Quantifying Hofmeister Effects on Polymer–Water Interactions through Freezing Point Depression

The freezing point depression of ternary salt–polymer–water mixtures is examined with a particular focus on how specific ion hydration modulates polymer–solvent interactions. Using poly(ethylene glycol) (PEG) as a model hydrophilic polymer and four sodium salts (NaF, NaCl, NaI, and NaSCN) spanning the Hofmeister series, we show that the freezing point of PEG solutions follows the Flory–Huggins equation and is independent of molecular weight for PEGs larger than 1000 g mol^–1. While binary salt-water systems display purely colligative behavior, ternary PEG–salt–water mixtures exhibit pronounced ion-specific effects. The addition of NaF, NaCl, and NaI produces synergistic freezing point depressions exceeding the sum of the individual PEG and salt contributions, consistent with strong ion hydration that withdraws water from the polymer hydration shell. In contrast, NaSCN yields a nearly additive response, reflecting its weak hydration and direct association with PEG chains. The effective interaction parameters and derived hydration numbers reveal a quantitative correlation between ion hydration strength and the macroscopic freezing behavior. This establishes a general thermodynamic framework that links molecular hydration to bulk phase transitions in aqueous polymer systems.