Quantifying Hydrophilicity in Polyelectrolytes and Polyzwitterions

The affinity of charged polymers for water is of central interest in polyelectrolyte science. Hydration controls the solution properties of polyelectrolytes as well as their performance in materials having a balance of positive and negative repeat units, such as polyelectrolyte complexes, PECs, and polyzwitterions, PZs. As with neutral polymers, a ranking of water affinity, loosely termed hydrophilicity, is often sought. Apart from the solubility in water, there are few methods for determining relative hydrophilicity. The scaling exponent of size with molecular weight provides, for polymers in general, a classical measure of solvent quality. In this work, using aqueous size exclusion chromatography coupled with static light scattering, the radius of gyration scaling with molecular weight was determined for a range of cationic and anionic polyelectrolytes and for some polyzwitterions. For a more definitive comparison of hydrophilicity, solution calorimetry was used to measure the enthalpy of solution, Δ<i>H</i> _sol, when rigorously dried samples of these polymers were dissolved in aqueous 0.1 M NaCl. All polymers yielded strongly exothermic Δ<i>H</i> _sol, which provided a ranking of hydrophilicity. The first four molecules of water appear to generate almost all of the heat. Methacryl versions of polymers were more hydrophilic, as Δ<i>H</i> _sol was 3-5 kJ mol^-1 more exothermic than the nonmethacryl polymer. Polyzwitterions were shown to be strongly hydrated, consistent with the proposed mechanisms for their antifouling properties, although water is not necessarily more strongly held for PZs compared to PEs.