Synthesis and inhibitive performance of novel cationic and gemini surfactants on carbon steel corrosion in 0.5 M H2SO4 solution

Article abstract of DOI:10.1039/c5ra06473b

Novel cationic and gemini surfactants were synthesized and characterized. The corrosion inhibition efficiency of the synthesized surfactants was studied, on carbon steel in 0.5 M H₂SO₄, by weight loss, potentiodynamic polarization an

Full text of DOI:10.1039/c5ra06473b

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: M. A. E. Hegazy, A.
El-Tabei, A. Hamam and M. Sadeq, RSC Adv., 2015, DOI: 10.1039/C5RA06473B.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
Synthesis and inhibitive performance of novel cationic
and gemini surfactants on carbon steel corrosion in 0.5
M H SO solution
1
2
3
4
2
4
a,
a
b
b
M.A. Hegazy *, A.S. ElꢀTabei , A.H. Bedair , M.A. Sadeq
a
Egyptian Petroleum Research Institute (EPRI), Nasr City, 11727, Cairo, Egypt
Faculty of Science, AlꢀAzhar University, Chemistry Dept., Nasr City, 11787, Cairo,
5
6
b
Egypt
7
8
9
Abstract
Novel cationic and gemini surfactants were synthesized and characterized. The corrosion
inhibition efficiency of the synthesized surfactants was studied, on carbon steel in 0.5 M
H SO , by weight loss, potentiodynamic polarization and EIS. The results revealed that
10
11
12
13
14
15
16
17
18
2
4
both cationic and gemini surfactants were effectively inhibited the corrosion of carbon
steel in 0.5 M H SO . The corrosion inhibition efficiencies obtained were calculated by
2
4
weight loss and electrochemical experiments. The potentiodynamic polarization studies
revealed that the inhibitors acted as mixedꢀtype inhibitors. Thermodynamic parameters of
adsorption and kinetic were obtained from weight loss at different temperatures (20–60
o
C). The inhibitors adsorption on carbon steel surface obeyed Langmuir isotherm.
Keywords: Surfactants; Carbon steel; EIS; Weight loss; Polarization; Acid inhibition; 19
Surface properties.
20
21
22
*
Corresponding author. Tel.: +20 1002653529; fax: +20 222747433.
Eꢀmail address: mohamed_hgazy@yahoo.com (M.A. Hegazy).
1

RSC Advances
Page 2 of 62
View Article Online
DOI: 10.1039/C5RA06473B
1
. Introduction
1
Carbon steel is the most widely used as a constructional material in many industries due
to its excellent mechanical and low cost. It is used in large tonnages in marine
applications, chemical processing, petroleum production (refining construction) and
metal processing equipment. Acid solutions are widely used in industry, e.g., chemical
cleaning, descaling, pickling and oilꢀwell acidizing, which leads to corrosive attack.
Therefore, the consumption of inhibitors to reduce corrosion has increased in recent
years. The corrosion control by inhibitors is one of the most common effective and
economic methods to protect metals in acidic media [1–7]. The majority of the wellꢀ
known inhibitors are organic compounds containing heteroatom, such as O, N, or S, and
multiple bonds, which allow adsorption on the metal surface [8–13].
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Gemini surfactants exhibit many unique properties in comparison to singleꢀchained
conventional surfactants, so it is reasonable to design and synthesize novel gemini
surfactants with different structures and study their effects on the corrosion inhibition of
metals. Gemini surfactants become the focus of study in recent years [8,14–16], although
numerous corrosion inhibitors have been developed and reported. Gemini surfactants
containing heterocyclic ring and their applications in corrosion inhibition of metals are
rarely reported.
Schiff bases, the condensation product of an amine and a ketone or aldehyde with general
formula of R C=NR are wellꢀknown organic inhibitors [17,18]. Some researchꢀworks
2
revealed that, the inhibition efficiency of the Schiff bases was much greater than that of
corresponding amines and aldehydes due to the presence of a ꢀCH=Nꢀ group in the
molecules [17].
2

Page 3 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
Our goal of this work was directed to synthesize a cationic surfactant ((Z)ꢀ1ꢀdodecylꢀ2ꢀ
2ꢀhydroxybenzylideneamino) pyridinium bromide), in addition to, creation of a new type
1
(
2
of bisꢀSchiff base based on gemini cationic surfactant (2,2'ꢀ(pentaneꢀ1,5ꢀdiylidene
bis(azanꢀ1ꢀylꢀ1ꢀylidene))bis(1ꢀdodecylpyridinium bromide)). The inhibition performance
of the novel synthesized cationic and gemini surfactants for carbon steel in acidic
medium was investigated using weight loss, potentiodynamic polarization and
electrochemical impedance spectroscopy (EIS). The Ccmc values of the prepared
surfactants were determined by surface tension and conductivity measurements. The
surface parameters were calculated by surface tension measurements. The relation
between surface properties and the metal corrosion inhibition efficiencies of the prepared
surfactants was also discussed.
3
4
5
6
7
8
9
10
11
12
2
. Materials and Experimental Methods
13
14
2
.1. Materials
2
.1.1. Carbon steel specimens
15
16
17
18
19
20
21
22
Carbon steel specimens of the following chemical composition (wt. %) were used in the
experiment: 0.19% C, 0.05% Si, 0.94% Mn, 0.009% P, 0.004% S, 0.014% Ni, 0.009%
Cr, 0.034% Al, 0.016% V, 0.003% Ti, 0.022% Cu, and balance Fe.
A preꢀtreatment was carried out prior to each experiment, in which specimen surface was
mechanically grinded with 340, 400, 600, 800, 1000 and 1200 grades of emery paper,
rinsed with bidistilled water, degreased in ethanol and dried at room temperature before
use. They were also degreased with acetone and rinsed with distilled water two times and
finally dried.2.2. Synthesis of novel surfactants
23
3

