How do the hydrocarbon chain length and hydroxyl group position influence the solute dynamics in alcohol-based deep eutectic solvents?

Abstract

Deep eutectic solvents (DESs) have received considerable attention in recent years as new sustainable green media and some of their interesting properties have stimulated investigations on the microscopic solution structure, solute-solvent interactions and solute/solvation dynamics in these media. Even though the alcohol-based DESs, due to their low viscosity, serve as useful media in various applications, little is known about the structure and dynamics of these solvents. In order to obtain insight into the microscopic structure and interactions operating in these media, we have studied the rotational and translational diffusion dynamics of some carefully chosen molecular systems (both dipolar and nonpolar) using time-resolved fluorescence anisotropy and fluorescence correlation spectroscopy techniques in a series of choline chloride/alcohol based DESs differing in hydrocarbon chain length and positioning of the hydroxyl group on the hydrogen bond donor. The results reveal an increase of both spatial and dynamic heterogeneity upon an increase in chain length of one of the components of these solvents. No significant variation of heterogeneity, however, could be observed with the change in the hydroxyl group position. The analysis of the experimental results indicates that solute-solvent hydrogen-bonding interaction plays a dominant role in determining both rotational and translational diffusion dynamics of AP in these DESs.