Synthesis, characterization and corrosion inhibition studies of polyunsaturated fatty acid derivatives on the acidic corrosion of mild steel: Experimental and computational studies

Article abstract of DOI:10.1016/j.molliq.2020.114162

In an effort to make an efficient and benign CI for the purpose of acidizing, a novel range of new derivatives of polyunsaturated fatty acids (PUFA) were prepared from a group of amines and Z-9,12-octadecadienoic acid with Excellent yields. Elemental analysis, FTIR, ¹³C NMR and ¹H NMR was employed to realize a description for the newly manufactured compound. The inhibitive action of synthesized amides was examined by means of potentiodynamic polarization techniques and weight loss measurements in 1.00 M HCl. Derivatives of Z-9,12-octadecadienoic acid amides (DA) were found to obey the Langmuir adsorption model. The hydrophobic nature of mild steel (MS) was revealed by measurement of the contact angle in the presence of CI. The experimental findings were found to be supported by quantum chemical calculations. Inhibition efficiencies were computed for various DA concentrations for inhibition against the wear of MS in 100.00 ml of 1.00 M HCl, with exposure for four days at temperatures ranging from 298 to 333 K. For a DA concentration of 100 ppm, every inhibitor molecule showed outstanding percentage inhibition efficiencies in 1.00 M HCl. Compounds 2, 3, 4, 5, 6, 7, 8 and 9 offered a robust percentage inhibition efficiency of 92.90, 86.6, 49.8, 82.7, 85.9, 96.70, 94.30 and 91.30, correspondingly, at 100 ppm. The interaction of the p-electrons in compounds with low-energy, empty Fe d-orbitals helped the inhibitive molecules (IMs) to experience adsorption and inhibit the process of anodic dissolution. When Tafel plots were employed for the compounds used in the electrochemical method, similar findings were obtained for the percentage inhibition efficiencies. Compound adsorption on the MS surface was discovered to obey Arrhenius and Transition state plots in 1.00 M HCl.

Full text of DOI:10.1016/j.molliq.2020.114162

Journal of Molecular Liquids 319 (2020) 114162
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis, characterization and corrosion inhibition studies of
polyunsaturated fatty acid derivatives on the acidic corrosion of mild
steel: Experimental and computational studies
a
b
c
Asma M. Elsharif a, , Samar A. Abubshait , Ismail Abdulazeez , Haya A. Abubshait
⁎
a
Department of Chemistry, College of Science, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, Dammam 31441, Saudi Arabia
Department of Chemistry, King Fahd University of Petroleum and Mineral, Dhahran 31261, Saudi Arabia
Department of Basic Science, Deanship of Preparatory Year, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, Dammam 31441, Saudi Arabia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2020
Received in revised form 2 August 2020
Accepted 26 August 2020
Available online 29 August 2020
In an effort to make an efficient and benign CI for the purpose of acidizing, a novel range of new derivatives of poly-
unsaturated fatty acids (PUFA) were prepared from a group of amines and Z-9,12-octadecadienoic acid with Excel-
lent yields. Elemental analysis, FTIR, 13C NMR and 1H NMR was employed to realize a description for the newly
manufactured compound. The inhibitive action of synthesized amides was examined by means of potentiodynamic
polarization techniques and weight loss measurements in 1.00 M HCl. Derivatives of Z-9,12-octadecadienoic acid
amides (DA) were found to obey the Langmuir adsorption model. The hydrophobic nature of mild steel (MS) was
revealed by measurement of the contact angle in the presence of CI. The experimental findings were found to be
supported by quantum chemical calculations. Inhibition efficiencies were computed for various DA concentrations
for inhibition against the wear of MS in 100.00 ml of 1.00 M HCl, with exposure for four days at temperatures ranging
from 298 to 333 K. For a DA concentration of 100 ppm, every inhibitor molecule showed outstanding percentage
inhibition efficiencies in 1.00 M HCl. Compounds 2, 3, 4, 5, 6, 7, 8 and 9 offered a robust percentage inhibition effi-
ciency of 92.90, 86.6, 49.8, 82.7, 85.9, 96.70, 94.30 and 91.30, correspondingly, at 100 ppm. The interaction of the
p-electrons in compounds with low-energy, empty Fe d-orbitals helped the inhibitive molecules (IMs) to experience
adsorption and inhibit the process of anodic dissolution. When Tafel plots were employed for the compounds used
in the electrochemical method, similar findings were obtained for the percentage inhibition efficiencies. Compound
adsorption on the MS surface was discovered to obey Arrhenius and Transition state plots in 1.00 M HCl.
© 2020 Elsevier B.V. All rights reserved.
Keywords:
Corrosion inhibition
Mild steel
Acidic media
DFT
Z-9,12-Octadecadienoic acid amides
1. Introduction
In contemporary years, some CIs have attained substantial consider-
ation as they offer efficient CI because of robust adsorption on the sur-
HCl is extensively employed in numerous industrial procedures such
as acidic cleaning, acid pickling, petrochemical processes and the acidifi-
cation of oil wells [1–3], which normally results in grave metallic wear.
As the most economic and effective technique, CIs are used in these acid
solutions to regulate the dissolution of metal [4,5]. A substantial determi-
nation is employed to produce efficient and unique CIs. The function of in-
hibitors can be effectively realized by organic compounds, which usually
save the metal from wear and corrosion by creating a protective film on
its surface [6]. Most of the famous CIs for the case of organic compounds
are those which comprise p-bonds as well as oxygen, sulfur and nitrogen
atoms [7–10]. A significant part can be played by these structures with
polar groups in the adsorption of IMs on the surface of metal [8–18].
A continuous determination has been conducted to establish a CI
that shows a better effect of inhibition at small concentrations in corro-
sion media in addition to an eco-friendly aspect [18].
face of metal [10,19–22].
Moreover, long-chain amidoamines are typically employed as MS
corrosion inhibitors. These amidoamines experience adsorption onto
the surface of metal via the amino group in which the chain of hydrocar-
bons results in the formation of a single protecting film [23].
The aromatic ring and the nitrogen atom in the molecular structure
are probably the enablers of the adsorption onto the surface of the
metal [9,24–27].
Generally, IMs adsorb onto the surface of metal by chemical or phys-
ical interaction, resulting in an adsorption sheath with the purpose of
saving the metal from erosion [28–30], and the behavior of adsorption
intensely relies on the electronic/chemical configuration of organic
compounds [31]. In general, it is deliberated that the process of adsorp-
tion brings about the development of back bonding and dative bond by
accepting and denoting electrons amid the surface of the metal and the
organic compounds [28,32], and the bond strength is associated with
the inhibition of corrosion. It is usually acknowledged that plane conju-
gated moieties such as simple arenes, unsaturated bonds and atoms
⁎
Corresponding author.
E-mail address: aelsharif@iau.edu.sa (A.M. Elsharif).
https://doi.org/10.1016/j.molliq.2020.114162
0167-7322/© 2020 Elsevier B.V. All rights reserved.

