the preparation and characterization of some novel qua ternary iminium salts based on schiff-base as...
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Petroleum Science and Technology, 28:1158–1169, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1091-6466 print/1532-2459 online
DOI: 10.1080/10916460902967718
The Preparation and Characterization of Some
Novel Quaternary Iminium Salts Based on
Schiff-base as a Corrosion Inhibitor
M. M. A. EL-SUKKARY,1 I. AIAD,1 A. DEEB,2
M. Y. EL-AWADY,1 H. M. AHMED,1 AND S. M. SHABAN1
1Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt2Faculty of Science, Chemistry Department, Zagazig University,
Zagazig, Egypt
Abstract Novel quaternary iminium compounds, namely, N-(4-methoxybenzylidene)-
N-benzyldodecyliminium chloride (Ia), N-(4-methoxybenzylidene)-N-benzylhexadecyliminium chloride (Ib ), N-(4-methoxybenzylidene)-N-benzyloctadecyliminium chloride
(Ic ), and N-benzylidene-N-benzyldodecyliminium chloride, were prepared. The surfaceproperties such as surface and interfacial tension, foaming, and emulsifying power
of these surfactants were investigated. The surface parameters including criticalmicelle concentration (CMC), maximum surface excess (�max), minimum surface area
(Amin), efficiency (PC20), and effectiveness (�CMC) were calculated. Free energy ofmicellization (�Go
mic) and adsorption (�Goads
) were calculated.
Keywords cationic surfactants, corrosion inhibitors, quaternary iminium salts, sur-face parameters
1. Introduction
Cationic surfactants are of widespread of interest due to their ability to self-assemble
in supermolecular structures such as micelles. The aggregates formed create sharp po-
larity gradients at the interface and define clear hydrophobic regions in an aqueous
solution. Those properties are of fundamental importance for the creation of new materials
(Uhlman, 1991).
Surface activity and micellar properties of surfactants (Alxandridis, 1994; Voicu et al.,
1994; Sugihara et al., 1995; Al-Sabagh, 1998; Burczyk et al., 2001; Chatterjee et al., 2002;
Osman et al., 2003; El-Ghazawy et al., 2004) are of potential importance and scientific
interest. Surfactants micellize in solution after a critical micelle concentration (CMC) de-
pending on their molecular structure and environmental conditions (Clint, 1991; Moulik,
1996). The concept of the CMC value is very useful and perhaps the most frequently
measured and discussed micellar parameter (Lindman and Wennerstrom, 1980). Surface
tension is a widely used physicochemical property at the gas–liquid interface to determine
CMC value without serious difficulty (Adamson, 1990). Through surface tension versus
surfactant concentration curves (SVC), several surface and thermodynamic parameters are
Address correspondence to S. M. Shaban, Egyptian Petroleum Research Institute, Nasr City,11727 Cairo, Egypt. E-mail: samyshaban2010@yahoo.com
1158
Some Novel Quaternary Iminium Salts 1159
derived. Detailed thermodynamics of micellization and adsorption processes are essential
for understanding their stability, spontaneity of formation, and the state of environmental
order or disorder. The corrosion of iron is a fundamental academic and industrial concern
that has received a considerable amount of attention. Quaternary ammonium salts have
been used extensively as inhibitors against the acid corrosion of iron and steel (Kawa
et al., 1996). The action of this class of compounds is commonly attributed to their
adsorption on the surface of the metal. Quaternary ammonium cations are known to be
effective corrosion inhibitors for iron in hydrochloric acid but ineffective in sulfuric acid
(Iofa et al., 1964).
This study aimed to prepare some novel cationic surfactants based on Schiff base
and to determine its surface properties, including CMC, minimum surface area (Amin),
efficiency (PC20), maximum surface excess (�max), and effectiveness (�CMC). Determina-
tion of the foaming power and the corrosion inhibition as well as the biocidal efficiency
of the prepared compounds represent an additional objective of this investigation.
2. Material and Experimental Techniques
2.1. Synthesis
Synthesis of the surfactants compounds was carried out in two steps:
2.1.1. Synthesis of Schiff Base. Different alkyl Schiff bases were synthesized throughout
condensation reactions of different alkyl fatty amines, mainly dodecyl, hexadecyl, and
octadecyl amine with anisaldehyde and benzaldehyde (Erk et al., 2000).
