How Is Charge Transport Different in Ionic Liquids and Electrolyte Solutions?
How Is Charge Transport Different in Ionic Liquids and Electrolyte Solutions?
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DOI：
10.1021/jp204182c
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发表时间：
2011-11-17
影响因子：
3.3
通讯作者：
Margulis, Claudio J.
中科院分区：
文献类型：
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作者：
Kashyap, Hemant K.;Annapureddy, Harsha V. R.;Margulis, Claudio J.
在这篇文章中，我们表明，在一个重心参考系分析，直接从阻抗实验相对于间接估计从NMR扩散实验测量的电导率的偏差有不同的起源在电解质溶液和纯盐。在电解质溶液的情况下，溶剂+离子满足动量守恒定律。相反，在熔融盐或离子液体中，动量守恒必须仅由离子满足。这具有重大意义。虽然相反电荷的离子的正相关运动是电解质溶液中阻抗电导率降低的合理解释，但在离子液体和熔融盐的情况下并非如此。这项工作提出了一组方程，在离子液体和熔融盐的情况下，可以用来获得从阻抗和NMR的直接测量的扩散系数矩阵的不同部分在重心参考系中。换句话说，通过使用实验可测量的量，这些方程使我们能够访问的离子之间的运动耦合，没有一个单一的直接实验测量技术。虽然这种类型的方程已经提出了之前，这里提出的那些可以很容易地从动量守恒定律和线性响应理论。我们的研究结果表明，在离子液体和熔融盐中的阻抗电导率相对于NMR电导率的降低是由于相同电荷的离子的质子相关运动。这种情况在电解质溶液中有所不同，其中相反电荷离子的正相关运动对阻抗电导率的降低做出了显着贡献。相比之下，在包含单一二元盐（室温离子液体或熔融盐）的体系中，阳离子-阴离子相异扩散系数是负定的，并且与阳离子-阳离子和阴离子-阴离子相异扩散系数的贡献相反。在只包含两种离子成分的系统中，阳离子-阴离子扩散系数不同的性质不仅在重心参考系中成立，而且在任何非平衡热力学的内部参考系中也成立。
In this article we show that, analyzed in a barycentric reference frame, the deviation in conductivity measured directly from impedance experiments with respect to that estimated indirectly from NMR diffusion experiments has different origins in electrolyte solutions and pure salts. In the case of electrolyte solutions, the momentum conservation law is satisfied by solvent + ions. Instead, in a molten salt or ionic liquid momentum conservation must be satisfied solely by the ions. This has significant implications. While positively correlated motion of ions of opposite charge is a well justified explanation for the reduction in impedance conductivity in the case of electrolyte solutions, it is not so in the case of ionic liquids and molten salts. This work presents a set of equations that in the case of ionic liquids and molten salts can be used to obtain from direct measurements of impedance and NMR the distinct part of the diffusion coefficient matrix in the barycentric reference frame In other words, by using experimentally measurable quantities, these equations allow us to access the motional coupling between ions for which there is no single direct experimental measurement technique. While equations of this type have been proposed before, the ones presented here can be easily derived from the momentum conservation law and linear response theory. Our results indicate that the decrease in the impedance conductivity with respect to NMR conductivity in ionic liquids and molten salts is due to anticorrelated motion of ions of same charge. This scenario is different in electrolyte solutions, where the positively correlated motion of ions of opposite charge makes a significant contribution to the decrease in the impedance conductivity. In contrast, in a system comprising a single binary salt (a room temperature ionic liquid or a molten salt), the cation-anion distinct diffusion coefficient is negative definite and opposes the contribution from the cation-cation and anion-anion distinct diffusion coefficients. This property of the cation-anion distinct diffusion coefficient in systems comprising just two ion-constituents holds true not just in the barycentric reference frame but also in any of the internal reference frames of nonequilibrium thermodynamics.