2
A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
showing electronegativity such as O, S and N play the most vital part in
the adsorption of IMs onto the surface of metal because of their empty
orbitals, which can accept or donate electrons, or the capability to
offer a lone pair of electrons [33]. Quantum chemical computations
counting density functional theory (DFT), ab initio and semi-empirical
approaches have been extensively employed in several studies with
the intention of quantitatively characterizing the electron configura-
tions of CIs based on organic compounds [33–35]. The inhibition process
for these CIs can be effectively elucidated by theoretical computations
[33,36–39].
Additionally, the inhibition efficiency of CIs based on organic com-
pounds has been determined in several circumstances via quantum
chemical indices similar to in the case of some electrons being shifted
from CIs to the dipole moment and metal and the frontier molecular or-
bital energy [40–44].
In recent times, nevertheless, Kokalj et al. suggested that the exam-
ination of the electronic configuration of CIs is not adequate to forecast
the tendency for their inhibition process despite the accomplishment in
examining the process of Cis [45–47]. In numerous circumstances, for
example, straight association between the investigational efficiency of
inhibition and calculated electronic parameters cannot be recognized
for a specified set of Cis [7,45,48–51].
0.5 ml of sample in 0.6 ml of CDCl3. Chemical shifts (δ) are presented
in parts per million using tetramethylsilane (TMS) as internal standard.
Moreover, elemental analysis was conducted using a Carlo-Erba Ele-
mental analyzer model 1106.
2.3. Corrosion study
2.3.1. Specimens preparation
Tests for corrosion inhibition were conducted by gravimetric mea-
surements in 1.00 M HCl by means of coupons made from mild steel hav-
ing the following percentage composition: 99.47 (Fe); 0.010 (P); 0.005
(V); 0.005 (Cu); 0.007 (Mo); 0.022 (Ni); 0.037 (Cr); 0.34 (Mn) and
0.089 (C). For the electrochemical measurement, a test specimen was
made into the shape of a flag from a mild steel sheet with a width of
1 mm. The flag base was approximately 3 cm in size and insulated via
araldite, which is an insulating paint. The rest of the area was 1 cm2,
with an exposed area of 2 cm2. The mild steel specimen was roughened
with emery papers having different grades (grit size of 100, 400, 600,
800 and 1200), degreased by sonication in acetone and cleaned with dis-
tilled water. The specimen was then dried and stored in a desiccator.
2.3.2. Solutions
Actually, a robust model for the collaboration amid the surface of a
metal and CIs must be developed in relating the theoretical findings to
investigational inhibition efficiency. Such a collaboration model could
offer info for both the adsorption energy and the interfacial structure
of organic molecules on the surface of a metal. It is extensively acknowl-
edged that the adsorption energy value, retrieved from either molecular
dynamic simulations or quantum chemical computations (QCC), is sim-
ilar in its forecast for the inhibition efficiency of Cis [47,52–59].
In this paper, eight new amides are synthesized and the CI efficiency
for MS in HCl solutions is examined by the weight loss method. The
mechanism is examined by QCC, and then molecular dynamic simula-
tions are employed to relate their inhibition efficiency to the adsorption
energy values. Furthermore, the electronic structures of those molecules
were also deliberated for enhanced knowledge of their adsorptive effec-
tiveness on the surface of MS.
Solutions of 1.00 M hydrochloric acid were prepared from concen-
trated HCl via dilution with deionized water. The inhibitor concentra-
tion ranged from 0 to 500 ppm. Each corrosion test was conducted
with electrolyte solution in equilibrium.
2.4. Gravimetric studies
Gravimetric corrosion measurements were carried out on a mild
steel specimen AISI 1018 with dimensions of 2.0 × 2.0 × 0.5 cm. Freshly
polished specimens were hanged in a 250 ml test solution (1.00 M HCl)
at 298–353 K in the absence and presence of 50–500 ppm inhibitors for
96 h. Thereafter, the specimens were cleaned thoroughly to remove
adsorbed corrosion products. The washing steps comprised of cleaning
with deionized water, followed by acetone, and, finally, drying in a vac-
uum oven at room temperature. Corrosion rates were calculated using
Eq. (1):
2. Experimental
8:76 ꢀ 104 ꢀ W
CR ðmmpyÞ ¼
ð1Þ
D ꢀ A ꢀ t
2.1. Materials and methods
where W is the weight loss (g), D is the mild steel density (g/cm3), A is
the specimen area (cm2) and t is the exposure time (h). The specimen
weight loss in the absence of inhibitor (blank) and in the presence of in-
hibitor molecules was computed accordingly. The procedure was re-
peated thrice to attain the anticipated standard deviation SD.
Inhibitor surface coverage (θ) and corrosion inhibition efficiencies
(η%) were calculated using Eqs. (2) and (3), respectively:
9,12-Octadecadienoic acid (linoleic acid), phosphoryl chloride,
aniline, o-anisidine, methylamine, ethylamine, diethylamine, 1-
naphthylamine, 3-amino-1,2,4-triazole-5-thiol and thiophene-2-
ethylamine from sigma Aldrich were employed for the creation of the
derivatives of 13-docosenoic acid. A Spectra/Por Standard Regenerated
Cellulose (RC) Membrane was bought from Spectrum Laboratories Inc.
and used for dialysis. DMSO (Dimethyl sulfoxide) was dehydrated
over calcium hydride for the duration of one night and then distilled
under decreased pressure at a b.p. of 180–189 °C (4 mm Hg). Every sol-
vent used was of HPLC (High Performance Liquid Chromatography)
grade. The equipment composed of glass was washed by means of
water having no ions or DI water. Each of the reactions was carried
out with a positive atmosphere of nitrogen. H2SO4 (Fisher Scientific
Company), concentrated HCl (A.C.S.) and distilled or DI water was
employed to develop solutions HCl (1 M). The research included exper-
iments involving the use of electrolyte solutions in open air by means of
aninhibitor witha concentration range between0and 500parts permil-
lion (ppm).
ꢀ
ꢁ
CRo−CRi
CRo
θ ¼
ð2Þ
ð3Þ
ꢀ
ꢁ
CRo−CRi
η% ¼
ꢀ 100
CRo
where CRo and CRi are the corrosion rates in the absence and presence of
inhibitors, respectively.
2.5. Computational procedure
Quantum chemical geometry optimizations of the isolated inhibitor
molecules were carried out using density functional theory (DFT) at the
B3LYP functional and 6-31G* basis set. The 6-31G* basis set was chosen
because compared to higher basis sets, it provides the best estimation
for solvation energies when using an implicit solvent model and it
often provides data in good correlation with experimental results at a
2.2. Spectral characterization of inhibitors
Infrared spectrum was recorded using a Shimadzu IR-Affinity FTIR
spectrometer. 13C NMR and 1H spectra were recorded using a Bruker
spectrometer, 400 MHZ. The samples were prepared by dissolution of

