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Building a better salt trap: IU researchers synthesize a molecular ‘cage’ to trap chloride

New molecule could help reduce growing level of salt contaminants flowing into freshwater streams and lakes across the U.S.

For Immediate Release May 23, 2019

BLOOMINGTON, Ind. – Indiana University researchers have created a powerful new molecule for the extraction of salt from liquid. The work has the potential to help increase the amount of drinkable water on Earth.

Built using chemical bonds previously regarded as too weak, the new molecule is about 10 billion times improved compared to a similar structure created over a decade ago at IU. The molecule’s design is reported today in the journal Science.

“If you were to place one-millionth of a gram of this molecule in a metric ton of water, 100 percent of them will still be able to capture a salt,” said Yun Liu, who led the study as a Ph.D. student in the lab of Amar Flood, the James F. Jackson Professor of Chemistry and Luther Dana Waterman Professor in the IU Bloomington College of Arts and Sciences’ Department of Chemistry.

The molecule is designed to capture chloride, which is formed when the element chlorine pairs with another element to gain an electron. The most familiar chloride salt is sodium chloride, or common table salt. Other chloride salts are potassium chloride, calcium chloride and ammonium chloride.

At the same time that the human population continues to grow, the seepage of salt into freshwater systems is reducing access to drinkable water across the globe. In the U.S. alone, the U.S. Geological Survey estimates about 272 million metric tons of dissolved solids, including salts, enter freshwater streams per year. Contributing factors include the chemical processes involved in oil extraction, the use of road salts and water softeners, and the natural weathering of rock. It only takes one teaspoon of salt to permanently pollute five gallons of water.

The new salt-extraction molecule created at IU is composed of six triazole “motifs” – five-membered rings composed of nitrogen, carbon and hydrogen – which together form a three-dimensional “cage” perfectly shaped to trap chloride. In 2008, Flood’s lab created a two-dimensional molecule, shaped like a flat doughnut, that used four triazoles. The two extra triazoles give the new molecule its three-dimensional shape and 10 billionfold boost in efficacy.

The molecule is also unique because it binds chloride using carbon-hydrogen bonds, previously regarded as too weak to create stable interactions with chloride compared to the traditional use of nitrogen-hydrogen bonds. Despite expectations, the researchers found that the use of triazoles created a cage so rigid as to form a vacuum in the center, which draws in chloride ions.

By contrast, cages with nitrogen-hydrogen bonds are often more flexible, and their vacuum-like center needed for chloride capture requires energy input, lowering their efficiency compared to a triazole-based cage.

“If you were to take our molecule and stack it up against other cages that use stronger bonds, we’re talking many orders of magnitude of performance increase,” Flood said. “This study really shows that rigidity is underappreciated in the design of molecular cages.”

The rigidity also enables the molecule to retain its shape after the central chloride has been lost, compared to other designs that collapse under the same circumstances due to their flexibility. This gives the molecule greater efficacy and versatility.

Lastly, the work is reproducible. The first molecule took nearly a year to synthesize, said Liu, who was shocked to discover that the crystals required to confirm the molecule’s unique structure had formed after the experiment was left alone in the lab for several months – a surprising occurrence since that process typically requires careful monitoring. Later, Wei Zhao, a postdoctoral researcher in Flood’s lab, was able to re-create the molecule in a span of several months.

The formation of the crystal represented a “eureka” moment, proving that the molecule’s unique design was actually viable, Liu said.

The final author on the study is Chun-Hsing “Josh” Chen, an associate scientist at the IU Molecular Structure Center at the time of the study, who confirmed the molecule’s structure using X-ray crystallography. Liu is currently a postdoctoral research associate at the University of Illinois at Urbana-Champaign.

This work was supported in part by the U.S. Department of Energy. A patent application on the work has been filed by IU’s Innovation and Commercialization Office.

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