Jurnal Tribologi 21 (2019) 63-81

Molecular dynamics analysis of adsorption process of anti-coppercorrosion additives to the copper surface
Kohei Nishikawa 1, Hirotoshi Akiyama1, Kazuhiro Yagishita 2, Hitoshi Washizu 1,3,*

1 Graduate School of Simulation Studies, University of Hyogo, 7-1-28 Minatojima-minamimachi,

Chuo-ku, Kobe, Hyogo 650-0047, JAPAN.

2 JXTG Nippon Oil & Energy Corporation, 8 Chidoricho, Naka-ku, Yokohama, Kanagawa 231-

7153, JAPAN.

3 Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University - 1-30 Goryo-

Ohara, Nishikyo-ku, Kyoto 615-8245, JAPAN.

*Corresponding author: h@washizu.org

KEYWORDS

Anti-corrosion additive
Benzotriazole
Copper
Molecular dynamics
Physical adsorption
Chemical adsorption

ABSTRACTAs a model system of the adsorption process of anticorrosion additives on a metal surface, molecular
dynamics simulations of benzotriazole (BTA) moleculeswith copper slabs were completed. As a force field, ReaxFF
was used to simulate both adsorption dynamics and thecharge transfer on the solid surface. Two simulations arepresented. In order to investigate the physical adsorption
on the surface, the simulation was done for the adsorption
process of BTA molecules on the copper (II) oxide slab.
BTA molecules formed on adsorbed layer in parallel to thesurface, and aggregation of the molecules due to thesurface diffusion was found. In order to investigate theselective and chemical adsorption on the surface, a hybridslab, which has both a copper (Cu) area and a copper (I)oxide (Cu 2 O) area, was used. A selective adsorption
phenomenon was found. The number of BTA molecules
adsorbed on the Cu area is 5 times greater than that on the
Cu 2 O area. Detailed dynamics focused on charge transfer
showed a surface diffusion and enhancement ofpolarization due to charge transfer from the metal surface
caused the selective adsorption. In a real phenomenon, thereason why a few anti-corrosion additives are able toprotect a metal surface is postulated to be due to thisselective nature of adsorption onto a newly formed metal
surface.

Received 8 September 2018; received in revised form 9 September 2018; accepted 20 September 2018.
To cite this article: Nishikawa et al. (2019). Molecular dynamics analysis of adsorption process of anti-coppercorrosion additives to the copper surface. Jurnal Tribologi 21, pp.63-81.

© 2019 Malaysian Tribology Society (MYTRIBOS). All rights reserved.

