Molecular mechanism of the Hofmeister series
In plain words
Since 1888 it has been known that salts fall in a fixed order, the Hofmeister series, in how strongly they make proteins precipitate. Why ions of the same charge rank this way is still debated.
Precise statement
Explain the anion ordering of the Hofmeister series (roughly SO4^2- > F- > Cl- > Br- > NO3- > I- > ClO4- > SCN-) in protein salting-out, nonpolar solute solubility and electrolyte surface tension, choosing among bulk water structure making and breaking, direct ion-solute pairing, interfacial adsorption driven by polarizability and partial dehydration, and ion-specific dispersion forces. An answer is a quantitative model with parameters from first principles or independent data that reproduces the ordering and its reversal on positively charged or hydrophobic surfaces.
What would settle it
Ion-resolved measurements at protein and model interfaces compared with simulations using polarizable or ab initio potentials across the whole series.
Status in the literature
Explanations based on bulk water structure lost support in the 2000s in favor of direct ion-interface interactions, but no single quantitative model covers the series (Jungwirth and Cremer, Nature Chemistry 2014, https://doi.org/10.1038/nchem.1899).