In-Depth Physico-Chemical and Structural Investigation of a Dicarboxylic Acid/Choline Chloride Natural Deep Eutectic Solvent (NADES): A Spotlight on the Importance of a Rigorous Preparation Procedure

In this study, we report the density, viscosity, conductivity, and X-ray diffraction profile of the deep eutectic solvent (DES) formed by choline chloride and different dicarboxylic acids (i.e., malonic, oxalic, succinic, or fumaric acid) at room temperature. These compounds have recently found several applications in green chemistry methodologies and consequently require the highest purity standards. Yet, we found that malonic DES, differing from the other DESs that were prepared (containing short alkyl-chained dicarboxylic acids), is especially sensitive to thermal treatment and undergoes sizable decarboxylation beyond 60 °C. Such release of CO₂ would become a serious limit in the potential process scale-up and prompt researchers to avoid heating during the preparation. The analysis of X-ray diffraction patterns of malonic and oxalic acid DESs, that are optimally reproduced by molecular dynamics simulations, suggests that the charged species (choline and chloride ions) are held firmly by the acid molecules, with interaction distances shorter in oxalic acid DES but with higher coordination number in the malonic system. This complies closely with the smaller hydrodynamic radius of the aggregate hypothesized from the conductivity data.