Surface properties, thermodynamic aspects and antimicrobial activity of some novel iminium surfactants

Article abstract of DOI:10.1007/s11743-011-1317-9

Novel iminium compounds namely p-benzyli-dene benzyldodecyl iminium chloride (I), p-benzylidene benzylhexadecyl iminium chloride (II), p-benzylidene ben-zyloctadecyl iminium chloride (III) were prepared and characterized using FTIR, ¹H-NMR and mass spectroscopy. The surface properties such as surface and interfacial tension, and biological activity of these surfactants were investigated. The surface parameters including critical micelle concentration, maximum surface excess, and minimum surface area. Efficiency and effectiveness were calculated, as well as free energies of micellization and adsorption. The prepared cat-ionic surfactants exhibit a better biological activity than the used reference cetyl trimethyl ammonium bromide.

Full text of DOI:10.1007/s11743-011-1317-9

J Surfact Deterg (2012) 15:359–366
DOI 10.1007/s11743-011-1317-9
ORIGINAL ARTICLE
Surface Properties, Thermodynamic Aspects and Antimicrobial
Activity of Some Novel Iminium Surfactants
•
Ismail Aiad Mohamed M. El-Sukkary
•
•
Ali El-Deeb Moshira Y. El-Awady
•
Samy M. Shaban
Received: 19 September 2011 / Accepted: 11 October 2011 / Published online: 25 November 2011
Ó AOCS 2011
Abstract Novel iminium compounds namely p-benzyli-
dene benzyldodecyl iminium chloride (I), p-benzylidene
benzylhexadecyl iminium chloride (II), p-benzylidene ben-
zyloctadecyl iminium chloride (III) were prepared and char-
acterized using FTIR, 1H-NMR and mass spectroscopy. The
surface properties such as surface and interfacial tension, and
biological activity of these surfactants were investigated. The
surface parameters including critical micelle concentration,
maximum surface excess, and minimum surface area. Effi-
ciency and effectiveness were calculated, as well as free
energies of micellization and adsorption. The prepared cat-
ionic surfactants exhibit a better biological activity than the
used reference cetyl trimethyl ammonium bromide.
There has been a broad range of research performed on
surfactants over the years due to their importance in many
fields. Surfactants are used for detergency [2–4], mineral
and petrochemical recovery, paints and coatings formula-
tion, demulsification [5], metal flotation [6], solubilization
[7] and biocides against bacteria [8], fungi and yeast [9].
The biocidal activity of the cationic surfactants has been
greatly developed over the last 20 years due to an increase
in immunity of microorganisms toward the conventional
biocides [10–13].
This study aimed to prepare of some novel cationic
surfactants based on Schiff bases and determine their sur-
face properties including the CMC, the maximum surface
excess }max, the minimum surface area Amin, the efficiency
PC20, the effectiveness pCMC and biological activities.
Keywords Cationic surfactants ꢀ Quaternary ammonium
salts ꢀ Surface parameters ꢀ Thermodynamic aspects and
biological activity
Experimental Section
Introduction
Materials
Surfactants are amphiphilic compounds that contain polar
or ionic head groups and non-polar residues. In water or
similarly strongly hydrogen-bonded solvents, they self-
associate at concentrations above the critical micellar
concentration (CMC) to form association colloids called
micelles, which have interfacial regions containing ionic or
polar head groups [1].
All the reagents were of analytical grade and used as
received. Dodecyl amine, hexadecyl amine and octadecyl
amine were purchased from Aldrich Chemicals Co. and the
benzaldehyde was purchased from AL-Nasr Chemical Co.
Solvents (ethyl alcohol absolute, diethyl ether and acetone)
were of pure grade and purchased from Algomhoria
Chemical Co.
