(733e) Elucidating Composition-Dependent Thermophysical and Structural Properties of Ionic Liquid-Ionic Liquid Mixtures from Atomistic Simulations

Room Temperature Ionic liquids (RTILs) are a class of salts that are liquid at room temperature comprised of exclusively of ions. They are one of the most studied solvents due to their unique properties such as negligible volatility, electrochemical stability, low melting point, high thermal stability, and ionic conductivity. In addition to all these properties, they also offer the possibility of combining different cations and anions from a huge list of available choices to form task-specific ionic liquids, which is why they are also known as designer solvents. Tuning of their characteristics, for example, can be simply achieved by increasing the alkyl chain length attached to the cation or replacing the central atom with another atom in the cation or by changing the chemical composition of the anions and so on. This tuning of ionic liquid properties can be taken a step further by forming a mixture of two or more ionic liquids to form binary mixtures. Several studies have shown that mixing of ionic liquids leads to enhancement and improvement in properties such as decreasing the overall viscosity, increasing conductivity. However, very few studies have been carried to understand on a molecular level how the non-polar domain consisting of the alkyl chains attached to the cation, changes with the mixing of binary ionic liquids, which has implications on key ionic liquid properties such as dielectric constants important for the usage of ionic liquids in supercapacitors.

In this presentation, we will discuss our efforts at using molecular dynamics simulations to study binary mixtures consisting of imidazolium-based cations with pyridinium- and pyrrolidinium-based cations paired with the anion bis(trifluoromethanesulfonyl)imide [NTf₂]^-. Thermodynamic properties such as densities and excess molar volumes are calculated to investigate the volumetric effects of mixing ionic liquids with different alkyl chain lengths and cation types. Transport properties such as ionic conductivity and self-diffusion constant are also calculated to examine the changes in the dynamics as the composition is varied in the system. Our results indicate that the self-diffusion coefficients of imidazolium cation with n=2 and 4 are significantly impacted with the system composition. We will show that this observation can be tied to the changes in the number of non-polar domains as a function of the mole fraction. Additional insights into the structure of these system will be provided in terms of radial distribution function plots, spatial distribution function plots and coordination number plots.