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Oguike et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (177-186) July 2014
International Journal of
Research in Chemistry and Environment
Available online at: www.ijrce.org
ISSN 2248-9649
Research Paper
Computational Simulation and Inhibitive Properties of Amino acids for Mild
Steel Corrosion: Adsorption in Gas Phase onto Fe (110)
*Oguike R.S. and Oni O.
Corrosion Protection and Materials Science Laboratory, Department of Chemistry, Abubakar Tafawa Balewa University
Bauchi, PMB 0248, Bauchi, NIGERIA
(Received 17th June 2014, Accepted 25th June 2014)
Abstract: A theoretical study has been performed on five selected amino acid molecules, arginine, asparaginine,
aspartic acid, glutaminine and Tryptophan on mild steel surface from gas phase. The study is computed using
Density Functional Theory (DFT) by quantum chemical calculation and molecular dynamics simulation. The
properties of these molecules relevant to their potential action as corrosion inhibitor were calculated such as
EHOMO, ELUMO, energy gap (∆E), electronegativity (χ), global hardness (η), softness (σ) and the fractional number
of electrons transferred (∆N) using the Dmol3 code. The theoretical order of inhibition efficiency was found to be
comparable with experimental result reported. The quench molecular dynamics simulations were applied to
discover the equilibrium adsorption configurations between single inhibitor molecule and mild steel surface using
supermolecule approach.
Keywords: Density Functional Theory, Corrosion inhibition, Molecular dynamics, Mild steel, Amino acid
© 2014 IJRCE. All rights reserved
the overall process is a function of the metal type,
corrodent molecular and electronic structure as well as
concentration of the inhibitor molecules while
temperature and other environmental conditions have
their contributions to the overall process [9-16, 22].
Introduction
Material corrosion has been one of the factors
that undermine our modern development, more especially
in the industrial sector. Researches have shown that
corrosion inhibitors when applied are an economic and
effective technique that retards metals and alloys from
deteriorating. Several studies have been carried out on
amino acids as corrosion inhibitors for metals and its
alloys and they have been found to inhibit its corrosion in
aggressive media. Predictions by several authors have
been made that amino acids as an organic inhibitor which
adsorbs on the metals surface to mitigate corrosion rate
either by the blocking effect of adsorbed inhibitor
molecules on the metal surface and/or by the effects
attributed to the change in the activation barriers of the
anodic and cathodic reactions of the corrosion process [18]
.
Despite the widespread increasing interests in
the application of these organic inhibitors, experimental
results (as of now) reveal that the inhibition process is
neither uniform with respect to all the classes of
compounds studied nor are they consistent in a given
environment. The main objective of theoretical research
is to gain insight into the mechanisms by which inhibitor
molecules added to aqueous environment retard the
metal/corrodent interaction. Indeed, the effectiveness of
Density functional theory (DFT) has become a
useful theoretical method that is applied to successfully
describe the chemical reactivity of inhibitors and their
adsorption efficiency on metal surface. A DFT-based
quantum-chemical computational simulation of suitable
models is now a prevailing tool readily available to
corrosion scientists for theoretical investigation of
corrosion-inhibition mechanism. Such computations have
been widely used to analyze the molecular electronic
structures of adsorption-type inhibitors using a number of
quantum chemical descriptors which gives important
insights on corrosion inhibition mechanisms [17-19]. The
effectiveness of an inhibitor to provide corrosion
protection depends to a large extent on the interaction
between the inhibitor and the metal surface. The adsorbed
inhibitors can affect the corrosion reaction, either by the
blocking effect of the adsorbed inhibitor on the metal
surface or by the effects attributed to the change in the
activation barriers of the anodic and cathodic reactions of
the corrosion process. Organic compounds, which can
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Oguike et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (177-186) July 2014
donate electrons to unoccupied d orbitals of metal surface
to form coordinate covalent bonds and can also accept
free electrons from the metal surface by using their
antibonding orbitals to form feedback bonds, constitute
excellent corrosion inhibitors. The most effective
inhibitors are those compounds containing heteroatoms
like nitrogen, oxygen, sulfur and phosphorus, as well as
aromatic rings. The inhibitory activity of these molecules
is accompanied by their adsorption to the metal surface.
