Relations between the Fractional Stokes-Einstein and Nernst-Einstein Equations and Velocity Correlation Coefficients in Ionic Liquids and Molten Salts
Relations between the Fractional Stokes-Einstein and Nernst-Einstein Equations and Velocity Correlation Coefficients in Ionic Liquids and Molten Salts
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DOI：
10.1021/jp102687r
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发表时间：
2010-07-29
影响因子：
3.3
通讯作者：
Harris, Kenneth R.
中科院分区：
文献类型：
--
作者：
Harris, Kenneth R.
人们经常断言，离子液体（ILs）和熔盐中摩尔电导率（Lambda）和离子自扩散系数（D(i)）之间偏离能斯特-爱因斯坦关系（NE）是离子配对的证据。NE最初是用于非相互作用离子，如在无限稀释的电解质溶液中。实际上，液态液体和熔盐中的质量、电荷、动量和能量传输过程与其他致密液体一样，涉及相关的离子间碰撞、笼化和涡旋运动。利用非平衡热力学和文献分子动力学模拟的现象学理论表明，由于离子速度互相关的差异，与简单NE表达式的偏差会发生。与一般的分子液体和Lennard-Jones等模型流体一样，il也被证明符合Stokes-Einstein关系的分数形式，D(i)/T与（1/eta）(T)和Lambda (1/eta)(T)成比例，其中eta是剪切粘度。如图所示，在这种情况下，NE偏差参数Delta是一个常数，与温度和压力无关（与实验一致），参数t的值；它是离子电荷和体积的函数，而不是质量的函数。因此，Delta不是“离子性”的度量：有必要寻找其他独立的证据来确定在给定的离子液体或熔盐中是否存在离子配对。用自扩散系数推导出的“表观”输运数来描述纯盐中的电荷输运被认为是不必要的。
It is often asserted that deviation from the Nernst-Einstein relation (NE) between the molar conductivity (Lambda) and ion self-diffusion coefficients (D(i)) in ionic liquids (ILs) and molten salts is evidence for ion pairing. The NE was originally derived for noninteracting ions, as in an infinitely dilute electrolyte solution. In reality, mass, charge, momentum, and energy transport processes in ILs and molten salts involve correlated interionic collisions, caging, and vortex motions, as in any other dense liquid. Phenomenological theory using nonequilibrium thermodynamics and literature molecular dynamics simulations shows that deviations from the simple NE expression occur due to differences in cross-correlations of ionic velocities. ILs have also been shown, like molecular liquids generally, and model fluids such as the Lennard-Jones, to fit the fractional form of the Stokes-Einstein relation, D(i)/T proportional to (1/eta)(t) and Lambda (1/eta)(t) where eta is the shear viscosity. Here, it is shown that when this is the case, the NE deviation parameter Delta is then a constant, independent of temperature and pressure (consistent with experiment) and the value of the parameter t; it is a function of the ionic charges and volumes, but not the masses. Therefore, Delta is not a measure of "ionicity": it is necessary to seek other independent evidence to determine whether ion pairing is present in a given ionic liquid or molten salt. The use of "apparent" transport numbers derived from self-diffusion coefficients to describe charge transport in pure salts is argued to be unnecessary.