Preparation strategy and stability of deep eutectic solvents: A case study based on choline chloride-carboxylic acid

Deep eutectic solvents (DESs) with designable physicochemical properties are expected to be successfully used in wide fields. Different preparation methods for DESs based on various feedstocks could result in different expectations. In this study, we focused on the preparation strategy for choline chloride-carboxylic acid to get homogeneous liquid DESs and to understand the effects of some process parameters on the formation and the impurities of DESs. For the preparation of DESs, heating is more suitable for liquid acids, while freeze-drying is more suitable for DESs with thermally unstable components. For the hydrogen bond donor (HBD) with more carboxyl or hydroxyl groups, freeze-drying or rotary evaporation are more suitable. The impurities in DESs are different from various preparation methods due to the different process conditions. Besides, the molecular interactions of choline chloride-carboxylic acid DESs were measured by FT-IR spectroscopy and analyzed with the aid of density functional theory (DFT) calculations. Our results provide the underlying insights to guide the selection of preparation methods for DESs with different structures.