A Molecular-Based Framework for Studying the Thermodynamic Properties of Ionic Liquids

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CBET-0829062McCabeIonic liquids (ILs) are liquids comprised entirely of ions that have melting points at or below room temperature, distinguishing them from high temperature molten salts. ILs are at the forefront in the use of alternative and greener solvents due to their negligible volatility, which minimizes the risk of atmospheric contamination and reduces associated health concerns in comparison with conventional organic solvents. ILs show promise in a wide range of applications from catalysis and biocatalysis, to separations, and photochemical cells. The interest in ILs stems from their unique physical properties, that extend beyond the range of normal molecular solvents, and can include (in addition to no or low volatility) thermal stability, a wide range of temperature over which they are liquid, and the ability to solubilize a wide variety of organic and inorganic materials, including macromolecules. ILs generally consist of a large, organic cation with a weakly coordinating inorganic or organic anion, which frustrates packing and lowers the melting point. The cations and anions each impart different physiochemical properties to the IL and can be substituted to obtain the properties desired, and if needed, functionalized to provide further control. While initial interest in ILs focused primarily on their solvent properties, more recently the potential to use their novel, tunable, physical and chemical properties in the design of new functional materials for a wide range of applications has fueled the interest in ILs. As a result, there has been a great deal of progress to date in IL design; however, much of that progress has been achieved through an empirical approach to property modification. The vast number of possible cation-anion combinations makes it vitally important to be able to rationally predict IL properties based on their constituents. Thus, there is considerable incentive to develop an accurate tool for designing task-specific ILs based upon a physical understanding of the structure and interactions within ILs. The goal of our work is to address this important need through the development of a molecular-based theoretical framework with which to study the thermodynamic properties of ILs. The complex nature of the interactions present in ILs makes their study using computational tools a fundamentally challenging subject. The overall goal is to develop a framework with which to accurately model IL systems and their mixtures with other molecular species, and enable the prediction of their thermodynamic properties. In this work we will focus on developing a molecular-based model for pure ILs, which will lay the foundation for future work on modeling mixtures of ILs and molecular solutes/solvents and the predictive design of task-specific ILs. The approach developed will be optimized against experimental data and ab initio calculations and will enable the prediction of IL properties from the chemical composition, thus eliminating guess work in determining the properties of a given combination of ions and, ultimately, their mixture behavior with molecular species. Specifically, the project e will create a theoretical tool to describe ILs built on the SAFT framework for modeling fluid phase behavior. SAFT, in contrast to the various engineering based equations of state available in the literature, provides a molecular-based approach and is truly unparalleled in its ability to capture the effects of molecular shape, size and interactions into an analytical equation of state. In order to describe the thermodynamic properties of ILs with the molecular-based model outlined above, the PI will develop a theoretical framework for IL systems that combines analytical solutions from integral equation theory (that accurately describe the structure of dipolar and ionic fluids) with the molecular framework of the SAFT-VR approach. The centerpiece of the proposed approach will be the hetero-segmented SAFT model developed by the PI,12 which contrasts with SAFT models that are based on a homo-segmented approach(i.e., each segment in the model chain has the same size and energy of interactionparameters). Intellectual Merit of the Proposed Research - The proposed work will lead to new methods to map properties obtained from quantum chemistry calculations and molecular simulations onto the parameters needed in statistical-mechanics-based models and new developments in liquid state theory. The work will lay the foundation for a significant advance in molecular thermodynamics by providing a predictive IL design methodology based upon a deep physical understanding of IL structure and interactions.Broader Impact of the Proposed Research - The integration of the proposed researchwith the PIs educational activities will ensure that students at Vanderbilt are exposed to, and participate in, the frontiers of molecular modeling and molecular thermodynamics. Participation of minorities will be sought through the NSF TLSAMP4 program at Vanderbilt and high school teachers involved through the PIs participation in a School of Engineering RET site program.Overall plan - AIM 1: A molecular-based model for pure ILsAIM 2: A molecular-based model for mixtures of ILs and molecular solvents/solutesAIM 3: Predictive design of task-specific ILsThe PI will limit the scope of this revised exploratory project to aim one, a molecular-based model for pure ILs since it represents the most challenging and novel part of the work. The work will focused on (1) the derivation and application of new developments in liquid state theory to describe the combined effects of electrostatic interactions, charge delocalization, and polarity in ILs; (2) the development of new methods to map quantum chemistry calculations and molecular simulation results onto the parameters in the underlying theoretical models; and (3) application of these new developments to experimental IL systems.