Interfacial Polarization and Ionic Structure at the Ionic Liquid–Metal Interface Studied by Vibrational Spectroscopy and Molecular Dynamics Simulations
Interfacial Polarization and Ionic Structure at the Ionic Liquid–Metal Interface Studied by Vibrational Spectroscopy and Molecular Dynamics Simulations
复制标题
通过振动光谱和分子动力学模拟研究离子液体与金属界面的界面极化和离子结构
DOI：
10.1021/acs.jpcb.0c11232
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
Dawlaty, Jahan M.
中科院分区：
文献类型：
--
作者：
Voegtle, Matthew J.;Pal, Tanmoy;Pennathur, Anuj K.;Menachekanian, Sevan;Patrow, Joel G.;Sarkar, Sohini;Cui, Qiang;Dawlaty, Jahan M.
离子液体在界面科学和电化学领域具有基础和实用价值。然而，了解它们在表面附近的行为是具有挑战性的，因为强库仑相互作用和大而不规则的离子尺寸会影响它们的结构和能量学。为了理解这个问题，我们提出了一个结合实验和计算的研究，使用一个振动探针分子，4-巯基苯腈，插入在金属和各种il之间的连接处。探针分子中腈的振动频率反映了该接点的局部溶剂化环境和静电场。在乙基甲基咪唑（EMIM+）阳离子家族中，我们改变了一系列大小和类型的阴离子。作为表面光谱学的补充，我们还对这些界面进行了分子动力学模拟，以更好地理解产生测量场的离子结构。频移的幅度和场的大小与阴离子的大小一般相关，较大的阴离子对应较小的场。我们发现这种相关性的来源是较小的阴离子部分嵌入探针单层，导致靠近表面的离子层堆积更紧密。较大的阴离子降低了表面附近的总体横向离子堆积密度，从而降低了单位面积的净电荷，并解释了观察到的较小的场。这项工作的见解对于开发界面附近浓缩电解质的基本图像非常重要，并且可以帮助设计ILs以在电极附近创建定制的电场。
Ionic liquids (ILs) have both fundamental and practical value in interfacial science and electrochemistry. However, understanding their behavior near a surface is challenging because of strong Coulomb interactions and large and irregular ionic sizes, which affect both their structure and energetics. To understand this problem, we present a combined experimental and computational study using a vibrational probe molecule, 4-mercaptobenzonitrile, inserted at the junction between a metal and a variety of ILs. The vibrational frequency of the nitrile in the probe molecule reports on the local solvation environment and the electrostatic field at this junction. Within the ethylmethyl imidazolium (EMIM+) cation family of ILs, we varied the anions over a range of sizes and types. Complementing our surface spectroscopy, we also ran molecular dynamics simulations of these interfaces to better understand the ionic structures that produced the measured fields. The magnitude of the frequency shifts, and thereby fields, shows a general correlation with the size of anions, with larger anions corresponding to smaller fields. We find that the source of this correlation is partial intercalation of smaller anions into the probe monolayer, resulting in tighter packing of ionic layers near the surface. Larger anions reduce the overall lateral ion packing density near the surface, which reduces the net charge per unit area and explains the smaller observed fields. The insight from this work is important for developing a fundamental picture of concentrated electrolytes near interfaces and can help with designing ILs to create tailored electric fields near an electrode.