Comprehensive physicochemical evaluation of choline chloride-based natural deep eutectic solvents

Natural deep eutectic solvents (NADES) represent a class of novel mixtures formed between two or more compounds acting as hydrogen bond donors (HBD) and acceptors (HBA). These solvents are regarded as green, sustainable, and harmless, so they overcome the toxic nature of organic solvents and have plenty of applications in different scientific and industrial areas. In the presented study, the basic physicochemical properties of twelve deep eutectic solvents composed of choline chloride (HBA) and different HBDs (sugars, alcohols, urea, and organic acids) with 20% (w/w) of initially added water were investigated. The obtained data suggested that NADES properties: pH, conductivity, polarity, and density were associated with the structure of HBD presented in the solvents. FT-IR results confirmed that H-bonds, occurring between two components in NADES, were the main force leading to the eutectic formation. Thermal analysis showed that dehydration, which started at room temperature, was the first step in the thermal decomposition of the DES containing 20% of water. The thermal stability of dehydrated systems was between the stabilities of their components and depended on the HBD structure. Our study confirmed different physicochemical features of DES, which are related to the type of HBDs, so they can be considered as tailor-made solvents, which properties may be easily adjusted to the multitude of applications.