Jurnal Tribologi 21 (2019) 63-81

1.0
INTRODUCTION
Newly formed metal surfaces are often unstable and become stable when they are terminatedwith other molecules, however, the original color and properties may be diminished when they
are covered with oxygen or other gases or liquids in the atmosphere. Anti-copper-corrosion
additives adsorb onto the surface of copper and are used in order to prevent these phenomena inorder to retain its color and other properties. Benzotriazole (BTA, C 6 H 5 N 3 ) is one of the famous
anti-copper-corrosion additives, and BTA has been used to prevent corrosion and discolorationof copper and copper alloys for a long time (Cotton and Scholes, 1967). There are a huge numberof studies concerning this phenomenon (Antonijević and Petrović, 2008, Petrović et al., 2015), andthey are applied to novel systems such as ionic liquids (Liu et al., 2006, Zhang et al., 2017), but themechanism of protecting surfaces is still under study.
From an experimental point of view, the adsorption process of molecules can be analyzed bya surface technique. Recently, a real-time instrumentation technique using Otto-SPR was
proposed, and it is becoming possible to observe how additives adsorb onto a surface on amolecular level (Maegawa et al., 2016). Surface-enhanced Raman spectroscopy is also a wellestablished technique to investigate the surface adsorption process of additives (Ali et al., 2017).
The mechanism is also studied by computational chemistry. In the beginning, a molecularorbital (MO) simulation is used to calculate the partial charges, dipole moment, HOMO-LUMO
energy gap, and free volume of the single additive molecule in order to consider the relation withexperiments (Edwards et al., 1994). The placement of the additive molecule on the copper surface
is studied using a Monte Carlo simulation (Bartley et al., 2003). In this simulation, the dependenceof the surface binding energy with an alkyl chain length on alkyl esters of 5-carboxybenzotriazole
are obtained. In the Monte Carlo simulation, the classical force field in which the partial charges
are fixed is used.
In the adsorption process, electron transfer between solid surface and additive molecule may
be important. Density functional theory (DFT) calculations are useful to calculate the interaction
between the solid slab and the additive molecule. Using DFT calculation, Jiang et al., 2004 foundthat the standing, i.e., the plane of the double ring of the BTA molecule is perpendicular to the slab,
structure is stable on the Cu 2 O slab. The other calculation also shows that the N-Cu bond andintermolecular aggregate structure are important for adsorption of the BTA on the copper surface(Kokalj et al., 2010, Kokalj and Peljhan, 2015). In these studies, since the simulation cell is set to aminimum size to calculate the slab-additive structure, the "aggregate" means that the interactions
are calculated between single molecules in the periodic boundary. DFT calculations are used inother groups including van der Waals interactions (Gattinoni and Michaelides, 2015). In order todiscuss the solvent effect, molecular dynamics (MD) (Zeng, 2011), a combination of DFT withmolecular dynamics (Wang et al., 2012, Guo et al., 2017), and combination of MO simulation withmolecular dynamics are used (Khaled 2008, Khaled 2009). In all these simulations, adsorption ofa single or small number of additive molecules are discussed.
Although the MD simulation is useful to discuss the dynamic process, such as diffusion
coefficients in the many molecules systems (Hassan, 2016), this method was a subsidiary tool,
since the charge transfer is unable to treat in usual MD. Recently, a force field called ReaxFF (vanDuin et al., 2001), which can treat a chemical reaction with van der Waals and a long-range
Coulomb interaction, is growing by including parameters for many elements (Senftle et al., 2016).
In this paper, we use the ReaxFF MD simulation to analyze the mechanism of the adsorption
dynamics of the ensemble of the BTA anti-copper-corrosion additive molecules on the surface ofcopper. Most of the previous researches are focused on adsorption of single BTA molecule on the64Jurnal Tribologi 21 (2019) 63-81

surface, or single BTA molecule with solvents, we can discuss the adsorption process of themolecular ensemble of the BTAs. By using ReaxFF, we can analyze not only the physicaladsorption but the chemical adsorption process. The objective of this paper is to clarify themechanism of the selective adsorption on the newly formed copper surface by analyzing the
dynamics of the ensemble of the BTA molecules to the copper surfaces.
Our simulation shows two new aspects of the surface adsorption. The first one is theadsorption, surface diffusion, and aggregation of the BTAs on the copper (II) oxide (CuO) slab
substrate surface. In this simulation, the physical adsorption and aggregation of many molecules
are discussed. The second is the selective adsorption process of the BTAs on the copper (I) oxide(Cu 2 O) and pure copper (Cu) hybrid surfaces. In this simulation, both the physical adsorption andthe chemical adsorption, charge transfer between the slab and the BTA molecules, andaggregation of many molecules are discussed. This simulation gives insight into the long standing
question of why the anti-copper-corrosion additive protects the newly formed surface.

