RUI: Bimolecular Collisions in Ionic Liquids

The University Corporation, Northridge, Northridge CA

Investigators

Abstract

In this project funded by the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) program of the Chemistry Division, Professor Miroslav Peric and his students at the California State University (CSU) Northridge are studying the collisions between molecules dissolved in room temperature ionic liquids (ILs). ILs are a class of liquids which are being used as environmentally-friendlier substitutes for volatile organic solvents. The frequency of molecular collisions in a liquid can provide an idea of how quickly chemical reactions can take place in that liquid. ILs are different from other liquids because they are composed of positively and negatively charged molecules (charged atoms or molecules are called "ions", hence the name ionic liquid). The current understanding of molecular collisions in IL liquids is limited. Water, alcohol, and gasoline are common examples of liquids composed of neutral (non-charged) molecules. Professor Peric dissolves "probe" molecules that have magnetic properties into the ILs. He then uses Electron Paramagnetic Resonance (EPR) spectroscopy to detect changes in the probe molecule's magnetic properties as the probe collides with the IL. EPR measures the magnetic properties of electrons in atoms or molecules. A detailed understanding of collision events in ILs, and how these relate to the structure of the ILs may ultimately allow us to design ILs for specific technological applications. The undergraduate and graduate students working on this research project are gaining valuable exposure to a wide range of disciplines, from solution chemistry to physics and quantum mechanics to complex data analysis. The project is based on the EPR Heisenberg Spin Exchange (HSE) technique that can separate the effects of dipole-dipole and spin exchange interactions in viscous media. This separation enables the measurement of the bimolecular collisions of neutral and charged nitroxides in viscous liquids such as ILs. The translational and rotational diffusion of neutral, singly-charged and doubly-charged nitroxide spin probes in ILs composed of cations and anions of variable sizes is measured as a function of temperature. The choice of the nitroxide spin probes allows for an understanding of the effect of charge on bimolecular collisions of spin probes in ILs. The cations are of a homologous series of substituted imidazoles (ethyl to octyl). This series of cations allows the Peric team to clarify the role of the hydrophobic part of ILs in solution phase dynamics. Similarly, the anions are of two different sizes (single atom anions and large multi-atom anions) in order to understand the influence of the anion on the translation of solute molecules in ILs. The results are interpreted in terms of the fractional Stokes-Einstein relation and the Cohen-Turnbull free volume equation. The broader impacts of this project include an understanding of transport properties and the kinetics of chemical reactions in ILs, as well as an opportunity for the CSU Northridge students to gain experience in experimental research, cutting-edge instrumentation, and theory, all of which are valuable as the students enter the modern chemical sciences workforce. 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.