Understanding the Microscopic Behavior of Binary Mixtures of Ionic Liquids through Various Spectroscopic Techniques

In recent times, it has been shown that certain binary mixtures of pure ionic liquids having appropriate chemical composition can behave like a new chemical entity. However, current knowledge about the microscopic behavior of these interesting systems is rather limited. The present study is undertaken with an objective to understand the microscopic behavior in terms of intermolecular interaction, structure, and dynamics of these systems. In the present study, few (IL + IL) mixtures are chosen with a common cation and a variation of anion. The investigations are also carried out by taking individual pure ILs so that the difference in the behavior of pure IL and (IL + IL) mixtures is understood. Initially, the systems have been investigated by studying the thermophysical properties of the concerned mixtures. The synergistic effect between combining pure ILs through photochromism has also been studied. These mixtures have been investigated further through steady-state and time-resolved fluorescence spectroscopy, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and fluorescence correlation spectroscopy (FCS). Interestingly, time-resolved fluorescence data also pointed out that (IL + IL) mixtures are not only spatially heterogeneous but also dynamically heterogeneous. EPR measurements have suggested that the micropolarity ( E_T(30)) of the (IL + IL) mixture is close to that of aliphatic polyalcohols. Measurements of translational diffusion coefficients of the diffusing species through NMR and FCS studies have provided an idea about the nanostructural organization within (IL + IL) binary mixtures. The analysis of data essentially reveals that the mixtures of ILs that are used in the current study do not behave like a nonideal solution. The behavior of the IL mixtures is observed to be more like quasi-ideal type.