Effect of Electric Fields on the Decomposition of Phosphate Esters

Phosphate esters are important lubricant additives that decompose on metal surfaces and form protective polyphosphate films. For many applications, such as electric vehicles and wind turbines, an understanding of the molecular decomposition of lubricant additives in the presence of electric fields is urgently required. Experimental investigations have yielded contradictory results, with some suggesting that electric fields improve tribological performance, while others report the opposite effect. Here, we use non-equilibrium molecular dynamics (NEMD) simulations to study the decomposition of tri-n-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 study high temperatures (1000-1350 K) and electric field strengths (0.25-1.00 V/Å) to accelerate decomposition. We show that the charge transfer with polarization current equalization (QTPIE) method leads to a more accurate prediction of the dissociation behaviour than the standard charge equilibration (QEq) method under applied electric fields. The rate of TNBP decomposition via carbon-oxygen bond dissociation is faster in the nanoconfined systems than in the bulk due to the catalytic action of the surfaces. In all cases, the application of an electric field accelerates TNBP decomposition due to elongation of the carbon-oxygen bonds and increased molecule-surface collisions. When electric fields are applied to the confined systems, the phosphate anions are pulled towards the surface with high electric potential, while the alkyl cations are pulled to the surface with lower potential. Analysis of the temperature- and electric field strength-dependant dissociation rate constants using the Arrhenius equation suggests that, on reactive iron surfaces, the increase in reactivity is driven mostly by an increase in the pre-exponential factor, which is linked to the number of successful collisions. Conversely, the increase in reactivity on iron oxide surfaces is attributable 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, through stabilisation of the charged transition state. The simulation results are consistent with experimental observations of enhanced tribofilm growth under electrostatic fields.