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j.matchemphys.2018.08.041

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Accepted Manuscript
Adsorption and performance assessment of some imine derivatives as mild
steel corrosion inhibitors in 1.0�M HCl solution by chemical, electrochemical and
computational methods
Emad A. Badr, M.A. Bedair, Samy M. Shaban
PII:
S0254-0584(18)30717-X
DOI:
10.1016/j.matchemphys.2018.08.041
Reference:
MAC 20884
To appear in:
Materials Chemistry and Physics
Received Date: 25 January 2018
Revised Date:
14 May 2018
Accepted Date: 19 August 2018
Please cite this article as: E.A. Badr, M.A. Bedair, S.M. Shaban, Adsorption and performance
assessment of some imine derivatives as mild steel corrosion inhibitors in 1.0�M HCl solution by
chemical, electrochemical and computational methods, Materials Chemistry and Physics (2018), doi:
10.1016/j.matchemphys.2018.08.041.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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Adsorption and performance assessment of some imine
derivatives as mild steel corrosion inhibitors in 1.0 M HCl
solution by chemical, electrochemical and computational
methods.
Emad A. Badr (a), M.A. Bedair (b,*) and Samy M. Shaban (a)
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(a) Petrochemical Department, Egyptian Petroleum Research Institute, Egypt.
(b) Chemistry Department, Faculty of Science (Men’s Campus), Al-Azhar University, 11884,
Egypt
Abstract:
This study provided a thermodynamic study of the adsorption process and the corrosion
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inhibition performance of three cationic surfactant based on imine surfactant corrosion inhibitor
in 1.0 M HCl solution. The results showed that the tested imine surfactants inhibitors are
promising because of their high inhibition efficiency at low concentrations and high
temperatures. According to the obtained results, the adsorption behavior of the surfactant imine
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inhibitors onto the mild steel surface follow Langmuir model isotherm. The potentiodyanamic
polarization parameters showed that the surfactant imine inhibitors act as mixed type inhibitors.
A reduction in the double layer capacitance and an increase in the charge transfer resistance are
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occurring as a result for the imine inhibitor adsorption. DFT, DMol3 and molecular dynamic
simulations were also used for investigation of inhibitive and adsorption properties for the
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studied compounds.
Key words: imine cationic surfactants; Tafel Polarization; DFT; Electrochemical Impedance
Spectroscopy; SEM; Molecular Dynamics.
(*) Corresponding author Tel.: +20 1014134321.
E-mail address: m_bedier@yahoo.com, m_bedier@hotmail.com
dr.samyshaban@yahoo.com (Samy M.Shaban)
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(M. A. Bedair) and
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1. Introduction
Mild steel considers the essential constructing material for vast of industries, so its protection
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from corrosion presents an essential case to save the safety, money and time. [1]. In spite of the
hydrochloric acid cause severe corrosion, it's used in many jobs like acid pickling, acid
descaling, oil well acidizing and acid cleaning, consequently these jobs must be accompanied
with an effective corrosion inhibitors for protecting the infrastructure [2-4]. The chemical
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corrosion inhibitor materials consider the most effective and economical method for controlling
the metal dissolution, especially in petroleum production processes for protecting the pipeline,
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tanks, shipping pipeline lines, vessels, etc. [5, 6]. The chemical corrosion inhibitors are the
materials which can be injected with the fluid phase in low concentration to achieve a good
protection for the main constructing materials. The effective corrosion inhibitor should have high
adsorption ability on the corroded surface which can be achieved electronic rich function groups
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like heteroatoms (oxygen, sulfur, nitrogen, phosphorous and so….), unsaturated bonds and
aromatic system, etc. which can form an intense electronic cloud [7, 8]. The Schiff base
corrosion inhibitor compounds proved high efficiency in acidic medium, where nitrogen atom is
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capable to form coordinate-covalent bonds with metal surface by its unshared lone pairs of
electrons; While the π-bond interact physically enhancing their adsorption affinity to the metal
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surface [9-11]. The surfactant compounds are characterized by their high ability to adsorb at the
interfaces due to their unique amphipathic structure, which consist of two opposing parts head
and tail [12, 13]. The head is characterized by its high electronic rich, which is about set of some
electronic rich functional group like carbonyl, double, imine group, nitrogen and oxygen, etc…,
[14, 15]. A successive layer on the mild steel surface can be produced by the surfactant
hydrocarbon which has the ability to isolate the corroded surface from the contact with
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aggressive environments [16-18]. In addition the cationic surfactant is characterized by its
biocidal activity which enhance their ability in petroleum sector [19].
This research focused on preparing and evaluating the inhibition efficiency of the three imine
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compounds in the surfactant structure on the mild steel corrosion and using the 1 M HCl solution
as an aggressive medium via weight loss tests, electrochemical experiments (polarization and
impedance spectroscopy). The thermodynamic parameters governing the adsorption of inhibitor
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on the metal steel surface and the mechanism of adsorption were determined. Also, we are going
to explain the inhibiting behavior of the studied compounds by means of DFT, DMol3 and
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molecular dynamic computational methods.
2. Materials and Experimental Methods
2.1. Preparation of corrosion inhibitors
The amphipathic inhibitors were prepared as follows:
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2.1.1. Preparation of Schiff-base based on dimethoxybenzaldhyde:
The Schiff base compound has been prepared through a condensation reaction between the 3, 4dimethoxybenzaldehyde and 2-dimethylamino ethylamine. In a typical procedure, 0.05 mole (4.4
ethylamine
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gm) from 2-dimethylamino
was mixed
with
0.05
mole from 3, 4-
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dimethoxybenzaldehyde (8.3 gm) in a round flask 250 ml containing 80 ml absolute ethanol as a
solvent. The reaction mixture was heated to reflux for 6 hrs. The solvent was evaporated and the
obtained Schiff base was purified and crystallized using diethyl ether [20].
2.1.2. Preparation of cationic Schiff amphipathic corrosion inhibitor
Equimolar from the prepared Schiff base (0.01 mol, 2.36 gm) from the first step was refluxed
with equimolar from Octyl bromide (0.01 mol, 1.93 gm), Dodecyl bromide (0.01 mol, 2.49 gm)
and Hexadecyl bromide (0.01 mol, 3.05 gm) separately in a round flask containing 90 ml from
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the absolute ethanol as a solvent for 30 hours. The product was purified and recrystallized from
diethyl ether, then filtrated and dried to obtain the synthesized cationic amphipathic inhibitors as
clarified in Fig 1 [21]. The prepared three amphipathic inhibitors named N-(2-(3,4bromide,
N-(2-(3,4-
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dimethoxybenzylideneamino)ethyl)-N,N-dimethyloctan-1-ammonium
dimethoxybenzylideneamino)ethyl)-N,N-dimethyldodecan-1-ammonium bromide and N-(2-(3,4-
labeled as DMAOB, DMADB and DMAHB respectively.
2.2. Solutions and Mild Steel
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dimethoxybenzylideneamino)ethyl)-N,N-dimethylhexadecan-1-ammonium bromide which are
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The used aggressive solution is 1.0 M HCl, which prepared by dilution of concentrated
hydrochloric acid 37% using distilled water. The different concentrations ranged from (5x10-7 to
5x10-3 M) from the prepared amphipathic inhibitors (DMAOB, DMADB and DMAHB
individually) were prepared. Stock solution 5x10-3 M from the surfactant were prepared using
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1.0 M HCl, then diluting it with 1.0 M HCl to obtain on the required doses. The tested mild steel
was immersed in the prepared solutions. The used mild steel used in gravimetrical experiment
and in electrochemical measurements is mild steel with a chemical composition (weight % ) C
and the rest is Fe.
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(0.23%), Cr (0.05), Mo (0.01%), Mn (1.35%), P (0.017%), Ni (0.02%), Si (0.22%), S (0.01%),
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2.3. Corrosion Measurements
The weight loss, electrochemical impedance spectroscopy and potentiodynamic polarization
have been used for determining the ability of the synthesized Schiff base surfactant DMAOB,
DMADB and DMAHB to inhibit corrosion of the mild steel in 1.0 M HCl.
