Shi Junqin, Zhou Qing, Sun Kun, Liu Guoqiang, Zhou Feng
State Key Laboratory of Solidification Processing, Center of Advanced Lubrication and Seal Materials, Northwestern Polytechnical University, Xi'an 710072, China.
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China.
Langmuir. 2020 Jul 28;36(29):8543-8553. doi: 10.1021/acs.langmuir.0c01386. Epub 2020 Jul 16.
Molecular dynamics simulations are used to investigate the physisorption of organic friction modifiers (OFMs) lubricated by 1-decene trimer (PAO4) representing a base oil and confined between hydroxylated SiO (001) surfaces. The results indicate that OFM molecules form dense, tendentiously vertical monolayer films at low temperature but loose adsorption layers at high temperature, particularly for R-NH with weaker molecular polarity. The structural information is quantitatively clarified by mass density profiles, radial distribution function, and probability distributions of an end-to-end distance at a perpendicular-to-surface direction. The movement performance of lubricant, reflected by the thickness of the organic part and radius of gyration of PAO4 molecules, strongly depends on temperature. The adsorption amount of OFM molecules decreases dramatically with lowering OFM polarity and increasing temperature above the critical desorption temperatures of about 320, 373, and 453 K for amine (R-NH), alcohol (R-OH), and acid (R-COOH), respectively. The interaction energies of the OFM-surface decrease continuously for the R-NH system with temperature and decrease rapidly as temperature exceeds a critical value for both R-OH and R-COOH systems. The single-molecule geometry optimization validates the significant role of the electrostatic and hydrogen-bond attractions in molecular adsorption. Therefore, the OFMs with stronger polarity (like R-COOH) present stronger adsorption and better temperature resistance. The findings in this work are of particular value and provide a guideline in designing and engineering novel OFM additives for extreme lubrication conditions.
分子动力学模拟用于研究由代表基础油的1-癸烯三聚体（PAO4）润滑并限制在羟基化SiO（001）表面之间的有机摩擦改进剂（OFM）的物理吸附。结果表明，OFM分子在低温下形成致密的、倾向于垂直的单分子层薄膜，而在高温下形成松散的吸附层，特别是对于分子极性较弱的R-NH。通过质量密度分布、径向分布函数以及垂直于表面方向上的端到端距离的概率分布，定量地阐明了结构信息。由PAO4分子的有机部分厚度和回转半径反映的润滑剂的运动性能强烈依赖于温度。对于胺（R-NH）、醇（R-OH）和酸（R-COOH），OFM分子的吸附量分别在约320、373和453K的临界解吸温度以上，随着OFM极性的降低和温度的升高而急剧下降。对于R-NH体系，OFM-表面的相互作用能随温度持续降低，而对于R-OH和R-COOH体系，当温度超过临界值时，相互作用能迅速降低。单分子几何优化验证了静电和氢键吸引力在分子吸附中的重要作用。因此，极性较强的OFM（如R-COOH）表现出更强的吸附和更好的耐热性。这项工作中的发现具有特别的价值，并为设计和制造用于极端润滑条件的新型OFM添加剂提供了指导。