A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
3
lower cost [60]. Full geometry optimizations and vibrational frequency
calculations were carried out to the minima on the potential energy sur-
faces without imposing any constraints. Aqueous media were simulated
using Tomasi's polarized continuum model-self consistent reaction field
(PCM-SCRF) model [61] and water was chosen as the solvent. In this
model, the solvent is depicted as a continuum of uniform dielectric con-
stants and the cavity where the solute is placed is regarded as a contin-
uous series of interlocking spheres [62]. Performance of the inhibitors in
acidic medium was simulated by carrying out similar calculations for
the protonated forms of the molecules and the effects of protonation
were studied. Quantum chemical reactivity descriptors were computed
from the energies of the highest occupied molecular orbital (EHOMO) and
the lowest unoccupied molecular orbital (ELUMO) as follows [63]:
presence of the inhibitor molecules. The CA readings were averaged
and the RSD was less than 1.0% in all cases.
2.7. Synthesis of inhibitors
2.7.1. Synthesis of Z-octadeca-9,12-dienoyl chloride 1
In a single-neck round bottom flask, 5 ml of DMF in dry benzene and
a solution of 12-octadecadienoic acid (0.5 g, 0.0015 mol) were placed.
The flask was equipped with a condenser. A dropping funnel was used
to slowly add SOCl2 to the mixture as the reaction is endothermic. The
mixture was stirred for 10 min and heated in oil for 1 h at 90 °C. It
took 10 min on a rotary to remove all solvent from the mixture. The
cis-9, cis-12-octadecadienoyl chloride thus made was used for addi-
tional reaction as shown in the second part.
1H NMR: (δ, ppm): 0.86 (t, 3H, -CH3), 1.28–1.30 (m, 14H, -CH2-),
1.92 (m,2H, -CH2-CH2-C=O), 2.02 (q, 4H, -CH2-CH=), 2.73 (t, 2H, =
A ¼ −EHOMO
I ¼ −ELUMO
ð4Þ
ð5Þ
CH-CH2-CH=), 2.84 (t, 2H, -CH2-CO-Cl), 5.32 (t, 4H, -CH=CH-); 13
C
NMR: (δ, ppm): 14.1 (1C, -CH3), 23.2 (1C, -CH2-CH3), 24.3 (1C, -CH2-),
25.0 (1C, -CH2-CH2-CO-), 27.2 (2C, -CH2-CH=CH-), 28.9–32.6 (6C,
-CH2-), 47.1 (1C, -CH2- CO-), 129.8–137.2 (4C, -CH=CH-), 172.6 (1C,
-C=O). Exact mass: m/e 298.21 (100.0%), 300.20 (32.0%), 299.21
(20.5%), 301.21 (6.6%), 300.21 (2.1%). Anal. calcd. for C18H31ClO
(298.89); C, 72.33; H, 10.45; Cl, 11.86; found: C, 72.43; H, 10.25; Cl,
11.90.
ðI−AÞ
η ¼
ð6Þ
2
where I, A and η represent the ionization energy, electron affinity and
global hardness, respectively. Other descriptors are electronegativity
(χ) and electrophilicity index (ω), which depict the inhibitors ability
to attract electrons when chemically combined with other molecules
and are computed as follows:
2.7.2. Preparation of 9,12-octadecadienoic amides derivatives
In a round bottom flask, a mixture of some primary, secondary ali-
phatic and aromatic amines; namely; aniline (0.16 ml, 0.0017 mol), o-
anisidine (0.2 ml, 0.0017 mol), methylamine (0.053 ml, 0.0017 mol),
ethylamine (0.0766 ml, 0.0017 mol), diethyl amine (0.124 ml,
0.0017 mol), 1-naphthylamine (0.243 g, 0.0017 mol), 3-amino-1,2,4-
triazole-5-thiol (0.197 g, 0.0017 mol) and, thiophene-2-ethylamine
(0.21 ml, 0.0017 mol) respectively in (30 ml) dry benzene were used.
A guard tube was used with the flask. A dropping funnel was used for
pouring the mixture, which was subsequently mixed for 45 min. At re-
flux temperature, the mixture was mixed for 12 h until it reached room
temperature (TLC, n-hexane: ethyl acetate, 7:3, v:v). The mixture was
first washed with petroleum ether (40–60°) and then (80–100°). All
solvents were removed and evaporated with rotary evaporator to afford
Z-9,12-octadecadienoic amide 2, 3, 4, 5, 6, 7, 8, and 9 respectively.
ðI þ AÞ
χ ¼
ω ¼
ð7Þ
ð8Þ
2
μ2
2η
During inhibitor-metal interactions, electrons flow from the lower
electronegativity inhibitors to the more electronegative iron metals
until the chemical potentials become equal. According to Pearson [63],
the fraction of electrons transferred during this process is calculated
using the equation:
½χFe−χinh
ꢁ
À
Á
ΔN ¼
ð9Þ
2 ηFe−ηinh
where χFe, χinh, ηFe and ηinh represent the electronegativities and global
hardness of the bulk iron and the inhibitor molecules, respectively. The-
oretical values of 7.0 eV and 0 eV were assigned to χFe and ηFe accord-
ingly. All calculations were carried out using the Gaussian 09 package
[64] while data extraction, visualization and analysis were accom-
plished using the Gauss View 5.0 graphical user interface [65].
2.7.2.1. Synthesis of Z-octadeca-9,12-dienoic acid phenyl amide 2. Yield
(99%), as a brown oil. IR (KBr): v = 2916, 2843, 1445, 1375, 3010,
3022, 1630, 1620, 1590, 3300, 1668, 1575 and 1266 cm−1 1H NMR:
.
(δ, ppm): 0.96 (t, 3H, -CH3), 1.20–1.33 (m, 14H, -CH2-), 1.67 (m,2H,
-CH2-CH2-C=O), 1.06 (q, 4H, -CH2-CH=), 2.26 (t, 2H, -CH2-CO-NH),
2.73 (t, 2H, =CH-CH2-CH=), 5.31 (t, 4H, -CH=CH-), 7.20–7.64 (m,
5H, Ar-H), 8.3 (br, 1H, -NH-); 13C NMR: (δ, ppm): 13.0 (1C, -CH3), 22.2
(1C, -CH2-CH3), 24.7 (1C, -CH2-), 26.0 (1C, -CH2-CH2-CO), 27.1 (2C,
-CH2-CH=CH-), 28.9–32.6 (6C, -CH2-), 33.9 (1C, -CH2- CO-),
120.1–140.8 (6C, Ar-C), 123.8–130.2 (4C, -CH=CH-), 176.9 (1C, -C=
O). Exact mass: m/e: 355.29 (100.0%), 356.29 (27.3%), 357.29 (3.7%).
Anal. calcd. for C24H37NO (355.56): C, 81.07; H, 10.49; N, 3.94; found:
C, 81.27; H, 10.59; N, 4.00.
2.6. Surface characterization
Analysis of the surface of mild steel coupons immersed in 1.00 M HCl
for 24 h in the absence and presence of inhibitor molecules was con-
ducted using a field emission scanning electron microscope (FESEM).
Coupons with dimensions of 1.0 × 1.0 × 0.2 cm were immersed in
50 ml of test solutions with and without inhibitors. At the end of 24 h,
the coupons were removed, rinsed several times with water followed
by ethanol to remove adsorbed corrosion products and then later
dried in a vacuum oven. Micrographs were recorded using a TESCAN
LYRA 3 FESEM equipped with an energy-dispersive X-ray spectroscope
(EDX) detector.
To investigate the hydrophilicity/hydrophobicity of the surface of
the mild steel specimen upon adsorption of inhibitor molecules, contact
angle (CA) measurements were carried out using a One Attension Theta
Optical Tensiometer. Using the sessile drop technique with a drop size
of ~5.0 μL and water as solvent, an average of 3 points were measured
on coupons immersed in the test medium for 24 h in the absence and
2.7.2.2. Synthesis of Z-octadeca-9,12-dienoic acid (2-methoxy-phenyl)
amide 3. Yield (93%), as a brown oil. IR (KBr): v = 2952, 2842, 1443,
1371, 3010, 3013, 1630, 1589, 1520, 3270, 1670, 1575, 1210 and
1274 cm−1 1H NMR: (CDCl3, δ, ppm): 0.86 (t, 3H, -CH3), 1.26–1.33
.
(m, 14H, -CH2-), 1.67 (m,2H, -CH2-CH2-C=O), 2.02 (q, 4H, -CH2-CH=),
2.30 (t, 2H, -CH2-CO-NH), 2.75 (t, 2H, =CH-CH2-CH=), 3.73 (s, 3H,
-O-CH3), 5.43–5.43 (t, 4H, -CH=CH-), 6.75–7.53 (m, 4H, Ar-H), 8.0
(br, 1H, -NH-); 13C NMR: (δ, ppm): 14.0 (1C, -CH3), 22.6 (1C, -CH2-
CH3), 24.3 (1C, -CH2-), 26.2 (1C, -CH2-CH2-CO), 27.4 (2C, -CH2-CH=
CH-), 28.9–32.6 (6C, -CH2-), 33.9 (1C, -CH2- CO-), 56.0 (1C, -OCH3),
121.0–126.4 (5C, Ar-C), 123.8–133.2 (4C, -CH=CH-), 153.9 (1C, -C-

