Nonadditive Ion Effects Drive Both Collapse and Swelling of Thermoresponsive Polymers in Water

When a mixture of two salts in an aqueous solution contains a weakly and a strongly hydrated anion, their combined effect is nonadditive. Herein, we report such nonadditive effects on the lower critical solution temperature (LCST) of poly( N-isopropylacrylamide) (PNiPAM) for a fixed concentration of Na₂SO₄ and an increasing concentration of NaI. Using molecular dynamics simulations and vibrational sum frequency spectroscopy, we demonstrate that at low concentrations of the weakly hydrated anion (I^-), the cations (Na⁺) preferentially partition to the counterion cloud around the strongly hydrated anion (SO₄^2-), leaving I^- more hydrated. However, upon further increase in the NaI concentration, this weakly hydrated anion is forced out of solution to the polymer/water interface by sulfate. Thus, the LCST behavior of PNiPAM involves competing roles for ion hydration and polymer-iodide interactions. This concept can be generally applied to mixtures containing both a strongly and a weakly hydrated anion from the Hofmeister series.