One-millionth of a gram of a new molecule in a metric ton of water will be able to capture all of the salt. The new salt-extraction molecule created at Indiana University
(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.
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.
It only takes one teaspoon of salt to permanently pollute five gallons of water.
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 metric tons of dissolved solids, including salts, enter freshwater streams per year.
Researchers have created a powerful new molecule for the extraction of salt from liquid.
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.”
Tight binding and high selectivity are hallmarks of biomolecular recognition. Achieving these behaviors with synthetic receptors has usually been associated with OH and NH hydrogen bonding. Contrary to this conventional wisdom, we designed a chloride-selective receptor in the form of a cryptand-like cage using only CH hydrogen bonding. Crystallography showed chloride stabilized by six short 2.7-Å hydrogen bonds originating from the cage’s six 1,2,3-triazoles. Atto-molar affinity (10^17 M–1) was determined using liquid-liquid extractions of chloride from water into nonpolar dichloromethane solvents. Controls verified the additional role of triazoles in rigidifying the 3D structure to effect recognition affinity and selectivity. This cage shows anti-Hofmeister salt extraction and preliminary corrosion inhibition.
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