Specific ion effects on the permselectivity of sulfonated poly(ether sulfone) cation exchange membranes

Water uptake and permselectivity were measured for five sulfonated poly(ether sulfone) cation exchange membranes with varying degrees of functionalization from 20% to 60%. Experiments were conducted in aqueous salt solutions of LiCl, NaCl, KCl, Li₂SO₄, Na₂SO₄ and K₂SO₄ to isolate the effect of counter-ions and co-ions on membrane permselectivity. Water uptake ranged from 0.13g_water/g_polymer to 0.76g_water/g_polymer depending on the degree of functionalization and salt used, but was not found to describe the permselectivity differences between salts as the counter-ion and co-ion were varied. This lack of correlation between water uptake and permselectivity is counter to expectations for swollen polymers. Counter-ion binding affinity, charge density and dilute solution mobility were identified as factors influencing membrane permselectivity. Specifically, counter-ions with higher binding affinities to the fixed charge group of the polymer showed lower permselectivities due to counter-ion condensation. Co-ion polarizability was identified as the primary factor for co-ion effects on permselectivity, with more polarizable co-ions resulting in lower membrane permselectivities.