Ionic liquids can be useful alternatives to water or organic solvents, e.g., as electrolytes or for the execution of different reactions. However, they are often more toxic due to cationic components with long alkyl chains that can disrupt cell membranes.

In zwitterions, cationic and anionic subunits are covalently connected in a single molecule via a spacer. They generally have lower toxicities and can show similar useful properties to ionic liquids in some respects. However, they are mostly solid at room temperature. Known examples that are technically/thermodynamically “liquid” at room temperature have such high viscosities that they cannot easily be stirred, pipetted, or used to dissolve solutes very well—they act more like, for example, peanut butter than like a traditional organic solvent. “Flowable”, lower-viscosity variants could make the practical application of room-temperature zwitterionic liquids significantly more realistic.

Kosuke Kuroda, Kanazawa University, Japan, and colleagues have developed a room-temperature zwitterionic liquid that is “flowable” and stirrable, called OE₂imOE₃C (pictured above in blue). Starting from the structure of the known zwitterionic liquid OE₂imC₅C (pictured above in red), the team formally replaced the simple, relatively short alkyl chain that acts as a spacer and somewhat hinders the independent movement of the cation and the anion. Instead, they used a more flexible oligoether spacer in OE₂imOE₃C, which better promotes the free movement of the ionic units for a lower overall viscosity.

The desired viscosity reduction was achieved: OE₂imOE₃C is a liquid at room temperature, and its viscosity at 80 °C is only one-seventeenth that of OE₂imC₅C. It maintains a comparable thermal stability to OE₂imC₅C, which cannot be magnetically stirred at room temperature. OE₂imOE₃C also showed low toxicity towards yeast, comparable to another low-toxicity zwitterion model. As an example for a possible application, the researchers demonstrated that OE₂imOE₃C shows high performance as a cellulose solvent, dissolving 11 wt% cellulose at 100 °C within 60 min, a drastic improvement over OE₂imC₅C.

• A low-viscous and flowable zwitterionic liquid,
  Mitsuhiro Shimizu, Tetsuo Fujie, Mayu Shibata, Tetsuo Komori, Kazuaki Ninomiya, Kenji Takahashi, Takuya Uto, Kosuke Kuroda,
  Chem. Commun. 2025.
  https://doi.org/10.1039/D4CC06448H