Synthesis and characterization of novel Cu (II) and Zn (II) complexes of 5-{[(2-Hydroxyethyl) sulfanyl] methyl}-8-hydroxyquinoline as effective acid corrosion inhibitor by experimental and computational testings

Article abstract of DOI:10.1016/j.cplett.2020.137771

Two new complexes were synthesized, characterized and tested as inhibitors for mild steel corrosion in 1.0 M HCl. The anticorrosive effect has been studied using the potentiodynamic polarization (PDP) and impedance spectroscopy (EIS). The inhibitory efficiency increases with increasing concentration to reach 96.0percent for Q-Cu and 94.6percent for Q-Zn at 10^?3 M. The two inhibitors are mixed type and follows the Langmuir isotherm adsorption. The surface of the mild steel has been examined by SEM/EDS and the corrosive solution by ICP-OES. DFT calculations and molecular dynamic simulation (MD) show a good correlation with the experimental results for both complexes.

Full text of DOI:10.1016/j.cplett.2020.137771

Journal Pre-proofs
Research paper
Synthesis and characterization of novel Cu (II) and Zn (II) complexes of 5-
{
[(2-Hydroxyethyl) sulfanyl] methyl}-8-hydroxyquinoline as effective acid
corrosion inhibitor by Experimental and Computational testings
M. Rbaa, F. Benhiba, M. Galai, Ashraf S. Abousalem, M. Ouakki, Chin-Hung
Lai, B. Lakhrissi, C. Jama, I. Warad, M. Ebn Touhami, A. Zarrouk
PII:
S0009-2614(20)30686-2
DOI:
https://doi.org/10.1016/j.cplett.2020.137771
CPLETT 137771
Reference:
To appear in:
Chemical Physics Letters
Received Date:
Revised Date:
Accepted Date:
22 May 2020
25 June 2020
4 July 2020
Please cite this article as: M. Rbaa, F. Benhiba, M. Galai, A.S. Abousalem, M. Ouakki, C-H. Lai, B. Lakhrissi, C.
Jama, I. Warad, M. Ebn Touhami, A. Zarrouk, Synthesis and characterization of novel Cu (II) and Zn (II)
complexes of 5-{[(2-Hydroxyethyl) sulfanyl] methyl}-8-hydroxyquinoline as effective acid corrosion inhibitor
by Experimental and Computational testings, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/
j.cplett.2020.137771
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Synthesis and characterization of novel Cu (II) and Zn (II) complexes of 5-
[(2-Hydroxyethyl) sulfanyl] methyl}-8-hydroxyquinoline as effective acid
corrosion inhibitor by Experimental and Computational testings
{
1
2,10
3
4,5
6
M. Rbaa **, F. Benhiba
, M. Galai , Ashraf S. Abousalem , M. Ouakki , Chin-
7
1
8
9
3
10
Hung Lai B. Lakhrissi , C. Jama , I. Warad , M. Ebn Touhami , A. Zarrouk *
,
1Laboratory of Agro-Resources, Polymers and Process Engineering, Faculty of Sciences, Ibn Tofail
University, PO Box 133, 14000, Kenitra, Morocco.
2Laboratory of Separation Processes, Faculty of Sciences, Ibn Tofail University, PO Box 133, 14000,
Kenitra, Morocco.
3Laboratory of Materials Engineering and Environment: Modeling and Application, Faculty of
Sciences, Ibn Tofail University BP. 133-14000, Kenitra, Morocco.
4
Chemistry Department, Faculty of Sciences, Mansoura University, El-Mansoura 35516, Egypt.
Quality Control Laboratory, Operations Department, JOTUN, Egypt.
Laboratory of Materials Engineering and Environment: Application and Modeling, Faculty of
5
6
Sciences, Ibn Tofail University, PO Box 133, 14000, Kenitra, Morocco.
7
Department of Medical Applied Chemistry, Chung Shan Medical University, 402, Taichung, Taiwan.
Univ. Lille, CNRS, INRAE, ENSCLCentrale Lille, UMR 8207, - UMET - Unité Matériaux et
8
Transformations, F-59000 Lille, France.
9
Department of Chemistry and Earth Sciences, PO Box 2713, Qatar University, Doha, Qatar.
1
0
Laboratory of Materials, Nanotechnology and Environment, Mohammed V University, Faculty of
Sciences, P.O. Box. 1014, Rabat, Morocco.
✉
Corresponding authors
Prof. Dr. Abdelkader Zarrouk *
E-mail: azarrouk@gmail.com (AZ)
Dr. Mohamed Rbaa**
E-mail:mohamed.rbaa10@gmail.com (MR).
1

Abstract
Two new complexes were synthesized, characterized and tested as inhibitors for mild steel
corrosion in 1.0 M HCl. The anticorrosive effect has been studied using the potentiodynamic
polarization (PDP) and impedance spectroscopy (EIS). The inhibitory efficiency increases
-3
with increasing concentration to reach 96.0% for Q-Cu and 94.6% for Q-Zn at 10 M. The
two inhibitors are mixed type and follows the Langmuir isotherm adsorption. The surface of
the mild steel has been examined by SEM/EDS and the corrosive solution by ICP-OES. DFT
calculations and molecular dynamic simulation (MD) show a good correlation with the
experimental results for both complexes.
Keywords: Synthesis; Complexes; Electrochemical techniques; ICP-OES/SEM; DFT/MD
simulations.
1
. Introduction
Organic synthesis attracted considerable attention because this procedure is suitable for the
production of new polyfunctional heterocyclic compounds [1-3]. The objective of productions
of heterocyclic compounds to be the subject of two considerations: the first consideration is to
enrich the heterocyclic chemotherapy, the second consideration of increasing the applications
of heterocyclic compounds to a broader spectrum [4]. The use of heterocyclic compounds has
become widespread in several application fields such as biological, electrochemical,
analytical and pharmaceutical field [5-8]. Today, researchers in this field making a shift from
synthesis of classical organic compounds towards the synthesis of organic compounds that
2