4
A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
OCH3), 172.0 (1C, -C=O). Exact mass: m/e: 385.30 (100.0%), 386.30
(28.8%), 387.30 (4.2%). Anal. calcd. for C25H39NO2 (385.58C), 77.87; H,
10.19; N, 3.63; found: C, 77.90; H, 10.29; N, 3.65.
4H, -CH2-CH=), 2.14 (t, 2H, -CH2-CO-NH), 2.32 (t, 2H, =CH-CH2-CH=),
5.25 (t, 4H, -CH=CH-), 7.26–7.86 (m, 7H, Ar-H), 8.0 (br, 1H, -NH-); 13
C
NMR: (δ, ppm): 14.0 (1C, -CH3), 23.2 (1C, -CH2-CH2-CH3), 24.3 (1C,
-CH2-), 26.0 (1C, -CH2-CH2-CO), 27.4 (2C, CH2-CH=CH-), 28.9–32.6
(6C, -CH2-), 33.9 (1C, -CH2- CO-), 109.4–142.0 (10C, -Ar-C),
123.8–133.2 (4C, -CH=CH-), 178.9 (1C, -C=O). Exact mass: m/e:
405.30 (100.0%), 406.31 (31.8%), 407.31 (5.1%). Anal. calcd. for
2.7.2.3. Synthesis of Z-octadeca-9,12-dienoic acid methyl amide 4. Yield
(98%), as a light brown oil. IR (KBr): v = 2920, 2860, 1445, 1376,
3011, 1630, 3140, 1668, 1520 and 1226 cm−1 1H NMR: (CDCl3, δ,
.
C28H39NO (405.62): C, 82.91; H, 9.69; N, 3.45; found: C, 82.95; H, 9.90;
ppm): 0.82 (t, 3H, -CH3), 1.24–1.33 (m, 14H, -CH2-), 1.57 (m,2H,
-CH2-CH2-C=O), 1.96 (q, 4H, -CH2-CH=), 2.13 (t, 2H, -CH2-CO-NH),
2.63 (t, 2H, =CH-CH2-CH=), 2.71 (s, 3H, -NH-CH3); 5.29–5.43 (t, 4H,
-CH=CH-); 7.2 (br, 1H, -NH-); 13C NMR: (δ, ppm): 14.0 (1C, -CH3),
23.0 (1C, -CH2-CH3), 24.6 (1C, -CH2-), 26.8 (1C, -CH3), 26.0 (1C, -CH2-
CH2-CO), 27.4 (2C, -CH2-CH=CH-), 29.0–32.6 (6C, -CH2-), 34.0 (1C,
-CH2- CO-), 123.8–133.2 (4C, -CH=CH-), 175.0 (1C, -C=O). Exact
Mass: m/e: 293.27 (100.0%), 294.28 (21.7%), 295.28 (2.4%). Anal.
calcd. for C19H35NO (293.49): C, 77.76; H, 12.02; N, 4.77; found: C,
77.81; H, 11.98; N, 4.57.
N, 3.75.
2.7.2.7. Synthesis of Z-octadeca-9,12-dienoic acid (5-mercapto-1H-[1,2,4]
triazol-3-yl) amide 8. Yield (82%), as a brown oil. IR (KBr): v = 2924,
2853, 1445, 1378, 3011, 1620, 3270, 1668, 1514, 1220 and 2540 cm−1
.
1H NMR: (δ, ppm): 0.84 (t, 3H, -CH3), 1.26–1.33 (m, 14H, -CH2-), 1.59
(m,2H, -CH2-CH2-C=O), 1.99 (q, 4H, -CH2-CH=), 2.01 (t, 2H, -CH2-
CO-NH), 2.75 (t, 2H, =CH-CH2-CH=), 5.33 (t, 4H, -CH=CH-),
6.80–7.30 (br, 1H, -NH-), 7.25 (br, 1H, -SH); 13C NMR: (δ, ppm): 14.0
(1C, -CH3), 23.2 (1C, -CH2-CH2-CH3), 24.3 (1C, -CH2-), 26.0 (1C, -CH2-
CH2-CO), 27.4 (2C, -CH2-CH=CH-), 28.9–32.6 (6C, -CH2-), 33.9 (1C,
-CH2-CO-), 123.8–133.2 (4C, -CH=CH-), 148 (2C, -CH-triazine), 179.0
(1C, -C=O). Exact mass: m/e: 378.25 (100.0%), 379.25 (22.8%), 380.24
(4.5%), 380.25 (3.2%), 379.24 (2.3%), 381.24 (1.1%). Anal. calcd. for
2.7.2.4. Synthesis of Z-octadeca-9,12-dienoic acid ethyl amide 5. Yield
(76%), as a brown oil. IR (KBr): v = 2916, 2843, 1435, 1375, 3010,
1631, 3200, 1700, 1515 and 1200 cm−1 1H NMR: (δ, ppm): 0.87 (t,
.
3H, -CH3), 1.24 (t, 3H, -NH-CH2-CH3), 1.29–1.33 (m, 14H, -CH2-), 1.61
(m,2H, -CH2-CH2-C=O), 2.00 (q, 4H, -CH2-CH=), 2.15 (t, 2H, -CH2-
CO-NH), 2.72 (t, 2H, =CH-CH2-CH=), 3.34 (q, 2H, -NH-CH2-CH3),
5.33 (t, 4H, -CH=CH-), 8.6 (br, 1H, -NH-); 13C NMR: (δ, ppm): 14.0
(1C, -CH3), 15.3 (1C, -CH2-CH3), 23.2 (1C, -CH2-CH2-CH3), 24.3 (1C,
-CH2-), 26.0 (1C, -CH2-CH2-CO), 27.4 (2C, -CH2-CH=CH-), 28.9–32.6
(6C, -CH2-), 34.3 (1C, -CH2- CO-), 35.9 (1C, -NH-CH2-CH3),
123.8–133.2 (4C, -CH=CH-), 179.8 (1C, -C=O). Exact Mass: m/e:
307.29 (100.0%), 308.29 (22.8%), 309.29 (2.6%). Anal. calcd. for
C
20H34N4OS (378.58): C, 63.45; H, 9.05; N, 14.80; S, 8.47; found: C,
63.60; H, 9.15; N, 14.60; S, 8.67.
2.7.2.8. Synthesis of Z-octadeca-9,12-dienoic acid (2-thiophen-2-yl-ethyl)
amide 9. Yield (99%), as a brown oil. IR (KBr): v = 2916, 2845, 1445,
1377, 3010, 1630, 3300, 1670, 1530 and 1200 cm−1 1H NMR: (δ,
.
ppm): 0.96 (t, 3H, -CH3), 1.29–1.33 (m, 14H, -CH2-), 1.57 (m,2H,
-CH2-CH2-C=O), 1.96 (q, 4H, -CH2-CH=), 2.23 (t, 2H, -CH2-CO-NH),
2.63 (t, 2H, =CH-CH2-CH=), 2.81 (t, 2H, -NH-CH2-CH2-), 3.53 (t, 2H,
-NH-CH2-CH2-), 5.43 (t, 4H, -CH=CH-), 6.60–6.91 (m, 3H, -CH- thio-
phene), 8.0 (br, 1H, -NH-); 13C NMR: (δ, ppm): 14.0 (1C, -CH3), 23.2
(1C, -CH2-CH2-CH3), 24.3 (1C, -CH2-), 26.0 (1C, -CH2-CH2-CO), 27.4
(2C, -CH2-CH=CH-), 28.9–32.6 (6C, -CH2-), 34.3 (1C, -CH2- CO-), 31.9
(1C, -NH-CH2-CH2-), 45.9 (1C, -NH-CH2-CH2-), 123.4–126.5 (4C, thio-
phene), 123.8–133.2 (4C, -CH=CH-), 174.7 (1C, -C=O). Exact mass:
m/e: 389.28 (100.0%), 390.28 (27.3%), 391.27 (4.4%), 391.28 (4.1%),
392.27 (1.2%), 390.27 (1.2%). Anal. calcd. for C24H39NOS (389.64): C,
73.98; H, 10.09; N, 3.59; S, 8.23; found: C, 74.00; H, 10.10; N, 3.40;
S, 8.23.
C
20H37NO (307.51): C, 78.11; H, 12.13; N, 4.55; found: C, 78.00; H,
12.03; N, 4.72.
2.7.2.5. Synthesis of Z-octadeca-9,12-dienoic acid diethyl amide 6. Yield
(86%), as a yellow oil. IR (KBr): v = 2952, 2862, 1454, 1376, 3011,
1628, 1711 and 1226 cm−1 1H NMR: (δ, ppm): 0.87 (t, 3H, -CH3),
.
1.20 (t, 6H, -NH-CH2-CH3), 1.29–1.33 (m, 14H, -CH2-), 1.59 (m,2H,
-CH2-CH2-C=O), 1.96 (q, 4H, -CH2-CH=), 2.03 (t, 2H, -CH2-CO-NH),
2.77 (t, 2H, =CH-CH2-CH=), 4.02 (q, 4H, -NH-CH2-CH3), 5.35–5.43 (t,
4H, -CH=CH-); 13C NMR: (δ, ppm): 14.0 (1C, -CH3), 13.2 (2C, -CH2-
CH3), 23.2 (1C, -CH2-CH2-CH3), 24.3 (1C, -CH2-), 26.0 (1C, -CH2-CH2-
CO), 27.4 (2C, -CH2-CH=CH-), 28.9–32.6 (6C, -CH2-), 32.1 (1C, -CH2-
CO-), 40.3 (2C, -N-CH2-CH3), 123.8–133.2 (4C, -CH=CH-), 172.9 (1C,
-C=O). Exact Mass: m/e: 335.32 (100.0%), 336.32 (25.5%), 337.33
(3.0%). Anal. calcd. for C22H41NO (335.57): C, 78.74; H, 12.32; N, 4.17;
found: C, 79.00; H, 12.10; N, 4.27.
3. Results and discussion
3.1. Characterization of 9,12-octadecadienoic amides derivatives
The new amide derivatives 2, 3, 4, 5, 6, 7, 8 and 9 were prepared in
excellent yields from low-cost materials: aniline, o-anisidine,
methylamine, ethylamine, diethyl amine 1-naphthylamine, 3-amino-
1,2,4-triazole-5-thiol and, thiophene-2-ethylamine respectively and
9,12-octadecadienoic acid as shown in Schemes 1, 2 and 3.
2.7.2.6. Synthesis of Z-octadeca-9,12-dienoic acid naphthalen-1-yl amide 7.
Yield (94.40%), as honey oil. IR (KBr): v = 2916, 2843, 1454, 1378, 3011,
1630, 3200, 1686, 1578 and 1227 cm−1. 1H NMR: (δ, ppm): 0.84 (t, 3H,
-CH3), 1.24–1.33 (m, 14H, -CH2-), 1.58 (m, 2H, -CH2-CH2-C=O), 1.97 (q,
CH3
CH3
HO
O
Cl
O
POCl3
DMF/benzene
90oC
(1)
Scheme 1. Schematic diagram of synthesis of acid chloride (1).

