Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, D-64287 Darmstadt, Germany.
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
J Phys Chem B. 2021 Jan 21;125(2):680-688. doi: 10.1021/acs.jpcb.0c11076. Epub 2021 Jan 7.
Salt effects on the solubility of uncharged polymers in aqueous solutions are usually dominated by anions, while the role of the cation with which they are paired is often ignored. In this study, we examine the influence of three aqueous metal iodide salt solutions (LiI, NaI, and CsI) on the phase transition temperature of poly(-isopropylacrylamide) (PNIPAM) by measuring the turbidity change of the solutions. Weakly hydrated anions, such as iodide, are known to interact with the polymer and thereby lead to salting-in behavior at low salt concentration followed by salting-out behavior at higher salt concentration. When varying the cation type, an unexpected salting-out trend is observed at higher salt concentrations, Cs > Na > Li. Using molecular dynamics simulations, it is demonstrated that this originates from contact ion pair formation in the bulk solution, which introduces a competition for iodide ions between the polymer and cations. The weakly hydrated cation, Cs, forms contact ion pairs with I in the bulk solution, leading to depletion of CsI from the polymer-water interface. Microscopically, this is correlated with the repulsion of iodide ions from the amide moiety.
盐对不带电聚合物在水溶液中溶解度的影响通常受阴离子主导，而与之配对的阳离子的作用往往被忽视。在这项研究中，我们通过测量溶液浊度的变化，考察了三种水合金属碘化物盐溶液（LiI、NaI 和 CsI）对聚（异丙基丙烯酰胺）（PNIPAM）相变温度的影响。弱水合阴离子，如碘离子，已知与聚合物相互作用，从而导致低盐浓度下的盐溶行为，随后在更高盐浓度下出现盐析行为。当改变阳离子类型时，在较高盐浓度下观察到出乎意料的盐析趋势，Cs > Na > Li。使用分子动力学模拟表明，这源于在本体溶液中形成接触离子对，这导致聚合物和阳离子之间对碘离子的竞争。弱水合阳离子 Cs 在本体溶液中与 I 形成接触离子对，导致 CsI 从聚合物-水界面耗尽。从微观上看，这与碘离子从酰胺部分的排斥有关。
