Preparation, Characterization and Properties of Novel Cationic Gemini Surfactants with Rigid Amido Spacer Groups

Article abstract of DOI:10.1007/s11743-015-1752-0

A series of novel cationic gemini surfactants with rigid amido groups inserted as the spacers, named C ₁₂ -PPDA-C ₁₂, C ₁₄ -PPDA-C ₁₄ and C ₁₆ -PPDA-C ₁₆, were synthesized by a two-step rea

Full text of DOI:10.1007/s11743-015-1752-0

J Surfact Deterg
DOI 10.1007/s11743-015-1752-0
ORIGINAL ARTICLE
Preparation, Characterization and Properties of Novel Cationic
Gemini Surfactants with Rigid Amido Spacer Groups
1
1
2
2
2
Dongqing Xu
•
Banghua Qi
•
Di Fang
•
Xuemei Zhang
•
Ting Zhang
Received: 1 May 2015 / Accepted: 6 October 2015
Ó AOCS 2015
Abstract A series of novel cationic gemini surfactants
with rigid amido groups inserted as the spacers, named C -
surfactants exhibit superior aqueous solution properties,
such as lower critical micelle concentration (CMC), higher
surface activity, better emulsifying performances, lower
Krafft point and unusual aggregation morphologies [1–5].
To identify highly efficient surfactant structures, great
efforts have been taken in the design and synthesis of
gemini surfactants and their properties have been resear-
ched [6–9]. It is well-known that the different spacers [10–
12], head groups [13, 14], and hydrophobic groups can
make up of different molecular structures of gemini
surfactants.
1
2
PPDA-C , C -PPDA-C
1
and C -PPDA-C , were
16 16
2
14
14
synthesized by a two-step reaction with dimethyl tereph-
thalate, N,N-dimethyl propylene diamine and alkyl bromide
as raw materials. The chemical structures of the prepared
1
13
compounds were confirmed by IR, H and C NMR and
element analysis. Surface activity properties of the synthe-
sized compounds were investigated by surface tension,
electrical conductivity and fluorescence. Increasing the
number of carbon atoms in the hydrophobic alkyl chain,
decreased the critical micelle concentration (CMC), surface
tension at the CMC and the minimum surface area. Other
relevant properties including foaming ability and emulsion
stability were investigated. The results indicated that the
synthesized gemini surfactants possess good surface prop-
erties, emulsifying properties and steady foam properties.
Recently, gemini surfactants with different types of
spacers have been developed with different physical and
chemical properties [15–17]. Zhu et al. [15] prepared four
anionic gemini surfactants with long rigid or semi-rigid
spacers, and found that all these gemini surfactants showed
lower CMC and C20 values, and that gemini surfactants
with rigid spacers preferred to form vesicles. Gao and co-
workers [16] reported a series of novel cationic gemini
surfactants with diethylammonium headgroups and a dia-
mido spacer. This was the first report of sponge-like
aggregates in the cationic gemini surfactant-water binary
system which was probably caused by adhesion and fusion
of vesicles at high surfactant concentration. Hu et al. [17]
synthesized a class of quaternary ammonium salt gemini
surfactants with para-xylene as a rigid spacer. These
gemini surfactants possessed high surface activity.
Keywords Gemini surfactants Á Rigid amido spacer
groups Á Surface activity properties Á Synthesis
Introduction
Surfactants are widely applied in many fields from con-
sumer products to industrial applications. In comparison
with conventional single-chain surfactants, gemini
In this paper, we designed and prepared a class of
cationic gemini surfactants with rigid amido spacer groups.
The hydrophilic radical (amido bond) was selected as the
important building block for the rigid spacer, which was
hoped to improve the solubility of gemini surfactant in
aqueous solution, and further enhance their range of
applications. Scheme 1 shows the chemical structures and
the synthetic routes used to prepare these gemini
&
Ting Zhang
tingzhang84@163.com
1
2
Center of Analysis and Measurement, Anhui Science and
Technology University, Fengyang, Anhui 233100, China
School of Chemistry and Material Engineering, Anhui
Science and Technology University, Fengyang,
Anhui 233100, China
1
23

J Surfact Deterg
Scheme 1 Synthetic routes for
the gemini surfactants
surfactants. The surface and physical–chemical properties
of these gemini surfactants were investigated, including
their surface tension, electrical conductivity, micropolarity,
foaming and emulsion properties.
