Synthesis, assessment and corrosion protection investigations of some novel peptidomimetic cationic surfactants: Empirical and theoretical insights

Article abstract of DOI:10.1016/j.molliq.2020.113672

Three novel peptidomimetic cationic surfactants were synthesized in good yields. The chemical configurations of these surfactants were clarified using ¹H, ¹³C NMR and FT-IR spectroscopy. The inhibition capacity and adsorption performance of these compounds on C-steel were studied by electrochemical techniques (Electrochemical impedance spectroscopy (EIS) and Potentiodynamic polarization (PDP) methods). The prepared compounds demonstrated outstanding protection power for the erosion of C-steel in 0.5 M HCl at 323 K. The PDP studies demonstrated that the novel surfactants behaved as mixed-type additives. The protection capacity rises with an increasing surfactant dose, with values ranging from 93.10 to 98.25percent at 100 ppm. The adsorption of additives on the electrode interface follows the Langmuir model and contains chemisorption modes. The Monte Carlo (MD) simulations and density functional theory (DFT) calculations support the experimental findings and provide insight into the understanding of the adsorption features and protection performance mechanisms of the examined surfactants.

Full text of DOI:10.1016/j.molliq.2020.113672

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Synthesis, assessment and corrosion protection investigations of
some novel peptidomimetic cationic surfactants: Empirical and
theoretical insights
Wessam S. Abdrabo, Bahaa Elgendy, Kamal A. Soliman, Hany M.
Abd El-Lateef, Ahmed H. Tantawy
PII:
S0167-7322(20)32352-7
DOI:
https://doi.org/10.1016/j.molliq.2020.113672
MOLLIQ 113672
Reference:
To appear in:
Journal of Molecular Liquids
Please cite this article as: W.S. Abdrabo, B. Elgendy, K.A. Soliman, et al., Synthesis,
assessment and corrosion protection investigations of some novel peptidomimetic cationic
surfactants: Empirical and theoretical insights, Journal of Molecular Liquids (2018),
https://doi.org/10.1016/j.molliq.2020.113672
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Synthesis, assessment and corrosion protection investigations of some novel
peptidomimetic cationic surfactants: empirical and theoretical insights
a
a,c,d
a
b, e
Wessam S. Abdrabo , Bahaa Elgendy *, Kamal A. Soliman , Hany M. Abd El-Lateef *,
a,f
Ahmed H. Tantawy *
a
Department of Chemistry, Faculty of science, Benha University, Benha-13518, Egypt
b
Department of Chemistry, College of Science, King Faisal University, P.O. Box 380 Al
Hofuf 31982 Al-Ahsa, Saudi Arabia
c
Department of Pharmaceutical and Administrative Sciences, St. Louis College of Pharmacy,
St. Louis, MO 63110, USA
d
Center for Clinical Pharmacology, Washington University School of Medicine and St. Louis
College of Pharmacy, St. Louis, MO 63110, USA
e
Chemistry Department, Faculty of Science, Sohag University, Sohag-82534, Egypt
f
Department of Chemistry, College of Science, Huazhong Agricultural University, Wuhan,
4
30070, China
*
Corresponding authors: ahmed.tantawy@mail.hzau.edu.cn (Ahmed H. Tantawy),
hmahmed@kfu.edu.sa (H. M. Abd El-Lateef), bahaa.elgendy@stlcop.edu (Bahaa
Elgendy).
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Abstract
Three novel peptidomimetic cationic surfactants were synthesized in good yields. The
1
13
chemical configurations of these surfactants were clarified using H, C NMR and FT-IR
spectroscopy. The inhibition capacity and adsorption performance of these compounds on C-
steel were studied by electrochemical techniques (Electrochemical impedance spectroscopy
(EIS)and Potentiodynamic polarization (PDP)
methods). The prepared compounds
demonstrated outstanding protection power for the erosion of C-steel in 0.5 M HCl at 323 K.
The PDP studies demonstrated that the novel surfactants behaved as mixed-type additives.
The protection capacity rises with an increasing surfactant dose, with values ranging from
9
3.10 to 98.25% at 100 ppm. The adsorption of additives on the electrode interface follows
the Langmuir model and contains chemisorption modes. The Monte Carlo (MD) simulations
and density functional theory (DFT) calculations support the experimental findings and
provide insight into the understanding of the adsorption features and protection performance
mechanisms of the examined surfactants.
Keywords: Peptidomimetics; Surfactants; Acid inhibition; MD simulations; DFT
2

