Supplementary materials
1. Materials and Experimental techniques
1.1. Surface tension measurements
Surface tension was measured with a Du Nouy Tensiometer (Kruss Type 6) for different
concentrations of the synthesized cationic surfactants. Doubly distilled water from an allglass apparatus with a surface tension of 72 mN cm-1 at 25 oC was used to prepare all
solutions. The readings were taken in triplicate for each solution to check repeatability and
the surface tension values were within ±1 mN m-1. Concentration ranges of the synthesized
surfactants varied from 1×10-6 to 1×10-3 M.
1.2. Electrochemical measurements
The electrochemical experiments were carried out in a conventional three-electrode cell with
a platinum counter electrode (CE) and a saturated calomel electrode (SCE) as a reference
electrode. A working electrode (WE) was a carbon steel rod with the same composition
which illustrated earlier which embedded in PVC holder using epoxy resin so that the flat
surface was the only exposed surface in the electrode [1]. The exposure surface area of the
working electrode was 0.7 cm2. This area was abraded with emery paper (grade 320, 400,
600, 800, 1000, 1200) on the test face, rinsed with distilled water, degreased with acetone,
and dried. Before measurement, the electrode was immersed in a test solution at open circuit
potential (OCP) for 30 min until a steady state was reached. All electrochemical
measurements were recorded by a Voltalab 40 Potentiostat PGZ 301. Each experiment was
repeated three times to check the reproducibility.
Electrochemical impedance spectroscopy (EIS) measurements were carried out as described
before. A small alternating voltage perturbation (10 mV) was imposed on the cell over the
frequency range of 100 kHz to 30 mHz at open circuit potential at 20 oC. Potentiodynamic
1
polarization experiments were carried out in the potential range (from -0.8 to -0.3 V vs.
Ag/AgCl) with scan rate of 0.2 mV s-1.
1.3. Weight loss measurements
Weight loss measurements were performed (in triplicate) on the carbon steel sheets with a
rectangular form of dimensions 7 cm × 3 cm × 0.5 cm in 1 M HCl solution with and without
addition of different concentrations of the tested cationic surfactants IV(4N), II(4N) and
I(4N) (5×10-3, 1×10-3, 5×10-4, 1×10-4 and 5×10-5 M). The average weight loss of three
parallel carbon steel sheets was obtained. Immersion time was 24 h at all studied
temperatures.
2. Mathematical treatments
2.1. Surface tension measurements
The maximum surface excess surfactant ions concentration, (Γmax), at the interface was
calculated from the following equation [2]:
𝛤max =
1
d
(
)
𝑛𝑅𝑇 dln C
(1)
where R is the gas constant, T is the absolute temperature, C is the surfactant concentration, γ
is surface tension at given concentration and n is the species ions number in solution.
Also, the minimum surface area per adsorbed molecule, Amin is defined as the area occupied
by each molecule in nm2 at the liquid/air interface. Amin was calculated from the following
equation:
𝐴min =
1014
NA 𝛤max
(2)
where NA is the Avogadro’s number.
2
The effect of the surfactant ions on the interface is better described by surface pressure or
effectiveness of the surfactant (πcmc) and can be calculated by using the following equation
[3]:
𝜋CMC = 𝛾o − 𝛾
(3)
where γo is the surface tension of distilled water and (γ) is the surface tension of surfactant
solution at the critical micelle concentration CMC.
2.2. Electrochemical measurements
2.2.1. The inhibition efficiency (ηI)
The corrosion inhibition efficiency (ηI) was calculated from the values of Rct using the
following equation [4]:
Rct -Roct
ηI = (
) ×100
Rct
(4)
where Rct and Roct are the charge transfer resistance values in 1 M HCl in the presence and
absence of the inhibitor, respectively.
2.2.2. The double layer capacitance (Cdl)
The double layer capacitance, Cdl, for a circuit including a CPE was calculated from the
following equation:
Cdl = 𝑌o (ωmax )n-1
(5)
where ωmax = 2πfmax and fmax is the frequency at which the imaginary component of the
impedance is maximal.
Also, the impedance, ZCPE, for a circuit including a CPE was calculated from the following
equation:
ZCPE =
1
𝑌o (Jω)n
(6)
3
where 𝑌ₒ is a proportional factor, J2 = -1, ω = 2πf and n is the phase shift. For n = 0, ZCPE
represents a resistance with R = Yₒ-1, for n = 1 a capacitance with C = Yₒ, for n = 0.5 a
Warburg impedance with W= Yₒ and for n = -1 an inductive with L = Yₒ-1.
2.2.3. The Helmholtz model equation:
δorg =
εεo A
Cdl
(7)
where ε is the dielectric constant of the medium, εo is the vacuum permittivity, A is the
electrode surface area and δorg is the thickness of the protective layer.
Potentiodynamic polarization measurements were obtained by changing the electrode
potential automatically from -1000 to -200 mV vs. SCE with scan rate 2 mV s-1 at 20 oC.