pressure, the white residue was recrystallized from a
mixture of ethyl acetate and methanol (4:1) to obtain the
white solid product (15.13 g, 90.60 % yield based on the
1
dimethyl terephthalate). H NMR (400 MHz, CDCl3,
TMS) d ppm: 1.76–1.78 (m, 4H, 2 NHCH CH CH N),
2
2
2
2.28 (s, 12H, 4CH ), 2.47 (t, J = 6.0 Hz, 4H, 2 NHCH
3 2-
Experimental Section
Materials
CH CH N), 3.53–3.55 (m, 4H, 2 NHCH CH CH N),
2 2 2 2 2
7
.80–7.81 (s, 4H at the benzene ring), 8.764 (s, 2H, 2 NH).
1
3
C MNR (100 MHz, CDCl , TMS) d ppm: 166.47,
3
-
36.99, 126.99, 58.89, 45.29, 40.34, 25.38. IR (KBr) cm :
1
1
Dimethyl terephthalate (AR, Analytical Reagent), 1-bro-
mododecane (CP, Chemical Pure), 1-bromotetradecane
3347, (N–H, amide); 1632 (C=O, amide); 2942 (CH –);
3
2857 (–CH –); 1552, 1500, 1457 (benzene ring); 664
2
(
(
CP), acetonitrile (AR), 1-bromohexadecane (CP), acetone
AR), Diethyl phthalate (CP) and methanol (AR) were
(benzene ring hydrogen). Found %: C, 64.80; H, 8.98; N,
16.69. Theory %: C, 64.64; H, 9.04; N, 16.75.
purchased from Sinopharm Chemical Reagent Corporation.
N, N-dimethyl propylene diamine (AR) and pyrene (AR)
were purchased from commercial sources without further
purification. Double distilled water was used in all
experiments.
Synthesis of Gemini Surfactant (C -PPDA-C )
6
1
16
A mixture of bis(3-(dimethylamino)propyl)terephthala-
mide (3.34 g, 10 mmol) and 1-bromohexadecane (7.32 g,
24 mmol) in acetone (20 mL) was placed in a dried
3-necked flask (150 mL) equipped with a magnetic stirrer
and a condenser closed with a anhydrous calcium chloride
drying tube. After stirring for 36 h at reflux (60 °C), the
reaction mixture was cooled to room temperature, and a
light yellow precipitate was collected by filtration. The
crude product was recrystallized twice from a mixture of
acetonitrile and acetone (3:1) and dried under reduced
pressure to give C -PPDA-C as a white solid (7.60 g,
1
13
H- and C-NMR spectra were recorded on a Bruker
AV 400 NMR Spectrometer (Bruker Corporation,
Switzerland, 400 or 100 MHz). The infrared (IR) spectra
were measured on a Japan FT/IR-430 spectrometer as KBr
pellets. Elemental analyses were carried out on a Vario EL-
III Spectrometer (Elementer Corporation, Germany).