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1
. Introduction
Metallic corrosion is a major problem for the application of engineering materials in
various environments.[1,2]. As a result, there are ongoing efforts to develop protective tools
that can effectively protect the materials from corrosion.[3] There is no one solution to be
used against the corrosion of all metals across different aggressive media. Surfactants are
used as corrosion inhibitors to slow down the deterioration of metals [4–6].
Cationic surfactants are widely known for their corrosion inhibition activity,[7–13]
especially quaternary ammonium compounds, which are important corrosion inhibitors in
acidic media [7–9]. The anti-corrosive effect of adding halide[14] or increasing the alkyl
chain length have been extensively studied [15]. Cationic surfactants are amphiphilic
compounds with self-assembling properties and can dissociate in water to form surface-active
cations. Because of these cationic amphiphilic properties, cationic surfactants have high
application potential in many fields [16]. For example, they can be used as follows: in drug
delivery as drug and gene nano-carriers, [17] stabilizers of nanoparticles and nano-
carriers,[18–20] antimicrobial and bioimaging agents, [21,22] supramolecular
catalysts;[23,24] additionally, they can be applied for the removal of drugs [25] and organic
pollutant [26] from aqueous media and in thermal energy storage applications through
enhancing the heat storage capacity of metal hydroxide[27]. Quaternary ammonium
compounds have a high affinity for biopolyanions, such as DNA [28–30].
Amino acids and peptide-based surfactants or peptidomimetic surfactant have many
promising applications that have been explored in various ways, especially for their anti-
microbial activities[31–33] and self-assembly nano-structure formation [34]. Toxicity,
biodegradability, and bioaccumulation are now highly important criteria for deciding to apply
any surfactant.[35] The combination of quaternary ammonium compounds with
peptidomimetic moiety should produce a promising class of peptidomimetic surfactants that
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combine the anti-corrosive activity of quaternary ammonium compounds with the
antimicrobial and antifungal activities of the peptidomimetic compounds, while also having
quick biodegradation properties.[16]
It is worth noting that the alkyl chain length of cationic surfactants may affect the
interactions between the molecules and metals, resulting in the effect of corrosion
inhibition.[11,36,37]. Therefore, three peptidomimetic compounds with different alkyl chain
lengths were designed and employed for C-steel corrosion protection for the first time.
The molecular structures of the synthetic peptidomimetic surfactants are presented in
Fig. 1. The corrosion inhibition performance of the synthetic molecules was studied by taking
electrochemical measurements and performing a surface tension analysis. DFT calculations,
MD simulations and adsorption isotherms were utilized to further study the corrosion
inhibition mechanism. In this report, we studied the anticorrosion properties of newly
developed peptidomimetic cationic surfactants with different alkyl chains.
2
2
. Experimental Technique
.1 Materials
Benzoyl benzotriazole, acetylacetone (98.0%), N,N-dimethylpropane-1,3-diamine
99.9%) and 2-chloroacetylchloride (98.0%) were obtained from Aldrich company. Benzoic
acid, 1-decanol, 1-tetradecanol and 1-hexadecanol were purchased from Acros Organics
Belgium). Diethyl ether (99%), triethylamine (97%), K CO (anhydrous) benzene (98%),
(
(
2
3
,
and ethanol (99%) were obtained from El Nasr Company, Egypt.
The structure compositions of the C-steel used for the erosion experiments are (in
weight percent): Mn (0.72%), C (0.18%), Ni (0.02%), S (0.05%), Cr (0.01%), and rest iron.
The aggressive medium of hydrochloric acid (0.5 M) was designed by mitigating the
3
7% HCl (analytical grade) with bidistilled H O. Different concentrations (10–100 ppm) of
2
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the synthesized surfactants were similarly prepared in double-distilled H O and utilized as
2
additives for the C-steel corrosion in 0.5 M HCl.
2
.2. Instruments
The FTIR analysis was performed at Benha University, Egypt via a thermo Nicolet
iS10 FTIR spectrophotometer using potassium bromide. The NMR spectra were recorded on
