Atomic-scale imaging reveals an easy route to crystal formation

Atomic-scale imaging reveals an easy route to crystal formation

Aluminum hydroxide, shown here in orange, undergoes fluctuations between structures before forming an ordered crystal. Credit: Nathan Johnson | Pacific Northwest National Laboratory

What do clouds, televisions, pharmaceuticals and even the dirt under our feet have in common? All have or use crystals in some way. Crystals are more than luxury gemstones. Clouds form when water vapor condenses into ice crystals in the atmosphere. Liquid crystal displays are used in a variety of electronic products, from televisions to instrument panels. Crystallization is an important step for drug discovery and purification. Crystals also form rocks and other minerals. Their crucial role in the environment is a focus of materials science and health science research.

Scientists do not yet fully understand how crystallization occurs, but the importance of surfaces in promoting the process has long been recognized. Research from the Pacific Northwest National Laboratory (PNNL), the University of Washington (UW) and Durham University is shedding new light on how surface crystals form. Their results were published in Advances in science.

Earlier studies of crystallization led scientists to form the classical nucleation theory—the prevailing explanation of why crystals begin to form or nucleate. When crystals nucleate, they start out as very small ephemeral clusters of only a few atoms. Their small sizes make clusters extremely difficult to detect. Scientists have managed to collect only a few images of such processes.

“New technologies make it possible to visualize the crystallization process like never before,” said Ben Legg, PNNL physical sciences division chemist. He collaborated with PNNL Battelle Fellow and UW Affiliate Professor James De Yoreo to do just that. With the help of Professor Kislon Voitchovsky from Durham University in England, they used a technique called atomic force microscopy to watch the nucleation of an aluminum hydroxide mineral on a mica surface in water.

Mica is a common mineral found in everything from drywall to cosmetics. It often provides a surface for other minerals to nucleate and grow. For this study, however, its most important feature was its extremely flat surface, which allowed the researchers to detect clusters of a few atoms as they formed on the mica.

What Legg and De Yoreo observed was a crystallization pattern not expected from classical theory. Instead of a rare event in which a group of atoms reaches a critical size and then grows on the surface, they saw thousands of fluctuating clusters that joined together in an unexpected pattern with voids that persisted between crystalline “islands.”

After careful analysis of the results, the researchers concluded that while certain aspects of the current theory were valid, their system ultimately followed a non-classical path. They attribute this to electrostatic forces from small surface charges. Because many types of materials form charged surfaces in water, the researchers hypothesize that they have observed a widespread phenomenon and are excited to look for other systems where this non-classical process might occur.

“The assumptions in classical nucleation theory have far-reaching implications in disciplines ranging from materials science to climate prediction,” De Yoreo said. “The results of our experiments can help produce more accurate simulations of such systems.”

Order up: New study reveals importance of liquid structural ordering in crystallization

More information:
Benjamin A. Legg et al., Hydroxide films on mica form charge-stabilized microphases that bypass nucleation barriers, Advances in science (2022). DOI: 10.1126/sciadv.abn7087

Provided by Pacific Northwest National Laboratory

Citation: Atomic-scale imaging reveals facile route to crystal formation (2022, September 23) Retrieved September 23, 2022, from .html

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