Hossain Sk Saddam, Samanta Anunay
School of Chemistry, University of Hyderabad , Hyderabad 500 046, India.
J Phys Chem B. 2017 Nov 22;121(46):10556-10565. doi: 10.1021/acs.jpcb.7b08472. Epub 2017 Nov 14.
Deep eutectic solvents (DESs) are an emerging class of environment-friendly media useful in a variety of applications. However, the microscopic structure of these liquids is quite unclear, and issues like whether spatial and dynamic heterogeneity is a generic feature of the ionic DESs and whether the solute molecules experience similar environment and interactions with the medium irrespective of their charge are still open. In this work, we have attempted to address some of these issues for ethaline, a less viscous and ionic DES consisting of a mixture of 1:2 mole ratio of choline chloride and ethylene glycol by monitoring the fluorescence response of a number of carefully chosen neutral and charged probe molecules in ensemble and single molecule conditions. Specifically, we have examined the liquid state structure of ethaline by studying the rotational and translational diffusion dynamics of the solutes measured by monitoring the time dependence of fluorescence anisotropy in ensemble condition and fluorescence correlation signal of extremely dilute samples diffusing through confocal volume. These studies clearly reveal dynamic heterogeneity of the medium, though no spatial heterogeneity is observable through excitation wavelength dependent fluorescence measurements. The insights obtained from this study will be helpful in understanding the nature of solute-solvent interactions in this type of complex media.
深共熔溶剂（DESs）是一类新兴的环境友好型介质，在多种应用中都很有用。然而，这些液体的微观结构尚不清楚，诸如空间和动态非均质性是否是离子型DESs的普遍特征，以及溶质分子无论其电荷如何是否都经历相似的环境并与介质发生相似的相互作用等问题仍然没有答案。在这项工作中，我们试图通过监测在整体和单分子条件下一些精心挑选的中性和带电探针分子的荧光响应，来解决关于乙胺（一种由摩尔比为1:2的氯化胆碱和乙二醇混合物组成的低粘度离子型DES）的其中一些问题。具体而言，我们通过研究溶质的旋转和平动扩散动力学来研究乙胺的液态结构，这些动力学是通过监测整体条件下荧光各向异性的时间依赖性以及极稀样品通过共焦体积扩散的荧光相关信号来测量的。这些研究清楚地揭示了介质的动态非均质性，尽管通过依赖激发波长的荧光测量无法观察到空间非均质性。从这项研究中获得的见解将有助于理解这类复杂介质中溶质 - 溶剂相互作用的本质。