1
13
a Bruker NMR spectrometer operating at 400 MHz for H NMR and 100 MHz for C NMR.
The chemical shifts were referenced to the solvent peak.
2
.3. Surfactants Synthesis
Synthesis of benzoyl benzotriazole
SOCl (0.15 mL, 4.2 mmol) was added with stirring at 25°C to a benzotriazole
2
solution (2.0 g, 16.8 mmol) in dichloromethane (DCM) (15.0 mL). After 0.5 h, a
solution of benzoic acid (0.512 g, 4.2 mmol) in DCM (10 mL) was added to one
portion and stirred for 120 min. The white precipitate was filtered off, followed by
washing with DCM (20 mL), and the collective organic solution was washed with aq.
NaOH (2N) (3.0 × 15.0 mL) and desiccated over Na SO ; then, the DCM was
2
4
evaporated under a vacuum to obtain benzoyl benzotriazole (0.81 g, 86.4%).
Synthesis of hippuric acid
To a solution of benzoyl benzotriazole (0.447 g, 2.0 mmol) in acetonitrile (6
mL), glycine (0.15 g, 2 mmol) in acetonitrile/water (6/4 mL) and triethylamine (0.102
g, 1 mmol) were added under stirring at 25°C. After the stirring the, mixture was
allowed to react for 2 h; then, it was dilute by water (10 mL) and extracted by DCM (3
×
15 mL) to remove the excess of benzotriazole. The water was concentrated to obtain
the crystalline hippuric acid (0.275 g, 77%). The reaction was repeated with the same
conditions on (1.24 g, 5.0 mmol) of benzoyl benzotriazole, and the yield was 74%.
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Synthesis of 4-(4-methoxybenzylidene)-2-phenyloxazol-5(4H)-one (3)[38].
A mixture of sodium acetate (0.82 g, 10 mmol), (0.9 g, 5 mmol) hippuric acid and
acetic anhydride (5 mL) was stirred at 25°C for 30 min. 4-Methoxy benzaldehyde (0.61
mL, 5 mmol) was added to the reaction solution and stirring of the solution continued for an
additional 6 h. The reaction was quenched by frozen H O and washed by sodium
2
carbonate. The precipitate was collected and recrystalized from ethanol to obtain
yellow fluffy crystals of compound
3 (1.1 g, 78.7%).
Synthesis (Z)-N-(3-((3-(dimethylamino)propyl)amino)-1-(4-methoxyphenyl)-3-oxoprop-1-
en-2-yl)benzamide (4)
Compound 3 (0.56 g, 2 mmol) was dissolved in 5 mL DCM; then N,N-dimethyl
propyl amine (0.38 mL, 3 mmol) was added. The reaction mix was stirred for 0.5 h. at 25°C,
and then the solvent was separated under vacuum to give the desired product as white crystals
(0.57 g, 75%); mp 139-141 ℃. IR(KBr) νmax: 3247, 3070, 2973, 2942, 2855, 2763, 1655,
1
1
8
5
3
640, 1606,1507, 1478, 1278, 1254. H NMR (400 MHz, DMSO-d6) δ 9.82 (s, 1 H, -NH-),
.15 (t, J = 5.5 Hz, 1 H, -NH-), 8.04 (dt, J = 7.0, 1.4 Hz, 2 H, aromatic CH), 7.65 – 7.47 (m,
H aromatic CH), 7.26 (s, 1 H, aromatic CH), 6.95 – 6.86 (m, 2 H, aromatic CH), 3.73 (s,
H, OCH ), 3.19 (q, J = 6.5 Hz, 2 H, NH-CH -), 2.22 (t, J = 6.8 Hz, 2 H, CH -N), 2.01 (s, 6
3
2
2
1
3
H, N(CH ) ), 1.57 (p, J = 6.8 Hz, 2 H, -CH -). C-NMR (DMSO-d 101 MHz) δ 213.85,
3
2
2
6,
2
1
08.54, 201.77, 183.56, 167.49, 165.46, 164.64, 159.39, 133.63, 131.35, 130.73, 129.07,
28.05, 127.82, 127.60, 126.61, 113.81, 57.27, 55.00, 44.86, 38.19, 26.35.
Synthesis of 2-chloroacetate derivatives: It was prepared according to our previous work [39].
Synthesis of (Z)-3-(2-benzamido-3-(4-methoxyphenyl)acrylamido)-N-(2-(hexadecyloxy)-2-
oxoethyl)-N,N-dimethylpropan-1-aminium (5a-c)
A solution of 2-chloroacetate derivatives (1.51 mmol) in 10.0 mL of ethyl ethanoate
was prepared and mixed with a solution of 4 (0.38 g, 1 mmol) in 20 mL of ethyl acetate with
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continuous stirring. The reaction system was taken to reflux at 70°C for 48 h. The product
precipitated upon allowing the reaction solution to cool. The obtained yields were washed via
diethyl ether to separate the undesired reactants. By column chromatography (CH Cl :
2
2
MeOH, 10:1.5), the product was purified to generate the target cationic surfactants (5a-c)
with varying yields and colors.
(Z)-3-(2-Benzamido-3-(4-methoxyphenyl)acrylamido)-N-(2-(hexadecyloxy)-2-oxoethyl)-
N,N-dimethylpropan-1-aminium (WA-1, 5a).Yield: 55%, pale yellow crystals; mp 148-150
℃. IR(KBr) νmax: 3243, 3072, 2972, 2947, 2852-2767, 1758, 1644, 1606, 1513, 1477, 1283,
1
1
254. H NMR (400 MHz, DMSO-d ) δ 9.93 (s, 1 H, NH), 8.14 (s, 1 H, NH), 7.53 (d, J = 7.7
6
Hz, 4 H, aromatic CH), 7.25 (s, 1 H, aromatic CH), 7.16 (s, 1 H, aromatic CH), 6.92 (t, J
8.8 Hz, 4 H, aromatic CH), 4.45 (s, 2 H, CH ), 4.17 (d, J = 7.8 Hz, 2 H, CH ), 3.75 (s, 3
