Harris Kenneth R, Kanakubo Mitsuhiro
School of Science, The University of New South Wales, P.O. Box 7916, Canberra BC, Australian Capital Territory 2610, Australia.
Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan.
J Phys Chem B. 2024 May 9;128(18):4504-4512. doi: 10.1021/acs.jpcb.4c01100. Epub 2024 Apr 29.
In electrolytes, the self- and interdiffusion coefficients, transport numbers, and electrical conductivity can be used to determine velocity cross-correlation coefficients (VCC) that are also accessible through molecular dynamics simulations. In an ionic liquid or molten salt, there are only three, corresponding to correlations between the velocities of distinct ion pairs (cation-anion, cation-cation, and anion-anion) averaged over both the ensemble and time, calculable from experimental ion self-diffusion coefficients and the electrolyte conductivity. Most usually, the mass-fixed frame of reference (with velocities relative to that of the center of mass of the system) is used to discuss the VCC and the distinct diffusion coefficients (DDC) derived from them. Recent work in the literature has suggested a dependence of the DDC on the ratio of the anion to cation mass. Here, we demonstrate, using our own and literature transport property data for a large number of ionic liquids and molten salts, that the trends observed depend on the particular choice of velocity reference frame, mass-, number-, or volume-fixed. The perception of ion-ion interactions may be distorted in the mass- and volume fixed frames when the co-ions have very different masses or volumes, particularly for systems containing light lithium ions.
在电解质中，自扩散系数和互扩散系数、迁移数以及电导率可用于确定速度交叉相关系数（VCC），这些系数也可通过分子动力学模拟获得。在离子液体或熔盐中，只有三个这样的系数，分别对应于不同离子对（阳离子 - 阴离子、阳离子 - 阳离子和阴离子 - 阴离子）速度之间的相关性，它们是在系综和时间上平均得到的，可根据实验离子自扩散系数和电解质电导率计算得出。最常见的是，使用质量固定的参考系（速度相对于系统质心的速度）来讨论VCC以及由它们推导出来的不同扩散系数（DDC）。文献中的最新研究表明，DDC与阴离子和阳离子质量之比有关。在此，我们利用我们自己以及文献中大量离子液体和熔盐的传输性质数据表明，观察到的趋势取决于速度参考系的特定选择——质量固定、数量固定或体积固定。当同离子具有非常不同的质量或体积时，尤其是对于含有轻锂离子的系统，在质量固定和体积固定的参考系中，离子 - 离子相互作用的认知可能会被扭曲。
