Lee Alpha A, Vella Dominic, Perkin Susan, Goriely Alain
†Mathematical Institute, University of Oxford, Oxford OX2 6GG, United Kingdom.
‡Department of Chemistry, University of Oxford, Oxford OX1 3QZ, United Kingdom.
J Phys Chem Lett. 2015 Jan 2;6(1):159-63. doi: 10.1021/jz502250z. Epub 2014 Dec 19.
An important question in understanding the structure of ionic liquids is whether ions are truly "free" and mobile, which would correspond to a concentrated ionic melt, or are rather "bound" in ion pairs, that is, a liquid of ion pairs with a small concentration of free ions. Recent surface force balance experiments from different groups have given conflicting answers to this question. We propose a simple model for the thermodynamics and kinetics of ion pairing in ionic liquids. Our model takes into account screened ion-ion, dipole-dipole, and dipole-ion interactions in the mean-field limit. The results of this model suggest that almost two-thirds of the ions are free at any instant, and ion pairs have a short lifetime comparable to the characteristic time scale for diffusion. These results suggest that there is no particular thermodynamic or kinetic preference for ions to reside in pairs. We therefore conclude that ionic liquids are concentrated, rather than dilute, electrolytes.
理解离子液体结构的一个重要问题是,离子是真正“自由”且可移动的(这对应于一种浓离子熔体),还是更多地“结合”成离子对,即一种含有少量自由离子的离子对液体。不同研究团队最近进行的表面力平衡实验对这个问题给出了相互矛盾的答案。我们提出了一个关于离子液体中离子配对的热力学和动力学的简单模型。我们的模型在平均场极限下考虑了屏蔽的离子 - 离子、偶极 - 偶极和偶极 - 离子相互作用。该模型的结果表明,在任何时刻几乎三分之二的离子是自由的,并且离子对的寿命很短,与扩散的特征时间尺度相当。这些结果表明,离子没有特别的热力学或动力学偏好来形成离子对。因此,我们得出结论,离子液体是浓电解质,而非稀电解质。