Thermodynamic equilibrium constants of alkali metal ion-hydrogen ion exchanges and related swelling free energies in perfluorosulphonate ionomer membrane (Nafion-117) in aqueous medium

Abstract

Li⁺/<em>H</em>⁺, Na⁺/<em>H</em>⁺, <em>K</em>⁺<em>/</em><em>H</em>⁺, Rb⁺<em>/</em><em>H</em>⁺ and Cs⁺<em>/</em>H⁺ exchange equilibria on a perfluorosulphonate exchanger (Nafion-117) have been investigated at 0.1 <em>M </em>ionic strength in aqueous medium and thermodynamic equilibrium constants of 0.78 to 12.6 (Li⁺/<em>H</em>⁺ to Cs⁺<em>/</em>H⁺) have been evaluated and the alkali metal selectivity sequence has been found to be Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺. The various ionic forms of Nafion-117 involved have also been subjected to isopiestic water vapour sorption investigations and the hydration numbers and swelling free energies determined. The exchange selectivities have been found to be consistent with the sequence of ionic hydration and swelling free energies, the exchange free energies showing a linear relationship with the difference in free energies of hydration of the exchanging ions as per Eisenman's model. The expanded selectivity as well as swelling free energy ranges observed in Nafion- 117 compared to Dowex 50W type of resins have been interpreted in terms of hydrative and osmotic swelling behaviours of the two exchangers. Existence of a solvent shared ion pair in Nafion-117, i.e. – SO (H₂O)Cs⁺ observed in an earlier study has been supported by present selectivity data.