Gemini cationic surfactants: Synthesis and influence of chemical structure on the surface activity

Article abstract of DOI:10.1007/s11743-013-1478-9

Three cationic gemini surfactants were synthesized and characterized using different methods. Their surface activities were measured using surface and interfacial tension measurements. The effect of the spacer chain length on the surface activity, emulsif

Full text of DOI:10.1007/s11743-013-1478-9

J Surfact Deterg (2013) 16:733–738
DOI 10.1007/s11743-013-1478-9
ORIGINAL ARTICLE
Gemini Cationic Surfactants: Synthesis and Influence of Chemical
Structure on the Surface Activity
Nabel A. Negm
Dalia E. Mohamed
Manal M. Khowdiary
•
Maher A. El-Hashash
•
•
John M. Marquis
•
Received: 10 January 2013 / Accepted: 8 April 2013 / Published online: 18 April 2013
Ó AOCS 2013
Abstract Three cationic gemini surfactants were syn-
thesized and characterized using different methods. Their
surface activities were measured using surface and inter-
facial tension measurements. The effect of the spacer chain
length on the surface activity, emulsification power and
interfacial tension was studied. The thermodynamic
parameters showed the tendency towards micellization and
adsorption. The results showed that longer spacers
increased the micellization tendencies of the surfactants,
while shorter spacers increased the adsorption tendency at
the air–water interface.
recent development, gemini surfactants are already a hot
topic in colloids and surface science and are considered to
be a type of surfactant that will be most widely used in the
21st century [4–6]. Gemini cationic surfactants have been
extensively studied due to their easy preparation. This has
also made them particularly interesting from an industrial
point of view, and much of the published information
concerning the solution properties of gemini surfactants is
based on cationic species [7–10].
Experimental
Keywords Gemini surfactants ꢀ Spacer group ꢀ Surface
activity ꢀ Thermodynamics
Synthesis of Aminopyridine Schiff Base (SB)
A mixture of benzaldehyde (0.15 mol, 15.75 g), p-amino-
pyridine (0.15 mol, 14.1 g) and xylene (150 mL) was
added to a round flask and heated with continuous removal
of water of the reaction [11]. The reaction was stopped
after removal of 2.7 mL of water. The obtained Schiff base
was viscous brown color, C H N O, elemental analysis
Introduction
Gemini surfactants are a novel type of surfactant synthe-
sized in recent years. Their advent has greatly broadened
the perspective of interfacial science [1–3]. Compared with
conventional surfactants, gemini surfactants are more
efficient at lowering surface tension and have much lower
critical micelle concentration values (CMC). Despite their
1
2 10 2
% (calc/found): C = 77.6/77.1, H = 5.9/5.8, N = 16.5/
16.3.
Synthesis of Cationic Gemini Surfactants
N. A. Negm (&) ꢀ D. E. Mohamed ꢀ M. M. Khowdiary
Department of Petrochemicals, Egyptian Petroleum Research
Institute, Nasr City, Cairo, Egypt
A mixture of the prepared Schiff base (SB) (0.05 mol,
8
.51 g), 0.025 mol of dibromoalkanes namely: 1,2-dibro-
moethane, 1,6-dibromohexane and 1,12-dibromododecane
4.7 g, 6.1 g and 8.2 g, individually) and 100 mL of ace-
e-mail: nabelnegm@hotmail.com
(
M. A. El-Hashash
Department of Chemistry, Faculty of Science, Ain Shams
University, Cairo, Egypt
tone was put into a round flask and refluxed for 12 h. The
reaction mixture was allowed to cool overnight, and then
the reaction products were filtered off and washed twice by
diethyl ether and dried under vacuum [12]. The obtained
products were designated as: SB-2, SB-6 and SB-12, in
J. M. Marquis
Cooperation Petroleum, Cairo, Egypt
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Scheme 1 Synthesis of the
gemini cationic surfactants
H
C
H
H O
2
O
H N
2
N
C
N
N
Xylene
reflux
Acetone
Br CH2 CH2 n CH2 Br
2-
H
C
H
_
N
N
CH2 CH2 n CH2
N
N
C
2Br
Scheme 1. The molecular weights and the elemental
analysis of the obtained surfactants were listed in Table 1.
light paraffin oil at 25 °C using the same procedures of the
surface tension measurements [14].
Measurements
Emulsion Stability
TM
FTIR spectra using an ATI
Mattsonm Infinity series,
TM
Bench top 961 instrument controlled by Win First V2.01
Emulsion stability was measured by vigorously stirring a
mixture of 10 mL (0.5 %) of the synthesized cationic
surfactant solutions and 10 mL of paraffin oil at 25 °C
[15]. The emulsion was formed by hand shaking of the
cylinder containing the oil and the surfactant solution for
10 min. Emulsifying power (emulsion stability) of the
surfactant solutions was expressed as the time required for
the separation of 9 mL of pure surfactant solution.