A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
5
CH3
NH
O
(2)
NH2
dry benzene
reflux/12 h
CH3
CH3
CH3
CH3CH2NH
O
Cl
O
CH3NH
O
CH3NH2
CH3CH2NH2
dry benzene
reflux/12 h
dry benzene
reflux/12 h
(5)
(4)
(1)
NH2
OCH3
dry benzene
reflux/12 h
CH3
OCH3
O
NH
(3)
Scheme 2. Schematic diagram of synthesis of acid amide derivatives (2–5).
The FTIR spectra for the newly synthesized compounds showed
bands in the regions at (2915–2955) cm−1 and (2840–2860) cm−1 for
C\\H stretching and bands at (3010–3015) cm−1 and (1575–1630)
cm−1 for = C-H, C_C aromatic and alkene stretching, respectively. Ad-
ditionally, new amide bands appeared at the regions (1670–1700)
cm−1 and (3140–3300) cm−1 for –C=O, N\\H, respectively. Compound
6 did not show N\\H amide stretching at this region. A new band ap-
peared at 1210 cm−1 due to the methoxy group for compound 3.
Furthermore, the 1H NMR spectra for the amide derivatives 2, 3, 4, 5,
6, 7, 8 and 9 showed major signals for (-CH3) at δ 0.82–0.96; (-CH2-
CONH-) at δ 2.01–2.30; (-CH2-CH2-CH2-) at δ 1.20–1.33; (=CH-
Compound 3 showed methoxy signals at δ 56.05 for (-O-CH3) and at
δ 153.90 for (=C-OCH3), respectively. Furthermore, compounds 2, 3
and 7 showed aromatic carbon signals at 109.4–130.36 to (=CH- Ar)
and 128.45–170.04 to (=C-NH- Ar).
Compound 8 showed signals at δ 142.0 and 148.0 for (1C, =C-SH)
and (1C, =C-NH-triazole), respectively. On the other hand, compound
9 showed major signals at δ 32.60; 42.90; 123.76–126.97 and 141.46
for (1C, -NH-CH2-CH2-); 42.9 (1C, -NH-CH2-CH2-); 123.76–126.97 (3C,
=CH- thiophene) and (1C, =C-S- thiophene), respectively. The results
obtained from measurement of the preliminary weight loss corrosion
rate after 96 h of immersion of mild steel in 1.00 M HCl in the absence
and presence of the inhibitors are presented in Fig. 1. The results reveal
that in the presence of molecules 2 and 7 having one and two aromatic
functionalities in their molecular structure, the corrosion rate for the
mild steel was minimal, indicative of higher corrosion inhibition effi-
ciencies. Hence, these species were selected for further study.
Surface characterization of coupons immersed in 1.00 M HCl for 24 h
without and in the presence of 100 ppm of selected inhibitor molecules
2, 7, 8 and 9 was carried out and the results are presented in Fig. 2. The re-
sults revealed that the coupon immersed in 1.00 M HCl without inhibitor
molecules (Fig. 2b) suffered a severe corrosion attack, which damaged the
microstructure compared to the freshly polished coupon (Fig. 2a). These
attacks were however significantly minimized for the coupons immersed
in the test media containing 100 ppm of inhibitor molecules, as shown in
Fig. 2c–f. This demonstrates the adsorption propensity for the molecules
CH2CH2−
) at δ 1.96–2.02; (=CH-CH2-CH=) at δ 2.63–2.77;
(CH2CH2CONH-) at δ 1.57–1.61; (-CH=CH-) at δ 5.29–5.43; (br, -NH-)
at δ 6.80–8.6 for 2, 3, 4, 5, 7, 8 and 9. Compounds 2, 3 and 7 showed
new aromatic protons (=CH- Ar) at δ 6.85–7.86. Moreover, compound
6 showed signals for (-N(CH2)2-) at δ 4.02. New signals appear at δ 2.81
and δ 3.53 for (-CH2-CH2-NH-) and (-NH-CH2-), respectively, for com-
pound 9.
13C NMR spectra for compounds 2, 3, 4, 5, 6, 7, 8 and 9 showed major
carbon signals at δ 13.07–14.10 to (-CH3); δ 22.20–24.69 to (CH2-CH3);
δ 26.01–26.20 to (-CH2-CH2CO-); δ 27.10–27.40 to (-CH2-CH=); δ
32.10–34.30 to (-CH2-CO-NH); δ 172.01–179.80 (1C, -CONH); δ
123.80–137.28 to (-CH=CH-). New signals appeared at δ 25.79, 35.70,
40.36 to (1C, -NH-CH3), (1C, -NH-CH2), and (2C, -N-CH2) for com-
pounds 4, 5 and 6, respectively.

6
A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
S
CH3
CH2CH2
NH
O
(9)
dry benzene
CH2CH2NH2
reflux/12 h
S
HS
NH
CH3
N
N
CH3
CH3
NH
N
CH2CH3
H
N
Cl
O
NH2
HS
NH
O
CH3CH2
N
O
N
dry benzene
reflux/12 h
dry benzene
reflux/12 h
(1)
(6)
(8)
NH2
dry benzene
reflux/12 h
CH3
NH
O
(7)
Scheme 3. Schematic diagram of synthesis of acid amide derivatives (6–9).
on the surface of mild steel to serve as protective shields against corrosion
attack, as found from weight-loss studies.
interaction or repulsive forces that exist between them. Values of <90°
are associated with hydrophilic surfaces while values of >90° are due to
hydrophobic surfaces. The results from CA measurements for coupons
immersed in a test medium for 24 h in the absence and presence of in-
hibitor molecules are presented in Fig. 3. A freshly polished coupon ex-
hibited a CA of 67.5°. After immersion in 1.00 M HCl for 24 h, there was a
significant drop in the CA value, indicating an increase in surface wetta-
bility as a result of acid attack on the coupon. The coupon immersed in
the test medium containing 100 ppm of inhibitor (7), however, revealed
a high degree of hydrophobicity with a CA value of 106.2° as a result of
adsorption of the inhibitor molecule on the surface of the coupon, which
forms a protective film against corrosion attack. Film formation and the
consequent increase in hydrophobicity of the coupons is derived from
the molecular structures of the inhibitor molecules, which contain
heteroatomic adsorption centers that bind with the free metal ions,
and the hydrophobic alkyl chains, which isolate the metal surface
from the corrosive medium. Results from CA measurements are, there-
fore, in good agreement with those obtained from weight-loss studies
and FESEM analysis.
Adsorption of the inhibitor molecules on the surface of mild steel
was further investigated by contact angle measurements. The contact
angle (CA) reveals the hydrophobicity/hydrophilicity of a given solid
surface. It is a physical parameter which is determined by the properties
of both the solid and the liquid droplet suspended on top as well as the
10
100
80
60
40
20
0
96.7
94.3
92.9
91.3
86.6
85.9
82.7
8
6
4
2
0
49.8
3.2. Effect of concentration and temperature
Effect of increase in inhibitor concentration for mild steel corrosion
in the presence of selected molecules (2, 7, 8 and 9) was studied and
the results are presented in Table 1. A corresponding decrease in corro-
sion rate for increasing inhibitor concentration was observed for all the
molecules. This is because increasing the inhibitor concentration
Test medium
Fig. 1. Weight loss corrosion rates and inhibition efficiencies in 1.0 M HCl in the absence
and presence of 100 ppm inhibitor molecules.

A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
7
Fig. 2. FESEM micrographs of (a) freshly polished mild steel coupon and coupon immersed in 1.00 M HCl for 24 h without inhibitors (b) and with 100 ppm of inhibitors 2 (c), 7 (d), 8
(e) and 9 (f).
increases the surface coverage on the mild steel coupon, providing more
protection from the acid medium. An optimum IE of 93.8%, 96.3%, 96.8%
and 94.5% was recorded at 500 ppm and 298 K for molecules 2, 7, 8 and
9, respectively. Furthermore, the role of temperature on the corrosion
inhibitive performance for the molecules was studied by increasing
the temperature of the test medium up to 80 °C (353 K). While the mol-
ecules were able to inhibit mild steel corrosion at higher temperature, it
was observed that as the temperature increased their efficiencies de-
creased except for molecule 7 Table 1. This is because increasing the
temperature of the test medium increases the kinetic energy of the mol-
ecules, which consequently weakens the electrostatic force of attraction
between the metal atoms and the molecules. As a result, the rate of dis-
solution of the metal increases. The exception with molecule 7 could be
attributed to the structural orientation of the molecule, which unlike
other molecules does not contain bulky groups around the heteroatoms
that serve as the basic anchor centers to the atoms of the metal. This en-
ables proper coordination with the metal atoms, which leads to an effec-
tive surface coverage [66]. Remarkable inhibitive efficiencies of 92.7%,
97.0%, 94.9% and 93.4% were recorded at 500 ppm and 353 K for mole-
cules 2, 7, 8 and 9, respectively.
In addition, energies of activation for mild steel corrosion in the ab-
sence and presence of inhibitor molecules were evaluated following the
Arrhenius Eq. (10):
−Ea
2:303RT
logCR ¼
þ logA
ð10Þ
where T is the absolute temperature, A is the Arrhenius pre-exponential
factor and R is the ideal gas constant (8.314 J K−1 mol−1). A plot of log
CR against the reciprocal of temperature gave straight lines for both a
blank medium and a medium containing inhibitor, as shown in Fig. 4.
The corresponding Ea values presented in Table 2 revealed that the acti-
vation energy for mild steel dissolution increases from a value of 12.9 kJ/
mol to maximum values of 24.2 kJ/mol, 24.8 kJ/mol, 29.3 kJ/mol and
20.9 kJ/mol in the presence of inhibitor molecules 2, 7, 8 and 9, respec-
tively. This reveals that the addition of the inhibitors heightens the acti-
vation barrier for mild steel dissolution, slowing down the corrosion
rate. Other thermodynamic functions that include the standard entropy
of activation (ΔS*) and the standard enthalpy of activation (ΔH*) were
evaluated by means of the Transition state Eq. (11):
Fig. 3. Sessile drop CA measurements of mild steel coupon (a) freshly polished, (b) after immersion in 1.0 M HCl solution for 24 h in the absence of inhibitors, and (c) after immersion in
1.0 M HCl containing 100 ppm of inhibitor 7.

