Effects of Ions on the OH Stretching Band of Water as Revealed by ATR-IR Spectroscopy

The effects of various cations (Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, and Ni2+) and anions (Cl-, Br-, I-, , , , and ) on the molar absorptivity of water in the OH stretching band region (2,600-3,800 cm(-1)) were ascertained from attenuated total reflection infrared spectra of aqueous electrolyte solutions (22 in all). The OH stretching band mainly changes linearly with ion concentrations up to 2 mol center dot L-1, but several specific combinations of cations and anions (Cs2SO4, Li2SO4, and MgSO4) present different trends. That deviation is attributed to ion pair formation and cooperativity in ion hydration, which indicates that the extent of the ion-water interaction reflected by the OH stretching band of water is beyond the first solvation shell of water molecules directly surrounding the ion. The obtained dataset was then correlated with several quantitative parameters representing structural and dynamic properties of water molecules around ions: Delta G (HB), the structural entropy (S (str)), the viscosity B-coefficient (B (eta) ), and the ionic B-coefficient of NMR relaxation (B (NMR)). Results show that modification of the OH stretching band of water caused by ions has quasi-linear relations with all of these parameters. Vibrational spectroscopy can be a useful means for evaluating ion-water interaction in aqueous solutions.