Length scales in electrolytes

Ioannis Skarmoutsos, Stefano Mossa
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Abstract

The elusive presence of an anomalously increasing screening length at high ionic concentrations hampers a complete picture of interactions in electrolytes. Theories which extend the diluted Debye-Huckel framework to higher concentrations predict, in addition to the expected decreasing Debye length, an increasing significant scale of the order of at most a few ionic diameters. More recent surface force balance experiments with different materials succeeded in measuring increasing length scales which, however, turn out to extend over tenths or even hundreds of ionic diameters. While simulation work has managed to characterize the former, the latter still avoid detection, generating doubts about its true origin. Here we provide a step forward in the clarification of such a conundrum. We have studied by extensive Molecular Dynamics simulation the properties of a generic model of electrolyte, lithium tetrafluoroborate dissolved in ethylene-carbonate, in a vast range of salt concentrations continuously joining the Debye non-interacting limit to the opposite over-charged ionic liquid-like states. On one side, we have accurately determined the macroscopic concentration-induced structural, dielectric and transport modifications, on the other we have quantified the resulting nano-scale ions organization. Based only on the simulation data, without resorting to any uncontrolled hypothesis or phenomenological parameter, we identify a convincing candidate for the measured anomalously increasing length, whose origin has been possibly misinterpreted.
电解质中的长度标度
在高离子浓度下,屏蔽长度异常增加,这种难以捉摸的现象阻碍了对电解质相互作用的全面了解。将稀释的 Debye-Huckel 框架扩展到更高浓度的理论预测,除了预期的 Debyelength 递减之外,还有一个最多为几个离子直径数量级的显著增大尺度。最近使用不同材料进行的表面力平衡实验成功地测量出了不断增大的长度尺度,但结果表明这些长度尺度超过了十分之一甚至数百个离子直径。虽然模拟工作已经成功地描述了前者的特征,但后者仍然没有被探测到,这使人们对其真正的起源产生了怀疑。在这里,我们在澄清这一难题方面向前迈出了一步。我们通过大量的分子动力学模拟,研究了溶解在碳酸乙烯酯中的四氟硼酸锂这一通用电解质模型在从德拜非相互作用极限到相反的过电荷离子液体状态的巨大盐浓度范围内的特性。一方面,我们精确测定了宏观浓度引起的结构、介电和传输变化,另一方面,我们对由此产生的纳米尺度离子组织进行了量化。我们仅根据模拟数据,而不诉诸任何不可控的假设或现象学参数,就确定了一个令人信服的候选方案,即测量到的异常增大长度,其起源可能被误解了。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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