(20d) Thermodynamics of Ion and Water Uptake in Phase-Separated Ion-Exchange Membranes

Partitioning of ions and solvent from electrolyte solutions into polymer membranes is an important phenomena for water treatment and numerous electrochemical technologies. Consequently, controlling the partition coefficients of species in these materials provides an avenue to optimize performance of these technologies by tuning selectivity. To understand this process, we present and validate a mathematical model for multicomponent thermodynamic activity in phase-separated cation-exchange membranes (e.g., Nafion). Specifically, this work formulates the membrane and electrolyte solution free energy. The model accounts for long-range electrostatic and short-range solvation and physical interactions between ions. A mechanics-based term accounts for the elastic swelling energy of the membrane’s hydrophobic matrix. We also include steric interactions that exclude larger species from the membrane. A thermodynamically consistent treatment of the Donnan potential accounts for species charge. Externally validated solution and membrane properties parameterize the model. The model’s calculations agree with measured water and ion uptake into the membrane in dilute and concentrated binary and ternary salt electrolytes. The results show that water and ion uptake is a balance between osmotic and elastic pressures with ion specificity and membrane pretreatment affecting this balance.