Insight into the role of hydrogen bond donor in deep eutectic solvents

Deep eutectic solvent (DES) is considered as one of the green solvents with bright prospect in this century. Its excellent performance is closely related to the hydrogen bonds formed between hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD). Through changing the type of HBD and adjusting its molar fraction, it makes a difference to the intensity of the hydrogen bonds. In this work, tetrabutylammonium chloride (TBAC) was chosen as HBA, decanoic acid (DEC) and octanoic acid (OC) were chosen as HBD. With the molar ratios ranging from 3:1 to 1:3, seven TBAC-DEC-based DES and seven TBAC-OC-based DES were synthesized. Based on the study of the structural characteristics and the thermophysical properties, such as the eutectic point, the thermal stability, the density and the viscosity, the effect of HBD on DES was investigated. The increased HBD destroyed the orderly configuration of DES. Meanwhile, the elongation of the HBD's alkyl chain enhanced the interaction between the components. Moreover, the mass-connectivity-based model and the hard sphere model were introduced to predict the density and viscosity, respectively. Both of the two models exhibited good predictivity, with the average absolute relative deviation less than 0.2 % and 5 %, respectively.