Liquid Structure and Dynamics of Tetraalkylammonium Bromide-Based Deep Eutectic Solvents: Effect of Cation Chain Length

Deep eutectic solvents (DESs) have emerged in recent years as environmentally sustainable media across several fields. However, knowledge of liquid structure, dynamics, and solute-solvent interactions in many DESs that is essential for exploiting their potential is still lacking. In this work, we make an attempt to obtain some insight into these aspects of a set of less-explored DESs comprising tetraalkylammonium bromide salts and ethylene glycol (EG) by monitoring the fluorescence response of some carefully chosen dipolar (C153 and 4-AP) and nonpolar (9-PA) solutes in these media. Specifically, we have studied the translational and rotational diffusion dynamics of these molecular systems using single-molecule-based fluorescence correlation spectroscopy technique and ensemble-based time-resolved fluorescence anisotropy measurements. These results point to spatial and dynamic heterogeneity of these DESs, which becomes prominent in systems comprising cations with a longer alkyl chain length. This study reveals that diffusion dynamics of the probe molecules is determined not only by the solvent bulk viscosity but also dependent on their microenvironments and solute-solvent interactions experienced in these media.