Insights into the Intermolecular Interactions of Monoethanolamine–p-Hydroxyanisole DES Aqueous Solution through Volumetric and Viscometric Properties

Deep eutectic solvents (DESs) have been increasingly studied due to their wide application in many fields, such as metal treatment, extraction and separation, chemical synthesis, refinement of biological and natural components, and gas absorption. Water, as an abundant, environmentally friendly, and low-cost resource, can effectively regulate the structure and thermodynamic properties of DES; thus, it is important to study the effect of water on DESs. In this work, DESs with different water contents composed of monoethanolamine (MEA) and p-hydroxyanisole (MEHQ) in various mole ratios (1.0000:1.0000 to 5.0000:1.0000) were successfully prepared, and the density and viscosity were measured at temperatures ranging from 293.15 to 323.15 K using a U-shaped vibrating tube densitometer and a falling ball viscometer. Intermolecular interactions in DES aqueous solutions were investigated by using volumetric and viscometric properties: excess molar volume, thermal expansion coefficient, viscosity deviation, viscous flow activation energy, and excess Gibbs activation energy. The effect of the water content on the structure of DES could be roughly classified into three zones: the zone of structural integrity of DES, the zone of partial destruction of DES, and the zone of freeing of DES components.