Raman Spectrum of the Li₂SO₄-MgSO₄-H₂O System: Excess Spectrum and Hydration Shell Spectrum

Lithium, as a green energy metal used to promote world development, is an important raw material for lithium-ion, lithium-air, and lithium-sulfur batteries. It is challenging to directly extract lithium resources from brine with a high Mg/Li mass ratio. The microstructure study of salt solutions provides an important theoretical basis for the separation of lithium and magnesium. The changes in the hydrogen bond network structure and ion association of the Li₂SO₄ aqueous solution and Li₂SO₄-MgSO₄-H₂O mixed aqueous solution were studied by Raman spectroscopy. The SO₄^2- fully symmetric stretching vibration peak at 940~1020 cm^-1 and the O-H stretching vibration peak at 2800~3800 cm^-1 of the Li₂SO₄ aqueous solution at room temperature were studied by Raman spectroscopy and excess spectroscopy. According to the peak of the O-H stretching vibration spectrum, with an increase in the mass fraction of the Li₂SO₄ solution, the proportion of DAA-type and DDAA-type hydrogen bonds at low wavenumbers decreases gradually, while the proportion of DA-type hydrogen bonds at 3300 cm^-1 increases. When the mass fraction is greater than 6.00%, this proportion increases sharply. Although the spectra of hydrated water molecules and bulk water molecules are different, the spectra of the two water molecules seriously overlap. The spectrum of the anion hydration shell in a solution can be extracted via spectrum division. By analyzing the spectra of these hydration shells, the interaction between the solute and water molecules, the structure of the hydration shell and the number of water molecules are obtained. For the same ionic strength solution, different cationic salts have different hydration numbers of anions, indicating that there is a strong interaction between ions in a strong electrolytic solution, which will lead to ion aggregation and the formation of ion pairs. When the concentration of salt solution increases, the hydration number decreases rapidly, indicating that the degree of ion aggregation increases with increasing concentration.