I. Aiad (&) ꢀ M. M. El-Sukkary ꢀ M. Y. El-Awady ꢀ
S. M. Shaban
Instruments
Petrochemical Department, Egyptian Petroleum Research
Institute, Cairo, Egypt
e-mail: yiaiad@yahoo.co.uk
The chemical structure of the synthesized compounds was
characterized by the following analysis. FTIR spectra using
ATI Mattsonm Infinity seriesTM, Bench top 961 controlled
by Win FirstTM V2.01 software. (Egyptian Petroleum
A. El-Deeb
Faculty of Science, Zagazig University, Zagazig, Egypt
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J Surfact Deterg (2012) 15:359–366
Research Institute). 1H-NMR spectra were obtained in
DMSO-d6 by Spect Varian, GEMINI 200 (1H 200 MHz).
(Micro-analytical Center, Cairo University. Mass spectra
were measured by GC MS-OP1000EX (Micro Analytical
Center, Cairo University).
surfactant solution. The reading was recorded when the ring
detached itself from the solution surface. The platinum ring
was removed after each reading, washed with diluted HCl
followed by distilled water. Apparent surface tension values
were taken as the average of three replicates at different
temperatures (15, 35, and 45 °C) [7].
The interfacial tension was measured between the sur-
factant solution of 0.1% concentration and light paraffin oil
at 15 °C.
Synthesis
Synthesis of Schiff Base Compounds
Different alkyl Schiff bases were synthesized through con-
densation reactions of different alkyl fatty amines, mainly:
dodecyl, hexadecyl and octadecyl amine with benzaldehyde.
Equimolar amounts of the desired fatty amines and aldehyde
were refluxed in ethanol for 6 h. The reaction mixture was
left to cool to ambient temperature and filtered. The products
were recrystallized twice from ethanol, washed with water
and dried in a vacuum oven at 40 °C [14, 15].
Determination of Antimicrobial Activities of Prepared
Compounds
The antimicrobial activities of synthesized products were
measured against a wide range of bacterial and fungal
organisms. The bacteria tested were Gram-positive (Staph-
ylococcus albus, Bacillus subtilis and Streptococcus fae-
calis) and Gram-negative (Escherichia coli), the fungi used
were Candida albicans and Aspergillus flavus. Cetyl tri-
methyl ammonium bromide was taken as a reference [16].
The different species of tested organisms were obtained
from the unit of the Micro-Analytical Center, Cairo Uni-
versity, Cairo, Egypt.
Synthesis of Cationic Schiff Base Compounds
To 0.1 mol of Schiff base in absolute ethanol, 0.1 mol
benzyl chloride was added. The reaction mixture was
refluxed for 45–90 h depending on the fatty amine chain
length. The solution was evaporated under reduced pres-
sure. The products were crystallized from acetone three
times to obtain the purified iminium salts (I, II & III).
The chemical structures of the synthesized compounds
are shown in Scheme 1.
An assay was made to determine the ability of an anti-
biotic to kill or inhibit the growth of living microorgan-
isms, the technique which was used consists of the
following filter-paper disc-agar diffusion test [17].
1. Inoculate flask of melted agar medium with the
organism to be tested.
2. Pour this inoculated medium into a Petri dish.
3. After the agar has solidified, a multilobed disc that has
been impregnated with different antibiotics is laid on
top of the agar.
Measurements
Surface (c) and Interfacial Tension (cIT)
4. The antibiotic in each lobe of disc diffuses into the
medium and if the organism is sensitive to a particular
antibiotic, no growth occurs in a large zone surround-
ing that lobe (clear zone).
Surface and interfacial tensions of the prepared compounds
solutions weremeasuredusing a Du-Nouytensiometer (Kru¨ss
type 845, Hamburg, Germany). Freshly prepared aqueous
cationic surfactant solutions with a concentration range of
0.01–0.000001 mol/liter were poured into a clean 25-mL
Teflonholderandallowedtoequilibrate for30–45 min.Then,
the platinum ringwas adjusted atthe air–water interface ofthe
5. The diameters of inhibition zones are measured after
24–48 h at 35–37 °C (for bacteria) and 3–4 days at
25–27 °C (for yeast and fungi) of incubation at 28 °C.
6. The clear zones are measured and comparison between
them is made to determine which antibiotic is a better
inhibitor.
