Effect of Water on a Hydrophobic Deep Eutectic Solvent

Abstract

Deep eutectic solvents (DESs) formed by hydrogen bond donors and acceptors are a promising new class of solvents. Both hydrophilic and hydrophobic binary DESs readily absorb water, making them ternary mixtures, and a small water content is always inevitable under ambient conditions. We present a thorough study of a typical hydrophobic DES formed by a 1:2 mole ratio of tetrabutyl ammonium chloride and decanoic acid, focusing on the effects of a low water content caused by absorbed water vapor, using multinuclear NMR techniques, molecular modeling, and several other physicochemical techniques. Already very low water contents cause dynamic nanoscale phase segregation, reduce solvent viscosity and fragility, increase self-diffusion coefficients and conductivity, and enhance local dynamics. Water interferes with the hydrogen-bonding network between the chloride ions and carboxylic acid groups by solvating them, which enhances carboxylic acid self-correlation and ion pair formation between tetrabutyl ammonium and chloride. Simulations show that the component molar ratio can be varied, with an effect on the internal structure. The water-induced changes in the physical properties are beneficial for most prospective applications but water creates an acidic aqueous nanophase with a high halide ion concentration, which may have chemically adverse effects.

Chiral Solvent Discovery: Exploring Chiral Eutectic Mixtures and Deep Eutectic Solvents

“…First, since the mixtures were screened in ambient conditions, every sample has some amount of environmental water, but sample size (~0.1 g) precludes quantitative water determination. [43][44][45][46] This means that many of the mixtures in the database should be considered a ternary mixture of HBA:HBD:water, and it is unclear what role water content plays in the formation of the liquid at 55 or 20 °C. Second, only a limited number of mole ratios have been explored for each of the mixtures, where different molar ratios may form liquids at 55 or 20 °C.…”

“…this screening cannot be used to distinguish between DES and simple eutectic mixtures, or even understand how environmental water may play a role in liquid formation. 43,44 Over the course of this study to identify chiral solvents, >100 mixtures have been prepared in larger quantities to measure water content, thermal and physical properties. Since it is beyond the scope of this manuscript to document the properties of all of the mixtures, a subset (< 20) will be characterized in the remaining sections of this manuscript.…”

“…The samples with the highest water content show that water is the largest component by moles of the mixture, but the mole fractions are within a range that has been characterized as "water in DES". 43,44 It is probably best to characterize most of the chiral mixtures developed as a ternary eutectic mixture that includes HBA, HBD, and water. In the absence of complete solid-liquid equilibrium curves, a measure of the melting point depression provides a method to classify the mixtures (eutectic mixtures, or DES).…”

Chiral Solvent Discovery: Exploring Chiral Eutectic Mixtures and Deep Eutectic Solvents

“…Hydrogen bond donors that formed liquids at all molar ratios HBD a HBA (liquid in all ratios) HBD a HBA (liquid in all ratios) (S)/(R)-2acetoxypropionic acid this screening cannot be used to distinguish between DES and simple eutectic mixtures, or even understand how environmental water may play a role in liquid formation. 43,44 Over the course of this study to identify chiral solvents, >100 mixtures have been prepared in larger quantities to measure water content, thermal and physical properties. Since it is beyond the scope of this manuscript to document the properties of all of the mixtures, a subset (< 20) will be characterized in the remaining sections of this manuscript.…”

Chiral Solvent Discovery: Exploring Chiral Eutectic Mixtures and Deep Eutectic Solvents

“…First, since the mixtures were screened in ambient conditions, every sample has some amount of environmental water, but sample size (~0.1 g) precludes quantitative water determination. [44][45][46][47] This means that many of the mixtures in the database should be considered a ternary mixture of HBA:HBD:water, and it is unclear what role water content plays in the formation of the liquid at 55 or 20 °C. Second, only a limited number of mole ratios have been explored for each of the mixtures, where different molar ratios may form liquids at 55 or 20 °C.…”