*4.2.4. Conclusions*

evolved its adsorption geometry to a bi-dentate configuration. At 400 time steps, molecular fragment showed up a uni-dentate configuration. Interestingly, during this decomposition, neither bi-dentate nor tri-dentate configuration depicted in above decomposed pieces of butanoic acid were observed in butanol alcohol configurations. Additional AIMD simulations for a butanol alcohol molecule slightly rotated relative to the Al (111) slab also confirmed this observation, see discussions in Section 4.2.3. According to this, we concluded that the decomposition pathway for butanol alcohol just occurred to oxidize the surface by means of its dissociated OH group to form an alcoholate on the surface if other additives were not involved in this reaction. **Figure 18** shows the distribution of potential energy for M–4 starting at 300 K. Here a sharp peak around 40 time steps represented the dissociation of an H ion from molecular main piece. The dip in the curve near 220 time steps was due to the re-arrangement of the decom-

**Figure 17.** Dynamic decomposition pathways for M–4 starting at 300 K.

Other several AIMD simulations were carried out on decomposition pathways with above additive molecules rotating their initial configurations by 180° clockwise (group-1) and 90° counterclockwise (group-2) toward Al (111) surface rather than their functional groups

**Figure 18.** Distribution of potential energy vs. time steps for M–4 starting at 300 K.

posed pieces on the surface.

*4.2.3. Other decomposition pathways*

18 Lubrication - Tribology, Lubricants and Additives

Simulations of ab-initio molecular dynamics starting at room temperature (300 K) for the decomposed intermediates of aliphatic butanoic acid and butanol alcohol on clean Al (111) surface, indicated that, (1) Initial decomposition pieces of these additive molecules involved attachments of residual alkyl-chains to Al ions in the surface via their oxygen ions; (2) In further decomposition reactions, the remaining alkyl-chains would anchor to the surface via its end C ion, with the complete liberation of oxygen ions or OH group from the carboxyl (O═C─OH) group to oxidize the surface; (3) The remaining alkyl-chains did not participated in reactions with the surface, but may serve as molecular caps to inhibit migrations of corrosive species into the oxide surface, and let these chains accessible react with other general lubricants in base oil, which may form effective boundary thin-films on the surface.