I : n=8
Results and Discussion
II : n=12
III : n=14
CH2
N
Chemical Structure
CH
CH2CH2(CH2)nCH2CH3
-
Cl
The chemical structure of the prepared cationic surfactants
1
was confirmed by the FTIR, H-NMR spectra and mass
Scheme 1 The general chemical structure of the prepared
compounds
spectra.
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361
The 1H-NMR spectra data (d ppm) of N–benzylidene-N-
benzyloctadecyliminium chloride (III) showed the follow-
ing signals d = 0.88 (t,3H, —CH3); d = 1.268 (m,30H, —
(CH2)15CH2CH3); d = 1.6 (m, 2H, —CH2CH2Nꢁ);
FTIR Spectra
FTIR spectroscopy was used to identify the functional
groups of the prepared cationic surfactants. The IR spectra
of the synthesized Schiff bases showed the disappearance of
the characteristic (N–H) absorption band at 3,200 cm-1 and
the appearance of 1,620–1,660 cm-1 band which charac-
terizes the azomethine group (–CH=N–), that proves the
occurrence of a coupling reaction. The absorption bands of
d = 2.53 (s,2H, —CH2Ph); d = 2.714 (t,2H,
—
CH2CH2Nꢁ); d = 7.42 (m,5H, —CH2–Ph); d = 7.62
(m,5H, ꢁN = CH —Ph); d = 8.21 (s,1H, ꢁN = CH—Ph).
Mass Spectroscopy
the phenyl groups appeared in the range of 826–903 cm-1
.
The prepared cationic surfactants have the nearly same
main characteristic bands, for example compound I shows
stretching vibration bands of –C–H aliphatic symmetric and
asymmetric at 2,844 and 2,913 cm-1 respectively, =C–H
aromatic stretching at 3,004 cm-1, –N=CH– stretching at
1,620 cm-1, C=C aromatic stretching at 1,509 cm-1, -CH2
bending at 1,463 cm-1, C–O stretching at 1,091 cm-1 and
showed a sharp absorption band at 3,040 cm-1 corresponded
to the quaternary nitrogen atom (N?) as showed in Fig. 1.
Mass spectroscopy confirmed the chemical structure of
the prepared compounds, where the mass spectroscopy
of N-benzylidene-N-benzyldodecyliminium chloride (I)
showed molecular peaks at m/z: 400 (corresponding to
molecular weight) and m/z: 91 (corresponding to the benzyl
cation).
Surface Properties
Critical Micelle Concentration (CMC)
1H-NMR Spectra
Critical micelle concentration values of the prepared cat-
ionic surfactants were obtained graphically by plotting the
surface tension (c) of aqueous solutions of the prepared
surfactants versus their bulk concentrations in mol l-1 at
15, 35 and 45 °C. The CMC values are shown in Table 1
and show a decrease in the CMC of prepared cationic
surfactants with increases in both the temperature and the
hydrophobic chain length [18, 19] as showed in Figs. 2, 3
and 4. It was found that increasing the temperature causes a
decrease in the hydration of the hydrophilic group, which
favors micellization. However, temperature increases cause
disruption of the water-structure surrounding the hydro-
phobic group, so the system disfavors micelle formation.
The relative magnitude of these two opposing effects
determines the trend of CMC over a particular temperature
range.
1
The H-NMR spectra data (d ppm) of N-benzylidene-N-
benzyldodecyliminium chloride (I) showed the following
signals d = 0.84–0.872 (t,3H, —CH3); d = 1.261 (m,18H,
—(CH2)9CH3); d = 1.571 (m,2H, —CH2CH2Nꢁ);
d = 2.527 (S,2H, —CH2ph); d = 2.7–2.773 (t,2H, —
CH2CH2Nꢁ); d = 3.381 (S,3H, —OCH3); d = 7.414 (5H,
—CH2–ph); d = 7.599 (4H, ph-OCH3); d = 8.176 (S,1H,
ꢁN = CH —ph).