meets modern application. Among these compounds, metal complexes were on top. The metal
complexes have a multi-purpose use for example in metallurgy for precipitation or
maintenance of a species in a solution [9], photography [10], recovery of noble metals [11],
pigment staining [12], electroplating bath and antitumoral chemistry [13,14].
Today, several industries face major corrosion problems, particularly the ones
operating in acidic environments such hydrochloric acid [15]. In order to mitigate this
problem, several researchers have been proposed the protection of metals by chemical
inhibitors because this method is more efficient and economical, especially the protection by
organic inhibitors, because it is less toxic than inorganic inhibitors [16]. Among the organic
compounds that are used for the protection of metals, we find the 8-hydroxyquinoline and its
derivatives have the potentials to act as the corrosion inhibitors. The 8-hydroxyquinoline
derivatives have been presented in several scientific publications as inhibitors for corrosion of
metals in different acid and neutral media [17-20].
The investigations of the molecular properties related to the corrosion inhibition
performances of the inhibitor molecules are performed by the quantum chemical calculations
and molecular dynamics (MD) simulation. These two simulation techniques are very efficient
among other available computer-aided-simulations [21-23]. With respect to quantum
chemical calculation, inhibition effectiveness of the inhibitors can be determined by studying
optimized geometries of the inhibitor molecules, electron distribution in the frontier molecular
orbitals (FMOs) and adsorption related electronic properties like FMOs energies, energy gap,
total energy and fraction of transferred electron between inhibitors and metal surfaces etc.
The obtained results provided by the above investigations have exuded more light on
the active sites, inhibition mechanism and reactivity of the inhibitor molecules. However, the
modern researchers have proposed that in order to predict the inhibition efficiency inherited
by inhibitors, only quantum chemical approach cannot be sufficient enough [24,25]. It has
3

been found that the results obtain from DFT calculations cannot be correlated well with the
experimentally obtained results in many cases [26,27]. In this purview, it is needed to
emphasize the modelling of experimentation for the accurate prediction of chemical behavior
and inhibition performance of the inhibitor molecules. A precise modelling of
+
-
experimentation will take care of all concerned species (like H O, H O , Cl , Fe surfaces etc.)
2
3
which actually take part in corrosion related process. In this perspective, in recent time, MD
simulation is employed as a modern technique to envisage actual interfacial configuration of
inhibitor molecules on the metal/solution interface [28,29].
This work aimed the preparation of new metal complexes based on 8-
hydroxyquinoline in order to enrich the library of heterocyclic compounds and to find the new
physical-chemical, biological and electrochemical properties. The metal complexes were
1
13
analyzed out by the following spectral methods: IR, NMR ( H, C), EA and X-Ray. Also,
these compounds were tested as a corrosion inhibitor of mild steel in 1.0 M HCl medium,
Potentio-dynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), density
functional theory (DFT) and molecular dynamic simulation (MDS). The surface of the steel
after corrosion test has been characterized by scanning electron spectroscopy (SEM) coupled
with the energy dispersive spectroscopy (EDS), And the gravimetric solutions analyzed by the
Inductively Coupled Plasma-Optical Spectroscopy (ICP-OES).
2
2
2
. Materials and methods
.1. Chemical study
.1.1. Chemicals and equipment
All chemicals and materials used in this work followed international standards. The
qualitative analysis of all the products synthesized in this work was carried out by the
4

1
13
following spectral methods: IR, NMR ( H, C), EA and X-Ray. Further details concerning
these materials and products have been published in another work [30].
The chemical compositions of the steel used in this work are: [Si (0.24); Mn (0.47); C (0.11);
Mo (0.02); Ni (0.1); Cr (0.12); Al (0.03); Co (< 0.012); Cu (0.14); V (< 0.003); Fe (balance);
W (0.06)]. Corrosive solutions were prepared from the dilution of a concentrated hydrochloric
acid (37 %) with the distilled water (H O). The metal parts are cleaned with technical acetone
2
and alcohol. The gravimetric tests were repeated three times for the reliability of the results.
2
.1.2. Ligand Preparation
The ligand [5-{[(2-hydroxyethyl)sulfanyl]methyl}quinolin-8-ol] (HL) was synthesized in two
steps; the first step consists of the conversion of 8-hydroxyquinoline (HQ) to 5-chloromethyl-
8
-hydroxyquinoline hydrochloride (5-CMHQ) by acid wading (HCl ) at room temperature
g
for 24 h. The second step is the reaction of the product 5-CMHQ with the mercaptoethanol,
in tetrahydrofuran (THF) in the presence of NaHCO at reflux for 8 h (Figure 1).
3
Cl
S
OH
THF, NaHCO3
Reflux, 8 h
H CO, HCl
2
HS
OH
rt, 24 h
N
N
N
OH
HQ
Fig. 1. Ligand Preparation (HL)
OH
OH
HL
H Cl
5-CMHQ
2
.1.3. Preparation of metal complexes
The metal complexes (Q-Cu and Q-Zn) were prepared by the reaction of the product HL
with the zinc chloride or copper (II) chloride in ethanol (EtOH) in the presence of
triethylamine (Et N) as a base at reflux for 3 h (Figure 2).
3
5

S
OH
OH
S
N
M = Q-Cu
M = Q-Zn
EtOH
MCl , 2 H O
2
2
O M O
N
Reflux, 3 h
N
OH
OH
S
Fig. 2. Preparation of metal complexes (Q-Cu and Q-Zn).
2
.2. Electrochemical study
The corrosion tests were carried out by the usual electrochemical methods such as
electrochemical politicization and impedance spectroscopy. The details for electrochemical
methods have been published in another publication [31].
Inhibitory efficiencies for gravimetric studies, polarization studies, and impedance
spectroscopy, respectively, were designed using the following equations 1-3:
0
corr 
corr


%


100
(1)
(2)
0
corr
0
where corr and corr are the corrosion rates without and with the addition of inhibitors.
0
corr
i
i
corr
PP

100
0
i
corr
0
corr
where icorr and
i
are the corrosion current density with and without the addition of inhibitors.
The
corrosion
current
density
(i )
corr
was
calculated
by
extrapolating
the linear Tafel segments of anodic and cathodic curves back to the corrosion potential
0
Rp  Rp
EIS

100
(3)
Rp
0
where Rp and Rp are the polarization resistance with and without the addition of inhibitors.
6

