Anion Species Dependence of Undetected Water Molecules along the Hofmeister Series in Various Lithium Electrolytes Using Near-infrared and Nuclear Magnetic Resonance Spectroscopy

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The anion species dependence of the undetected solvent content in various aqueous lithium electrolyte solutions is discussed, and the quantitative behavior of near-infrared absorption due to intramolecular vibrations of dissolved species in the electrolyte is discussed in correlation with the Hofmeister series. It was observed that the number of water molecules measured by NIR spectroscopy was less than the theoretical value of the number of water molecules in solution, and the difference between the two increased with increasing concentration in the aqueous electrolyte solution. It is also observed that the undetected water amounts in ¹H qNMR and NIR spectra show clear anion dependences, specifically TFSA⁻ < ClO₄⁻ < Cl⁻ < NO₃⁻ < SO₄²⁻. The prominence of the undetected amount of water was found to be mainly due to the reduction of the peak at the low wavenumber region caused by water molecules with a well-developed hydrogen bonding network. It was observed that this feature almost follows the order of the Hofmeister series. Decrease in the absorbance of the water molecules in the NIR spectra are observed. The observed water molecules are predominantly those that have not interacted with the anions. It is suggested that the anion-water molecule interaction suppresses the generation of the combination band of OH symmetric stretching vibrations and OH asymmetric stretching vibrations of water molecules. The ¹H NMR quantitative analysis results are also consistent with the NIR results.