Influence of Polymer Hydrophilicity on Solvent-Driven Rare-Earth Element Binding Thermodynamics and Heat Capacity Changes

Rare-earth elements (REEs: La-Lu, Y, Sc) are critical to modern technology; however, their acquisition, purification, and separation from each other can be challenging, and current methods are often environmentally damaging. Metal-binding polymers are a promising material class to improve these methods, but fundamental structure-property relationships have yet to be fully developed. Inspired by nature's use of solvation- and conformation-driven effects to control metal-binding interactions, we hypothesized that modifying the molecular environment around the binding sites on metal-chelating polymers could offer a way to modify solvation structure and direct these interactions. Specifically, we hypothesized that increasing binding-site hydrophilicity would increase the enthalpic favorability of polymer-metal interactions, while maintaining the entropic driving force of metal-ion dehydration. To test this hypothesis, we synthesized a series of poly(amino acid acrylamides) with systematic changes in the hydrophilicity around the metal-binding sites, using a modular polymer approach to hold key polymer parameters constant. We then used isothermal titration calorimetry (ITC) to measure the solution thermodynamics of polymer-REE binding. Binding by all polymers in this series was entropically driven─consistent with substantial solvent reorganization─but binding site hydrophilicity did not significantly affect binding thermodynamics due to enthalpy-entropy compensation. Interestingly, poly(glutamic acid acrylamide) displayed a distinct binding behavior, possibly driven by reduced solvent reorganization and larger conformational contributions. In general, lower hydrophilicity was associated with larger desolvation signatures, which could indicate that hydrophobic groups more effectively dehydrate the metal ion than hydrophilic groups. Together, these results demonstrate how polymer structure can modulate desolvation and control polymer-REE binding while also highlighting thermodynamic constraints that limit straightforward tuning of binding affinity, thus providing new insights for the design of materials for critical metal purification.