2.3.1. Weight Loss Experiment
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All the mild steel coupons which have been used during the weight loss experiments have
the dimension of 6.0 cm x 3.0 cm x 0.4 cm. Each coupon was abraded with emery papers with
different grades (400, 600, 1000 and 1200), then degreased with acetone before immersing in the
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specified solution. The experiment duration was 24 hours at the four different temperatures,
which are 25, 40, 55 and 70oC using water bath of 0.2 oC accuracy. An analytical balance has
been used for weighting the coupons before and after the experiment. Each experiment was
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2.3.2. Electrochemical Measurements
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repeated and the average value was considered in the calculations [22, 23].
The Tafel and Nyquist curves have been obtained electrically at 25 ᴼC using voltalab 40
potentiostat PGZ 301. A three-electrode cell consisting from the working electrode (WE),
platinum electrode, and saturated calomel reference electrode (SCE) has been used. The WE was
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treated as it is outlined in the coupons, which used in weight loss and was left for 30 minutes in
the tested solution before starting the experiment. The polarization experiment was conducted
from -800 to -200 mV with scanting rate 0.2 mVs-1 [24, 25]. The electrochemical impedance
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measurements (Nyquist curves) have been carried out by changing the frequency from 100 kHz
to 50 mHz with amplitude of 5 mV peak-to-peak [26]. The exposed corroded area from the WE
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is 0.7 cm2.
2.4. Scanning electron microscopy (SEM)
The surface morphology has been examined after 24 h immersion of steel samples in the
tested aggressive medium (1.0 M HCl) with and without the 5x10-3 M concentration of the tested
Schiff base surfactant inhibitors using SEM techniques. The model of the instrument used for
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scanning the samples is Quanta 250 FEG (Field Emission Gun) attached with EDX Unit (Energy
Dispersive X-ray Analyses), magnification14x up to 1000000, accelerating voltage 30 K.V.
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2.5. Computational calculations
The computational calculations were performed using Gaussian 09 [27] at DFT/B3LYP level
using 6-31G + (d,p) basis set [28] and Materials Studio software version 7.0 (Accelrys Inc. USA)
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at DMol3 [29]. The calculations were done on the cationic surfactants in gas phase and an
aqueous phase. The self-consistent reaction field (SCRF) theory, with Tomasi’s polarized
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continuum model (PCM) was applied to perform DFT/B3LYP - 6-31G + (d,p) computations in
solution. The effect of solvent was considered in DMol3 calculations by including COSMO
controls. In order to study the adsorption of the cationic surfactants on steel surface, molecular
dynamic simulations were carried out. The Fe crystal was imported and cleaved along (110)
plane and a slab of 15 Å was employed. The simulation box (22.90 Å×57.26 Å ×26.68 Å) and
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COMPASS force field was used [30].
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3. Results and Discussions:
3.1. Characterization the Structure of the Synthesized Schiff base Inhibitor:
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The Chemical Structure of the synthesized Schiff base amphipathic inhibitors has been
confirmed using FTIR and 1HNMR spectroscopy, as it is outlined in Supplement Figs. 1, 2.
Supplement Fig. 1, represents the FTIR spectrum of the synthesized DMADB amphipathic
inhibitor as a representative example, which show the appearance of a new azomethine group at
1640 cm-1 as a result of condensation reaction between the aldehyde and the amine ( carbonyl
group and amino groups were disappeared). Some other bands are present confirming the
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chemical structure of the synthesized amphipathic inhibitors like 2853.2, 2923, 1513, 1139 and
1023 cm-1 which refer to asymmetrical aliphatic CH, symmetrical aliphatic CH, aromatic C=C,
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C-O and C-N bond respectively.
Supplement Fig. 2 represents the 1H-NMR spectrum data (δ ppm) of inhibitor DMADB as an
example showing signals at: δ= 0.82 (t,3H, ‫ ـ‬CH3); δ= 1.21 (m ,18H,‫( ـ‬CH2)9CH3); δ=1.61
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(m,2H, -CH2(CH2)9CH3); δ=2.47 (s,6H, -CH2N (CH3)2CH2-); δ=2.8 (t,2H,CH3(CH2)9CH2CH2N (CH3)2CH2-); δ=3.07 (t,2H,CH3(CH2)9CH2-CH2N (CH3)2CH2-CH2N=); δ=3.35 (s,6H,
δ=7- 7.49 (m,3H, CH2N=CH-
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=CH-Ph(OCH3)2); δ=3.77 (t,2H,-CH2N=CH-Ph(OCH3)2);
Ph(OCH3)2); δ=8.34 (s,1H, -N=CH -Ph(OCH3)2).
3.2. Weight loss measurements
3.2.1. Concentration influence
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The corrosion rate (k), and the corrosion inhibition efficiencies (ηw) of the synthesized imine
(Schiff base) surfactant inhibitors DMAOB , DMADB and DMAHB after 24 h of immersion at
25, 40, 55 and 70 ºC has been evaluated by the weight loss technique and calculated using
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equations 1 and 2 respectively [31, 32]. The experimental data have been listed in Table 1.
(1)
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=
Where the expression ∆W correspond to the weight loss average of three parallel mild steel
coupons immersed in protected or unprotected solution with different concentration of the imine
amphipathic inhibitor, S corresponds to the total exposed area of the working electrode and
finally t refer to the immersing time by hours.
=
× 100 =
ᴼ
ᴼ
× 100
(2)
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Where Wo and W represent the weight loss in the unprotected and protected mild steel coupon
respectively.
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The experimental weight loss ∆W, and corrosion rate (k) values which depicted in Table 1,
decrease with increasing the concentration of the imine surfactant derivatives DMAOB ,
DMADB and DMAHB, while inhibition efficiency and surface coverage were increased as it is
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obvious in Fig. 2. This might be owing to increasing the availability of electron donors (N), (O)
and aromatic rings by increasing the concentration of inhibitor. Imine derivatives showed
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maximum inhibition efficiencies in 5.0 mM of DMAOB, DMADB and DMAHB inhibitors.
Increasing the concentration of DMAOB, DMADB and DMAHB inhibitors lead to increasing
the migration rate of the surfactant to the interface which permitting a higher accumulation of
the prepared imine inhibitors at the interface. The electronic rich functional group located in the
hydrophilic heads of DMAOB, DMADB and DMAHB are responsible for forming a
35].
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coordination bond with the vacant d-orbital through -C=N-, Nitrogen, and oxygen groups [33-
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3.2.2. Temperature effect
It is obvious from the dataset in Table 1 that the corrosion rate (k) increases with increasing the
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solution temperature from 25 to 70 ᴼC. This can be ascribed to increasing the transfer rate of the
aggressive ions with elevating the solution temperature. Fig.2. and Table 1, showed that the
values of inhibition efficiency (ηw) (%), for each DMAOB , DMADB and DMAHB inhibitors
increase with raising the temperature. Which give more insight for strong adsorption of the imine
inhibitors on the mild steel surface which, enhanced by increasing the temperature, where some
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chemical change occur, lead to forming a stronger bond between the DMAOB , DMADB and
DMAHB inhibitors and the vacant d-orbital. [36, 37].
3.3.1.
Potentiodynamic polarization measurements:
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3.3. Electrochemical measurements
The obtained polarization curves for mild steel in 1.0 M HCl in the absence and presence of
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the synthesized imine surfactant inhibitors DMAOB, DMADB and DMAHB are shown in Fig.3
at 25 ᴼC. The addition of the synthesized imine surfactant inhibitors shifts the corrosion current
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density to lower values. This trend refers to decreasing the metal dissolution and retardation the
hydrogen evolution as a result for the covering the corroded surface with the used DMAOB ,
DMADB and
DMAHB
imine surfactant inhibitors. The Polarization electrochemical
parameters Ecorr (corrosion potential), Icorr (corrosion current density), βa and βc (anodic and
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cathodic Tafel slopes), have been obtained by extrapolation of the obtained experimental tafel
curves and were depicted in Table 2. The inhibition efficiency (
) of DMAOB , DMADB and
DMAHB imine surfactants acquired according to Eq. (3)
× 100 =
°
–
°
× 100
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% =
(3)
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Where iocorr and icorr are the corrosion current density of the uninhibited and inhibited mild steel
with the DMAOB , DMADB and
DMAHB imine surfactants inhibitors respectively. The
experimental depicted data set in Table 2, refer to decreasing the icorr values with increasing the
concentration of DMAOB, DMADB and DMAHB imine surfactants Fig. 3, hence the ηp
increases Table 2. The decreasing icorr, give an insight on increasing the electrode surface
covered area by DMAOB, DMADB and DMAHB imine inhibitors forming a protective layer.
