Synthesis, surface activity and thermodynamic properties of cationic gemini surfactants with diester and rigid spacers

Article abstract of DOI:10.1016/j.molliq.2017.01.063

A group of cationic gemini surfactants with diester and rigid spacers, 12-Ph-12, 14-Ph-14, 16-Ph-16, were synthesized and confirmed by IR, ¹H NMR and elemental analysis. The surface parameters of the synthesized gemini surfactants including: the critical micelle concentration (CMC), the surface tension value at CMC (γ_CMC), effectiveness (π_CMC), efficiency (pC₂₀), maximum surface excess (Γ_max) and minimum surface area (A_min), were obtained from surface tension measurements. These gemini surfactants showed higher surface activity than the traditional monomeric surfactants. The thermodynamic parameters of micellization process from conductivity measurements showed that the micellization was a spontaneous and exothermic process in environment, and the micellization process becomes less favorable with the decrease of alkyl chain length and increase in temperature. The micropolarity was evaluated from steady fluorescence spectra, and the dye solubilization was investigated by using UV–visible spectroscopy. The results revealed that the micropolarity of surfactant micelles is very low and these gemini surfactants can really enhance the solubility of water insoluble dyes.

Full text of DOI:10.1016/j.molliq.2017.01.063

Accepted Manuscript
Synthesis, surface activity and thermodynamic properties of
cationic gemini surfactants with diester and rigid spacers
Ting Zhang, Xianfei Cao, Xuchun Wang, Changchun Song
PII:
S0167-7322(16)33402-X
doi: 10.1016/j.molliq.2017.01.063
MOLLIQ 6865
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
30 October 2016
13 January 2017
18 January 2017
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Synthesis, surface activity and thermodynamic properties of cationic gemini surfactants
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ACCEPTED MANUSCRIPT
Synthesis, surface activity and thermodynamic properties of cationic
gemini surfactants with diester and rigid spacers

Ting Zhang Xianfei Cao Xuchun Wang Changchun Song
School of Chemistry and Material Engineering, Anhui Science and Technology University,
Fengyang 233100, Anhui, China
γ
π
Γ
Keywords Gemini surfactants, Diester and rigid spacers, Surface activity properties,
Thermodynamic parameters, Dye solubilization
1
. Introduction
Gemini surfactants represent a novel class of surfactants, which are made up of
two conventional monomeric surfactant units linked by a flexible or rigid spacer at or
very close to the head groups. It has been found that gemini surfactants can exhibit
superior solution properties compared to the corresponding single-chained surfactants
in many aspects, such as lower critical micelle concentration (CMC), more efficient in
lowering the surface tension of water, better emulsifying properties and unusual
aggregation morphologies [1-5]. Owing to their unique performance, the use of gemini
surfactants covers the substantial homogeneous catalytic reactions along with
solubilisation, emulsification, and many more technical applications in industries such
as industrial detergency, gene transfection, micellar catalysis and corrosion inhibitors
[
6-9].
Corresponding author. Tel.: +86 550 6732096; Fax: +86 550 6733389
E-mail address: tingzhang84@163.com (T. Zhang)
1

ACCEPTED MANUSCRIPT
In recent years, there has been a particular interest in gemini surfactants with rigid
spacers, partly because they possess particular performance and broad application
prospects [10-16]. Xie et al. [15] reported two novel carboxylate gemini surfactants
with a long and rigid spacer, and found that the gemini surfactants containing rigid
group spacer could form large size aggregation and spherical micelles in aqueous
solution. Zhu et al. [16] synthesized four anionic gemini surfactants with long rigid or
semi-rigid spacers and investigated their surface and aggregation properties in water,
all the gemini surfactants possessed high surface activities and excellent micelle
forming ability. Obviously, the existence of a rigid spacer plays an important role in
the properties of gemini surfactants.
Enlightened by the advantages of inserting the rigid spacers in the structure of
gemini surfactants, herein in this paper, we designed and synthesized a series of
cationic gemini surfactants with diester and rigid spacers (Scheme 1). The diester
group was also introduced to the spacers of our gemini surfactants with the expectation
that the properties of gemini surfactants can be further improved. The surface tension,
thermodynamic properties of micellization and micropolarity for the gemini
surfactants, which are essential for the prediction of applications, have been measured
by tensiometry, conductometry and steady-state fluorescence. Moreover, the effect of
different concentrations of gemini surfactants on the dye solubilization was also
investigated by using UV-visible spectroscopy.
Scheme 1. Synthetic routes for the gemini surfactants.
2
. Experimental Section
2
.1 Materials
1
-bromododecane (>98%), 1-bromotetradecane (>98%), 1-bromohexadecane
(>98%), acetone, dimethylamine (33% aqueous solution), chloroacetyl chloride
(≥98%), ethyl acetate, ethanol were purchased from Sinopharm Chemical Reagent
Corporation (Shanghai, China). Pyrene, Methyl red and Hydroquinone (>98%) were
purchased from Sigma-Aldrich and used as received without further purification.
Ultrapure water was used in all experiments.
1
H NMR spectra were recorded on a Bruker AV 400 NMR Spectrometer (Bruker
Corporation, Switzerland, 400 MHz). Elemental analyses were carried out on a vario
EL III Element Analyzer (Elementer Corporation, Germany). The infrared (IR)
spectra were measured on a Japan FT/IR-430 spectrometer as KBr pallets.
2

