Terahertz spectroscopy of aqueous ionic solutions to understand the role of hydrogen-bond network breaking and strengthening in the Hofmeister

1888年，弗朗茨·霍夫迈斯特在他的论文《Zur Lehre von der Wirkung der Salze》中发现了一系列盐——后来被命名为霍夫迈斯特系列——它们有越来越强的沉淀蛋白质的倾向。随着时间的推移，人们逐渐清楚地认识到，霍夫迈斯特级数与各种离子溶解水的倾向有关。经典观点认为，离子可分为共色性或混沌性：共色性被认为在周围的水中引起更像冰的有序，而混沌性则破坏局部四面体网络。然而，超快红外研究表明，水的结构在溶剂化离子周围的第一个溶剂化壳层（“溶剂型”模型）之外保持不变，这一清晰的图像被打乱了。本文提出利用太赫兹拉曼光谱和红外光谱结合时间分辨二维红外光谱（与Robin Hochstrasser合作）来研究溶剂化水分子的飞秒氢键动力学和离子周围的皮秒扩散动力学。初步结果表明，太赫兹拉曼数据与之前的泵浦探测实验和各种理论研究不一致，需要对理论模型进行彻底检查。我们与凯西·海因斯的合作将使我们能够影响离子溶剂化理论工作的发展。

In 1888, Franz Hofmeister in his paper 'Zur Lehre von der Wirkung der Salze' discovered a series of salts -- later named the Hofmeister series -- that have an increasing propensity to precipitate proteins. It became clear over time that the Hofmeister series is related to the propensity of various ions to solvate water. The classical view is that ions may be classed as kosmotropes or chaotropes: kosmotropes are thought to induce a more icelike ordering in the surrounding water whereas chaotropes disrupt the local tetrahedral network. However, this tidy picture was thrown into disarray by ultrafast infrared studies pertaining to show that the water structure remains unaltered beyond the first solvation shell around solvated ions (the 'solventberg' model). Here we propose to use terahertz Raman and infrared spectroscopy combined with time-resolved two-dimensional infrared spectroscopy (in collaboration with Robin Hochstrasser) to study femtosecond hydrogen-bond dynamics of solvated water molecules and picosecond diffusive dynamics around ions. Preliminary results indicate that the terahertz Raman data is at odds with previous pump-probe experiments and various theoretical studies, requiring an overhaul of theoretical models. Our collaboration with Casey Hynes will allow us to influence the development of theoretical work on ionic solvation.