Physical Properties and Thermal Behavior of Natural Deep Eutectic Solvents

Natural deep eutectic solvents (NADES) have been shown to be promising, sustainable media for a wide range of applications. Nonetheless, very limited data is available on the properties of these solvents. NADES based on choline chloride, organic acids, amino acids and sugars, were prepared, and their density, thermal behavior, conductivity and polarity were assessed, for different NADES compositions. The NADES studied are stable over a wide range of temperatures up to 170 ^oC, depending on their composition. Thermal characterization revealed that all the NADES are glass formers and some exhibit crystallinity. It was also observed that water has a plasticizing effect on NADES, with a decrease of the glass transition temperature with the addition of water. The morphological characterization of the crystallizable materials was done by polarized optical microscopy that provided also evidence of homogeneity/phase separation. The conductivity of the NADES was also assessed from 0 to 40 ^oC. The more polar, organic acid-based NADES presented the highest conductivities. The conductivity dependence with temperature was well described by the Vogel-Fulcher-Tammann equation for most of the NADES studied. It is crucial to characterize the fundamental physicochemical properties of NADES to provide data for more complex molecular simulation studies. To tailor the most suitable combination of compounds for a given application, the development of models able to predict their properties and the mechanisms that allow the formation of the deep eutectic mixture will be hereafter of utmost value. Only using computer simulation models it is possible to evaluate the 10⁶ possible combinations within a reasonable time frame and to truly produce tailor-made solvents.