The Chemical Structure, Dynamics and Mechanisms Program of the NSF Chemistry Division supports a Collaborative Research effort by Assoc. Prof. Claudio J. Margulis (P.I.) at the University of Iowa Dept. of Chemistry and by Prof. Edward W. Castner, Jr. (co-P.I.) at the Dept. of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey. The goal is to study in detail the structural and dynamical properties of ionic liquids using a combination of theoretical and computational methods from the Margulis group with experimental investigations done by the Castner group. Ionic liquids consist of flexible, asymmetric molecular cations and anions. Several reports have shown that very small modification of either the cation or the anion components of the ionic liquid can make dramatic changes to its properties. For example replacing the central nitrogen atom of a tetralkylammonium cation by a phosphorus atom can lead to a drop in the liquid viscosity by a factor of two. Our current understanding of these liquids does not provide a straightforward explanation of these results; our goal is to combine x-ray and NMR experiments on these liquids with detailed molecular simulations and theoretical analyses to explain this phenomenon.
The observed x-ray scattering patterns from ionic liquids that all share the same bis(trifluoromethylsulfonyl)amide anion are remarkably similar. This is because the anions have more electron-rich atoms than the hydrocarbon-based cations. Thus, whereas the cations may be the most important organizational components in many liquids, the information content in the x-ray scattering provides mostly detailed information about the anions with information about the cations being obtained only indirectly. We will invert this circumstance by preparing ionic liquids with smaller anions such as nitrate, dicyanamide and acetate, so that the observed x-ray scattering patterns result predominantly from scattering by the cations. This will provide a new perspective on the structure of ionic liquids. It will also help in testing and designing better parameters for simulations.
While the research is of a fundamental nature, the outcomes will inform the design, testing and selection of ionic liquids for a range of electrolyte applications and interaction with biological molecules and assemblies. These liquids are providing solutions to a number of significant challenges in energy storage, nanotechnology, processing of biomass, and pharmaceutical research. Undergraduate students, graduate students and postdoctoral researchers will receive training in the latest experimental methods, including synchrotron-based x-ray scattering experiments, two-dimensional NMR spectroscopy, molecular dynamics simulations, and theoretical methods applied to the statistical physics of complex liquids.