8
A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
Table 1
Weight loss data of mild steel corrosion at temperatures 298–353 K and inhibitors concentration 50–500 ppm.
Medium/inhibitor conc. (ppm)
298 K
313 K
333 K
353 K
CR (mmpy)
7.304
θ
η (%)
CR (mmpy)
9.752
θ
η (%)
CR (mmpy)
14.22
θ
η (%)
CR (mmpy)
15.96
θ
η (%)
Blank
(2)
50
100
200
500
(7)
50
100
200
500
(8)
50
100
200
500
(9)
50
–
–
–
–
–
–
–
–
0.986
0.518
0.474
0.453
0.865
0.929
0.935
0.938
86.5
92.9
93.5
93.8
1.541
0.731
0.692
0.634
0.842
0.925
0.929
0.935
84.2
92.5
92.9
93.5
2.915
1.237
1.065
1.024
0.795
0.913
0.925
0.928
79.5
91.3
92.5
92.8
4.389
1.596
1.245
1.165
0.725
0.900
0.922
0.927
72.5
90.0
92.2
92.7
0.760
0.243
0.226
0.109
0.896
0.955
0.965
0.963
89.6
95.5
96.5
96.3
1.463
0.438
0.341
0.166
0.850
0.955
0.965
0.983
85.0
95.5
96.5
98.3
2.389
0.711
0.540
0.298
0.832
0.950
0.962
0.979
83.2
95.0
96.2
97.9
3.112
1.197
0.846
0.479
0.805
0.925
0.947
0.970
80.5
92.5
94.7
97.0
0.803
0.419
0.350
0.234
0.890
0.943
0.952
0.968
89.0
94.3
95.2
96.8
1.706
0.585
0.536
0.361
0.825
0.940
0.945
0.910
82.5
94.0
94.5
91.0
3.470
1.024
0.867
0.597
0.756
0.928
0.939
0.958
75.6
92.8
93.9
95.8
5.011
1.659
1.356
0.814
0.686
0.896
0.915
0.949
68.6
89.6
91.5
94.9
1.446
0.633
0.540
0.402
0.802
0.913
0.926
0.945
80.2
91.3
92.6
94.5
2.389
0.897
0.877
0.585
0.755
0.908
0.910
0.940
75.5
90.8
91.0
94.0
3.669
1.621
1.336
0.910
0.742
0.886
0.906
0.936
74.2
88.6
90.6
93.6
5.506
2.154
1.564
1.053
0.655
0.865
0.902
0.934
65.5
86.5
90.2
93.4
100
200
500
ꢂ
ꢃ
CR
T
−ΔHꢂ
2:303RT
R
ΔSꢂ
of inhibitor molecules, which provides a protective film on the surface of
the metal, was able to affect the kinetics of dissolution [67]. Upon the
addition of inhibitor molecules, a gradual increase in the entropy of
the system (ΔS*) was also observed, which suggests that the degree of
disorderliness of the molecules decreases in the course of the transition
from the reactants to the activated complex, as is reflected in an overall
decrease in the corrosion rate.
Adsorption of the inhibitor molecules on the surface of mild steel
was further investigated by fitting the data to adsorption isotherms.
Commonly used adsorption isotherms are the Langmuir, Frumkin and
Temkin isotherms, each of which relates the surface coverage (θ) to
log
¼
þ
log
þ
ð11Þ
Nh 2:303R
where N is Avogadro's number and h is the Planck constant. By plotting
log (CR/T) against the reciprocal of temperature (Fig. 5), intercepts [log
(R / Nh) + ΔS* / 2.303R] and straight slope lines [−ΔH* / 2.303R] were
obtained from which the values of ΔS* and ΔH* were computed, as pre-
sented in Table 2.
From Table 2, it is obvious that the addition of inhibitor molecules re-
sults in a surge in the enthalpy of activation (ΔH*), which suggests that
the dissolution of the metal is the rate limiting step. Hence, the addition
Fig. 4. Arrhenius plots of mild steel corrosion in 1.00 M HCl in the absence (blank) and presence of 50–500 ppm of inhibitors (a) 2, (b) 7, (c) 8 and (d) 9.

A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
9
Table 2
molecules, as shown in Fig. 6. The corresponding adsorption data are
shown in Table 3.
Thermodynamic parameters of mild steel corrosion in the absence and presence of
50–500 ppm of inhibitor molecules.
Gibbs free energy of adsorption was computed using the eq:
Inhibitor Concentration (ppm)
E )
a (kJ/mol) ΔH (kJ/mol) ΔS (J mol−1 K−1
ꢄ
ꢅ
ΔGads ¼ −RT ln 1 ꢀ 106 Kads
ð12Þ
Blank
(2)
–
50
12.9
24.2
18.5
15.7
15.7
22.2
24.8
20.9
23.8
29.3
22.2
21.5
22.6
20.9
20.2
17.0
15.7
19.9
68.7
57.9
50.7
51.1
67.6
81.9
71.4
85.8
84.5
70.2
69.5
68.8
56.1
61.2
53.2
52.1
−185
−174
−180
−182
−182
−175
−174
−178
−175
−169
−176
−177
−178
−177
−178
−181
−182
100
200
500
50
100
200
500
50
100
200
500
50
100
200
500
where 1 × 106 is the water concentration in mg/L. Typical values of
ΔGads greater in magnitude than −40 kJ/mol are related to chemisorp-
tion, while values less than −20 kJ/mol are usually accredited to
physisorption [68]. As shown in Table 3, the ΔGads value for the mole-
cules fall amid the twofold classifications suggesting that the molecules
undergo both chemical and physical adsorption modes on the MS
surface.
(7)
(8)
(9)
3.3. Computational results
Molecular geometry optimizations for the inhibitor molecules were
carried out using density functional theory (DFT) calculations at the
B3LYP functional and 6-31G* basis set. The lowest energy conformers
representing the most stable geometries for the molecules as well as
the frontier orbitals distributions for the highest occupied molecular or-
bital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are
presented in Fig. 7. The corresponding reactivity descriptors for the neu-
tral isolated inhibitor molecules and the protonated forms of the mole-
cules are given in Table 4 and Table 5. It can be seen from Fig. 7 that the
frontier orbitals were mainly distributed over the heteroatoms and the
pi system on the molecules, except for molecules 1, 4, 5 and 6 where
they are equally distributed across the alkyl chain. This suggests the
propensity for the inhibitor molecules to donate charges into the vacant
d-orbital of the metal atoms and their equal tendency to accept back-
donation from the metal atoms, leading to strong attractions and the
consequent formation of adsorbed layers on the surface of mild steel.
the concentration of inhibitor molecules according to a set of given
equations [68]. The Langmuir isotherm suggests that a given adsorbate
only covers one substrate site at a time. According to this model, all the
sites on the substrate have equal surface free energies, and no interac-
tion occurs between the adsorbing molecules. However, once the con-
centration of the adsorbed molecules is sufficiently high to the extent
that there is the possibility of attractive or repulsive interactions
among them, this model fails and it becomes necessary to fit the data
using either the Frumkin or Temkin isotherm model. Weight-loss data
were fitted into the Langmuir isotherm for all inhibitor molecules and
the results gave linear fits with slopes almost equal unity for all
Fig. 5. Transition state plots of mild steel corrosion in 1.00 M HCl in the absence (blank) and presence of 50–500 ppm of inhibitors (a) 2, (b) 7, (c) 8 and (d) 9.

