Characterization of the Influence of Hydrated Ions on the Oxygen–Hydrogen Stretching Vibration of Water by Raman Spectroscopy

The Raman OH stretching vibration band of liquid water is asymmetric and multimode superimposed and is susceptible to variations in salinity, pressure, and temperature. Ions have non-negligible influences on the Raman O–H stretching band of water because these ion–water interactions are stronger than those between water molecules. The determination of molecular-level details of hydration processes remains a great challenge, both experimentally and theoretically. The effects of 11 common ions on the O–H stretching band of liquid water are characterized with a skewing parameter introduced to describe the distortion caused by ion hydration. With increasing salt concentration, the symmetry of the Raman O–H stretching band decreases as the skewing parameter value increases. Therefore, ions primarily break the tetrahedral hydrogen bonding and promote the formation of partly hydrogen-bonded and free water molecules. Additionally, the effects of anions are much stronger than those of cations due to the asymmetric charge distribution in the dipoles of H2O. It is revealed that the influences of the common ions on the Raman OH stretching band of water are in the following order: I– > Br– > Cl– > F–, SO42− > CO32−, K+ > Na+, and Sr2+ > Ca2+ > Mg2+. For ions with the same charge, the order of their effects on the Raman O–H stretching band of water is usually the same as the order of their ionic crystal radii but the reverse order of their hydrated radii.