2.2.4. The inhibition efficiency (ηp)
The corrosion inhibition efficiency (ηp) was calculated using the following equation [5]:
iocorr - icorr
ηp = ( o
) × 100
icorr
(8)
where iocorr and icorr are the corrosion current density values without and with various
concentrations of inhibitors, respectively, which determined by extrapolation of the cathodic
and anodic Tafel lines to the respective corrosion potential.
2.3. Weight loss measurements
Weight loss measurements were performed (in triplicate) on the carbon steel sheets with a
rectangular form with a dimensions of: 7 cm × 3 cm × 0.5 cm in 1 M HCl solution with and
without addition of different concentrations of IV(4N), II(4N) and I(4N) compounds (5×10-3,
1×10-3, 5×10-4, 1×10-4 and 5×10-5 M).
2.3.1. The corrosion rate (k)
The corrosion rate value (k) was calculated from the following equation [6]:
k =
∆W
(9)
St
4
where ΔW is the weight loss average of three parallel carbon steel sheets, S is the total area of
the specimen, and t is the immersion time.
2.3.2. The corrosion inhibition efficiency (ηw)
The corrosion inhibition efficiency (ηw) of carbon steel was calculated as follows:
ηw = (
𝑊o - W
) ×100
𝑊o
(10)
where Wo and W are the weight loss values without and with the addition of the inhibitor,
respectively.
2.4. Langmuir isotherm
The simplest isotherm equation is Langmuir [7]:
𝐶
1
=
+𝐶
𝜃 𝐾ads
(11)
where C is the inhibitor concentration in the bulk phase of the solution, θ is the surface
coverage and Kads is the binding constant of the adsorption reaction.
2.5. The degree of surface coverage (θ)
The surface coverage degree (θ) for different concentrations of inhibitor molecules in 1 M
HCl was calculated from weight loss measurements using the following equation [7]:
𝑊𝑜 − 𝑊
𝜃=(
)
𝑊𝑜
(12)
where Wo and W are the weight loss values without and with the addition of the inhibitor,
respectively.
2.6. The standard free energy (ΔGoads)
The standard free energy (ΔGoads) was obtained according to the following equation [8]:
∆Goads = − RT ln (55.5Kads )
(13)
where 55.5 value is the molar concentration of water.
2.7. The standard enthalpy (ΔHoads)
The standard enthalpy, ΔHoads, was calculated according to the Van’t Hoff equation [8]:
5
∆Hoads
ln Kads = − (
) + constant
RT
(14)
where ∆Hoads and Kads are the standard enthalpy and adsorptive equilibrium constant,
respectively.
2.8. The standard entropy (ΔSoads)
According to the thermodynamic basic equation, the standard entropy, ΔSoads, was calculated
from the following equation [8]:
∆Goads = ∆Hoads − T∆Soads
(15)
3. Results and discussion
3.1. Chemical structure confirmation
100
90
Relative intensity (%)
80
70
60
50
40
30
20
10
0
50
65
80
95
110 125 140 155 170 185 200 215 230 245 260 275 290 305 320
m/z
Fig. 1: Mass spectrum of the Schiff base (S).
6
3.1.2. Chemical structure confirmation of the synthesized cationic surfactants
Fig.2: FTIR spectrum of inhibitor IV(4N).
Fig. 3: 1HNMR spectrum of inhibitor IV(4N).
7
600
HCl
0.00001M
0.00005M
0.0001M
0.0005M
0.001M
II (4N)
-Zi (Ω cm2)
500
400
300
200
100
0
0
100
200
300
Zr (Ω
400
500
600
cm2)
400
HCl
0.00001M
0.00005M
0.0001M
0.0005M
0.001M
I (4N)
-Zi (Ω cm2)
300
200
100
0
0
100
200
Zr (Ω
300
400
cm2)
Fig. 4. Nyquist plots of carbon steel in 1 M HCl solutions without and with different
concentrations of inhibitors II(4N) and I(4N).
8
0.001M.txt
Model : R(QR) Wgt : Modulus
260
Z , Msd.
Z , Calc.
240
220
200
180
Iter #: 4
Chsq: 9.10E-03
160
- Z '' (ohm)
4
140
7.94
7.14
14
100
35.7
50
3.57
2.5
2
3.16
1.25
1.58
893m
1.12
794m
20
63.3
20
125
158m
400
0
714m
446m
562m
281m
357m
200m
250m
28.1
40
40
# of pars with
rel. std. errors
>10%:
0/4
>100%: 0 / 4
1.79
2.23
15.8
25
60
5
11.2
17.9
80
4.46
5.62
10
120
79.4m
56.2k
141m
112m
71.4m
50m
-20
0
50
100
150
200
250
300
350
400
Z ' (ohm)
0.001M.txt
Model : R(QR) Wgt : Modulus
1,000
65
60
55
|Z|, Msd.
|Z|, Calc.
Angle, Msd.
Angle, Calc.
