Mass transport coefficients play an important role in process design and in compositional grading of oil reservoirs. As experimental measurements of these properties can be costly and hazardous, Molecular Dynamics simulations emerge as an alternative approach. In this work, we used Molecular Dynamics to calculate the self-diffusion coefficients of methane/n-hexane mixtures at different conditions, in both liquid and supercritical phases. We evaluated how the finite box size and the choice of the force field affect the calculated properties at high pressures. Results show a strong dependency between self-diffusion and the simulation box size. The Yeh-Hummer analytical correction [J. Phys. Chem. B, 108, 15873 (2004)] can attenuate this effect, but sometimes makes the results depart from experimental data due to issues concerning the force fields. We have also found that different all-atom and united-atom models can produce biased results due to caging effects and to different dihedral configurations of the n-alkane.

传质系数在工艺设计和储油层组成分级中起着重要作用。由于这些性质的实验测量可能既昂贵又危险，因此，分子动力学模拟成为一种替代方法。在这项工作中，我们使用分子动力学计算了在液相和超临界相中不同条件下甲烷/正己烷混合物的自扩散系数。我们评估了有限的箱体尺寸和力场的选择如何影响高压下的计算性能。结果表明，自扩散与模拟盒尺寸之间存在很大的依赖性。Yeh-Hummer解析校正[J. 物理 化学 B，108，15873（2004）]可以减弱这种作用，但是由于涉及力场的问题，有时会使结果偏离实验数据。