RSC Advances
Page 4 of 62
View Article Online
DOI: 10.1039/C5RA06473B
2
.2.1. Synthesis of a novel cationic surfactant
A novel cationic surfactant was synthesized through two steps as shown in Fig. 1.
ꢀSynthesis of (Z)ꢀ2ꢀ((pyridinꢀ2ꢀylimino)methyl)phenol
1
2
1
3
Schiff base reaction between (1.22 g, 0.01 mol) of salicyldehyde and (0.94 g, 0.01 mol)
4
o
of 2ꢀaminopyridine in ethanol at 70 C for 6 h [19] to produce (Z)ꢀ2ꢀ((pyridinꢀ2ꢀ
5
ylimino)methyl)phenol. The mixture was allowed to cool down, then the obtained yellow
precipitate was further purified by diethyl ether then recrystallized from ethanol.
6
7
2
ꢀSynthesis of (Z)ꢀ1ꢀdodecylꢀ2ꢀ(2ꢀhydroxybenzylideneamino)pyridinium bromide
8
Quaternization reaction of (Z)ꢀ2ꢀ((pyridinꢀ2ꢀylimino)methyl)phenol (1.98 g, 0.01 mol)
9
o
and 1ꢀbromododecane (2.49 g, 0.01 mol) in ethanol at 70 C for 24 h [20] to produce (Z)ꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1
ꢀdodecylꢀ2ꢀ(2ꢀhydroxybenzylideneamino)pyridinium bromide. The mixture was allowed
to cool down. The obtained light yellow precipitate was further purified by diethyl ether
then recrystallized from ethanol.
2.2.2. Synthesis of a novel cationic gemini surfactant
A novel cationic gemini surfactant used in this study was synthesized as shown in Fig. 2.
This process was carried out in two steps as follow:
1
ꢀ Synthesis of N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine
Schiff base reaction was carried out between (1.00 g, 0.01 mol) of glutaraldehyde and
o
(
1.88 g, 0.02 mol) of 2ꢀaminopyridine in ethanol at 70 C for 6 h [21] in order to produce
N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine and a trace of byproduct (N,N'E,N,N'E)ꢀ
N,N'ꢀ((E)ꢀ2ꢀ((E)ꢀ5ꢀ(pyridinꢀ2ꢀylimino)pentylidene)pentaneꢀ1,5ꢀdiylidene)bis(pyridinꢀ2ꢀ
amine). The mixture was allowed to cool down. The obtained yellow precipitate was
further purified by diethyl ether then recrystallized from ethanol.
4

Page 5 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
2
ꢀ Synthesis
of
2,2'ꢀ(pentaneꢀ1,5ꢀdiylidenebis(azanꢀ1ꢀylꢀ1ꢀylidene))bis(1ꢀ
1
2
3
4
5
6
7
8
9
dodecylpyridinium bromide)
Quaternization reaction of N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine (2.52 g, 0.01
o
mol) and 1ꢀbromododecane (4.98 g, 0.02 mol) in ethanol at 70 C for 24 h [22] to
produce
2,2'ꢀ(pentaneꢀ1,5ꢀdiylidenebis(azanꢀ1ꢀylꢀ1ꢀylidene))bis(1ꢀdodecylpyridinium
bromide). The mixture was allowed to cool. The obtained light yellow precipitate was
further purified by diethyl ether then recrystallized from ethanol.
The chemical structure of the synthesized compounds was confirmed by elemental
1
analysis, FTIR, HNMR and Mass spectroscopy.
2
.3 Solutions
10
The 0.5 M H SO was prepared by dilution of analytical grade H SO (98 %/wt) with
11
12
13
14
15
16
2
4
2
4
distilled water. The molecular weights of the synthesized cationic and cationic gemini
surfactant are 447.45 and 750.66, respectively. The concentration range of used
ꢀ4
ꢀ2
synthesized inhibitors varied from 1×10 to 5×10 M for corrosion measurements and
ꢀ6
−2
from 1×10 to 1×10 M for both surface tension and conductivity measurements.
Double distilled water was used for preparing test solutions in all measurements.
2
.4. Weight loss measurements
17
The rectangular specimens of carbon steel (7 cm × 3 cm × 0.5 cm) were accurately
weighed and then immersed for up to 24 h in a solution containing 0.5 M H SO with and
18
19
20
21
2
4
without different concentrations of each surfactant. Afterwards, the steel sheets were
taken out, rinsed thoroughly with distilled water, dried, and accurately weighed.
2
.5. Electrochemical measurements
22
5

RSC Advances
Page 6 of 62
View Article Online
DOI: 10.1039/C5RA06473B
Polarization experiments were carried out using a conventional threeꢀelectrode cell with a
platinum counter electrode (CE) and a saturated calomel electrode (SCE) as a reference
electrode. The working electrode (WE) was a carbon steel rod embedded in PVC holder
using epoxy resin, so that the circular surface was the working area. Before each
measurement, the electrode was immersed in a test solution at open circuit potential
1
2
3
4
5
6
7
8
9
10
(OCP) for 30 min, until a steady state was reached. All polarization curves were recorded
by a Voltalab 40 Potentiostat PGZ 301 and a personal computer was used with
o
Voltamaster 4 software at 20 C. Each experiment was repeated at least three times to
check the reproducibility.
The potentiodynamic polarization measurements were obtained by changing the electrode
potential automatically from ꢀ800 to ꢀ300 mV vs. SCE at open circuit potential with a 11
ꢀ
1
o
scan rate 0.2 mV s at 20 C.
12
13
14
15
Electrochemical impedance spectroscopy (EIS) measurements were carried out as
described elsewhere [22]. A small alternating voltage perturbation (10 mV) was imposed
o
on the cell over the frequency range of 100 kHz to 30 mHz at 20 C.
2
.6. Surface tension measurements
16
The surface tension was measured for different concentrations of the synthesized
surfactants that dissolved in double distilled water and in 0.5 M H SO with a Du Nouy
17
18
19
20
2
4
Tensiometer (Kruss Type 6). All solutions were prepared in double distilled water with a
ꢀ1
surface tension equal to 72 mN m at 25 °C.
2
.7. Conductivity measurements
21
An electrical conductivity meter (Type 522; Crison Instrument, S.A.) was used to
measure the conductivity of the different surfactant solutions.
22
23
6

Page 7 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
3
. Results and discussion
1
2
3
.1. Confirmation of chemical structure
Schiff base ((Z)ꢀ2ꢀ((pyridinꢀ2ꢀylimino)methyl)phenol)
FTIR spectra
3
4
FTIR spectrum of (Z)ꢀ2ꢀ((pyridinꢀ2ꢀylimino)methyl)phenol (Fig. 3) showed that the
5
ꢀ1
characteristic bands (cm ) at 3431.12 (bonded –OH), 3051.41 (ArꢀH) and 1608.52
6
(CH=N).
7
1
HNMR spectra
8
1
HNMR (DMSO ꢀ d ) spectrum showed δ , ppm at: 12.99 (1H, s, OH), 9.47 (1H, s,
9
6
N=CH), 8.51, 8.50 (1H, dd, J=3.05, 1.5 Hz, PyꢀH); 7.90, 7.90, 7.89 (1H, ddd, J=7.6, 2.3,
.35 Hz, PyꢀH), 7.76495, 7.773 (1H, dd, J=1.55, 6.1 Hz, PyꢀH) and three sets multiples at
7.41–7.4425), (7.32–7.34), (6.95–6.97) (5H, PyꢀH + 4 ArꢀH).
10
11
12
13
14
15
16
17
18
19
20
21
5
(
Mass spectrum
The mass spectrum of (Z)ꢀ2ꢀ((pyridinꢀ2ꢀylimino)methyl)phenol (Fig. 4) showed that a
molecular ion peak m/z 198(17.12%), and other significant peaks are shown at m/z 171
(
4.86%) C H NO, 143 (5.63%) C H N, 128 (9.03%) C H .
11 9 10 9 10 8
1
According to the data of FTIR, HNMR and Mass spectroscopy, the product is confirmed
as (Z)ꢀ2ꢀ((pyridinꢀ2ꢀylimino) methyl) phenol.
Cationic
bromide)
surfactant
((Z)ꢀ1ꢀdodecylꢀ2ꢀ(2ꢀhydroxybenzylideneamino)pyridinium
Elemental analysis
7

