## 3 Results and Discussion
### Microstructure of Mixed Brine at Different Temperatures  
To validate the changes in the microstructure of single-salt solutions in a cascaded temperature field, as discussed in Section 3.1, we investigated the influence of interionic temperature on the magnesium-lithium separation structure in a multi-ion system and examined the microstructural variations in brine with different magnesium-lithium mass ratios. Figure 3 shows the microstructure of mixed brine with magnesium-lithium mass ratios of 5:1, 10:1, 20:1, 40:1, 80:1, and 160:1 at different temperatures. It can be seen from the combined results that with a decreasing magnesium-lithium mass ratio, the number of Cl \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Wavenumber/cm\({}^{-1}\)** & **Soluble Salt** & **20 \({}^{\circ}\)C** & **40 \({}^{\circ}\)C** & **60 \({}^{\circ}\)C** \\ \hline \multirow{8}{*}{3340} & MgSO4 & 1.00 & 0.98 & 1.08 \\  & Li2 SO4 & 1.00 & 2.23 & 2.29 \\  & LiCl & 1.00 & 1.08 & 1.01 \\  & MgCl2 & 1.00 & 1.00 & 1.02 \\  & LiCl & 1.00 & 1.08 & 1.01 \\  & MgSO4 & 1.00 & 0.98 & 1.07 \\  & Li2 SO4 & 1.00 & 2.35 & 2.20 \\  & LiCl & 1.00 & 1.01 & 1.00 \\  & MgCl2 & 1.00 & 1.01 & 1.00 \\  & MgSO4 & 1.00 & 0.98 & 0.99 \\  & Li2 SO4 & 1.00 & 1.79 & 1.60 \\  & MgSO4 & 1.00 & 0.90 & 0.82 \\  & Li2 SO4 & 1.00 & 1.06 & 1.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relative intensities of infrared spectral absorption peaks for each single-salt solution. ions in the solution increases, and the peak intensity and area of water peaks at 3340 cm\({}^{-1}\) and 1090 cm\({}^{-1}\) change less during temperature variation. This is due to the easier formation of a coordination relationship between Li\({}^{+}\) and Cl\({}^{-}\) in the solution, resulting in fewer Li\({}_{2}\)SO\({}_{4}\) clusters. It was shown in Section 3.1 that Li\({}_{2}\)SO\({}_{4}\) clusters play a major role in the change in water structure in the solution; therefore, the variation in the microstructure of the water structure in the solution is smaller. In a multi-ion system, as the magnesium-lithium ratio increases, the number of Cl\({}^{-}\) ions in the solution decreases, and with temperature change, the variation in water structure in the solution with hydrogen bond number becomes more significant. However, with more Cl\({}^{-}\) ions in the solution, the change in water structure with temperature is hindered, and the change in water structure around Li\({}_{2}\)SO\({}_{4}\) clusters is suppressed, which is not conducive to the change in magnesium-lithium structure in the solution. Figure 4 shows the radial distribution function of anions in simulated brine with a magnesium-lithium mass ratio of 40:1. Combined with the infrared spectra of brine, it can be seen that the peak areas of the characteristic bands of SO\({}_{4}\)\({}^{2-}\) at 1090 cm\({}^{-1}\) and 980 cm\({}^{-1}\) decrease as the temperature increases, indicating that the free ion peak and contact ion peak of SO\({}_{4}\)\({}^{2-}\) in the brine decrease with temperature increase. Correspondingly, in Figure 4, the radial distribution function of SO\({}_{4}\)\({}^{2-}\) in the brine has a clear shoulder peak at r = 3.5 A, and the stronger the shoulder peak, the stronger the interaction energy between SO\({}_{4}\)  
Figure 3: FTIR spectra of simulated brine at different temperatures. and H\({}_{2}\)O in the solution. In the radial distribution function, the distribution of water molecules around Cl\({}^{-}\) in the brine changes less. In the radial distribution function of Mg\({}^{2+}\)-SO\({}_{4}\)\({}^{2-}\), it can be clearly seen that with temperature change, the arrangement of SO\({}_{4}\)\({}^{2-}\) and H\({}_{2}\)O in the second hydration layer changes, and the ionic coordination structure undergoes a significant change.