Specific Ion Effects of Chaotropic and Superchaotropic Anions Probed by Raman Hydration-Shell Spectroscopy

Chaotropic ions enhance protein solubility and the associated Hofmeister effects are widely used in biotechnology and materials science. However, the characterization of the recently discovered class of superchaotropic borate cluster anions remains incomplete, limiting their rational application. Using Raman multivariate curve resolution spectroscopy, we measured the O─H stretching vibrational frequencies of hydration-shell water molecules surrounding both conventional and superchaotropic anions. For conventional monoanions, we observed dangling O-H bands in their hydration-shell spectra, whose vibrational frequencies increase with chaotropicity and follow the trend Br^- < I^- < ClO₄ ^-, BF₄ ^- < PF₆ ^-. This affords a useful molecular-spectroscopic scale for ion classification and differentiation from kosmotropic and hydrophobic solutes. In contrast, superchaotropic dodecaborates (B₁₂X₁₂ ^2-, X = H, F, Cl, Br, and I) emerged as significant outliers: their dangling O─H frequencies do not exceed those of classical chaotropes, despite displaying more pronounced chaotropic properties (such as salting-in effects) and they show an opposing trend along the halogen series. These deviations were attributed to higher charge and charge screening in solution which affect ion-water interactions to a different extent depending on the X substituents. These effects compete with general chaotropicity trends and affect the hydration pattern.