Salt takes a half step before falling out of solution as a crystal

Thursday, May 2, 2019
by Scott Lyon, Chemical and Biological Engineering

When a drop of sea spray lands on a rock and starts to evaporate under the midday sun, the salt solidifies and falls out of the water as a crystal—helping to power the Earth's atmosphere and leaving a delicious kernel of spice for dinner.

New computational research from a CBE team has shown that process to include an extra step, a finding that has implications for everything from climate models to the production of medicine.

"We were trying to understand how solids fall out of solution as crystals," said Athanassios Panagiotopoulos, the Susan Dod Brown Professor of Chemical and Biological Engineering and the project's lead researcher. "This happens when you are trying to make a pharmaceutical component, to get your desired ingredient in pure form. It is also relevant for atmospheric processes. So, both for environmental applications and technological applications, this is a very important process."

The results, published as a cover article in the Journal of Chemical Physics on March 22, show that when salts in a highly supersaturated solution precipitate as crystals, they first go through a brief intermediate phase. In this first quick step, salt ions in the solution form disordered clusters that the researchers have called "amorphous salt," which represent a semi-crystalline state. That state lasts between 10 and 100 nanoseconds, mere billionths of a second, before the semi-crystals begin to rearrange themselves into a more ordered state as true crystals.

The work required complex computational models running for several months at Princeton’s high-performance computing center to see how these solutions evolve beyond their saturation thresholds. The researchers believe their findings will allow scientists to have a better, more accurate framework for experimental results.

In addition to Panagiotopoulos, chair of chemical and biological engineering, the team included Hao Jiang, formerly a postdoctoral researcher in this department and now at the University of Pennsylvania; and Pablo Debenedetti, the Class of 1950 Professor in Engineering and Applied Science and Princeton's Dean for Research.

Financial support for this work was provided by the Office of Basic Energy Sciences, U.S. Department of Energy, under Award No. DE-SC0002128. Additional support was provided by the National Oceanic and Atmospheric Administration (Cooperative Institute for Climate Science Award No. AWD 1004131) and the U.S. National Science Foundation under Award No. CBET-1402166.