamic phase diagrams, which describe concentration and temperature-dependent transition points in solutions, can be used to illustrate the new theory, dubbed the transition-zone theory.
The theory demonstrates that crystallization happens in two steps: first a melt-like pre-growth intermediate forms. Then that intermediate can organize into the crystal structure.
“To grow a crystal out of a solution, you have to quickly separate the solvent and solute,” Martin says. “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.”
Applications and Practical Implications
Martin applied his theory to a number of different solutions, concentrations, and temperature conditions and found that it accurately describes 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,” Martin says. “Instead, it is necessary to understand cooperative ensembles of the solvent to describe crystal growth.”
According to Martin, the important aspect of the new theory is its focus on understanding how solute impurities disrupt that cooperative ensemble of solvent.
“By understanding the interplay of temperature and concentration, we can predict exactly how fast and large crystals will grow out of solution.”
Martin believes the phase diagrams could have important applications for not just crystal formation, but for preventing crystal formation, such as preventing kidney stones from growing.
“Crystals underpin technology – they’re all around us and impact our daily lives,” Martin says. “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 the foundation for solving all kinds of real-world problems.”
Reference: “Solutes don’t crystallize! Insights from phase diagrams demystify the “magic” of crystallization” by James D. Martin, 2 October 2024, Matter.
DOI: 10.1016/j.matt.2024.08.011
The paper was supported in part by the National Science Foundation.
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4 Comments
Help the semi-initiated with this dynamical terminology, please. Especially confusing is when solute becomes solvent, but then it seems to then remain referred to as the solute. I am baffled, but this chemistry jargon could be in parallel by example to the ‘debits and credits’ in finance.
That’s what I’ve been telling the chemistry teacher back in the day, but the old bat wouldn’t listen.
Crystals can also form inside other crystals. The late Dr Yuri Mnyukh wrote a book about that, that’s still on amazon, but no one seems interested because they prefer their own theories. I can’t tell from this article if it blames the water or the salt, but this book proves it’s the salt and shows how crystal defect is necessary for growth, even one crystal in another. So a flawless crystal with no defects cannot transition, and can remain in a superconductive state in room temperature, if it was formed in a superconductive (cold) state. Don’t blame me if you don’t agree, I’m not a PhD myself. Just helped edit the book as Mnyukh isn’t a native English speaker.
This information is more important to avoid crystal growth out of solvent such as water as you drink more and more water it prevents crystal growth that prevents kidney stone growth problems. If only laser assisted information in kifneys that msu prevent solid phase out of diluted water as a solvent more frequently taken. The ne medicine such as Punarva solution msy be subjected to such a study.