2.1.2. Synthesis of Quaternary Iminium Compounds. To 0.1 mol of Schiff base in
absolute ethanol, 0.1 mol benzyl chloride was added. The reaction mixture was refluxed
for 45 to 90 hr depending on the fatty alkyl chain length of the Schiff base. The solution
was evaporated under reduced pressure. Crystallization from acetone was carried out
three times to obtain the purified quaternary iminium compounds (Ia, Ib, Ic).
2.2. Structural Confirmations of the Prepared Compounds
The chemical structure of the synthesized compounds was characterized by:
1. Fourier transform infrared (FTIR) spectra using ATI Mattson Infinity seriesTM Bench-
top 961 controlled by Win FirstTM V2.01 software (Egyptian Petroleum Research
Institute Cairo, Nasr City, Egypt).
2. 1HNMR was measured in DMSO-d6 by Spect Varian, GEMINI 200 (1H 200 MHz).
(Micro Analytical Center, Cairo University, Cairo, Egypt).
2.3. Experimental Techniques
2.3.1. Surface and Interfacial Tension. Surface and interfacial tension of the prepared
compounds solutions were measured using a Du-Nouy tensiometer (Kruss type 8451).
The surface tension of different concentrations at different temperatures (15ıC, 35ıC,
45ıC) was measured.
1160 M. M. A. El-SukKary et al.
2.3.2. Surface Parameters of the Prepared Compounds
1. Critical micelle concentration: The values of the CMC of the prepared compounds
were determined using surface tension techniques. In this method, the surface tension
values were plotted against the corresponding concentrations. The interrupt change in
the surface tension concentration (SC) curves express on the CMCs.
2. Effectiveness: �CMC is the difference between the surface tension of the pure water
( o) and the surface tension of the surfactant solution ( ) at the critical micelle
concentration.
�CMC D o �
3. Maximum surface excess: The values of �max were calculated from surface or inter-
facial data by the use of the Gibbs equation (Shuichi et al., 1991).
�max D �1=2:303 RT.ı =ı log c/T
where �max is maximum surface excess in mol/cm2; R is universal gas constant 8.31 �
107 ergs mol�1 K�1; T is absolute temperature (273.2 CıC); ı is surface pressure
in dyne/cm; C is surfactant concentration; and (ı =ı log c)T is the slope of a plot of
surface tension vs. concentration curves below CMC at constant temperature.
4. Minimum surface area: The area per molecule at the interface provides information
on the degree of packing and the orientation of the adsorbed surfactant molecule.
The average area (in square angstroms) occupied by each molecule adsorbed on the
interface (Takeshita and Wakebe, 1982) is given by:
Amin D 1016=�max N
where �max is maximum surface excess in mol/cm2 and N is Avogadro’s number,
6.023 � 1023.
5. Thermodynamic parameters of micellization and adsorption: The thermodynamic pa-
rameters of adsorption and micellization of the synthesized cationic surfactants were
calculated according to Gibb’s adsorption equations as follows (Rosen, 1987):
�GOmic D RT ln.CMC/
�GOads D �GO
mic � 6:023 � 10�2� �CMC � Amin
�Smic D �d.�GOmic=�T/
�Sads D �d.�GOads=�T/
�Hmic D �GOmic C T�Smic
�Hads D �GOads C T�Sads
2.3.3. Corrosion Inhibition of the Prepared Compounds. A weight loss technique was
used to measure the corrosion inhibiting effect of the prepared quaternary compounds
for mild steel specimen having a composition as follows (wt%): 0.21 C, 0.035 Si, 0.51
Mn, 0.82 P, and the remainder Fe. Each specimen was sequentially machined into regular
shapes of 54.006 cm2 cross-sectional area. The specimens were sequentially abraded with
different emery papers, degreased with acetone, washed with distilled water, and then
Some Novel Quaternary Iminium Salts 1161
dried. Corrosive solutions of 1 M HCl in the absence and presence of the inhibitors at
concentrations between 1 � 10�5 and 1 � 10�3 M prepared from doubly distilled water
were used.
