Drawing out the structural information about the first hydration layer of the isolated Cl^- anion through the FTIR-ATR difference spectra

The Fourier transform infrared-attenuated total reflectance (FTIR-ATR) difference spectra of aqueous MgSO₄, Na₂SO₄, NaCl and MgCl₂ solutions against pure water were obtained at various concentrations. The difference spectra of the solutions showed distinct positive bands and negative bands in the O-H stretching region, indicating the influences of salts on structures of hydrogen-bonds between water molecules. Furthermore the difference spectra of MgCl₂ solutions against NaCl solutions and those of MgSO₄ solutions against Na₂SO ₄ solutions with the same concentrations of anions (Cl^- or SO ₄ ^2-, respectively) allowed extracting the structural difference of the first hydration layer between Mg²⁺ and Na ⁺. Using SO ₄ ^2- as a reference ion, structural information of the first hydration layer of the Cl^- anion was obtained according to the difference spectra of MgCl₂ solutions against MgSO₄ solutions and those of NaCl solutions against Na ₂SO₄ solutions containing the same concentrations of cations (Mg²⁺ or Na⁺, respectively). The positive peak at ∼3,407 cm^-1 and negative peak at ∼3,168 cm^-1 in these spectra indicated that adding Cl^- decreased the strongest hydrogen-bond component and increased the relatively weaker one.