Nextmol (Bytelab Solutions SL), Carrer de Roc Boronat 117, 08 018 Barcelona, Spain.
Barcelona Supercomputing Center (BSC-CNS), Plaça Eusebi Güell, 1-3, 08 034 Barcelona, Spain.
Langmuir. 2021 Dec 21;37(50):14582-14596. doi: 10.1021/acs.langmuir.1c02133. Epub 2021 Dec 8.
The performance of organic friction modifiers (OFMs) depends on their ability to adsorb onto surfaces and form protective monolayers. Understanding the relationship between OFM concentration in the base oil and the resulting surface coverage is important for improving lubricant formulations. Here, we use molecular dynamics (MD) simulations to study the adsorption of three OFMs─stearic acid (SA), glycerol monoostearate (GMS), and glycerol monooleate (GMO)─onto a hematite surface from two hydrocarbon solvents─-hexadecane and poly(α-olefin) (PAO). We calculate the potential of mean force of the adsorption process using the adaptive biasing force algorithm, and the adsorption strength increases in the order SA < GMS < GMO. We estimate the minimum area occupied by OFM molecules on the surface using annealing MD simulations and obtained a similar hard-disk area for GMS and GMO but a lower value for SA. Using the MD results, we determine the adsorption isotherms using the molecular thermodynamic theory (MTT), which agree well with one previous experimental data set for SA on hematite. For two other experimental data sets for SA, lateral interactions between surfactant molecules need to be accounted for within the MTT framework. SA forms monolayers with lower surface coverage than GMO and GMS at low concentrations but also has the highest plateau coverage. We validate the adsorption energies from the MD simulations using high-frequency reciprocating rig friction experiments with different concentrations of the OFMs in PAO. For OFMs with saturated tailgroups (SA and GMS), we obtain good agreement between the simulations and the experiments. The results deviate for OFMs containing -unsaturated tailgroups (GMO) due to the additional steric hindrance, which is not accounted for in the current simulation framework. This study demonstrates that MD simulations, alongside MTT, are an accurate and efficient tool to predict adsorption isotherms at solid-liquid interfaces.
有机摩擦改性剂 (OFM) 的性能取决于其在表面吸附并形成保护性单分子层的能力。了解基础油中 OFM 浓度与表面覆盖率之间的关系对于改进润滑剂配方非常重要。在这里，我们使用分子动力学 (MD) 模拟研究了三种 OFM（硬脂酸 (SA)、甘油单硬脂酸酯 (GMS) 和甘油单油酸酯 (GMO)）从两种碳氢化合物溶剂-十六烷和聚 (α-烯烃) (PAO) 吸附到赤铁矿表面的情况。我们使用自适应偏置力算法计算吸附过程的平均力势，并发现吸附强度按 SA < GMS < GMO 的顺序增加。我们使用退火 MD 模拟估计 OFM 分子在表面上的最小占据面积，并得到 GMS 和 GMO 的类似硬磁盘面积，但 SA 的值较低。使用 MD 结果，我们使用分子热力学理论 (MTT) 确定吸附等温线，与 SA 在赤铁矿上的一个先前实验数据集吻合良好。对于 SA 的另外两个实验数据集，需要在 MTT 框架内考虑表面活性剂分子之间的横向相互作用。在低浓度下，SA 形成的单分子层比 GMO 和 GMS 的表面覆盖率低，但也具有最高的平台覆盖率。我们使用高频往复式摩擦实验验证了 MD 模拟中的吸附能，实验中使用了不同浓度的 OFM 在 PAO 中的混合物。对于饱和尾基的 OFM（SA 和 GMS），我们得到了模拟和实验之间的良好一致性。对于含有 -不饱和尾基的 OFM（GMO），由于额外的空间位阻，结果会偏离，而目前的模拟框架并未考虑到这一点。这项研究表明，MD 模拟与 MTT 一起，是预测固液界面吸附等温线的准确而高效的工具。