=
2
2
H, OCH ), 3.20 (s, 6 H, N(CH ) ), 2.14 (d, J = 8.7 Hz, 2 H, CH ), 1.96 (s, 2 H, CH ), 1.63
3
3 2
2
2
1
3
(
t, J = 7.6 Hz, 4 H CH -CH ), 1.25 (s, 26 H, (CH ) ), 0.87 (m, 3 H, CH ). C NMR
2 2 2 13 3
(
DMSO-d 100 MHz) δ 166.19, 165.45, 165.19, 160.09, 132.04, 131.48, 129.70, 128.90,
6,
1
3
28.73, 128.51, 128.40, 128.31, 114.51, 66.37, 63.81, 61.47, 57.55, 55.70, 51.63, 45.14,
4.25, 31.70, 29.44, 29.08, 28.30, 26.70, 25.67, 23.14, 22.48, 14.31.
(Z)-3-(2-Benzamido-3-(4-methoxyphenyl)acrylamido)-N,N-dimethyl-N-(2-oxo-2-
(tetradecyloxy) ethyl)propan-1-aminium (WA-2, 5b). Yield: 61%, pale yellow crystals; mp
1
15-118 ℃.
(Z)-3-(2-Benzamido-3-(4-methoxyphenyl)acrylamido)-N-(2-(decyloxy)-2-oxoethyl)-N,N-
dimethylpropan-1-aminium (WA-3, 5c). Yield: 61%, pale yellow semisolid at room temp.
IR(KBr) νmax: 3241-3220, 3026, 2922, 2852, 1754, 1648, 1606, 1514, 1477, 1283, 1254.
IR(KBr) νmax: 3243, 3072, 2972, 2947, 2852-2767, 1758, 1644, 1606, 1513, 1477, 1283,
1
1
254. H NMR (DMSO-d , 400 MHz) δ 10.02 (s, 1 H, NH), 8.20 (s, 1 H, NH), 8.09 (s, 2 H,
6
aromatic CH), 7.56 (d, J = 22.0 Hz, 5 H, aromatic CH), 7.22 (s, 1 H, aromatic CH), 6.93
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(
s, 1 H, aromatic CH), 6.91 (s, 1 H, aromatic CH), 4.52 (s, 2 H, CH ), 4.19 (s, 2 H, CH ),
2
2
3
2
1
1
2
2
.76 (s, 3 H, OCH ), 3.63 (s, 2 H, CH ), 3.27 (s, 6 H, N(CH ) ), 1.97 (s, 2 H, CH ), 1.63 (s,
3 2 3 2 2
13
H, CH ), 1.26 (s, 16 H, (CH ) ), 0.87 (s, 3 H, -CH ). C NMR (DMSO-d 100 MHz) δ
2
2 8
3
6,
66.45, 166.17, 165.26, 160.09, 134.24, 132.05, 131.50, 129.21, 128.69, 128.58, 128.46,
27.24, 114.48, 66.33, 63.73, 61.43, 55.70, 51.53, 36.44, 31.71, 29.39, 29.08, 28.31, 25.67,
3.07, 22.48, 14.31.
.4. Corrosion inhibition measurements
All corrosion tests were conducted on the VersaSTAT4 potentiostat/galvanostat
connected with an electrochemical cell system containing 3-electrodes including working (C-
steel), reference (saturated calomel electrode; SCE) and counter electrodes (Pt wire).
Approximately 45 min was permitted for the corrosion system to reach the open circuit
potential (E ) after the exposure of the carbon steel prior to each electrochemical
ocp
experiment.
Electrochemical impedance spectroscopy (EIS) was implemented using a
VersaSTAT4 potentiostat/galvanostat, controlled by means of a frequency response analyzer
3
(
FRA). The frequency extended from 100 × 10 Hz to 0.5 Hz with an ac voltage capacity of
0
.010 V. The potentiodynamic polarization (PDP) plots were achieved by sweeping the
potential up to ±250 mV vs. Ecorr at a sweep rate of 0.2 mV/s. All the corrosion measurements
were performed under unstirred aerated environments at 50 °C. All the electrochemical tests
were repeated three times to ensure the accuracy of the experimental data.
2
.5. Surface activities
The surface tension (γ) values of the obtained cationic surfactants at 298 K were
investigated via a Tensiometer-K6 processor (Krüss Company, Germany). For calibration, the
ring was cleaned between the measurement runs with distilled water and acetone. Before the
measurements were taken, all the aqueous solutions of the obtained cationic surfactants
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continued to stand 1-2 h. Each concentration of the synthesized compounds was measured three
times at the same temperature, and the average value was recorded and taken. From the γ
values, the surface activity parameters will be calculated.
2
.6. Electrical conductivity
The electrical conductivity measurements of the synthesized compounds were
performed via a model Type AD3000 digital conductivity/temperature meter, with EC/TDS at a
certain temperature (298 K). All the measurement values were repeated three times for each
surfactant solution and the data were recorded.
2
.7. Theoretical Studies
In this work, the geometry of the two inhibitor molecules (WA-1 and WA-2) were fully
optimized via density functional theory (DFT) hybrid functional B3LYP[40] in the gas and
liquid phases. The polarizable conductor calculation model (CPCM) was used to simulate
water as a solvent [41]. The calculations were based on the 6-311G (d,p) basis set. Quantum
descriptors for the tested molecules were obtained using Gaussian 09 software [42].
According to Pearson [43,44], the global hardness (η) and the fraction of electrons transferred
(∆N) from the surfactant to the iron surface are calculated according the following equations:

