(253bb) Elucidation of Microstructure and Its Connection to Thermophysical Properties of Binary Ionic Liquid Mixtures, a Molecular Simulation Study

Authors: 
Kapoor, U., Oklahoma State University
Shah, J. K., Oklahoma State University
Ionic liquids (ILs) 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 through 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. For pure ionic liquids, a thorough understanding of structure-property relationship has become possible from experimental and molecular simulation studies conducted over the last 15 years. However, such relationships are only now beginning to be probed for binary ionic liquid mixtures. Using atomistic molecular dynamics simulations, a study to obtain the relation between different interactions and physical property trend of ionic liquid mixtures was carried out. The work focused on two different mixture systems containing common cation 1-n-butyl-3-methylimidazolium [C4mim]+ in combination with chloride Cl- and methylsulfate [MeSO4]- and Cl- and bis(trifluoromethanesulfonyl)imide [NTf2]-ô??? anions. Thermodynamic properties such as densities and excess molar volumes were computed. The binary mixture system [C4mim]Cl[MeSO4] showed negative excess molar volumes in contrast to positive excess volumes displayed by [C4mim]Cl[NTf2]. Cation-anion and cation-cation interactions were examined using various structural correlation techniques like radial distribution function, angular distribution function, spatial distribution functions in order to rationalize the nonideal behavior. The correlated motion between ion pairs, quantified by ion pair lifetime time constants, was found to affect the self-diffusion coefficients and ionic conductivities for both the mixture systems.