Synthesis of bis(3-
(
(
dimethylamino)propyl)terephthalamide
Compound A)
1
6
16
80.51 % based on bis(3-(dimethylamino)propyl)terephtha-
1
lamide). H MNR (400 MHz, CDCl , TMS) d ppm: 0.86 (t,
3
A mixture of N,N-dimethyl propylene diamine (20.40 g,
.20 mol) and dimethyl terephthalate (9.70 g, 0.05 mol)
was added to a 3-necked flask (150 mL). After stirring for
J = 6.0 Hz, 6H, 2(CH ) CH ), 1.23–1.30 (m, 52H,
3
2
13
0
2(CH ) CH ), 1.71 (m, 4H, 2CH (CH ) CH ), 2.21 (m,
2 13 3 2 2 13 3
4H, 2 NHCH CH CH N), 3.28 (s, 12H, 2 N(CH ) ),
2
2
2
3 2
1
6 h at reflux (125 °C), the reaction mixture was cooled to
room temperature. The excess N,N-dimethyl propylene
3.43–3.52 (m, 8H, 2 NCH CH (CH ) CH , 2 NHCH
2 2 2 13 3 2-
CH CH N), 3.72 (m, 4H, 2 NHCH CH CH N), 7.90 (s, 4H
2 2 2 2 2
1
at the benzene ring), 8.67 (s, 2H, 2 NH). C MNR
3
diamine and methanol were removed under reduced
1
23

J Surfact Deterg
(
100 MHz, CDCl , TMS) d ppm: 167.18, 136.16, 127.68,
4,62, 62.83, 50.95, 36.95, 31.88, 29.68, 29.62, 29.50,
50.94, 36.89, 31.80, 29.58, 29.52, 29.42, 29.38, 29.23,
-
3
1
6
29.15, 26.27, 22.86, 22.75. 14.01. IR (KBr) cm : 3478,
3434 (N–H, amide); 2922 (CH –); 2852 (–CH –); 1645
2
9.44, 29.31, 29.22, 26.33, 22.83, 22.63, 14.06. IR
3
2
-
KBr) cm : 3482, 3435 (2 N–H, amide); 2919 (CH –);
1
(
(C=O, amide); 1539, 1493, 1468 (benzene ring); 727
(benzene ring hydrogen). Anal. calcd. Found %: C, 60.29;
H, 9.636; N, 6.756. Theory %: C, 60.56; H, 9.681; N,
6.727.
3
2
851 (–CH –); 1644 (C=O, amide); 1539, 1493, 1468
2
(
benzene ring); 727 (benzene ring hydrogen). Anal. Calcd.
Found %: C, 63.82; H, 10.26; N, 5.901. Theory %: C,
3.54; H, 10.24; N, 5.928.
6
Surface Tension Measurement
Synthesis of Gemini Surfactant (C -PPDA-C )
14
1
4
The surface tensions of all the gemini surfactant solutions
were measured using the du No u¨ y ring technique (Kr u¨ ss
K20 Tensiometer, Germany) [18], and the value of c was
measured at least three times at 301.0 ± 0.1 K. The sur-
face tension of pure water was measured as being
71.99 ± 0.05 mN/m at 298.0 K. All solutions were pre-
pared with triple-distilled water.
C -PPDA-C was prepared according to the method used
4
1
14
for
C -PPDA-C
16
by
using
bis(3-(dimethy-
1
6
lamino)propyl)terephthalamide (3.34 g, 10 mmol) and
-bromotetradecane (6.65 g, 24 mmol) in acetone
20 mL). The crude product was recrystallized twice from
1
(
a mixture of acetonitrile and acetone (3:1) and dried under
reduced pressure to give C -PPDA-C as a white solid
14
1
4
(
6.80 g,
76.58 %
based
on
H
bis(3-(dimethy-
MNR (400 MHz,
Electrical Conductivity Measurement
1
lamino)propyl)terephthalamide).
CDCl , TMS) d ppm: 0.87 (t, J = 4.0 Hz, 6H, 2(CH )
3
2 11-
The CMC of the gemini surfactants were determined by the
electrical conductivity method. The conductivity analyzer
CH ), 1.24–1.30 (m, 44H, 2(CH ) CH ), 1.71 (m, 4H,
2 11
3
3
2
3
CH (CH ) CH ), 2.21 (m, 4H, 2 NHCH CH CH N),
2 2 11 3 2 2 2
(Model DDS-307, Shanghai Precision & Scientific Instru-
.28 (s, 12H, 2 N(CH ) ), 3.45-3.51 (m, 8H, 2 NCH
2-
3
2
ment Corporation) was used to measure the conductivity of
the gemini surfactant solutions at 301.0 ± 0.1 K, and each
experiment was repeated three times. The concentration at
the intersection of the curve of conductivity versus gemini
surfactants concentration was taken as the critical micelle
concentration [19].
CH (CH ) CH , 2NHCH CH CH N), 3.74 (m, 4H,
2
2
2 11
3
2
2
2
NHCH CH CH N], 7.91 (s, 4H at the benzene ring), 8.67
2 2 2
1
3
(
s, 2H, 2NH). C MNR (100 MHz, CDCl , TMS) d ppm:
3
1
2
2
2
1
67.16, 136.17, 127.67, 64.61, 62.81, 50.95, 36.93, 31.86,
9.64, 29.60, 29.48, 29.43, 29.30, 29.21, 26.32, 22.81,
-
1
2.62, 14.05. IR (KBr) cm : 3481, 3434 (N–H, amide);
920 (CH –); 2851 (–CH –); 1644 (C=O, amide); 1538,
3
2
493, 1469 (benzene ring); 727 (benzene ring hydrogen).
