Drawing Out the Structural Information About the First Hydration Layer of the Isolated Cl− Anion Through the FTIR-ATR Difference Spectra

Abstract

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 Na2SO4 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 Na2SO4 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.