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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。
Having good low temperature fluidity is one of the important features of biodiesel, and this property is related to the system’s freezing point. In this paper, seven kinds of common biodiesel components, such as methyl stearate and methyl oleate, were studied. Simulated liquid models of seven single-component biodiesel molecules were constructed by modeling. The solidification process of seven liquid model systems was studied by molecular dynamics method. Using PCFF force field, under certain simulation conditions, the system was simulated by molecular dynamics simulation. According to the trajectory of molecular dynamics, the heat capacity, the self-diffusion coefficient of carbon atom and the head-tail distance of the molecular chain are calculated under different temperatures. The calculated liquid heat capacity is reasonable. Using “heat capacity method” and “diffusion coefficient method” to judge the freezing point of different systems, it is found that the freezing point interval is basically the same, and basically coincides with the actual freezing point. The intersection point of the intersection of the two kinds of interval obtained the freezing point interval is very similar with the actual freezing point. Take methyl oleate as an example, the simulation shows that as the temperature decreases, the molecular chains tend to extend in general, in line with the basic principles of thermodynamics. In the oleic acid methyl ester before and after the freezing point, the chain length and chain microstructure there is a clear difference. The simulation results show that the experimental method is indeed effective for the study of biodiesel molecular system.