1
software. H H-NMR spectra were measured in DMSO-d
1
by Spect. Varian, Gemini 200 ( H 200 MHz).
6
Surface and Interfacial Tension Measurements
Surface and interfacial tension measurements were per-
formed using a Kr u¨ ss K6 tensiometer by the platinum ring
detachment method (± 0.5 mN/m). Freshly prepared
aqueous solutions of the synthesized gemini surfactants
were used with a concentration range of 0.01–0.00001 M/L
at 25 °C. The solutions were poured into a clean Teflon cup
with a mean diameter of 28 mm (Teflon cup was used to
prevent the adhesion of the surfactant to the glass cup
walls). The solutions were left for 2 h to allow the stabil-
ization and complete adsorption at the solution surface,
then the apparent surface tension values were measured a
minimum of three times and the recorded values were
taken as the average of these values [13]. The platinum ring
was then removed, washed with a diluted HCl solution
followed by distilled water.
Results and Discussion
Structure
The chemical structures of the synthesized gemini surfac-
tants were confirmed using elemental analysis, IR and
NMR spectra. Table 1 represents the calculated and
obtained ratios of the different elements in the chemical
structures of the different surfactants, which proves their
purity. FTIR spectra showed absorption bands at 3,035,
-
1
2,920, 2,830, 1,635, 780–920 cm . NMR spectra showed
signals at: d (ppm) = 1.3 (CH , m, nH), 4.5 (CH = N, s,
2
The interfacial tension measurements were obtained
between aqueous solutions of the synthesized gemini cat-
ionic surfactants at a concentration of 0.1 % by weight and
2H), 7.3 (phenyl, m, 18H). These data proved the expected
chemical structures of the synthesized surfactants as rep-
resented in Scheme 1.
Table 1 Characterization of the synthesized gemini surfactants
Compound
Molecular weight
M.Wt. (g/mol)
Carbon %
Calc
Hydrogen %
Nitrogen %
Calc
Bromine %
Calc
Found
Calc
Found
Found
Found
SB-2
SB-6
SB-12
554.32
608.41
691.57
56.34
59.22
62.52
54.32
58.93
63.05
4.73
5.30
6.28
4.90
5.32
6.43
10.11
9.12
8.12
9.97
9.30
8.18
28.83
26.27
23.11
28.62
27.01
23.15
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735
6
5
5
4
4
0
5
0
5
0
The surface pressure (qc/qlogC) indicates the pumping
of surfactant molecules to the interface of their solutions.
Increasing this term revealed that the surfactant molecules
are adsorbed at the air–water interface more preferably
[
16]. It is clear from the data in Table 2 that the increase in
spacer chain length decreases the surface pressure of the
surfactant towards the interface.
The maximum surface excess is defined as the maxi-
mum concentration of surfactant molecules at the interface
of their solutions in the saturation case. The maximum
surface excesses of the synthesized gemini surfactants were
calculated using the slopes of the pre-micellar part (qc/
qlogC) in Fig. 2, according to the Gibbs’ adsorption
equation as follows:
-6
-5
-4
-3
-2
Cmax ¼ ðoc=o log CÞ=2:303nRT
-log C, M/L
where, Cmax is the maximum surface excess under satura-
tion conditions, n is the number of active species in the
surfactant solution and equal to 3 in the case of gemini
surfactants, R is the universal gas constant and T is the
absolute temperature, the results were listed in Table 2. It
is clear from data in Table 2 that the gradual increase in the
spacer chain length decreases the concentration of the
surfactant molecules at the interface.
Fig. 1 Surface tension versus concentration of the synthesized
gemini cationic surfactants (square SB-2, triangle SB-6, circle SB-
1
2) at 25 °C
Surface Activity
The surface tension versus -log concentration profiles of
the aqueous solutions of the synthesized gemini surfactants
SB-2, SB-6 and SB-12 were obtained at 25 °C. The profiles
showed no minimum, which indicate the purity of the
surfactants. The minimum is appeared mainly in an impure
or contaminated surfactant solutions due to the residual
compounds have high surface activities than the surfactants
under investigation, and appeared in lower concentration
regions.
On the other hand, the average area occupied by each
surfactant molecule at the solution interface can be calcu-
lated using Cmax values according to the following
equation:
Amin¼1=ðNav:CmaxÞ
where, Nav is Avogadro’s number.