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Increasing the surfactant concentration, enhance their adsorption affinity to the interfaces and
hence the covered area on the corroded mild steel is increasing.
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The negative shift in the corrosion potential of the three DMAOB, DMADB and DMAHB imine
surfactants, not exceed 85 mV (vs. SCE), predicting that the three DMAOB, DMADB and
DMAHB imine surfactants act as mixed-type inhibitors with a predominant cathodic inhibitor,
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i.e the synthesized cationic surfactant reduce the anodic dissolution of mild steel and at the same
time it retards the cathodic hydrogen evolution reaction but the effect of DMAOB, DMADB and
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DMAHB surfactant on the cathodic hydrogen evolution reaction is more. The slight shift in the
depicted anodic (βa) and cathodic (βc) Tafel in Table 2, refers to the DMAOB, DMADB and
DMAHB imine surfactant inhibitors have an effect on the corrosion rate without changing the
inhibition mechanism [38, 39].
Electrochemical Impedance Spectroscopy (EIS):
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3.3.2.
Fig. 4 outlines the Nyquist plots for the used mild steel immersed in 1.0 M HCl containing
different concentrations from DMAOB, DMADB and DMAHB imine surfactants inhibitors at 25
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ᴼC. The equivalent circuit model used for fitting the obtained Nyquist plots is also presented in
Fig. 4. The Fig. 4 outline increasing the radii of the obtained semi-circle with the plethora of the
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DMAOB, DMADB and DMAHB imine inhibitor concentration as a result of increasing the
protected surface area of the mild steel by the tested imine which act as a barrier between the
mild steel and the aggressive medium, hence the corresponding inhibition efficiency increase
[40, 41]. Double-layer capacitance (Cdl) values were calculated using the equation (4)
Cdl = (Y0 Rct1-n)1/n
(4)
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Where Rct represent the charge transfer resistance, Y0 is CPE constant and n is CPE exponent.
The experimental inhibition efficiency for DMAOB, DMADB and DMAHB imine inhibitors (ηz)
have been calculated depending on the obtained Rct values according to equation (5) as follows
=
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[42]:
100
(5)
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Roct and Rct represent the charge transfer resistance of protected and unprotected solution with
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the synthesized DMAOB, DMADB and DMAHB imine inhibitors, respectively.
The obtained electrochemical corrosion kinetic parameters Rct, Cdl and ηz (%)from the Nyquist
plot were depicted in Table 3. The depicted values in the Table 3, declare that upon increasing
the inhibitor concentration, the Rct increases while the corresponding (Cdl) decreases which lead
to increasing the percentage inhibition efficiency. Decreasing Cdl with the plethora of the
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DMAOB, DMADB and DMAHB concentration may be ascribed to decreasing the local
dielectric constant and/or increasing the thickness of the formed electrical double layer by the
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imine surfactants inhibitors, which reflect their high adsorption ability on the metal/solution [42].
Both Bode and phase angle plots of the synthesized imine surfactant inhibitors (DMAOB,
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DMADB and DMAHB) are plotted in Fig. 5. The slopes of Bode impedance magnitude at an
intermediate frequencies log |Z| vs. log f (S) and maximum phase angle (α°) values were
depicted in the Table 3. The slopes of the Bode impedance magnitude at intermediate
frequencies (S) and maximum phase angle values (α°) for an ideal capacitive behavior are −1 and
−90° respectively. Inspection of the data revealed that the slopes are not equal to -1, which may
be a result of frequency dispersion of interfacial impedance. It’s also clear that slopes (S) and
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maximum phase angle values shifted toward −1 and −90° in the presence of the cationic
surfactants as an indication of the capacitive behavior at intermediate frequencies.
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3.4. Thermodynamic activation study:
The temperature increasing has a great influence on the electrochemical processes rate, the
adsorption equilibrium and kinetics. These thermodynamic activation functions were obtained
$%
&
(6)
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= exp
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throughout Arrhenius and the transition-state equations (6, 7):
Where R and A are the universal gas constant and the Arrhenius pre-exponential factor,
respectively, while k is the experimental weight loss corrosion rate as depicted in Table 1 and T
correspond to the absolute temperature (298, 313, 328, and 343 K). Fig. 6 exhibits the linear
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relationship between log k of the synthesized DMAOB, DMADB and DMAHB imine inhibitor
versus 1/T. The obtained straight lines are very close to 1, as an indication of following the
Arrhenius equation and from the slope the apparent activation energy is obtained. The calculated
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activation energy values (Ea) are collected in Table 4, where the values of Ea of the three imine
inhibitors DMAOB, DMADB and DMAHB were lower than blank. For example, the apparent
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activation energies for the DMAOB, DMADB and DMAHB inhibitors at 5x10-3 M are equal to
30.38, 26.68 and 24.9 kJ/mol respectively, while the corresponding activation energy for the
unprotected solution was equal to 43,14 kJ/mol. The lower values of Ea in the presence of the
imine inhibitors (Table 4) can be interpreted as an indication of chemical adsorption [44, 45].
The activation standard enthalpy of (∆H*) and the activation entropy (∆S*) were calculated
according to the transition state equation:
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)
ln = *ln
&
+, -
∆ ∗
+
1−
∆3 ∗
&
(7)
Where h corresponds to Planck’s constant, R to the ideal gas constant and NA refers to
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Avogadro's number. The calculated ∆H* and ∆S* values were calculated according to the
previous equation (7) and depicted in Table 4. Supplement Fig. 3 represents a relation between
log k/T versus reciprocal of the absolute temperature of tested mild steel immersed in
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unprotected and protected aggressive solution with various dosages from the synthesized
amphipathic imine inhibitors DMAOB, DMADB and DMAHB respectively. The obtained slope
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equal to (-∆ H*/2.303R) and the intersection is equal to [(log R/Nh) + (∆S*/2.303R)]. The
positive signs of (∆H*) infer to the process of the mild steel dissolution is endothermic, which
imply the difficulty of the mild steel corrosion in the protected solution with DMAOB, DMADB
and DMAHB amphipathic inhibitor. The ∆S* values in case of unprotected and protected mild
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steel synthesized imine inhibitors are negative (Table 4), which referring to more ordering are
occurring, ongoing from reactant species to activated complex [46, 47].
3.5. Adsorption isotherm model
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The adsorption isotherm calculations performed to present description about the inhibition
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mechanism throughout the adsorption nature of the synthesized imine derivatives DMAOB,
DMADB and DMAHB on the mild steel surface at the four tested temperatures. The
experimental weight loss surface coverage (θ) values (Table 1) have been considered fitting the
different adsorption isotherm models. The most fitted adsorption isotherm was Langmuir
isotherm through equation 8:
4
5
=7
6
89:
+;
(8)
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Where Kads is the adsorption equilibrium constant while C refer to the inhibitor concentration.
Fig. 7 present a relation between C/θ versus C for the prepared imine inhibitors DMAOB,
DMADB and DMAHB. The obtained correlation factors (R2) for the obtained straight lines are
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very close to 1 as an indication for obey the Langmuir equation 8. The obtained slopes, which
depicted in Table 5 are greater than 1 indicating that each imine inhibitor DMAOB, DMADB
and DMAHB occupy more than one adsorption center on the mild steel surface which is not
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matching with the postulates of the Langmuir theory. Consequently, the adsorption of the
synthesized DMAOB, DMADB and DMAHB imine inhibitors can be expressed through the new
4
5
=
<
789:
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modified Langmuir isotherm (Villamil isotherm), as described in equation 9 [22, 49]:
+ =;
(9)
The slope is equal to n which resemble to the number of the adsorbed water molecules displaced
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from the corroded mild steel surface. The equilibrium constant Kads for the DMAOB, DMADB
and DMAHB imine inhibitors at the four tested temperatures 25, 40, 55 and 70oC have been
determined from the intercept. The adsorption heat (∆Hoads) was calculated according to the van't
389:
&
+ CD=EFG=F
(10)
AC
C
>=?@AB =
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Hoff equation 10:
The calculated standard free energy of adsorption (∆Goads) and standard adsorption entropy
(∆Soads) for DMAOB, DMADB and DMAHB imine inhibitors were calculated throughout the
equations 11 and 12:
J
HI@AB
= −KL>=55.5?@AB
(11)
The value of 55.5 is the water molar concentration in the solution.
