Molecular size and shape effects: Rotational diffusion and the Stokes-Einstein-Debye relation
Molecular size and shape effects: Rotational diffusion and the Stokes-Einstein-Debye relation
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分子大小和形状效应：旋转扩散和斯托克斯-爱因斯坦-德拜关系
DOI：
10.1016/j.molliq.2020.113764
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发表时间：
2020
影响因子：
6
通讯作者：
Ishii Yoshiki
中科院分区：
文献类型：
--
作者：
Ohtori Norikazu;Kondo Yuta;Ishii Yoshiki
通过双中心Lennard-Jones （2CLJ）势的分子动力学模拟，给出了双原子分子液体的旋转扩散系数和Stokes-Einstein-Debye （SED）关系的表达式。剪切粘度η和旋转扩散系数表示为分子质量和分子数密度yn /V，或转动惯量、堆积分数、温度、相互作用能和分子伸长的函数l∗≡l/σ，其中，e为系统体积中包含的分子数V，l为双原子分子的键长，σ为LJ势中使用的尺寸参数。填充率和伸长率分别是表示分子大小和形状的变量。这些结果直接产生了基于分子的SED关系，即drη sv/T∝vm∗1/3l∗−3(N/V)，其中evm∗是无因次的分子体积，仅以伸长∗的解析函数表示。也就是说，这个SED方程不取决于大小，而取决于形状。这与基于大小的原始SED关系形成鲜明对比，这表明在分子尺度上对关系进行全面的重新考虑。形状项解释了一个悖论，即更多的球形分子，如n2，更强烈地偏离基于球形粒子的原始SED方程。此外，对于lenard - jones和2CLJ液体，没有尺寸的SED关系与Stokes-Einstein关系一致，表示为ηsv/T∝(N/V)1/3，其中为平移自扩散系数。
Formulation of rotational diffusion coefficient and the Stokes-Einstein-Debye (SED) relation is presented for diatomic molecular liquids by molecular dynamics simulation with two-center Lennard-Jones (2CLJ) potentials. Shear viscosityηsvand rotational diffusion coefficientDrare expressed as a function of molecular mass and number densityN/V, or moment of inertia, packing fraction, temperatureT, interaction energy, and molecular elongationl∗≡l/σ, whereNis the number of molecules included in the system volumeV,lthe bond length in the diatomic molecules, andσthe size parameter used in the LJ potentials. The packing fraction and elongation are the variables expressing molecular size and shape, respectively. These results produce directly a molecular-basis SED relation asDrηsv/T∝vm∗1/3l∗−3(N/V), wherevm∗is the dimensionless molecular volume expressed as an analytical function only of elongationl∗. That is, this SED equation depends not on the size but on the shape. This is highly contrasted with the original SED relation based on the size, which suggests overall reconsideration of the relation on a molecular scale. The shape term accounts for a paradox that more spherical molecules such as N2deviate more strongly from the original SED equation based on a spherical particle. In addition, the SED relation without the size is consistent with the Stokes-Einstein relation for both the Lennard-Jones and 2CLJ liquids expressed asDηsv/T∝ (N/V)1/3, whereDis the translational self-diffusion coefficient.