Novel synthesized Schiff Base-based cationic gemini surfactants: Electrochemical investigation, theoretical modeling and applicability as biodegradable inhibitors for mild steel against acidic corrosion

Article abstract of DOI:10.1016/j.molliq.2017.02.105

Three novel Schiff base compounds were synthesized and characterized using spectroscopic methods (FT-IR, ¹H NMR and ¹³C NMR). Their critical micelle concentrations were determined by conductimetric and surface tension measurements. The inhibition potentials of the prepared surfactants on mild steel corrosion in H₂SO₄ have been investigated using electrochemical and scanning electron microscopy techniques. The results showed that all surfactants are good inhibitors for mild steel corrosion with the percentage inhibition efficiency of up to 99.21% at 1.0?mM. The data showed that the studied surfactants are mixed-type inhibitors. Adsorption of the inhibitors obeyed the Langmuir isotherm. The results obtained from theoretical calculations support our experimental studies.

Full text of DOI:10.1016/j.molliq.2017.02.105

Accepted Manuscript
Novel synthesized Schiff Base-based cationic gemini surfactants:
Electrochemical investigation, theoretical modeling and
applicability as biodegradable inhibitors for mild steel against
acidic corrosion
Hany M. Abd El-Lateef, Kamal A. Soliman, Ahmed H. Tantawy
PII:
S0167-7322(16)34104-6
doi: 10.1016/j.molliq.2017.02.105
MOLLIQ 7018
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
17 December 2016
23 February 2017
25 February 2017
Please cite this article as: Hany M. Abd El-Lateef, Kamal A. Soliman, Ahmed H. Tantawy
Novel synthesized Schiff Base-based cationic gemini surfactants: Electrochemical
,
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steel against acidic corrosion. The address for the corresponding author was captured
as affiliation for all authors. Please check if appropriate. Molliq(2017), doi: 10.1016/
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ACCEPTED MANUSCRIPT
Novel Synthesized Schiff Base-based Cationic Gemini
Surfactants: electrochemical investigation, theoretical
modeling and applicability as biodegradable inhibitors for
mild steel against acidic corrosion
a, b
c
c
Hany M. Abd El-Lateef *, Kamal A. Soliman , Ahmed H. Tantawy
Department of Chemistry, College of Science, King Faisal University, Al Hufuf,
1982 Al Hassa, Saudi Arabia
a
3
b
c
Chemistry Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt
Chemistry Department, Faculty of science, Benha University, 13518Benha, Egypt
*
Corresponding author: Fax: (+2)-093 -4601159
Tel: (+2)-012-28-137-103
E-mail address: Hany_shubra@yahoo.co.uk (Hany M. Abd El-Lateef)
1

ACCEPTED MANUSCRIPT
Novel Synthesized Schiff Base-based Cationic Gemini
Surfactants: electrochemical investigation, theoretical
modeling and applicability as biodegradable inhibitors for
mild steel against acidic corrosion
Abstract
Three novel Schiff base compounds were synthesized and characterized using
1
13
spectroscopic methods (FT-IR, H NMR and C NMR). Their critical micelle
concentrations were determined by conductimetric and surface tension measurements.
The inhibition potentials of the prepared surfactants on mild steel corrosion in H SO
2
4
have been investigated using electrochemical and scanning electron microscopy
techniques. The results showed that all surfactants are good inhibitors for mild steel
corrosion with the percentage inhibition efficiency of up to 99.21 % at 1.0 mM. The
data showed that the studied surfactants are mixed-type inhibitors. Adsorption of the
inhibitors obeyed the Langmuir isotherm. The results obtained from theoretical
calculations support our experimental studies.
Keywords: Schiff Base Surfactants; mild steel ; EIS; SEM; adsorption ; acid
inhibition
2

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1
. Introduction
Mild steel is a widely used as the constituent materials in various industry
fields because of its high mechanical strength and low cost [1-3]. Industrial
infrastructures include reactors, drums, petrochemical process devices, boilers, and
heat exchangers. In oil and gas industries, aqueous acidic solution (especially
hydrochloric and sulphuric acids) is used for acid pickling of steel, de-scaling, acid
treatment, chemical improvement and processing or production, and oil well
acidification. In this case, the exposure of metal to aqueous acidic solution causes
corrosion, which puts safety and economic parts at high risk [3, 4]. Consequently,
corrosion inhibitors are used to minimize the rate of corrosion and prohibit the metal
from corrosion. This naturally means that it is very important for the workers in the
petroleum and related industries to find efficient corrosion inhibitors [5]. The majority
of the well-known inhibitors which are effective in acid media there are the
organic compounds containing heteroatoms, such as N, O, or S, and/or aromatic rings
and/or multiple bonds, which provide a physical or chemical adsorption zone for the
metal surface [6-8]. However, most of these compounds are toxic and expensive
[9−11]. For this reason, current research activities are being garnered towards
the development of cheap, readily available, and “green” corrosion inhibitors with
high inhibition efficiency even at much reduced concentrations [12, 13].
Schiff base-based gemini surfactants are considered nontoxic and water-
soluble organic compounds [14, 15]. These kinds of compounds have superior surface
activity, surface properties and have interesting rheological properties at low
concentrations [16-18]. Recent publications revealed that the inhibition efficiency of
Schiff bases is much greater than that of corresponding aldehydes and amines in
acidic media for different metals such as steel, aluminum and zinc due to the presence
3

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of a –CH=N- group in the molecules [19-21]. Schiff bases may also be changed to
surfactant and find application as potential inhibitors [22]. Gemini surfactants
containing imine group and their implementations in corrosion inhibition of metals
are rarely reported.
Recently, a quantum chemical calculation is extensively used for correlating
the molecular structure of inhibitors and their inhibitive performance and it is also
considered a very powerful method to probe the inhibitor/surface interaction and to
analyze empirical data [2, 21, 23].
In the present work, the efficiency of three novel Schiff base-based cationic
gemini surfactants namely, 53,3 '-(pentane-2,4-diylidenebis(azanylylidene))bis(N-(2-
(
(
(
decyloxy)-2-oxoethyl)-N,N-dimethylpropan-1-aminium) chloride (SBGS-10), 3,3'-
pentane-2,4-diylidenebis(azanylylidene))bis(N,N-dimethyl-N-(2-oxo-2-
tetradecyloxy)ethyl)propan-1-aminium) chloride (SBGS-14), and 3,3'-(pentane-2,4-
diylidenebis(azanylylidene))bis(N,N-dimethyl-N-(2-(hexadecyloxy)-2-
oxoethyl)propan-1-aminium) chloride (SBGS-16) as biodegradable corrosion
inhibitors for mild steel in 1.0 M H SO is discussed on the basis of electrochemical
2
4
impedance spectroscopy (EIS), potentiodynamic polarization (PDP) data and
scanning electron microscopy (SEM). The critical micelle concentration (CMC) and
the surface parameters of the prepared surfactants were determined by surface tension
and conductivity measurements. Some quantum chemical indices based theoretical
calculations were used to support our experimental investigations.
2
. Experimental Section
2
.1. Materials
Decyl, tetradecyl and hexadecyl alcohols were obtained from Acros organics
company (Belgium). 2-Chloroacetylchloride (97%), Acetylacetone (97%) and 3-(N, N
4

