Nonadditive Ion Effects Drive Both Collapse and Swelling of Thermoresponsive Polymers in Water
Nonadditive Ion Effects Drive Both Collapse and Swelling of Thermoresponsive Polymers in Water
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DOI：
10.1021/jacs.9b00295
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发表时间：
2019-04-24
影响因子：
15
通讯作者：
van der Vegt, Nico F. A.
中科院分区：
文献类型：
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作者：
Bruce, Ellen E.;Bui, Pho T.;van der Vegt, Nico F. A.
当水溶液中两种盐的混合物中含有一个弱水合阴离子和一个强水合阴离子时，它们的联合作用是不加和的。在此，我们报道了在固定浓度的Na2SO4和增加浓度的NaI下，这种非加性效应对聚n -异丙基丙烯酰胺（PNiPAM）的低临界溶解温度（LCST）的影响。通过分子动力学模拟和振动和频率谱分析，我们证明了在低浓度弱水合阴离子（I-）下，阳离子（Na+）优先分配到强水合阴离子（SO42-）周围的反离子云，使I-更水合。然而，当NaI浓度进一步增加时，这种弱水合阴离子被硫酸盐从溶液中挤出到聚合物/水界面。因此，PNiPAM的LCST行为涉及离子水合作用和聚合物-碘化物相互作用的竞争作用。这个概念一般适用于含有强水合阴离子和弱水合阴离子的霍夫迈斯特系列混合物。
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.