Department of Physical Chemistry II, Ruhr-University Bochum, 44780 Bochum, Germany.
J Am Chem Soc. 2009 Dec 30;131(51):18512-7. doi: 10.1021/ja9083545.
We present terahertz (THz) measurements of salt solutions that shed new light on the controversy over whether salts act as kosmotropes (structure makers) or chaotropes (structure breakers), which enhance or reduce the solvent order, respectively. We have carried out precise measurements of the concentration-dependent THz absorption coefficient of 15 solvated alkali halide salts around 85 cm(-1) (2.5 THz). In addition, we recorded overview spectra between 30 and 300 cm(-1) using a THz Fourier transform spectrometer for six alkali halides. For all solutions we found a linear increase of THz absorption compared to pure water (THz excess) with increasing solute concentration. These results suggest that the ions may be treated as simple defects in an H-bond network. They therefore cannot be characterized as either kosmotropes or chaotropes. Below 200 cm(-1), the observed THz excess of all salts can be described by a linear superposition of the water absorption and an additional absorption that is attributed to a rattling motion of the ions within the water network. By providing a comprehensive set of data for different salt solutions, we find that the solutions can all be very well described by a model that includes damped harmonic oscillations of the anions and cations within the water network. We find this model predicts the main features of THz spectra for a variety of salt solutions. The assumption of the existence of these ion rattling motions on sub-picosecond time scales is supported by THz Fourier transform spectroscopy of six alkali halides. Above 200 cm(-1) the excess is interpreted in terms of a change in the wing of the water network librational mode. Accompanying molecular dynamics simulations using the TIP3P water model support our conclusion and show that the fast sub-picosecond motions of the ions and their surroundings are almost decoupled. These findings provide a complete description of the solute-induced changes in the THz solvation dynamics for the investigated salts. Our results show that THz spectroscopy is a powerful experimental tool to establish a new view on the contributions of anions and cations to the structuring of water.
我们呈现了太赫兹（THz）测量的盐溶液，这为盐是作为亲溶体（结构形成者）还是疏溶体（结构破坏者）的争议提供了新的视角，亲溶体分别增强或降低溶剂有序性。我们对 15 种溶剂化的碱卤盐在 85 cm(-1)（2.5 THz）附近的浓度依赖性太赫兹吸收系数进行了精确测量。此外，我们使用太赫兹傅里叶变换光谱仪记录了六种碱卤盐在 30 到 300 cm(-1)之间的总览光谱。对于所有溶液，我们发现随着溶质浓度的增加，THz 吸收与纯水相比呈线性增加（THz 过剩）。这些结果表明，离子可以被视为氢键网络中的简单缺陷。因此，它们不能被描述为亲溶体或疏溶体。在 200 cm(-1)以下，所有盐的观察到的 THz 过剩可以通过水吸收的线性叠加和归因于离子在水网络中振动的额外吸收来描述。通过提供不同盐溶液的全面数据集，我们发现该模型可以很好地描述溶液，该模型包括水网络中阴离子和阳离子的阻尼谐振动。我们发现该模型预测了各种盐溶液的 THz 光谱的主要特征。THz 傅里叶变换光谱学对六种碱卤盐的研究支持了存在这些离子振动运动的假设，其时间尺度在亚皮秒范围内。在 200 cm(-1)以上，过剩部分可以根据水分子网络转动模式的翅膀变化来解释。使用 TIP3P 水模型的伴随分子动力学模拟支持我们的结论，并表明离子及其周围环境的快速亚皮秒运动几乎是解耦的。这些发现提供了对所研究盐的太赫兹溶剂化动力学中溶质诱导变化的完整描述。我们的结果表明，太赫兹光谱学是一种强大的实验工具，可以建立一种新的观点，即阴离子和阳离子对水结构的贡献。
