Dynamics of Deep Eutectic Mixtures of Tetraethylammonium Halides/Ethylene Glycol Investigated with Ultrafast Infrared Spectroscopy

Health and environmental risks posed by volatile organic solvents create an incentive to develop safer, less volatile solvents with the appropriate functionality. Deep eutectic solvents and other low-volatility organic mixtures offer a highly tunable alternative through a mixture composition selection. However, a significant gap exists in understanding the relationship between molecular-level properties and the resulting solvation and transport properties. Using ultrafast infrared (IR) polarization-selective pump–probe (lifetimes and orientational relaxation) spectroscopy, we investigated the dynamics of 1:3 molar mixtures of tetraethylammonium bromide (TEABr) and chloride (TEACl) with ethylene glycol (EG) and of pure EG using the anionic vibrational probe, the CN stretch of SeCN^–. The very high salt concentrations are in many respects analogous to water-in-salt solutions, e.g., LiBr and LiCl. These ion/water mixtures can have extremely high ratios of ions to solvating neutral molecules, similar to the 1:3TEABr and 1:3TEACl mixtures studied here. In 1:3TEABr/EG and 1:3TEACl/EG solutions, there are far too few EGs to solvate the ions. Therefore, like water-in-salt, 1:3TEABr/EG and 1:3TEACl/EG solutions will have solvent-separated ion pairs, contact ion pairs, and large ion/EG clusters, forming extended ion/solvent networks. The orientational dynamics experiments on 1:3TEABr/EG and 1:3TEACl/EG show striking similarities to experiments from the literature on 1:4 LiBr and LiCl aqueous solutions, even though the cations and solvents in the deep eutectic mixtures are vastly different.