Fluorescence Measurement
Anal. calcd. Found %: C, 61.96; H, 9.949; N, 6.279.
Theory %: C, 62.15; H, 9.978; N, 6.302.
Fluorescence measurements were carried out on a F-4600
Fluorescence spectrophotometer (Hitachi High-Technolo-
gies Corp, Tokyo Japan). Pyrene was used as the fluores-
cence probe at 336 nm photoexcitation. The emission
spectra were scanned over the spectrum range of
345–460 nm, and the slit widths of excitation and emission
were fixed at 5.0 and 2.5 nm, respectively. The ratio of the
intensities of the first peak (around 373 nm) to the third peak
(around 384 nm) was used as an index of the polarity of the
pyrene-solubilizing medium (I /I ) [20]. All solutions were
Synthesis of Gemini Surfactant (C -PPDA-C )
1
2
12
C -PPDA-C was prepared according to the method used
1
2
12
for
C -PPDA-C
16
by
using
bis(3-(dimethy-
1
6
lamino)propyl)terephthalamide (3.34 g, 10 mmol) and
-bromododecane (5.98 g, 24 mmol) in acetone (20 mL).
1
The crude product was recrystallized twice from a mixture
of acetonitrile and acetone (3:1) and dried under reduced
pressure to give C -PPDA-C as a white solid (5.93 g,
1
3
prepared by injecting the appropriate amount of ethanolic
1
2
12
-3
7
1.27 % based on bis(3-(dimethylamino)propyl)terephtha-
1
pyrene solution (1 9 10 M, 0.1 % of the volume of the
gemini surfactants solution) into the gemini surfactant
solutions, followed by ultrasonication for 2 h. The temper-
ature was kept at 28 °C throughout the experiments.
lamide). H MNR (400 MHz, CDCl , TMS) d ppm: 0.86 (t,
3
J = 4.0 Hz, 6H, 2(CH ) CH ), 1.23–1.29 (m, 36H,
2
9
3
2
2
(CH ) CH ), 1.70 (m, 4H, 2CH (CH ) CH ), 2.19 (m, 4H,
2 9 3 2 2 9 3
NHCH CH CH N), 3.26 (s, 12H, 2 N(CH ) ), 3.42–3.51
3 2
2
2
2
(
m, 8H, 2 NCH CH (CH ) CH , 2NHCH CH CH N),
2
Foamability and Foam Stability Measurement
2
2
2 9
3
2
2
3
.71 (m, 4H, 2NHCH CH CH N), 7.91 (s, 4H at the ben-
2 2 2
1
zene ring), 8.67 (s, 2H, 2NH). C MNR (100 MHz,
3
A calibrated 100-mL glass cylinder with a stopper and an
aqueous solution containing 0.1 % gemini surfactant
CDCl , TMS) d ppm: 167.14, 136.15, 127.64, 64.55, 62.70,
3
1
23

J Surfact Deterg
(
20 mL) was used for the measurement of foamability and
foam stability. The solution was shaken vigorously for 10 s
at room temperature by an inverted cylinder test and the
foam height measured [21]. Foam stability was assessed
using the time required for the foam to collapse to half its
initial height (half-life) [22, 23]. The experiments were
repeated at least five times.
Emulsion Stability Measurement
Emulsion stability measurement was performed using a
graduated cylinder with a plastic stopper and standard
ground-glass joint [21]. Emulsions were prepared by mix-
ing 20 mL of 0.1 % aqueous gemini surfactant solutions
with 20 mL of paraffin oil. The stoppered graduated
cylinder was inverted up and down five times successively
and allowed to equilibrate for 1 min. In order to get a good
reproducibility and high reliability, the experiment was
repeated five times, and the time for the separation of
Fig. 1 Surface tension (c) vs the concentrations (C) plot for the
gemini surfactants at 301.15 K
concentration for the new gemini surfactants at 301.15 K
are shown in Fig. 1. The CMC and cCMC values are sum-
marized in Table 1.
