Department of Physics, National University of Singapore, Lower Kent Ridge Road, S117550, Singapore, Singapore.
Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, S117552, Singapore, Singapore.
Nat Commun. 2023 Jul 5;14(1):3978. doi: 10.1038/s41467-023-39215-9.
The presence of water strongly influences structure, dynamics and properties of ion-containing soft matter. Yet, the hydration of such matter is not well understood. Here, we show through a large study of monovalent π-conjugated polyelectrolytes that their reversible hydration, up to several water molecules per ion pair, occurs chiefly at the interface between the ion clusters and the hydrophobic matrix without disrupting ion packing. This establishes the appropriate model to be surface hydration, not the often-assumed internal hydration of the ion clusters. Through detailed analysis of desorption energies and O-H vibrational frequencies, together with OPLS4 and DFT calculations, we have elucidated key binding motifs of the sorbed water. Type-I water, which desorbs below 50 °C, corresponds to hydrogen-bonded water clusters constituting secondary hydration. Type-II water, which typically desorbs over 50-150 °C, corresponds to water bound to the anion under the influence of a proximal cation, or to a cation‒anion pair, at the cluster surface. This constitutes primary hydration. Type-III water, which irreversibly desorbs beyond 150 °C, corresponds to water kinetically trapped between ions. Its amount varies strongly with processing and heat treatment. As a consequence, hygroscopicity-which is the water sorption capacity per ion pair-depends not only on the ions, but also their cluster morphology.
水的存在强烈影响含离子软物质的结构、动力学和性质。然而，这种物质的水合作用还没有被很好地理解。在这里，我们通过对单价π 共轭聚电解质的大量研究表明，它们的可逆水合作用，每个离子对可达几个水分子，主要发生在离子簇与疏水分子之间的界面处，而不会破坏离子堆积。这确立了适当的模型是表面水合作用，而不是经常假定的离子簇的内部水合作用。通过对解吸能和 O-H 振动频率的详细分析，以及 OPLS4 和 DFT 计算，我们阐明了吸附水的关键结合模式。在 50°C 以下解吸的 I 型水对应于构成二级水合作用的氢键结合水簇。通常在 50-150°C 以上解吸的 II 型水对应于在邻近阳离子的影响下结合到阴离子上的水，或者在簇表面处与阳离子-阴离子对结合的水。这构成了初级水合作用。在 150°C 以上不可逆解吸的 III 型水对应于在离子之间动力学捕获的水。其数量强烈地随加工和热处理而变化。因此，吸湿性（即每对离子的水吸附能力）不仅取决于离子，还取决于它们的簇形态。