RSC Advances
Page 8 of 62
View Article Online
DOI: 10.1039/C5RA06473B
Elemental analysis of (Z)ꢀ1ꢀdodecylꢀ2ꢀ(2ꢀhydroxybenzylideneamino)pyridinium bromide
as the following: (theoretical, %) C, 64.42; H, 7.88; N, 6.26; O, 3.58; Br, 17.86 and
1
2
(found, %) C, 64.55; H, 7.79; N, 6.24; O, 3.61; Br, 17.81.
3
FTIR spectra
4
FTIR spectrum of (Z)ꢀ1ꢀdodecylꢀ2ꢀ(2ꢀhydroxybenzylideneamino)pyridinium bromide
5
ꢀ1
ꢀ1
showed bonded –OH at 3293.31cm , (ArꢀH) aromatic at 3126 cm , (CꢀH) aliphatic at
6
ꢀ1
ꢀ1
2
921.20, 2852.63 cm and CH=N at 1660.14, 1612.91 cm .
7
1
HNMR spectra
8
1
HNMR (DMSO–d ) spectrum (Fig. 5) showed δ , ppm at: 10.21 ((1H, s, OH), 8.5 (1H,
9
6
s, N=CH), three sets of multiples at (7.85–8.07 ), (7.06–7.07), ( 6.78–6.95), (4H, PyꢀH +
10
11
12
13
14
15
16
17
18
19
20
21
22
+
4
H, Ar–H), 0.79 (3H, CH ), 1.15 (m, 16H, (CH ) ), 4.10 (2H, NCH ).
3
2 8
2
1
According to the data of FTIR, and HNMR spectroscopy, the product is (Z)ꢀ1ꢀdodecylꢀ
ꢀ(2ꢀhydroxybenzylideneamino) pyridinium bromide.
2
Schiff base (N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine)
FTIR spectra
FTIR spectrum of N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine showed CH aliphatic
ꢀ1
ꢀ1
ꢀ1
at 2953.9, 2923.3, 2843 cm , ArꢀH aromatic at 3078.23, 3015.49 cm , NH at 3285 cm
ꢀ1
and CH=N at 1650.78 cm .
1
HNMR spectra
1
HNMR (CDCl –d ) spectrum of N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine showed
3
6
different peaks at δ, ppm : 1.229, 1.942, 2.168 and 2.262 as multi sets for saturated CꢀH
protons; 4.87 and 4.941 for olefinic CꢀH protons. PyꢀH, as multi sets at δ: 6.49, 6.63,
8

Page 9 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
6
.84, 7.428, 8.057 ppm and N=CH at 8.19 and 8.20 ppm. Allylic proton shift indicated
1
2
by the presence of δ NꢀH at 4.416 ppm as broad absorption.
3
4
5
Mass spectra
The mass spectrum of N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine (Fig. 6a) showed
that a molecular ion peak m/z 252 (0.3%) together with a base peak at 157 while the
byproduct (Fig. 6b) gave a molecular ion peak at m/z 410 (28.6 %) together with a base
peak at m/z 91.
6
7
8
9
Fragmentation of major product as follows:
10
9

RSC Advances
Page 10 of 62
View Article Online
DOI: 10.1039/C5RA06473B
1
1
According to the data FTIR, HNMR, Mass spectroscopy, the product is a mixture of two
compounds: N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine (major) and (N,N'E,N,N'E)ꢀ
N,N'ꢀ((E)ꢀ2ꢀ((E)ꢀ5ꢀ(pyridinꢀ2ꢀylimino)pentylidene)pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀ
amine (trace). The formation of byꢀproduct was considered Aldol condensation followed
by Schiff base byꢀproduct.
2
3
4
5
6
Gemini
surfactant
(2,2'ꢀ(pentaneꢀ1,5ꢀdiylidenebis(azanꢀ1ꢀylꢀ1ꢀylidene))bis(1ꢀ
7
dodecylpyridinium bromide)
8
Elemental analysis
9
Elemental analysis of (Z)ꢀ1ꢀdodecylꢀ2ꢀ(2ꢀhydroxybenzylideneamino)pyridinium bromide
as the following: (theoretical, %) C, 62.39; H, 8.86; N, 7.46; Br, 21.29 and (found, %) C,
10
11
12
13
6
2.71; H, 8.91; N, 7.41; Br, 21.09.
FTIR spectra
10

Page 11 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
FTIR
spectrum
of
2,2'ꢀ(pentaneꢀ1,5ꢀdiylidenebis(azanꢀ1ꢀylꢀ1ꢀylidene))bis(1ꢀ
1
2
ꢀ1
ꢀ1
dodecylpyridinium bromide) showed two bands at 2926.12 cm , 2854.34 cm for CꢀH
ꢀ1
aliphatic and C=N stretching appeared band at 1662.41 cm .
3
1
HNMR spectra
4
1
HNMR (DMSO–d6) spectrum of 2,2'ꢀ(pentaneꢀ1,5ꢀdiylidenebis(azanꢀ1ꢀylꢀ1ꢀ
5
ylidene))bis(1ꢀdodecylpyridinium bromide) (Fig. 7) showed different peaks δ, ppm: 0.80
6
(
6H, 2CH ); 1.21 (46H, 23CH ), 3.46 (4H, 2CH N), 4.08 (2H, 2CHꢀBr), 6.816, 6.927,
7
3
2
2
7
.855, 7.901 (4 sets of multiplets, 2PyꢀH) ppm. The presence of δ CHꢀBr at 4.08 was
8
indicated the presence of nonꢀcationic ((N1E,N5E)ꢀ1,5ꢀdibromoꢀN1,N5ꢀbis(1ꢀ
dodecylpyridinꢀ2(1H)ꢀylidene)pentaneꢀ1,5ꢀdiamine) as trace.
9
10
1
The above data of FTIR, HNMR spectrum confirmed the proposed structure of the
11
12
13
gemini
surfactant
(2,2'ꢀ(pentaneꢀ1,5ꢀdiylidenebis(azanꢀ1ꢀylꢀ1ꢀylidene))bis(1ꢀ
dodecylpyridinium bromide).
3
.2. Weight loss measurement
14
15
The corrosion rate (k) was calculated from the following equation [23]:
Δꢁ
ꢀ
=
ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(1)
ꢂꢃ
where ꢁW is the average weight loss of three parallel carbon steel sheets, S is the total
area of the specimen and t is immersion time.
16
17
11

