Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 23, Issue 23, Pages 13136-13147Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/d1cp00573a
Keywords
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Funding
- National Natural Science Foundation of China [21733011]
- National Key Research and Development Program of China [2016YFA0203101]
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The study used ATR-FTIR spectroscopy to investigate the structural and interactive properties of zinc chloride-ethylene glycol mixtures, revealing that EG is most significantly affected by ZnCl2 to form a new complex, while the ZnCl2-4EG deep eutectic solvent exhibits unique interaction characteristics.
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.
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