A new theory has clarified the crystallization process, indicating that the primary material that forms crystals in a solution is actually the solvent, not the solute. This theory may have significant implications across various fields, including pharmaceutical development and climate change research.
Do you recall the classic high school chemistry experiments where salt crystals form from saltwater, or where rock candy is made from sugar water? It seems that what you thought you knew about crystal formation in these contexts might need a rethink.
A recently proposed theory sheds light on the crystallization process, asserting that the material that crystallizes is the dominant element of a solution — specifically, the solvent instead of the solute. This theory could influence a broad range of areas from medication formulation to climate studies.
“Crystals are everywhere — they play a role in technology as well as medicine — but our understanding of how they crystallize has been limited,” explains James Martin, a chemistry professor at North Carolina State University and author of a study published in Matter that presents this new theory.
“The common understanding posits that dissolving and crystallizing are essentially opposites; however, that is not the case. They are fundamentally different processes,” Martin clarifies.
Taking the high school chemistry experiment of creating a precipitate as an example, Martin illustrates: “When I dissolve salt (the solute) in water (the solvent), the water is the dominant component. It dissolves the salt by essentially breaking it down. If I aim to grow a salt crystal from that solution, the dominant phase must shift to the salt, which becomes the solvent at that stage and subsequently forms the crystal.”
The new concept, referred to as transition-zone theory, can be depicted using thermodynamic phase diagrams that highlight the relationship between concentration, temperature, and transition points within solutions.
The theory indicates that crystallization occurs in two phases: firstly, a melt-like intermediate appears, and then this intermediate can arrange itself into a crystallized structure.
“To facilitate crystal growth from a solution, you must effectively separate the solvent from the solute,” Martin explains. “The ‘melt’ refers to the pure phase of the solvent before crystals form. The key takeaway from my theory is that you can achieve better and faster crystal growth by adjusting the solution to emphasize the solvent; in essence, it’s the solvent, rather than any impurities it contains, that governs the rate at which crystals grow.”
Martin tested this theory across various solutions, concentrations, and temperature conditions, finding that it accurately captures both the growth rate and size of crystals.
“The main flaw in earlier explanations of crystallization was the assumption that crystals grow when individual solute particles diffuse toward and attach to an expanding crystal surface,” Martin states. “In reality, one must understand the collaborative behavior of the solvent to accurately describe how crystals develop.”
According to Martin, a significant aspect of this new theory is its emphasis on how solute impurities can disrupt that cooperative behavior of the solvent.
“By analyzing the interactions between temperature and concentration, we can precisely predict the speed and scale of crystal growth in a solution.”
Martin asserts that phase diagrams may be crucial not only for understanding crystal growth but also for preventing unwanted crystal formation, such as in the case of kidney stones.
“Crystals are fundamental to technology — they are ubiquitous and influence our daily lives,” Martin remarks. “This theory provides researchers with straightforward tools to grasp the complexities of crystal growth and make more accurate predictions. It’s a prime example of how foundational science can lay the groundwork for addressing a variety of real-world challenges.”
This research is published in Matter and received partial support from the National Science Foundation.