2.0
SIMULATION PROCEDURE
We simulate and visualize the adsorption process of BTA molecules onto the copper surface
by a MD simulation and analyze the dynamic behavior of the adsorbed molecules. Molecular
dynamics is a simulation technique to provide the information about the micro structure andchemical and physical properties. The information comes from solving the time integral of theequation of motion for many particle systems and obtain trajectories of the particles. For input
information, the potential function of interacting atoms, temperature, and pressure are used.
Under a given condition, the atoms move toward the distribution in thermal equilibrium fromsolving the equation of motion of many atom systems.
As a potential function of interacting atoms, we use the Reax Force Field (ReaxFF) potential(van Duin et al., 2001). The ReaxFF potential is able to represent the formation and cleavage ofbonds, because the parameter comes from quantum chemical calculations. In addition, the ReaxFF
potential is compatible with a lot of elements (Senftle et al., 2016). The parameters for Cu andorganic molecules are taken from the paper of Hu et al. (Hu et al, 2015) and their supplementary
information. To calculate the time integral of the copper (copper oxide) and anti-coppercorrosion additive, we use molecular dynamics calculations software LAMMPS (Large-scale
Atomic/Molecular Massively Parallel Simulator, Plimpton, 1995). LAMMPS is an open source thatis good at efficient execution of the time integral execution and highly diverse.
In this paper, we simulate two models. The first model includes a copper (II) oxide (CuO) slab
and BTA. In this model, we analyze the mechanism of the physical adsorption of BTA moleculeson the surface and aggregation due to the surface diffusion. Since the copper atoms are almost
polarized in this system, we believe aggregation without chemical adsorption can be discussed.
The second model includes pure (not oxidized) copper (Cu) and copper (I) oxide (Cu 2 O) in one
plate, and BTA. Using this hybrid surface model, we analyze the mechanism of the selective
chemical adsorption of the BTA molecules. Since the copper (I) oxide is the dominant structure in
real phenomena, and the copper atoms can provide charges to the adsorbed additive molecules,
thus copper (I) oxide is chosen. We describe the details of both models used in these simulations
as follows.

65Jurnal Tribologi 21 (2019) 63-81

2.1
METHOD FOR THE FIRST SIMULATION (CuO AND BTA)
To represent the dynamics of the adsorption process of the anti-copper-corrosion additive
molecules onto the surface of copper (II) oxide (CuO), we first model a single BTA molecule as ananti-copper-corrosion additive and copper oxide slab. The bottom atoms of the CuO slab are fixed.
Each part is stabilized using the ReaxFF potential (van Duin et al., 2010). The charge equilibration
method (Rappe & Goddard III, 1991) is used for calculating the partial charge. We then preform
molecular dynamics calculations with modeled anti-copper-corrosion additive molecules andCuO slab in the same space. The size of the simulation cell is (x, y, z) = (4.6837, 3.0347, 12.2) nm
and a periodic boundary condition is used for the x and y directions. The number of Cu and O
atoms is both 960 atoms. For the anti-copper-corrosion additive molecules, we disposed 20 BTA
molecules in a random order. The setup condition of the temperature is 300 K using the NoseHoover thermostat, the time step size is 0.1 fs, and the number of time steps is 2,000,000. Basedin these setups, the snapshot of the initial state of the model is shown in Figure 1.
In this simulation, we discuss the simulation results of the point BTA molecules absorbed fromthe aspect of their orientation to analyze the mechanism of the adsorption and the properties of
the anti-copper-corrosion additive.
In order to analyze the orientation, we calculated the order parameter function f defined byequation (1).

f =3<cos2 θ> −1
2(1)

Where θ is the angle between the focused axis and vertical vector from the surface of the BTA
molecule and <> denotes the statistical average.
When the BTA molecule is non-oriented, the coefficient of orientation < cos 2 θ > will be 1/3
which is the statistical average value. If the surface of the copper slab and the surface of the BTA
molecules are vertical (a), θ will be 0°or 180°, thus < cos 2 θ > equals 1. If the surface of thecopper slab and the surface of the BTA molecule are parallel (b), θ will be 90°or 270°, thus <cos2 θ > equals 0. Therefore, if the surface of the copper slab and the surface of the BTA molecule
are vertically oriented, the coefficient of orientation f equals 1, and if the BTA molecule israndomly non-oriented, f is 0, and if the surface of the copper slab and the surface of the BTA
molecule`s surface are parallel, it will be 0. Thereby, the lower the value, the more surface of thecopper substrate and the surface of BTA molecule become vertical.

