The Electrolyte Equilibrium in Ion Exchange Polymers

Recent developments in a number of technical areas, such as in grid-scale batteries, desalination and other water purification technologies and so on have revealed gaps in our understanding of the interaction of polymer electrolyte membranes with concentrated electrolyte solutions.  In such systems, the electrolyte concentration exceeds a threshold below which co-ions are excluded from the membrane.  Significant quantities of ‘molecular’ acid, salt and so on are then imbibed into membrane, dramatically altering its properties.  A step toward a quantitative explanation of the resulting phenomena is a description of the driving forces favoring and opposing the uptake of solution components into the membrane.  This balance is between osmotic pressure and activity of external solution components as a driving force for uptake balanced against the chemical potential of the pore fluid in the polymer membrane.  The chemical potential will be defined by terms related to the Donnan potential opposing acid or salt uptake, individual ion properties such as the enthalpy of solvation and the ‘swelling pressure’ of the membrane.  The latter is related to the mechanical properties of the polymer. Here we provide a first description of some of these properties, emphasizing the sorption equilibria and the mechanical properties of the membrane, for a membrane-acid system typical of that used in vanadium redox flow batteries.