1
0 mL water was recorded in seconds [24]. The measure-
ment was made at room temperature.
As shown in Table 1, the CMC values of C -IPDA-
1
2
C , C -IPDA-C and C -IPDA-C are 1.17, 0.218 and
1
2
14
14
16
16
Results and Discussion
0.0601 mmol/L at 301.15 K, respectively, indicating that
the gemini surfactants easily form micelles in the bulk
solution. Obviously, the gemini surfactants with longer
hydrophobic chain length have a lower CMC value. The
CMC of conventional ionic surfactants is known to
decrease with increasing number of carbon atoms in the
hydrophobic groups [26]. For comparison, the CMC values
of two cationic single-chain surfactants, CTAB (hexade-
cyltrimethylammonium bromide) and DTAB (dode-
cyltrimethylammonium bromide), were also measured as
0.846 and 15.31 mmol/L under the same conditions. Evi-
dently the CMC values for the synthesized gemini surfac-
tants were about one order of magnitude lower than those
of the single-chain surfactants with the same number of
carbon atom in the hydrophobic groups. In this work, the
CMC of the gemini surfactants decrease from 1.17 to
0.0601 mmol/L with increasing number of carbon atoms in
the hydrocarbon chain from 12 to 16, which is similar to
common surfactants in a homologous serious [27]. The
data indicate that the gemini surfactants with rigid spacers
have excellent micelle forming ability.
Synthesis
The synthetic route for preparing the gemini surfactants (C -
1
2
PPDA-C , C -PPDA-C C -PPDA-C ) is shown in
16
1
2
14
14,
16
Scheme 1. The three surfactants were easily prepared through
a two-step procedure from the starting materials. First, the
aminolysis reaction of commercially available dimethyl
terephthalate (1 equiv) with an excess molar ratio of N, N-
dimethylpropylene diamine (4equiv)generatedthe important
intermediate bis(3-(dimethylamino)propyl)terephthalamide
(Compound A). Subsequently, the target compounds C -
12
PPDA-C , C -PPDA-C and C -PPDA-C were pre-
1
2
14
14
16
16
pared at a high yield of 75.84–80.51 % by quaternization of
Compound A with alkyl bromide (1-bromododecane,
1
-bromotetradecane, 1-bromohexadecane) in the presence of
acetone, respectively. Their chemical structures were con-
1
13
firmed by H- and C-NMR spectroscopy, infrared spec-
troscopy and elemental analysis.
The Surface Active Properties of the Gemini
Surfactants
The surface tension at the CMC (cCMC) can be obtained
directly from Fig. 1 and the values are listed in Table 1.
The values of cCMC for these gemini surfactants in aqueous
solutions are 36.31, 40.89 and 43.41 mN/m at 301.15 K,
respectively. The data showed that the cCMC values
increased from 36.31 to 43.41 mN/m with increasing
number of carbon atom in the hydrocarbon chain from 12
to 16. Another important parameter (pC ) of surfactants is
The measurement of surface tension is a classical method
of studying the surface active properties of surfactants. The
critical micelle concentration (CMC) and the surface ten-
sion at the CMC (cCMC) are important parameters for
characterizing the surface activity of a surfactant [25]. The
curves of the surface tension as a function of the
2
0
also listed in Table 1. The pC20 represents the value of
1
23

J Surfact Deterg
Table 1 Surface active properties and parameters of the micellization of gemini surfactants
a
b
2
2
ꢀ
DG
m
Compounds
CMC (mM)
CMC (mM)
cCMC (mN/m)
C
max (lmol/m )
A
2
min (nm )
pC20
a
(KJ/mol)
2
3
3
C
C
C
a
12-PPDA-C12
14-PPDA-C14
16-PPDA-C16
1.17 ± 0.05
0.218 ± 0.01
0.0601 ± 0.002
1.253 ± 0.06
0.26 ± 0.02
36.31
40.89
43.41
2.22
1.86
1.16
1.48
1.24
0.77
0.75
0.89
1.43
1.12
1.34
2.16
3.57
4.13
4.90
0.4820
0.4408
0.3629
-54.54
-65.08
-76.93
0.0754 ± 0.003
Measured by tensiometry
b
Measured by conductometry
logarithm surfactant concentration required to reduce the
surface tension of water by 20 mN/m. The pC20 value
indicates the adsorption efficiency of surfactants at the air–
water interface, the larger the pC20 value, the more sur-
factant adsorbed at the air–water interface [28]. As shown
in Table 1, the pC20 values of the synthesized gemini
surfactants are 3.57, 4.13 and 4.90 at 28 °C, respectively. It
is obvious that the adsorption efficiency increases with
increasing length of the hydrophobic chain. In other words,
the gemini surfactant concentration required for reducing
the surface tension of water by 20 mN/m is decreased.