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dimethylamino)-1-propylamine (99%) was purchased from Aldrich company.
Diethylether (99%), K CO (anhydrous) triethylamine (97%), Dichloromethane
2
3
,
(
98%), and ethyl alcohol (99%), are high grade and obtained from AL-Nasr chemical
company.
.2. Instruments
The synthesized compounds were elucidated by spectroscopic tools (FTIR, H
2
1
1
3
and C NMR). The FTIR analysis was done in faculty of science, Benha university
1
using a thermo Nicolet iS10 FTIR spectrophotometer recorded in KBr. H NMR (400
1
3
MHz) and C NMR (100 MHz) spectra were done in Sohag university using FT-IR-
ALPHA-BRUKER-Platinum-ATR measured in DMSO-d6.
2
.3. Synthesis of Schiff Base cationic Gemini Surfactants (SBGS)
.3.1. Synthesis of alkyl 2-chloroacetate [compound I]
2
In 100 mL round-bottom flask, chloroacetyl chloride (3.86 g, 35 mmol) was
placed in an ice-water bath with continuous stirring. Decyl, tetradecyl and hexadecyl
alcohols separately, triethylamine and dichloromethane (4 g, 35 mmol), (20 mL) were
added slowly through 15 min in ice bath. The reaction mixture was remained under
°
stirring for 4 h at 25 C. The solvent was evaporated and separated by rotary evaporator
and the product was obtained and washed several times with distilled water to give the
desired product as a product in 97 % yield. The structure of Alkyl 2-chloroacetate was
elucidated through the following reported procedure [23].
2
.3.2.
Synthesis
of
3,3'-(pentane-2,4-diylidenebis(azanylylidene))bis(N,N-
dimethylpropan-1-amine) [compound II]
Acetyl acetone (10 mmol) was subjected to condensation reaction with 20 mmol
of 3-(N, N-dimethylamino)-1-propyl amine in the presence of 100 mL ethanol as
°
appropriate solvent. The reaction contents were refluxed for 4-5 h at 70 C and left to
5

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cool and a product in a viscous liquid form will be produced. Purification of product
will be done through extraction process by ethyl acetate to obtain the Schiff base
compounds (compound II).
2
.3.3.
Synthesis
of
3,3'-(pentane-2,4-diylidenebis(azanylylidene))bis(N-(2-
(alkyloxy)-2-oxoethyl)-N,N-dimethylpropan-1-aminium) chloride [SBGS-10,
SBGS-14 and SBGS-16 compounds]
In a single-neck flask containing magnetic bar, a prepared compound II (10
mmole) was added to containing compound I (20 mmole) in diethyl ether as a solvent
°
for 92 h at 25 C with continuous stirring. Solid and viscous products were obtained.
The unreacted materials were removed by washing the obtained products several
times with diethylether to afford the required Schiff base cationic gemini surfactants
(SBGS). The synthetic routes are represented in Fig. 1.
2
.4. Electrodes and aggressive solutions for corrosion inhibition studies
The corrosion tests were carried out on the mild steel specimens with the
chemical compositions (w %) Si 0.18%, C 0.16 %, Mn 0.71%, Ni 0.01 %, Cr 0.01%,
S 0.04 %and Fe (balance). This electrode was used for one time. The cylindrical
2
shaped of mild steel rod has a diameter of 0.5 cm, length of 3.0 cm and 4.55 cm
exposed area to fluid.
The aggressive solution of 1.0 M H SO was prepared by diluting the
2
4
analytical grade 98% H SO with bi-distilled water. Various concentrations (0.01–
2
4
1
.0 mM) of the synthesized Schiff base gemini surfactants (SBGS) were also
prepared in bi-distilled water and used as inhibitors for mild steel corrosion in 1.0
M H SO .
2
4
6

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2
.5. Electrochemical Measurements
Electrochemical experiments were performed in a conventional three-electrode
electrochemical cell with mild steel as working electrode, platinum sheet as counter
electrode and a saturated calomel electrode (SCE) as reference electrode. The
reference electrode was coupled with a lugging capillary to ensure suitable geometry
of cell electrodes for which potential drop is minimized. Before each electrochemical
measurement, the mild steel was immersed in the test solution for 50-60 min to
achieve a steady open circuit potential (OCP).
Electrochemical impedance spectroscopy measurements were performed with
a phase-sensitive detector (amplifier) (Model 5208) driven by using VersaSTAT4
potentiostat/galvanostat with a frequency response analyzer (FRA) contained in a
single unit under aerated conditions. Impedance spectra were recorded at Ecorr in the
frequency range 100 kHz to 0.5 Hz with the ac voltage amplitude of 10 mV.
Potentiodynamic polarization measurements were performed using
VersaSTAT4 potentiostat/galvanostat under aerated conditions. Potentiodynamic
-1
anodic and cathodic polarization curves were obtained at a scan rate of 0.001 Vs in a
potential range ±250 mV versus the corrosion potential (E ). All electrochemical
corr
experiments were performed at 323 K.
2
.6. Surface tension
The surface tension of SBGS compounds in water and 1.0 M H SO was
2
4
determined by Tensiometer- K6 processor (KRÜSS Company, Germany) via the ring
method [24, 25], between the measurement runs, the ring was initially cleaned with
pure water and acetone. All the measurements were performed at a constant
temperature (298 K), which has been shown to be higher than the Krafft point of the
investigated SBGS compounds (whereas the Krafft point of all three synthesized
7

ACCEPTED MANUSCRIPT
surfactants were lower than 5 °C). All aqueous solutions of the synthesized
compounds were remained to stand 2-3 h before determined. The values of surface
tension were determined 3 times for each sample within 3 min interval between each
reading and the values were taken as the average of these recorded values.
2
.7. Electrical conductivity
Electrical conductivity measurements of Schiff base gemini surfactant
solutions were done using a digital conductivity meter (model Type AD3000;
EC/TDS and Temperature meter) at 298 K (above the Krafft point of prepared
solutions). The measurement was repeated three times for each solution.
2
.8. Biodegradability measurements
Biodegradability test of the synthesized Schiff base gemini surfactant in river
water was measured by the surface tension method via Tensiometer-K6 processor
KRÜSS company, Germany) as reported in [23]. The calculation of biodegradation
(
percent (D/%) was done as the following equation:
γt  γ0
γbt  γ0
D/% 
100
(1)
where the surface tension at time t is γ , the surface tension at time zero is γ , and the
t
0
surface tension of river water without surfactant sample at time t is γbt.
2
.9. Surface morphological investigations
Freshly mild steel specimens were immersed in 1.0 M H SO without and with
2
4
1
.0 mM of SBGS-10, SBGS-14 and SBGS-16 inhibitors for 3 days at 323 K. The
specimens were retrieved after 3 days, dried and used for the SEM analyses. SEM
measurements were performed using scanning electron microscope (JEOL, model
5
300) by applying operating voltage of 5 kV.
8