Egap  ELUMO  EHOMO
(1)
(2)

Egap
ELUMO  EHOMO




2
2

 inh
N 
(3)
2
(  )
Fe
inh
where

, ,
inh Fe and inh are the function work (4.82 eV), the electronegativity of the
inhibitor, the chemical hardness of Fe(110) (0 eV) and the chemical hardness of the inhibitor,
respectively.
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2
.8. Monte Carlo (MC) simulations
The MC simulations were performed using a simulated annealing method to determine the
interaction of WA-2 and WA-1 with the surface of Fe (110). The adsorption locator module
in software 7 (Material Studio) from BIOVIA-Accelrys, USA is used for the MC simulations.
The iron surface (110) consisted of five layers of iron. A supercell of (15×15) was created
and a vacuum layer of 20 Å was built over the surface of the iron. Optimized WA-2 and WA-
1
inhibitors were placed close to the surface of the Fe (110) to adopt the simulated annealing
adsorption detection module with the "COMPASS" force field. The simulations were
performed under medium convergence conditions. The lowest adsorption energies were
obtained for WA-2 and WA-1 as they interact with the Fe surface.
3
3
. Results and discussion
.1. Chemistry
The synthetic procedure for the novel peptidomimetic cationic surfactants is described
in scheme 1. Oxazolinone 3 was synthesized according to Plöchl- Erlenmeyer azlactone
synthesis with some modification. Compound 4 was synthesized by the ring opening reaction
of the oxazolin-5-one ring using N,N-dimethylpropane-1,3-diamine in DCM as a solvent at
room temperature. The alkyl-chloroacetate compounds were synthesized according to our
previous work.[11] A catalyst free quaternization reaction was conducted between compound
4
3
and alkyl-chloroacetate in a closed system to produce compounds WA-1, WA-2, and WA-
. The structures of the synthesized compounds were investigated by various spectroscopic
1
13
techniques (i.e., mp, IR, H and C NMR spectroscopy).
<<<<<<Scheme 1>>>>>>
3
.2. Surface activities of the prepared cationic surfactants
As shown in Fig. 2, the surface tensions (γ) versus the logarithm of the concentration
(C) for the solutions of the cationic surfactant were determined at 25°C. The surface tension
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values were inversely proportional to the cationic surfactant’s concentration at low
concentrations. At higher concentrations, the surface tension values were relatively stable.
The critical micelle concentration (CMC) of a cationic surfactant is the concentration at the
break. The CMC values for compounds WA-1, WA-2 and WA-3 at 298 K are 0.050, 0.062
-1
and 0.069 mmol/L , respectively. These values show that our novel peptidomimetic cationic
surfactants have excellent micelle forming ability. The CMC values of compounds WA-1,
WA-2 and WA-3 decreased with the increase in the alkyl chain length, which occurs because
of the increase in the hydrophobicity of the investigated surfactant molecules with the raising
of the length of the carbon chain. This increase in hydrophobicity leads to an increase in the
mutual repulsion between the oppositely charged moieties, such as nonpolar hydrophobic
carbon chains and polar water molecules. A great repulsion between the longer hydrophobic
chain and the water phase reduced the CMC value[37,39]. The surface-active parameters of
the cationic surfactants are summarized in Table 1.
<<<<<<Figure 2>>>>>>
<<<<<<Table 1>>>>>>
The value of the degree of counter ion dissociation (α) of the prepared cationic
surfactants was calculated at 298 K. α was deduced from the specific electrical conductivity
concentration curves and can be defined as the ratio between the premicellar and postmicellar
slopes. The relationship between α and the degree of counter ion binding (β) is β=1-α. The α
values of synthesized peptidomimetic cationic surfactants WA-3, WA-2 and WA-1 at 25 °C
were found to be 0.59, 0.55 and 0.53, respectively. As shown in Table 1, the specific
electrical conductivity of the obtained surfactants was found to be inversely proportional to
their hydrophobicity. Normally, the specific conductivity of cationic surfactants in an
aqueous solution depends on the strength of the bond between the head of the surfactant and
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their counter ion. All the numbers of counter ions were also found to be important parameters
in determining the specific conductivity. To minimize the conductivity of the synthesized
surfactant in aqueous solutions, we increased the hydrophobic chain length to decrease the
number of water molecules around the hydrophilic head of the synthesized surfactant.
Decreasing the hydration improved the density of the head charge, which increased the bond
strength between their associated counterions and the surfactant's head. Consequently, the
counter ion degree of dissociation has been reduced. Additionally, raising the molecular
weight of the synthesized cationic surfactants by the increasing the length of the hydrophobic
tail reduced the counterions’ number and the electrical conductivity.[45–48]
The critical micelle concentrations (CMCs) of compounds WA-1, WA-2, and WA-3
°
were determined at 25 C by surface tension and electrical conductimetric techniques. The
CMC values were obtained from the break point in the specific electrical conductivity versus
concentration curve and in the surface tension- log[C] curve, as shown in Fig. 2.
The packing density of the surfactant molecules can be determined from the surface
area (Amin) unavailable by the surfactant molecules at the interface of air-water [49]. The
maximum surface excess (Γmax) of the obtained surfactants is the surfactant unimer
concentration in the interface/unit area (Table 1). The extreme surface excess (Γmax) was
calculated utilizing Gibb’s adsorption equation 4, as follows [50]:

1
 d

max





(4)
2
.303nRT d log C


T
where the gas constant is R, n is the number of ionic species (n= 2 for a mono-valent cationic
surfactant), (δ γ/δ log c) is the slope below the CMC and the absolute temperature is T.
Additionally, the minimum surface area (Amin) (in square angstrom) is known as the middling
area occupied by each surfactant species adsorbed at the system interface. The Amin has been
calculated at room temperature using Gibb’s adsorption Eqn. [51].
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1
4
10
Amin

(5)
max  NA
where N is Avogadro’s number. As the hydrocarbon chain length increased from 10 to 16, we
observed an increase in the value of Γmax and a decrease in the value of Amin (Table 1). This
clearly indicates that the synthesized cationic surfactant has a higher packing density.
Although the hydrophilic group size is a major factor to determine the values of Amin and
Γmax,[52] the hydrophobic group’s interactions increased by increasing the hydrocarbon chain
length, which led to closer packing of the cationic surfactant molecules and, hence, increased
the Γmax values.[53]
The effectiveness (ΠCMC) is calculated utilizing equation (6), and it is known as the
difference between γ (i.e., the value of the surface tension of distilled water) and γCMC (i.e., the
o
CMC of the investigated surfactant).
ΠCMC = γ0 – γCMC
(6)
All the values of ΠCMC are listed in Table 1, and they indicate that the synthesized cationic
surfactants are highly effective in reducing the water surface tension. The dissolution of
surfactant molecules is the main cause of the great water surface tension reduction [54].
o
0
The standard free energy changes of micellization ( G ) and adsorption (G ) of
mic
ads
the synthesized peptidomimetic cationic surfactants were calculated using the following eq.
23]:
[
o
mic