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J
HO@AB
=
P
389:
&
P
Q89:
(12)
All the calculated thermodynamic parameters ∆Goads, ∆Hoads and ∆Soads have been depicted in
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Table 5. The ∆Goads values are negative referring that the adsorption of DMAOB, DMADB and
DMAHB imine inhibitor on the metal surface is a spontaneous in nature. On analyzing the data
in Table 5, we disclose that the adsorption of DMAOB, DMADB and DMAHB on the tested
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mild steel surface is a mixture between physical and chemical as outlined from the ∆Goads values
which ranged from -37.08 to -46.17 kJ mol−1. In presence of DMAHB imine inhibitor, the ∆Goads
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value is equal to 46.17 kJ mol-1 at temperature 70 ᴼC. These findings may confirm that the upon
raising the solution temperature, some chemical change takes place in the imine inhibitor in 1.0
M HCl solution leading to a typical of chemical adsorption on the mild steel surface [49, 50].
The ∆Hᴼads values of DMAOB, DMADB and DMAHB imine inhibitors depicted in Table 5, is
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positive referring that their adsorption on the mild steel surface is an endothermic process. While
their ∆Soads values depicted Table 5 carry a positive sign, referring that the endothermic
adsorption process is accompanied by an increase of entropy, which controls the DMAOB,
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DMADB and DMAHB imine adsorption on the mild steel surface [51].
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3.6. Scanning electron microscopy
The surface morphology of the mild steel samples rinsed in 1.0M HCl solutions unprotected and
protected with 5x10-3 M from the DMAOB, DMADB and DMAHB imine inhibitors at the 25 ᴼC
are shown in Fig .8. We can notice that the surface morphology of the steel sample kept
immersed in 1.0 M HCl solution for 24 h without adding any inhibitor was extremely damaged
as a result of highly significant corrosion. On the contrary, in case of the protected mild steel
with DMAOB, DMADB and DMAHB imine inhibitors, the mild steel surface becomes
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smoother, reflecting a considerable prevention of the corrosion rate. This improvement in the
surface morphology is a result of the formation of a good protective layer from the DMAOB,
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DMADB and DMAHB inhibitor on the mild steel surface.
The results of EDX spectra in Fig .8 shows that the abraded mild steel surface immersed in HCl
without adding any inhibitor presents the characteristic peaks of the elements constituting mild
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mild steel sample and the corrosive medium (Fe, O and Cl) ascribed to general corrosion in
hydrochloric acid. While, for solutions containing surfactant inhibitors, the EDX spectra showed
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an additional peak characteristic for the existence of (N) and absence of (Cl) peak. This data
indicates that surfactant inhibitors (DMAOB, DMADB and DMAHB) have the ability to adsorb
on the mild steel surface with percentage contents listed in Table 6.
3.7. Computational calculations
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Geometric and electronic structures of the cationic surfactants in the gas phase and aqueous
phase are calculated by the optimization all structure parameters; bond lengths, bond angles and
dihedral angles to the lowest energy. The optimization energy curves by DMol3 method are
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shown in Supplement Fig. 4. The optimized structures, frontier molecular orbitals (HOMO and
LUMO), electrostatic potential and Mulliken atomic charges obtained from DFT calculations in
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the gas phase are shown in Fig .9. The structure parameters obtained from the calculation are
listed in Table 7.
3.7.1. Atomic bond distance
The calculations of geometrical structures of the cationic surfactants using DFT/ B3LYP/6-31+G
(d,p) showed that the bond distance within the aromatic ring moiety vary from 1.38 to 1.42 Å
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which is characteristic for conjugated system . For the substituted two methoxy groups on ring,
the O-Ar bond distance 1.35- 1.37 Ȧ and O-CH3 bond distance 1.43- 1.44 Ȧ. The two methyl
groups attached to the nitrogen atom that bears positive charge, the N-CH3 bond distance 1.50 –
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1.52 Ȧ (sp3 hybridization). The azo methane group N=CH bond showed a short bond distance
1.27 - 1.28 Å (formation of a double bond, sp2 hybridization). The aliphatic hydrocarbon chain
showed the longest bond length with values equal to 1.52- 1. 54 Å (saturated C-C single bond,
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sp3 hybridization).
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3.7.2. Atomic charges
Electric charges in the cationic surfactants are responsible for the polarity of the molecules and
so electrostatic interactions with metal surface. The electronic charges are important for
physicochemical properties of the cationic surfactants and its intermolecular interactions. It’s
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well known that the more negative the calculated charge, the greater is the ability of the cationic
surfactant molecule to donate its electrons to mild steel surface and so the its inhibition
efficiency increased. There are many methods for calculating the partial atomic charges like
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Mulliken and natural population analysis.
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3.7.2.1. Mulliken population analysis
The calculated Mulliken charges prove the presence of more than one active center [52]. The
data showed that the tetravalent N atom has a high negative charge in all cationic surfactants
with values equal to -0.546, -0.548 and -0.546 e for DMAOB, DMADB and DMAHB
respectively, using DFT/B3LYP/6-31G+ (d,p) method due to the electron donating ability of the
two adjacent methyl groups. It is also found that two O atoms have a high negative charge with
values from -0.36 to -0.37 e, using DFT/B3LYP/6-31G+(d,p) method and with values from 17
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0.42 to -0.45 e using DMol3 method. The N atom of azo methane group has also high negative
charge with values -0.28 e, in Dmol3 method. The presence of negative charges on carbon atoms
of the aromatic ring moiety and delocalization of electrons is good evidence that the conjugating
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system of the aromatic ring has the ability to donate π electrons to the mild steel surface, acting
also as an active center for the adsorption process. The total negative charge on the cationic
surfactants was calculated. The data show that TNC has values -6.20, -7.277 and -8.424 for
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DMAOB, DMADB and DMAHB respectively in the gas phase and values -6.248, -7.306 and 8.426 for DMAOB, DMADB and DMAHB respectively in the aqueous phase using
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DFT/B3LYP/6-31G+(d,p) method. From the view of TNC the order of inhibition efficiency will
be DMAHB > DMADB > DMAOB which is the same ranking obtained from experimental
results.
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3.7.2.2. Natural Bonding Orbital
NBO analysis is done to provide information about the electron distribution of the molecular
orbitals, electron delocalization within the molecule and atomic charge distribution. According to
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NBO analysis for DMAOB molecule as an example, the -CH=N- bonding orbital has σ bond
occupancy of 1.98938 electrons resulting from the overlap of sp1.98 (33.57% s, 66.33% p, 0.10%
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d) hybrid on C with sp1.44 (40.95% s, 58.99% p, 0.06% d) hybrid on N atom and π bond
occupancy of 1.94600 electrons resulting from the overlap of sp
99.99 14.74
d
(0.01 % s, 99.80 % p,
0.18 % d) hybrid on C with sp99.99d7.65 (0.02% s, 99.83% p, 0.15% d) hybrid on N atom. The
atomic charge distributions for the cationic surfactant structures were examined through natural
population analysis (NPA) as a result of NBO calculation [53]. The data of NPA charges showed
that hetero atoms (two N atoms and two O atoms), carbon atoms of the aromatic ring and
aliphatic chain carbon atoms bears negative charges. O atom exhibit the maximum negative
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charge value (-0.56 e) but the tetravalent N atom exhibit the minimum negative charge value (0.34 e). The N atom of azo methane group has value from -0.52 to -0.54 e and the aliphatic side
chain carbon atoms have value of – 0.46 e. TNC by NBO method has been also calculated and
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showed the values of -8.386, -10.734 and -12.678 for DMAOB, DMADB and DMAHB
respectively in the gas phase and values of -8.893, -10.759 and -12.707 for DMAOB, DMADB
and DMAHB respectively in the aqueous phase using DFT/B3LYP/6-31G+(d,p). It’s clear that
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TNC increases by the same order of increasing inhibition efficiency.
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3.7.2.3. Active sites by Fukui analysis
Local reactivity, structure–activity relationship of the cationic surfactant molecules with the mild
steel surface can be understood by means of condensed Fukui functions (CFF) [54]. It’s useful
for prediction the inhibitor atomic sites on which nucleophilic and electrophilic interaction can
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be occurred. The nucleophilic f + and electrophilic f - Fukui indices were calculated using the
finite difference approximation as follows [55].
