Abstract

The predictive capabilities of some existing theoretical models to quantify thermodiffusion have been investigated in this work. To do so, the tests have been performed on two model fluids, the hard-sphere and the Lennard-Jones (including spheres and dimers) ones, exploring different mixtures and thermodynamic conditions thanks to extensive molecular simulations. It has been confirmed that the thermal diffusion factor should be expressed as the sum of one term related to the isotope effect and one term related to the “chemical” effects and that a kinetic term is required to quantify thermodiffusion from the gas state to the liquid state. In addition, regarding the isotope effects, it has been obtained that none of the available theoretical models are able to yield a reasonable prediction relatively to the molecular simulations results and that the moment of inertia contribution is one order of magnitude smaller than the mass contribution in the liquid state. Finally, concerning the chemical effects, it has been shown the Shukla and Firoozabadi model, complemented with a kinetic term, is probably the most reasonable option to estimate the chemical contribution to the thermal diffusion factor, even if it fails in capturing the effect of the asymmetry in size and in shape between the species. Overall, this works confirms that there is still a lack of a generic model able to predict accurately thermal diffusion factors, or equivalently Soret coefficient, in simple binary mixtures from the gas state to the liquid state.

Graphical abstract

中文翻译：

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预测简单二元流体混合物中的热扩散

摘要

在这项工作中，已经研究了一些现有理论模型对热扩散进行量化的预测能力。为此，已对硬球和 Lennard-Jones（包括球体和二聚体）两种模型流体进行了测试，通过广泛的分子模拟探索了不同的混合物和热力学条件。已经证实，热扩散因子应表示为与同位素效应相关的一项和与“化学”效应相关的一项之和，并且需要一个动力学项来量化从气态到液态的热扩散状态。此外，关于同位素效应，已经发现，没有一个可用的理论模型能够相对于分子模拟结果产生合理的预测，并且惯性矩的贡献比液态的质量贡献小一个数量级。最后，关于化学效应，已经表明 Shukla 和 Firoozabadi 模型，加上一个动力学项，可能是估计化学对热扩散因子的贡献的最合理的选择，即使它未能捕捉到热扩散因子的影响。物种之间的大小和形状不对称。总体而言，这项工作证实，在从气态到液态的简单二元混合物中，仍然缺乏能够准确预测热扩散因子或等效的 Soret 系数的通用模型。

图形概要