Collaborative Research: Ionic Liquids at Interfaces: from Bulk Structure to Tunable Films

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The Chemical Structure, Dynamics and Mechanisms Program of the NSF Chemistry Division (CSDM-A) supports a Collaborative Research effort by Professors Claudio J. Margulis at the University of Iowa (P.I.) and Edward W. Castner, Jr. (co-P.I.) at Rutgers, The State University of New Jersey, with synthetic collaborations including Professor Sharon Lall-Ramnarine at Queensborough Community College. The focus of this research is to develop a more complete understanding of the molecular level structure of a special class of liquids called ionic liquids. Ionic liquids are gaining interest for their potential applications in various technologies for example, batteries and gas separation systems. Professors Margulis and Castner are studying how the molecular structure of ionic liquids affects such properties as their "thickness" (viscosity) and electrical conductivity. Ionic liquids are an interesting class of compounds that, like salts are composed of positively charged atoms or molecules (cations) and negatively charged atoms or molecules (anions). Ionic liquids are salts that are molten at or near room temperature. The research involves both experimental techniques (e.g., x-ray spectroscopy and synthesis) and computer modeling. The research project is expected to increase our understanding of ionic liquid properties, so that they can be better applied to a variety of problems in modern energy storage technologies, as well as other novel applications. The students involved in this research project are gaining experience in a highly interdisciplinary environment that challenges graduate students in non-traditional ways. For example, students in the Margulis group whose interests and expertise are primarily in theory and computation spend time in the Castner laboratory, where they gain experience in actual experiments. At the same time, the experimentalist graduate researchers from the Castner group are doing simulation work under Prof. Margulis' guidance. Finally, the Queensborough Community College collaboration is providing research experiences for selected undergraduate students from a minority-serving Associates-level institution. Liquids can exhibit structural order at various length scales, from simple pair-wise interactions between two molecules to hydrogen-bonded networks that may extend over several nanometers. Ionic liquids have been shown to have an intermediate range of structural order arising from the combination of the strong interactions between the positively charged cations and the negatively charged anions, with hydrophobic groups on these molecular ions. Some of the Margulis and Castner groups' recent work has shown that ionic liquids can be designed such that this intermediate range order is disrupted. Such structural disruptions are correlated to lower viscosities, which provide opportunities for the development of superior electrolytes for both enhanced battery and electrical double-layer technologies. In this project, the Margulis and Castner teams are seeking to understand whether ionic liquids with certain combinations of ions will further disrupt intermediate range order. Computational molecular dynamics studies guide and explain experimental results, including those from high-energy X-ray experiments at synchrotrons (such as the Advanced Photon Source at Argonne National Laboratory), from measurement of ion diffusivities using nuclear magnetic resonance techniques, and density and viscosity experiments. Another important aspect of the project is to obtain a quantitative understanding of the arrangement of the ionic liquid anions and cations in vacuum or specific solid interfaces. The Castner group is extending the bulk liquid simulations to include vacuum interfaces, while angle-resolved X-ray photoelectron spectroscopy measurements are providing the density profiles of specific atoms at the interface. Both the Margulis and Castner groups continue to participate in both computational studies of ionic liquids and synchrotron-based X-ray experiments, providing excellent collaborative cross-training research experiences for younger scientists. Inclusion of Professor Lall-Ramnarine and her students provides a first research experience for community college students.