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f + = q (N + 1) – q (N)
f - = q (N) – q (N - 1)
(13)
(14)
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Where q (N + 1), q (N) and q (N – 1) represent charge values of atom for anion, neutral, and
cation, respectively. The data of the Fukui indices for nucleophilic and electrophilic sites of
DMAOB, DMADB and DMAHB are listed in Table 8. In case of the electrophilic attack the
highest values are observed in O7 (0.115, 0.110 and 0.100), O8 (0.059, 0.062 and 0.066), N12
(0.050, 0.058 and 0.064) and the C atoms of the aromatic ring, which are the most which are the
most reactive sites for the investigated cationic surfactants. The presences of the methyl groups
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adjacent to O atoms in these cationic surfactants enhance their electron donation to the mild steel
surface. The most susceptible sites for nucleophilic attack are present in C11 (0.088, 0.119 and
0.125), C10 (0.120 for DMAOB), and N12 (0.085 and 0.079 for DMADB and DMAHB
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respectively).
3.7.3. Global reactivity descriptors
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It’s known that chemical interactions are either electrostatic (polar) or orbital (covalent).
Forming of any chemical bond is due to transition of electrons which is a result of interaction
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between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) of reacting species [56]. HOMO can be considered as the sites of donating electrons
while LUMO is the sites of accepting electrons between the inhibitors and metal surfaces.
Perfect inhibitors not only donate electrons to the empty d- orbitals of metal surface, but also can
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accept electrons from metal leading to the formation of a back donation bond. According to
previous results, the higher is the value of EHOMO of the inhibitor, the easier is its offering
electrons to the metal surface and the greater is its inhibition efficiency. On the other hand, the
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lower is the value of ELUMO of the inhibitor; the easier is its ability to accept electrons from the
metal surface and the greater is its inhibition efficiency [57]. The results from the Fig. 9 indicate
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that the HOMO and LUMO orbitals are largely distributed throughout the aromatic ring, the two
OCH3 groups and CH=N- group. According to data in Table 7, the calculated values of EHOMO
for the investigated cationic surfactants in gas and aqueous phases by DFT/B3LYP/6-31G+ (d,p)
and DMol3 methods showed that increasing values of EHOMO, are consistent with the same order
obtained from experimental results (DMAHB > DMADB > DMAOB). However, from the data
obtained for ELUMO in gas phase and aqueous phase showed that there is no clear relation
between the trend in the experimentally determined corrosion inhibition efficiency and ELUMO.
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The HOMO–LUMO separation energy gap is considered as an important stability factor and
function of reactivity of the investigated compounds towards adsorption on metal surface. Low
values of ∆E indicate high reactivity and so high inhibition efficiency. It was shown from Table
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7 in the aqueous phase that DMAHB has the lowest ∆E with values equal to 4.4896 and 3.2332
eV in DFT/B3LYP/6-31G+ (d,p) and DMol3 methods, respectively, which means that it is the
highest reactivity in the aqueous phase and accordingly the highest inhibition efficiency. It was
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also found that DMAOB has the highest ∆E in the aqueous phase with value equal to 4.5779 eV
in DFT/B3LYP/6-31+G(d,p) method, which means that it has the lowest reactivity and
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accordingly the lowest inhibition efficiency.
3.7.4. Electronegativity and global electrophilicty
The absolute electronegativity (χ) can be calculated from the energy of the HOMO and the
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LUMO by the following equations [58].
Ionization energy (I) = -EHOMO, Electron affinity (A) = -ELUMO
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(16)
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χ=
(15)
It’s well-known from Sanderson’s electronegativity equalization principle that the electrons will
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flow from molecules that had lower electronegativity values for that with higher
electronegativity values until the chemical potential becomes equalized [59]. So the perfect
inhibitor is the one that has low electronegativity and so higher values of the electronegativity
gap between metal and inhibitor, offering electrons to flow to metal surface. The calculated (χ)
data in Table 7 show that the values of electronegativity vary by the order DMAHB < DMADB
< DMAOB either in the gas phase or in an aqueous phase. The calculated values of
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electronegativity emphasize the same inhibition efficiency trend obtained from experimental
results.
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Electrophilicity (ω) can be defined as the ability of chemical molecules to gain electrons. It can
be calculated from chemical potential (µ) and global hardness (η) by the following equations
[60].
(17)
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VW
UX
(18)
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ω=
R T
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µ = - χ, η =
So a good electrophile possesses high values of ω and a good nucleophile possesses low values
of ω. Weak electrophile (strong nucleophile) will show high inhibition efficiency. Examination
of the calculated data of ω in Table 7 showed that, if ω was decisive for the inhibitor properties,
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the order of the investigated cationic surfactants should be DMAHB < DMADB < DMAOB ;(
ω). All The calculations either in gas phase or in aqueous phase indicate that DMAHB ( highest
inhibition efficiency) has the lowest ω in the all methods with values equal to 9.227, 3.452,
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13.747 and 4.981 eV, then DMADB ( ranks 2nd in inhibition efficiency) with values equal to
9.233, 3.459, 13.783 and 4.994 eV, and DMAOB ( ranks 3rd in inhibition efficiency ) has the
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highest ω with values equal to 9.248, 3.465, 14.021 and 5.008 eV in DFT/B3LYP/6-31+G (d,p)
gas phase, DFT/B3LYP/6-31+G (d,p) aqueous phase, DMol3 gas phase and DMol3 aqueous
phase methods, respectively.
3.7.5. Molecular volume and dipole moment
In the presence of adsorption centers, molecular volume is a very effective quantum factor that
contributes to inhibition efficiency. By increasing the molecular volume of the inhibitor
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molecules, the surface covered of metal increases and so inhibition efficiency. The calculated
values of molecular volume by DFT/B3LYP/6-31+G (d,p) are shown in Table 7. It’s clear from
the data that DMAHB has the highest volume by values 426.366 cm3/mol in gas phase and
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430.351cm3/mol in aqueous phase. The ranking of the investigated cationic surfactants according
to their molecular volume is the same one obtained from experimental inhibition efficiency
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(DMAHB > DMADB > DMAOB).
There’s a lack in literature either dipole moment (DM) can be utilized in estimating the corrosion
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inhibition properties of chemical species or not. Some research papers indicated that a clear
correlation between DM and inhibition efficiency is not found [61, 62]. On the other hand other
research papers indicated that corrosion inhibition efficiency can be stimulated by increasing the
value of DM [63, 64]. Our calculated data reveal that a higher value of DM corresponds with
higher inhibition efficiency. DMAHB has the highest value of DM (20.399 debye,
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DFT/B3LYP/6-31+G (d,p) in the gas phase) among the three investigated cationic surfactants,
while DMAOB that has the lowest inhibition efficiency, possess a DM value of 1.177 debye
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(DFT/B3LYP/6-31+G (d,p) in the gas phase).
3.7.6. Electron transfer electron back-donation
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We can calculate the fraction of electrons transferred from cationic surfactant molecules to the
mild steel surface using the equation [65];
∆N =
Z[\ Z]^_
(19)
U(a[\Sa]^_)
By application of the theoretical value, χFe = 7 eV/mol and value of η Fe = 0 eV/mol for iron
(steel). ∆N doesn’t give precise values express the actual number of electrons transferred but it
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expresses the tendency of the inhibitor molecules to provide the metal surface with its electrons.
Lukovits et al. [66] proposed that, the corrosion inhibition ability improved by increasing
electron donating ability if <3.6. The calculated data of ∆N have the ranking DMAHB >
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DMADB > DMAOB. As highest values of ∆N refers to DMAHB with highest inhibition
efficiency while smallest values of ∆N refer to DMAOB with lowest inhibition efficiency.
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The donation of electrons back from metal to inhibitor molecules may control the binding of
surfactant molecules to metal surface. Gomez et al [67] related the back donation energy (∆EBackby the hardness of the inhibitor molecules (η) for a charge transfer model, if the both
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donation)
processes of electron donation and electron back donation occur by the equation;
X
∆EBack-donation = - c
(20)
The negative values of ∆EBack-donation indicate that the electron follows from metal to inhibitor is
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favorable. The increase in the ∆EBack-donation may lead to strong adsorption and so high inhibition
efficiency. ∆EBack-donation values for the cationic surfactants molecules are -0.53632, -0.53625 and
the gas phase.
