Raman spectroscopy study for the systems (LiCl-H₂O and LiCl-MgCl₂-H₂O): Excess spectra and hydration shell spectra

The micro-structure of hydration shell of solute in water is significant for understanding the properties of aqueous solutions. Raman spectroscopy has been employed for studying the hydration shell structure of the solute for decades, however, Raman imaging data is still seriously overlapped, making it challenging to obtain information on the spectrum of hydrated water molecules. In this paper, Raman spectroscopy was employed to study the O-H vibration peaks of LiCl aqueous solution and LiCl-MgCl₂-H₂O mixed aqueous solution. The changes of stretching vibration peak of 2800 ∼ 3800 cm^-1O-H and hydrogen bond network structure in aqueous solution were analyzed at room temperature and ion association. With the increase of magnesium salt ratio, the damage of solute to the bulk water gradually decreases in the mixed solution, which indicated that LiCl has a more significant influence on the bulk water molecules. It is mainly due to the intense hydration of Li⁺, which can not only affect the water molecules in the first hydration shell but also affect the water molecules in the second hydration shell. The number of water molecules in the first hydration shell were obtained by extracting the spectra of different solute first hydration shells from the solution spectra. Those spectra of the hydration shell were employed to study the micro-structures of the first hydration shells of anions, and the aggregation behavior of ions in the the mixed solution.