Raman spectroscopy of alkali halide hydration: hydrogen bond relaxation and polarization

• Published in: Journal of Raman spectroscopy 2016-11, Vol.47 (11), p.1351-1359
• Main Authors: Gong, Yinyan, Zhou, Yong, Wu, Huacheng, Wu, Danping, Huang, Yongli, Sun, Chang Q.
• Format: Article
• Language: English
• Subjects: Abundance; Bonding; Contact angle; Dynamics; Hofmeister series; Hydration; hydrogen bond relaxation; Hydrogen bonds; Molecular dynamics; Phonons; Polarization; Raman spectroscopy; Spectrum analysis; surface tension
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.4963

The solute–solvent interaction of salts has a striking impact on various biological and industrial processes but its mechanism remains yet mysterious despite intensive studies since 1888 when Franz Hofmeister established the salt series. A combination of confocal Raman spectroscopy and contact angle measurements has enabled us to resolve the hydrogen bond relaxation (O:H―O, HB) and the associated charge polarization dynamics at different molecular site because of alkali halides hydration. Results show consistently that salt hydration softens the O:H phonon but stiffens H―O phonon cooperatively. The extent of HB relaxation and polarization is proportional to the electronegativity difference and ionic radius, following the order of Hofmeister series: X (R/η) = I (2.2/2.5) > Br (1.96/2.8) > Cl (1.81/3.0) > F (1.33/4.0) ≈ 0 for anions, and Y(R/η) = Na (0.98/0.9) > K (1.33/0.8) > Rb (1.49/0.8) > Cs (1.65/0.8) for cations. Observations suggest that ions create each an electric field that aligns, stretches, and polarizes water molecules, which relaxes the O:H―O bond cooperatively, depresses the molecular dynamics, and enhances the hydration shell viscosity and the skin stress. Exercises also demonstrate that Raman spectroscopy performs as a powerful tool for probing the molecular‐site‐resolved HB network relaxation dynamics in terms of phonon stiffness, molecular fluctuation dynamics, and phonon abundance transition under external stimulus. Copyright © 2016 John Wiley & Sons, Ltd. Sodium halides dissolve into ions that form each a hydration shell through polarization that depresses molecular dynamics and enhances skin stress. The hydration shortens the H―O bond (ΔdH  0) while the O:H nonbond reacts oppositely (ΔdL > 0, ΔωL