Cation Identity Affects Nonadditivity in Salt Mixtures Containing Iodide and Sulfate

Abstract: The cloud point temperature of poly(N-isopropylacrylamide) was measured in aqueous solutions containing salt mixtures. Solutions were made with SO42-, a strongly hydrated anion, I⁻, a weakly hydrated anion, or both anions together with a common alkali metal counterion: Li⁺, Na⁺, K⁺ or Cs⁺. Nonadditive behavior was observed when the MI concentration was increased in the presence of 0.2 mol⋅L⁻¹ M₂SO₄. Although complex changes in the cloud point temperature were observed with all four metal counter cations, the magnitude of the effects differed substantially amongst them. More specifically, SO42- was able to attract cations away from I⁻ in each case, but its propensity to do so depended on the relative strength of the cation–iodide versus the cation–sulfate interactions. When the counterion was stripped away from it, I⁻ became more hydrated and acted more like a strongly hydrated anion. A competitive binding model was employed to determine the fraction of cations bound to SO42- in the presence of I⁻, allowing for a qualitative comparison of the fraction of strongly hydrated I⁻ ions that was produced with all four cations. K⁺ showed the greatest relative affinity for SO42-. As such, experiments performed with this cation led to the greatest fraction of I⁻ that was more hydrated and displayed the strongest nonadditive behavior. By contrast, Cs⁺ showed the weakest relative affinity for SO42-, resulting in the least pronounced nonadditivity. This work demonstrates that the identity of the counter cation plays a critical role in the nonadditive behavior of salt mixtures when both weakly and strongly hydrated anions are present. Graphic Abstract: [Figure not available: see fulltext.]