Jaishankar Aditya, Jusufi Arben, Vreeland Jessica L, Deighton Shane, Pellettiere Joseph, Schilowitz Alan M
ExxonMobil Research and Engineering , 1545 Route 22 East , Annandale , New Jersey 08801 , United States.
Langmuir. 2019 Feb 12;35(6):2033-2046. doi: 10.1021/acs.langmuir.8b03132. Epub 2019 Jan 25.
Improved friction performance is an important objective of equipment manufacturers for meeting improved energy efficiency demands. The addition of friction-reducing additives, or friction modifiers (FMs), to lubricants is a key part of the strategy. The performance of these additives is related to their surface activity and their ability to form adsorbed layers on the metal surface. However, the extent of surface coverage (mass per unit area) required for effective friction reduction is currently unknown. In this article, we show that full coverage is not necessary for significant friction reduction. We first highlight various features of surface adsorption that can influence the surface coverage, packing, and free energy of adsorption of organic FMs on iron oxide surfaces. Using stearic acid in heptane and hexadecane as model lubricant formulations, we employ a combination of experiments and molecular dynamics (MD) simulations to show how the dimerization of acid molecules in the bulk solvent and the crystallographic orientation of the surface modifies surface adsorption. In addition, we show that the solvent can strongly influence the adsorption kinetics, and MD simulations reveal that hexadecane tends to align on the surface, increasing the energy barrier for the adsorption of stearic acid to the surface. Furthermore, we present a combined approach using MD and molecular thermodynamic theory to calculate adsorption isotherms for stearic acid on iron oxide surfaces, which agrees well with experimental data obtained with a quartz crystal microbalance (QCM). Our results suggest that while the friction of systems lubricated with organic FMs decreases with increasing coverage, complete coverage of the surface is neither practically achievable nor necessary for effective friction reduction for the systems and conditions studied here.
提高摩擦性能是设备制造商满足提高能源效率需求的一个重要目标。在润滑剂中添加减摩添加剂或摩擦改性剂（FM）是该策略的关键部分。这些添加剂的性能与其表面活性以及在金属表面形成吸附层的能力有关。然而，目前尚不清楚有效降低摩擦所需的表面覆盖率（单位面积质量）。在本文中，我们表明，显著降低摩擦并不需要完全覆盖。我们首先强调了表面吸附的各种特征，这些特征会影响有机摩擦改性剂在氧化铁表面的表面覆盖率、堆积和吸附自由能。以庚烷和十六烷中的硬脂酸作为模型润滑配方，我们结合实验和分子动力学（MD）模拟，展示了本体溶剂中酸分子的二聚作用以及表面的晶体取向如何改变表面吸附。此外，我们表明溶剂会强烈影响吸附动力学，MD模拟显示十六烷倾向于在表面排列，增加了硬脂酸吸附到表面的能垒。此外，我们提出了一种结合MD和分子热力学理论的方法来计算硬脂酸在氧化铁表面的吸附等温线，该方法与用石英晶体微天平（QCM）获得的实验数据吻合良好。我们的结果表明，虽然用有机摩擦改性剂润滑的系统的摩擦随着覆盖率的增加而降低，但对于本文研究的系统和条件，表面的完全覆盖在实际中既无法实现，也不是有效降低摩擦所必需的。
