Comparative study on the deep eutectic solvents formed by choline chloride and cresol isomers from theoretical and experimental perspectives

To provide some guidance for the selective separation of cresol isomers from oils, the effect of structure on the physicochemical properties of deep eutectic solvents (DESs) formed by choline chloride (ChCl) and cresol isomers were studied. In this study, DESs with different mole ratios of cresols to ChCl (1.68 to 9.00) were prepared. The physicochemical properties (viscosity and density) of DESs, which were related to the separation process, were measured at different temperatures. The results indicated that the density and viscosity were closely related to the structure of cresols, temperature, and mole ratios of cresols to ChCl. Interestingly, when the mole ratio of cresols to ChCl was large enough, such as > 5.00, the density and viscosity got correspondingly closer to that of the pure cresols. This meant that cresols could not only serve as hydrogen bond donors to form DESs but also act as a solvent to dilute DESs. Finally, density functional theory (DFT) was used to reveal the microscopic properties and binding interactions of DESs. The most stable conformations, corresponding interaction energies and hydrogen bond length of DESs were obtained from the Gaussian 16 software, and Multiwfn 3.8 program was used to perform wave function analysis of the system. The results showed that the interaction between ChCl and cresol isomers decreased in the order o-cresol, m-cresol, and p-cresol, and the hydrogen bond dominated the interaction. The results at the molecular/atomic level further verified the experiment results, and a better understanding of the structural characteristics and intermolecular interactions of DESs could provide opportunities for the application of DESs.