The obtained surface tension values are decreased with
the increasing of concentration at the lower ranges (pre-
micellar region), until reached almost constant values at
higher concentrations (post micellar region), Fig. 1. The
break points of these two regions are determined the critical
micelle concentrations (CMC) of the surfactants, Table 2.
The adsorption of the surfactant molecules at the air–
water interface can be interpreted by studying two
parameters: the surface pressure and the maximum surface
excess (Cmax).
The average area at the interface available for each
surfactant molecule is increased by increasing the spacer
chain length. The maximum area occupied at the inter-
2
face was 67.4 nm which obtained for the SB-12 sur-
2
factant, while the lowest area was 44.6 nm for SB-2
surfactant. The values of Amin were larger than the
published values for the saturated gemini surfactant [17].
That can be attributed to the geometry of gemini sur-
factant molecules at the interface. Analyzing the Amin
data in Table 2 showed that the increase in Amin was
Table 2 Surface activities of the gemini surfactants at 25 °C
Compound CMC c
CMC
(mM) (mNm
Interfacial
tension
p
CMC
(mNm
C
20
,
qc/
C
max
CMC/
A
min
Emulsification DGmic
(kJmol
DGads
-1
(kJmol )
-1
)
-1
)
11
2
(nm ) (Sec)
-1
)
qlogC (910 )
C
20
6
910 M
-1
(
mNm )
SB-2
SB-6
SB-12
1.33
0.61
0.28
50
47
41
15
13
6
21.8
24.8
30.8
316
-6.37 3.72
-5.03 2.94
-4.22 2.47
4.21 44.6
9.13 56.5
72.35 67.4
120
-16.4
-18.3
-20.3
-22.3
-26.8
-32.7
66.8
3.87
300
Stable
CMC critical micelle concentration in mM, cCMC surface tension at CMC, pCMC effectiveness, C20 efficiency, Cmax maximum surface excess, qc/
qlogC slope of surface tension vs concentration profile, CMC/C
micellization free energy, DGads adsorption free energy
2
ratio between CMC and efficiency, Amin minimum surface area, DGmic
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2
2
2.8 nm by increasing the spacer chain length from 2 to
2 methylene groups. Hence, the average Amin of the
1
2
methylene group at the interface is 2.28 nm , which is in
3
.5
a good agreement with our previously published data
2
2.2 nm ) [9].
(
The chemical structures of the gemini surfactants play
an important role in their surface activity, and mainly
regarding the CMC values. In the case of saturated
gemini surfactants, the terminal alkyl chains have a high
tendency towards geometrical conformation at the inter-
face [18], which decreases the area occupied by each
molecule at that interface. As a result, the surface con-
centration is increased gradually by increasing the ter-
minal alkyl chain length, and also by the gradual
increase in the spacer chain length. The increase in
surface concentration increases the concentration of sur-
factant molecules at the interface, which consequently
increases the CMC values. Gemini surfactant molecules
adopt different conformations depending on the spacer
length. Before micellization 12–2–12 monomers are in
the trans-conformation. While in case of 12–4–12 and
3
2.5
0
5
10
Spacer chain length
Fig. 2 Effect of spacer chain length on the critical micelle concen-
tration of the synthesized gemini surfactants at 25 °C
1
2–6–12, they are in the cis- conformation [19]. In the
Surface Tension Reduction, Effectiveness
case of the trans-conformation, the free energy of
transfer for the surfactant monomer from the aqueous
phase to the pseudo micellar phase is relatively lower as
compared to the cis-conformation case. Hence, micelli-
zation is relatively more easily facilitated for 12–2–12
and Efficiency (cCMC, pCMC, Pc20)
The surface tension reduction at the critical micelle con-
centration of the synthesized surfactants ranges between 50
and 41 mN/m (Table 2). That indicates a lower surface
activity of these surfactants compared to saturated cationic
gemini surfactants [19]. This can be attributed to the lower
surface concentration of the surfactants at the interface and
also to the highly hydrophobic character of the surfactant
molecules [10]. The higher surface tension reduction
accompanied by longer hydrophobic spacer chain length is
attributed to the fast interface saturation. That describes the
fast adsorption of surfactant molecules on the interface.
The SB-2 surfactant has the highest surface tension
reduction at the CMC, which indicates its lower adsorption
tendency than SB-6 and SB-12 surfactants.
(
lower CMC) and vice versa for 12–4–12 and 12–6–12.
In the case of the unsaturated or conjugated terminal
hydrophobic groups, the geometry of these groups
restricts their conformation and consequently they are
forced to be planar at the interface. Hence, increasing the
spacer chain length increases the area occupied by each
molecule at the interface. That decreases the surface
concentration of these molecules at the interface, which
forces the molecules to micellize at a lower concentra-
tion than the saturated molecules do.
