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Published on: March 24, 2018
Dynamics of Deep Eutectic Mixtures of Tetraethylammonium Halides/Ethylene Glycol Investigated with Ultrafast Infrared
Junkun Pan1, Kimberly A Carter-Fenk1,2, Samantha T Hung1
1Department of Chemistry, Stanford University, Stanford, California 94305, United States.
Abstract:
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.
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