4.6 Article

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

Journal

JOURNAL OF PHYSICAL CHEMISTRY C
Volume 124, Issue 5, Pages 3028-3036

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.9b09692

Keywords

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Funding

  1. Sir J. C. Bose Fellowship, DST, India
  2. CEBS
  3. HBNI

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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.

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