The inhibition efficiency (E%) of an inhibitor was calculated from the following
equation:
E% D .CR � CR0/=CR � 100
CR D KW=ATD
where CR and CR0 are the corrosion rate of the carbon steel in the absence and presence
of an inhibitor at given inhibitor concentration and temperature, W is the weight loss (g),
A is surface area of the mild steel species (cm�2), D is density of the species (g/cm3)
and is equal 7.88 g/cm3, K is a constant (3.45 � 106), and T is immersion time (hr).
3. Results and Discussion
3.1. Chemical Structure
The chemical structure of the prepared cationic surfactants was confirmed by the FTIR
and 1HNMR spectra.
3.1.1. FTIR Spectra. FTIR spectroscopy was used to identify the functional groups of
prepared cationic surfactants. The FTIR spectra of compound Ia is shown in Figure 1,
which shows the stretching vibration band of �C�H aliphatic symmetric and asymmet-
ric at 2,844 and 2,913 cm�1 , respectively; DC�H aromatic stretching at 3,004 cm�1 ;
�NDCH� stretching at 1,620 cm�1 ; CDC aromatic stretching at 1,509 cm�1 ; �CH2
bending at 1,463 cm�1 ; and C�O stretching at 1,091 cm�1. We observe the disappearance
of two absorption bands of CDO and �NH2 at 1,725 and 3,300 cm�1, respectively.
3.1.2. 1HNMR Spectra. The 1H-NMR spectra data (ı ppm) of the compound (Ia) are
shown in Figure 2, which shows that ı D 0.84–0.872 (t, 3H, �CH3); ı D 1.261 (m,
18 H, �(CH2)9CH3); ı D 1.571 (m, 2H, �CH2CH2N˚); ı D 2.527 (S, 2H, �CH2ph);
Figure 1. 1H-NMR spectra of N-(4-methoxybenzylidene)-N-benzyldecyliminium chloride (Ia).
1162 M. M. A. El-SukKary et al.
Figure 2. General chemical structure of prepared compounds.
ı D 2.7–2.773 (t, 2H, �CH2CH2N˚); ı D 3.381 (S, 3H, �OCH3); ı D 7.414 (5H,
�CH2-ph); ı D 7.599 (4H, ph-OCH3); ı D 8.176 (S, 1H, ˚N D CH-ph).
The data obtained from FTIR and 1HNMR spectra showed that the synthesized
inhibitors are as represented in Figure 3.
3.2. Surface Properties
3.2.1. Surface and Interfacial Tension. Surface tension values were measured for aque-
ous solutions of the prepared cationic surfactant with different concentrations at differ-
ent temperatures (15ıC, 35ıC, 45ıC) and the data are represented in surface tension–
concentration curves as shown in Figures 4 and 5. It is clear that surface tension
decreases with increasing concentration and increasing length of the hydrophobic chain
and temperature at constant concentration. This can be explained due to the adsorption
of surfactant molecules at the surface. When materials that contain a hydrophilic and
hydrophobic group attached together in the same molecule are dissolved in a solvent, the
structure of the solvent is distorted and therefore increases free energy of the system;
thus, molecules concentrate at the interface in a way that minimizes the free energy of
the system, where the hydrophobic part is oriented away from the solvent to avoid
energetically unfavorable contact with aqueous media, and the hydrophilic group is
directed toward the bulk; this adsorption at the interface provides an expanding force
Some Novel Quaternary Iminium Salts 1163
Figure 3. FTIR spectra of N-(4-methoxybenzylidene)-N-benzyldecyliminium chloride (Ia).
Figure 4. Surface tension values of Ia at different concentrations and temperatures.
Figure 5. Surface tension values of Ib at different concentrations and temperatures.
1164 M. M. A. El-SukKary et al.
acting against the normal surface tension, and thus surface tension is decreased (Rosen,
1989). So by increasing the concentration of surfactant, the adsorption at the interface
will increase, so surface tension decreases until a stable lower level is achieved. This
lower level of surface tension corresponds to maximum monolayer-level adsorption at
the air–water interface. Further increasing surfactant concentration above this maximum
interfacial adsorption level leads to formation of surfactant aggregation known as micelle
in solution. For example, the surface tension of Ia at concentration of 5 � 10�4 is
51.5 mN m�1 and at 5 � 10�3 is 27 mN m�1 at 15ıC.