RSC Advances
Page 12 of 62
View Article Online
DOI: 10.1039/C5RA06473B
Inhibition efficiency (η ) and surface coverage (θ) were calculated according to the
1
2
w
following equations [23]:
ꢁ
− ꢁꢈ
ꢁ
ꢅꢆ = ꢇ
ꢉ × 100ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(2)
ꢁ
− ꢁꢈ
ꢊ = ꢇ
ꢉ × 100ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(3)
ꢁ
where W and W are the weight loss of carbon steel in absence and presence of the
3
o
inhibitors respectively.
4
The obtained results showed that the corrosion efficiencies for the cationic and the
cationic gemini surfactants increased with increasing the inhibitors concentration.
5
6
The values of corrosion rate (k), percentage inhibition efficiency (η ) and surface
7
w
coverage (θ) were obtained from weight loss method in the presence and absence of
8
o
different concentrations of cationic and gemini surfactants at 20ꢀ60 C are summarized in
9
Table 1. It was found that, the corrosion rate values were decreased with increasing the
inhibitors concentration. The inhibition efficiency of all inhibitors was increased with
increasing the concentration. The maximum inhibition efficiency for each compound was
10
11
12
13
14
15
16
17
18
19
ꢀ2
obtained at 1 × 10 M. From data in Table 1, it was observed that, the synthesized gemini
surfactant was showed a good inhibition efficiency by increasing temperature from 20–60
o
C. This result can related to the chemical adsorption of the inhibitor molecules to the
metal surface [24]. On the contrary, the inhibition efficiency of the synthesized cationic
o
surfactant was decreased with increasing the temperature from 20–60 C. This result can
be attributed to desorption of the inhibitor molecules to the metal surface at high
temperature [25].
3
.3. EIS measurement
20
12

Page 13 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
Nyquist plots for carbon steel in 0.5 M H SO in the absence and presence of the
1
2
3
4
5
6
7
8
9
2
4
synthesized inhibitors at various concentrations are shown in Fig. 8. It was concluded
from these plots that the impedance response of carbon steel in 0.5 M H SO had
2
4
significantly altered after the addition of inhibitors into the test solutions. The semicircles
were obtained which cut the real axis at higher and lower frequencies. At higher
frequency end, the intercept corresponds to solution resistance (R ) and at lower
s
frequency end; the intercept corresponds to R + R . The difference between these two
s
ct
values gives the charge transfer resistance (Rct).
The Nyquist plots have similar shape for the synthesized inhibitors. The semicircle in all
cases was corresponded to a capacitive loop. The semicircle radii depend on the inhibitor 10
concentration. The diameter of the capacitive loop was increased with increasing of
inhibitor concentration.
11
12
13
14
15
16
The double layer capacitance (C ) and (R ) were calculated from Nyquist plots as
dl
ct
described elsewhere [26]. As R is inversely proportioned to the corrosion current
ct
density, it was used to determine the inhibition efficiency (η ) from the following
I
equation:
ꢈ
ꢍ − ꢍ
ꢎꢏ
ꢎꢏ
ꢅꢋ = ꢌ
ꢐ × 100ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(4)
ꢈ
ꢍ
ꢎꢏ
o
where R and R are the charge transfer resistance values in uninhibited and inhibited
17
18
19
20
21
22
ct
ct
solutions.
The results obtained from electrochemical impedance spectroscopy (EIS) method can be
interpreted in terms of the equivalent circuit of the electrical double layer which has been
shown in Fig. 9. Simulation of Nyquist diagrams was made using ZSimpWin program in
order to obtain the optimum electrical circuit. The Nyquist plots obtained in the real
13

RSC Advances
Page 14 of 62
View Article Online
DOI: 10.1039/C5RA06473B
system had a general behavior where the double layer on the interface of metal/solution
does not behave as a real capacitor. On the metal side electrons control the charge
distribution whereas on the solution side it is controlled by ions. Since ions were much
larger than the electrons, the equivalent ions to the charge on the metal will occupy quite
a large volume on the solution side of the double layer [27]. Therefore CPE used in place
1
2
3
4
5
6
7
of double layer capacitance, C , to represent the nonꢀideal capacitive behavior of the
dl
double layer. Its impedance is described by the expression [28]:
1
ꢑꢒꢓꢔ
=
ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(5)
ꢕꢈ(ꢖꢗ)ꢘ
ꢚ
where Y is a proportional factor, ꢙ = −1, ꢗ = 2ꢛꢜ and n is the phase shift. For ꢝ = 0,
8
o
ꢞꢟ
ZCPE represents a resistance with ꢍ = ꢕ , for ꢝ = 1 a capacitance with ꢠ = ꢕ, for
9
ꢝ = 0.5 a Warburg impedance with ꢁ = ꢕ and for ꢝ = −1 an inductive with ꢡ = ꢕꢞꢟ.
10
11
12
The double layer capacitances, C , for a circuit including a CPE were calculated from the
dl
following equation [28]:
ꢠ = ꢕ (ꢗ )ꢘꢞꢟꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(6)
ꢢꢣ
ꢈ
ꢤꢥꢦ
where Y is a proportional factor and ꢗ
= 2ꢛꢜꢤꢥꢦ and ꢜꢤꢥꢦꢄis the frequency at which
13
14
15
16
17
18
19
20
21
o
ꢤꢥꢦ
the imaginary component of the impedance is maximal.
The electrochemical impedance parameters derived from the Nyquist plots and the
inhibition efficiency (η ) are shown in Table 2. It is clear that R values were changed in
I
s
the presence of inhibitor in its absence. The increase in the synthesized surfactants
concentration can cause a change in a small percent of the overall solution conductivity.
From the Table 2, it is clear that charge transfer resistance values increased and the
capacitance values decreased with increasing inhibitors concentration. The decreasing in
the capacitance, which can resulted from a decrease in local dielectric constant and/or an
14