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2
.10. Computational simulation
The ground state geometries of the studied SBGS-10, SBGS-14 and SBGS-16
inhibitors were optimized with no constraints using the Density Functional Theory
DFT) based on Beck’s three parameter exchange functional and Lee–Yang–Parr non-
(
local correlation functional (B3LYP) [26-28] and the 6-31G(d,p) basis set. Quantum
chemical calculations were carried out using Gaussian-09 software package [29]. The
conductor-like polarizable continuum model (CPCM) was applied to water as solution
phase. The quantum chemical indices calculated were, total energy (Etotal), the energy
of the highest occupied molecular orbital (EHOMO), the energy of the lowest
unoccupied molecular orbital (ELUMO), the energy gap (ΔE= ELUMO-EHOMO), the
dipole moment (μdipole), the ionization potential (I ), electron affinity (E ), Chemical
P
A
potential (μpotential), Global hardness (η), Global softness (σ), Electronegativity (χ), the
fraction of electron transferred (∆N) and Polarizability <α>. The molecular
electrostatic potential (MEP) maps were generated in the range from deepest red color
to deepest blue color to help in the explanation of the experimental data obtained for
the corrosion process.
<
<Figure 1>>
3
. Results and discussion
3
.1. Structure elucidation of the prepared Schiff base gemini surfactants
SBGS)
The spectroscopic results obtained from analysis of the prepared compounds are
as follows:
,3'-(pentane-2,4-diylidenebis(azanylylidene))bis(N,N-dimethylpropan-1-amine)
(
3
-1
[compound II]: FT-IR (KBr pellet) shows characteristic bands at 2777-2946 cm
9

ACCEPTED MANUSCRIPT
-1
(
CH alipatic) and 1606 cm (CH=N), also showed the disappearance of two carbonyl
band and –NH band (Figure S1, supporting data).
2
3
,3'-(pentane-2,4-diylidenebis(azanylylidene))bis(N-(2-(decyloxy)-2-oxoethyl)-
N,N-dimethylpropan-1-aminium) chloride [SBGS-10]: Brownish-yellow
-1
semisolid, yield = 85%, FT-IR (KBr pellet) cm = 2920, and 2853 (C–H, aliphatic
1
fatty chain), 1748 (C=O of ester), 1211 (C-O-C) and 1611 (CH=N). H NMR (400
MHz) δ (ppm) (DMSO-d ): 0.77(t, 6H, 2[CH -CH ]), 1.17(s, 28H, 2[(CH -CH ) ]),
6
3
2
3
2 7
1
2
2
.42(t, 4H, 2[CH -N=C]) 1.62(s, 6H, 2[CH -C=N]), 1.75(m, 8H, 2[CH -CH O],
2 3 2 2
+
[CH -CH N]), 3.05(s, 12H, 4[CH -N ]), 3.12(s, 2H, [CH -C=N]), 3.81(m, 4H,
2
2
3
2
+
13
[CH -N ]), 4.41(s, 4H, 2[CH -O]), 4.94(t, 4H, 2[CH -CO]). C NMR (100 MHz) δ
2
2
2
(
ppm) (DMSO-d ): 14.31[2(CH -CH )], 18.81[2(CH -C=N)], 22.41[2(CH -CH -
6 3 2 3 2 2
+
N )], 25.44[2(CH -CH )], 28.28[2(-CH -CH -CH O)], 29.11[2(-CH -CH -CH O)],
3
2
2
2
2
2
2
2
+
2
6
1
3
9.48[2(CH CH -CH ) ], 39.58[CH -C=N]) 51.61[4CH -N ], 61.46[2CH -N=C],
3 2 2 5 2 3 2
+
+
2.63[2CH -N ], 66.28[2CO-CH -N )], 95.57 [2CH -O], 164.21[2C=N],
2
2
2
65.34[2C=O, carbonyl ester].
,3'-(pentane-2,4-diylidenebis(azanylylidene))bis(N,N-dimethyl-N-(2-oxo-2-
tetradecyloxy)ethyl)propan-1-aminium) chloride [SBGS-14]: Buff color, m.p =
(
°
-1
8
8 C, yield = 87%, FT-IR (KBr pellet) cm = 2955, and 2850(C–H, aliphatic fatty
chain), 1749 (C=O of ester), 1224 (C-O-C) and 1606 (CH=N) (Figure S2a;
1
supporting data). H NMR (400 MHz) δ (ppm) (DMSO-d ): 0.61(t, 6H, 2[CH -
6
3
CH ]), 1.1(s, 44H, 2[(CH -CH ) ]), 1.37(t, 4H, 2[CH -N=C]) 1.62(s, 6H, 2[CH -
2
3
2 11
2
3
+
C=N]), 1.75(m, 8H, 2[CH -CH O], 2[CH -CH N]), 3.04(s, 12H, 4[CH -N ]), 3.14(s,
2
2
2
2
3
+
2
H, [CH -C=N]), 3.93(m, 4H, 2[CH -N ]),4.37(s, 4H, 2[CH -O]), 4.71(t, 4H, 2[CH -
2 2 2 2
1
3
CO]) (Figure S2b; supporting data). C NMR (100 MHz) δ (ppm) (DMSO-d ):
6
+
1
4.26[2(CH -CH )], 18.79[2(CH -C=N)], 22.47[2(CH -CH -N )], 25.66[2(CH -
3
2
3
2
2
3
1
0

