**4.2. Dynamic simulations**

In this section, dynamic decomposition pathways for two selected aliphatic boundary-layer lubricant additives: butanoic acid and butanol alcohol on Al (111) surface, are discussed using ab-initio molecular dynamics.

Corresponding to **Figure 2**, **Figure 12** shows isosurfaces of charge density at the ELF = 0.93 for these two molecules. In **Figure 12(a)**, O2 had two e<sup>−</sup> lone pairs (a circular-lunar lobe) because of its *aniso-sp3* hybrid bonding to C and H neighbors; O1 formed a strong double bond with C, leading to lower charge density (a semi-lunar lobe) than around O2; Both O1 and O2 behaved more reactive with the clean Al (111) surface than other ions on butanoic acid, which were very similar to those in **Figure 4(b)**. In **Figure 12(b)**, O2 also had two e<sup>−</sup> lone pairs (a circular-lunar lobe) because of its *aniso-sp3* hybrid bonding to C and H neighbors; O2 showed more reactive with the clean Al (111) surface than other ions on butanol alcohol. In a word, the ELF analyses indicated that functional groups O═C─OH and C─OH on these two molecules were electronrich, and hence were more likely to react with the Al surface than other C─H and C─C twigs.

**Figure 12.** Side views of isosurfaces of charge density at the ELF = 0.67 for (a) butanoic acid and for (b) butanol alcohol.

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

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

*4.1.5. Conclusions*

14 Lubrication - Tribology, Lubricants and Additives

However, the formation of gaseous H2

**4.2. Dynamic simulations**

ab-initio molecular dynamics.

lobe) because of its *aniso-sp3*

of its *aniso-sp3*

face was more favorable in adsorption ends.

these two molecules. In **Figure 12(a)**, O2 had two e<sup>−</sup>

similar to those in **Figure 4(b)**. In **Figure 12(b)**, O2 also had two e<sup>−</sup>

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

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.

In this section, dynamic decomposition pathways for two selected aliphatic boundary-layer lubricant additives: butanoic acid and butanol alcohol on Al (111) surface, are discussed using

Corresponding to **Figure 2**, **Figure 12** shows isosurfaces of charge density at the ELF = 0.93 for

leading to lower charge density (a semi-lunar lobe) than around O2; Both O1 and O2 behaved more reactive with the clean Al (111) surface than other ions on butanoic acid, which were very

with the clean Al (111) surface than other ions on butanol alcohol. In a word, the ELF analyses indicated that functional groups O═C─OH and C─OH on these two molecules were electronrich, and hence were more likely to react with the Al surface than other C─H and C─C twigs.

**Figure 12.** Side views of isosurfaces of charge density at the ELF = 0.67 for (a) butanoic acid and for (b) butanol alcohol.

hybrid bonding to C and H neighbors; O1 formed a strong double bond with C,

hybrid bonding to C and H neighbors; O2 showed more reactive

molecules by means of H ions desorbed from the sur-

lone pairs (a circular-lunar lobe) because

lone pairs (a circular-lunar

(111) surface were in the order: bi-bridged coordination > uni-dentate coordination.

**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.

this main piece remained anchoring to the surface. However at 200 time steps, this coordination was becoming uni-dentate as the main piece rebounded the surface under its consistent collisions, and with an Al ion pulled upward by one of O ions in it. At 310 time steps, the fragment in a bi-dentate configuration resembled a soap, i.e., a product of "R─COOM (here "R─ "represented the alkyl-chain and "M" was a metal ion in the surface)" was formed in simulation [33]. Soap formation with fatty acids had been observed in the Al forming process where nominal pressures were in the vicinity of 2.5 times the material flow strength [3, 34]. The temperature of this system increased from 300 to 960 K throughout the whole simulation.

In **Figure 14(c)** for the M−3, at 60 time steps, the O ion in O═C group had dissociated and was subsequently adsorbed in a tri-dentate configuration on Al (111) surface. The same was true for the OH group. The residual alkyl-chain formed bi-dentate and tri-dentate configurations in the order at 100, 150, and 310 time steps throughout the whole simulation. One H ion dissociated from the adsorbed OH group and migrated about the surface. The temperature of this system increased from 300 to 1330 K throughout the whole simulation.

