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Non-Additive Ion Effects Drive Both Collapse and Swelling of Thermoresponsive Polymers in Water.

Research paper by Ellen E EE Bruce, Pho T PT Bui, Bradley A BA Rogers, Paul S PS Cremer, Nico F A NFA van der Vegt

Indexed on: 28 May '19Published on: 29 Mar '19Published in: Journal of the American Chemical Society



Abstract

When a mixture of two salts in aqueous solution contain a weakly and a strongly hydrated anion, their combined effect is non-additive. Herein, we report such non-additive effects on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) for a fixed concentration of NaSO 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 (SO), 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.