ACCEPTED MANUSCRIPT
CH )], 28.30[2(-CH -CH -CH O)], 29.09[2(-CH -CH -CH O)], 29.44[2(CH CH -
2
2
2
2
2
2
2
3
2
+
+
CH ) ], 39.56[CH -C=N]) 51.63[4CH -N ], 61.41[2CH -N=C], 62.61[2CH -N ],
2
9
2
3
2
2
+
6
6.26[2CO-CH -N )], 95.54 [2CH -O], 163.14[2C=N], 165.29 [2C=O, carbonyl
2
2
ester] (Figure S2c; supporting data).
,3'-(pentane-2,4-diylidenebis(azanylylidene))bis(N,N-dimethyl-N-(2-
hexadecyloxy)-2-oxoethyl)propan-1-aminium) chloride [SBGS-16]: Pale brown
3
(
°
-1
color, m.p = 98 C, yield = 90%, FT-IR (KBr pellet) cm = 2918, and 2851 (C–H,
aliphatic fatty chain), 1749 (C=O of ester), 1196 (C-O-C) and 1608 (CH=N) (Figure
1
2
5
8
a). H NMR (400 MHz) δ (ppm) (DMSO-d ): 0.86(t, 6H, 2[CH -CH ]), 1.25(s,
6
3
2
2H, 2[(CH -CH ) ]), 1.62(t, 4H, 2[CH -N=C]) 1.91(s, 6H, 2[CH -C=N]), 1.95(m,
3
2 13
2
3
H, 2[CH -CH O], 2[CH -CH N]), 2.51(s, 2H, [CH -C=N]), 3.23(s, 12H, 4[CH -
2
2
2
2
2
3
+
+
N ]), 4.18(m, 4H, 2[CH -N ]), 4.59(s, 4H, 2[CH -O]), 4.96(t, 4H, 2[CH -CO])
2
2
2
1
3
(
Figure 2b). C NMR (100 MHz) δ (ppm) (DMSO-d ): 14.26[2(CH -CH )],
6 3 2
+
1
8.79[2(CH -C=N)], 22.47[2(CH -CH -N )], 25.67[2(CH -CH )], 28.31[2(-CH -
3 2 2 3 2 2
CH -CH O)], 29.09[2(-CH -CH -CH O)], 29.45[2(CH CH -CH ) ], 39.57[CH -
2
2
2
2
2
3
2
2 11
2
+
+
+
C=N]) 51.63[4CH -N ], 61.44[2CH -N=C], 62.64[2CH -N ], 66.28[2CO-CH -N )],
3
2
2
2
9
5.55 [2CH -O], 163.14[2C=N], 165.29 [2C=O, carbonyl ester] (Figure 2c).
2
<
<Figure 2>>
3
.2. Critical micelle concentration and surface-active properties
Studying the critical micelle concentration (CMC) of synthesized Schiff base
gemini surfactants is a conventional method for the measurement of surface tension.
The variations of the surface tension γ as a function of the surfactant molar
concentration in water and 1.0 M H SO at 298 K is shown in Fig. 3 a, b. It is noted
2
4
that the initial raise in the concentration of surfactant leads to the surface tension of
the surfactant solutions decreased rapidly. At the break point, the surface tension
1
1

ACCEPTED MANUSCRIPT
began to decrease slowly, indicating the formation of surfactant aggregations. The
concentration value at the break point is shown as the CMC of the synthesized
surfactants. Also it is found that the synthesized Schiff base gemini surfactants with
longer hydrophobic chain have low CMC. Moreover, the values of CMC were
decreased in 1.0 M H SO compared with the values in water solutions (Table 1).
2
4
These data indicated that the prepared Schiff base gimini surfactants are strongly
adsorbed at steel/H SO interface.
2
4
Figure 4 represents the relation between electrical conductivity of synthesized
Schiff base gemini surfactants solution (k) and concentration. In each of the plots,
intersect at the point CMC have been obtained from two straight lines with different
slopes. The CMC values calculated from electrical conductivity plots are recorded in
Table 1.
The degree of counter ion dissociation (α) can be obtained from the ratio of
the slopes above and below the break indicative of the CMC, as shown in Fig 4.
Normally, the degree of counter ion dissociation and the degree of counter ion binding
(β) have the next relationship: β=1-α. The β parameter is an important for expression
of how many counter ions are contained in the Stern layer to counterbalance the
electrostatic force that opposes to the micelle formation [30]. All values of α and β are
observed in Table 1, it also found that β values decreases slightly with longer
hydrophobic alkyl chain at the certain temperature [31]. As well as from Table 1, it is
noted that using electrical conductivity, the CMC values determined were in
agreement with those obtained using surface tension.
From the critical micelle concentration data and surface tension values, the
effectiveness values (the difference between the surface tension values of pure water
1
2

ACCEPTED MANUSCRIPT
(γ0) and the surface tension values at CMC (γCMC), πCMC) were calculated according to
the following expression (Eq. 2) [31]:
πCMC= γ0 – γCMC
(2)
The maximum reduction of surface tension can be evaluated the effectiveness of
synthesized surfactant to reduce the surface tension of water. The data in Table 1
showed that the increasing the hydrocarbon chain length led to increase πCMC values.
In addition to, the results showed that the synthesized SBGS-16 more effective
surfactant and the maximum reduction of the surface tension at CMC reach to 36.27
-1
mN m [32].
As well as at the air-water interface, the surface area Amin showed the packing
densities of Schiff base gemini surfactants, which is crucial to interpreting the surface
activities of gemini surfactants. The value of Γmax describes the accumulation of
surfactant molecules at the air–water interface and can be calculated via the Gibbs
adsorption equation (3) [32]:

1
 d 
Γ 



max

(3)
2
.303nRT dlogC


T
where R is a gas constant; T is the absolute temperature; C is the concentration of
surfactant; γ is the surface tension at given concentration; dγ/dlogC is the slope of γ vs
log C; and n is the number of ionic species whose concentration at the interface varies
with the surfactant concentration in the solution. The value of n is taken to be 3 for
divalent gemini surfactants (the gemini amphiphile species and the two counter ions)
[33, 34]. The minimum area occupied per surfactant molecule at the air/water
2
interface (Amin in nm ) is calculated by the following equation (4) [34]:
1
0
4
1
A 
(4)
min
Γmax NA
where N is Avogadro’s number. The values of Γmax and Amin are listed in Table 1.
A
From Table 1, it is found that the surfactant with a longer hydrophobic chain had a
1
3

ACCEPTED MANUSCRIPT
higher Amin value, suggesting that the decreasing in packing densities of gemini
surfactants at the air-water interface with longer the hydrophobic tails. This rule is
agreed with other gemini surfactants, and a possible explanation is that the increasing
in hydrophobic chains length leads to a more prone to curl and thus make the Amin
value larger [35].
From Gibbs equations (Eqs. 5 and 6), the standard free energy change of micelle
o
o
formation ( G ) and adsorption ( G ) of synthesized Schiff base surfactants were
mic
ads
calculated according the following equations [36]:
0
mic