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Jurnal Tribologi 21 (2019) 63-81

Cu of oxide copper
O of oxide copper
C of BTA
H of BTA
N of BTA

Figure 1: Snapshot of initial distribution of the first simulation.

2.2
METHOD FOR THE SECOND SIMULATION (Cu-Cu 2 O AND BTA)
To prepare for the initial condition (Figure 2), we first modeled the copper (Cu), oxide copper(Cu 2 O), and the anti-copper-corrosion additive. Each one was stabilized using the ReaxFF
potential the same as in the first simulation. We then combined the copper (Cu) and oxide copper(Cu 2 O) in the same simulation cell. The size of the simulation cell is (x, y, z) = (6.4924, 3.2535,
11.6) nm. The number of atoms for the slab models are set not to make a large dislocation. The
number of Cu atoms in the Cu area and Cu 2 O area are 1,296 and 784, respectively. The lowermost
atoms are fixed. We finally preform the molecular dynamics calculation with modeled anticopper-corrosion additive and slab in same simulation cell. For anti-copper-corrosion additives,
we dispose 60 BTA molecules in a random order. The setup condition of temperature is 300 K, thetime step size is 0.1fs, and the number of steps is 3,000,000.

Cu of copper
Cu of oxide copper
O of oxide copper
C of BTA
H of BTA
N of BTA

Figure 2: Snapshot of initial distribution of the second simulation.

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Jurnal Tribologi 21 (2019) 63-81

3.0
RESULTS AND DISCUSSION
In both simulations, the BTA molecules are all adsorbed on the slab surfaces and show a quasiequilibrium state, i.e., the total energy of the system converges to a single value and fluctuates dueto thermal motion.

3.1
Results of The First Simulation (CuO AND BTA)
In the first simulation, from the initial state shown in Figure 1, we simulate the process ofadsorption of the BTA molecules onto the surface of the copper (II) oxide (CuO). Figure 3 shows
snapshots of the adsorption process of the BTA molecules onto the CuO surface. In 100 ps, 90 %of the BTA molecules are adsorbed and in 200 ps, all the BTA molecules are adsorbed. From the
trajectory of the molecules in this process, the adsorption phenomenon is divided into 3 types.
Figure 4 shows a schematic snapshot of each type. In the first type, the BTA molecule adsorbs onto
the surface of CuO alone (Figure 4a). In the second type, the BTA molecules first form a group ina vacuum then adsorb onto the CuO surface (Figure 4b). In the third type, each BTA molecule firstabsorbs onto the surface, then forms a group by surface diffusion (Figure 4c). In addition to thesetypes, after the absorption onto the surface of the CuO, the BTA molecules do not stay on the sameplace, but move on the surface (Figure 5).
Cu of oxide copper
O of oxide copper
C of BTA
H of BTA
N of BTA(a) 0 step(b) 100 steps(c) 200 steps

Figure 3: Snapshots of the adsorption process of BTA molecules onto the surface of CuO in thefirst simulation.

68Jurnal Tribologi 21 (2019) 63-81(b) Formed a group before
adsorption(t = 78.05 ps)(a) Adsorbed alone(t = 46.35 ps)(c) Adsorbed and formed a group(t = 167.2 ps)

Figure 4: schematic snapshot of each adsorption type.

(t = 123.85, 156.10, 166.65 ps, from left to right).
Figure 5: Snapshots of a molecule moving on the surface by surface diffusion.

We analyzed the trajectory of the anti-copper-corrosion additive BTA molecules anddetermined the mechanism of adsorption. From the snapshot of the last step shown in Figure 3c,the BTA molecules that adhere to the copper surface make a layer a parallel to the surface. In
order to analyze this process, we compare molecules that absorb onto the copper surface alone(type 1) and molecules that form a group before adsorbing (type 2). The average of the orientation
factor at each time step is plotted in Figure 6 for the two types. The number of molecules is 3, forthe type that absorbs alone (type 1, Figure 6a), and 11 molecules for the type which form a group
before adsorption (type 2, Figure 6b).

