Effect of Electric\nFields on the Decomposition of\nPhosphate Esters

Phosphate esters decompose on metal surfaces and form\nprotective\npolyphosphate films. For many applications, such as in lubricants\nfor electric vehicles and wind turbines, an understanding of the effect\nof electric fields on molecular decomposition is urgently required.\nExperimental investigations have yielded contradictory results, with\nsome suggesting that electric fields improve tribological performance,\nwhile others have reported the opposite effect. Here, we use nonequilibrium\nmolecular dynamics (NEMD) simulations to study the decomposition of\ntri-<i>n</i>-butyl phosphate (TNBP) molecules nanoconfined\nbetween ferrous surfaces (iron and iron oxide) under electrostatic\nfields. The reactive force field (ReaxFF) method is used to model\nthe effects of chemical bonding and molecular dissociation. We show\nthat the charge transfer with the polarization current equalization\n(QTPIE) method gives more realistic behavior compared to the standard\ncharge equilibration (QEq) method under applied electrostatic fields.\nThe rate of TNBP decomposition via carbon–oxygen bond dissociation\nis faster in the nanoconfined systems than that in the bulk due to\nthe catalytic action of the surfaces. In all cases, the application\nof an electric field accelerates TNBP decomposition. When electric\nfields are applied to the confined systems, the phosphate anions are\npulled toward the surface with high electric potential, while the\nalkyl cations are pulled to the surface with lower potential, leading\nto asymmetric film growth. Analysis of the temperature- and electric\nfield strength-dependent dissociation rate constants using the Arrhenius\nequation suggests that, on reactive iron surfaces, the increased reactivity\nunder an applied electric field is driven mostly by an increase in\nthe pre-exponential factor, which is linked to the number of molecule–surface\ncollisions. Conversely, the accelerated decomposition of TNBP on iron\noxide surfaces can be attributed to a reduction in the activation\nenergy with increasing electric field strength. Single-molecule nudged-elastic\nband (NEB) calculations also show a linear reduction in the energy\nbarrier for carbon–oxygen bond breaking with electric field\nstrength, due to stabilization of the charged transition state. The\nsimulation results are consistent with experimental observations of\nenhanced and asymmetric tribofilm growth under electrostatic fields.