Also, by increasing the length of the hydrophobic chain, the surface tension decreases
as a result of increasing of hydrophobicity, thus increasing the free energy of adsorption
so more reduction in surface tension. For example, the surface tension of compound Ia
is 51.5, whereas Ib is 43.7 mN m�1 at a concentration of 5 � 10�4 and at 15ıC.
Also, there is a decrease in surface tension by rising temperature as shown in
Figures 4 and 5, due to rising temperature causes disruption of the structured water
surrounding the hydrophobic group, an effect that disfavors micellization. For example,
the surface tension of compound Ia at concentration of 5 � 10�4 is 51.5, 43.7, and 42
mN m�1 at 15ıC, 35ıC, and 45ıC, respectively.
3.2.2. The Critical Micelle Concentration. Critical micelle concentration values of the
prepared cationic surfactant were determined by plotting the surface tension ( ) of
surfactant solutions versus their bulk concentrations in mol/L at 15ıC, 35ıC and 45ıC.
The CMC values listed in Table 1 show a decrease with increase in temperature and
the length of the hydrophobic chains. The rise of temperature probably decreases in the
hydration of the hydrophilic group, which favors micellization. On the other hand, the
rise of temperature causes disruption of the structured water surrounding the hydrophobic
group, an effect that disfavors micellization. The relative magnitude of these two opposing
effects determines whether the CMC increase or decrease over a particular temperature
range. The data in Table 1 reveal that with a decrease in CMC with rising temperature,
micellization is enhanced.
The increase of the hydrophobic chain length decreases the solubility and conse-
quently increases the free energy of the system, leading to concentration of surfactant
molecules at the surface. Also, aggregation of surfactant molecules into micelles with
their hydrophilic part directed to the solvent and the hydrophobic parts directed to the
interior of the micelle.
3.2.3. Maximum Surface Excess (�max). The values of �max are represented in Table 1.
When increasing the temperature and hydrophobic moiety length of the prepared surfac-
Table 1
Surface properties of the synthesized cationic surfactants at different temperatures
CMC � 10�4,
mol � L�1…cmc , mN m�1
�max � 10�10,
mol � cm�2Amin , nm2
Comp. 15ıC 35ıC 45ıC 15ıC 35ıC 45ıC 15ıC 35ıC 45ıC 15ıC 35ıC 45ıC
Ia 59.5 25.7 24.95 49.4 41.2 39.7 4.55 3.038 2.502 0.36 0.546 0.66
Ib 20.96 6.88 3.36 40.2 41.2 38.2 2.253 2.025 1.513 0.736 0.82 1.097
Ic 0.18 0.12 0.0367 31.8 35.3 34 1.658 1.348 0.703 1.00 1.23 2.36
Some Novel Quaternary Iminium Salts 1165
Table 2
The thermodynamic parameters of micellization of the prepared cationic surfactants
�Gmic, kJ mol�1
�S D ��G/T,
kJ mol�1 K�1 �HMIC, kJ mol�1
Comp. 15ıC 35ıC 45ıC 15ıC 35ıC 45ıC 15ıC 35ıC 45ıC
Ia �12.27 �15.28 �15.85 0.1236 0.1236 0.1236 23.34 22.81 23.48
Ib �14.77 �18.65 �21.15 0.21 0.21 0.21 45.74 46.07 45.67
Ic �26.19 �29 �33.09 0.217 0.217 0.217 36.36 37.9 35.98
tants, �max shifts to lower concentrations and thus the surfactant molecules are directed
to the interface, which decreases the surface energy of their solutions.
3.2.4. Minimum Surface Area (Amin). The minimum area per molecule at the aqueous
solution/air interface for the prepared surfactants is listed in Table 1. When Amin increases
with increasing length of hydrophobic moiety and temperature due to decreasing �max,
the distances between molecules are increased, so Amin increased.
3.2.5. Effectiveness (˘CMC). Efficiency values of the prepared cationic surfactants are
given in Table 1. From these data it was observed that increasing the alkyl chain length
and temperature decreases the efficiency. This is because the efficiency of adsorption at
the interface increases linearly with an increase in the carbon atoms in the hydrophobic
group and temperature as illustrated in the discussion on surface tension (Rosen, 1987).
