Zhu Zhaoran, Ewen James P, Kritikos Efstratios M, Giusti Andrea, Dini Daniele
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, U.K.
Department of Applied Physics and Materials Science, California Institute of Technology, Pasadena, California 91125, United States.
J Phys Chem C Nanomater Interfaces. 2024 Sep 12;128(38):15959-15973. doi: 10.1021/acs.jpcc.4c04412. eCollection 2024 Sep 26.
Phosphate esters decompose on metal surfaces and form protective polyphosphate films. For many applications, such as in lubricants for electric vehicles and wind turbines, an understanding of the effect of electric fields on molecular decomposition is urgently required. Experimental investigations have yielded contradictory results, with some suggesting that electric fields improve tribological performance, while others have reported the opposite effect. Here, we use nonequilibrium molecular dynamics (NEMD) simulations to study the decomposition of tri--butyl phosphate (TNBP) molecules nanoconfined between ferrous surfaces (iron and iron oxide) under electrostatic fields. The reactive force field (ReaxFF) method is used to model the effects of chemical bonding and molecular dissociation. We show that the charge transfer with the polarization current equalization (QTPIE) method gives more realistic behavior compared to the standard charge equilibration (QEq) method under applied electrostatic fields. The rate of TNBP decomposition via carbon-oxygen bond dissociation is faster in the nanoconfined systems than that in the bulk due to the catalytic action of the surfaces. In all cases, the application of an electric field accelerates TNBP decomposition. When electric fields are applied to the confined systems, the phosphate anions are pulled toward the surface with high electric potential, while the alkyl cations are pulled to the surface with lower potential, leading to asymmetric film growth. Analysis of the temperature- and electric field strength-dependent dissociation rate constants using the Arrhenius equation suggests that, on reactive iron surfaces, the increased reactivity under an applied electric field is driven mostly by an increase in the pre-exponential factor, which is linked to the number of molecule-surface collisions. Conversely, the accelerated decomposition of TNBP on iron oxide surfaces can be attributed to a reduction in the activation energy with increasing electric field strength. Single-molecule nudged-elastic band (NEB) calculations also show a linear reduction in the energy barrier for carbon-oxygen bond breaking with electric field strength, due to stabilization of the charged transition state. The simulation results are consistent with experimental observations of enhanced and asymmetric tribofilm growth under electrostatic fields.
磷酸酯在金属表面分解并形成保护性的多磷酸盐薄膜。在许多应用中，例如在电动汽车和风力涡轮机的润滑剂中，迫切需要了解电场对分子分解的影响。实验研究得出了相互矛盾的结果，一些研究表明电场可改善摩擦学性能，而另一些研究则报告了相反的效果。在此，我们使用非平衡分子动力学（NEMD）模拟来研究在静电场作用下，纳米限域在铁表面（铁和氧化铁）之间的磷酸三丁酯（TNBP）分子的分解。采用反应力场（ReaxFF）方法来模拟化学键合和分子解离的影响。我们表明，与在施加静电场时的标准电荷平衡（QEq）方法相比，使用极化电流平衡电荷转移（QTPIE）方法能给出更符合实际的行为。由于表面的催化作用，纳米限域体系中通过碳 - 氧键解离的TNBP分解速率比本体中的更快。在所有情况下，施加电场都会加速TNBP的分解。当对限域体系施加电场时，磷酸根阴离子被拉向高电势表面，而烷基阳离子被拉向低电势表面，导致薄膜生长不对称。使用阿伦尼乌斯方程对温度和电场强度相关的解离速率常数进行分析表明，在活性铁表面，施加电场时反应活性的增加主要是由指前因子的增加驱动的，这与分子 - 表面碰撞的数量有关。相反，TNBP在氧化铁表面的加速分解可归因于随着电场强度增加活化能的降低。单分子推挤弹性带（NEB）计算还表明，由于带电过渡态的稳定，碳 - 氧键断裂的能垒随电场强度呈线性降低。模拟结果与静电场下增强和不对称摩擦膜生长的实验观察结果一致。