The packing density of surfactants at the air–water
interface is important to interpret the surface activities of
surfactants [29]. The surface area (Amin) occupied by the
gemini surfactant molecules at the air–water interface is a
measure of their packing density [30]. The surface excess
concentration (Cmax) and the area occupied by a surfactant
molecule (Amin) at the air–water interface were calculated
using the Gibbs adsorption Eqs. (1) and (2) [31, 32] as
following:
longer hydrophobic chains are not vertically aligned at the
air–water surface and make the Amin values larger [29].
The Degree of Counter Ion Dissociation (a)
and Thermodynamic Properties of Micellization
The CMC values of the synthesized gemini surfactants in
aqueous solutions were also investigated by conductivity
measurements at 301.15 ± 0.1 K. As shown in Fig. 2, the
CMC values were determined as the sharp break points in
the plots of specific conductance (j) versus gemini sur-
factants concentration (c) [19], and the degree of counte-
rion dissociation (a) was determined from the pre-micellar
slope (S = dj/dc) to the post-micellar slope (S = dj/dc),
1
2
a = S /S [35, 36]. The values of CMC and a for the
2
1
synthesized gemini surfactants are tabulated in Table 1,
respectively. As shown in Table 1, the CMC values of the
three synthesized gemini surfactants measured by the
conductometry are 1.252, 0.26 and 0.0754 mmol/L at
301.15 K, respectively. It should be noted that the CMC
ꢀ
ꢁꢀ
ꢁ
1
dc
d log C
results determined by tensiometry are nearly identical those
determined by the electrical conductivity method. The data
in Table 1 show that the a values of gemini surfactant
decrease significantly with increasing hydrophobic tail
length for the gemini surfactants. The long hydrophobic
tails favor the formation of micelles and result in a
decrease in CMC values.
Cmax ¼ À
ð1Þ
ð2Þ
2
:303 nRT
1
016
Amin ¼
NACmax
where R is the gas constant (8.314 J/mol K), T is the
absolute temperature (K), C is the surfactant concentration
The CMC and a values calculated for all the synthetic
gemini surfactants at 28 °C in aqueous solutions have been
used to calculate the different thermodynamic parameters
(
mol/L), dc/dlogC is the slope below the CMC in the sur-
face tension vs concentration curves, N is Avogadro’s
A
2
3
-1
number (6.02 9 10 mol ). The values of n = 2 and 3
the number of species at the interface whose concentration
of micellization. The standard Gibbs energies of micel-
ꢀ
(
lization for ionic surfactants DG was calculated using
Eq. (3):
changes with the surfactant concentration) are used for
calculating the Cmax values [16, 33, 34]. The calculation
results of Cmax and Amin are given in Table 1 for each
gemini surfactant.
m
ꢀ
m
DG ¼ RTð3 À 2aÞ ln xCMC
ð3Þ
where XCMC is the CMC in mole fraction, R is the gas
constant and T is temperature on the Kelvin scale. The
XCMC is calculated by using the equation of XCMC = CMC/
As shown in Table 1, the values of Cmax of these gemini
surfactants decreases with increasing the length of the
hydrocarbon chains from 12 to 16 and the values of Amin
increase, which suggested that the gemini surfactants with
the longer hydrophobic chains had lower packing densities
at the air–water interface. A possible explanation is that the
(
CMC ? number of moles for the solvent).
