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

该研究项目的主要目的是通过实验和理论方法准确测定二元气体混合物中的二元扩散系数。为此，激光-光学测量技术动态光散射（DLS）和全息干涉法应用于一个罗氏细胞（HILC），以及分子动力学（MD）模拟应用于系统甲烷-二氧化碳和甲烷-丙烷。为了提高MD模拟在预测这种混合物的扩散系数方面的潜力，其目的是通过使用针对特定混合物的新的优化力场（FFs）来进一步发展该方法。这些FF应该是由罗斯托克大学的赫尔曼博士在一个平行的提案中在零密度极限下的高度精确的从头计算量子计算得出的。为了验证从MD计算获得的结果，需要从DLS和HILC获得的Fick二元扩散系数。这两种实验方法的应用提供了在宽范围的混合物密度的扩散传质的信息，针对典型的不确定性的二元扩散系数低于1%。从HILC获得的数据库也应该与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。Hellmann博士利用这些理论数据为我们的MD模拟开发合适的FF，作为气相密度的函数。基于这些FF，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方法的原理类似。在分子动力学模拟中，气体在纯态和混合态下的自扩散系数以及混合物的Maxwell-Stefan扩散系数和热力学因子应分别进行分析。结合后两个属性的结果中的菲克二元扩散系数的预测，应与实验数据进行比较。通过在MD模拟中解耦分子扩散过程，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与文献中的常见预测模型相关。从从头计算得到的FF的纯组分和特定的混合物与那些通过使用已建立的文献FF的纯组分的模拟扩散率数据的比较应进行广泛的热力学状态。通过这一点，它应该揭示是否特别是使用对特定的FF是更精确的计算的质量扩散系数，以及如何以及这些结果与从量子计算获得的零密度极限，以及那些从实验中获得的扩展的密度范围。

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.