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Jurnal Tribologi 21 (2019) 63-81

f

1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6

50

0

50

100

150

200

100

150

200

times [ps]

(a)

f

1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6

0

times [ps](b)
Figure 6: Time development for average of the orientation factor in each time step. (a) Average of3 molecules for type 1: absorbed alone (b) Average of 11 molecules for type 2: form a group before
adsorption

In both cases, the averages of the orientation factor are near 0 before the absorption (about 50ps), which indicate that the orientation of the molecules is random. In type 1 shown in Fig. 6a, as
the number of adsorbed molecules increase as the time step increases, because the parallel
adsorption structure is dominant, the orientation factor shows a negative value. In type 2 shownin Figure 6b, the orientation factor fluctuates around 0, which means the direction of the BTA
molecules is random after the adsorption.
We could not find the DFT calculation of the stable structure of the BTA molecule on the CuO
surface, therefore, the result of the MD simulation cannot be compared to the other theoreticalstudies. However, we can understand the MD result as followings. In the CuO surface, the electronsof the copper atoms totally moved to the oxygen atoms, thus the nitrogen atoms in the BTA do not
obtain electrons from the copper atoms. This means that the adsorption is a physical adsorption.
If the inter-molecular interaction, such as the van der Waals and Coulomb, are dominant to theadsorption process, the face-to face structure may be stable, as have been shown in classical MD
simulation of adsorption on Fe surface by Guo et al., 2017. The random orientation shown in thetype 2 (Figure 6b) can be understood by the excluded volume effect between the BTA molecules.
At room temperature, the BTA molecules show an amorphous-like structure on the surface. Oneof the newest experimental research (Zhao et al., 2018) shows that the CuO is the intermediate in
the anti-corrosion process and Cu(I) BTA is dominant rather than Cu(II)BTA, which is consistent
with our results that the BTA physically adsorb on the Cu(II) O surface.

70Jurnal Tribologi 21 (2019) 63-81

3.2
Results of The Second Simulation (Cu-Cu 2 O AND BTA)
From the initial state shown in Figure 3, we simulate the adsorption process of the BTA
molecules onto the surface of the copper (Cu) or copper (I) oxide (Cu 2 O) slab. The snapshots ofthe time evolution of this simulation are shown in Figure 7.
In this simulation, we investigated the trend onto which area the BTA molecules adsorb. The
number of BTA molecules adsorbed onto the copper (Cu) or copper (I) oxide (Cu 2 O) is shown in
Figure 8. At the beginning of the simulation (0 ps to 40 ps), the number of BTA molecules
increased as the time increased on both sides. This is because the adsorption process is a physicaladsorption and is governed by a random process. The number of BTA molecules on the Cu area
then quickly increased and the increased ratio decrease in the Cu 2 O area. Note that the decreasein the number of adsorbed BTA molecules on the Cu 2 O area is around 40 ps. This is due to both
the removal from the surface of the Cu 2 O area to the vacuum, and movement from the Cu 2 O area
to the Cu area by surface diffusion. After this turning point, the number of BTA molecules on the
Cu area quickly increased due to the long-range molecular interactions between the adsorbed
molecules and the molecules in the vacuum. The surface diffusion from the Cu 2 O area to the Cu
area also increased the number of molecules in the Cu area. Finally, about a 5 times greaternumber of BTA molecules was adsorbed on the Cu area.
In addition, the binding part of each BTA molecule to the surface of the molecules in the Cu
area seemed different from that in the Cu 2 O area. As shown in Figure 7b, during the initial process,the imino-group (N atom) of BTA molecules comes lower side when it adsorbs onto the Cu area
than the Cu 2 O area. Both experiments and the theoretical analysis which we have shown in the
introduction of this paper, have proven that in many cases, the imino-group face to the copper
surface and the charge transfer occur. The percentage of the molecules which imino-group isbound on the slab atoms in both areas is shown in Figure 9. During the process of the initialadsorption (0 to 150 ps), the binding part of the BTA molecules are different in the Cu area andthe Cu 2 O area. In case of BTA molecules adsorb onto Cu 2 O, the binding part of initial adsorption
is not specific. Whereas in case of BTA molecules adsorbed onto Cu, the percentage of BTA
molecules which N atoms are bound is high. This is because the initial adsorption process between
Cu surface and BTA molecules is thought to be due to the transfer of charge between the Cu atom
and N atom in the BTA. After the initial adsorption process, in both areas, the majority of thebinding part is N atoms in the BTA. This is because the Cu atoms in both areas are able to giveelectrons to N atoms, and the molecules are chemically adsorbed.