ꢀ
The DG values for all the synthetic gemini surfactants
m
ꢀ
in aqueous solutions are also listed in Table 1. The DGm
1
23

J Surfact Deterg
Fig. 2 Electrical conductivity (J) versus the concentration (C) for a C12-PPDA-C12, b C14-PPDA-C14 and c C16-PPDA-C16 at 301.15 K
values of C -PPDA-C , C -PPDA-C and C -PPDA-
1
exceeds the CMC, there is an obvious decrease in I /I for
1 3
2
12
14
14
16
C16 are -54.54, -65.08 and -76.93 kJ/mol at 301.15 K,
respectively, indicating that the process of micellization is
spontaneous. The results also revealed that the driving
force for the formation of micelles increases with
increasing carbon atoms in the hydrocarbon chain, which
all the gemini surfactant solutions, which demonstrates that
the microenvironment has changed due to the formation of
aggregates. When gemini surfactant molecules start to form
micelles, the extreme hydrophobic pyrene molecules are
transferred from the water environment to the hydrophobic
core. As shown in Fig. 3a, the minimum I /I values for all
ꢀ
leads to a DG value for C -PPDA-C .
m
16
16
1 3
three gemini surfactants decrease with increasing length of
the hydrocarbon chains, indicating that hydrophobic tail
length has a large effect on the aggregate microenviron-
ment. The aggregate micropolarity can be attributed to
specific molecular structures. The long and rigid spacer
with a certain rigidity and hydrophilicity may increase the
steric hindrance of the self-assembly procedure and hinder
the gemini surfactant molecules from close packing [16].
As shown in Fig. 3b, the pyrene fluorescent intensities
of C12-PPDA-C12 and C14-PPDA-C14 are far greater than
that of C16-PPDA-C16 at 0.1 mmol/L, which results from
the change in microenvironment due to the micellization of
Micropolarity and Aggregation
Pyrene is often used as a probe to investigate gemini sur-
factant aggregates in aqueous solution [28, 37]. The spec-
tral profiles and intensity of the photoluminescence (PL)
spectra for pyrene is sensitive to its microenvironment at
the site of fluorophore solubilization. The intensity ratio of
the first peak to the third peak (I /I ) can be taken as a
1
3
measure of the microenvironment polarity, which is high in
polar media and low in hydrophobic environments. When
the gemini surfactant concentration is lower than the CMC,
the intensity ratio I /I value remains essentially constant,
C
16-PPDA-C16 in aqueous solution. For C12-PPDA-C12
1
3
which indicates that surfactant molecules exist as mono-
mers in aqueous solutions, and the microenvironment does
not change. When the gemini surfactant concentrations
or C14-PPDA-C14, the concentration is lower than the
CMC, so micelles can not form in the bulk. But for C16-
PPDA-C16, the concentration exceeds the CMC and great
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J Surfact Deterg
Fig. 3 a Pyrene fluorescence intensity ratio I1/I3 vs the concentrations (C) for the Gemini Surfactants at 301.15 K and b the fluorescent intensity
of pyrene in aqueous solutions with different gemini surfactants at the concentration of 0.1 mmol/L
Table 2 Surfactants properties of the synthesized gemini surfactants in aqueous solutions
Compounds
0.5 CMC
CMC
2 CMC
Emulsion stability
time) (s)
(
Initial foam
height (mL)
Half-life
(s)
Initial foam
height (mL)
Half-life
(s)
Initial foam
height (mL)
Half-life
(s)
C12-PPDA-C12
C14-PPDA-C14
C16-PPDA-C16
15 ± 1
13 ± 1
202 ± 6
410 ± 9
472 ± 10
28 ± 2
26.6 ± 1.0
4.4 ± 0.3
216 ± 7
456 ± 8
503 ± 11
33 ± 2
30 ± 1
232 ± 7
476 ± 9
528 ± 11
350 ± 8
586 ± 10
637 ± 13
4.3 ± 0.2
4.5 ± 0.5
number of micelles and aggregates have been formed,
which results in the formation of a hydrophobic microen-
vironment. The hydrophobic pyrene molecules are solubi-
lized in the hydrophobic microdomain which leads to a
decrease in microenvironment polarity [38] and pyrene
fluorescence intensity.
driving force which increases bilayer adsorption and thus
the formation of stable films.