The gravimetric solutions have been investigated by the UV-visible spectrometry (UV-vis)
and inductively coupled with plasma optical spectroscopy (ICP-OES). The metal surface was
identified by the scanning electron microscope (SEM) connected with energy dispersive
spectroscopy (EDS).
2
.3. Details of DFT and MD simulation modeling methods
Theoretical methods, namely density functional theory (DFT) and molecular dynamics (MD)
simulation, are the most commonly used approaches to explain the mode of action of an
inhibitor on a metal support [32].
The DFT modeling of the Q-Cu and Q-Zn complexes have been accomplished by B3LYP
hybrid functional to consider the electron exchange-correlation interactions. The interactions
of copper and zinc atoms in the DFT calculations have been described by the effective
potential of the core using the LanL2DZ base set [33]. The Gaussian 09 program was used to
perform these DFT calculations [34]. When the calculation is completed, some important
reactivity descriptors in quantum chemistry, namely EHOMO (the energy of the highest
occupied molecular orbital), ELUMO (the energy of the lowest unoccupied molecular orbital),
ΔEgap (the energy difference), ∆N110 (the amount of the transferred electrons) and the total
energy are calculated and interpreted [35-38].
The MD simulation cells adapted for the low interaction energy complex corrosion
configurations of our studied complexes were constructed with consideration of the
experimental conditions [39]. For this purpose, this dynamic simulation was carried out in an
acidic medium. The vacuum created of 40 Å was filled with a solvent layer containing 500
water molecules, a molecule of complex optimized and minimized for at least 5000 steps
-
+
using the smart algorithm for each simulation and 5 Cl and 5 H O corrosive ions. Thereafter,
3
3
the constructed cells of 32.27 × 32.27 × 30.13 Å size and of 13 × 13 unit were exposed to 400
ps with the NVT ensemble simulations which were run at 298 K using the Forcite module
7

which is adopted in the Materials Studio software suite. The COMPASS force field was
selected to model the energies of Fe (110) atoms, water molecules, corrosive ions and Q-Cu
or Q-Zn [40-42].
The interaction (Einteraction) and binding (Ebinding) energies of Q-Cu and Q-Zn complexes in
relation to Fe (110) surface in HCl medium are calculated as follows [43]:
Einteraction  Etotal  Esurfacesolution  Einhibitor
Ebinding =−E interaction
(4)
(5)
3
3
. Results and Discussions
.1. Chemical study
General procedures for ligand preparation (HL)
In a 100 mL flask, containing 40 mL of THF, 0.01 mol of 5-chloromethyl-8-hydroxyquinoline
hydrochloride 5-CMHQ and 0.01 mol of mercaptoethanol (C H OS) were added in the
2
6
presence of 0.01 mol NaHCO . The reaction mixture was refluxed with stirring for 8 h and
3
then separated by a TLC plate. The reaction mixture was concentrated under vacuum and then
extracted with dichloromethane. The resulting residue was purified by silica column
chromatography (CH Cl C H = 85:15). Yield: 85 %; mp 110-112 °C; F 0.67; Aspect;
2
2:
6
14
r
1
White crystal; IR 1625.62 (C=C), 3239.18 (OH), 2036.29 (CH ), 2398.10 (C=N). H NMR:
2
δppm = 1.01 (s, 2H, CH -C), 4.72 (s, 2H, CH -S), 6.99 (s, 1H, OH-Ar), 4.83 (s, 1H, OH), 7.01-
2
2
1
3
7
.34-7.56-8.83-9.77 (m, 5 H, ArHquinoline). C NMR: δ = 153.21 (C-OH-Ar), 34.28 (CH2-
ppm
S), 61.28 (CH -OH), 32.80 (C-C), 110.62-121.92-124.73-128.94-148.29 (ArCHquinoline),
2
1
2
5
33.75-139.49 (ArCquinoline). Elemental Analysis: Molecular Formula: C H NO S (M =
12 13 2
35). Calculated: C, 61.25 %; H, 5.57 %; N, 5.95 %. Found: C, 61.02 %; H, 5.70 %; N:
.37 %.
X-Ray Crystal Structures Description of Ligand
8

3
A white colored crystal of dimensions 0.44 × 0.43 × 0.23ꢀmm of HL was chosen for an X-ray
diffraction study. The single crystal X-ray diffraction data show that it crystallizes in a space
group, P2 /c. The crystal belongs to the monoclinic system with the following lattice
1
parameters: a = 11.8362 (7) Å, b = 11.7958 (7) Å, c = 7.8459 (5) Å, β = 91.673 (1) (°) and its
3
unit cell volume is 1094.96 (12) Å . The details of the crystal data and structure refinement
were given in Table 1. Moreover, the obtained molecular structure of the title compound was
depicted as Fig. 3 shows.
Fig. 3. The title molecule with labeling scheme and 50% probability ellipsoids. The
intramoleculart O—H···N hydrogen bond is shown by a dashed line.
Table 1
Crystallographic and refinement data for HL.
Crystal data
Chemical formula
Mr
C H NO S
12 13 2
235.29
Crystal system, space group
Temperature (K)
Monoclinic, P2 /c
1
120
9

a, b, c (Å)
β (°)
11.8362 (7), 11.7958 (7), 7.8459 (5)
91.673 (1)
1094.96 (12)
4
V (Å3)
Z
Radiation type
Mo Kα
−
1
µ (mm )
0.28
Crystal size (mm3)
0.44 × 0.43 × 0.23
Data collection
Diffractometer
Bruker Smart APEX CCD
Absorption correction
Multi-scan
SADABS (Krause et al., 2015)
T , T
min
0.86, 0.94
max
No. of measured, independent and
33009, 2961, 2797
observed [I > 2σ(I)] reflections
Rint
0.025
0.687
−
1
(sinθ/λ)max (Å )
Refinement
2
2
2
R[F > 2σ(F )], wR(F ), S
No. of reflections
0.032, 0.093, 1.05
2961
No. of parameters
H-atom treatment
191
Only H-atom displacement parameters refined
−
3
Δρmax, Δρmin (e Å )
0.56, −0.20
General procedure for metal complexes (Q-Cu and Q-Zn)
.02 mole of [5-{[(2-hydroxyethyl)sulfanyl]methyl}quinolin-8-ol] (HL) is dissolved in 50 mL
0
of absolute ethanol in the presence of triethylamine (0.01 mole), and the mixture is then
heated until the mixture is completely dissolved. After 30 minutes of heating with stirring, a
solution containing 0.01 mole copper (II) chloride or zinc chloride was added dropwise with
stirring until the appearance of the solid with a color change of the solution. Then, the mixture
1
0