G
 (2 )RT ln CMC
(5)
(6)
0

G  (2)RT ln CMC-0.06 Amin
ads
CMC
where CMC is the critical micelle concentration, R is a gas constant, T = t+273, πCMC
is the effectiveness; α is a degree of counter ion dissociation and Amin is the minimum
area occupied per surfactant molecule at the air/water interface. The free energies
o
o
values of micellization ( G ) and adsorption ( G ) of the synthesized Schiff base
mic
ads
gemini surfactants (SBGS-10, SBGS-14 and SBGS-16) were calculated and are listed
o
mic
o
ads
in Table 1. When the values of G compared with the values of G , it is noted
o
o
that slightly increasing in G values than G values and the more negativity of
ads
mic
values indicates the ability of these surfactants to adsorbed than micellization
tendency due to the preference of adsorption is governed by the thermodynamic
stability of the molecules at the air-water interface [37].
<
<
<Figure 3>>
<Figure 4>>
<
<Table 1>>
1
4

ACCEPTED MANUSCRIPT
3
.3. Biodegradability measurements of synthesized SBGS compounds
Using the surface tension method, the biodegradability in the river water of the
synthesized cationic gemini surfactants was evaluated for 25 day. From Fig. 5, it is
clear that gradually increasing in the biodegradability by increasing the time and
reached to the maximum values after 25 day returned to the synthesized Schiff base
gemini surfactants under investigation have ability to loss the surface activity. In
addition to, there is a direct relationship between the hydrophobic chain length and the
rate of biodegradation. So it is noted from Fig. 5 that the biodegradability increased
by increasing the length of hydrophobic chain. Also due to the presence of an
oxycarbonyl group (-O-C=O-) leads to increase the ability of investigated SBGS
compounds to degrade by the action of the environmental microorganisms.
Furthermore, the highest biodegradation extent was obtained in the case of SBGS-16
at 93%, SBGS-14 at 88% compared to SBGS-10 surfactants at 80%. The values of
biodegradation of these synthesized Schiff base gemini surfactants specified them as
biodegradable compounds. So the synthesized Schiff base gemini surfactants will be
high biodegradability than the other synthesized surfactants and less toxicity, i.e.,
environment friendly [38]. In fact, the biodegradation affinity is which has the ability
to degrade by the action of the environmental microorganisms.
<
<Figure 5>>
3
.4. Open circuit potential (Ecorr) vs. time measurement
E -time curves for mild steel in 1.0 M H SO solution at 50 °C containing
corr
2
4
1
.0 mM of synthesized SBGS inhibitors is presented in Fig. 6. It is obvious from the
curve that, Ecorr of mild steel in the studied solution without surfactant is shifted to
more noble direction until steady state potential is determined and the Ecorr stabilized
at around −471 mV (SCE). Upon the addition of surfactant molecules to the
1
5

ACCEPTED MANUSCRIPT
aggressive system, a positive shift in Ecorr is produced, i.e.; the potential was shifted to
more noble direction [39]. In the presence of 1.0 mM of SBGS-10, SBGS-14 and
SBGS-16, Ecorr stabilized at -452, -459 and -466 mV (SCE), respectively. The
divergence in Ecorr in the aggressive medium with and without investigated inhibitors
indicates that the metal surface was protected and covered by the surfactant
adsorption.
<
<Figure 6>>
3
.5. Electrochemical impedance measurements (EIS)
The Nyquist, Bode phase and Bode modulus plots of mild steel in 1.0 M H SO4
2
in the absence and presence of various concentrations of the investigated SBGS
inhibitors is presented in Fig. 7 a-f. Nyquist diagram shown in Fig. 7 a, c and e
exhibited that, the impedance plots consists of an inductive loop at low frequencies
(LF) and a large capacitive loop at high frequencies (HF) both in the absence and
presence of studied inhibitors. The inductive loop at LF generated from the adsorption

ads
2
relaxation of intermediates such as
H
andSO 4 , displaying negative change in the
surface coverage with potential on the metal surface [40]. The large capacitive loop at
HF could be imputed to the double layer capacity in parallel with the charge transfer
resistance (R ). The occurrence of two time constants for steel corrosion at the
ct
corrosion potential value (E ) in the absence and presence of inhibitors has been
corr
reported [41]. Inspection of Fig. 7 a, c and e reveals that increase in the diameter of
the Nyquist semicircles in the presence of the surfactants as compared to that without
the inhibitors. This also increases with increase in the inhibitors dose. This suggests
that the studied SBGS surfactants change the impedance response of mild steel in 1.0
M H SO possibly due to adsorption of the surfactant molecules on the steel surface
2
4
1
6

ACCEPTED MANUSCRIPT
and the degree of surface coverage by the adsorbed molecules increases with increase
in inhibitor concentration in solution.
The depressed semicircular feature of the Nyquist plots is the characteristic
of solid electrodes, and it is often regarded as frequency dispersion due to different
possible forms of inhomogeneity [42, 43]. One other overt behavior of the Bode and
phase modules in Fig. 7 b, d, f especially in the presence of the surfactants is the
appearance of the second time-constant at higher frequency, which suggests the
formation and/or adsorption of intermediate corrosion products and/or steel-surfactant
complexes. Moreover, phase plots reflect apparently higher values of the phase angles
in the presence of the surfactants. These features can be attributed to the formation of
protective films of the inhibitor molecules on the steel surface making it to exhibit
relatively closer resemblance to an ideal capacitor than the metal surface in the acid
blank system without the inhibitor molecules [44].
The impedance data were analyzed by fitting to the equivalent circuit
model as depicted in Fig. 8 for mild steel corrosion in 1.0 M H SO and containing
2
4
1
.0 mM SBGS-14. The circuit (Fig. 8 inset) consists of solution resistance R , the
s
constant phase element (CPE) in parallel to the parallel charge transfer resistor, the
charge transfer resistance, R , in addition the inductive elements, R and L for H SO
4
ct
L
2
corrosive medium. A CPE was used allowance of double-layer capacitance (C ) in
dl
the equivalent circuits in order to fit the data more accurately [45]. CPE have been
used intensively to account for deviations brought about by surface roughness. The
impedance of CPE can be represented as [46]:
1
1
Z  
(7)
CPE
n
Y ( j)
0
1
7

ACCEPTED MANUSCRIPT
−
1
where j=(-1) is an imaginary number; ω is the angular frequency in rad s Q; Y is the
0
CPE constant and n is the CPE exponent which can be used as a criterion of the
heterogeneity or roughness of the surface. For n = 1, a capacitance with C = Y , for
0
1
n = -1, an inductance with L =Y0 and for n = 0.5, a Warburg element with W = Y . In
0
this study, we can obtain Y and n values, and transform Y into C using the
0
0
dl
following equation (8) [44]:
n1
C Y (2f )
(8)
dl
0
max
Here fmax is the frequency at a maximum point of imaginary part impedance. The
impedance parameters including the R , R , C , Y , R , n, L, R and the percentage
ct
s
dl
0
L
P
inhibition efficiency values (P %) are reported in Table 2. The PEIS and R were
EIS
p
calculated from the R values as [47]:
ct
i
ct
w
ct