As shown in Table 2, increasing the hydrocarbon chain
length from 12 to 14, the initial foam height gradually
decreases and the half-life of foam collapse shows a
gradual increase. In addition, when the concentration of
gemini surfactant increases from 0.5 CMC to 2 CMC, both
the initial foam height and the half-life of foam are grad-
ually increased. In comparison with other synthesized
gemini surfactants, the lowest foam-producing ability is
C -IPDA-C , which is similar to common homologous
Foamability and Foam Stability
Many authors have suggested that foamability and foam
stability of gemini surfactants can be attributed to proper-
ties of the interfacial film and the interfacial tension [39–
1
6
16
surfactants [41]. However, foam stability improves as the
hydrocarbon chain length increasing from 12 to 16. The
low foamability and foam stability of these synthetic
gemini surfactants is not desirable. It is probably due to the
presence of the rigid spacer linking group, which consid-
erably increases the area per molecule which produces less
cohesive force at the interface.
4
1]. When generating foam of the same total surface area,
low surface tension benefits foam formation, and it is also
good for foam rupture (foam instability). Foam stability is
mainly due to elasticity of the interfacial film that separates
the two phases. In recent reports [42, 43], we demonstrated
that foam stability correlates well with the high limit
interfacial elasticity of the interfacial film. This agrees with
the finding by Sonin et al. [44] who reported that the high-
frequency interfacial elasticity plays an important role in
the film thinning process. Under aerated conditions, sur-
factants adsorb to form bilayers with the hydrophilic head
groups forming a hydration layer within the liquid film.
Increasing the chain length increases the hydrophobic
Emulsion Stability
The stability of paraffin oil-in-water emulsions in the
presence of the three gemini surfactants was investigated.
Emulsification relies on surfactant adsorption forming a
protective film at the surface of the dispersed droplets
1
23

J Surfact Deterg
which can delay or prevent coalescence. Cationic gemini
surfactants enhance stability by imparting an electrostatic
charge on the droplet surface thus reducing the physical
contact between the droplets. The amount of surfactant
adsorption at the oil–water interface in O/W emulsions
depends on the degree to which the hydrophobic groups of
the gemini surfactants damage the structure of the water
phase. Therefore, the stability of the emulsion increases
with increasing length on the hydrophobic chain. As shown
in Table 2, for C -PPDA-C , it takes 350 s for separation
14. Nieh MP, Kumar SK, Fernando RH, Colby RH, Katsaras J
(
2004) Effect of the hydrophilic size on the structural phases of
aqueous nonionic gemini surfactant solutions. Langmuir
0:9061–9068
2
15. Zhu DY, Cheng F, Chen Yu et al (2012) Preparation, charac-
terization and properties of anionic gemini surfactants with long
rigid or semi-rigid spacers. Colloids Surf A Physicochem Eng
Asp 397:1–7
1
6. Zhang Q, Gao ZN, Tai SX et al (2012) Surface tension and
aggregation properties of novel cationic gemini surfactants with
diethylammonium headgroups and a diamido spacer. Langmuir
2
8:11979–11987
1
2
12
1
7. Hu DH, Guo XF, Jia LH (2013) Synthesis, surface active prop-
erties of novel gemini surfactants with amide groups and rigid
spacers. J Surfactants Deterg 16:913–919
of the aqueous layer from the emulsion, whereas for C -
4
1
PPDA-C14 and C -PPDA-C 586 and 637 s is required,
16
16,
respectively. Thus, the emulsion stability increases with
increasing the length of the hydrophobic tail from 12 to 16.
18. Cui ZG, Yang LL, Cui YZ, Binks BP (2010) Effects of surfactant
structure on the phase inversion of emulsions stabilized by
mixtures of silica nanoparticles and cationic surfactant. Langmuir
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6:4717–4724
Acknowledgments We thank the Research Funds of Anhui Science
and Technology University (ZRC2014432, AKZDXK2015A01,
ZRC2014401) and the Natural Science Foundation of Anhui Province
1
2
9. Williams R, Phillips JN, Mysels KJ (1955) The critical micelle
concentration of sodium lauryl sulphate at 25 °C. Trans Faraday
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on vibronic band intensities in pyrene monomer fluorescence and
their application in studies of micellar systems. J Am Chem Soc
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publishing house of light industry, Beijing, pp 456–462
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gemini surfactants: physical–chemical properties. Langmuir
(
1508085QB41) for financial support of this work.
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Ting Zhang is a lecturer at the School of Chemistry and Material
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