Vibrational Raman Spectroscopy of the Hydration Shell of Ions

Abstract

Ionic perturbation of water has important implications in various chemical, biological and environmental processes. Previous studies revealed the structural and dynamical perturbation of water in the presence of ions, mainly with concentrated electrolyte solutions having significant interionic interactions. These investigations highlighted the need of selective extraction of the hydration shell water from a dilute electrolyte solution that is largely free from interionic interactions. Double-difference infrared (DDIR) and Raman multivariate curve resolution (Raman-MCR), as well as MD simulation, provided valuable insight in this direction, suggesting that the perturbed water mainly resides in the immediate vicinity of the ion, called the hydration shell. Recently, we have introduced Raman difference spectroscopy with simultaneous curve fitting (Raman-DS-SCF) analysis that can quantitatively extract the vibrational response of the perturbed water pertaining to the hydration shell of fully hydrated ions/solute. The DS-SCF analysis revealed novel hydrogen-bond (H-bond) structural features of hydration water, such as the existence of extremely weakly interacting water–OH (νmax ~ 3600 cm−1) in the hydration shell of high-charge-density metal ions (Mg2+, Dy3+). In addition, Raman-DS-SCF retrieves the vibrational response of the shared water in the water–shared-ion pair (WSIP), which is different from the hydration shell water of either the interacting cation and anion. Herein, we discuss the perturbation of water H-bonding in the immediate vicinity of cation, anion, zwitterion and hydrophobes and also the inter-ionic interactions, with a focus on the recent results from our laboratory using Raman-DS-SCF spectroscopy.

Roles of Hydrogen Bonds in Ionic Conductivity in the LiNO3–Poly(vinyl alcohol) Electrolyte: A Real-Time Raman Spectroscopic Study

“…As the LiNO 3 molarity increases, the HOH band of LiNO 3 solution shifted to higher wavenumbers for both strongly and weakly coordinated water molecules (Figure a). This indicates the reduction in strength and number of hydrogen bonds, which has also been shown in the HOH band of similar salts like NaNO 3 and NaCl . As a result, the relative intensity ratio of these strongly/weakly coordinated hydrogen bonds also decreases at a higher concentration.…”

Electro-Freezing of Supercooled Water Is Induced by Hydrated Al3+ and Mg2+ Ions: Experimental and Theoretical Studies

“…As a result of this difference, diffraction, scattering, and spectroscopic methods, as well as theoretical calculations, also show differences in the strength of the binding of the coordinated water molecules with neighboring water molecules. ,− Furthermore, due to the high polarizing power of the Al 3+ ion, one can observe the formation of a second stable water shell. , A less rigid water layer was reported near the Mg 2+ ion. ,, On the other hand, the monovalent Na + hydrated ions weaken the hydrogen bond interactions with neighboring water molecules in comparison to those in bulk water …”

Enabling uniform zinc deposition by zwitterion additives in aqueous zinc metal anodes

“…The red shi of the Raman peak results from the relatively free molecular motion, which indicates a loose solvation structure in the 6-AA added electrolyte. 42 Also, in Fig. 1e, the hydrogen bond peak and H 2 O-SO 4 2− interaction also show a blue shi in the spectrum of the 6-AA added electrolyte, which also shows strong interaction among water molecules and other ions by the 6-AA additive.…”

“…1e, the hydrogen bond peak and H 2 O–SO 4 2− interaction also show a blue shift in the spectrum of the 6-AA added electrolyte, which also shows strong interaction among water molecules and other ions by the 6-AA additive. 42 This can be interpreted as due to the introduction of 6-AA molecules which suppress the H 2 O induced side reactions because of the weak solvation structure and relatively intensive interaction between water molecules. The reduced side reactions of water molecules are further investigated as below.…”