Terpenoid-capric acid based natural deep eutectic solvent: Insight into the nature of low viscosity

Abstract

The viscosity of natural deep eutectic solvent (NADES) plays an important role in determining how they are used industrially. However, most of NADES systems exhibit a relatively high viscosity. Terpene-based mixtures have attracted widespread interest as relatively low-viscosity solvents. The goal of this work was to shed light on the nature of their unique properties by exploring the relationship between viscosity and non-covalent interactions. The microstructure and characteristics of different NADES were investigated through multiple methods and theories. Among these systems, ketone-based and alkenyl alcohol-based terpenoids act as hydrogen bond acceptors, and phenol-based terpenoids are prone to be hydrogen bond donors. The viscosity of studied NADES was low and was synthetically influenced by the number and lifetime of hydrogen bond. To some extent, their viscosity was consistent with the hydrogen-bond lifetime. Additionally, a positive correlation was found between their viscosity and non-bonded interactions energy. The relatively high viscosity mainly was driven by the enhancement of van der Waals force. Through visualization of non-covalent interactions, the results indicated that the appropriate strength and high fluctuations of interactions in the systems were necessary for forming low-viscosity NADES. Furthermore, the vertices on the electrostatic potential of terpenoids are pivotal factors influencing the property of final complex via quantitative analysis of molecular surface. It can be utilized to preliminarily assess the viscosity of NADES without the study of the entire system. The discovery of intrinsic rules paves the way for rational and effective design of novel low-viscosity NADES in the future. • The microstructure of NADES was investigated by multiple methods and theories. • Viscosity is affected by hydrogen-bond lifetime and non-bonded interactions energy. • Appropriate strength and high fluctuations of interactions are necessary for forming low-viscosity systems. • Extreme points of electrostatic potential are pivotal factors influencing their properties. • It paves the way for rational and effective design of novel NADES with low viscosity.