Page 15 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
increase in the thickness of the electrical double layer, was suggested that the inhibitor
molecules acted by adsorption at the metal/solution interface [29]. This indicated the
formation of a surface film on the carbon steel. The addition of synthesized inhibitors
1
2
3
4
5
6
7
provided lower C values, probably as a consequence of replacement of water molecules
dl
by inhibitors bases at the electrode surface. Also the inhibitor molecules may reduce the
capacitance by increasing the double layer thickness according to the Helmholtz model
[
28].
ꢪꢪ ꢫ
ꢈ
ꢧꢈꢨꢩ
=
ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(7)
ꢠꢢꢣ
where ε is the dielectric constant of the medium, ε is the vacuum permittivity, A is the
8
o
electrode surface area and δorg is the thickness of the protective layer.
9
The value of C was always smaller in the presence of the inhibitor than in its absence,
10
11
12
13
14
15
16
17
18
19
20
21
dl
which due to result from the effective adsorption of the synthesized inhibitors [30].
As seen from Fig. 10, Bode plots refer to the existence of an equivalent circuit that
contains a single constant phase element in the metal/solution interface. The increase of
absolute impedance at low frequencies in Bode plot confirmed the higher protection with
increasing the concentration of the inhibitor, which is related to adsorption of the
inhibitor on the carbon steel surface [31]. Fig. 10 was showed that the phase angle
depression at relaxation frequency decreased with increasing the inhibitor concentration
which indicated the capacitive response increased with increasing the inhibitor
concentration. This behavior can attributed to the corrosion activity that was decreased
with increasing the concentration of both cationic and gemini surfactants. The standard
deviation average of three replicates was 0.34.
3
.4. Potentiodynamic polarization measurements
22
15

RSC Advances
Page 16 of 62
View Article Online
DOI: 10.1039/C5RA06473B
The anodic and the cathodic polarization curves, for carbon steel in 0.5 M H SO in the
1
2
4
absence and presence of different concentrations of inhibitors, are shown in Fig. 11. It
was observed from Fig. 11 that both of cathodic and anodic curves showed a lower
current density in the presence of the prepared surfactants than those recorded in the 0.5
M H SO solution alone. This behavior indicates that these surfactants had significant
2
3
4
5
2
4
effects on both of cathodic and anodic reactions of corrosion process which suggest that
these inhibitors acts as mixed type inhibitors [32]. This behavior is due to the synthesized
cationic and gemini surfactants adsorbed to the metal surface via the quaternary nitrogen
6
7
8
+
ꢀ
atom (N ), counter ion (Br ) and πꢀelectrons of aromatic ring and lone pair of electrons of
9
+
nitrogen atoms of azomethine group (ꢀCH=Nꢀ). Quaternary nitrogen atom (N ) is
10
11
12
13
14
15
adsorbed on the cathodic sites to decrease the evolution of hydrogen while the counter
ꢀ
ion (Br ) and πꢀelectrons of aromatic ring and lone pair of electrons of nitrogen atoms of
azomethine group (ꢀCH=Nꢀ) are adsorbed on the anodic sites to reduce the anodic
dissolution.
The inhibition efficiency, (η ), was calculated from the following equation [33]:
p
ꢭꢎꢈꢨꢨ − ꢭꢈ
ꢎꢈꢨꢨ
ꢅꢬ = ꢇ
ꢉ × 100ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(8)
ꢭꢎꢈꢨꢨ
o
where icorr and i
are the corrosion current density values without and with inhibitor,
16
17
18
corr
respectively.
The electrochemical parameters such as corrosion potential (E ), corrosion current
corr
density (i ), cathodic and anodic Tafel slopes (β and β ) and inhibition efficiency (η ) 19
corr
c
a
p
values were obtained from polarization curves are given in Table 3. It was clearly 20
reported in Table 3 that, the inhibition efficiency was increased and the corrosion current
densities was decreased by increasing the inhibitors concentration. Moreover, Table 3
21
22
16

Page 17 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
results were showed that the corrosion potentials of inhibitors were slightly shifted in the
positive direction. The electrochemical processes on the metal surface were likely to be
closely related to the adsorption of the inhibitors, and the adsorption depends on the
1
2
3
4
5
6
7
8
9
chemical structure of the inhibitors [34]. The values of cathodic Tafel slope (β ) and
c
anodic Tafel slope (β ) for the synthesized inhibitors were slightly shifted in the presence
a
of inhibitor. The slight variations in Tafel slope was suggested that the synthesized
inhibitors affects the kinetics of the hydrogen evolution reaction without change the
mechanism of iron dissolution [35]. The average of the standard deviation for the three
replicates was 0.37.
3
.5. Adsorption isotherm and thermodynamic parameters
10
The mechanism of corrosion inhibition may be explained on basis of adsorption behavior.
The adsorption of organic inhibitor molecules from the aqueous solution can be regarded
as a quasiꢀsubstitution process between the organic compound in the aqueous phase
11
12
13
14
15
16
17
18
19
20
21
22
[
Org(sol)] and water molecules at the electrode surface [H O(ads)]
2
Org(sol) + × H O
↔ Org(ads) + × H O
(sol)
(9)
2
(ads)
2
where × is the size ratio, that is, the number of water molecules replaced by one organic
inhibitor.
In this situation, the adsorption of the synthesized surfactants was accompanied by
desorption of water molecules from the surface. The degree of surface coverage (θ) for
different inhibitor concentrations of all inhibitors were evaluated from the weight loss
data. Several adsorption isotherms were tested to describe the adsorption behavior of all
compounds used in this study. The Langmuir isotherm is given by following equation:
ꢠ
ꢊ
1
=
+ ꢠꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(10)
ꢮꢥꢢꢯ
17

RSC Advances
Page 18 of 62
View Article Online
DOI: 10.1039/C5RA06473B
where C is the concentration of inhibitor, Kads is the equilibrium constant of the
adsorption process, and θ is the surface coverage.
1
2
The Langmuir adsorption plots (Fig. 12) of the synthesized inhibitors gave straight lines
with slope equal 1 which indicated that the studied compounds obeyed Langmuir
3
4
adsorption isotherm. The adsorption equilibrium constant (K ) equal reciprocal intercept
5
ads
in Fig. 13 for both cationic and gemini surfactants at different concentrations and
temperatures was calculated and listed in Table 4. It was found that the high values of
both cationic and gemini surfactants reflected the high adsorption ability of these
inhibitors on carbon steel surface in 0.5 M H SO . But the adsorption of the gemini
6
7
8
9
2
4
surfactant was more efficient than the cationic surfactant. This is due to the gemini
surfactant has more adsorption centers than the cationic surfactant.
10
11
12
13
o
The standard free energy of adsorption (ꢁG ads) was calculated using the following
equation [36] and listed in Table 4.
ꢈ
ꢥꢢꢯ
∆
ꢰ
= −ꢍꢱ ln(55.5ꢮꢥꢢꢯ)ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(10)
where ꢁGads is the free energy of adsorption, Kads is the equilibrium constant for
adsorption desorption process, R is the gas constant, T is the absolute temperature and
14
15
16
17
18
19
20
21
22
5
5.5 is the molar concentration of water.
o
ads
The negative value of ꢁG
suggested that the adsorption of the inhibitors, in 0.5 M
o
ads
H SO on the carbon steel surface, was more stable. Generally, ꢁG
values around ꢀ20
2
4
ꢀ1
kJ mol or lower were consistent with the electrostatic interaction between the charged
ꢀ1
molecules and the charged metal (physisorption) while those around ꢀ40 kJ mol or
higher were involved sharing or transferring of electrons from organic molecules to the
o
ads
metal surface to form a coordinate bond (chemisorption) [1,37,38]. The calculated ꢁG
18