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-0.53612 eV for DMAHB, DMADB and DMAOB respectively by DFT/B3LYP/6-31+G (d,p) in
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3.7.7. Molecular dynamic simulation
MD simulations were performed to get a better understanding for the interaction process between
the cationic surfactant molecules and steel surface. Supplement Fig. 5 presents a typical energy
profile plot for DMAHB surfactant molecule on Fe (110) surface at the simulation process. The
lowest energy adsorption configurations of DMAHB, DMADB and DMAOB on the Fe (110)
interface are shown in Fig. 10. It’s clear from Fig. 10 that all cationic surfactant molecules can
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adsorb on steel surface. The adsorption of the surfactants occur side by side (the surfactant
molecules parallel to steel surface) which leads to more surface coverage and so high inhibition
efficiency. The adsorption occurs mainly through π electrons of the aromatic ring, lone pairs of
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electrons of the two O-CH3 groups, the two nitrogen atoms and so the aliphatic hydrocarbon
chain. The calculated adsorption energy by its different forms (total adsorption, rigid adsorption,
and deformation energies) and so binding energy are listed in Table 9. The data in Table 9 show
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that the cationic surfactants possess negative adsorption energy indicating the spontaneous nature
of the adsorption process. The binding energy values of the cationic surfactants are 317.074,
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278.967 and 237.638kcal/mol for DMAHB, DMADB and DMAOB respectively. The
order of binding energy confirms the order of laboratory findings.
Radial distribution functions of Fe (110) - surfactant structures, g(r), were also calculated and
plotted in Fig. 11. The peaks in the g(r)~r plot within 3.5 Å represent chemical adsorption,
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otherwise that located outside 3.5 Å represent columbic and Van der Waals attraction [68]. Our
data indicated that, the length between the cationic surfactant molecules and Fe (110) surface is
steel surfaces.
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about 2.49 Å so chemical bonds can be occurred between the cationic surfactant molecules and
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4. Conclusion
From the chemical, electrochemical and computational data obtained throughout the paper, we
can conclude that:
1- The prepared three cationic surfactants (DMAOB, DMADB and DMAHB) act as
successful corrosion inhibitors for steel in 1 M HCl solution.
2- Their inhibition effects increase with increases in both concentration and temperature.
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3- Tafel polarization curves proved that DMAOB, DMADB and DMAHB act as mixed-type
inhibitors.
due to chemisorption and follows the Langmuir model.
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4- The adsorption process of DMAOB, DMADB and DMAHB on steel surface is highly
5- EIS results showed that the charge transfer resistance Rct increases and double layer
capacitance Cdl decreases in
the presence of the cationic surfactants, which suggests
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their adsorption on steel surface.
experimental results.
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[60] M.A. Bedair, J. Mol. Liq., 219 (2016) 128.
[62] M.A. Bedair, M.K. Awad, M.S. Metwally, S.A. Soliman, A.A. El-Zomrawy, J. Ind. Eng. Chem, 20
M
AN
U
(2014) 796.
[63] I.B. Obot, N.O. Obi-Egbedi, Corros. Sci., 52 (2010) 657.
[64] Y. Qiang, S. Zhang, S. Xu, W. Li, J. Colloid Interface Sci., 472 (2016) 52.
[65] S. Martinez, Mater. Chem. Phys. 77 (2002) 97.
TE
D
[66] I. Lukovits, E. Kalman, F. Zucchi, Corrosion. 57 (2001) 3.
[67] B. Gomez, N.V. Likhanova, M.A. Dominguez-Aguilar, R. Martinez-Palou, R. Vela, J. Gasquez, J
.Phy. Chem B. 110 (2006) 8928-894.
AC
C
EP
[68] J.P. Zeng, J.Y. Zhang, X.D. Gong, Comput. Theor. Chem., 963 (2011) 110.
30
ACCEPTED MANUSCRIPT
Table 1 Corrosion rate, surface coverage and percentage inhibition efficiency of the mild steel in 1.0 HCl of synthesized surfactants
(DMAOB , DMADB and DMAHB) at different temperatures.
CR,
(mg cm-2h-1)
ηw
θ
(%)
-----
0.9110
-----
-----
5x10-3
0.0611
0.8285
82.85
0.1282
0.8593
85.93
5x10-4
0.0879
0.7533
75.33
0.1752
0.8077
80.77
5x10-5
0.1081
0.6965
69.65
0.2124
0.7668
76.68
5x10-6
0.1251
0.6488
64.88
0.2636
0.7106
5x10-7
0.1572
0.5585
55.85
0.3178
5x10-3
0.0560
0.8428
84.28
0.1177
0.0779
0.7813
78.13
0.1576
0.8270
0.0959
0.7307
73.07
0.1912
0.1164
0.6732
67.32
0.2348
0.1384
0.6113
61.13
0.0452
0.8730
87.30
0.0609
0.8290
0.0836
5x10
-4
5x10
-5
5x10
-6
5x10
-7
5x10
-4
5x10
-5
5x10
-6
5x10
-7
θ
70 oC
ηw
CR,
(mg cm-2h-1)
θ
(%)
ηw
(%)
-----
-----
3.4054
-----
-----
0.2212
0.8914
89.14
0.3004
0.9118
91.18
0.3194
0.8432
84.32
0.4706
0.8618
86.18
0.3989
0.8041
80.41
0.6083
0.8214
82.14
71.06
0.5138
0.7477
74.77
0.7529
0.7789
77.89
0.6511
65.11
0.6449
0.6833
68.33
0.9812
0.7119
71.19
0.8708
87.08
0.2041
0.8998
89.98
0.2480
0.9272
92.72
82.70
0.2860
0.8596
85.96
0.3837
0.8873
88.73
0.7901
79.01
0.3454
0.8303
83.03
0.5055
0.8516
85.16
0.7423
74.23
0.4431
0.7824
78.24
0.6645
0.8049
80.49
0.6857
68.57
0.5544
0.7277
72.77
0.8377
0.7540
75.40
0.0918
0.8992
89.92
0.1281
0.9371
93.71
0.1730
0.9492
94.92
82.90
0.1270
0.8606
86.06
0.2027
0.9004
90.04
0.2711
0.9204
92.04
0.7653
76.53
0.1721
0.8111
81.11
0.2816
0.8617
86.17
0.4068
0.8805
88.05
0.1036
0.7090
70.90
0.2264
0.7515
75.15
0.3843
0.8112
81.12
0.5525
0.8378
83.78
0.1260
0.6462
64.62
0.2625
0.7119
71.19
0.5034
0.7528
75.28
0.7115
0.7911
79.11
0.2863
2.0361
SC
-----
CR,
(mg cm-2h-1)
M
AN
U
0.3561
RI
PT
ηw
(%)
0
5x10-3
DMAHB
θ
θ
55 oC
TE
D
DMADB
CR,
CR,
(mg cm-2h-1)
40 oC
EP
DMAOB
25 oC
AC
C
Inhibitor
Inhibitor
Conc.
(M)
1
ACCEPTED MANUSCRIPT
Table 2 Potentiodynamic polarization parameters of corrosion of the mild steel in 1.0M HCl of
synthesized cationic surfactants at 25oC at scanning rate 0.2 mV s-1.