10
A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
Fig. 6. Langmuir adsorption plots for inhibitor molecules (a) 2, (b) 7, (c) 8 and (d) 9 at temperatures 298–353 K.
Reactivity descriptors: energy of the highest occupied molecular or-
increases with a decrease in global hardness. This implies that mole-
cules having higher values of η are mostly less reactive compared to
those with lower η values due to their lower propensity to donate
charges. Hence, for effective charge donation, an inhibitor molecule
should possess minimal hardness. The dipole moment (μ) is a measure
of the polarizability of a molecule. It provides an insight into the distri-
bution of charges on the surface of a molecule. Lower values of μ there-
fore imply that a molecule is less susceptible to undergo dipole-dipole
interactions in the presence of an electrostatic force [70–77]. As seen
from Table 5, protonation of the inhibitor molecules resulted in an in-
crease in their overall surface charge making them more prone to
charge transfer processes. Molecule 7 has the highest value for dipole
moment in an acidic medium, 43.90 Debye, and is, therefore, predicted
to have higher corrosion inhibition efficiency, in agreement with results
from weight-loss studies.
bital (EHOMO), energy of the lowest un-occupied molecular orbital
(ELUMO), energy gap (ΔE), electronegativity (χ), global hardness (η),
electrophilicity index (ω), fraction of transferred electrons (ΔN) and di-
pole moment (μ) were computed for both the neutral molecules and the
protonated forms of the molecules. Whereas most of the descriptors did
not show a significant change upon protonation of the molecules, the
global hardness and dipole moment were observed to show reasonable
discrimination among the non-protonated and protonated molecules.
Global hardness (η) expresses the resistance of a given molecule to elec-
tron density distortion. It is a relevant reactivity descriptor that ex-
presses the readiness of a molecule to donate its non-bonding
electrons when interacting with other molecules. In line with the
hard-soft acid-base (HSAB) principle [69], reactivity of molecules
Table 3
4. Conclusion
Langmuir adsorption data of inhibitor molecules at temperatures 298–353 K.
A group of new amides derivatives; 9,12-octadecadienoic amides in-
hibitors were synthesized for the first time at high yields (>90%) from
9,12-octadecadienoic acid with some primary and secondary aliphatic
and aromatic amines for the first time. The main objective of the study
was to synthesize inhibitor molecules from poly unsaturated fatty
acids that can provide effective protection from corrosion of mild steel
in acidic media. Inhibitors 2, 7, 8 and 9 show %IE at 92.7, 97.0, 94.9
and 93.4 at 500 ppm, respectively, which serve as donor-centers for
the empty d-orbitals of the metal atoms. The adsorption of inhibitor
molecules onto the surface of mild steel followed a Langmuir adsorption
isotherm and the values for the free energy of adsorption ΔGads indi-
cated that the inhibitors adsorb through a mix of physisorption and
chemisorption mechanisms. The results are indeed very promising
and pave the way to exploit the excellent ability of synthesized com-
pounds to inhibit corrosion of mild steel. Further work has to be carried
Inhibitor
Temperature (K)
Kads × 10−2 (Lmol−1
)
ΔGads (kJ/mol)
(2)
298
313
333
353
298
313
333
353
298
313
333
353
298
313
333
353
32.8
24.7
17.2
10.2
42.7
15.1
13.9
10.8
21.8
14.7
9.91
6.37
12.7
9.49
8.29
5.54
−31.5
−32.3
−33.4
−33.8
−32.1
−31.0
−32.8
−34.0
−30.5
−31.0
−31.8
−32.5
−29.1
−29.8
−31.4
−32.1
(7)
(8)
(9)

A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
11
(1)
(2)
(3)
Optimized
geometry
HOMO
LUMO
(4)
(5)
(6)
Optimized
geometry
HOMO
LUMO
(7)
(8)
(9)
Optimized
geometry
HOMO
LUMO
Fig. 7. Optimized geometries and frontier orbital distribution of inhibitor molecules at B3LYP/6-31G*.
Table 4
out to investigate the effectiveness of inhibitor molecules exposed in
acidic media for longer durations and with flowing systems using addi-
tional techniques for the study of corrosion inhibition.
Quantum chemical reactivity descriptors of non-protonated isolated inhibitor molecules.
Inhibitor EHOMO
(eV)
ELUMO
(eV)
ΔΕ
(eV)
η
(eV)
χ
(eV)
ω
(eV)
ΔN
μ
(Debye)
Declaration of competing interest
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
−6.448 −1.081 5.367 2.683 3.765 2.641 0.603 3.968
−6.197 −0.416 5.782 2.891 3.306 1.891 0.639 4.145
−6.114 −0.405 5.709 2.855 3.259 1.861 0.655 6.104
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Also, all authors declare that this work is original and has not been
published elsewhere nor is it currently under consideration for publica-
tion elsewhere. There is No conflict of interest regarding the publication
of this paper.
−6.433
−6.432
−6.372
0.461 6.893 3.447 2.986 1.293 0.582 4.857
0.460 6.892 3.446 2.986 1.294 0.582 4.990
0.458 6.830 3.415 2.957 1.280 0.592 4.802
−5.877 −1.302 4.575 2.288 3.590 2.816 0.745 4.488
−6.243 −0.230 6.013 3.007 3.236 1.742 0.626 7.599
Acknowledgements
Table 5
Quantum chemical reactivity descriptors of protonated isolated inhibitor molecules.
The authors gratefully acknowledge the financial support provided
by Imam Abdulrahman Bin Faisal University through project number
2019-340-Sci. Authors would also like to acknowledge the support re-
ceived from Dr. Khalid Alhooshani, Head of the Chemistry Department,
King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia.
Inhibitor EHOMO
(eV)
ELUMO
(eV)
ΔΕ
(eV)
η
(eV)
χ
(eV)
ω
(eV)
ΔN
μ
(Debye)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
−6.451 −3.759 2.691 1.346 5.105 9.683 0.704 42.63
−6.445 −1.728 4.718 2.359 4.086 3.540 0.618 37.86
−6.442 −1.708 4.734 2.367 4.075 3.507 0.618 30.86
−6.450 −1.549 4.901 2.451 4.000 3.264 0.612 36.28
−6.450 −1.494 4.955 2.478 3.972 3.184 0.611 42.15
−6.448 −1.416 5.032 2.516 3.392 3.073 0.610 43.90
−6.423 −1.886 4.557 2.279 4.145 3.770 0.627 33.24
−6.451 −2.142 4.309 2.155 4.297 4.284 0.627 38.85
References
[1] H. Keles, M. Keles, I. Dehri, O. Serindag, Mater. Chem. Phys. 112 (2008) 173.
[2] H. Wang, H. Fan, J. Zheng, Mater. Chem. Phys. 77 (2003) 655.
[3] S.A.A. El-Maksoud, A.S. Fouda, Mater. Chem. Phys. 93 (2005) 84.