Page 19 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
ꢀ
1
values at various temperatures were found to be from ꢀ31.84 to ꢀ35.92 kJ mol for
1
2
3
4
5
6
ꢀ1
cationic surfactant and from ꢀ31.94 to ꢀ38.43 kJ mol for gemini surfactant at various
o
ads
temperatures. ꢁG
values indicated that the adsorption process involved both the
physical and chemical adsorption [39].
o
The standard enthalpy of adsorption, ꢁH ads, was calculated according to the Van’t Hoff
equation [40,41]:
ꢈ
ꢥꢢꢯ
−
∆ꢲ
ln ꢮꢥꢢꢯ = ꢌ
ꢐ + constantꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(11)
ꢍꢱ
To obtain the standard enthalpy of adsorption, the relationship between ln Kads and 1/T is
7
8
9
o
ads
shown in Fig. 13. The negative sign of ꢁH
values for cationic surfactant indicated that
the adsorption of inhibitor molecules was an exothermic process [20] while the positive
o
ads
sign of ꢁH
values for gemini surfactant indicated that the adsorption of inhibitor 10
molecules was an endothermic process [19].
11
12
13
o
According to the thermodynamic basic equation, the standard adsorption entropy, ꢁS ads,
was calculated from the following equation [42]:
ꢈ
ꢥꢢꢯ
= ∆ꢲꢈ − ꢱ∆ꢂ ꢥꢈ ꢢꢯꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(12)
∆
ꢰ
ꢥꢢꢯ
All the obtained thermodynamic parameters were given in Table 4. It was found that the
14
o
ads
adsorption of inhibitor molecules is accompanied by positive values of ꢁS
which may 15
16
be explained by an ordered layer onto the steel surface.
3
.6. Mechanism of inhibition
17
Corrosion inhibition of carbon steel in acidic solutions by different inhibitors can be
explained on the basis of molecular adsorption. The compounds were inhibited corrosion
by controlling both anodic as well as cathodic reactions. The different types of adsorption
may be considered for the adsorption of cationic and gemini surfactants molecules at the
18
19
20
21
19

RSC Advances
Page 20 of 62
View Article Online
DOI: 10.1039/C5RA06473B
metal surface: (i) electrostatic attraction between the charged molecules and the charged
metal, (ii) interaction of unshared electron pairs in the molecule with the metal, and (iii) a
combination of the above [43,44].
1
2
3
The synthesized cationic and gemini surfactants adsorbed to the metal surface via the
4
+
ꢀ
+
quaternary nitrogen atom (N ) and counter ion (Br ). Quaternary nitrogen atom (N )
5
adsorbed on the cathodic sites to decrease the evolution of hydrogen while the counter
6
ꢀ
ion (Br ) adsorbed on the anodic sites to reduce the anodic dissolution. The adsorption on
7
anodic site occurred through πꢀelectrons of aromatic ring and lone pair of electrons of
nitrogen atoms of azomethine group (ꢀCH=Nꢀ) in both cationic and gemini surfactant
which decreased the anodic dissolution of carbon steel. The high performance of two
inhibitors was attributed to the presence of many adsorption centers, larger molecular size
and the planarity of compounds. The inhibition efficiency of the gemini surfactant were
higher than those of the cationic surfactant at different concentrations and temperatures.
The gemini surfactant has been found to give the better performance. This may be
attributed to the higher dipole moment than cationic surfactant and due to the presence of
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
ꢀ
two azomethine groups, two quaternary nitrogen atoms and two Br ions in this
compound while cationic surfactant has one azomethine group, one quaternary nitrogen
ꢀ
atom and one Br ion.
The adsorption of studied cationic and gemini inhibitors on carbon steel surface obeyed
Langmuir adsorption isotherm because the inhibitors form monolayer in the
concentrations range used in this study as shown in the relation between inhibition
efficiency and concentration (where there one S curve only). The adsorption of gemini
surfactants on metal surface is more complicated than that of conventional surfactants
20

Page 21 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
because the gemini surfactants contain two hydrophilic groups and two hydrophobic
1
groups.
2
The adsorption of gemini surfactant on the steel surface in acidic medium reflects three
different modes of adsorption:
3
4
(1) At low concentrations, it seems that the adsorption takes place by horizontal binding
5
to hydrophobic region (Fig. 14a). This adsorption is favored by an electrostatic
6
+
interaction between the two ammonium groups (N ) and cathodic sites on one hand
7
ꢀ
and Br ion on the metallic surface on the other hand.
8
(
2) When the inhibitor concentration increases, a perpendicular adsorption takes place as
a result of an interꢀhydrophobic chain interaction (Fig. 14b).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
(
3) At higher inhibition concentrations, a perpendicular adsorption of surfactant continues
with the hydrophilic group protruding into the solution and the hydrocarbon tail
mingling with the adsorbed monomers, driven by the hydrophobic force, until the
surface is saturated (Fig. 14c).
3
.7. The comparison between the synthesized surfactants and some Schiff bases,
synthesized as precursors
The anticorrosive effect of the Schiff bases, synthesized as precursors ((Z)ꢀ2ꢀ((pyridinꢀ2ꢀ
ylimino)methyl)phenol and N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine), on the
corrosion of carbon steel in 0.5 M H SO was examined, in comparison with the
2
4
anticorrosive effect of the synthesized surfactants by weight loss measurements at the
ꢀ2
o
same conditions. It was found that at 1 × 10 M and 20 C, the inhibition efficiency of
the synthesized cationic surfactant (94.0 %) is higher than Schiff bases (((Z)ꢀ2ꢀ((pyridinꢀ
ꢀ2
o
2
ꢀylimino)methyl)phenol) (88.7 %). Also, at 1 × 10 M and 20 C, the inhibition
21

