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Materials

Ionic liquids bring order to themselves

Room-temperature liquid salts arrange their ions into surprisingly thick layers that could be useful for energy storage

by Matt Davenport
May 30, 2016 | APPEARED IN VOLUME 94, ISSUE 22

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Credit: Shaw Group
Researchers observed long-range ordering of ions in ionic liquids as they oozed down a silver surface.
Credit: Shaw Group
Researchers observed long-range ordering of ions in ionic liquids as they oozed down a silver surface.

Like Paul McCartney, researchers studying ionic liquids have found wisdom in three simple words: Let it be. Ionic liquids are viscous fluids made of salts that are unable to crystallize at or near room temperature. Despite their inability to package themselves into solids, ions within the liquids can arrange themselves into orderly fluid layers near interfaces, for instance between the liquid and air. These layers can stretch for dozens of nanometers, or so researchers believed. Radhika S. Anaredy and Scott K. Shaw of the University of Iowa have now discovered that ionic liquids can form micrometer-thick ordered layers, when researchers are patient (Langmuir 2016, DOI: 10.1021/acs.langmuir.6b00304). The Iowa chemists typically analyze interfaces in ionic liquids using infrared spectroscopy and other optical techniques as the materials ooze down rotating, mirrored silver surfaces. But Anaredy and Shaw found that stopping the rotation and letting the liquid be for 10 to 30 minutes permitted the ions to self-organize into thicker layers. The duo is uncertain as to what drives the ordering, but Shaw says this discovery could help researchers use ionic liquids to design better capacitors for energy storage or proton-conducting solvents that help convert carbon dioxide into methane and other fuels.

Credit: Langmuir
An example of time-dependent ordering of the negative charges in ionic liquids containing bis(trifluoromethylsulfonyl)imide anions, which orient themselves parallel to a surface.
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