Temperature Dependent Optical Acidity–Basicity Scale of LiCl–KCl–CsCl–LaCl₃ Molten Salt and Its Correlations with Salt Structure and Properties

Electrochemical separation of spent fuel utilizing molten salt as a medium is an efficient approach to achieve the sustainable development of nuclear energy. The Lewis acidity and basicity of molten salts are closely related to the local structure of the molten salt and significantly influence its properties. In this study, the acidity–basicity scales of LiCl–KCl–CsCl–LaCl3 molten salts have been quantitatively determined using in situ UV–vis spectrophotometry with Tl+ probes, and the relationship between the acidity–basicity scale and temperature was deduced. The influence of molten salt’s acidity–basicity scale on its structural evolution was investigated through Deep Potential Molecular Dynamics (DPMD) simulations and in situ Raman spectroscopy. As the acidity–basicity scale of molten salt increases, the short-range ordering of the melt diminishes, and the intermediate-range ordering enhances. Moreover, based on the accumulation of data at various temperatures, the relationships between the temperature-dependent acidity–basicity scale and the thermal transport properties were derived. The reduction potential of La was analyzed through electrochemical tests to evaluate the corrosiveness of the salt melt, and it was found that molten salt with a higher acidity–basicity scale exhibited greater corrosivity. This study elucidates the correlation of the acidity–basicity scale with salt structure and properties, providing an effective method for comprehensive assessment and real-time monitoring of salt structure and properties through the direct measurement of the salt acidity–basicity scale.