In the case of the synthesized gemini cationic Schiff
bases, the terminal groups are two phenyl groups linked to
each other by a double bond (azomethine group), while
the spacer groups vary from 2 to 12 methylene groups, as
seen in Scheme 1. The geometry of these molecules is
forced to be planar at the interface due to lack of flexi-
bility. The rigidity of the targeted surfactant molecules
increases their area at the interface, which consequently
decreases the surface concentration. The decrease in sur-
face concentration indicates that the interface is com-
pletely covered by adsorbed surfactant molecules. Hence,
the molecules in the bulk of their solution form micelles.
The critical micelle concentrations of the synthesized
surfactants were decreased gradually by increasing the
spacer chain length from 2 to 12 methylene groups as
shown in Fig. 2.
The effectiveness values (pCMC) of the prepared sur-
factants are listed in Table 2, where the efficiency values
are slightly increased from 21 to 30 mN/m by increasing
the number of methylene groups in the spacer chains from
2 to 12. The efficiency of adsorption at interfaces increases
linearly by increasing the number of methylene groups in
the hydrophobic groups [20, 21]. While, the efficiency
values (Pc20) were decreased gradually by increasing the
hydrophobic chain length. The efficiency parameter indi-
cates the adsorption behavior of surfactant molecules at the
interface. Higher efficiency values revealed a lower
adsorption tendency.
The ratio between CMC and the efficiency determines
the tendency of surfactant molecules towards micellization
and adsorption in their solutions. Increasing this ratio
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J Surfact Deterg (2013) 16:733–738
737
indicates the higher tendency of surfactant molecules
towards adsorption at the air-solution interface than mic-
ellization in the bulk, and vice versa. It is clear from CMC/
C20 values listed in Table 2 that the surfactants have a
tendency towards adsorption. SB-2 has a 4-fold lower
tendency towards adsorption at the air–water interface than
micellization. SB-12 has a higher adsorption tendency than
micellization by 67-folds.
DGmic ¼ 2:303nRTðlog CMCÞ
DGads ¼ DGmicꢁð0:6xpCMCxAminÞ
The listed values in Table 2 revealed that the adsorption
and micellization processes occur spontaneously with a
higher preference towards adsorption than micellization.
That was indicated by the more negative values of the
adsorption free energy than the micellization. Furthermore,
SB-12 showed a large difference between the adsorption
and micellization free energies, with more negativity in the
adsorption free energy. This showed the stability of the
adsorbed monolayer of SB-12 at the air–water interface
rather than the aggregated molecules in the bulk of the
solution.
Emulsification Power and Interfacial Tension
The emulsification power of the synthesized gemini cat-
ionic surfactants is determined as the time required for
breakdown of the emulsion formed between surfactant
solution and paraffin oil. Emulsification is one of the most
important surface properties of the surfactants at the
interface. The emulsification tendency in some cases is an
important property which is needed in surfactants to form
stable emulsions, as in the case of solubilization, emulsi-
fication processes, cosmetics and drug formulations.
Contrarily, some applications do not favor this, especially
in the case of petroleum applications including demulsi-
fication and corrosion inhibition. Table 2 lists the emul-
sification tendency of the different synthesized surfactants
in the presence of light paraffin oil. It is clear that the
emulsification power of the surfactants is completely
dependent on the spacer chain length. Short spacer chain
which contains two methylene groups (SB-2) has the
lowest emulsification tendency at 120 s, while increasing
the spacer chain length to six methylene groups consid-
erably increases the emulsification power of SB-6 to
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towards bacteria, fungi and yeast. His activities now are concerned
with studying the interaction of different types of surfactants with
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Maher A. El-Hashash Is a professor of organic chemistry, on the
Faculty of Science at Ain-Shams University, Egypt. His interests are
organic chemistry and the synthesis of new compounds.
Dalia Emam Mohamed Received her B.Sc. and M.Sc. from Cairo
University and her Ph.D. from Ain Shams University, Egypt. She is
currently a Researcher at the Egyptian Petroleum Research Institute
(Surfactant Laboratory). Her research interests are in synthesis,
properties, and applications of new surfactants.
2
2
John M. Marquis Is an M.Sc. student. His interests are the synthesis
and use of surfactants in corrosion inhibition of petroleum industries.
Author Biographies
Manal M. Khowdiary Is a researcher in the petrochemicals
department at the Egyptian Petroleum Research Institute. Her
interests are the synthesis and evaluation of surfactants in several
applications including detergency and biocidal activity.
Nabel Abdelmoneem Negm Received his Ph.D. in 2000 from Ain
Shams University, Egypt. He is currently a professor of applied
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