71Jurnal Tribologi 21 (2019) 63-81

Cu of copper
Cu of oxide copper
O of oxide copper
C of BTA
H of BTA
N of BTA(a) 0 ps(b) 100 ps(c) 200 ps(d) 300 ps

Figure 7: Snapshots of the adsorption process of BTA molecules onto the surface of Cu2O and Cu
hybrid surface in the second simulation.

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Number of adsorbed BTA molecules

60

Cu2O

50

Cu

40
30
20
10
0

0

50

100

150

times [ps]

200

250

300

Figure 8: Number of BTA molecules adsorbed onto the copper (Cu) area or the copper (I) oxide(Cu 2 O) area.
100

90

percentage of BTA [%]

80
70
60
50
40
30

Cu2O

20
10
0

0

50

100

150

times [ps]

200

Cu
250

300

Figure 9: Percentage of BTA molecules in which the imino-group is bound to the slab atoms in thecopper (Cu) area or the copper (I) oxide (Cu2O) area.

In order to understand the dynamics of the charge transfer, we plotted the time developmentof the averaged charge of the copper atoms in the top layer of the slab and the middle of the slab
in each area (Figure 10). Initially, the copper atoms in the Cu 2 O area are already polarized, while
the copper atoms in the Cu area are not. After the simulation, the charge of the Cu atoms in thesurface of the Cu area increases due to the charge transfer to the imino-group of the BTA
molecules and become positive value. The charge in the Cu 2 O area slightly increases. This means

73Jurnal Tribologi 21 (2019) 63-81

the chemical adsorption occur in the Cu 2 O area too. However, the number of BTA molecules in
the Cu 2 O area are restricted since the charge on copper atoms are already polarized, and there isa slight tendency to transfer the charge to the adsorbed molecules.
0.7

0.6

CuSurface
Cu2O Surface

Cu MidCu2O Mid150

250

average of charge [e]

0.5
0.4
0.3
0.2
0.1
0

0

50

100

times [ps]

200

300

Figure 10: Time development of averaged charge of the copper atoms in the top layer of the slab
and the middle of the slab in the copper (Cu) area or the copper (I) oxide (Cu 2 O) area.

Comparing the final snapshot of the two simulations (Figure 3c and Figure 7d), one candistinguish that the BTA molecules are standing in both areas in Figure 7d and they are laying in
Figure 3c. To analyze this difference, we calculated the order parameter function of the BTA
molecules which adsorb onto the Cu area.
Figure 11 shows the orientation of the first layer of the BTA molecules adsorb onto copper
plate. In the first simulation, we calculated the orientation of the BTA molecules adsorbed onto
the copper (II) oxide (CuO), and the result of orientation was parallel with the surface (Figure 6b).
Although the both f values are almost 0, the average in Figure 11 is higher than that shown in
Figure 6b. This is due to the oriented adsorption, which is consistent with a previous theoreticalprediction using DFT (Kokalj et al., 2010). In our simulation, the reason why the orientation factor
decreases from 1, is the number of adsorbed molecules is high and making a complex structure
under the realistic thermal effect.
We next calculated the amount of charge transfer of each of the atoms belonging to the BTA
molecules to figure out the precise mechanism of the BTA molecule adsorption onto the copper
slabs.
We first calculated an average of the charge transfer amount of the N atoms and H1 atom
(Figure 12) belong to imino-group of the BTA molecules which are the absorbed point BTA
molecules. These results are shown in Figure 13. An average of the amount of charge of the Natoms (Figure 13a) does not show a large difference between the BTA molecules adsorbed onto74