is refluxed for 3h and then poured into 150 mL of cold water. The metal complex is thus
formed, filtered under vacuum, washed with ethanol and dried in the oven.
Q-Zn Complex:
1H NMR: δppm = 3.70 (s, 2 H, CH -OH), 2.27 (s, 2 H, CH -S), 3.36 (s, 1 H, OH), 3.77 (s, 2 H,
2
2
1
3
Ar-CH -S), 6.97-7.52-7.57-7.61-7.64 (m, 5 H, ArHquinoline). C NMR: δ
= 162.83 (C-O-
ppm
2
Ar), 31.28 (CH -S), 36.20 (C-C), 113.70-121.71-123.98-129.72-135.14-139.11-149.58-
2
1
51.29 (ArCquinoline). EA: Molecular Formula: C H N O S Zn (M = 533.97). Calculated
24 24 2 4 2
C, 53.98 %; H, 4.53 %; N, 5.25 %. Found C, 53.25 %; H, 4.07 %; N, 5.73 %.
Q-Cu Complex:
1H NMR: δppm = 3.82 (s, 2 H, CH -OH), 2.29 (s, 2 H, CH -S), 3.33 (s, 1 H, OH), 3.85 (s, 2 H,
2
2
1
3
Ar-CH -S), 7.11-7.14-8.28-8.81-8.83 (m, 5 H, ArHquinoline). C NMR: δ
= 164.05 (C-O-
ppm
2
Ar), 56.23 (CH -S), 53.58 (CH -OH), 21.44 (C-C), 115.44-122.19-124.41-129.73-134.01-
2
2
1
5
5
3
3
34.33-136.53-155.02 (ArCquinoline). EA: Molecular Formula: C H CuN O S (M =
24 24 2 4 2
32.13). Calculated C, 54.17 %; H, 4.55 %; N, 5.26 %. Found C, 54.56 %; H, 4.51 %; N:
.05 %.
.2. Electrochemical studies
.2.1. EIS studies
The EIS tests in the absence and in the presence of various concentrations of metal complexes
Q-Cu and Q-Zn) in the solution of 1.0 M HCl was carried out at 298 K. The representative
(
of the Nyquist diagrams from this test, together with the fitted curve are shown in Fig. 4.
Moreover, we also have represented the Bode Modulus plots and Bode phase angle of the
Nyquist diagrams in Fig. 4. The curves in Figure 4 are not perfectly semicircular and show a
compression state. The incompleteness of the semicircles can be ascribed to the roughness of
the electrode’s surface and their compression can be attributed to the time constant of the
electrical double layer and the charge transfer reaction [44]. In addition, the slopes of the
1
1

Bode modulus plots (logZ versus log f) are not equal to -1 (Figure 4), which are usually
observed for solid electrodes as a result of frequency dispersion of interfacial impedance.
Moreover, as Fig.4 depicts, the changes of inhibitory concentrations are followed by the
changes of the diameters for the semicircles compared to the blank solution; which means that
a protective layer was formed and thickened on the surface. Figure 4 reveals that by
increasing the concentration of the inhibitor to the corrosive solution, the absolute value of the
impedance increases at lower frequencies. In other words, by increasing the inhibitory
concentration, the protecting potential of the metallic surface was increased against corrosion
by adsorbing the inhibitory molecules on the surface and creating a layer [45]. The single
peak in the phase angle diagrams in Fig. 4 represents the existence of a time constant. The
depressed semicircles are generally attributed to the frequency dispersion, arising from
roughness and inhomogeneity of the electrode’s surface [44]. So, in such cases a constant
phase element (CPE) is used instead of a pure capacitor to compensate for a non-ideal
capacitive response of the interface to get a more fit to the experimental data and the
impedance is given by Eq. 6:
1
n
ZCPE Q (i)
(6)
-1 n
-2
where Q is the proportionality coefficient (in Ω s cm ), ω is the angular frequency and i is
the imaginary number, n is the deviation parameter (-1 ≤ n ≤+1) which has the meaning of a
phase shift. When n =0 indicates the CPE represents a pure resistor, while n = -1 indicates the
CPE represents an inductor and n = +1 indicates that the CPE represents a pure capacitor [45].
In this study, the best fit of the extracted experimental data is obtained for the equivalent
circuit given in Figure 4. Fig. 4 shows the electrical equivalent circuit diagram to model the
mild steel/1.0 M HCl interface which consists of a CPE, polarization resistance (R ), and
p
solution resistance (Rs).
The impedance parameters from this figure (Figure 4) are collected in Table 2.
1
2

500
400
300
200
100
0
-80
-
3
1
1
1
1
0
0
0
0
M
M
M
M
3.0
2.5
2.0
Q-Cu
Q-Cu
-4
-5
-
60
40
-6
Blank
Fitting curves
-
1.58 Hz
1.5
3
.16 Hz
1
00 KHz
-20
0
1.0
2
.5 Hz
-
3
1
0
M
M
M
M
4
.0 Hz
-4
0
.5
.0
10
10
10
-
5
6
1
00 mHz
-
0
Fitting curves
20
-0.5
0
100
200
300
400
500
600
700
-1
0
1
2
3
4
5
1
0
10
10
10
10
10
10
2
ZRe ( cm )
f (Hz)
500
400
300
200
100
0
-80
-
-
-
3
4
5
10
10
10
10
M
M
M
M
Q-Zn
Q-Zn
3.0
2.5
-60
-6
Blank
Fitting curves
2.0
1.5
1.0
0.5
0.0
-40
5
.0 Hz
-20
1
00 KHz
-
3
10
M
M
M
M
7
.93 Hz
-4
1
0
0
0
-
5
6
1
-
100 mHz
10
Fitting curves
20
10 Hz
0
100
200
300
400
2
500
600
700
-1
0
1
2
3
4
5
1
0
10
10
10
10
10
10
ZRe ( cm )
f (Hz)
1.6
1.2
0.8
0.4
0.0
-
-
-
60
40
20
Rs
CPE
Rp
0
2
0
Blank HCl 1.0 M
Fitting curves
40
-1
0
1
2
3
4
5
10
10
10
10
10
10
10
f (Hz)
Fig. 4. EIS curves, Bode diagrams and Equivalent circuit for M-steel in 1.0 M HCl in the
presence and absence of metal complexes Q-Cu and Q-Zn.
Table 2
The EIS parameters of MS before and after the addition of metal complexes (Q-Cu and Q-
Zn).
C
Rs
(Ω cm )
Rp
(Ω cm )
Cdl
(µF cm )
Q
ηEIS
%
Medium
n
θ
2
2
-2
n-1
(M)
(µF S )
1
3