DMADB
βc
mA cm-2
mV dec-1
mV dec-1
-496
0.2488
176.7
-159.3
-----
-----
5x10-3
-534.7
0.0494
169.2
-198.10
0.8014
80.14
-4
5x10
-526.8
0.0612
173.6
-182.40
0.7540
75.40
5x10-5
-507.2
0.0797
135.3
-156.80
0.6797
67.97
-6
5x10
5x10-7
-514
-501.8
0.0905
0.1111
153
148.2
-154.30
-157.30
0.6363
0.5535
63.63
55.35
5x10-3
-528.20
0.0462
174.7
-189.10
0.8145
81.45
-4
5x10
-523.10
0.0572
117.9
-332.00
0.7699
76.99
5x10-5
-508.80
0.0715
161.4
-142.70
0.7127
71.27
-6
5x10
5x10-7
-502.90
-516.50
0.0807
0.0919
157.7
157.4
-201.60
-162.50
0.6755
0.6308
67.55
63.08
5x10-3
5x10-4
-538.9
-530.6
0.0391
0.0507
164.9
200.6
-163.90
-178.40
0.8428
0.7961
84.28
79.61
5x10-5
-524.8
0.0563
176.5
-196.70
0.7738
77.38
5x10-6
-506.7
0.0716
135.5
-155.50
0.7121
71.21
0.0847
182.6
200.40
0.6593
65.93
EP
DMAHB
0.00
βa
-7
-515.6
AC
C
5x10
2
%
RI
PT
Ө
SC
M
AN
U
DMAOB
(SCE)
Icorr
mV
TE
D
Inhibitor
name
Ecorr
Conc. of
inhibitor
(M)
ACCEPTED MANUSCRIPT
Table 3 EIS parameters, slopes of the Bode impedance magnitude at intermediate frequencies (S)
and maximum phase angle values (α°) for corrosion of steel in 1.0 M HCl in the absence and
ηz
%
-0.528
-0.599
-0.681
-58.92
-60.87
-59.39
----0.833
0.750
----83.37
75.02
119.80
-0.682
-68.24
0.697
69.78
112.88
-0.651
-62.93
0.647
64.78
0.8372
169.82
-0.613
-65.64
0.548
54.81
270.8
0.7284
147.87
-0.647
-58.8
0.849
84.95
467.8
458.3
0.7735
225.75
-0.585
-55.2
0.799
79.96
1.415
403.7
150.9
0.7044
67.123
-0.560
-59.71
0.767
76.78
12.47
265.8
258.0
0.7988
115.08
-0.489
-49.96
0.647
64.74
1.969
238.4
530.7
0.7755
259.03
-0.640
-61.01
0.606
60.68
1.440
701.4
181.1
0.6932
72.662
-0.613
-57.03
0.866
86.63
1.001
466.9
100.2
0.7968
45.889
-0.716
-67.02
0.799
79.92
1.003
427.7
156.1
0.7222
55.112
-0.632
-58.22
0.780
78.08
3.797
334.0
280.8
0.7143
108.96
-0.517
-55.59
0.719
71.94
357.1
0.7422
156.52
-0.492
-54.49
0.640
64.03
1.581
1.381
4.593
93.72
563.8
375.2
530.7
340.3
289.9
0.8071
0.7254
0.7933
1.442
310.2
199.9
0.8445
1.371
266.1
228.6
0.7987
1.568
207.4
292.8
1.377
622.9
11.97
5x10
-4
5x10
-5
-6
5x10
5x10-7
5x10
-3
5x10
-4
5x10-5
-6
5x10
5x10-7
3.677
-2
s cm )
260.6
n
Cdl
(µF cm-2)
259.03
182.15
162.63
M
AN
U
5x10
-3
(µ Ω
Yo
-1 n
TE
D
5x10
5x10-7
Rct (Rp)
(Ω cm2)
3
S
SC
0.00
5x10-3
5x10-4
5x10-5
-6
RI
PT
θ
Rs (Ru)
(Ω cm2)
EP
DMAHB
DMADB
DMAOB
blank
αo
o
( )
Conc
(M ).
AC
C
Inhibitor
name
presence of different concentrations of the prepared cationic surfactants at 25oC.
ACCEPTED MANUSCRIPT
Table 4 Activation parameters values of the mild steel in 1.0M HCl of different concentrations of
the synthesized inhibitors.
Conc. of
inhibitor
(M)
Ea
(kJ mol-1)
Linear
regression
coefficient
∆H*
(kJ mol-1)
∆S*
(J mol-1 K-1)
Blank
0.00
43.14
0.9927
40.48
-117.22
5x10-3
5x10-4
5x10-5
5x10-6
5x10-7
30.38
32.07
33.06
34.47
35.26
0.9807
0.9935
0.9961
0.9904
0.9941
27.73
29.41
30.40
31.81
32.61
-174.51
-166.13
-161.22
-155.09
-150.70
5x10-3
5x10-4
5x10-5
5x10-6
5x10-7
28.68
30.68
31.76
33.36
34.49
0.9603
0.9826
0.9929
0.9942
0.9937
26.03
28.02
29.10
30.70
31.84
-180.75
-171.60
-166.45
-159.52
-154.25
5x10-3
5x10-4
5x10-5
5x10-6
5x10-7
24.90
28.26
29.86
31.65
33.31
0.9678
0.9755
0.988
0.9842
0.9874
22.24
25.60
27.20
29.00
30.65
-195.26
-181.58
-173.75
-165.84
-158.87
DMADB
AC
C
EP
TE
D
DMAHB
SC
M
AN
U
DMAOB
RI
PT
Inhibitor
name
4
ACCEPTED MANUSCRIPT
Table 5 Thermodynamic parameters from Villamil adsorption isotherm of the mild steel surface
in 1.0M HCl containing different concentrations of the synthesized inhibitors at different
temperatures.
M
kJ mol
-1
1.20
0.9999
5.64
-37.08
40
1.16
0.9999
9.12
-40.19
55
1.12
1.10
25
1.18
1.15
40
55
25
55
0.9999
0.9999
-42.51
-44.51
kJ mol
-1
12.02
KJ mol-1
K-1
0.1647
0.1667
0.1662
0.1647
-37.69
-40.71
13.16
13.87
-43.12
-45.24
0.9999
10.33
12.84
-38.58
-41.08
1.07
0.9999
15.17
-43.51
0.1676
1.05
0.9999
19.23
-46.17
0.1680
1.14
1.11
0.9999
0.9999
0.9999
0.9999
AC
C
EP
70
10.54
10.75
TE
D
40
0.9999
∆Sads
7.23
11.11
1.11
1.08
70
DMAHB
-1
25
70
DMADB
R
∆Hads
RI
PT
DMAOB
C
Slope
∆Gads
SC
Name
o
Kads x10-5
2
M
AN
U
Inhibitor
Temp.
5
0.1675
0.1691
12.24
11.48
0.1687
0.1675
0.1679
0.1679
ACCEPTED MANUSCRIPT
Table6. Quantitative analysis for steel surface after 24 hr immersion in 1.0 M HCl in the
presence and absence of the prepared surfactant inhibitors obtained from EDX.
Blank (HCl- Fe)
DMAOB - Fe
DMADB - Fe
DMAHB - Fe
Element
Mass %
Atom %
Mass %
2.73
1.58
----5.07
90.62
100
3.58
6.14
----14.75
75.53
100
----4.12
0.65
2.19
93.05
100
----15.65
2.11
6.23
76.01
100
-----
100
M
AN
U
TE
D
EP
6
Mass %
Atom %
-----
-----
-----
6.48
3.13
6.09
84.29
1.60
0.77
1.46
96.17
6.67
2.76
4.55
86.03
100
100
100
SC
1.58
0.89
1.98
95.55
Atom %
RI
PT
Atom %
AC
C
Cl
C
N
O
Fe
Total
Mass %
ACCEPTED MANUSCRIPT
EHOMO
ELUMO
∆E
∆E back donation
DM
TNC (e)
T.E.
3
M.V. cm /mol
(eV)
(eV)
(eV)
(eV)
(D)
(eV)
Mulliken
∆N
(eV-1)
(eV)
(eV)
(e)
∆N max
(e)
η
IE*
(eV)
(%)
-29427.8 290.941
-6.200
-8.386 0.46631 9.248 6.298 0.163
2.936 2.1444 83.37
DMADB
-8.4386 -4.148
4.2900 -0.53625 9.969
-33704.0 376.846
-7.277
-10.734 0.46619 9.233 6.293 0.164
2.934 2.1450 84.95
DMAHB
-8.4373 -4.146
4.2905 -0.53632 20.399 -37980.2 426.366
-8.424
-12.678 0.46613 9.227 6.292 0.165
2.932 2.1452 86.63
DMAOB
-6.2721 -1.6942 4.5779 -0.57225 2.106
-29429.7 278.011
-6.248
-8.893 0.43687 3.465 3.983 0.658
1.740 2.2889 83.37
DMADB
-6.2297 -1.6933 4.5363 -0.56705 9.536
-33705.8 333.363
-7.306
-10.759 0.4408 3.459 3.961 0.669
DMAHB
-6.1816 -1.6920 4.4896
-0.5612 20.039 -37982.0 430.351
-8.426
-12.707 0.4454 3.452 3.936 0.682
1.746 2.2681 84.95
1.753 2.2448 86.63
M
AN
U
4.2889 -0.53612 1.177
Hirshfeld
-7.5632 -4.8263 2.7368 -0.34211 1.8732
-28869.2
------
-7.259
-1.0728 0.7307 14.021 6.194 0.2941 4.526 1.3684 83.37
DMADB
-7.7175 -4.8518 2.8656 -0.35821 9.0466
-7.7204 -4.8478 2.8726 -0.35908 19.399
-33047.9
------
-8.542
-1.2919 0.6979 13.783 6.285 0.2496 4.386 1.4328 84.95
-37226.5
------
-10.035 -1.5257 0.6962 13.747 6.284 0.2492 4.375 1.4363 86.63
DMAOB
EP
Mulliken
DMAOB
-5.6455 -2.4080 3.2374 -0.40468 1.5191
-28869.2
------
-7.461
-1.1506 0.61776 5.008 4.026 0.9183 2.487
1.618
83.37
DMADB
-5.6416 -2.4024 3.2391 -0.40490 10.022
-33047.9
------
-8.301
-1.3741 0.61743 4.994 4.022 0.9193 2.483
1.619
84.95
DMAHB
-5.6298 -2.3965 3.2332 -0.40416 20.355
-37226.5
------
-10.508 -1.6146 0.61856 4.981 4.013 0.9237 2.482
1.616
86.63
DMAHB
AC
C
gas phase
X
-8.4420 -4.153
DMol3
aqueous phase
ω
DMAOB
TE
D
aqueous phase
gas phase
DFT/B3LYP/6-31G+(d,p)
NBO
σ
SC
Molecule
RI
PT
Table 7The calculated quantum chemical parameters in eV for the investigated inhibitors at DFT/B3LYP/6-31G+(d,p) and DMol3 in gas phase and
in aqueous phase.
