Cation Identity Affects Nonadditivity in Salt Mixtures Containing Iodide and Sulfate

Pho T. Bui, Paul S. Cremer

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5 Scopus citations


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.]

Original languageEnglish (US)
Pages (from-to)1443-1456
Number of pages14
JournalJournal of Solution Chemistry
Issue number11-12
StatePublished - Dec 2021

All Science Journal Classification (ASJC) codes

  • Biophysics
  • Biochemistry
  • Molecular Biology
  • Physical and Theoretical Chemistry


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