1
.0 M HCl ──
1.12
0.9
1.6
1.2
1.3
1.2
1.0
1.0
1.4
34.7
351
522
610
714
153
226
238
642
121.0
84.9
65.3
33.7
15.9
100.4
93.2
84.8
59.4
0.773
0.783
0.856
0.814
0.844
0.811
0.800
0.808
0.841
419
231
223
195
159
220
204
196
100
──
──
-
-
-
-
-
-
-
-
6
1
1
10
1
1
1
0
0
0.901 90.1
0.933 93.3
0.943 94.3
0.951 95.1
0.773 77.3
0.846 84.6
0.854 85.4
0.946 94.6
5
4
3
6
5
4
3
Q-Cu
0
0
0
Q-Zn
10
1
0
As per data in Table 2, it is observed that the surface coverage increases with increasing the
inhibitory concentrations, which is due to the increase in the adsorption capacity of the
inhibitors on the mild steel surface. In fact, the formation of a protective layer on the surface
and the blocking of the active sites on the surface can be speculated accordingly. Moreover, it
is observed that by increasing the concentration of inhibitors in the corrosive solution, the
value of R is increased, while that of C is decreased. This behavior is probably due to the
p
dl
adsorption of inhibitory molecules on the metallic surface and the decrease in the local
dielectric constant, i.e. absorption of the inhibitor on the metallic surface reduces the electrical
capacity by replacing the water molecules and other adsorbed ions on the surface [46,47]. The
values of the index "n" close to unity show that the inhomogeneity of the surface has been
increased by the adsorption of the two new complexes on the metallic surface [48]. The data
about impedance showed that inhibitory efficacy increases from 90 % to 95 % by increasing
-6
-3
the concentration of Q-Cu from 10 to 10 M.
3
.2.2. Polarization studies
The polarization study is useful to obtain information on the kinetics of the substrate/inhibitor
system on the slow stage. The polarization is distinguished by two main reactions such as in
the anodic domain (iron oxidation) and in the cathodic domain (reduction of H+).
2


Fe  Fe  2e
(7)
(8)


2
H  2e  H
2
1
4

The electrochemical parameters of the steel extracted from the polarization curves (Figure 5)
are given in Table 3.
3
2
1
0
1
2
3
4
3
1
1
1
1
0
0
0
10
Q-Cu
Q-Zn
2
1
1
1
0
1
0
0
0
0
-
10
-1
-3
-
3
10
10
10
10
10
M
M
M
M
1
0
M
M
M
M
-4
-
-
-
-4
10
10
10
10
-
5
6
-5
-6
1
0
-2
10
-
10
Blank
Blank
-3
10
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
E (V/Ag/AgCl)
E (V/Ag/AgCl)
Fig. 5. Polarization curves before and after the addition of metal complexes Q-Cu and Q-Zn.
Table 3
Polarization parameters of MS before and after the addition of metal complexes (Q-Cu and
Q-Zn).
Medium
.0 M HCl
Conc.
M
Ecorr
mV/Ag/AgCl
icorr
µA cm
c
ηPP
%
-2
-1
mV dec
1
──
-498
983
86
62
47
-140
──
-
-
-
-
-
-
6
5
4
3
6
5
10
10
10
10
10
10
10
10
-525
-542
-540
-552
-541
-545
-541
-525
-130
-125
-120
-106
-135
-132
-130
-124
91.2
93.6
95.2
96.3
77.8
84.7
85.7
Q-Cu
Q-Zn
36
218
150
140
-
4
3
-
52
94.7
The polarization studies show that both complexes have a protective effect on the corrosion of
MS. This inhibitory effect can be justified by the decrease in the values of the corrosion
current intensity (i) after the addition of the two metal complexes to the corrosive solution.
The inhibitory efficiency increases with increasing the concentrations of the complexes up to
1
5

an optimal value of 96 % for the complex Q-Cu. After the addition of the two complexes, it is
noted that there is a slight decrease in the value of the potential and in the cathodic slopes
with respect to the blank solution. This shows the mixed reactivity character of the two
complexes with the steel surface [49]. According to Table 3, the corrosion potential, as
shown in Fig. 5, changes to negative values by increasing the concentration of inhibitors, but
this potential change is less than 85 mV. In addition, increasing the concentration of the
inhibitors (Q-Cu and Q-Zn) in the corrosive solution causes the displacement of both anodic
and cathodic branches. Therefore, these two inhibitors can be classified in the category of
mixed inhibitors based on research done by other groups [50,51].
It is very clear from the Table 3 that the presence of Q-Cu and Q-Zn in the 1.0 M HCl
solution affects the values of the Tafel slopes. However, it is noticed that the change in βc
values of the inhibited systems with respect to that of the blank is independent of inhibitory
concentration, that is, there is no pattern in β values of the inhibited solution with respect to
c
that of the blank as increasing the concentration of Q-Cu and Q-Zn. This means that the
kinetics of the cathodic reactions are not affected and the inhibition is by a simple geometric
blocking mechanism [49].
In this study, the copper complex Q-Cu is more effective in the corrosion inhibition than the
zinc complex Q-Zn, which can be explained by the electronegativity of copper is greater than
that of zinc (χ = 1.69 > χZn = 1.65). In addition, the results obtained by the study of the
Cu
potentiodynamic polarization (PDP) confirm the results of impedance spectroscopy (EIS).
3
.2.3. Temperature effect on corrosion process and activation parameters
For more than a century, scientists have been interested in the study of the corrosion of steels
as a function of temperature, since temperature is essential for measuring the degree of
protection of chemical inhibitors for steel surfaces [52]. The electrochemical data as a
function of temperature are shown in Table 4 and Figure S2.
1
6