The values of effectiveness are listed in the Table 1. It is clear that …CMC decreases
by increasing the hydrophobic chain; for example, …CMC of Ia, Ib, and Ic are 49.4, 40.2,
and 31.8, respectively.
3.2.6. Standard Free Energies of Micellization and Adsorption (�Gomic; �Go
ads). The
thermodynamic parameters of micellization and adsorption are summarized in Tables 2
and 3, respectively, where theses parameters are calculated at 15ıC, 35ıC, and 45ıC.
From thermodynamic parameter of micellization it was observed that the �GOmic are
negative and point to micellization as a spontaneous process.
Also �GOmic increase in the negative direction by increasing both temperature and
length of hydrophobic moiety, which implies that by increasing temperature from 15ıC
Table 3
Thermodynamic parameter of adsorption of the prepared cationic surfactants
�Gads, kJ mol�1
�Sads D ��G/T,
kJ mol�1 K�1 �Hads, kJ mol�1
Comp. 15ıC 35ıC 45ıC 15ıC 35ıC 45ıC 15ıC 35ıC 45ıC
Ia �13.36 �16.63 �17.43 0.1399 0.1399 0.1399 26.95 26.47 27.077
Ib �16.55 �20.68 �23.67 0.233 0.233 0.233 50.59 51.12 50.463
Ic �28.11 �31.61 �37.930 0.3055 0.3055 0.3055 59.93 62.53 59.27
1166 M. M. A. El-SukKary et al.
to 45ıC and the length of hydrophobic moiety, the hydrophobicity increasees, and thus
free energy of the system will increase so molecules tend to form micelles in a way
that minimizes free energy so the change in the free energy of the system as a result of
micelle formation will increase.
Also, we can observe that �Smic is positive and it increases by increasing the
hydrophobic chain length, which is an indication that by increasing the hydrophobic
chain length, there is an increase in the randomness in the system upon transformation of
surfactant molecules into micelles in another means the compounds favor micellization
by increasing hydrophobic character, and thus CMC is decreased.
From the thermodynamic parameter of adsorption it was observed that standard
free energy of adsorption (�GOads) is negative and points to adsorption as a spontaneous
process. Also, �GOads increases in the negative direction by increasing both temperature
and length of hydrophobic moiety, which implies that by increasing temperature from
15ıC to 45ıC and the length of hydrophobic moiety, the compounds favor adsorption
at the interface (adsorption at the interface lower surface and interfacial tension that by
expanding force acting against the contracting force resulting from surface and interfacial
tension) where the hydrophobic part directed away from solvent in a way that minimizes
free energy of the system, and thus the change in free energy as a result of the adsorption
process will increase.
It can be observed that �Sads is positive and increases by increasing the hydrophobic
chain length.
In comparing the thermodynamic parameter of micellization and adsorption we
observed that �GOads is more negative direction �GO
mic, indicating the tendency of the
molecules to be adsorbed at the interface and the adsorption process at the interface is
associated with a decrease in the free energy of the system.
3.3. Corrosion Inhibition of the Prepared Compounds
The efficiency values of the prepared cationic surfactant corrosion inhibitors are shown
in Table 4. We found that by increasing the concentration of the surfactant, the inhibition
efficiency increased as a result of increased adsorption at the interface (solid–liquid
interface). The surfactants adsorb on the metal surface through heteroatoms such as
nitrogen, oxygen, or sulfur and multiple bonds or aromatic rings that block the active
sites of local galvanic cell, which decreases the corrosion rate of the metal (Ramesh and
Rajeswari, 2002).
Table 4
Inhibition efficiency of different concentrationsa
Efficiency
Comp.
1 �
10�5 M
5 �
10�5 M
7 �
10�5 M
1 �
10�4 M
5 �
10�4 M
1 �
10�3 M
Ia 28.92 89.96 91.9 98.38 98.07 98.6
Ib 38.61 90.3 94.18 97.16 97.18 97.23
Ic 40.146 92 95.55 97.37 97.52 97.47
aThe corrosion rate of the control sample is 342.8 mpy.
