Hydration of Nafion and Dowex in liquid and vapor environment: Schroeder's paradox and microstructure

Functioning of ion-exchange resins and ionomers strongly depends the state of hydration (equilibrium water uptake), which strongly affects their performance. The relation between water activity and uptake by such polymers (isotherm) has usually been measured by equilibration with water vapor of known activity. However, at the point of saturation (pure water) hydration by liquid water and water vapor differ, i.e., depend on the mode of equilibration. This effect, known as Schroeder's paradox, leads to an ambiguity in determination of the chemical potential and calculating the solvent transport in such systems. In order to verify the general nature of this phenomenon, we carried out a systematic examination of the paradox in a wide range of water activities using solutions of a polyelectrolyte (polyvinylsulfonic acid salts) as an osmotic stressor. This allowed measuring hydration isotherms in parallel vapor- and liquid-equilibration modes for two solid polymer electrolytes: Dowex (a cross-linked resin) and Nafion (an ionomer). The results indicate that for the studied polymers Schroeder's paradox extends to the whole range of the examined water activities. Furthermore, the difference between the isotherms could be related to the polymer microstructure, however, it suggested that a correction due to the difference between the surface and bulk microstructure is necessary for Nafion.