RSC Advances
Page 22 of 62
View Article Online
DOI: 10.1039/C5RA06473B
efficiency of the synthesized gemini surfactant (96.7 %) is higher than Schiff bases
N,N'ꢀ(pentaneꢀ1,5ꢀdiylidene)dipyridinꢀ2ꢀamine)), synthesized as precursors, (90.2 %).
1
(
2
This is due to the synthesized compound regard as cationic and gemini surfactants,
however, Schiff bases reported in the literature are ordinary organic compounds.
Surfactants that lower the surface tension (or interfacial tension) between corrosive
medium and steel surface and also act as dispersants. Therefore, surfactants up to critical
micelle concentration (Ccmc) will diffuse out of the bulk water phase and are
3
4
5
6
7
adsorbed at the interfaces between carbon steel and corrosive medium. On the other side,
organic compounds will diffuse both in bulk solution and interface by the same rate
nearly. In addition, surfactant up to Ccmc form thin film on steel surface involved two
inhibitive factors; one hydrophilic group involved hetero atoms and other waterꢀinsoluble
hydrophobic group. But Schiff bases adsorbed on steel surface through only one
8
9
10
11
12
inhibitive factor, hetero atoms. 3.7. Surface active properties
13
3
.7.1. The surface tension
14
15
16
17
18
19
20
21
22
23
The surface tension values (γ) of the surfactants were measured for a range of
concentrations above and below the critical micelle concentration (Ccmc). A
representative plot of γ versus ꢀlog concentration of cationic and gemini surfactants in
both bidistilled water and 0.5 M H SO is shown in Figs. 15 and 16. A linear decrease in
2
4
surface tension was observed with the increasing the surfactant concentration. This
observation was recorded for the synthesized surfactants up to the Ccmc, beyond which no
considerable changes were noticed. This common behavior is shown by surfactants in
solution and was used to determine their purity and Ccmc's. The Ccmc values obtained from
the break point in the γ–log C plots are listed in Table 5. The γ–log C plots also provided
22

Page 23 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
information about area per molecule at airꢀwater interface, effectiveness and surface
excess concentration of surfactant ions of the synthesized surfactants. Ccmc and other
surface properties of all synthesized surfactants were determined also in 0.5 M H SO
1
2
3
2
4
and are listed in Table 5.
4
Comprising Ccmc for both cationic and gemini surfactants demonstrated that increasing
the number of the hydrocarbon chain had the tendency of lowering the concentration at
which aggregation was initiated. Therefore Ccmc of gemini surfactant is smaller than
cationic surfactant due to the hydrocarbon chain length.
5
6
7
8
3
.7.2. Effectiveness (πcmc
)
9
The maximum surface pressure (πcmc) which is defined as the effectiveness of a surfactant
10
11
in reducing surface tension was calculated from the following equation [45]:
ꢛꢎꢤꢎ = ꢳ − ꢳꢎꢤꢎꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(13)
ꢈ
where γ and γ
are the surface tensions of pure water and surface tension at Ccmc
,
12
13
14
15
16
17
18
19
20
21
22
o
cmc
respectively. πcmc values are listed in Table 5. It was found that, effectiveness of gemini
surfactant is higher than cationic surfactant due to the increasing the number of alkyl
chain. Also, Ccmc and surface pressure (πcmc) values decreased in 0.5 M H SO than in
2
4
water. These phenomena are related to the famous Hofmeister series [46ꢀ48], which is an
empirical measure of ions' hydration degree. The Hofmeister series orders ions with
increased saltingꢀin potency from left to right as follows:
ꢀ2
ꢀ2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
SO4 > HPO > OH > F > HCOO > CH COO > Cl > Br > NO > I > SCN > ClO
4
4
3
3
The basis for this ordering was related to an individual anion’s ability to penetrate the
head group region of the monolayer, thereby disrupting the hydrocarbon packing [49,
ꢀ
5
0]. The species to the left of Cl were referred to as kosmotropes, while those to its right
23

RSC Advances
Page 24 of 62
View Article Online
DOI: 10.1039/C5RA06473B
are called chaotropes. These terms originally referred to an ion’s ability to alter the
hydrogen bonding network of water [51]. The kosmotropes, which were believed to be
1
2
‘water structure makers’, were strongly hydrated and have stabilizing and saltingꢀout
3
effects on surfactant. On the other hand, chaotropes (‘water structure breakers’) are
known to destabilize folded hydrophobic part and give rise to saltingꢀin behavior.
4
5
ꢀ2
Kosmotropic anions as SO , producing high electric fields at short distances, bind their
6
4
water molecules strongly and compete efficiently for water with the hydrophilic part of
both the synthesized cationic and cationic gemini surfactants. This phenomenon leads to
the dehydration of the surfactant molecule's hydrophilic part, an effect which depresses
the Ccmc value of the synthesized cationic and cationic gemini surfactants and increases
attractive interactions between these micelles.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
3
.7.3. The surface excess (Γmax)
The surface excess concentration of surfactant ions, Γmax, was provided an effective
measure of the surfactant adsorption at the air/water interface. The maximum value of the
surface excess concentration was in corresponds with the maximum concentration that a
surfactant can attain at the interface, Γmax, and it was defined as the effectiveness of
adsorption at an interface. The concentration of the surfactant was always higher at the
surface phase than that in the bulk solution. The Γmax values were calculated from the
slope of the straight line in the surface tension plot (dγ/d lnC) below Ccmc, using the
appropriate form of Gibbs adsorption equation [52]:
−
1
ꢵꢳ
ꢴ
ꢤꢥꢦ
= ꢇ
ꢉ ꢇ
ꢉꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(14)
ꢝꢍꢱ ꢵ ln ꢠ
24

Page 25 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
where Γmax is the surface excess concentration of surfactant ions, R is gas constant, T is
absolute temperature, C is concentration of surfactant, γ is surface tension at given
concentration and n is number of species ions in solution.
1
2
3
The values of the surface excess concentration were calculated and listed in Table 5. It
was found that, surface excess concentration increased by increasing the number of
carbon chain length, which could be due to the hydrophobic effect of carbon chain.
4
5
6
3
.7.4. The area per molecule (Amin
)
7
2
The minimum surface area per adsorbed molecule, Amin (nm ), is defined as the area
occupied by each molecule at the liquid/air interface. Amin was calculated from the
following equation [53,54]:
8
9
10
1
0ꢟꢸ
ꢴ
ꢫꢤꢶꢷ
=
ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(15)
ꢹꢺ
ꢤꢥꢦ
where N is the Avogadro's number and Γ is the maximal surface excess of adsorbed
11
12
13
14
15
16
17
A
max
surfactant molecules at the interface.
The values of area per molecule were calculated and listed in Table 5. It was found that
Amin values of the gemini surfactant were smaller than that of the cationic surfactant. That
can be attributed to the increasing numbers of head groups and alkyl chains. Amin value of
both cationic and gemini surfactants in 0.5 M H SO were smaller than in water due to
2
4
the increase in dehydration of the surfactant molecule's hydrophilic part.
3
.8. Conductivity measurements
18
Specific conductivity (K) measurements were performed for the prepared cationic and
19
20
21
22
o
gemini surfactants at 20 C in order to evaluate the C and the degree of counter ion
cmc
dissociation, β. It is well known that, the specific conductivity was linearly correlated to
the surfactant concentration in both the premicellar and in the postmicellar regions, and
25