Some Novel Quaternary Iminium Salts 1167
Corrosion inhibitors such as cationic compounds are charged surfactants, and when
adsorbed on a metal surface, only a monolayer of its molecules is possible because the
positive charge-containing molecules would repel each other. Therefore, the adsorption
of the prepared compounds can be expected to follow a Langmuir isotherm.
The efficiency of the compound Ia at concentration of 1 � 10�5 M is 25.9% and at
a concentration of 1 � 10�3 is 98.6%.
By increasing the hydrophobic chain length, the inhibition efficiency increases before
the CMC values of the compounds, which is due to increasing the adsorption of the
surfactants on the iron surfaces, which protects it from the corrosive ions and increases
its resistance; however, after the CMC not increases this may be due to formation
monolayer on the metal surface. The inhibition efficiency of compounds Ia, Ib, and Ic at a
concentration of 7 � 10�5 are 91.9, 94.2, and 95.5, respectively. The maximum efficiency
of these molecules may be around the CMC. The increase in inhibitor concentrations may
not enhance the efficiency of these compounds.
3.3.1. Thermodynamic Parameters of Adsorption Process. The adsorption of the in-
hibitor depends on its chemical structure, the type of metal and the nature of its surface,
the nature of the corrosion medium (its pH value), temperature, and the electrochemical
potential of the metal–solution interface.
Adsorption provides information about the interaction among the adsorbed molecules
themselves as well as their interaction with the electron surface. The mathematical
relationship for the adsorption isotherms suggests fitting the experimental data of the
present work to a Langmuir equation (Langmuir, 1918):
�=1 � � D KŒI�
where K is the equilibrium constant of the adsorption reaction, [I] is the inhibitor
concentration in the bulk of the solution, and � is the surface coverage.
The surface coverage, � , is the fraction of the surface covered by the inhibitor
molecules. It is calculated from the following equation:
� D 1 � W 0=W
where W 0 and W are the corrosion rates in the presence and absence of the inhibitor,
respectively.
Figure 6 shows a plot of (� /1 � �) vs. [I] (Langmuir adsorption plots) for adsorption
of dodecyltriethyliminium bromide on the surface of the carbon steel sample in 1 M HCl
solution.
The above relations are straight lines, which indicates that the Langmuir isotherm
is valid for this system. Langmuir’s isotherm is applied for an ideal case of physical
and chemical adsorption on a smooth surface with no interaction between the adsorbed
molecules.
3.3.2. Relation between the Surface Properties and Corrosion Inhibition. The data
listed in Table 1 reveal that the maximum effective concentrations of the inhibitors range
from 1 � 10�4 to 7 � 10�5 M; i.e., around the CMC. The greatest reduction of surface
tension (effectiveness, …CMC) was achieved by Ia compared with that obtained by the
other two compounds. This is in good agreement with the inhibition efficiency result
achieved by Ia (Tables 1–4).
1168 M. M. A. El-SukKary et al.
Figure 6. Relation between (� /1 � � ) and inhibitor concentrations in 1 M HCl.
It seems that the investigated surfactants favor adsorption rather than micellization.
The fact that �Gads is more negative compared with the corresponding �Gmic may be
taken as strong evidence for the greater feasibility of the adsorption of the investigated
surfactants. It remains now to point out that �max of the prepared compounds is decreased
with an increase in the alkyl chain length; on the other hand, Amin is increased. �max of Ia
is higher, which indicates that a greater number of molecules are adsorbed. This implies
close packing of the adsorbed molecules associated with less area on the metal surface
for each molecule, thus leading to more electrostatic interaction well packed adsorbed
layer and more homogenous adsorbed film. All these parameters explain why Ia is the
most effective corrosion inhibitor.
4. Conclusions
From the obtained results we can conclude that:
1. All the prepared compounds have good surfactant properties.
2. The prepared cationic surfactants show good inhibition properties for the corrosion
of low carbon steel in 1 M HCl solution at room temperature, and the inhibition
efficiency increases with increasing concentration of the inhibitors until it reaches a
maximum value near the critical micelle concentration.
3. The corrosion inhibition of the prepared compounds increases with an increase in the
alkyl chain length.
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