The inspection of both Table 4 and Figure S2 apparently indicates that an increase in temperature
causes an increase in the values of the current density (icorr) if adding the two complexes. Although
there is a decrease in their inhibitory efficiencies, both inhibitors remain effective at high
temperatures. Therefore, the temperature effect confirms that both complexes have a corrosion
inhibiting effect even at high temperature. However, the study suffers from possible imprecision on
the parameters of activation of corrosive reaction processes, for example, activation energy (Ea),
activation enthalpy (ΔH ) and activation entropy (ΔS ).
a
a
The three activation parameters were calculated and compiled by the following Eqs. 9 and 10 [53]:

E 
a
icorr  Aexp
(9)



RT

RT
 S 
 H 
a
a
icorr

exp
exp
(10)








Nh
R
RT
The variation of Ln (icorr) = f (1/T) gives the activation energy value (E ) (Figure S3A). The
a
extrapolation of the Arrhenius equation and the variation of Ln (icorr/T) = 1000/T (Figure
S3B) give the values of the activation enthalpy (ΔH ) and the activation entropy (ΔS ). These
a
a
three parameters [(E ), (ΔH ) and (ΔS )] are gathered in Table S1.
a
a
a
From this data (Table S1), we can see that the activation energy (E ) of both complexes Q-Cu
a
and Q-Zn is greater than that of the blank solution. This shows that the inhibitors cause the
formation of electrostatic fields (energy barrier) between the inhibitors and the steel surface
[54]. The recent study about the temperature effect has also showed that the values of
activation enthalpy (ΔH ) for Q-Cu and Q-Zn are more positive than that for the blank
a
solution. This indicates that Q-Cu and Q-Zn both increase the energy barrier of the corrosion
of MS in 1.0 M HCl [55]. The values of the activation entropy (ΔS ) for Q-Cu and Q-Zn are
a
more positive than that for the control solution. This confirms that the two complexes Q-Cu
1
7

and Q-Zn are adsorbed well to the surface of the steel. This is in good agreement with the
literature [56].
3
.2.4. Adsorption isotherm model study
The investigation about the adsorption isotherm is a continuing essential to identify the mode
of protection of steels by the chemical inhibitors [57]. As well, knowledge of the types of
interactions of the inhibitor in the system, inhibitor/metal surface/HCl is thus very important.
2
After plotting the data from the EIS methods, the regression coefficient (R ) for Langmuir
isotherm was very close to 1. Also, the slopes of the two straight lines for the two metal
complexes (Q-Cu and Q-Zn) are also tending towards unity, suggesting that adsorption of
inhibitors on the metal surface follows the Langmuir adsorption isotherm. The adsorption
parameters are shown in Table S2 and Figure S4.
In this study, we tested several isotherms but the best isotherm was found to be the Langmuir
isotherm (R² is approximately unity) [58]. The adsorption parameters extracted in this study
are collected by the following equations [59]:
1
 G 
ads
Kads


exp
(11)
(12)


5
5.5
RT


Cinh
1
 Cinh

Kads
where

,
Cinh and Kads are the fraction of the surface covered by the inhibitor, the
concentration of the inhibitor and the adsorption constant, respectively.
kJ

ads
Reports from other researchers show that if G is more negative than 20
means the
mol
physical absorption of the inhibitor occurs on the surface. This kind of absorption occurs due
to the electrostatic reaction between the inhibitory molecules and the metallic surface. While
kJ

ads
if G is less negative than 40
shows a charge transfer occurs from the inhibitory
mol
1
8

molecules to the metallic surface, the inhibitor adsorb on the metallic surface by the chemical
kJ
kJ

ads
absorption. Moreover, if G has a value between 40
and 20
, the adsorption of
mol
mol
the inhibitor on the surface is a mixture of the physical and chemical absorption [60-62]. As
listed in Table S2, it can be seen that the values of the adsorption free energy of the two
kJ
complexes are greater than 40
. These results show that both inhibitors adsorb to the
mol
surface of the steel by chemical bonds, which confirms the good resistance at high
temperature. The data of the adsorption isotherm showed that both complexes were good
corrosion inhibitors for M-steel (MS) in 1.0 M HCl. This confirms that the results obtained in
the polarization study, impedance spectroscopy, temperature effect and activation study.
3
.2.5. ICP-OES analysis
In order to measure the amount of dissolved irons in the corrosive solution before and after
the addition of the tested complexes (Q-Cu and Q-Zn), we performed an ICP-OES analysis
[63]. In the case of irons in the corrosive solution, the amount of dissolved irons is only in the
order of 250.569 mg/L. On the other hand, in the presence of both complexes, the amount of
dissolved irons has been decreased to 5.089 mg/L for the Q-Cu complex and 6.058 mg/L for
the Q-Zn complex. The ICP-OES analysis showed that our complexes have an excellent
inhibitory effect on ordinary steel in molar hydrochloric acid.
3
.2.6. Surface analysis (SEM and EDS)
To evaluate the degree of protection of the MS surface by the two tested complexes (Q-Cu
and Q-Zn), we carried out a scanning electron spectroscopy analysis [64]. The immersion of
the ironic substrate in molar hydrochloric acid for 6 hours shows that the surface of the M-
steel was destroyed by corrosion (Fig. 6A). While, in the case of adding the tested complexes
(Q-Cu and Q-Zn) in the corrosive solution, it is observed that the MS surface becomes to be
well protected (Fig. 6B, C). The scanning electron microscope analysis also showed that our
1
9

complexes have a corrosion inhibiting effect for M-steel (MS) in 1.0 M HCl. The surface
images in the presence of the tested complexes (Q-Cu and Q-Zn) are in the form of clouds.
This can be explained by the presence of copper (Cu) and zinc (Zn) atoms will move when the
current passes through them (the two complexes are semi-conductors).
The EDS spectra showed that in the case of addition of the complexes Q-Cu and Q-Zn (Fig.
6
B and C), there are the presence of carbon (C), oxygen (O), nitrogen (N), copper (Cu) and
zinc (Zn) atoms. In addition, in the case of adding the tested complexes (Fig. 6B and C), the
percentage of dissolved irons was reduced compared to the blank surface (Fig. 6A). This
confirms the presence of the tested complexes (Q-Cu and Q-Zn) on the surface of the steel.
2
0