RSC Advances
Page 26 of 62
View Article Online
DOI: 10.1039/C5RA06473B
the slope in the premicellar region greater than that in the postmicellar region [55]. The
intersection point between the two straight lines gave the Ccmc while the ratio between the
slopes of the postmicellar region to that in the premicellar region gives counter ion
dissociation, β.
1
2
3
4
Fig. 17 showed the relation between specific conductivity and concentration of the
synthesized surfactants. The degree of counter ion dissociation values were calculated
and listed in Table 5. It was found that, the degree of dissociation of the gemini surfactant
was higher than that of the cationic surfactant, due to the increase in the cation bulkiness.
The Ccmc values, which determined using electrical conductivity, were in agreement with
5
6
7
8
9
those obtained using surface tension.
10
o
3
.9. The standard free energy of micelle formation (ꢁG mic)
11
The Ccmc of a surfactant was regarded as a measure of the stability of its micellar form
relative to its monomeric form. In the charged pseudophase model of micelle formation,
the standard free energy of micelle formation per mole of surfactant was calculated by the
12
13
14
15
16
17
18
19
20
21
22
following equation [22]:
ꢈ
∆
ꢰ
=ꢄ(2ꢄ − ꢄꢻ)ꢍꢱꢄ ln ꢠꢎꢤꢎ ꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄ(16)
ꢤꢶꢎ
where R is the gas constant, T is the temperature, β is the degree of counter ion
dissociation and Ccmc is expressed in the molarity of the surfactant.
o
mic
o
ꢁG
values were calculated and listed in Table 5. It is clear that, ꢁG
values of the
mic
gemini surfactant were lower than the cationic surfactant. This result means that the
micelle formation was thermodynamically favored for the gemini surfactant than the
cationic surfactant.
26

Page 27 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
3
.10. The relation between corrosion inhibition and surface properties
of the prepared surfactants
1
2
Generally, surfactant is amphiphilic compound that contains hydrophobic group (tail) and
hydrophilic groups (head) [56]. Ordinary cationic surfactant contains one
hydrophobic group and one hydrophilic group while gemini surfactant contains two
hydrophobic group and two hydrophilic group connected by spacer. Gemini surfactants
are more efficient at low surface tension and have much lower critical micelle
concentration values (Ccmc) than conventional surfactants. Surfactants, up to critical
micelle concentration (Ccmc), will diffuse out of the bulk waterꢀphase and adsorb at the
interfaces between carbon steel and corrosive medium. Therefore, migration of gemini
surfactant is faster than cationic surfactant. Thus, concentration of gemini surfactant at
interface, between steel surface and corrosive solution, is higher than cationic surfactant
at the same concentration [52,56]. This is due to surface tension and Ccmc of gemini
surfactant that are smaller than cationic surfactant. The highest reduction in the surface
tension (effectiveness, πcmc) was achieved by the gemini surfactant compared to that
obtained by the cationic surfactant. This is in a good agreement with the inhibition
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
efficiency results which achieved by the gemini surfactant. It seems that the synthesized
o
ads
surfactants were favored adsorption rather than micellization. The fact that ꢁG
was
o
mic
more negative compared to the corresponding ꢁG
could be taken as a strong evidence
on the feasibility of the adsorption of the synthesized surfactants. It was noticed that, the
Γmax value of the gemini surfactant was higher than the cationic surfactant. On the other
hand, the Amin value of the gemini surfactant was lower than the cationic surfactant
considering this one explain why gemini surfactant was more effective than cationic
27

RSC Advances
Page 28 of 62
View Article Online
DOI: 10.1039/C5RA06473B
surfactant. All these parameters explain why gemini surfactant was effective inhibitor
than cationic surfactant.
1
2
4
. Conclusions
3
1
. Novel cationic and gemini surfactants were successfully synthesized, purified and
characterized.
4
5
2
. The corrosion inhibition efficiency of the gemini surfactant was higher than the
cationic surfactant at the same concentration and temperature. The maximum
6
7
ꢀ2
inhibition efficiency was at 1 × 10 M.
8
3
4
5
. Inhibition efficiency increases with increase in the concentration of both the
synthesized cationic and gemini surfactants and decreases with increasing the
temperature.
9
10
11
12
13
14
15
. Inhibition efficiency increases with increase in the temperature for the
synthesized cationic surfactant and decreases with increase in the temperature for
the synthesized gemini surfactant.
. The adsorption of the prepared inhibitors, on the carbon steel, obeyed the
Langmuir adsorption isotherm model. The adsorption process involved both the 16
physical as well as chemical adsorption but physical adsorption more efficient
than chemisorption for cationic surfactant and chemisorption more efficient than
physical adsorption for gemini surfactant
17
18
19
20
21
22
6
. The synthesized cationic and gemini surfactants had good surface properties but
the gemini surfactant was better than the cationic surfactant. Ccmc and surface
pressure values were decreased in 0.5 M H SO than in water.
2
4
28

Page 29 of 62
RSC Advances
View Article Online
DOI: 10.1039/C5RA06473B
7
. The prepared inhibitors acted as mixedꢀtype inhibitor for carbon steel in 0.5 M
H SO .
1
2
3
4
5
6
7
8
2
4
8
. The value of C was always smaller in the presence of the inhibitor than in its
dl
absence, which due to result from the effective adsorption of the synthesized
inhibitors.
9
. The inhibition efficiency calculated from potentiodynamic polarization,
electrochemical impedance spectroscopy and electrochemical frequency
modulation measurements are in good agreement.
References
9
[
[
[
[
[
1] M.A. Hegazy and A.S. ElꢀTabei, Synthesis, surface properties, synergism parameter
and inhibitive performance of novel cationic gemini surfactant on carbon steel
corrosion in 1 M HCl solution, J. Surfact. Deterg. 16 (2013) 221–232.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2] P.C. Okafor, Y. Zheng, Synergistic inhibition behaviour of methylbenzyl
quaternary imidazoline derivative and iodide ions on mild steel in H SO solutions,
2
4
Corros. Sci. 51 (2009) 850ꢀ859.
3] H.L. Wang, R.B. Liu, J. Xin, Inhibiting effects of some mercaptoꢀtriazole
derivatives on the corrosion of mild steel in 1.0 M HCl medium, Corros. Sci. 46
(2004) 2455ꢀ2466.
4] O. Benali, L. Larabi, B. Tabti, Y. Harek, Influence of 1ꢀmethyl 2ꢀmercapto
imidazole on corrosion inhibition of carbon steel in 0.5 M H SO , AntiꢀCorros.
2
4
Method Mater. 52 (2005) 280ꢀ285.
5] A.S. Fouda, A.A. AlꢀSarawy, E.E. ElꢀKatori, Pyrazolone derivatives as corrosion
inhibitors for Cꢀsteel in hydrochloric acid solution, Desalination 201 (2006) 1ꢀ13.
29