Molecular-level freezing and melting dynamics of aqueous lithium chloride solutions using in-situ Raman spectroscopy

Lithium chloride (LiCl) aqueous solutions are widely used in chemical, biological, and industrial applications due to their high solubility, hygroscopicity, and antifreeze properties. This study investigates how Li⁺ and Cl^- ions influence the structural and dynamic behavior of water molecules during freezing and melting processes. In-situ Raman spectroscopy was employed to analyze these solutions with varied concentrations and temperature ramping rates. Vibrational features, including OH stretching, HOH bending, and low-frequency lattice modes, were monitored to elucidate ion-induced changes. For the 1.5 M LiCl solution, a visual indicator, methylene blue, was used to track freezing and melting transitions. A unique behavior of LiCl, characterized by chain-like aggregation, was observed at low temperatures in the liquid phase, where increasing ion concentration with decreasing temperature coincided with the disappearance of the HOH bending band. The spectral shifts correlated with increased salt concentration, confirming freezing point depression and two-step structural transformations. This work deepens the understanding of ion-specific effects on the vibrational behavior and phase transitions of water.