Thermophysical Properties and Solubility of Different Sugar-Derived Molecules in Deep Eutectic Solvents

Deep eutectic solvents (DESs) are designer solvents analogous to ionic liquids but with lower preparation cost. Most known DESs are water-miscible, but recently water-immiscible DESs have also been presented, which are a combination of hydrogen bond donors and acceptors with long hydrophobic alkyl chains (e.g., decanoic acid 4-quaternary ammonium salts). These hydrophobic DESs are very interesting as solvents for recovering molecules from aqueous solutions. In this study the solubility of the sugar derived molecules furfural (FF), hydroxymethylfurfural (HMF), dimethyladipate, glucose, fructose, cyclopentanediol, cyclopentanone, and tetrahydrofurfurylalcohol was experimentally screened in six different DESs (five hydrophobic and one hydrophobic) at 30-50-80 degrees C, for the first time. The Kamlet-Taft parameters of the DESs were also determined, and correlations with the solubility data were established. Moreover, the thermophysical properties (viscosity, decomposition temperature) of the six DESs were measured. All DESs showed Newtonian viscosity behavior. Their thermal stability was good but decreased when sugars were added to the DES phase. The hydrophobic DES had the most interesting solubility properties (highest solubility for FF and HMF, and lowest solubility for the monosaccharides glucose and fructose) and is water-immiscible. Moreover, the hydrophobic DES has the highest Kamlet-Taft pi parameter (measure of dipolarity/polarizability ratio) that can be related to the high selectivity for HMF and FF over glucose. Thus, especially the hydrophobic DES is a promising extractant that can be used for selective removal of FF and HMF by liquid-liquid extraction from aqueous biomass solutions, e.g. in biorefineries.