Observation of Extremely Weakly Interacting OH (∼3600 cm-1) in the Vicinity of High Charge Density Metal Ions (Mz+; z=1, 2, 3): A Structural Heterogeneity in the Extended Hydration Shell

Hydration of metal ions is critical to their adsorption, complexation, and discharge but remained elusive due to counterion interference. We introduce Raman difference spectroscopy with simultaneous curve fitting (RD-SCF) analysis to quantitatively retrieve the counterion-free OH stretch spectra of water in the hydration shell of various metal ions such as Li+, Mg2+, Ca2+, Sr2+, Ba2+, La3+, Gd3+ Dy3+, and Lu3+ as well as that of a proton (H+). The RD-SCF-extracted spectra show that extremely weakly (OH stretch similar to 3600 cm(-1)) and strongly (similar to 3300 cm(-1)) interacting OHs coexist in the hydration shell of the high charge density metal ions and proton. The weakly interacting OH originates from the second hydration shell water-OH (W-2hs-OH) that donates the hydrogen-bond to the electron-deficient oxygen at the first hydration shell (W-1hs-O) and is largely localized at the boundary between the first and second hydration shells. For high charge density ions, such as Mg2+ and La3+, at least 4 and 8 water molecules are affected in their respective second hydration shell, which is similar to 70% and 90% of their first hydration shell. The coexistence of different water-OHs discloses a heterogeneous water structure in the extended hydration shell of metal ions.