Jurnal Tribologi 21 (2019) 63-81

the Cu area and the Cu 2 O area, however, the average of the Cu area is slightly greater than that of
the Cu 2 O area. Comparing the charge of H1 atom (Figure 13b), the charge increases about 0.05 [e]in the Cu 2 O area, whereas it slightly increases in Cu area. In order to understand this result, we
calculated the average of the charge of oxygen atoms exist on top layer and intermediate layer of
the Cu 2 O area which BTA molecules adsorb and the charge of the H1 atom has changed (Figure14). The charge on the surface of the O atoms decreases as the adsorption occur. This means thatthe charge transfer from the H1 atom to the O atoms occurs.
1

0.8
0.6
0.4

f

0.2
0

-0.2
-0.4
-0.6

0

50

100

150

times [ps]

200

250

300

Figure 11: Time development of orientation function calculated from BTA molecules which
adsorbed onto the copper area.

Figure 12: Chemical structure and the number of atoms in this simulation of the BTA.

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Jurnal Tribologi 21 (2019) 63-81

0

-0.05

Cu
Cu2O

-0.1

charge [e]

-0.15

-0.2

-0.25

-0.3

-0.35

-0.4

0

50

100

150

times [ps]

200

250

300

(a) Charge of N atoms.

0.4

Cu
Cu2O

0.35

0.3

charge [e]

0.25

0.2

0.15

0.1

0.05

0

0

50

100

150

times [ps]

200

250

300

(b) Charge of H1 atoms.
Figure 13: Time development of the average of the charge on each BTA molecule during thesimulation.

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Jurnal Tribologi 21 (2019) 63-81

-0.6

-0.65

O Surface
O Mid

-0.7

-0.75

charge [e]

-0.8

-0.85

-0.9

-0.95

-1

0

50

100

150

times [ps]

200

250

300

Figure 14: Time development of the average of the charge of O atoms present on the surface andmiddle of the copper (I) oxide (Cu 2 O) area.

We finally summarize the details of each charge of the atoms belonging on the BTA molecule,before and after this simulation (Figure 15). The blue atom indicates that the atom is charged
negatively, and red indicates a positive charge. Also, the deeper color means a higher value. The
result is consistent with the partial charge distribution of previous DFT calculations (Jiang et al.,2004) or MO calculations (Zeng, 2011), at least qualitatively. A slight increase in the positive
charges of the hydrogen atoms in the benzene ring during the adsorption process may due to thecharge normalization effect in each molecule.

(a) BTA molecule which adsorb onto the copper (Cu) area.

77Jurnal Tribologi 21 (2019) 63-81(b) BTA molecule which adsorb onto the copper (I) oxide (Cu 2 O) area.

Figure 15: Average of charge of each atom belonging to the BTA molecule. At the initial state (t =0, left) and at the final state (t = 300 ps, right).