Free electron pairs on heteroatoms or π electrons are
readily available for sharing to form a bond and act as
nucleophile centers of inhibitor molecules and greatly
facilitate the adsorption process over the metal surface,
whose atoms act as electrophiles [20-21].
Accordingly, inhibition efficiency is correlated
to the molecular and structural properties of inhibitor
compounds. These parameters which could be obtained
through theoretical calculations which includes, chemical
reactivity, charge distribution and the frontier molecular
orbit theory, HOMO (higher occupied molecular orbital)
energy, the LUMO (lower unoccupied molecular orbital)
energy, the energy of the gap, (∆E), chemical hardness
(η) and softness (σ), electronegativity and electron
transfer number (∆N). Lesar and Milošev[23] studied
corrosion inhibition properties of 1,2,4-triazole and its
amino derivatives while Gece and Bilgic[24] studied
inhibition efficiencies of some amino acids as corrosion
inhibitors of nickel. Khaled studied molecular simulation
of trizaoles as corrosion inhibitor for mild steel[7].
Asparaginine (Asn)
Arginine (Arg)
Glutaminine (Gln)
Aspartic acid (Asp)
Tryptophan (Trp)
Figure 1: (a) Lewis structures of the investigated amino acids, (b) Name, (c) Abbreviation
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herein, χFe and χinh represent the absolute
electronegativity of iron and the inhibitor molecule,
respectively, ηFe and ηinh represent the absolute hardness
of iron and the inhibitor molecule. These quantities are
associated with electron affinity (A) and ionization
potential (I) which are useful in their ability to help
evaluate chemical behavior.
In this present study, we are investigating on
theoretical methods to elucidate the inhibition action of
arginine (Arg), asparaginine (Asn), aspartic acid (Asp),
glutaminine (Gln) and tryptophan (Trp) as corrosion
inhibitor for mild steel cleaved along Fe (110) plane
from gas phase using molecular and structural properties.
This was done by discussing quantum chemical
parameters, local reactivity indices such as Fukui
Function and the binding characteristics of these amino
acid compounds on the mild steel surface using quench
molecular dynamics simulations.
(5)
(6)
Computation
The geometry optimization process is carried for
the studied amino acids, Arg, Asn, Asp, Gln, Trp and the
Fe surface using an iterative process, in which the atomic
coordinates are adjusted until the total energy of the
structure corresponds to a local minimum at surface
potential energy. These were modeled by Materials
Studio Modeling 4.0 [25], a high quality quantum
mechanics computer program (available from Accelrys,
San Diego, CA). The electronic structures of inhibitor
molecules and the Fe surface were modeled by means of
the DFT electronic structure program DMol3 using a
Mulliken population analysis as well as a Hirshfeld
numerical integration procedure. Electronic parameter for
the simulation includes restricted spin polarization using
the DNP basis set and the Perdew Wang (PW) local
correlation density functional. [22] DFT has also been
found to be successful in providing insights into the
chemical reactivity and selectivity, in terms of global
parameters as electronegativity (χ), hardness (η) and local
ones as the Fukui function f(r). The local reactivity of the
molecules was studied through an evaluation of the Fukui
indices using Dmol3 code which computes the measure
of the molecules’ chemical reactivity indicating the
possible reactive sites on the molecule. The condensed
Fukui function calculations are based on the finite
difference approximations and partitioning of the electron
density ρ(r) between atoms in a molecular system.[26]
f + = ʠk(N + 1) – ʠk(N)
f – = ʠk(N) – ʠk(N – 1)
the global softness σ as the inverse of chemical hardness
(7)
According to DFT- Koopmans’ theorem, I can
be approximated as the negative of EHOMO and A is also
related to the negative ELUMO. Molecular dynamics (MD)
simulation of the interaction between a single inhibitor
molecules and the Fe surface was performed using
Forcite quench molecular dynamics[22] to sample different
low energy configurations and identify the low energy
minima. Calculations were carried out, using the
COMPASS (Condensed phase Optimized Molecular
Potentials for Atomistic Simulation Studies) force field
and the Smart algorithm, in a simulation box 24.79 Å ×
24.05 Å × 29.79 Å with periodic boundary conditions to
model a representative part of the interface, devoid of
arbitrary boundary effects. The box was comprised of the
Fe slab and a vacuum layer of 20 Å height. The Fe crystal
was cleaved along the 110 plane and relaxed by
minimizing its energy using molecular mechanics, while
its periodicity was changed by constructing a supercell 12
x 10. The temperature was fixed at 298.15 K, with NVE
ensemble. The system was quenched every 250 steps
with convergence tolerance energy at 1.0-3kcal/mol.
