Chremos Alexandros, Krekelberg William P, Hatch Harold W, Siderius Daniel W, Mahynski Nathan A, Shen Vincent K
Chemical Sciences Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8320, United States.
J Phys Chem B. 2025 Apr 3;129(13):3443-3453. doi: 10.1021/acs.jpcb.5c00536. Epub 2025 Mar 21.
We develop coarse-grained models for carbon dioxide (CO) and nitrogen (N) that capture the vapor-liquid equilibria of both their single components and their binary mixtures over a wide range of temperatures and pressures. To achieve this, we used an equation of state (EoS), namely Statistical Associating Fluid Theory (SAFT), which utilizes a molecular-based algebraic description of the free energy of chain fluids. This significantly accelerates the exploration of the parameter space, enabling the development of coarse-grained models that provide an optimal description of the macroscopic experimental data. SAFT creates models of fluids by chaining together spheres, which represent coarse-grained parts of a molecule. The result is a series of fitted parameters, such as bead size, bond length, and interaction strengths, that seem amenable to molecular simulation. However, only a limited set of models can be directly implemented in a particle-based simulation; this is predominantly due to how SAFT handles overlap between bonded monomers with parameters that do not translate to physical features, such as bond length. To translate such parameters to bond lengths in a coarse-grained force-field, we performed Wang-Landau transition-matrix Monte Carlo (WL-TMMC) simulations in the grand canonical ensemble on homonuclear fused two-segment Mie models and evaluated the phase behavior at different bond lengths. In the spirit of the law of corresponding states, we found that a force field, which matches SAFT predictions, can be derived by rescaling length and energy scales based on ratios of critical point properties of simulations and experiments. The phase behavior of CO and N mixtures was also investigated. Overall, we found excellent agreement over a wide range of temperatures and pressures in pure components and mixtures, similar to TraPPE CO and N models. Our proposed approach is the first step to establishing a more robust bridge between SAFT and molecular simulation modeling.
我们开发了用于二氧化碳（CO₂）和氮气（N₂）的粗粒度模型，该模型能够在广泛的温度和压力范围内捕捉其单一组分及其二元混合物的气液平衡。为实现这一目标，我们使用了一种状态方程（EoS），即统计缔合流体理论（SAFT），它利用基于分子的链状流体自由能代数描述。这显著加速了参数空间的探索，从而能够开发出能对宏观实验数据进行最佳描述的粗粒度模型。SAFT通过将代表分子粗粒度部分的球体连接在一起创建流体模型。结果得到了一系列拟合参数，如珠子大小、键长和相互作用强度，这些参数似乎适用于分子模拟。然而，只有有限的一组模型可以直接在基于粒子的模拟中实现；这主要是由于SAFT处理具有无法转化为物理特征（如键长）参数的键合单体之间的重叠方式。为了在粗粒度力场中将这些参数转化为键长，我们在巨正则系综中对同核融合两段米氏模型进行了王-兰道转移矩阵蒙特卡罗（WL-TMMC）模拟，并评估了不同键长下的相行为。根据对应状态定律，我们发现通过基于模拟和实验的临界点性质比率重新缩放长度和能量尺度，可以导出与SAFT预测相匹配的力场。我们还研究了CO₂和N₂混合物的相行为。总体而言，我们发现纯组分和混合物在广泛的温度和压力范围内与TraPPE CO₂和N₂模型具有高度一致性。我们提出的方法是在SAFT和分子模拟建模之间建立更强大桥梁的第一步。