Based on these analyses, let us explain the difference in the adsorption amount between the
Cu area and the Cu 2 O area shown in Figure 8. The copper atoms and N atoms of the BTA molecules
are exchanging their charge, so this type of adsorption between the copper plate and BTA
molecules is thought of as chemical adsorption in both the Cu and Cu 2 O areas. Since the N atomsof BTA molecules that adsorb onto the Cu 2 O area are already negatively charged, and the O atomsof the Cu 2 O area are also negatively charged, the BTA molecules in the Cu 2 O area are difficult tophysically adsorb due to the repulsive interaction between these atoms. Only when the N atoms
are on the Cu site, and the H1 atoms are on the O site is the adsorbed structure stable, which isconsistent with the previous DFT calculation (Jiang et al., 2004).
Compared with the Cu 2 O area, the BTA molecules are easily adsorbed onto the Cu area,
because the Cu atoms, which exist on the top layer of the Cu 2 O area, are originally polarized, andthat on the top layer of the Cu area are not yet charged. In spite of this difficulty, if the conditions
to adsorb are matched, chemical adsorption will occur in the Cu 2 O area by changing charges
between the Cu and N atoms of the BTA molecules.
Due to the surface diffusion of the physically adsorbed BTA molecules on the Cu 2 O area, the
BTA molecules chemically adsorb when the molecules reach the Cu area. The chemical adsorbed
molecules change its permanent dipole moment due to the charge transfer from the Cu atoms inthe slab. This effect enhances the adsorption between the molecules in the vacuum area and themolecules already adsorbed. The more polarized BTA molecules attract the other molecules tothe surface. Although the chemical adsorbed molecule in the Cu 2 O area also attracts the othermolecules in a vacuum, the total effect due to the already adsorbed molecules is different in bothareas. These two effects, i.e., surface diffusion and polarization effect, make the turning point
shown in Figure 8, more critical. Finally, the number of BTA molecules becomes very different.
Every simulation study should be related to the experimental results. Our simulation seems
that we succeeded in explaining the most important question why the anti-copper-corrosion
additive protects the newly formed surface. Even if the mass of the anti-copper-corrosion additive
is low in an aqueous or oil solution, the additive molecules show a selective adsorption process tofind the newly formed surface, and not the oxidized surface. This phenomenon is found by78

Jurnal Tribologi 21 (2019) 63-81

calculating the many number of molecules with a charge transfer between atoms. Although a more
precise method, such as the ab initio MD, will provide more detailed results, we think the resultsin this paper will not qualitatively change.
For the further work, many aspects must be studied. Since the real phenomena occur in theliquid phase, the effect of the solvent must be studied. The solvent is treated by classical molecular
dynamics, and as in many previous studies, the results will change in the electrolyte solution or
in a high or low pH aqueous solution. The results in this paper will slightly change in the oilsolution, however, we need to change the additive molecule which has a long hydrocarbon chain
to be soluble. In such further simulation, the ReaxFF may still be useful by the feature explained
above.

4.0
CONCLUSIONS
Two models of the adsorption process of the anti-copper-corrosion additive, benzotriazole(BTA), onto the surfaces of a copper (II) oxide (CuO) slab, and hybrid slab with copper (Cu) andcopper (I) oxide (Cu 2 O) are simulated using molecular dynamics with the ReaxFF potential which
includes the charge calculation.
From the result of the first simulation, adsorption between the BTA molecules and copperoxide surface, the BTA molecules form an adsorbed layer in parallel on the surface of the CuO. The
BTA molecules which adsorb alone, show a surface diffusion due to the weak physical adsorption.
In addition, some of BTA molecules aggregate on the surface.
From the result of the second simulation, a selective adsorption phenomenon is found. The
number of BTA molecules adsorbed on the Cu area is 5 times greater than that on the Cu 2 O area.
An analysis of the time evolution of the atomic charge showed that in both areas, the BTA
molecules finally undergo a chemical adsorption. The partially charged nature of the Cu 2 O andthe originally charged Cu atoms make the adsorption less than in the Cu area, and as a result, themajor part of the BTA molecules aggregate on the Cu area.
In real phenomena, the reason why a few anti-corrosion additives are able to protect a metal
surface is thought to be the selective nature of the adsorption onto the newly formed metal
surface.

ACKNOWLEDGEMENT
The authors gratefully acknowledge Dr. Kunichika Kubota, Dr. Junpei Yoshida, Dr. Takafumi
Ishii, Dr. Takao Kudo, and Prof. Masabumi Masuko for their useful discussions. This study was
supported by JSPS (Japan Society for the Promotion of Science) KAKENHI (Grants-in-Aid for
Scientific Research) Challenging Research (Exploratory) Grant Number 18K18813.
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