Freezing Point Depression of Aqueous Polyelectrolyte Solutions: Effects of Chain Length, Charge Density, and Counterion Type

The freezing point depression of aqueous polyelectrolyte solutions was investigated using an in-house-built apparatus. The influence of molecular design was systematically studied by varying the molecular weight, charge density, and counterion type. A series of poly(METAC- co -HEMA) copolymers with different ratios of charged (METAC) and noncharged (HEMA) monomers and with different molecular weights were synthesized using RAFT polymerization, ensuring control over the degree of polymerization and the statistical distribution of monomers. To explore ion-pairing effects, the counterions F –, Cl –, I –, and SCN – were compared, representing a range of hydration and ion-pairing tendencies with the quaternary amine groups on METAC. Two existing thermodynamic models were evaluated for their ability to describe the effects of polymer–solvent interactions and ionization behavior on the freezing point depression. While an extended Flory–Huggins model that explicitly includes counterions significantly underestimated the effective degree of ionization, leading to unphysical results, the traditional Flory–Huggins equation provided a more consistent description of the experimental data. Within the investigated range, the freezing point depression was independent of molecular weight but showed a weak dependence on charge density. A substantial increase in charge density led only to a minor decrease in the freezing point, indicating a limited effect of charge and counterion quantity. In contrast, the counterion type had a pronounced effect. Strongly hydrated counterions, like F – and Cl –, which are more likely to remain dissociated from the polymer, produced large freezing point depressions, whereas weakly hydrated counterions, like I – and SCN –, resulted in much smaller effects. These findings indicate that the freezing behavior is more consistent with counterion-specific hydration and ion-pairing effects than with a description dominated by the translational entropy of fully dissociated counterions.