期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：24
页码：7432-7437
DOI：10.1073/pnas.1508366112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceLiquid solutions with high concentrations of electrically charged ions are key elements of many energy storage technologies and are prevalent in biology. Nevertheless, they remain poorly understood. We study ionic liquids--liquids composed solely of ions--with the goal of providing a general picture of concentrated ionic solutions. Using molecular-scale experiments, we show that, surprisingly, less than 0.1% of the ions in ionic liquids are "free" to contribute to electrostatic screening, with the remainder "stuck" as neutral aggregates. Our temperature-dependent results provide fundamental guidance for designing high-performance ionic liquids for numerous applications. More broadly, we provide a novel way of envisioning concentrated ionic solutions with wide-ranging implications, such as elucidating the nanoscale properties of underwater bioadhesives and other self-assembled biomolecules. Electrolyte solutions with high concentrations of ions are prevalent in biological systems and energy storage technologies. Nevertheless, the high interaction free energy and long-range nature of electrostatic interactions makes the development of a general conceptual picture of concentrated electrolytes a significant challenge. In this work, we study ionic liquids, single-component liquids composed solely of ions, in an attempt to provide a novel perspective on electrostatic screening in very high concentration (nonideal) electrolytes. We use temperature-dependent surface force measurements to demonstrate that the long-range, exponentially decaying diffuse double-layer forces observed across ionic liquids exhibit a pronounced temperature dependence: Increasing the temperature decreases the measured exponential (Debye) decay length, implying an increase in the thermally driven effective free-ion concentration in the bulk ionic liquids. We use our quantitative results to propose a general model of long-range electrostatic screening in ionic liquids, where thermally activated charge fluctuations, either free ions or correlated domains (quasiparticles), take on the role of ions in traditional dilute electrolyte solutions. This picture represents a crucial step toward resolving several inconsistencies surrounding electrostatic screening and charge transport in ionic liquids that have impeded progress within the interdisciplinary ionic liquids community. More broadly, our work provides a previously unidentified way of envisioning highly concentrated electrolytes, with implications for diverse areas of inquiry, ranging from designing electrochemical devices to rationalizing electrostatic interactions in biological systems.
关键词：electrostatic interactions ; intermolecular interactions ; interfacial phenomena ; Boltzmann distribution ; activation energy