R  R 
(9)
PEIS
/
%
 



100
i
R

ct

RL  Rct
RL  Rct
(10)
Rp 
i
ct
w
where
R
and Rct are the charge transfer resistances with and without inhibitors,
respectively. Results presented in the Table 2 reveal that the addition of studied Schiff
base gemini surfactants to the aggressive solution increased the magnitude of R , with
ct
corresponding decrease in C , which implies that the SBGS inhibitors minimize the
dl
rate of charge transfer process associated with the corrosion reaction, and it is
indicative of the corrosion inhibition characteristics of the studied SBGS surfactants
[
48, 49]. More so, the decrease in the C in the presence of surfactants, which can
dl
result from a decrease in local dielectric constant or an increment in the thickness of
the electrical double layer (or both), suggesting the formation of the protective layer
of the inhibitor molecules on the steel surface act by adsorption at the steel/H SO
2
4
1
8

ACCEPTED MANUSCRIPT
interface [50]. The decrease in C is in accordance with Helmholtz model given by
dl
the following equation [49]:

 A
d
0
(11)
C 
dl
where ε is the dielectric constant of the medium, d is the thickness of the protective
layer, A is the effective area of the electrode and ε is the vacuum permittivity. Based
0
on Helmholtz model (Eq. 11), the decrease in the C value demonstrates that the
dl
protective layer becomes thicker with increasing of inhibitor dose; this makes the
corrosion inhibition more effective. Inspection of the data in Table 2, it was found
that, the values of inhibition efficiency for mild steel corrosion in 1.0 M H SO
2
4
increases with increase in concentration of SBGS inhibitors till reaching their
maximum value at 1.0 mM. It is also visible from the data that P /% (SBGS-16) >
EIS
P /% (SBGS-14)> P /% (SBGS-10) at all concentrations, which implies a better
EIS
EIS
interaction of SBGS-16 on steel surface than the other compounds.
<
<Figure 7>>
<Figure 8>>
<Table 2>>
<
<
3
.6. Potentiodynamic polarization (PDP) measurements.
The potentiodynamic polarization (PDP) curves for mild steel in 1.0 M
solution of H SO without and with various concentrations of the Schiff base gemini
2
4
surfactants, and at 50 °C are shown in Fig. 9 a, b and c for SBGS-10, SBGS-14 and
SBGS-16, respectively. The polarization plot for mild steel in 1.0 M H SO in the
2
4
absence of the inhibitors shows initial sharp increase in both cathodic and anodic
current densities attributed to the cathodic hydrogen reduction reaction and anodic
partial reaction involving the early dissolution of mild steel [51]. The polarization
1
9

ACCEPTED MANUSCRIPT
curves in the presence of the inhibitors show that SBGS-10, SBGS-14 and SBGS-16
compounds reduce both the cathodic and anodic corrosion current densities compared
with those in H SO free inhibitors. This indicates that the inhibitors act as adsorptive
2
4
rust preventing materials, without proton reduction or metal dissolution at the
metal/acid interface. That is, the investigated surfactants cover the active sites on the
steel surface, which leads to the protection of metal surface against acid corrosive
attacks and reduces oxidative dissolution and retards a hydrogen evolution reaction
[
52, 53].
Electrochemical kinetic parameters such as corrosion potential (Ecorr),
corrosion current density (J ) and cathodic and anodic Tafel slopes (β and β ,
corr
c
a
respectively) for the mild steel in all solutions (with and without inhibitors) were
obtained using the Tafel extrapolation method. These values in addition to the value
of the percentages of the inhibition efficiency (PPDP, %) are listed in Table 3. The
0
corr
i
values of corrosion current density in the absence (
J
) and presence (
J
) of the
corr
inhibitors were used to determine PPDP and the surface coverage (θ) according to the
equation (12) [1]:
o
corr
i

J  J

corr
P /%  
100  100
(12)
PDP
o


J

corr

Results in the Table 3 demonstrate that Jcorr decreases significantly in the presence of
SBGS compounds compared to the blank solution and also decreases with increasing
concentration of the surfactant. In other words, the PPDP increases with increase in
concentration of the studied SBGS inhibitors. This indicates that as the concentration
of the investigated surfactants increases, more molecules of the inhibitors or
steel/surfactant complexes adsorb on the surface of mild steel thereby increasing the θ
and also the PPDP. This observation is a direct indication of the corrosion inhibition
2
0

ACCEPTED MANUSCRIPT
characteristics of the studied SBGS compounds. The magnitude of the relative shift in
Ecorr after the addition of an inhibitor compared to the blank solution is often used to
characterize an inhibitor an anodic, cathodic or mixed-type. A shift in E > ±85 mV
corr
is generally attributed to an inhibitor of anodic or cathodic mitigating effect, while a
shift in Ecorr < ±85 mV implies that the inhibitor is of mixed effect, that is, it inhibits
both anodic metal dissolution and cathodic reduction reaction [54, 55]. The data in
Table 3 showed that for the studied Schiff base gemini surfactants, the shifts in Ecorr
over the whole concentration range are less than 85 mV, which implies that SBGS-10,
SBGS-14 and SBGS-16 compounds are mixed-type inhibitors. The values of the
cathodic (β ) and anodic (β ) Tafel slopes change with change in concentration. This
c
a
suggests that the inhibitive actions of the studied surfactants are concentration
dependent. The change in β values with change in concentration are generally more
a
pronounced than change in β values. Moreover, β are also found to be greater than
c
c
the respective β . This suggests that the inhibitors reduce the rate of the cathodic
a
partial reaction by blocking the active sites involved in the cathodic hydrogen
evolution without altering the mechanism of the cathodic reaction, while on the other
hand, the anodic mild steel dissolution might have witnessed some change in
mechanism as informed by the large change in β values, which is also inferable from
a
the appearance of the anodic limiting current plateau in the polarization curves. A
comparison of PPDP % values of SBGS-10, SBGS-14 and SBGS-16 as shown in Table
3
revealed that SBGS-16 exhibits better corrosion inhibition potentials than SBGS-10
and SBGS-14 over the concentration ranges considered in this study. Since the three
studied surfactants differ in the length of the alkyl group “R” (R = 16 in SBGS-16; R
=
14 in SBGS-14; R = 10 in SBGS-10) as shown in Fig. 9, it can be inferred that the
higher PPDP/% of SBGS-16 compared to SBGS-10 and SBGS-14 is as a result of the
2
1

