生物柴油分子系统凝固过程的分子动力学模拟

Molecular dynamics simulation on solidification process of bio-diesel molecular systems

具有良好的低温流动性是生物柴油的重要特性之一,而这一性质与系统的凝点相关联。本文以硬脂酸甲酯、油酸甲酯等7种常见生物柴油组成分子为研究对象,通过建模构建了7种单组份生物柴油分子的模拟液态模型。利用分子动力学方法对7种液态模型系统的凝固过程进行了研究。采用PCFF力场,在一定的模拟条件下,对系统进行逐次降温分子动力学模拟。根据分子动力学轨迹,计算了不同温度下系统的热容、碳原子自扩散系数以及分子链的头尾距。模拟计算得到的液态热容合理。利用"热容法"和"扩散系数法"对不同系统的凝点进行判断,发现凝点区间基本一致,且与实际凝点基本相吻合。而采用2种区间交集的方法得到的凝点区间则与实际凝点十分相近。以油酸甲酯为例,模拟观察到,随着温度降低,分子链总体呈伸展趋势,符合热力学基本原理。在油酸甲酯的凝点前后,链长及链的微观结构存在明显差异。模拟结果表明,本实验方法对研究生物柴油分子系统确实有效。

The favorable cold flow property hich is associated with the solidification point is one of the important characteristics of the biodiesel.In this paper,seven common biodiesel composition molecules were investigated by molecular dynamics simulation on their solidification processes, such as methyl stearate and methyl oleate.The PCFF force field was adopted and the liquid models were build as the initial ones.The heat capacities, self-diffusion constants of carbon atoms and the head-to-end distances（L） of the titled biodiesel molecules were calculated,and the results were reasonable.According to the sharp changes of heat capacities and self-diffusion constants with the simulation temperatures,the solidification point was identified for each system.It is found that the solidification point ranges identified by two methods were roughly consistent with the experimental values.When integrate the two ranges to identify the solidification point,the results were better.Take the methyl oleate as an example,it is found that the molecular chains tend to arrange orderly during the temperature lowing process,which is consistent with the thermodynamics fundamental.The chain lengths and the microstructures before and after the solidification point were obvious different.The simulated results reveal that the simulation methods in this paper are effective to use to research the solidification processes of biodiesels.