The Influence of Ion Solvation and Association Interactions on Mean Ionic Activity Coefficients in Neutral Polymeric Membranes

The influence of material properties on ion-hydrated polymer thermodynamic interactions is not fully understood. In this study, we probed how polymer properties (i.e., the network mesh size and dielectric constant) contribute to interactions between ions, water molecules, and the solvated polymer by synthesizing polymer networks with varied cross-link density and functionality (e.g., hydroxyl, ether, or nitrile). We characterized the hydration-dependent network mesh size, relative permittivity (i.e., dielectric constant), and the sodium chloride mean ionic activity coefficients in the polymers and related these properties to each other using a theoretical model that describes quantitatively the influence of ion solvation and ion pairing interactions on ionic activity coefficients in neutral polymers. These results and analysis help to explain the relationship between the network mesh size, polymer dielectric constant, and mean ionic activity coefficients in solvated polymers, which may be useful to guide molecular engineering strategies for polymer membrane materials.