The 1H-NMR spectra data (d ppm) of N-benzylidene-N-
benzyl hexadecyliminium chloride (II) showed the follow-
ing signals d = 0.77 (t,3H, —CH3); d = 1.27 (m,26H, —
(CH2)13CH2CH3); d = 1.41 (m,2H, —CH2CH2Nꢁ);
d = 2.52 (s,2H, —CH2Ph); d = 2.714 (t,2H,
—
CH2CH2Nꢁ); d = 7.41 (m,5H, —CH2Ph); d = 7.61
(m,5H, ꢁN = CH—Ph); d = 8.15 (s,1H, ꢁN = CH—Ph).
Fig. 1 FTIR of the prepared p-
benzylidene benzyldodecyl
ammonium chloride (I)
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Table 1 The surface properties of synthesized cationic surfactant at different temperatures
Compound
Temp. °C
CMC * 10-4
pCMC (mN m-1
)
PC20 * 10-5
Umax * 10-10
Amin (nm2)
cIT (mN m-1
)
(mol L-1
)
(mol L-1
)
(mol cm-2
)
I
15
35
45
15
35
45
15
35
45
57.4
52.3
29.7
8.66
7.4
38.5
42.5
41
47.7
43
3.86
3.27
2.38
2.82
2.13
2.05
1.93
1.1
0.43
0.51
0.69
0.59
0.78
0.81
0.86
1.5
4.8
37.3
5.16
1.74
1.15
4.31
0.23
0.03
II
39.86
44
4
5.34
6.8
42
III
35.5
37.5
38.4
3.5
6.1
0.89
0.89
1.86
Fig. 4 Variation of surface tension of compound (III) against
concentration at different temperatures
Fig. 2 Variation of surface tension of compound (I) against concen-
tration at different temperatures
Increasing the length of the hydrophobic chain leads to
an increase in the hydrophobicity thus the free energy of
the system increases [20]. Hence, by increasing the
hydrophobic chain length, the tendency of the compound to
form micelles increases, and the CMC of the surfactant
compounds decreases [21].
Efficiency (Pc20)
The efficiency values of synthesized surfactants are shown
in Table 1: it is clear that the efficiency of prepared sur-
factant compounds decreases by increasing the hydropho-
bic character and solution temperature. This is due to the
fact that the efficiency of adsorption at interfaces increases
linearly with an increase in the carbon atoms in the
hydrophobic group and the test temperature [22].
Effectiveness (pCMC
)
The effectiveness pCMC is the difference between the sur-
face tension of pure water (co) and the surface tension of
Fig. 3 Variation of surface tension of compound (II) against
concentration at different temperatures
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363
the surfactant solution (c) at the critical micelle
concentration.
pCMC ¼ co ꢂ c
Effectiveness values are presented in Table 1. The most
efficient surfactant is the one which gives the largest
reduction of the surface tension at the CMC. According to
the results of effectiveness the compound having a C12
carbon chain length at 15 °C was found to be more
efficient; it achieved the maximum reduction of the surface
tension at the CMC [22].
Fig. 5 Relation between temperature and Amin of compounds I, II and
III
Maximum Surface Excess (Cmax
)
area occupied by each molecule increases, so that Amin
increased [25].
The maximum surface excess is expressed as the concen-
tration of surfactant molecules at the interface per unit area
(Cmax), which depend mainly on the hydrophobic chain
length and the temperature. The values of maximum sur-
face excess Cmax are calculated from surface or interfacial
data by the use of Gibb’s equation [23].
Interfacial Tension
The interfacial tension was measured between an aqueous
solution of surfactant of 0.1% concentration and light
paraffin oil at 25 °C. The data are shown in Table 1.
From these data it was observed that by increasing the
hydrophobic chain length of the prepared cationic surfac-
tants, the interfacial free energy decreased [26].
Thermodynamics of micellization and adsorption
(DG°mic, DG°ads):
The thermodynamic parameters of adsorption and mic-
ellization of the synthesized cationic surfactants were cal-
culated according to Gibb’s adsorption equations as
follows [18]:
}
¼ ꢂð1=2:3RTÞðdc=dlogcÞ
max
T
where (dc/d log c) is the surface pressure, R is the universal
gas constant and T is the absolute temperature.
