TY - JOUR
T1 - Nonadditive Ion Effects Drive Both Collapse and Swelling of Thermoresponsive Polymers in Water
AU - Bruce, Ellen E.
AU - Bui, Pho T.
AU - Rogers, Bradley A.
AU - Cremer, Paul S.
AU - Van Der Vegt, Nico F.A.
N1 - Funding Information:
The simulations for this research were conducted on the Lichtenberg high performance computer at the TU Darmstadt. Furthermore, the Darmstadt authors thank the LOEWE project from iNAPO which is funded by the Ministry for Higher Education, Research and the Arts (HMWK) of the state of Hessen. The Penn State authors thank the National Science Foundation (CHE-1709735) for support. Additionally, the authors thank William Hunn, Kelvin Rembert, and Halil Okur for the initial observation of the nonadditive ion effects on lower critical solution temperature (LCST) measurements and Dr. Tinglu Yang for maintenance of the vibrational sum frequency spectroscopy system.
Publisher Copyright:
© 2019 American Chemical Society.
PY - 2019/4/24
Y1 - 2019/4/24
N2 - 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 (SO42-), 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.
AB - 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 (SO42-), 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.
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U2 - 10.1021/jacs.9b00295
DO - 10.1021/jacs.9b00295
M3 - Article
C2 - 30919630
AN - SCOPUS:85064993868
SN - 0002-7863
VL - 141
SP - 6609
EP - 6616
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 16
ER -