12
A.M. Elsharif et al. / Journal of Molecular Liquids 319 (2020) 114162
[4] M. Lagrenée, B. Mernari, M. Bouanis, M. Traisnel, F. Bentiss, Corros. Sci. 44 (2002)
573.
[43] M. Yadav, D. Behera, S. Kumar, R.R. Sinha, Ind. Eng. Chem. Res. 52 (2013)
6318–6328.
[5] A. Abdallah, Corros. Sci. 45 (2003) 2705.
[6] I.B. Obot, N.O. Obi-Egbedi, Corros. Sci. 52 (2010) 657.
[44] A. Döner, R. Solmaz, M. Özcan, G. Kardas, Corros. Sci. 53 (2011) 2902–2913.
[45] A. Kokalj, S. Peljhan, M. Finšgar, I. Milošev, J. Am. Chem. Soc. 132 (2010)
16657–16668.
[46] A. Kokalj, Electrochim. Acta 56 (2010) 745–755.
[47] N. Kovacˇević, A. Kokalj, Corros. Sci. 53 (2011) 909–921.
[48] N. Khalil, Electrochim. Acta 48 (2003) 2635–2640.
[49] E.E. Ebenso, M.M. Kabanda, L.C. Murulana, A.K. Singh, S.K. Shukla, Ind. Eng. Chem.
Res. 51 (2012) 12940–12958.
[50] Sudheer, M.A. Quraishi, Corros. Sci. 70 (2013) 161–169.
[51] W. Li, Q. He, C. Pei, B. Hou, Electrochim. Acta 52 (2007) 6386–6394.
[52] M.A. Chidiebere, C.E. Ogukwe, K.L. Oguzie, C.N. Eneh, E.E. Oguzie, Ind. Eng. Chem.
Res. 51 (2012) 668–677.
[53] E.E. Oguzie, K.L. Oguzie, C.O. Akalezi, I.O. Udeze, J.N. Ogbulie, V.O. Njoku, ACS Sust.
Chem. Eng. 1 (2013) 214–225.
[54] W. Guo, S. Chen, Y. Feng, C. Yang, J. Phys. Chem. C 111 (2007) 3109–3115.
[55] A.Y. Musa, R.T.T. Jalgham, A.B. Mohamad, Corros. Sci. 56 (2012) 176–183.
[56] B. Gómez, N.V. Likhanova, M.A.D. Aguilar, O. Olivares, J.M. Hallen, Martínez-Magadán, J.
Phys. Chem. A 109 (2005) 8950–8957.
[57] A. Kokalj, S. Peljhan, Langmuir 26 (2010) 14582–14593.
[58] D. Turcio-Ortega, T. Pandiyan, J. Cruz, E. Garcia-Ochoa, J. Phys. Chem. C 111 (2007)
9853–9866.
[59] F. Zhang, Y. Tang, Z. Cao, W. Jing, Z. Wu, Y. Chen, Corros. Sci. 61 (2012) 1–9.
[60] I. Abdulazeez, A. Zeino, C.W. Kee, A.A. Al-Saadi, M. Khaled, M.W. Wong, A.A. Al-
Sunaidi, Appl. Surf. Sci. 471 (2019) 494–505.
[61] J. Tomasi, B. Mennucci, R. Cammi, ChemInform 36 (2005) 2999–3093.
[62] A.A. Al-Saadi, Arab. J. Sci. Eng. 45 (2020) 153.
[7] Y. Tang, X. Yang, W. Yang, R. Wan, Y. Chen, W. Yin, Corros. Sci. 52 (2010) 1801.
[8] L. Wang, Corros. Sci. 43 (2001) 2281.
[9] A. Popova, M. Christov, T. Deligeorgiev, Corrosion 59 (2003) 756.
[10] K.F. Khaled, Electrochim. Acta 55 (2010) 6523–6532.
[11] S.A. Ali, A.M. El-Shareef, R.F. Al-Ghamdi, M.T. Saeed, Corros. Sci. 47 (2005)
2659–2678.
[12] S.A. Ali, A.M. El-Sharif, Corros. Eng. Sci. Technol. 47 (2012) 265–271.
[13] S.A. Ali, S.A. Haladu, A.M. El-Sharif, J. Polym. Res. 23 (2016) 167.
[14] A.M. El-Sharif, Asian J. Chem. 29 (2017) 1779–1784.
[15] S.A. Ali, S.A. Haladu, A.M. El-Sharif, Macromol. Res. 24 (2016) 163–169.
[16] S.A. Ali, M.T. Saeed, A.M. El-Sharif, Polym. Eng. Sci. 52 (2012) 2589–2596.
[17] S.A. Ali, S.M. Zaidi, A.M. El-Sharif, A.A. Altaq, Polym. Bull. 69 (2012) 491–507.
[18] H.J. Guadalupe, E. García-Ochoa, P.J. Maldonado-Rivas, J. Cruz, T. Pandiyan, J.
Electroanal. Chem. 655 (2011) 164–172.
[19] X. Wang, H. Yang, F. Wang, Corros. Sci. 53 (2011) 113–121.
[20] B.A. Baviskar, B. Baviskar, M.R. Shiradkar, U.A. Deokate, S.S. Khadabadi, E. J. Chem 6
(2009) 196–200.
[21] A. Ghanbaria, M.M. Attara, M. Mahdavian, Mater. Chem. Phys. 124 (2010)
1205–1209.
[22] Y. Abboud, A. Abourriche, T. Saffaj, M. Berrada, M. Charrouf, A. Bennamara, A.
Cherqaoui, D. Takky, Appl. Surf. Sci. 252 (2006) 8178–8184.
[23] Kirk-Othmer, Encyclopaedia of Chemical Technology, 2nd edvol. 6, Wiley, New
York, 1965 321.
[24] A. Popova, M. Christov, S. Raicheva, E. Sokolova, Corros. Sci. 46 (2004) 1333–1350.
[25] A. Popova, M. Christov, Corros. Sci. 48 (2006) 3208–3221.
[26] A. Popova, M. Christov, A. Zwetanova, Corros. Sci. 49 (2007) 2131–2143.
[27] A. Popova, Corros. Sci. 49 (2007) 2144–2158.
[63] R.G. Pearson, Inorg. Chem. 27 (1988) 734–740.
[64] M.J. Frisch, et al., Gaussian 09, Revision B.01, 2009 (Wallingford CT).
[65] R. Dennington, T. Keith, J. Millam, Gauss View, Version 5, Semichem Inc., Shawnee
Mission, 2009.
[66] C. Verma, L.O. Olasunkanmi, E.E. Ebenso, M.A. Quraishi, J. Mol. Liq. 251 (2018)
100–118.
[67] R.M. Hassan, S.M. Ibrahim, H.D. Takagi, S.A. Sayed, Carbohydr. Polym. 192 (2018)
356.
[68] I. Abdulazeez, M. Khaled, A.A. Al-Saadi, J. Mol. Struct. 1196 (2019) 348–355.
[69] A. Kokalj, Chem. Phys. 393 (2012) 1–12.
[28] H. Ju, Z.P. Kai, Y. Li, Corros. Sci. 50 (2008) 865–871.
[29] M. Elayyachy, B. Hammouti, A. El Idrissi, Appl. Surf. Sci. 249 (2005) 176–182.
[30] E.E. Foad El Sherbini, Mater. Chem. Phys. 60 (1999) 286–290.
[31] Y. Tang, L. Yao, C. Kong, W. Yang, Y. Chen, Corros. Sci. 53 (2011) 2046–2049.
[32] A. Ajmal, A.S. Mideen, M.A. Quaraishi, Corros. Sci. 36 (1994) 79–84.
[33] L. Yan, W. Li, C. Cai, B. Hou, Electrochim. Acta 53 (2008) 5953–5960.
[34] S.A. Ali, H.A. Al-Muallem, M.T. Saeed, S.U. Rahman, Corros. Sci. 50 (2008) 664–675.
[35] S.A. Ali, H.A. Al-Muallem, S.U. Rahman, M.T. Saeed, Corros. Sci. 50 (2008)
3070–3078.
[70] P. Chandra Ray, Chem. Phys. Lett. 395 (2004) 269–273.
[71] M. Yang, P. Senet, C. Van Alsenoy, Int. J. Quantum Chem. 101 (2005) 535–542.
[72] A. Echevarria, Corros. Sci. 67 (2013) 281–291.
[36] M.K. Awad, M.R. Mustafa, M.M. Abo Elnga, THEOCHEM J. Mol. Struct. 959 (2010)
66–74.
[73] K.F. Khaled, Electrochim. Acta 48 (2003) 2493.
[37] Z.E. Adnani, M. Mcharfi, M. Sharia, M. Benzakour, A.T. Benjelloun, M.E. Touhami,
Corros. Sci. 68 (2013) 223–230.
[74] N. Kovacˇevic, A. Kokalj, J. Phys. Chem. C 115 (2011) 24189–24197.
[75] A. Elsharif, S. Abubshait, I. Abdulazeez, H. Abubshait, Arab. J. Chem. 13 (2020)
5363–5376.
[76] M. Majd, M. Ramezanzadeh, B. Ramezanzadeh, G. Bahlakeh, J. Hazard. Mater. 382
(2020), 121029.
[38] C.M. Goulart, A. Esteves-Souza, C.A. Martinez-Huitle, C.J.F. Rodrigues, M.A.M. Maciel,
B.D. Mert, M.E. Mert, G. Kardaßs, B. Yazıcı, Corros. Sci. (2011) 4265–4272.
[39] S.H. Pournazari, M.H. Moayed, M. Rahimizadeh, Corros. Sci. 71 (2013) 20–31.
[40] M.A. Hegazy, A.M. Badawi, S.S. Abd El Rehim, W.M. Kamel, Corros. Sci. 69 (2013)
110–122.
[77] A. Dehghani, G. Bahlakeh, B. Ramezanzadeh, Bioelectrochemistry 130 (2019),
107339.
[41] G. Gece, S. Bilgiç, Ind. Eng. Chem. Res. 51 (2012) 14115–14120.
[42] N.O. Obi-Egbedi, I.B. Obot, Corros. Sci. 53 (2011) 263–275.