50
45
40
100
35
|Z| (ohm)
25
20
15
10
Iter #: 4
Chsq: 9.10E-03
Angle (deg)
30
# of pars with
rel. std. errors
>10%:
0/4
>100%: 0 / 4
10
5
0
-5
-10
-15
1
0.01
0.1
1
10
100
1,000
10,000
100,000
Frequency (Hz)
Fig. 5a: Simulation of Nyquist and Bode diagrams with suggested model (Fig. 3a and Fig. 3b)
of 4,4'-((1Z,11E)-5,8-didodecyl-2,5,8,11-tetraazadodeca-1,11-diene-5,8-diium-1,12diyl)bis(1-dodecylpyridin-1-ium) bromide (IV(4N)).
9
0.00001M.txt
Model : R(QR) Wgt : Modulus
Z , Msd.
Z , Calc.
75
70
65
60
55
50
44.6
56.2
- Z '' (ohm)
45
71.4
40
35.7
40
28.1
22.3
31.6 25
15.8
12.5
20
50
14
11.2
89.3
35
79.4
112
30
250
15
7.14
5.62
3.57
223
2.5
357
10
633m
1.12k
0
15.8k
71.4k
-5
2.81
2.23
1.58
1.4
562
5
# of pars with
rel. std. errors
>10%:
0/4
>100%: 0 / 4
4
5
125
158
179
20
7.94
100
140
25
Iter #: 4
Chsq: 7.53E-03
8.93
893m
500m
200m
-10
0
20
40
60
80
100
120
140
160
Z ' (ohm)
0.00001M.txt
Model : R(QR) Wgt : Modulus
1,000
60
55
50
|Z|, Msd.
|Z|, Calc.
Angle, Msd.
Angle, Calc.
45
40
100
35
|Z| (ohm)
25
20
15
10
10
5
Angle (deg)
30
Iter #: 4
Chsq: 7.53E-03
# of pars with
rel. std. errors
>10%:
0/4
>100%: 0 / 4
0
-5
-10
-15
1
0.1
1
10
100
1,000
10,000
100,000
Frequency (Hz)
Fig. 5b: Simulation of Nyquist and Bode diagrams with suggested model (Fig. 3a and Fig.
3b) of N1,N2-didodecyl-N1-(2-((E)-(pyridin-4-ylmethylene)amino)ethyl)-N2-(2-((Z)(pyridin-4-yl methylene)amino)ethyl)ethane-1,2-diaminium bromide (II(4N)).
10
IV (4N) 0.00001 M.txt
Model : R(QR) Wgt : Modulus
Z , Msd.
Z , Calc.
90
80
70
- Z '' (ohm)
60
31.6
50
25
17.9
44.6
28.1
63.3
40
50
89.3
30
12.5
10
7.94
11.2
4.46
5
2.5
4
1.4
714m
141m
500
200m
2.81k
71.4k
0
# of pars with
rel. std. errors
>10%:
0/4
>100%: 0 / 4
1.79
1
2.23
158
250
10
Iter #: 4
Chsq: 7.77E-03
5.62
7.14
40
71.4
100
140
200
20
20 15.8
40m
-10
0
20
40
60
80
100
120
140
160
180
200
Z ' (ohm)
IV (4N) 0.00001 M.txt
Model : R(QR) Wgt : Modulus
1,000
55
50
45
|Z| (ohm)
100
10
5
0
-5
-10
-15
10
1
0.01
0.1
1
10
100
1,000
10,000
Angle (deg)
40
35
30
25
20
15
|Z|, Msd.
|Z|, Calc.
Angle, Msd.
Angle, Calc.
Iter #: 4
Chsq: 7.77E-03
# of pars with
rel. std. errors
>10%:
0/4
>100%: 0 / 4
100,000
Frequency (Hz)
Fig. 5c: Simulation of Nyquist and Bode diagrams with suggested model (Fig. 3a and Fig. 3b)
of
N-(2-((E)-(pyridin-4-ylmethylene)amino)ethyl)-N-(2-((2-((Z)-(pyridin-4-
ylmethylene)amino)ethyl)amino) ethyl)dodecan-1aminium bromide (I(4N)).
11
-1.0
II (4N)
log i (A cm-2)
-2.0
-3.0
-4.0
1 M HCl
0.00001 M
0.00005 M
0.0001 M
0.0005 M
0.001 M
-5.0
-6.0
-0.82
-0.72
-0.62
-0.52
-0.42
-0.32
-0.22
E vs Ag/AgCl (mV)
-1.0
I (4N)
log i (A cm-2)
-2.0
-3.0
-4.0
1 M HCl
0.00001 M
0.00005 M
-5.0
0.0001 M
0.0005 M
0.001 M
-6.0
-0.82
-0.72
-0.62
-0.52
-0.42
-0.32
-0.22
E vs Ag/AgCl (mV)
Fig. 6. Anodic and cathodic polarization curves obtained at 25 oC in 1 M HCl in different
concentrations of inhibitors II(4N) and I(4N).
12
3.3. Langmuir isotherm
0.0012
IV(4N)
C/θ (M)
0.0010
0.0008
0.0006
0.0004
0.0002
25 °C
40 °C
55 °C
70 °C
0.0000
0
0.0002
0.0004
0.0006
0.0008
0.001
C (M)
Fig. 7: Langmuir’s adsorption plots for carbon steel in 1 M HCl containing different
concentrations of inhibitor IV(4N) at various temperatures.
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