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Ionic liquids: a step closer to clean technology


Conventional solvents are highly flammable and volatile materials, which also pollute the environment. A novel class of liquids, room temperature ion liquids (RTILs), have emerged recently as a very promising, environment-friendly replacement for conventional solvents and working fluids in many applications. Using synchrotron radiation, an international team of scientists has solved the liquid-air interface structure for RTILs, setting them a step closer to use in industrial applications.

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Many industrial processes employ organic solvents, like alcohols and halo-organics, which are highly volatile and thus pose an environmental pollution hazard. Room temperature ionic liquids, organic salts that remain liquid to below 100 C, are a completely different class of solvents, which have practically no, or very low, vapour pressure. This is only one of their many advantageous properties which render them solvents and working fluids of choice for numerous applications in petrochemistry, pharmaceutics, industrial waste treatment, batteries, supercapacitors, and solar cells.


An image from the experiment showing X-ray reflection from the surface of an ionic liquid. ©Bar-Ilan University.

Scientists from Bar-Ilan University (Israel), the ESRF, Kiel University (Germany) Stanford University (USA)  and Brookhaven National Laboratory (BNL) (USA) have now determined with sub-molecular resolution the liquid-air interface structure of a broad homologous series of RTILs, using mainly synchrotron x-ray reflectivity at ESRF (France), DESY (Germany) and APS (USA).

The results were published in PNAS this week. “The RTIL surface was found to evolve upon  increasing the cation's hydrocarbon chain length from  an unstructured  simple-liquid surface to a layered structure, then to a liquid-crystalline phase, indication an increasing dominance of the van der Waals interaction.” Says  Prof. Moshe Deutsch (Bar-Ilan University), who led the research together with Dr. Ben Ocko (BNL). “Understanding RTILs’ molecular organization and the underlying interactions will allow for better matching a specific RTIL to a specific application as  solvent in a chemical process or as an electrolyte in batteries and supercapacitors.” adds Dr. Ocko.

The next step for the team is to study the temperature evolution of the surface structure of these materials. Harald Reichert, director of research at the ESRF and co-author of the study, explains that “RTILs have been in the limelight for applications as solvents since the late 90s. Despite the cost of manufacturing, we have made great progress in the last decade and I see this as a big contender to replace conventional solvents, especially in the ever growing field of batteries”.

Haddad, J. et al, PNAS Early Edition, 22 January 2018. Doi:10.1073/pnas.1716418115