## 1 Introduction  
The Qinghai magnesium-sulfate-subtype salt lake in China contains abundant mineral resources such as potassium, magnesium, and lithium [1]. However, lithium in the lake brine is usually present in trace amounts. Due to the high magnesium-lithium ratio (generally more than 40:1) in the brine [2] and their similar physicochemical properties, efficient separation is difficult to achieve [3]. Current separation methods result in significant lithium losses when applied to high-magnesium-lithium-ratio brines, posing a challenge for achieving enhanced separation and a reduction in lithium losses. The traditional natural evaporation process directly utilizes MgCl\({}_{2}\) as the raw material for magnesium-lithium separation [4]. However, the high concentration of MgCl\({}_{2}\) solution leads to high viscosity, resulting in a lithium loss of 36-70% in the liquid phase [5]. Despite this, there exists a type of magnesium-sulfate-type brine in salt lake production that can achieve effective magnesium removal and enhance magnesium-lithium separation [6] due to its properties of being supersaturated and having a low viscosity under high-temperature conditions [7]. In fact, the temperature is higher, so the separation of salt components is easier; if the system is supersaturated, the separation effect will be significantly enhanced, and the separation performance will even be increased by 30% [7]. The properties of low viscosity and supersaturation at high temperatures are possessed by sulfate-type salt lake brines [8], which can provide convenient conditions for magnesium-lithium separation. However, the lithium loss mechanism and the process-enhanced separation mechanism of this process are still unclear, requiring clarification of the microstructure, hydrogen bonding, and complexation. Therefore, it is necessary to elucidate the effects of enhanced magnesium-lithium separation and the causes of lithium losses from changes in solution structure and molecular levels. Methods for studying the solution structure include Raman spectroscopy, infrared spectroscopy [9], and X-ray diffraction. Generally, inorganic salts do not produce infrared characteristic peaks. However, in salt solutions, the water structure near ions can be altered. Water preferentially solvates cations with oxygen and hydroxyl groups solvate anions, increasing the complexity of the water structure and leading to spectral changes. Therefore, infrared spectroscopy can be used to characterize the microstructure of water in inorganic salt solutions by analyzing the shifts [10] and intensities of the O-H stretching band [11]. [PERSON] et al. [12] studied the O-D stretching band in K\({}_{2}\)SO\({}_{4}\) and MgSO\({}_{4}\) solutions using FTIR and analyzed the water structure changes induced by these salts. [PERSON] et al. [13] used two-dimensional infrared spectroscopy to investigate the effects of halide salts as solutes on the diffusion dynamics of the OD stretching vibrational spectra in the water medium at different temperatures. [PERSON] et al. [14] used FTIR to demonstrate the strength of interactions between water and solvent molecules in the hydration layer by observing the frequency shift of the O-H bond. [PERSON] et al. [15] observed the water structure surrounding ions in aqueous salt solutions using FTIR technique. [PERSON] et al. [16] observed changes in the water vibrational spectra in halide salt solutions using FTIR. Therefore, infrared spectroscopy can be used to analyze the changes in water structure caused by soluble salts with varying temperatures, providing support for magnesium-lithium separation during cooling processes. Experimental infrared spectroscopy involves the absorption or scattering of specific wavelengths of infrared radiation to reveal the molecular composition and structural characteristics of a sample. On the other hand, simulated infrared spectroscopy can predict the infrared spectral features of a sample through theoretical calculations and various computational methods.