Optimized structures of inhibitor molecules and the Fe
surface were used for all simulations.
Results and Discussion
The Lewis structures of the amino acid
molecules studied are given in Figure 1. The optimized
mild steel surface cleaved along 110 plane is shown in
Fig. 2 while optimized molecular structures of the studied
molecules using Dmol3 code by high convergence
criteria are shown in Fig. 3. The computed quantum
chemical
parameters,
EHOMO,
ELUMO,
∆E,
electronegativity (χ), global chemical hardness (η), global
softness (σ) and number of electrons transferred (∆N) are
given in Table 1. A practical route to the complex
processes occurring between adsorbed inhibiting species
and metal surfaces at the molecular level involves
computer simulations of suitable models that calculate
the molecular reactivity. According to Yan et al [27], the
frontier molecular orbital theory, the formation of a
transition state of chemical specie is due to an interaction
between EHOMO and ELUMO of the reacting species.
(1)
(2)
herein, ʠk is the gross charge of atom k in the molecule
and N is the number of electrons. The condensed Fukui
function is local reactivity descriptor and can be used
only for comparing reactive atomic centres within the
same molecule. The binding energy (–Ecomplex) of iron
surface with inhibitor molecules was calculated according
to the following equation:[13]
EBind = EFe–inh – (EFe + EInh)
(3)
herein, EFe–inh is the total energy of the Fe crystal together
with the adsorbed inhibitor molecule, EFe and EInh is the
total energy of the iron crystal and free inhibitor
molecule, respectively. The number of electrons
transferred (∆N) from the inhibitor molecule to the
metallic atom was also calculated using the following
equation:[23]
The energy of HOMO characterizes the
susceptibility of the molecule towards attack by
electrophiles. High value of EHOMO indicates a tendency
of the molecule to donate electrons to appropriate
(4)
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acceptor molecules with low energy MO or empty
electron orbital. Increasing values of EHOMO is likely to
indicate adsorption and therefore enhanced inhibition
efficiency by an influence at the adsorbed layer through
transport process. The energy of LUMO characterizes the
propensity of the molecule towards attack by
nucleophiles. Low value of ELUMO indicates an electron
accepting ability of an inhibitor molecule. The binding
ability of an inhibitor to a metal surface increases with
increasing EHOMO and decreasing ELUMO values. This
suggests that the EHOMO facilitate electron donation
through the adsorbed layer while low E LUMO induces a
back-donation of electron from the metal to inhibitor
molecules[28]. The mild steel surface optimized along
Fe(110) plane acts as an electrophile centers which might
be due to the Fe atoms exhibiting an electron deficiency
in the corner atoms while rich electron density in the
center atom (Figure 2). This suggests that adsorption
behaviour of the corner atoms would be different to that
of the center atom, a phenomenon that could yield
marked effect on nanocrystalline structures.
molecular orbital theory indicate the following order of
inhibition efficiencies for the molecules: Trp > Arg > Asn
> Gln > Asp which is comparable to the studies of
Obiukwu et al. The HOMO densities of all the studied
molecules were virtually found to be around the whole
molecule. According to Yan [27], this kind of structure is
difficult to form chemical bond active sites which suggest
a physisorption for all studied molecules. This conforms
to the findings of Obiukwu et al[32].
The energy band gap (∆E), the difference of
ELUMO and EHOMO is another important factor that
describes the reactivity of inhibitor molecules towards
adsorption on metallic surfaces. It has been reported [33]
that the low values of ∆E will provide a good inhibition
efficiency, because the energy for removing an electron
from the last occupied orbital will be low. As ∆E
decreases, the reactivity of the molecule increases leading
to increase in inhibition efficiency of the molecule [34].
