Study on the Correlation between the Microstructure and Physical Properties of ZnCl₂ Aqueous Solution

This article focuses on the study of the correlation between the microstructure and physical properties of aqueous zinc chloride solutions. Macroscopic physical properties of zinc chloride aqueous solution were determined, and its microstructure was analyzed by Raman spectroscopy, molecular dynamics simulations, and density functional theory (DFT) calculations. The experimental results of macroscopic physical properties show that with the increase of ZnCl₂ concentration, the conductivity of aqueous solution first increases and then decreases, and the viscosity gradually increases. Raman spectrum analysis shows that with the increase of solute concentration, double donor-acceptor (DDAA)-type hydrogen bonds are continuously destroyed and the proportion of DA-type hydrogen bonds increases. The results of molecular dynamics simulations show that with the increase of solution concentration, contact ion pairs of Zn²⁺-Cl^- (2.28 Å) gradually appear in ZnCl₂ aqueous solution, and the diffusion coefficients of Zn²⁺ and Cl^- gradually decrease. The correlation between the Raman shift and the hydration cluster model of Zn²⁺ was calculated theoretically by the DFT method. With the increase of the concentration, the cluster structure of Zn²⁺ in aqueous solution gradually changed from [Zn(H₂O)₆]²⁺ to [ZnCl₂(H₂O)₄]. Based on experimental data and molecular dynamics simulation results, it can be concluded that the decrease in conductivity is related to the formation of Zn²⁺-Cl^- contact ion pairs in the solution. The interactions between Zn²⁺, Cl^-, or contact ion pairs and water molecules, namely, hydrated ions or hydrated contact ion pairs, are the microscopic essential reason for the increase in viscosity.