Fig. 6. SEM image and EDS spectra of surface (MS) after immersion in 1.0 M HCl for 6 h
-3
without inhibitor (A), and with 10 M inhibitor (B) Q-Cu and (C) Q-Zn.
3
.3. The DFT study
The electronic properties of Q-Cu and Q-Zn complexes were explored using the DFT method
23]. The optimized structures of these complexes are represented in top and side views in
[
Figure 7. The two optimized complexes shown in Figure 7 are in the minimum on the
potential surface, which is proven by the positive values of the frequencies. Visual findings
2
1

showed that the Q-Cu geometry is almost planar around the copper, indicating that this
element stabilizes both ligated quinolines. While in the presence of Zn, the geometry of the
Q-Zn structure is non-planar, with the two quinolines were almost perpendicular to each
other. In addition, the comparison revealed that the computed vibrational frequencies at
B3LYP/LanL2DZ showed acceptable deviations from the experimental results (Figure S1).
Indeed, this result showed that the complexes obtained are well identified theoretically and
experimentally.
Fig. 7. Optimized structures of Q-Cu and Q-Zn complexes represented in top and side views.
2
2

The bond lengths, bond and dihedral angles of the optimized complexes appeared in Figures
and 9, respectively. As depicted in Figure 8, the bond lengths between the heteroatoms (O
8
and N) and the metal in the Q-Cu complex (1.97-1.98 Å) were smaller than those in the Q-Zn
complex (1.97-2.12 Å). This result indicates that the stability of the Q-Cu complex is higher
than that of the Q-Zn complex. In addition, the comparison showed the bond lengths in the
complexed quinoline ligands were the same as those in the uncomplexed ones. The
complexation doesn’t significantly affect the geometry of quinoline and only influence the
heteratom-metal distance. The change in the geometry caused by the complexation is also
justified by the values of bond and dihedral angles as shown in Figure 9.
Fig. 8. Calculated bond angles descriptors of Q-Cu and Q-Zn at the B3LYP /LanL2DZ
theoretical level.
2
3

H
Angle
Dihedral
-35.5°
Dihedral
-0.6°
116.2°
95.5°
-0.6°
-35.5°
Fig. 9. Calculated dihedral angles of Q-Cu and Q-Zn at the B3LYP/LanL2DZ theoretical level.
For the purpose of clarifying the adsorption mechanism of the two metal complexes, namely Q-
Cu and Q-Zn, on the MS surface in 1.0 M hydrochloric acid, we were very interested in
investigating the electron density distribution of frontier molecular orbitals (FMOs) (HOMO and
LUMO) (Figure 10) to predict the spatial disposition of these orbitals in which the type(s) of
atoms that are responsible for corrosion inhibitory reactivity [65,66]. Indeed, the overall visual
analysis of the electron density of HOMOs and LUMOs indicates that they are distributed over the
atoms on the skeletions of quinolines in the studied complexes. Therefore, this distribution of
FMOs showed that the tested complexes carry electron donating and accepting sites, reinforcing
the adsorption of these species on the iron metal surface [67]. Specifically, it is clear that the Q-
Cu complex is characterized by two distributions of electronic densities of frontier molecular
orbital densities (FMOs) such as alpha FMOs (spin up) and beta FMOs (spin down). This behavior
is possibly due to the presence of the copper (Cu) element that is responsible for this separation of
spins. This produces a more active property of the Q-Cu complex with respect to the Q-Zn
2
4

complex. All these results can inform us that the two tested complexes adsorb on the iron surface
by the skeleton of the quinoline in these complexes except the substituent (2-(methylthio)ethanol).
Fig. 10. Distribution of the FMO electron densities of the complexes using DFT with the
B3LYP/LanL2DZ theoretical level.
Concerning the quantum chemical descriptors that are obtained from the graphical evaluations
of FMO distributions, the energy values of HOMO (EHOMO) and LUMO (ELUMO) and their
difference (ΔE ) allow a quantitative evaluation of the chemical reactivity of the selected
gap
complexes [68]. In general, the higher EHOMO values of a molecule reflect a high electron
donating power to another element that carries high electron deficiencies [69]. While a lower
ELUMO value of the same molecule implies that this molecule is capable of accepting more
electrons [70]. And when the smaller energy gap reflects that the molecule studied is more
reactive [71]. In addition, the fraction of the transferred electrons (ΔN ) and total energy
1
10
(ET) were calculated for each inhibitory complex. All the values of these descriptors are
summarized in Table 4. It is very notable from the results listed in this table that the Q-Cu
2
5

complex had lower values of ELUMO (-2.626 eV), ΔEgap (2.689 eV) and E (-1555.648 au) with
T
respect to the Q-Zn complex. These observations inform that Q-Cu is more reactive toward
the iron metal surface with respect to the Q-Zn. While the high values of EHOMO and ΔN110 of
Q-Cu indicate that this complex shares more electrons with the surface of the iron substrate
[72]. Therefore, Q-Cu inhibits corrosion more efficiently than Q-Zn. This result is logical
because copper is a noble metal that effectively resists corrosion, whereas zinc is an active
metal. Accordingly, Q-Cu complex is a good corrosion inhibitor.
Table 4
Chemical quantum descriptors of Q-Zn and Q-Cu complexes in the gaseous state.
Complex
EHOMO
eV)
ELUMO
(eV)
ΔEgap
(eV)
ΔN110
ET
(au)
(
Q-Cu(α(spin ↑))
Q-Cu(β(spin ↓))
Q-Zn
-5.365
-5.315
-5.421
-2.147
-2.626
-2.188
3.218
2.689
3.233
0.331
0.316
0.314
-
1555.648
-1425.116
The active centers of two complexes were further investigated by using the Fukui functions
Fukui(+) and Fukui(-) which provide a demonstration of nucleophilic and electrophilic
interactions, respectively [73]. Table 5 represents all Fukui centers related to
electrophilic/nucleophilic attacks of the tested complexes. Figure 11 shows the most relevant
sites responsible for the interaction with the metal surface. As depicted in Figure 11, in the
Q-Cu complex, the local reactivity is clearly defined by the oxygen atoms bound by the
copper metal, i.e. O (11), O (22) and Cu (23) atoms play the important role as the sites
responsible for electrophilic and nucleophilic attacks. This enhances the adsorption of the Q-
Cu and Q-Zn complexes on the surface of the iron substrate. In addition, the copper atom has
a high Fukui density, which indicates that this atom is considered a friendly element by iron
atoms. Concerning the local reactivity of the active sites in Q-Zn, it is notable that the
element zinc has no remarkable trend in Fukui density, which is why the inhibitory efficacy of
2
6