For example, molecules with large HOMO-LUMO gap
are generally stable and unreactive while those with small
are ∆E reactive[35]. Table 1 show that Trp has a high
energy band gap (70.85kcal/mol).
The nucleophile centers of the inhibitor
molecules are normally heteroatoms with free π-electrons
that are readily available for sharing in bond formation
[29-31]
. The computed results of EHOMO and ELUMO are
shown in Fig. 3. The results in table 1 indicates that Trp
has the highest EHOMO, while inspection of Fig. 3 reveals
that the HOMO orbital is chiefly located around the
indole ring while the LUMO orbital is predominant at the
aldehyde function. This suggests that Trp can interact
with the vacant orbital of Fe using these sites to form
feedback bonds via adsorption. The total electron density
(charge distribution) shown in Figure 3 reveals that the
electron density is saturated all around the molecule,
hence a flat-lying adsorption configuration was used for
the computation. The HOMO orbital for Asp is found to
saturate around carboxylate function and the LUMO
orbital is localized around the aldehyde function.
Similarly, the HOMO orbital of Arg is principally found
around the amines, N3 and N10 while the LUMO orbital
is localized around the aldehyde function.
For the purpose of investigating the active site of
the inhibitor molecules, the Fukui indices are considered
in table 2 and their computations shown in Fig. 4. Among
the theoretical models proposed to compute local
reactivity indices, Fukui functions make it possible to
rationalize the reactivity of individual molecular orbital
contributions which accounts for the response of the
whole molecular spectrum. The f – measures reactivity
with respect to the ability of the inhibitor molecule to
donate electrons, while the f + is a measure of reactivity
relating to the propensity of the molecule to accept
electrons.[36] The optimized geometries obtained from the
calculations of condense Fukui functions are shown in
Fig. 4, the result for Fukui (Mulliken analyses) function
support the trend of the frontier molecular orbital theory
observed for the studied inhibitor molecules indicating
the sites through which these molecules will be adsorbed
onto the Fe(110) surface.
The HOMO orbital of Asn and Gln is found
mostly around N7, O8 and C5, O7 respectively while
their LUMO orbital density is largely around O1, C2 and
C6, O7 respectively. It is interesting that we observed
that all of the molecules studied used the aldehyde
function to interact with the vacant molecular orbital of
Fe using antibonding orbitals to form feedback bonds.
This suggests that these inhibitor molecules can retard the
corrosion process of mild steel. Obiukwu et al[32] studied
some amino acids as corrosion inhibitor for mild steel
corrosion in 1M H2SO4. The results from frontier
Figure 2: Optimized structure of mild steel cleaved
along Fe(110) plane
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Table 1
Electronic parameters (EHOMO, ELUMO, ∆E, electronegativity (χ),global chemical hardness (η), global softness (σ)
and number of electrons transferred (∆N) of Arg, Asn, Asp, Gln, Trp in gas phase
Molecule
Arg
Asn
Asp
Gln
Trp
EHOMO
(kcal/mol)
-122.93
-129.33
-136.54
-131.21
-116.78
ELUMO
(kcal/mol)
-43.55
-47.38
-67.46
-48.57
-45.93
∆E
(kcal/mol)
79.38
81.95
69.08
82.64
70.85
∆N
(kcal/mol)
0.9849
0.8915
0.8602
0.8656
1.1299
χ
(kcal/mol)
83.24
88.36
102.0
89.89
81.36
η
(kcal/mol)
39.69
40.98
34.54
41.32
35.43
σ (10–2)
(kcal/mol)
2.52
2.44
2.90
2.42
2.82
Arg
Asn
Asp
Gln
Trp
(a) Optimized structure
(b) HOMO orbital
(c) LUMO orbital
Figure 3: Electronic properties of Arg, Asn, Asp, Gln, Trp: [C, gray, H, white, N, blue, O, red],
structures , (b) HOMO orbital , (c) LUMO orbital
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(a) optimized
Oguike et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (177-186) July 2014
Table 2
Condensed Fukui function of active sites for Arg, Asn, Asp, Gln, Trp in gas phase
