Why do ammonium salt/phenol-based deep eutectic solvents show low viscosity?

Abstract

• Glycolic acid (GA) DESs exhibit higher activation energy compared with phenolics DESs. • A strong charge transfer complex was found between GA and ammonium salt. • Hydrogen bonds of GA DES are partially covalent and partially electrostatic. • Interaction network stability of GA DES is more robust owing to strong covalency. • This work paves the way for rational design of novel DES with low viscosity. The viscosity of deep eutectic solvents (DESs) plays an important role in determining how they are used industrially. In order to gain a deeper insight into the parameters which affect the viscosity of ionic DES, a series of systems composed of ammonium salts and two types of representative donors were prepared and characterized. They were investigated by quantum-chemistry calculations and molecular dynamics simulations. The viscosity of phenol/4-methylphenol-based system is much lower than that of glycolic acid-based system. Moreover, DESs containing glycolic acid exhibit higher activation energy values compared with DESs containing phenolics. It was found the existence of a strong charge transfer complex between glycolic acid and ammonium salt, thus suggesting its vital role in the fluidity difference of studied mixtures. The hydrogen bonds of glycolic acid-based system are partially covalent and partially electrostatic, manifested via atoms in molecules (AIM) analysis. Additionally, Cl − ⋯HO phenolic hydroxyl is expected to be less covalent than Cl − ⋯HO carboxyl , which is also identified by lower delocalization index in the AIM basin. The interaction network stability of glycolic acid-based DES is more robust than that of phenolics-based one due to the strong covalency of hydrogen bond. This is the main reason that ammonium salt/phenol-based DESs show low viscosity. This work gives new perspectives on more rational design of novel DES with low viscosity.