Silva Savannah, Singh Siddharth, Cao Ethan, Fourkas John T, Siwy Zuzanna S
Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, USA.
Faraday Discuss. 2023 Oct 12;246(0):508-519. doi: 10.1039/d3fd00063j.
The development of modern membranes for ionic separations and energy-storage devices such as supercapacitors depends on the description of ions at solid interfaces, as is often provided by the electrical double layer (EDL) model. The classical EDL model ignores, however, important factors such as possible spatial organization of solvent at the interface and the influence of the solvent on the spatial dependence of the electrochemical potential; these effects in turn govern electrokinetic phenomena. Here we provide a molecular-level understanding of how solvent structure can dictate ionic distributions at interfaces using a model system of a polar, aprotic solvent, propylene carbonate, in its enantiomerically pure and racemic forms, at a silica interface. We link the interfacial structure to the tuning of ionic and fluid transport by the chirality of the solvent and the salt concentration. The results of nonlinear spectroscopic experiments and electrochemical measurements suggest that the solvent exhibits lipid-bilayer-like interfacial organization, with a structure that is dependent on the solvent chirality. The racemic form creates highly ordered layered structure that dictates local ionic concentrations, such that the effective surface potential becomes positive in a wide range of electrolyte concentrations. The enantiomerically pure form exhibits weaker ordering at the silica surface, which leads to a lower effective surface charge induced by ions partitioning into the layered structure. The surface charge in silicon nitride and polymer pores is probed through the direction of electroosmosis that the surface charges induce. Our findings add a new dimension to the nascent field of chiral electrochemistry, and emphasize the importance of including solvent molecules in descriptions of solid-liquid interfaces.
用于离子分离和超级电容器等储能装置的现代膜的发展依赖于固体界面处离子的描述，这通常由双电层（EDL）模型提供。然而，经典的EDL模型忽略了一些重要因素，例如界面处溶剂可能的空间组织以及溶剂对电化学势空间依赖性的影响；这些效应反过来又控制着电动现象。在这里，我们使用一种极性非质子溶剂碳酸丙烯酯的对映体纯形式和外消旋形式在二氧化硅界面的模型系统，从分子层面理解溶剂结构如何决定界面处的离子分布。我们将界面结构与溶剂手性和盐浓度对离子及流体传输的调节联系起来。非线性光谱实验和电化学测量结果表明，溶剂呈现出类似脂质双层的界面组织，其结构取决于溶剂手性。外消旋形式形成高度有序的层状结构，该结构决定了局部离子浓度，使得在广泛的电解质浓度范围内有效表面电势变为正值。对映体纯形式在二氧化硅表面表现出较弱的有序性，这导致离子分配到层状结构中所诱导的有效表面电荷较低。通过表面电荷诱导的电渗方向探测了氮化硅和聚合物孔隙中的表面电荷。我们的发现为新兴的手性电化学领域增添了新的维度，并强调了在固液界面描述中纳入溶剂分子的重要性。