ACCEPTED MANUSCRIPT
increased alkyl chain length. Longer alkyl chain has been reported to favor higher
PPDP% due to increased adsorption at surface layers [56]. The efficiency order of the
tested inhibitors at all concentrations is as follows: SBGS-16 > SBGS-14 > SBGS-10.
This order is the same as the one obtained from EIS measurements.
<
<Figure 9>>
<Table 3>>
<
3
.7. Adsorption Isotherm and thermodynamic parameters
In order to investigate the adsorption mechanism of corrosion inhibitor on
steel/acid interface, adsorption isotherms were plotted. In the present study, the
experimental data for the adsorption of SBGS-10, SBGS-14 and SBGS-16 inhibitors
were subjected to various adsorption isotherms including the Flory-Huggins,
Langmuir, Frumkin, Temkin and Freundlich isotherms. Among the different isotherm
models tried, the most suitable one was selected with the help of regression
2
coefficient R (Table 4). Langmuir isotherm was found to be the most suitable mode
2
to fit experimental data, with all linear R very close to unity, indicating that
adsorption process of I BGS-10, SBGS-14 and SBGS-16 on mild steel in 1.0 M
H SO obeys Langmuir isotherm. The linear form of the Langmuir isotherm is
2
4
expressed as [2]:
Ci
1

C 
(13)
i

Kads
where C is the concentration of the inhibitors, θ, the degree of the coverage, K is
i
ads
the equilibrium constant of adsorption process of corrosion inhibitors. The values of θ
can be determined from the potentiodynamic polarization measurements by equation
1
2. Plots of C /θ vs. C at 323 K give straight line as presented in Fig. 10. The change
i i
2
2

ACCEPTED MANUSCRIPT
o
ads
in Gibb’s free energy of adsorption ( G ) is related to the Kads by the equation (14)
[3]:
0
ads
(14)

G  RT ln(C K )
W
ads
where C is the molar concentration of water in acid solution which is 55.5 M [57], R
W
o
ads
is the gas constant and T is the absolute temperature. The values of G and Kads for
the studied SBGS inhibitors were calculated. Large values of Kads imply better
adsorption due to strong electrostatic interactions or enhanced charge transfer between
the inhibitor molecules and steel surface. It can be seen that the values of Kads follow
4
4
the order: Kads (SBGS-16; 15.38×10 ) > K (SBGS-14, 10.52×10 ) > K (SBGS-10;
ads
ads
4
6
.82×10 ), which reveal that SBGS-16 possess stronger adsorption affinity onto the
steel surface than SBGS-14 and SBGS-10, and thus SBGS-16 exhibits better
inhibitive performance. Generally, adsorption of inhibitors on the surface of metal is
classified as a physical adsorption mechanism, involving electrostatic interactions
between the charged inhibitor molecules and the charged metal surface, when the
o
ads
o
ads
value of G is less negative or around -20 kJ/mol. A value of G more negative
or around -40 kJ/mol suggests that the adsorption of an inhibitor on the metal surface
features chemisorption mechanism, involving charge sharing or charge transfer from
organic inhibitor molecule to the metal surface to form a coordinate type of bond [57].
o
In the present study, the values of G obtained for SBGS-10, SBGS-14 and SBGS-
ads
-1
1
6 were found to be -40.67, -41.84 and -42.85 kJ mol , respectively, suggests that the
surfactant molecules can adsorb on the metal surface with a typical of chemisorption
mechanism, implying the formation of a stable protection layer [58]. This type of
adsorption mechanism can be discussed in terms of the interaction between π-
electrons of surfactant molecules and vacant d-orbitals of metal surface to form
coordination bonds by charge or electrons transfer [59].
2
3

ACCEPTED MANUSCRIPT
<
<Figure 10>>
<Table 4>>
<
3
.8. Surface characterization by SEM
The SEM images of mild steel specimens after the immersion in 1.0 M H SO4
2
for 3 days in the absence and presence of 1.0 mM of SBGS-10, SBGS-14 and SBGS-
6 compounds are shown in Fig. 11. It can be seen from Fig. 11a that the steel
specimen in 1.0 M H SO solution without inhibitors was strongly corroded and
1
2
4
damaged due to free acid corrosion of the steel surface in the uninhibited corrosive
environment, resulting in a heterogeneous and rough surface of mild steel in the
investigated aggressive solution. In contrast, in presence of 1.0 mM of the
investigated inhibitors, the surface morphology of the specimens was uncommonly
improved resulting in the observation of smoother and less corroded surfaces (Fig. 11
b, c and d). These micrographs confirmed the formation of protective layer on the
surface of mild steel by the surfactant molecule, which shielding it from the direct
acid attack and thereby minimizing the corrosion rate effectively. Moreover, the mild
steel surface with SBGS-16 inhibitor (Fig. 11d) is smoother slightly than counterpart
with SBGS-10 and SBGS-14 compounds, which also support the results obtained
from EIS and PDP measurements.
<
<Figure 11>>
3
.9. Corrosion inhibition mechanism by SBGS compounds
The adsorption mechanism of the surfactant molecules at steel/acid interfaces
is affected by the chemical structures of the surfactants, the nature and charged metal
surface and the allocation of charge over the whole surfactant molecule. The
hydrophobic chain length plays an important role in the corrosion inhibition of the
2
4

ACCEPTED MANUSCRIPT
surfactant molecules. The results in Tables 2, 3 reveal that the inhibition efficiency of
the prepared SBGS compounds increments with increasing the hydrophobic chain
length at the studied temperature. These chains are situated vertical to the steel surface
[14], that repulse the polar molecules from the surface of metal and consequently
decreases the aggressive effects of the acid solution on the metal surface by formation
of adsorbed monolayer on the metal surface [60-62]. The studied surfactants can
adsorb on the steel surface through one and/or more of the following:
I. Donor–acceptor interaction between the π-electron of double bond of
carbonyl group and lone pair of electrons of nitrogen atoms of azomethine
group (–CH N–) and vacant d-orbital of the steel surface, which decreased
the anodic dissolution of mild steel.
II. Electrostatic interaction between the positive charge (quaternary nitrogen
+
2
4
atom; N ) on the prepared SBGS inhibitor and the already-adsorbed SO
ions, which decreased the evolution of hydrogen.
III.Interaction between unshared electron pairs located at nitrogen and oxygen
atoms and the vacant d-orbital of the steel surface.
The stability of the adsorbed monolayer on the steel surface may be due to the
attraction forces occurring between the negatively charged metal surface and the
positively charged head groups [63]. In addition to, the hydrophobic tail of the
synthesized surfactants forms consecutive protective layers, which isolate the surface
of steel from the aggressive acid solution.
3
.10. Quantum chemical calculations
In this work theoretical measurements was used to investigate the inhibitory
action of SBGS-10, SBGS-14 and SBGS-16 compounds in the gas and water media
using B3LYP methods with 6-31G (d,p) basis set. Theoretical calculations can spread
2
5

