Density, acidity and theoretical calculation of hydrophobic deep eutectic solvents formed by amide and lactic acid

Hydrophobic deep eutectic solvents (HDESs) were considered as a kind of green media and were widely used for extraction, synthesis and catalytic reactions. Exploring the physicochemical property and intermolecular interaction was of great significance. In this work, we synthesized a series of HDESs using amide as hydrogen bond acceptor (HBA) and lactic acid as hydrogen bond donor (HBD). The density, acidity and intermolecular interactions of HDESs were investigated from experimental and theoretical viewpoints. Firstly, density results indicated that the increase of HBD content as well as the decrease of HBA alkyl chain could increase the density of HDESs. Secondly, acidity was found to be more accurate using milliequivalents calculation than pH measurement. The increase of HBD content as well as the decrease of HBA alkyl chain could also enhance the acidity of HDESs. In addition, the intermolecular hydrogen bond was investigated by multiple theoretical approaches, including electrostatic potential distribution map, the length of molecular bond, independent gradient model based on Hirshfeld partition and topology analysis of atoms. Ultimately, Fourier-transform infrared spectroscopy (FT-IR) was further applied to prove the existence of hydrogen bond. We believe this work would be valuable for understanding the nature of HDESs and providing guides for practical applications of HDESs.