Q-Zn is lower than that of Q-Cu. In addition, the results shown in Figure 11 and grouped in
Table 6 show those oxygen atoms (O (11) and O (22)) and the appearance of new carbon
atoms are responsible for the electrophilic attack.
Table 5. Fukui (+) and Fukui (-) of Q-X (X = Cu or Zn) complexes calculated at
DFT/GGA/DNP using Material studio 2016.
Atoms
X = Cu
Fukui(-)
X = Zn
Fukui(-)
Fukui(+)
0.023
0.029
0.004
0.005
0.012
0.027
0.015
0.022
0.022
0.031
0.057
0.023
0.029
0.004
0.005
0.012
0.027
0.015
0.022
0.022
0.031
0.057
0.163
0.023
0.029
0.004
0.005
0.012
0.027
0.015
0.022
0.022
0.031
Fukui(+)
0.030
0.029
0.015
0.014
0.013
0.021
0.033
0.040
0.032
0.061
0.017
0.030
0.029
0.015
0.014
0.013
0.021
0.033
0.040
0.032
0.061
0.017
0.034
0.013
0.008
0.003
0.008
0.002
0.013
0.008
0.003
0.008
0.030
C ( 1)
C ( 2)
C ( 3)
C ( 4)
C ( 5)
C ( 6)
N ( 7)
C ( 8)
C ( 9)
C ( 10)
O ( 11)
C ( 12)
C ( 13)
C ( 14)
C ( 15)
C ( 16)
C ( 17)
N ( 18)
C ( 19)
C ( 20)
C ( 21)
O ( 22)
X ( 23)
C ( 24)
S ( 25)
C ( 26)
C ( 27)
O ( 28)
C ( 29)
S ( 30)
C ( 31)
C ( 32)
O ( 33)
0.023
0.029
0.004
0.005
0.012
0.027
0.017
0.021
0.022
0.030
0.061
0.023
0.029
0.004
0.005
0.012
0.027
0.017
0.021
0.022
0.030
0.061
0.154
0.003
0.017
0.008
0.004
0.007
0.003
0.017
0.008
0.004
0.007
0.024
0.046
0.024
0.012
0.012
0.050
0.004
0.022
0.024
0.022
0.058
0.024
0.046
0.024
0.012
0.012
0.050
0.004
0.022
0.024
0.022
0.058
0.015
0.026
0.009
0.004
0.008
0.005
0.026
0.009
0.004
0.008
0.024
2
7

Fig. 11. Atom-condensed Fukui functions of Q-Cu and Q-Zn complexes calculated at
DFT/GGA/DNP using Material studio 2016.
3
.4. MD simulation
The molecular dynamics (MD) study was used to examine interactions at the interface
between Q-Cu or Q-Zn and the contact surface Fe (110) [74]. In addition, based on the study
of these interactions occurring at the interface, we have endeavored to explain the
phenomenon of the adsorption of the complexes studied on the metallic surface [75]. Figure
1
2 showed the side/top views of the adsorption of the two inhibitory complexes on the first
iron layer relative to the iron plane (110). Visual analysis of the images depicted in Figure 12
2
8

showed that the two Q-Cu and Q-Zn complexes adsorb on the iron surface in the same
manner such that these two species are situated above Fe (110), which results in a large
coverage of the metallic surface and thus has a high degree of corrosion inhibition
performance [76]. In addition, it is evident that the copper element is very close to the iron
atoms of the first layer than the zinc element, which shows that Q-Cu inhibits corrosion better
than Q-Zn. When the systems studied are in the most stable state, the energy values of the
interaction energy (Einteraction) and binding energy (Ebinding) can be extracted from these values
and the inhibitory capacity of each complex against corrosion can be clarified. The simulation
results showed that the interaction (binding) energy values of Q-Cu and Q-Zn are -229.127
(229.127) and -214.453 (214.453) kJ/mol, respectively. The more negative value of Einteraction
and the more positive value of Ebinding for Q-Cu inform that this complex reacts more strongly
with the Fe (110) surface than Q-Zn [77]. This result is possibly proven that the copper atom
interacts more effectively with iron atoms, indicating a better corrosion inhibitory
performance. All these simulation results were in parallel with the experimentally-found order
of the inhibitory efficacy for the studied complexes. The adsorption of Q-Cu and Q-Zn on Fe
(110) was reinforced by inhibitor-Fe (110) interatomic bonds [78]. This phenomenon has been
studied using the radial distribution function (RDF) method [78]. Figure 13 shows the
variation of g(r) as a function of the atomic radius (inhibitor-Fe (110)) of the most active
atoms (N7, O11, N18, O22, Cu and Zn) of the mentioned complexes and the atoms of the first
iron layer. The values of bond lengths which were obtained from the most intense first peak
for each spectrum, were noted in Figure 13. All these values were included in the
chemisorption range (1 - 3.5 Å). As shown in Figure 13, it was very noticeable that the
distance of Cu-Fe is smaller than that of Zn-Fe, which shows Q-Cu should adsorb more
strongly on Fe (110) surface than Q-Zn. This behavior might promote the inhibitory efficacy
of Q-Cu.
2
9