Aggregation of Asphaltene Subfractions A1 and A2 in Different Solvents from the Perspective of Molecular Dynamics Simulations

The aggregation behavior of model molecules of asphaltene subfractions A1 and A2 dissolved in heptane, toluene, and tetrahydrofuran (THF) were studied using molecular dynamics simulations. The proposed asphaltene molecular models are based on previously studied structures with two new models, including a highly aromatic model with a prominent island-type molecule and another molecule with a prominent archipelago-type architecture. The aggregation mechanisms in toluene, THF, and heptane solvents were studied. The results in heptane and toluene were consistent with the solubility of asphaltenes and their subfractions in these solvents. The size of the aggregates is well-correlated with aromaticity. When considering THF, large aggregates are broken down into smaller aggregates. This could lead to the mixture of high, medium, and low molecular weight distribution bands usually observed when gel permeation chromatography (GPC) analyses are conducted on asphaltene samples in THF. The size distribution extracted from the simulations shows a bimodal distribution with profiles similar to the size distribution profiles usually found in GPC analysis for asphaltene samples in THF. The distributions of dipole moments of the aggregates against the number of molecules in aggregates were constructed in both THF and toluene and reveal that the dipole moment of the aggregates vanishes when the number of molecules increases as a result of a random structure of the aggregates. The contributions of different molecular interactions to the aggregation mechanism, such as π stacking, van der Waals, and hydrogen bonds, are described.

本研究采用分子动力学模拟，探究了沥青质亚组分A1与A2的模型分子在正庚烷、甲苯、四氢呋喃（tetrahydrofuran, THF）中的聚集行为。本研究提出的沥青质分子模型基于已有研究的结构，新增了两种模型：一种为具有典型岛型结构的高芳香性分子模型，另一种为具有典型群岛型结构的分子模型。研究分别考察了甲苯、四氢呋喃与正庚烷三种溶剂中的聚集机制。正庚烷与甲苯中的模拟结果与沥青质及其亚组分在这两种溶剂中的溶解特性相符。聚集体的尺寸与芳香性呈现显著相关性。在四氢呋喃溶剂中，大型聚集体会分解为小型聚集体，这一现象可解释为何在以四氢呋喃为流动相的沥青质样品凝胶渗透色谱（gel permeation chromatography, GPC）分析中，通常会同时出现高分子量、中分子量与低分子量的分布峰。从模拟中提取的聚集体尺寸分布呈现双峰特征，其分布轮廓与以四氢呋喃为溶剂的沥青质样品凝胶渗透色谱分析中常见的尺寸分布轮廓高度一致。本研究还构建了四氢呋喃与甲苯溶剂中，聚集体偶极矩随聚集体内分子数变化的分布曲线，结果显示：随着聚集体内分子数增加，由于聚集体结构趋于随机化，其偶极矩会逐渐消失。本研究同时阐明了π堆叠、范德华力与氢键等不同分子间相互作用对聚集机制的贡献。