Observation of the first hydration layer of isolated cations and anions through the FTIR-ATR difference spectra

The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) difference spectra of the dilute aqueous (NH4)2SO4, Na2SO4, MgSO4, ZnSO4, NaClO4, and Mg(ClO4)2 solutions by pure water were obtained at various concentrations. In the difference spectra of aqueous (NH4)2SO4 solutions, a peak at approximately 3039 cm(-1), two shoulders at approximately 3155 and approximately 2894 cm(-1), and a peak at approximately 1445 cm(-1) were ascribed to N-H stretching and bending vibrations, respectively. A small negative peak was resolved at approximately 3660 cm(-1) in the difference spectra of (NH4)2SO4, which is the sole contribution of SO4(2-) either in the O-H stretching or in the O-H bending region. The positive peaks of the difference spectra in the O-H stretching region for Na2SO4, MgSO4, and ZnSO4 systems, which constantly appeared at approximately 3423, approximately 3136, and approximately 3103 cm(-1) respectively, were suggested to be the contribution of the interactions between metal cations (Na+, Mg2+, and Zn2+) and water molecules, especially from the first hydrated layer of the cations. In the region of 800-1200 cm(-1), the normally infrared-prohibited nu1 (SO4(2-)) band was observed as a weak peak at approximately 981 cm(-1) even at very dilute concentrations (0.10 mol dm(-3)) due to the disturbance of the water molecules hydrated with SO4(2-), even though such a feature may increasingly result from associated ions with increasing concentration. The spectra of the water molecules directly influenced by ClO4-, i.e., mostly the first layer of hydrated water, in NaClO4 and Mg(ClO4)2 solutions were obtained by subtracting the corresponding spectra of the same metal sulfate solutions at the same concentrations from the perchlorate solutions. A positive peak at approximately 3583 +/- 6 cm(-1) and a negative peak at approximately 3184 +/- 25 cm(-1) were obtained as the result of the subtraction. The positive peak was attributed to the water molecules weakly hydrogen-bonded with ClO4-, while the negative one to the reduction of water molecules with fully hydrogen-bonded five-molecule tetrahedral nearest neighbor structure on the introduction of ClO4-.