Spectroscopic and Computational Study of ZnCl₂-Methanol Low-Melting-Temperature Mixtures

Alcoholic electrolyte mixtures have wide applications in industries. In this study, a series of mixtures composed of ZnCl₂ and methanol (MeOH) with ZnCl₂ mol % from 6.7 to 25 were prepared, and their spectral, structural, and thermodynamic properties were studied using infrared (IR) spectroscopy, differential scanning calorimetry (DSC), and density functional theory (DFT) calculations. The DFT-assisted analysis of excess spectra, supported by 2D-correlation spectroscopy, led to the identification of the major constituents of ZnCl₂-MeOH mixtures, namely, MeOH monomer, MeOH dimer, and ZnCl₂-3MeOH complex, produced after dissociation of MeOH trimer which represents the bulk methanol. The Hirshfeld charge analysis showed that in the competition between the O-H···Cl hydrogen bond (H-bond) and Zn ← O coordination bond to transfer charge in ZnCl₂-MeOH complexes, the latter always dominates, making MeOH positively charged. The phase diagram of the binary system showed the presence of V-shaped glass transition temperatures (T_g), characteristic of low-melting mixture solvents (LoMMSs). The present study provides insights into the microscopic properties of the system and sheds light on the understanding of the general principles to prepare deep-eutectic solvents (DESs) or LoMMSs using inorganic salts and alcoholic compounds.