f–
f+
Arg
N(10) 0.127
N(3) 0.103
C(8)
O(9)
0.285
0.234
Asn
N(7)
O(1)
0.232
0.155
C(2)
O(1)
0.284
0.231
Asp
O(1)
C(2)
0.363
0.101
C(6)
O(7)
0.278
0.236
Gln
O(7)
O(8)
0.165
0.133
C(6)
O(7)
0.283
0.237
Trp
C(8)
N(9)
0.065
0.058
C(12) 0.279
O(13) 0.224
Mulliken atomic
charges
N( 3) -0.409
C( 8) 0.243
O( 9) -0.329
N(10) -0.485
O( 1) -0.339
C( 2)
0.243
N( 7) -0.454
O( 1) -0.393
C( 2)
0.491
C( 6)
0.244
O( 7) -0.301
C( 6)
0.256
O( 7) -0.315
O( 8) -0.438
C( 8) -0.010
N( 9) -0.344
C(12) 0.241
O(13) -0.331
Figure 4: Chemical reactivity properties of Arg, Asn, Asp, Gln, Trp: [C, gray, H, white, N, blue, O, red],
(b) f +, (c) Electron density
182
(a) f – ,
Oguike et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (177-186) July 2014
The highest f – value associated with the indole
ring C8 atom for Trp indicates the site most prone to
nucleophilic attack and through which the molecules will
interact with the Fe(110) surface. This indicates the
propensity to donate electrons to vacant molecular orbital
on the Fe surface to form coordinate bond. This agrees
with the results of the computed HOMO density.
Likewise, the highest f – values found for Arg and Asn
are related to amine N10 atom and amine N7 atom
respectively which also agrees with the computed HOMO
density. Gln and Asp were found to be at aldehyl O7
atom and carboxyl O1 atom respectively with small
densities, which could be attributed to their low
reactivities (inhibition efficiency). The high f + value for
Trp is linked with the aldehyl C12 atom signify the site
most susceptible for an electrophilic attack that is through
which the molecule accepts electrons to form feedback
bonds with Fe surface. This also conforms to the
computed LUMO orbital density. The values got for Arg
(a) Arg
and Asn where found to be around their aldehyl C atoms,
C8 atom and C2 respectively while that of Gln and Asp
had their f + values to be found around aldehyl C atoms
C6 and C6 atom respectively. A close inspection would
reveal that all studied molecules had the back-donation
process at their aldehyl C atoms in agreement with the
frontier orbital results obtained. Table 1 provides some
important calculated quantum-chemical parameters using
Eqn. (4), number of electrons transferred (∆N).
Values of χ and η were calculated by using the
values of I (-EHOMO) and A (-ELUMO) obtained from
quantum chemical calculations. In order to calculate ∆N,
a theoretical value for the electronegativity of bulk iron
according to Zarrouk et al[37] was used, that is χFe ≈
161.42 kcal/mol, and a global hardness of ηFe ≈ 0, by
assuming that for a metallic bulk I = A, because they are
softer than the neutral metallic atoms [5].
(b) Asn
(c) Asp
(d) Gln
(e) Gln
Figure 5: Representative snapshots of (a) Arg, (b) Asn, (c) Asp, (d) Gln and (e) Trp adsorbed on Fe(110). Inset
images show the on-top views, emphasizing the molecular backbone aligning with vacant sites on the fcc lattice
atop the metal surface
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Oguike et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (177-186) July 2014
According to Lukovits, if ∆N < 3.6, the
inhibition efficiency increased with increasing electron
donating ability at the metal surface[33].
inhibition efficiency results got showed that the length of
alkyl chain had influence on molecular reactivity and the
presence of indole ring further advanced this reactivity.
This verity shows Trp to have higher inhibition efficiency
than all studied amino acid molecules. The quench
molecular dynamics demonstrated that the inhibitor
molecules conform to physisorption type of corrosion
inhibition. Also, our simulations showed that the
adsorption energy increased with the presence of the
indole ring and with elongation of alkyl chain. The values
of number of transferred electrons also agree with the
order of inhibition efficiency.
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