G
 (2 )RT ln CMC
(7)
(8)
o
ads
G
 (2 )RT ln CMC  0.06CMC Amin
It can be inferred from the negative sign of the calculated thermodynamic parameters (Table
1
) that the free energy of micellization and adsorption are both spontaneous processes. The
o
mic
G
of our novel peptidomimetic cationic surfactants WA-3, WA-2, and WA-1 were -
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0
ads
3
3
3.37, -34.80 and -35.27 kJ/mol, respectively. The G of WA-3, WA-2, and WA-1 were -
7.08, -38.54 and -38.69 kJ/mol, respectively. From these data, it is noted that the
micellization and adsorption processes increase by raising the hydrophobicity because of the
increase in the repulsion between the polar aqueous solution and hydrocarbon group. This
kind of change leads to disruption in the water structure, in which the surfactant molecule is
dissolved and eventually leads to an increase in the system free energy. As expected, the
surfactant molecules migrate to the interface or aggregate in micelles to reduce that repulsion
process, and the free energy of the system consequently decreases[54].
3
.3. Potentiodynamic polarization (PDP) experiments.
PDP experiments were carried out to investigate the influence of WA-1, WA-2 and WA-
on the cathodic and anodic corrosion reactions of C-steel in 0.5 M hydrochloric acid
3
solution. The polarization profiles in blank 0.5 M HCl and with various doses of the studied
additives at 50 º C are presented in Fig. 3. The addendum of all surfactants to the corrosive
medium influenced the anodic, as well as the cathodic reactions, which indicate mixed type
inhibitions. The values of the electrochemical parameters estimated from the polarization
profiles, such as the cathodic and anode Tafel slopes (β β ), corrosion current density (I ),
c,
a
corr
and corrosion potential (E ) are presented in Table 2. The protection power (η%) was
corr
obtained from the Icorr values using the following eqn.[55]:
o
corr
/

I
 I

corr

/ %  
100
(9)

o

I

corr

0
corr
/
where
I
and
I
are the Icorr of the uninhibited and inhibited systems, respectively.
corr
According to the previous studies,[56,57] if the modification in Ecorr value is more than ±85
mV in the solutions containing inhibitors related to blank system, the compound could be
defined as a cathodic or anodic type inhibitor; if it is below ±85 mV, it might be defined as
the mixed type inhibitor. In this report, the supreme shifts in Ecorr with the addition of WA-1,
1
4

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WA-2 and WA-3 were found to be 31, 30 and 7 mV, respectively, thus leading to the
interpretation that the compounds behaved as mixed type inhibitors. It is noticed from Fig. 3
that the decrease in current density is greater at the cathode rather than at the anode, and the
β values are greater than the β values, which suggests the predominance of cathodic
c
a
inhibition.
<<<<<<Figure 3>>>>>>
<<<<<<Table 2>>>>>>
As shown in Table 2, the Icorr values in the presence of WA-1, WA-2 and WA-3 were
-2
higher than that of the uninhibited medium. The value of Icorr decreases from 2.189 mA cm
-2
-2
to 0.15104 mA cm by means of WA-3 to 0.05932 mA cm using WA-2 and to 0.03831 mA
-2
cm in the presence of WA-1 at 100 ppm. Consistently, the protection capacity rises with the
surfactant dose, and the maximum values obtained for WA-1, WA-2 and WA-3 were
observed to be 98.25%, 97.29% and 93.1%, respectively.
The increment in η% with increasing the surfactant dose could owe to a rise in the
number of adsorbed molecules at metal/medium interface. The number of molecules that are
adsorbed on the active sites on the metal surface increase with the increasing concentration
the surfactant and lead to an increase in the coverage surface (θ) and η%. The η% of carbon
steel with WA-1 was also higher than those of WA-2 and WA-3. The values of η% at all
investigated concentrations were rated as WA-1 > WA-2 > WA-3.
3
4. EIS data
The EIS spectra for C-steel in a 0.5 M solution of HCl with and without the
occurrence of different amounts of the examined surfactants are presented by Nyquist
diagrams and are exemplified in Fig. 4A, 4B and 4C for WA-1, WA-2 and WA-3,
respectively. The EIS spectra display analogous behavior in the inhibited and uninhibited
1
5

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systems, which indicates that the C-steel corrosion inhibition in the HCl by the investigated
inhibitors does not change the corrosion process mechanism.[58] The diameters of the
semicircles of the Nyquist profiles with the occurrence of the surfactants are usually greater
than those of the free inhibitors medium. This suggests the carbon steel/HCl interface shows
greater impedance to charge movements in the presence of the studied compounds. This is
suggestive of the protective action of the investigated inhibitors. Additionally, from Fig. 4,
the Nyquist profile displays one depressed semicircle conformable to a single time constant.
The Nyquist semicircle depression is due to frequency dispersal impacts that could be an
outcome from the nonhomogeneity and coarseness of the surface of the electrode and/or
further formulas of interstitial phenomena.[59]
<<<<<<Figure 4>>>>>>
The EIS data for the Nyquist plots was evaluated using the equivalent circuit routine,
and the consistent equivalent circuit model (ECM) is depicted in Fig. 4D. In the ECM, R is
s
the electrolyte resistance. The constant phase element (CPE) [60] for the metal/HCl interface
model was applied in place of the “perfect” capacitance. R represents the resistance of the
ct
charge transfer. The CPE impedance was stated as follows [58]:
1
ZCPE