The values of Cmax are shown in Table 1. Where
increasing the temperature and hydrophobic moiety length
of prepared cationic surfactants, shifts Cmax to lower con-
centrations. It is clear that surfactant molecules are directed
towards the interface which indicates the maximum com-
pactness of these molecules. As a result, the adsorbed
monolayer formed is expected to be very dense [24]. The
compacted and highly dense adsorbed monolayer plays an
important role in several interfacial applications of sur-
factant solutions including emulsification, solubilization,
corrosion inhibition and biological activity.
DGꢃmic ¼ RT ln ðCMCÞ
DGaꢃds ¼ DGmꢃ ic ꢂ 6:023 ꢄ 10ꢂ2 ꢄ pCMC ꢄ Amin
À
Á
DSmic ¼ ꢂd DGꢃmic=DT
À
Á
DSads ¼ ꢂd DGꢃads=DT
Minimum Surface Area (Amin
)
DHmic ¼ DGꢃ þ TDSmic
mic
The area per molecule at the interface provides information
on the degree of packing and the orientation of the adsor-
bed surfactant molecule. The average area (in square
angstrom) occupied by each molecule adsorbed on the
interface [21] is given by:
DHads ¼ DGꢃ þ TDSads
ads
The thermodynamic parameters of micellization and
adsorption are summarized in Table 2, where theses
parameters were calculated at 15, 35 and 45 °C. From
thermodynamic parameter of micellization it was observed
that the DG°mic values are negative and the micellization is
a spontaneous process [27].
Amin ¼ 1016=}max
N
}
maximum surface excess in mol/cm2. N Avogadro’s
max
number 6.023 9 1023.
Also DG°mic increases in the negative direction by
increasing both temperature and hydrophobic character,
thus the free energy of the system would increase and
molecules would be more likely to form micelles.
Also we can observe that DSmic values are positive and
increase by increasing the hydrophobic chain length. This
The minimum area per molecule at the aqueous solu-
tion/air interface for the prepared surfactants is listed in
Table 1 and shown in Fig. 5. Where Amin increases with
increasing length of the hydrophobic moiety and temper-
ature due to increasing hydrophobic chain length and the
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Table 2 The thermodynamic parameters of micellization and adsorption of the prepared cationic surfactants
Compound
Temp. °C
ꢅGmic (Kj mol-1
)
ꢅHMIC (Kj mol-1
)
ꢅS = –ꢅG/T
ꢅGads
ꢅHads
ꢅHads
(Kj mol-1 K-1
)
(Kj mol-1
)
(Kj mol-1
)
(Kj mol-1
)
I
15
35
45
15
35
45
15
35
45
-12.36
-13.46
-15.39
-16.89
-18.45
-19.93
-17.46
-18.95
-24.66
14.84
15.63
14.64
11.35
11.75
11.25
44.87
47.71
44.16
-13.63
-14.79
-17.06
-18.30
-20.57
-21.98
-19.08
-22.35
-28.96
16.99
17.97
16.75
16.65
16.81
16.6
0.0944
0.098
0.1063
0.1213
0.3054
II
III
68.93
71.77
68.21
0.2163
indicates that increasing the hydrophobic chain length,
increases the randomness in the system upon aggregation
of surfactants molecules into micelles, thus CMC decreases
[22].
motion of the hydrocarbon chain at the air-aqueous solu-
tion interface.
Antimicrobial Activity
From the thermodynamic parameter of adsorption, it
was observed that the standard free energy of adsorption
(DG°ads) is negative and points to the adsorption process
being spontaneous.
The prepared quaternary iminium salts were tested against
some pathogenic Gram-positive (Staphylococcus aureus
and Bacillus subtilis) and Gram-negative (Escherichia coli
and Desulfomonas pigra) bacteria and also, some patho-
genic fungi (Candida albicans and Aspergillus flavus).
Zone inhibition results are recorded in Table 3, indicating
that the synthesized compounds have antimicrobial activ-
ities, and that the difference in their activities depends on
the length of hydrophobic chains of prepared surfactants.
The optimal length of alkyl chain has been noted to be
twelve carbon atoms, which exhibit the maximum inhibi-
tion zone. These results are in agreement with results
reported elsewhere [28–32].
