Depolarized Dynamic Light Scattering and Dielectric Spectroscopy: Two Perspectives on Molecular Reorientation in Supercooled Liquids
Depolarized Dynamic Light Scattering and Dielectric Spectroscopy: Two Perspectives on Molecular Reorientation in Supercooled Liquids
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DOI：
10.1007/978-3-319-72706-6_7
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发表时间：
2018-01-01
期刊：
影响因子：
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通讯作者：
Blochowicz, T.
中科院分区：
文献类型：
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作者：
Gabriel, J.;Pabst, F.;Blochowicz, T.
宽带介电光谱（BDS）可以被认为是标准的和最广泛的方法来实验获得分子定向在过冷液体，因为它涵盖了时间常数范围从亚皮秒对应的高度流体的液体到几千秒低于玻璃化转变温度。以类似的方式，去极化动态光散射（DLS）能够探测分子的重定向。通过将串联法布里珀罗干涉测量（TFPI）和光子相关光谱（PCS）与最近的多斑技术相结合，可以覆盖相当的时间尺度范围，甚至可以访问低于T-g的非遍历状态。因此，DLS代表了覆盖玻璃动力学全范围的另一种途径。此外，由于两种方法耦合不同的分子性质，通过比较两种技术的实验数据可以获得特定动态过程背后的运动机制的额外信息。在目前的工作中，我们探讨了这种方法的几个例子，包括离子液体和单羟基醇，并讨论了不同的弛豫过程的含义。例如，在过冷离子液体的情况下，即熔融盐，在室温下是液态的，这两种技术的结合可以明确地将分子重定向的贡献从其他极化特征中分离出来，这些特征通常掩盖了介电光谱中的重定向，并且详细的分析揭示了涉及α弛豫的运动机制中的交叉迹象。在单羟基醇中，我们讨论了两种技术中Johari-Goldstein β过程的出现以及观察结果对潜在运动机制的含义。此外，我们考虑了在单醇的介电光谱中经常观察到的德拜弛豫，它通常被归因于瞬态超分子结构。在这里，这样的数据比较揭示了超分子结构形成条件下的分子细节。
Broadband dielectric spectroscopy (BDS) can be considered the standard and most widespread method to experimentally access molecular reorientation in supercooled liquids, as it covers a range of time constants from sub picoseconds corresponding to the highly fluid liquid to several thousand seconds below the glass transition temperature. In a similar fashion, depolarized dynamic light scattering (DLS) is able to probe molecular reorientation. A comparable range of time scales is covered by combining Tandem Fabry Perot Interferometry (TFPI) and Photon Correlation Spectroscopy (PCS) with recent multispeckle techniques allowing to access even the non-ergodic regime below T-g. Thus, DLS represents an alternative route to cover the full range of glassy dynamics. Moreover, due to the fact that both methods couple to different molecular properties, extra information in particular on the motional mechanism behind a certain dynamic process can be obtained by comparing experimental data from both techniques. In the present work we explore this approach for several examples, including ionic liquids and monohydroxy alcohols, and discuss the implications for different relaxation processes. For instance in the case of supercooled ionic liquids, i.e., molten salts, which are liquid at room temperature, the combination of both techniques allows to unambiguously disentangle the contribution ofmolecular reorientation from other polarization features that often mask reorientation in the dielectric spectra, and a detailed analysis reveals indications for a crossover in the motional mechanism involved in the alpha-relaxation. In monohydroxy alcohols we discuss the appearance of the Johari-Goldstein beta-process in both techniques and what the observations imply for the underlying motional mechansim. Furthermore, we consider the Debye relaxation, which is frequently observed in the dielectric spectra of monoalcohols and is usually ascribed to transient supramolecular structures. Here, such a comparison of data reveals molecular details about the conditions under which the supramolecular structures are formed.