(4)
n
Yo (J)
where j represents the imaginary unit, Y is the frequency independent admittance of CPE and
0
ω characterizes the angular frequency. n is the exponent of CPE, which is generally close to
1
. The CPE exponents were between 0.862 and 0.950, which designates a coarse medium on
the steel surface. The C was calculated by the following equation [61]:
dl
n1
n1
C  Y ()  Y (2fmanx)
(11)
dl
0
0
where fmax is the frequency at which the maximum imaginary occurs on the Nyquist profiles
in Fig. 4. The values of C , R , R , n and protection power (η %) were estimated and
dl
ct
s
E
1
6

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recorded in Table 3. The η % values of the inhibitors were intended from the R values using
E
ct
the following eqn. [47]:
i
ct
0
ct

R  R 
E / % 100 
 100 
(12)

i

R

ct

i
ct
o
ct
where
R
and
R
are the R in the inhibited and uninhibited systems. It is apparent from
ct
Table 3 that the R value increases and the C value decreases with the rise in the surfactant
ct
dl
doses of all the studied additives. The maximum η % achieved at the optimum concentration
E
of 100 ppm was found to be 96.96%, 95.36% and 92.38% for WA-1, WA-2 and WA-3,
respectively. These results are related to the development of a shielding film of the surfactant
species on the electrode surface. The preventative layer of the surfactant diminishes the
immediate approach of the C-steel with the corrosive medium.
<<<<<<Table 3>>>>>>
According to the Helmholtz model,[62,63] the C is inversely proportional to the
dl
protective layer intensity, which indicates that the decrease in C demonstrates the rise in the
dl
intensity of the covering film with an increment in the inhibitor dose. Subsequently, the
surfactant molecules replace more H O molecules of high dielectric constant present in the
2
metal surface vicinity when inhibitor molecules with low dielectric constants are added in the
HCl solution, which is indicative of reduction in the C value. Additionally, the maximum
dl
inhibition was detected at 100 mg/L, which is in good correspondence with the data achieved
from the PDP experiments.
3
.5 Adsorption isotherms and corrosion inhibition mechanism
To provide more information regarding the corrosion inhibition mechanism,
adsorption isotherm experiments were performed.[64] The surfactant molecule’s trend to
dislodge H O molecules from the steel interface in a reversible route is demonstrated via the
2
following formula[65]:
1
7

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Sur(ads)+nH O ↔ Sur(sol)+nH O
(ads)
(13)
2
(sol)
2
where n represents the number of H O molecules. The adsorption performance of the
2
examined surfactants was inspected by subjugation of the potentiodynamic polarization
results into different adsorption models, such as the Temkin, Frumkin, Langmuir and
Freundlich isotherms [65]. The results confirmed that the appropriate model was the
Langmuir adsorption isotherm model, as follows:
Cinh
1

C 
(14)
inh

Kads
where Cinh is the [inhibitor], Kads represents the equilibrium adsorption constant. The
diagrams of Cinh versus C /θ give a straight line, as presented in Fig. 5, and the K values
inh
ads
are obtained from the intercept. The straight-line relationship was obtained for the Temkin,
2
Frumkin, Langmuir, and Freundlich isotherms with a correlation coefficient (R ) over 0.8172.
2
The Langmuir adsorption isotherm (R = 0.9996) was found to be suitable for the
2
experimental findings close to the Frumkin adsorption isotherm (R = 0.8542), the Temkin
2
2
adsorption isotherm (R = 0.9112), and the Freundlich adsorption isotherm (R = 0.8172). In
addition, we mentioned that the slope of the Langmuir isotherm slightly deviated from the
unity (i.e., 0.971 (WA-1), 0.979 (WA-2), and 0.997 (WA-3)) expected of an ideal Langmuir
isotherm, which indicates the validity of Langmuir model [66]. The standard adsorption free
o
ads
energy ( G ) and Kads are related by the equation given below:
0
ads
1
G
Kads

exp(
)
(15)
5
5.5
RT
5
The Kads values obtained for the investigated inhibitors were high, i.e., 1.21×10 (WA-1),
5
4
1
.001×10 (WA-2) and 7.14×10 (WA-3). The observations indicate the replacement of the
H O species from the metal substrate by the surfactant molecules, and accordingly, the
2
o
ads
inhibitors adsorb powerfully on the surface of the steel. The trend of the G values is such
1
8

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-1
-1
-1
that WA-1 (-42.16 kJ mol ) > WA-2 (-41.51 kJ mol ) > WA-3 (-40.79 kJ mol ), which is
o
ads
the same as the relative order of the inhibition efficiency. The G values more than -40.16
-1
kJ mol indicate chemisorption.
<<<<<<Figure 5>>>>>>
3
.6. DFT calculations
In the present work, we performed quantum calculations to find the relationship
between the electronic features of the studied inhibitors and their experimental inhibition
efficiencies. Recently, quantum calculations have been used to investigate the adsorption of
new surfactants on the metallic interface [67,68].
The distribution density of the EHOMO (i.e., the highest occupied molecular orbital),
ELUMO (i.e., the lowest unoccupied molecular orbital), in the gas phase, and MEP (i.e., the
molecular electrostatic potential) are shown in Fig. 6. The HOMO density was localized on
the chlorine atom and the LUMO density was distributed on the aromatic rings. Molecular
electrostatic potential (MEP) is a method to analyze the reactive sites for nucleophilic and
electrophilic attacks in the molecules. The red color shows the electrophilic reactivity, and
the blue color shows the nucleophilic reactivity of the molecules (Fig. 6). Therefore, the most
favorable sites for electrophilic attacks are the carbon atoms attached to chlorine and oxygen
atoms where the negative regions are localized. Similarly, the most favorable sites for
nucleophilic attack are the positive regions of the inhibitor. The quantum descriptors are
listed in Table 4. The energies of HOMO and LUMO reflect the capability of the molecule to
donate and to receive electrons to/from the vacant 3d-orbital of iron.[69,70]
<<<<<<Figure 6>>>>>>
Increasing the alkyl chain length caused an increase in the EHOMO and a decrease in the ELUMO
values. Therefore, WA-1 is predicted to have a corrosion inhibition activity higher than WA-
1
9

