*4.1.5. Conclusions*

In summary, if ignoring entropy contributions, for the VPA, the best favorable adsorption geometries on Al (111) surface were in the order: tri-bridged coordination > bi-bridged coordination > uni-dentate coordination. While for the EA, the best adsorption geometries on Al (111) surface were in the order: bi-bridged coordination > uni-dentate coordination.

**Figure 13** shows initial geometries of butanoic acid and butanol alcohol interactions with Al (111) surface slab in four models. In each model, an additive molecule was positioned with its carbon backbone directly above the center of the equilibrated Al (111) surface, molecular orientations along [111] direction were shown in **Figure 13(a)**–**(d)**, respectively. In **Figure 13**, the first three models were for butanoic acid reactions with Al (111) slab surface: In model-1 (M−1), *V*<sup>d</sup> = −15.0 Å/ps (1500 m/s) and the O ion on O═C bond was set at one 2.30 Å spacing above the Al (111) slab surface. In model-2 (M−2), *V*<sup>d</sup> = −15.0 Å/ps and the OH group was positioned at one 2.30 Å spacing above the Al (111) slab surface. In model-3 (M−3), *V*<sup>d</sup> = −20.0 Å/ps and all components in O═C─OH functional group were positioned directly at one 2.30Å spacing above the Al (111) slab surface. The last model was for butanol alcohol reaction with the Al (111) slab: In model-4 (M−4), *V*<sup>d</sup> = −20.0 Å/ps

Ab-Initio Modeling of Lubricant Reactions with a Metal Al (111) Surface

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and the OH group was positioned at one 2.30 Å spacing above the Al (111) slab surface.

**Figure 14** shows decomposition pathways for M−1, M−2 and M−3 models starting at 300 K. In **Figure 14(a)** for the M−1, at 50 simulation time steps, the O ion in O═C group on butanoic acid molecule began anchoring to the surface in a bi-dentate coordination. However, the OH group did not appear to interact with the surface at this time. At 90 time steps, the O ion in O═C group had dissociated from molecular main piece and was adsorbed onto the surface, with the residual molecular main piece anchoring to the surface through its alkyl-chain. At 200 time steps, the OH group had dissociated from molecular main piece and began interacting with the surface through its O ion. At 310 time steps, the OH group fully dissociated and was adsorbed onto the surface in a uni-dentate coordination. The residual alkyl-chain finally anchored to the surface in a tetra-coordination via its carboxyl C ion, with the O ion (dissociated from the O═C group) adsorbing on the surface in tri-dentate coordination. Note that the temperature of this system increased from 300 to 970 K throughout the whole simulation.

In **Figure 14(b)** for the M−2, at 40 time steps, the OH group interacted with Al (111) surface, resulting in dissociation of one H ion from molecular main piece. Main piece of butanoic acid interacted with the surface in a bi-bridged coordination through its O ions. At 130 time steps,

**Figure 13.** Four initial geometries of butanoic acid and butanol alcohol interactions with Al (111) slab: (a) M−1; (b) M−2;

(c) M−3; (d) M−4.

*4.2.1. Dynamic decomposition pathways for butanoic acid on Al (111) slab surface*

In addition, comparing these two adsorption types, in each of above adsorption geometries, the binding state of VPA on Al (111) surface was always stronger than that of equivalent EA on the surface. The main reason could be due to a more highly reactive 2(O)─P═O functional group with the surface than an O─C═O functional one. H ions liberating from molecular main pieces and adsorbing onto the surface, also influenced all of the binding states in above. However, the formation of gaseous H2 molecules by means of H ions desorbed from the surface was more favorable in adsorption ends.
