The structural properties of a ZnCl2–ethylene glycol binary system and the peculiarities at the eutectic composition

Abstract

ATR-FTIR spectroscopy was performed on a series of ZnCl₂–ethylene glycol (EG) mixtures with a wide-range of compositions (1 : 1.5–1 : 14 in molar ratios), involving the stable ZnCl₂–4EG deep-eutectic solvent (DES) composition, to explore the spectral variations, structural heterogeneity, and hydrogen bonding (H-bonding) properties. To enhance the resolution of the spectra, excess absorption and two-dimensional correlation spectroscopies were employed. In the initial IR spectra, a quasi-isosbestic point was identified, signaling that the major disturbance on EG microstructures by adding ZnCl₂ is to form a distinct complex. Further analysis uncovered the main transformation process to be from the EG tetramer to the ZnCl₂–4EG complex. It was also found that as the EG content increases, negative charge increasingly transfers to ZnCl₂, resulting in the strengthening of the Zn ← O coordination bonds and the weakening and finally dissociation of Zn–Cl bonds. Regarding the ZnCl₂–4EG DES, several incomparable specificities were observed. It was found that ZnCl₂ destructed the H-bonding network of pure EG to the largest extent, resulting in the highest production of the dimer and trimer of EG. Moreover, in comparison with other compositions, the ZnCl₂–4EG DES showed abrupt increases in the negative charge of the salt, the length of the Zn–Cl bond, and the strength of the Zn ← O coordination bond. All these imply the strongest intermolecular interactions and the highest solvation of ZnCl₂ in EG at the eutectic composition compared to those of other mixtures, resulting in a super-stable liquid mixture. The work provides physical insights into the structural and interactive properties of deep-eutectic solvents.