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2
(Table 4). The energy gap (ΔE) value is associated with the stability of the synthesized
inhibitor and the chemical reactivity of the complex between the metal and the inhibitor. The
lower the value of ΔE, the higher the molecule’s protection capacity. Therefore, WA-1 is
predicted to be a much better corrosion inhibitor than WA-2.
<<<<<<Table 4>>>>>>
The dipole moment (µ) results from the irregular distribution of charges on various atoms of
the molecule.[71] A high dipole moment is desired to increase the adsorption of the molecule
on metal surfaces.[72] The dipole moment of the WA-1 molecule is higher than that of the
WA-2 molecule in the gas phase, which supports the prediction that the WA-1 molecule has
a higher corrosion inhibitor (Table 4). The dipole moments of WA-1 and WA-2 are not
similar in the liquid and gas phases
We have also calculated the global hardness (ƞ). The soft inhibitor is usually a highly reactive
inhibitor that possess a lower ΔE value, and the opposite is true for the hard inhibitor.[73]
Since the WA-1 inhibitor has lower ƞ value, this compound has a higher propensity to be
adsorbed over the iron surface. We calculated the fraction of electrons that is transferred from
the inhibitor molecule to the steel interface (∆N) using Pearson’s formula (Eq. 3). The
inhibitor molecules having higher ∆N values easily transfer electrons to the unoccupied d-
orbital of the metal interface. The electron donation of the WA-1 inhibitors was found to be
higher than that of WA-2.
3
.7. MD simulations
The interaction between the inhibitors and the iron surface was simulated using the Monte
Carlo method and was calculated by the following relation (16):
E =E -(E +E )
(16)
int
tot
Fe
inh
2
0

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where Etot, EFe and Einh are the total energy, energy of the Fe surface and energy of the
inhibitor molecules, respectively. The binding energy of the inhibitors was given by the
following:
Ebinding=-Eint
(17)
The side and top visions of the adsorption of the WA-2 and WA-1 inhibitors on Fe (110) are
shown in Fig. 7. The two inhibitors are adsorbed in a planar manner over Fe (110) by an alkyl
chain while the benzoyl and methoxybenzene groups were adsorbed in the perpendicular
direction on the iron interface. The adsorption energy values of the WA-2 and WA-1
inhibitors on Fe (110) are presented in Table 4. The adsorption energy of WA-1 is higher
than that of WA-2. This indicates that the WA-1 molecule adsorbs better than the WA-2
molecule on the Fe surface and confirms the corrosion inhibition obtained empirically.
<<<<<<Figure 7>>>>>>
3
.8. Comparison with earlier reports
A comparative investigation of the performance of the cationic surfactants-based corrosion
inhibitors is presented in Table 5 [36, 74–77]. From the obtained experimental findings and
the comparative chart, it can be observed that the as-prepared peptidomimetic cationic
surfactants display significant corrosion inhibition behavior, which supports its applicability
as efficient inhibitors in hydrochloric acid pickling solutions. The higher protection capacity
of the synthesized peptidomimetic surfactants compared to ordinary organic inhibitors is due
to the surfactant, up to the CMC, will diffuse out of the bulk water phase and is adsorbed at
the carbon steel/HCl interfaces. Additionally, the thin layer of surfactant formed on the metal
surface involved two inhibitive factors; one hydrophilic group involved a lone pair of
+
electrons of oxygen atoms, quaternary nitrogen atom (N ), π-electrons of the aromatic ring
and other water-insoluble hydrophobic groups.
2
1

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4
. Conclusions
In the present work, experimental and theoretical approaches were used to study the
protection actions of three novel peptidomimetic cationic surfactants on C-steel corrosion. All
the examined peptidomimetic cationic surfactants act as good additives for carbon steel
erosion, and their protection capacity increased with the increase in their concentration. The
combination of peptidomimetic moiety with a long alkyl chain improved the anti-corrosion
properties of the studied compounds. The studied peptidomimetic compounds showed a
protection power of 98.25% at 0.17 mM (at 0.5 M HCl and 50 °C temperature). The PDP
studies revealed that the new peptidomimetic surfactants behaved as mixed type inhibitors.
The impedance showed increases in the R values due to the adsorption of additives on the
ct
steel/medium interfaces following the Langmuir model and comprises chemisorption action.
The DFT calculations were found to be in agreement with the experimental findings. The MD
simulations demonstrated that the compounds adsorb powerfully on the iron (110) interface
by parallel or flat modes, and the Ebinding trend agrees with the obtained protection capacity.
This study introduced novel peptidomimetic compounds as a new class of cationic surfactants
that could be useful in the field of anti-corrosion applications.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
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