ACCEPTED MANUSCRIPT
light on experimental investigations or even predict some experimentally unknown
properties [64]. The gas phase optimized structures and the respective HOMO and
LUMO electron density surfaces of SBGS-10, SBGS-14 and SBGS-16 inhibitors are
shown in Fig. 12.
3
.10.1. Frontier molecular orbital (HOMO and LUMO orbitals) characteristic of
the investigated inhibitors
According to the theory of quantum chemistry [65, 66], the interaction
between reactants only happens in frontier molecular orbital. To analyze the
adsorption process of the corrosion inhibitors we take HOMO and LUMO of the
compound molecules into consideration.
The EHOMO indicates the ability of the molecule to donate electrons to an
appropriated acceptor with empty molecular orbitals, whereas the ELUMO indicates its
ability to accept electrons. The lower the value of ELUMO, the more ability of the
molecule is to accept electrons [67].
The higher the value of EHOMO of the inhibitor, the easier is its ability to offer
electrons to the unoccupied d-orbital of metal surface, and the greater is its inhibition
efficiency. As is seen from Table 5 by increasing the alkyl chain, SBGS-14 and
SBGS-16 have the highest value of EHOMO which indicates that have high inhibition
efficiency. The ELUMO of SBGS-14 and SBGS-16 inhibitors are lower than SBGS-10
which implies that better inhibition efficiency, but in water medium there is no trend
observed.
The frontier molecular orbitals (FMO) distributions are of major importance in
characterizing the adsorption priority of the inhibitors. The HOMO and LUMO
molecular orbitals of the studied inhibitors are given in Fig. 12. The results display
that the HOMO orbitals are mostly localized on nitrogen and hydrogen (methyl
2
6

ACCEPTED MANUSCRIPT
ammonium), which will probably give to the inhibitors a strong adsorption onto the
mild steel surface throughout this site. furthermore, the HOMO orbitals are partially
localized on the oxygen (carbonyl) indicating that it should be a probable adsorption
site onto the mild steel surface as well but with less importance when compared to
nitrogen and hydrogen (methyl ammonium). However, the LUMO is almost
completely localized on the two chlorides and the second nitrogen (methyl
ammonium).
The energy gap, ΔE between the HOMO and LUMO orbitals pointed the
chemical reactivity of the studied inhibitors. The obtained values of ΔE indicate that
the substitution of the hydrogen atom in SBGS-10 by butyl group (SBGS-14) or by
hexyl group (SBGS-16) decreases the value of the energy gap. The smaller the value
of ΔE, the more is the probable inhibition efficiency the compound has [68-70]. It was
shown from Table 5 that inhibitors SBGS-14 and SBGS-16 have the smallest
HOMO–LUMO gap compared with SBGS-10. Accordingly, SBGS-14 and SBGS-16
molecules have more inclination to adsorb on the metal surface than the SBGS-10
molecule.
3
.10.2 .The dipole moment (μdipole
)
The dipole moment, μdipole, is the measurement of net molecular polarity.
Popova et al., [71] declared that the high value of dipole moment encourage
adsorption by affecting the transfer process through adsorbed layer. The energy of the
deformability increases with the increase in μdipole, making the molecule easier to
adsorb at the mild steel surface. The dipole moment of three inhibitors SBGS-10,
SBGS-14 and SBGS-16 show the same trend as obtained for the energy gap.
3
.10.3 .Total energy (Etotal)
2
7

ACCEPTED MANUSCRIPT
The total energy values of the studied SBGS models are listed in Table 5. The
order of increasing total energy of the undertaken inhibitors is as follows:
Etotal (SBGS-10) < Etotal (SBGS-14) < Etotal (SBGS-16)
This result indicated that SBGS-16 inhibitor is favorably adsorbed through the active
centers of adsorption on onto the mild steel surface.
3
.10.4 .Ionization potential (I ) and electron affinity (E )
P A
Ionization potential (I ) and electron affinity (E ) are related to the energy of
P
A
the HOMO and the LUMO according to Koopman’s theorem [72].
IP= -EHOMO
EA= -ELUMO
(15)
(16)
Ionization potential is defined as the amount of energy required to remove an electron
from a molecule. High Ionization energy show chemical inertness and small
ionization energy show high reactivity of the atoms and molecules [73]. In this study
ionization potential of SBGS-14 and SBGS-16 have lower value than SBGS-10
indicates that SBGS-14 and SBGS-16 possesses high inhibition efficiency.
3
.10.5. Electronegativity (χ), chemical hardness (η) and chemical softness (σ)
The electronegativity (χ) and chemical hardness (η) were estimated as linear
combinations of the calculated I and E [72]:
p
A
IP  E
A

 potential

(17)
(18)
2
IP  E
A


2
Chemical softness (σ) is the inverse of the hardness [74] estimated by using the
equation (19):
1


(19)

The order of electronegativity of the studied inhibitors are shown in Table 5 as SBGS-
1
0 > SBGS-14 ≈ SBGS-16. According to Sanderson’s electronegativity equalization
2
8

ACCEPTED MANUSCRIPT
principle [75], an increase in the difference of electronegativity between the metal and
the inhibitor is observed in the order SBGS-10 < SBGS-14 ≈ SBGS-16. With a high
electronegativity and low difference of electronegativity speedily reaches equalization
and low reactivity is expected which in turn marks low inhibition efficiency.
The molecular reactivity of the corrosion inhibitors are investigated from
chemical hardness (η) and softness (σ) [76]. It is obvious that the chemical hardness
principally signifies the resistance towards the deformation or polarization of the
electron cloud of the atoms, ions, or molecules under small perturbation of chemical
reaction [77, 78]. These quantities are connected with the Lewis theory of acid and
bases and Pearson’s hard and soft acids and bases [79]; a hard molecule has a large
energy gap ΔE and a soft molecule has a small energy gap ΔE. Soft molecules are
more interactive than hard ones because they could easily offer electrons to an
acceptor. Adsorption occurs most likely at the region of the molecule where σ has the
highest value [80]. In a corrosion system, the inhibitor acts as a Lewis base while the
metal acts as a Lewis acid. Bulk metals are soft acids and thus soft base inhibitors are
most effective for acidic corrosion of these metals. The order of the inhibitors
according to softness value was found to be SBGS-16 ≈ SBGS-14 > SBGS-10.
3
.10.6. The global electrophilicity (ω)
The global electrophilicity index (ω) introduced by Parr [80] as a
measurement of energy reducing due to maximal electron flow between donor and
acceptor and is given by equation (20) [80]:
2

potential


(20)
2
Electrophilicity provide information on the nucleophilic or elecrophilic nature of the
molecule. A good nucleophilic is recognized by lower value of chemical potential and
electrophilicity and reciprocally a good electrophilic is recognized by a high value of
2
9