A comprehensive review on the physicochemical properties of deep eutectic solvents

Deep eutectic solvents (DESs) are a class of ionic liquid analogues that have received heightened research attention in recent years. These binary or ternary mixtures are composed of quaternary salts, metal salts, or organic compounds that exhibit hydrogen bond donating and accepting behaviours within the DES system, forming a hydrogen bond network that results in melting point depression. The facile preparation of DESs coupled with the virtually unlimited combination of components enables a large number of unique solvents to be produced, characterized, and applied; accordingly, highly tailorable agents can be prepared for specific applications. Numerous applied studies have utilized DESs; however, current fundamental knowledge of DES physicochemical and phase equilibria properties is relatively limited compared to the wide spectrum of possible formulations. Furthermore, available reporting of DES physicochemical properties encompasses a multitude of parameters, from density and ionic conductivity to more niche properties such as vapour pressure and acoustic properties. Therefore, a comprehensive review that compiles and evaluates recent works documenting the physicochemical properties of DESs is required to improve our understanding of fundamental DES properties and their prospective applications, and to identify gaps in this critical research area. Accordingly, this paper aims to act as a reference on the physicochemical properties of DESs and illuminate efforts dedicated to this foundational research area.