Solute Rotation and Translation Dynamics in an Ionic Deep Eutectic Solvent Based on Choline Chloride

Deep eutectic solvents (DESs) are an emerging class of environment-friendly media useful in a variety of applications. However, the microscopic structure of these liquids is quite unclear, and issues like whether spatial and dynamic heterogeneity is a generic feature of the ionic DESs and whether the solute molecules experience similar environment and interactions with the medium irrespective of their charge are still open. In this work, we have attempted to address some of these issues for ethaline, a less viscous and ionic DES consisting of mixture of 1:2 mole ratio of choline chloride and ethylene glycol by monitoring the fluorescence response of a number of carefully chosen neutral and charged probe molecules in ensemble and single molecule conditions. Specifically, we have examined the liquid state structure of ethaline by studying the rotational and translational diffusion dynamics of the solutes measured by monitoring the time dependence of fluorescence anisotropy in ensemble condition and fluorescence correlation signal of extremely dilute samples diffusing through confocal volume. These studies clearly reveal dynamic heterogeneity of the medium, though no spatial heterogeneity is observable through excitation wavelength dependent fluorescence measurements. The insights obtained from this study will be helpful in understanding the nature of solute solvent interactions in this type of complex media.