Jiménez-Serratos Guadalupe, Totton Tim S, Jackson George, Müller Erich A
Department of Chemical Engineering , South Kensington Campus Imperial College London , London SW7 2AZ , U.K.
BP Exploration Operating Co. Ltd. , Sunbury-on-Thames TW16 7LN , U.K.
J Phys Chem B. 2019 Mar 14;123(10):2380-2396. doi: 10.1021/acs.jpcb.8b12295. Epub 2019 Mar 4.
Fully atomistic simulations of models of asphaltenes in simple solvents have allowed the study of trends in aggregation phenomena to understand the underlying role played by molecular structure. The detail included at this scale of molecular modeling is, however, at odds with the required spatial and temporal resolution needed to fully understand asphaltene aggregation. The computational cost required to explore the relevant scales can be reduced by employing coarse-grained (CG) models, which consist of lumping a few atoms into a single segment that is characterized by effective interactions. In this work, CG force fields developed via the statistical associating fluid theory (SAFT-γ) [ Müller , E. A. ; Jackson , G. Annu. Rev. Chem. Biomol. Eng. 5 , 2014 , 405 - 427 ] equation of state (EoS) provide a reliable pathway to link the molecular description with macroscopic thermophysical data. A recent modification of the SAFT-VR EoS [ Müller , E. A. ; Mejía , A. Langmuir 33 , 2017 , 11518 - 11529 ], which allows for the parameterization of homonuclear rings, is selected as the starting point to develop CG models for polycyclic aromatic hydrocarbons. The new aromatic-core models, along with others published for simpler organic molecules, are adopted for the construction of asphaltene models by combining different chemical moieties in a group-contribution fashion. We apply the procedure to two previously reported asphaltene models and perform molecular dynamics simulations to validate the coarse-grained representation against benchmark systems of 27 asphaltenes in a pure solvent (toluene or heptane) described in a fully atomistic fashion. An excellent match between both levels of description is observed for the cluster size, radii of gyration, and relative-shape-anisotropy-factor distributions. We exploit the advantages of the CG representation by simulating systems containing up to 2000 asphaltene molecules in an explicit solvent investigating the effect of asphaltene concentration, solvent composition, and temperature on aggregation. By studying large systems facilitated by the use of CG models, we observe stable continuous distributions of molecular aggregates at conditions away from the two-phase precipitation point. As a further example application, a widely accepted interpretation of cluster-size distributions in asphaltenic systems is challenged by performing system-size tests, reversibility checks, and a time-dependence analysis. The proposed coarse-graining procedure is seen to be general and predictive and, hence, can be applied to other asphaltenic molecular structures.
对简单溶剂中沥青质模型进行的全原子模拟，使得对聚集现象的趋势进行研究成为可能，从而了解分子结构所起的潜在作用。然而，这种分子建模尺度所包含的细节，与全面理解沥青质聚集所需的空间和时间分辨率不一致。通过采用粗粒度（CG）模型，可以降低探索相关尺度所需的计算成本，该模型由将几个原子聚集成一个由有效相互作用表征的单个片段组成。在这项工作中，通过统计缔合流体理论（SAFT-γ）[Müller, E. A.; Jackson, G. Annu. Rev. Chem. Biomol. Eng. 5, 2014, 405 - 427]状态方程（EoS）开发的CG力场，提供了一条将分子描述与宏观热物理数据联系起来的可靠途径。最近对SAFT-VR EoS[Müller, E. A.; Mejía, A. Langmuir 33, 2017, 11518 - 11529]的修改，允许对同核环进行参数化，被选为开发多环芳烃CG模型的起点。新的芳香核模型，以及为更简单的有机分子发表的其他模型，通过以基团贡献的方式组合不同的化学部分，被用于构建沥青质模型。我们将该程序应用于两个先前报道的沥青质模型，并进行分子动力学模拟，以针对以全原子方式描述的27种沥青质在纯溶剂（甲苯或庚烷）中的基准系统验证粗粒度表示。对于簇尺寸、回转半径和相对形状各向异性因子分布，在两种描述水平之间观察到了极好的匹配。我们通过在显式溶剂中模拟包含多达2000个沥青质分子的系统，利用CG表示的优势，研究沥青质浓度、溶剂组成和温度对聚集的影响。通过研究使用CG模型所促进的大系统，我们在远离两相沉淀点的条件下观察到了分子聚集体的稳定连续分布。作为进一步的示例应用，通过进行系统尺寸测试、可逆性检查和时间依赖性分析，对沥青质系统中簇尺寸分布的一种广泛接受的解释提出了挑战。所提出的粗粒度程序被认为是通用的且具有预测性，因此可以应用于其他沥青质分子结构。
