New Chains For Ionic Liquids

Ionic liquids can be manipulated to have lower than expected melting points by harnessing the same approach that nature uses to keep cell membranes fluid at low temperatures, according to a study. Building on what is known about fatty acid and phospholipid behavior, the findings create new opportunities for ionic liquids, the researchers say, and provide food for thought for scientists interested in membrane properties and soft synthetic materials.

Ionic liquids are salts commonly consisting of an organic cation such as an imidazole coupled with an inorganic anion. By definition, they have a melting point below 100 °C. The asymmetry of the cation-anion pair and delocalized charges in the crystal lattice produce a softening of the lattice, leading to a low melting point.

Chemists have been challenged, however, to design ionic liquids that incorporate progressively more lipophilic components while keeping the melting points below room temperature—the melting points start to increase once saturated alkyl side chains exceed about seven carbon atoms. Kevin N. West and James H. Davis Jr. of the University of South Alabama and their colleagues realized that a solution might come from the kinked structure of unsaturated alkyl chains found in phospholipids, which disrupt tight chain packing to help keep cell-membrane bilayers fluid.

To test their hunch, the researchers prepared imidazolium salts with an array of N-substituted fatty acid side chains, including versions with linear saturated chains, double bonds in different locations, and double bonds with different cis-trans geometries (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200906169). They found that double-bond geometries leading to more extreme kinked structures make an enormous difference. For example, an imidazolium salt with a C₁₈ stearic acid side chain has a melting point of 53.5 °C, whereas the corresponding C₁₈ linoleic analog with a significant bend in the alkyl chain has a melting point of -46.8 °C.

The lipidlike ionic liquids are good solvents for cholesterol, a major membrane component in many organisms, the researchers note. With this property, they suggest, ionic liquids could solubilize drugs and help them cross cell membranes or perhaps serve as solvents for lipase-mediated chemistry.

The research will help "bridge the gaps between knowledge in far-flung fields with what we know or need to know about ionic liquids and their future applications," says Robin D. Rogers, an ionic liquids specialist at the University of Alabama, Tuscaloosa.

"Everyone wants to compartmentalize knowledge and stick to truisms such as longer alkyl chains are more toxic, increase hydrophobicity, and raise melting points," Rogers adds. "I like the fact that these researchers are using these examples to get the ionic liquids community to broaden its thinking."