**Figure 15** shows the initial evolution of potential energy computed for each of three models in **Figure 14**. In each case, butanoic acid molecule usually began interacting with the surface within 30–50 time steps. The potential energy of each system increased during this time interval so as to overcome the barrier to the adsorption. This energy then decreased as the surface defused molecular consistent impacts and decomposition. Beyond the dip point, the energy increased once again as molecular fragment and its decomposed components in functional group re-arranged themselves on the surface. The sharp peaks on M−1 and M−3 curves indicated larger and more rapid exchanges between potential and kinetic energies than were

**Figure 15.** Potential energies of M−1, M−2 and M−3 vs. time steps for butanoic acid decomposition on Al (111) surface.

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

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**Figure 16** shows potential energies of three models during the first 350 time steps of thermal annealing after their distributions of 500 time steps as shown in **Figure 15**. In **Figure 15**, over this 350 time steps, each curve decreased and ultimately approached an asymptotic value after about 250 time steps. After 350 time steps, the potential energy of M−3 was slightly lower than M−1, but M−2 still had the highest energy level. In addition, our DFT energy minimiza-

**Figure 17** shows dynamic decomposition pathways for butanol alcohol on Al (111) surface, M–4, starting at 300 K. In **Figure 17**, at 40 time steps, 1 H ion dissociated from the OH group and interacted with an Al ion in the surface. At 100 time steps, molecular fragment was adsorbed on the surface plus a lone H ion moved below the surface. At 250 time steps, molecular fragment

**Figure 16.** An effect of further annealing at 500 K on potential energies of M−1, M−2 and M−3 for butanoic acid

tion found that M−3 was the most stable configuration followed by M−1 and M−2.

*4.2.2. Dynamic decomposition pathways for butanol alcohol on Al (111) surface*

observed in M−2 during the decomposition.

decomposition on Al (111) surface.

**Figure 14.** Three models of decomposition pathways starting at 300 K: (a) M−1; (b) M−2 and (c) M−3.

Ab-Initio Modeling of Lubricant Reactions with a Metal Al (111) Surface http://dx.doi.org/10.5772/intechopen.72512 17

this main piece remained anchoring to the surface. However at 200 time steps, this coordination was becoming uni-dentate as the main piece rebounded the surface under its consistent collisions, and with an Al ion pulled upward by one of O ions in it. At 310 time steps, the fragment in a bi-dentate configuration resembled a soap, i.e., a product of "R─COOM (here "R─ "represented the alkyl-chain and "M" was a metal ion in the surface)" was formed in simulation [33]. Soap formation with fatty acids had been observed in the Al forming process where nominal pressures were in the vicinity of 2.5 times the material flow strength [3, 34]. The temperature of this system increased from 300 to 960 K throughout the whole simulation. In **Figure 14(c)** for the M−3, at 60 time steps, the O ion in O═C group had dissociated and was subsequently adsorbed in a tri-dentate configuration on Al (111) surface. The same was true for the OH group. The residual alkyl-chain formed bi-dentate and tri-dentate configurations in the order at 100, 150, and 310 time steps throughout the whole simulation. One H ion dissociated from the adsorbed OH group and migrated about the surface. The temperature of

**Figure 15** shows the initial evolution of potential energy computed for each of three models in **Figure 14**. In each case, butanoic acid molecule usually began interacting with the surface within 30–50 time steps. The potential energy of each system increased during this time interval so as to overcome the barrier to the adsorption. This energy then decreased as the surface defused molecular consistent impacts and decomposition. Beyond the dip point, the energy increased once again as molecular fragment and its decomposed components in functional

this system increased from 300 to 1330 K throughout the whole simulation.

16 Lubrication - Tribology, Lubricants and Additives

**Figure 14.** Three models of decomposition pathways starting at 300 K: (a) M−1; (b) M−2 and (c) M−3.

**Figure 15.** Potential energies of M−1, M−2 and M−3 vs. time steps for butanoic acid decomposition on Al (111) surface.

group re-arranged themselves on the surface. The sharp peaks on M−1 and M−3 curves indicated larger and more rapid exchanges between potential and kinetic energies than were observed in M−2 during the decomposition.

