Investigation of the Phase Formation Mechanism of the Sugar-Based Natural Deep Eutectic Solvent + Water-Soluble Alcohol + Water Aqueous Biphasic System Based on the Excluded Volume Theory

Abstract

To investigate the phase formation mechanism of sugar-based natural deep eutectic solvent (DES) aqueous biphasic systems, an excluded volume theory (EEV) model was extended to the sugar-based (sucrose, glucose, fructose)-choline chloride DES + tert-butyl alcohol and isopropyl alcohol + water systems. In this paper, phase density measurement of sugar-based natural deep eutectic solvents (NADESs) + water-soluble alcohols + water systems revealed that the densities of NADES-rich and alcohol-rich phase presented significant differences. On the basis of these results, a modified EEV model was correlated with the density and phase equilibrium data of each system. Subsequently, the parameters of the excluded volume obtained were used to quantify the interaction force between the NADES and alcohol, and the relationship between parameters and functional groups of phase components was used to explore the mechanism of phase formation and predict the possible liquid structure of the DES-rich phase. The analysis of the Va – DES – w* and fa – DES – w values showed that the interaction between the components of each system was influenced by the functional groups of hydrogen bond donors (HBDs) and alcohols, and it was beneficial to reduce the compatibility of the system and the space unfilled of tight packing of molecules by increasing the number of hydroxyl groups of HBDs and alkyl groups of alcohol, leading to improvement of the phase formation ability. From the analysis of the parameters given by the EEV model, the adjustment of the number of hydroxyl groups of HBDs and alkyl groups of alcohols was beneficial to regulate the biphasic coexistence region of sugar-based DES/alcohol systems.