Accurate determination of binary gas diffusion coefficients by using laser-optical measurement methods and molecular dynamics simulations

本课题的主要目的是通过实验和理论方法精确测定二元气体混合物中的二元扩散系数。为此，对甲烷-二氧化碳和甲烷-丙烷体系应采用激光光学测量技术（Loschmidt cell， HILC）动态光散射（dynamic light scattering， DLS）和全息干涉测量技术以及分子动力学（Molecular Dynamics， MD）模拟。为了提高MD模拟在预测此类混合物扩散系数方面的潜力，我们的目标是通过对特定混合物使用新的优化力场（FFs）进一步发展该方法。罗斯托克大学的赫尔曼博士在一项平行提案中提出，这些ff应该是在零密度极限下通过高度精确的从头算量子计算得出的。为了验证从MD计算得到的结果，需要从DLS和HILC得到的菲克二元扩散系数。这两种实验方法的应用提供了在较宽的混合密度范围内扩散质量输运的信息，旨在使二元扩散系数的典型不确定性低于1%。从HILC获得的数据库还应该与Hellmann博士的理论二元扩散系数数据进行比较，以验证他在零密度极限下的计算。这些理论数据随后被Hellmann博士用于开发适合我们的MD模拟的FFs，作为气相密度的函数。基于这些FFs， MD模拟应该允许通过研究微观波动来可靠地计算质量扩散系数，这与DLS方法的原理相似。在MD模拟中，应分别分析纯态和混合态气体的自扩散系数，以及混合物的麦克斯韦-斯蒂芬扩散系数和热力学因子。结合后两种性质可以得到菲克二元扩散系数的预测结果，并与实验数据进行比较。通过解耦MD模拟中的分子扩散过程，还可以证明不同扩散率在多大程度上一致，以及它们如何相互关联以及如何与文献中常见的预测模型相关联。在广泛的热力学状态下，应将纯组分和特定混合物的从头算出的FFs模拟扩散率数据与利用已有文献的纯组分FFs得到的模拟扩散率数据进行比较。通过这一点，应该揭示是否特别使用对特异性FFs更适合更准确地计算质量扩散率，以及这些结果与零密度极限的量子计算以及在更大密度范围内的实验结果的一致程度。

The major aim of the proposed research project is the accurate determination of the binary diffusion coefficient in binary gas mixtures by experimental and theoretical methods. For this, the laser-optical measurement techniques dynamic light scattering (DLS) and holographic interferometry applied for a Loschmidt cell (HILC) as well as Molecular Dynamics (MD) simulations should be used for the systems methane-carbon dioxide and methane-propane. To improve the potential of MD simulation in predicting diffusion coefficients of such mixtures, it is the aim to further develop the method by using new optimized force fields (FFs) for the specific mixtures. These FFs should be derived from highly accurate ab initio quantum calculations in the zero-density limit by Dr. Hellmann from the University of Rostock in a parallel proposal. To verify the results obtained from the MD calculations, the Fick binary diffusion coefficients obtained from DLS and HILC are required. The application of the two experimental methods provides information on the diffusive mass transport over a broad range of mixture densities, aiming at typical uncertainties of the binary diffusion coefficients below 1%. The data base obtained from HILC should also be compared with Dr. Hellmann's theoretical binary diffusion coefficient data to validate his calculations at the limit of zero density. These theoretical data are then employed by Dr. Hellmann to develop suitable FFs for our MD simulations performed as a function of density in the gas phase. Based on these FFs, the MD simulations should allow for a reliable calculation of mass diffusivities by studying microscopic fluctuations, which is similar to the principle of the DLS method. With MD simulations, the self-diffusion coefficients of the gases in their pure and mixed states as well as the Maxwell-Stefan diffusion coefficient and the thermodynamic factor for the mixtures should be analyzed separately. Combining the two latter properties results in the prediction of the Fick binary diffusion coefficient which should be compared with the experimental data. By decoupling the molecular diffusion process in the MD simulations, it can also be proven to which extent the different diffusivities agree and how they can be related to each other and to common predictive models in literature. A comparison of the simulated diffusivity data obtained from the ab initio calculations-derived FFs for the pure components and the specific mixtures with those obtained by using established literature FFs for the pure components should be carried out for a broad range of thermodynamic states. By this, it should be revealed whether particularly the use of pair-specific FFs is preferable for more accurate computations of mass diffusivities and how well these results agree with those obtained from quantum calculations for the zero-density limit as well as those from the experiments over an extended density range.