Collaborative Research: Simple Paradigms for Understanding Viscosity in Ionic Liquids

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The overarching goal of the joint project by Professors Margulis (U. of Iowa) and Castner (Rutgers), funded by the NSF Chemical Structure and Dynamics Mechanisms (CSDM-A) program, is to understand how the molecular structure of a special class of liquids determines their viscosity. Viscosity is a measure of how well a liquid flows (for example, pure honey is more viscous than water). Professors Margulis and Castner are specifically interested in Ionic Liquids (ILs). ILs are unique materials; like table salt they are solely composed of positive (cationic) and negative (anionic) species, but unlike table salt, which is a solid at ambient conditions, they are liquids at room temperature. The reason why understanding and controlling the viscosity of ILs is important relates to their many possible applications as lubricants, as solvents for industrial processes or in advanced battery materials. On a microscopic length scale these liquids are complex, with networks of positive and negative charges separated by uncharged domains. The research team is using a battery of techniques including computer simulations, nuclear magnetic resonance, as well as X-ray and neutron scattering to attack these challenging questions. Researchers in this study range from undergraduate level students to graduate students to postdoctoral fellows, each contributing and gaining unique experiences on experimental and theoretical tools. One of the most pressing questions about ionic liquids is how to control their viscosity for specific applications. These liquids are structurally and dynamically heterogeneous with networks of charge being the stiffer component, and it is the relaxation with time of the charge alternation pattern within these that is directly linked to viscosity. The research team is seeking to understand how this process of “charge-blurring” occurs. For this, they are using theoretical calculations and molecular dynamics simulations as well as X-ray scattering, neutron scattering techniques that allow for temporal resolution of the dynamics of the different liquid structural motifs, and NMR techniques. The ultimate goal is to derive a predictive and pictorial view of what causes the viscosity for specific ILs. The broader impacts of the study are manifold; they include supplying the community of researchers working on ILs with an understanding of how viscosity might be controlled for different scientific and technological applications. In addition to providing formal training for graduate students and post-doctoral fellows, the project is providing research experience for undergraduate students, including some from Queensborough Community College.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.