**Figure 16** shows potential energies of three models during the first 350 time steps of thermal annealing after their distributions of 500 time steps as shown in **Figure 15**. In **Figure 15**, over this 350 time steps, each curve decreased and ultimately approached an asymptotic value after about 250 time steps. After 350 time steps, the potential energy of M−3 was slightly lower than M−1, but M−2 still had the highest energy level. In addition, our DFT energy minimization found that M−3 was the most stable configuration followed by M−1 and M−2.

#### *4.2.2. Dynamic decomposition pathways for butanol alcohol on Al (111) surface*

**Figure 17** shows dynamic decomposition pathways for butanol alcohol on Al (111) surface, M–4, starting at 300 K. In **Figure 17**, at 40 time steps, 1 H ion dissociated from the OH group and interacted with an Al ion in the surface. At 100 time steps, molecular fragment was adsorbed on the surface plus a lone H ion moved below the surface. At 250 time steps, molecular fragment

**Figure 16.** An effect of further annealing at 500 K on potential energies of M−1, M−2 and M−3 for butanoic acid decomposition on Al (111) surface.

facing the surface, respectively. For group-1 with the CH3

*4.2.4. Conclusions*

**5. Summary**

it initially bounced off the surface without decomposing, but intermediately rotated its functional groups toward the surface when it bounced back the surface. And then, it carried on decomposition pathways discussed in above Sections. For group-2 with the carbon backbones aligning with the surface, it initially bounced off the surface without decomposing, but immediately rotated its functional groups toward the surface when it bounced back the surface.

Comprehensively, the final thermal equilibration at 500 K for the decomposed species of butanoic acid and butanol alcohol on Al (111) surface indicated that (1) for butanol alcohol decomposed pieces in M-4: a butanol alcoholate was adsorbed on the surface in a uni-dentate coordination with the dissociated H ion adsorbed onto the surface; (2) for butanoic acid decomposed pieces in M−1, M−2 and M−3: residual alkyl-chains usually anchored to the surface in tri-dentate coordination via their carboxyl C ions, with the OH group and dissociated H and O ions interacting with the surface, which were consistent with experimental observations using the X-ray photoelectron spectroscopy (XPS) at temperatures from 300 to 750 K [8].

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.

field. Some main points from discussions may include, without any loss of generality:

face were in the order: bi-bridged coordination > uni-dentate coordination.

In this chapter, an ab-initio modeling of lubricant reactions with a metal Al (111) surface are comprehensively discussed using density functional theory (DFT) and ab-initio molecular dynamics (AIMD) based upon the DFT, to illuminate some reasonable reaction outputs in this lubrication

**1.** If ignoring entropy contributions, for the VPA, the best favorable reaction geometries on Al (111) surface were in the order: tri-bridged coordination > bi-bridged coordination > uni-dentate coordination. While for the EA, the best reaction geometries on Al (111) sur-

**2.** 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 for equivalent EA on the surface. The main reason could be due to a more highly reactive functional group of 2(O)─P═O with the surface than that of O─C═O. H ions liberating from molecular main

And then it carried on decomposition pathways discussed in above Sections.

end pointing toward the surface,

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

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

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 decomposed pieces on the surface.

### *4.2.3. Other decomposition pathways*

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.

facing the surface, respectively. For group-1 with the CH3 end pointing toward the surface, it initially bounced off the surface without decomposing, but intermediately rotated its functional groups toward the surface when it bounced back the surface. And then, it carried on decomposition pathways discussed in above Sections. For group-2 with the carbon backbones aligning with the surface, it initially bounced off the surface without decomposing, but immediately rotated its functional groups toward the surface when it bounced back the surface. And then it carried on decomposition pathways discussed in above Sections.

Comprehensively, the final thermal equilibration at 500 K for the decomposed species of butanoic acid and butanol alcohol on Al (111) surface indicated that (1) for butanol alcohol decomposed pieces in M-4: a butanol alcoholate was adsorbed on the surface in a uni-dentate coordination with the dissociated H ion adsorbed onto the surface; (2) for butanoic acid decomposed pieces in M−1, M−2 and M−3: residual alkyl-chains usually anchored to the surface in tri-dentate coordination via their carboxyl C ions, with the OH group and dissociated H and O ions interacting with the surface, which were consistent with experimental observations using the X-ray photoelectron spectroscopy (XPS) at temperatures from 300 to 750 K [8].
