(453e) Is Difference in Hydrogen Bond Ability of Anions an Exclusive Descriptor of Nonideality in Binary Ionic Liquid Mixtures?

Ionic liquids (ILs) are molten salts composed entirely of ions that exist as liquids under ambient conditions. The traditional approach for tuning the property behavior of ionic liquid has taken advantage of a proper ion selection. Recently, the use of ionic liquid mixtures has been proposed as a strategy to obtain task-specific properties of ionic liquid solvent systems. As a result of which, a thorough understanding of the structure-property relationships is only now beginning to be probed for binary ionic liquid mixtures. One of outstanding questions in the binary ionic liquid mixture research is whether there are molecular descriptors that can be used to predict nonideal behavior of such mixture systems. In an effort to answer this question, atomistic molecular dynamics (MD) simulations were carried out to evaluate if the difference in hydrogen bonding ability of the anions can serve as a primary descriptor for nonideal in binary ionic liquid systems. For this study, three sets of binary IL systems containing the common cation 1-n-butyl-3-methylimidazolium ([C₄mim]⁺)in combination with chloride (Cl^-) and acetate ([CH₃COO]^-), Cl^- and trifluoroacetate ([CF₃COO]^-), and [CH₃COO]^- and [CF₃COO]^- _­were considered. The excess molar volumes were used as a metric for assessing the nonideal behavior of these systems. The Cl-[CH₃COO] system exhibited nearly ideal behavior as the hydrogen bonding ability of the anions were similar. On the contrary, positive deviation was observed for the Cl-[CF₃COO] system which is not consistent with difference in the hydrogen bonding ability of anions. Further, the [CH₃COO]-[CF₃COO] system displayed negative excess molar volume despite very small difference in the hydrogen bonding ability of the two anions and the fact that considerable structural changes were observed in first solvation shell of the cation. Distribution of specific interaction energies, combination of van der Waals and electrostatic interaction experienced by cation and anions individually were evaluated and will be presented to rationalize the excess volume behavior for these binary IL systems. The results obtained for the three systems will be compared to those reported in our previous study¹ where nonideal behavior was accounted for by differences in the hydrogen bonding strength that leads to changes in structural organization of ions.

Overall, our results from MD simulations of binary IL mixtures indicate that changes in local microstructure of ILs can provide only a partial understanding of the nonideality in the binary ionic liquid mixtures, and a delicate balance between hydrogen bonding, electrostatic and dispersion interactions is required to explain the origin of nonideality.

1. Kapoor, U. and Shah, J. K., “Preferential ionic interactions and microscopic structural changes drive nonideality in binary ionic liquid mixtures as revealed from molecular simulations” Ind. Eng. Chem. Res., 55, 13132-13146 (2016).