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Probing Structural Origin of Deviations from Ideality in Binary Ionic Liquid Mixtures Using Molecular Dynamics Simulations

Probing Structural Origin of Deviations from Ideality in Binary Ionic Liquid Mixtures Using Molecular Dynamics Simulations

Name:
Utkarsh Kapoor

Department:
Chemical Engineering

Abstract:
Ionic liquids are molten salts composed entirely of ions and have inherent wide chemical variety. The property behavior of these so-called designer solvents can be appropriately tuned by proper ion selection or use of ionic liquid mixtures. An array of intermolecular interactions is known to govern the structure of ILs which affects its physical and chemical properties. Although a comprehensive understanding of structure-property relationship has become possible from experimental and molecular simulation studies conducted over the last two decades for pure ionic liquids, such relationships are only now beginning to be probed for binary ionic liquid mixtures. In an effort to obtain a relation between molecular level interactions and physical properties of such mixtures, three binary ionic liquid systems composed of the common cation 1-n-butyl-3-methylimidazolium [C4mim]+ in combination with anions pairs obtained from chloride Cl-, acetate [CH3COO]-, and trifluoroacetate [CF3COO]- were simulated using atomistic molecular dynamics methodology. Excess molar volumes were computed as a metric to characterize the nonideal behavior as a function of the anion mole fractions. The binary mixture system of [C4mim]+, [CH3COO]-, and [CF3COO]- showed negative excess volumes in contrast to positive excess volumes displayed by the binary ionic liquid system of [C4mim]+, Cl- ,and [CF3COO]-, and near ideal behavior seen for the mixture containing [C4mim]+, Cl-, and [CH3COO]- at 353 K. Interaction energies of the cation were found to vary as a function of the anion molar ratio. On the other hand, the distributions of energies experienced by the anions were observed to be much less sensitive to the anion compositions. These results will be discussed and rationalized using structural correlation techniques such as radial distribution functions, angular distribution functions, and spatial distribution functions.