7
ACCEPTED MANUSCRIPT
Table 8 The Fukui functions of studied inhibitors, calculated by DMol3 method at GGA function
with BOP/DNP basis set
DMADB
DMAHB
f-
f+
f-
f+
0.042
0.023
0.051
0.013
0.042
0.033
0.115
0.059
-0.043
-0.036
0.018
0.05
-0.024
-0.004
-0.002
0.002
-0.005
-0.01
-0.004
-0.003
-0.002
-0.002
-0.002
-0.002
-0.001
0.015
0.037
-0.022
0.051
0.021
0.021
0.04
-0.026
-0.032
0.12
0.088
-0.037
-0.007
-0.009
-0.002
-0.014
-0.014
-0.006
-0.004
-0.003
-0.003
-0.002
-0.003
-0.002
0.015
0.045
0.027
0.04
0.017
0.036
0.039
0.110
0.062
-0.053
-0.045
0.018
0.058
-0.027
0.001
-0.002
0.004
-0.008
-0.008
0.01
-0.003
-0.003
-0.001
-0.002
-0.001
-0.002
-0.001
-0.001
-0.001
0.000
0.065
0.014
0.044
-0.018
0.049
0.02
0.013
0.04
-0.015
-0.024
0.119
0.085
-0.034
-0.012
-0.009
-0.004
-0.004
-0.016
-0.027
-0.004
-0.003
-0.003
-0.001
-0.002
-0.001
-0.001
-0.001
-0.001
-0.001
0.054
0.025
0.039
0.038
0.018
0.06
0.100
0.066
-0.047
-0.04
0.01
0.064
-0.023
0.001
0.003
0.003
-0.014
-0.006
0.01
-0.003
-0.001
-0.002
-0.002
-0.003
-0.005
-0.007
-0.009
-0.011
-0.012
-0.012
-0.013
-0.011
-0.010
0.065
0.014
0.044
-0.023
0.058
0.015
0.011
0.036
-0.01
-0.021
0.125
0.079
-0.032
-0.014
-0.013
-0.004
0.002
-0.015
-0.026
-0.005
-0.005
-0.005
-0.003
-0.003
-0.002
-0.002
-0.002
-0.002
-0.002
-0.002
-0.002
-0.002
-0.001
TE
D
M
AN
U
SC
RI
PT
f+
EP
C ( 1)
C ( 2)
C ( 3)
C ( 4)
C ( 5)
C ( 6)
O ( 7)
O ( 8)
C ( 9)
C ( 10)
C ( 11)
N ( 12)
C ( 13)
C ( 14)
N ( 15)
C ( 16)
C ( 17)
C ( 18)
C ( 19)
C ( 20)
C ( 21)
C ( 22)
C ( 23)
C ( 24)
C ( 25)
C ( 26)
C ( 27)
C ( 28)
C ( 29)
C ( 30)
C ( 31)
C ( 32)
C ( 33)
DMAOB
f-
AC
C
2
Aliphatic chain
N(CH3)
C=N-C-C (O – CH3)2 Aromatic ring
atom
8
ACCEPTED MANUSCRIPT
Table 9 The outputs and descriptors calculated by the Molecular dynamic simulation for
adsorption of DMAHB, DMADB and DMAOB on Fe (110) (in kcal/ mol).
DMAOB
-323.710
-274.338
-221.233
-317.074
-278.967
-237.638
-288.494
-253.537
-216.390
SC
-28.579
RI
PT
DMADB
-25.429
-21.247
-317.074
-278.967
-237.638
317.074
278.967
237.638
86.63
84.95
83.37
AC
C
EP
TE
D
Total energy
(kcalmol-1)
Adsorption energy
(kcalmol-1)
Rigid adsorption energy
(kcalmol-1)
Deformation energy
(kcalmol-1)
(dEads/dNi)
(kcalmol-1)
Binding energy
(kcalmol-1)
IE* (%)
DMAHB
M
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U
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Fig. 1: The synthetic route for the three tested amphipathic inhibitors.
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Fig.2: Variation of corrosion rate and so inhibition efficiency against logarithm C of the
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Fig.3. Potentiodynamic polarization curves for the corrosion of mild steel in 1.0 M HCl in the
absence and presence of different concentrations of DMAOB, DMADB and DMAHB.
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Fig. 4. Nyquist plots for the mild steel in 1.0 M HCl in the absence and presence of different
concentrations of surfactants (a) for DMAOB, (b) for DMADB and (c) for DMAHB at 25oC and
(d) the equivalent circuit model used to fit the EIS data.
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Fig. 5. Bode and phase angle plots of impedance spectra for the mild steel in 1.0 M HCl in the
absence and presence of different concentrations of surfactants (a) for DMAOB, (b) for DMADB
and (c) for DMAHB) at 25 oC.
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Fig. 6 Arrhenius plots (log k vs. 1/T) for steel dissolution in the absence and the presence
of different concentrations of DMAOB, DMADB and DMAHB in 1.0 M HCl solution.
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Fig. 7. Langmuir isotherm adsorption model of the mild steel surface of compound DMAOB,
DMADB and DMAHB in 1.0 M HCl at different temperatures.
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Fig.8. SEM images and EDX spectra of the mild steel in 1 M HCl after 24 h immersion
without inhibitor (blank) and with the surfactant inhibitors.
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Fig.9. The optimized structures, frontier molecular orbitals (HOMO and LUMO), electrostatic potential and Mulliken atomic charges
obtained from DFT calculations in the gas phase.
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Fig.10. Top and side views of the most stable low energy configurations for the adsorption of the
three cationic surfactants on Fe (110) interface obtained using Molecular dynamic simulations.
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Fig. 11. Radial distribution functions of the three cationic surfactants on Fe (110) surface.
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Supplement Fig. 1: IR spectrum of N-(2-((3,4-dimethoxybenzylidene)amino)ethyl)-N,Ndimethyldodecan-1- ammonium bromide (DMADB).
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Supplement Fig. 2: 1H-NMR spectrum of N-(2-((3,4-dimethoxybenzylidene)amino)ethyl)-N,Ndimethyldodecan-1-ammonium bromide (DMADB).
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Supplement Fig. 3 Transition state plots of the mild steel dissolution in the absence and the presence of
different concentrations of DMAOB, DMADB and DMAHB in 1.0 M HCl solution.
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Supplement Fig. 4. Optimization energy curves for the cationic surfactants in gas phase and aqueous
phase obtained from DMol3
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Supplement Fig. 5. The typical energy profile for the adsorption of DMAHB on Fe (110) surface
obtained using Molecular dynamic simulations.
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Three surfactants were synthesized and characterized by FTIR and 1HNMR.
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The adsorption of these compounds is chemical adsorption and obeys Langmuir adsorption
isotherm.
Calculated quantum chemical parameters showed a good correlation with inhibition
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efficiencies.
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2018, 041, matchemphys
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