Unusual Hydration Properties of Choline Fluoride-Based Deep Eutectic Solvents

The hydration properties of the fluoride-based deep eutectic solvent ethalineF [a solution of choline fluoride in ethylene glycol (EG) at a 1:2 molar ratio] are studied and compared to the most common deep eutectic solvent ethaline (the solution of choline chloride in EG at 1:2 molar ratio). The densities of the deep eutectic solvent (DES) based on choline fluoride in EG (ethalineF) and its mixtures with water as cosolvent are measured over the temperature range of 298-323 K. The excess properties, including excess molar volumes, excess partial molar volumes, and viscosity deviations from ideal behavior, are calculated for ethalineF/water and ethaline/water mixtures and compared. The experimental excess molar volumes and viscosity deviations of the studied pseudobinary mixtures are fitted using the Redlich-Kister (R-K) equation. The results of the R-K model successfully reproduced the experimentally calculated values with minimal standard deviations. All excess molar volumes and viscosity deviations had negative values, indicating stronger solvation interactions between the mixture components than between each pure DES or water. The excess partial molar volumes show that water molecules are preferentially solvated by the DES environment. We show that the disruption of the DES interactions (primarily OH...halide interactions) by high mole fractions of water is related to the peak ionic conductivity. The stark differences in hydration behavior between fluoride- and chloride-based ethaline are analyzed and discussed.