Using the surface sensitive synchrotron X-ray capabilities at the U.S. Department of Energy’s Advanced Photo Source, researchers were able to figure out that the structure of the vapor/liquid interface of an ionic liquid is actually made up of tiny crystals even 100 °F above the liquid’s melting point. Ionic liquids consist of positive and negative ions. Because of this, they have many interesting properties, like low volatility, that make them potential green alternatives to conventional solvents for commercial syntheses. These liquid salts have many different possible types of short and long-range interactions thus complicating studies to understand their definitive molecular level structure. The X-ray results show that the negative ions of the ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate interact with the rings of the molecules in such a way that their 4-carbon chains are forced out towards the vapor side, which leads to the very thin nanocrystalline layer at the interface. This finding has interesting implications for the design of new ionic liquids and for understanding their inhomogeneous structure.
Y. Joen, D. Vaknin, W. Bu, J. Sung, Y. Ouchi, W. Sung, and D. Kim, “Surface Nanocrystallization of an Ionic Liquid”, Physical Review Letters, 2012, 108, 055502. DOI 10.1103/PhysRevLett.108.055502