Also DG°ads increases in the negative direction by
increasing both temperature and length of the hydrophobic
chain. This implies that increasing both temperature from
15 to 45 °C and the length of hydrophobic chain, favor the
adsorption at the interface.
Also it can be observed that DSads values are positive
and increase by increasing the hydrophobic chain length.
Comparing the free energies of micellization and
adsorption, it can be observed that DG°ads values are more
negative than DG°mic, indicating the tendency of the mol-
ecules to adsorb at the air–water interface until there is a
complete surface coverage. Beyond this, the molecules
diffuse to the bulk of their solution to form micelles.
Hence, the micellization and adsorption processes are
governed by the thermodynamic aspects. The chemical
structure of these molecules is the main factor influencing
their thermodynamic aspects [26]. The DSads values are
positive and slightly greater than the DSmic values for the
same compounds. This may reflect the greater freedom of
The bacterial cell membrane is composed of a thick wall
containing many layers of peptideglycan and teichoic
acids, which are glycerol-ribitol (polyhydric alcohol)
through a phosphorus bond surrounded by lipids of lipo-
polysaccharides and proteins [33, 34].
So the mode of action of that type of compound on
different microorganisms can be attributed to the adsorp-
tion of amphiphile molecules on the outer cellular mem-
brane of the microorganism due to their amphipathic
Table 3 Antimicrobial activity of synthesized surfactants against pathogenic bacteria and fungi
Samples
Inhibition zone diameter (mm/mg sample)
Bacillus subtilis
(G?)
Desulfomonas pigra
(G-)
Escherichia coli
(G-)
Staphylococcus
aureus (G?)
Candida albicans
(fungus)
Aspergillus flavus
(fungus)
Control DMSO
0.0
28
14
14
0.0
21
13
14
0.0
23
14
13
0.0
24
13
15
0.0
21
14
14
0.0
18
I
II
III
12
0.0
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365
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Chemical Services and Development Center for Oil Field Chemicals.
He is also a professor at the Egyptian Petroleum Research Institute.
His research interests deal with the synthesis and applications of
surfactants.
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E (2000) Synthesis and surface and antimicrobial properties of
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Mohamed M. El-Sukkary received his B.Sc. in chemistry and
geology (1965) and his M.Sc. in chemistry (1970) from the Faculty of
Science, Cairo University. His Ph.D. in applied and organic chemistry
was awarded by Al-Azhar University (1976). At the Egyptian
Petroleum Research Institute (EPRI), he was head of the surfactants
laboratory (1988–1991), head of the petrochemicals department
(1991–1997), and deputy director (2000). He is currently Supervisor
of Scientific Affairs and Relations and a member of the board of
directors of EPRI and of the Petroleum Research Council of Egyptian
Academy of Scientific Research and Technology.
34. Walsh F, Amyes S (2004) Microbiology and drug resistance
mechanisms of fully resistant pathogens. Curr Opin Microbiol
7:439–444
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biological membrane fusion. J Membrane Biol 146:1–14
36. Saier MH Jr (1987) Enzymes in metabolic pathways. A com-
parative study of mechanism, structure, evolution, and control.
Harper & Row, New York
Ali El-Deeb is a professor of organic chemistry at Zagazig
University, Faculty of Science. He is interested in organic syntheses
particularly of heterocompounds and chemotherapy.
Moshira Y. El-Awady received her B.Sc. (1978), M.Sc. (1988), and
Ph.D. (2000) from the College of Science at Ain Shams University,
Cairo, Egypt. She is now an assistant professor at the Egyptian
Petroleum Research Institute (Surfactants Laboratory).
Samy M. Shaban received his M.Sc. from Zagazig University
(2009). He is an assistant researcher in the surfactant laboratory at the
Egyptian Petroleum Research Institute. He is a member of the board
of the Chemical Services and Development Center for Oil Field
Chemicals.
Author Biographies
Ismail Aiad received his Ph.D. in physical chemistry from the
Faculty of Science, Zagazig University (1998). He is the deputy head
of the petrochemical department, and a board member of the
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