ACCEPTED MANUSCRIPT
2
.1.1 Synthesis of 1,4-phenylene bis(2-chloroacetate) (1)
Chloracetyl chloride (0.21 mol, 23.73 g) was slowly added to a stirred solution of
hydroquinone (0.11 mol, 11.57 g) and pyridine (0.21 mol, 16.62 g) in 100 mL
dichloromethane at 0 ℃. Then, the reaction mixture was stirred for 3 h at 40 ℃ and
neutralized to a pH = 7 with saturated sodium bicarbonate solution. The organic layer
was separated, washed with water and dried over anhydrous magnesium sulfate. The
solvent was removed under vacuum and the residue was recrystallized from a mixture
solution of acetone and petroleum (3:1) to give compound 1 as white crystals (24.87
1
g, 93.13% yield based on hydroquinone). H NMR (400 MHz CDCl , TMS) δ ppm:
4
8
3
-1
.30 (s, 4H), 7.16 (s, 4H); IR (KBr) cm : 3033, 2953, 1761, 1507, 1410, 1312, 942,
61, 814, 737, 684, 528;
2
.1.2 Synthesis of compounds 2-4
A mixture of 1-bromoalkane (0.10 mol) and dimethylamine (33% aqueous
solution, 0.50 mol, 5 equiv) in ethanol (40 mL) was added to a dried 100 mL 2-necked
flask. The solution was heated at 80 ℃for 24 h. After the reaction mixture was cooled
to room temperature, the solvent was removed under vacuum, and the residue was
poured into the saturated sodium bicarbonate solution (60 mL). The mixture was
extracted with ethyl acetate, and the extract phase was dried with anhydrous
magnesium sulfate. The solvent was removed by distillation under reduced pressure
and the solid residue was purified by distillation to give compounds 2-4 as colorless
oily products.
N,N-Dimethyldodecylamine (2, 20.15 g, 93.62% yield based on
1
1
-bromododecane). H NMR (400 MHz, CDCl , TMS) δ ppm: 0.86-0.90 (t, J = 8.0
3
Hz, 3H), 1.28-1.26 (d, J = 8.0 Hz, 18H), 1.47 (s, 2H), 2.23-2.30 (m, 8H); IR (KBr)
-1
cm : 2925, 2854, 1464, 721.
N,N-Dimethyltetradecylamine (3, 22.10 g, 91.08% yield based on
1
1
-bromotetradecane). H NMR (400 MHz, CDCl , TMS) δ ppm: 0.86-0.90 (t, J = 8.0
3
-1
Hz, 3H), 1.26 (s, 22H), 1.47 (s, 2H), 2.23-2.30 (m, 8H); IR (KBr) cm : 2924, 2853,
1
1
465, 721.
N,N-Dimethylhexadecylamine (4, 24.24 g, 89.59% yield based on
-bromohexadecane). H NMR (400 MHz CDCl , TMS) δ ppm: 0.86-0.90 (t, J = 8.0
1
3
-1
Hz, 3H), 1.26 (s, 22H), 1.47 (s, 2H), 2.21-2.26 (m, 8H); IR (KBr) cm : 2924, 2853,
1
463, 720.
2
.1.3 Synthesis of gemini surfactants
A solution of 1,4-phenylene bis(2-chloroacetate) (10.00 mmol) and
N,N-dimethylalkylamine (44.00 mmol, 2.2 equiv) in ethyl acetate (40mL) was stirred
and refluxed for 8 h. Upon cooling to room temperature, a white precipitate was
formed. The precipitate was filtered and washed with plenty of ethyl acetate, then
dried under reduced pressure. The crude product was recrystallized two times from
acetone/acetonitrile (3:1) to afford the gemini surfactants as white crystals.
1
2-Ph-12 (5.86 g, 85.05 % yield based on 1,4-phenylene bis(2-chloroacetate)).
1
H NMR (400 MHz, CDCl , TMS) δ ppm: 0.86-0.90 (t, J = 8.0 Hz, 6H), 1.25-1.36
3
3

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(
m, 36H), 1.83 (s, 4H), 3.65 (s, 12H), 3.85-3.91 (t, J = 8.0 Hz, 4H), 5.97 (s, 4H), 6.94
-1
(s, 4H); IR (KBr) cm : 2920, 2851, 1767, 1503, 1465, 1398, 1160, 1016, 988, 892;
Anal. Calcd. for C H O N Cl %: C, 66.16; H, 10.23; N, 4.06. Found %: C, 65.98;
3
8
70
4
2
2
H, 10.28; N, 4.01.
1
4-Ph-14 (6.49 g, 87.11 % yield based on 1,4-phenylene bis(2-chloroacetate)).
1
H NMR (400 MHz, CDCl , TMS) δ ppm: 0.86-0.90 (t, J = 8.0 Hz，6H), 1.25-1.36
3
(
m, 44H), 1.83 (s, 4H), 3.65 (s, 12H), 3.85 (s, 4H), 5.95 (s, 4H), 6.87 (s, 4H); IR
-1
(KBr) cm : 2920, 2850, 1767, 1504, 1465, 1399, 1158, 1016, 996, 892; Anal. Calcd.
for C H O N Cl %: C, 67.62; H, 10.54; N, 3.76. Found %: C, 67.04; H, 10.59; N,
4
2
78
4
2
2
3
.71.
1
6-Ph-16 (7.11 g, 88.76 % yield based on 1,4-phenylene bis(2-chloroacetate)).
1
H NMR (400 MHz, CDCl , TMS) δ ppm: 0.89-0.90 (t, J = 8.0 Hz，6H), 1.25-1.82
3
(
m, 52H), 1.82 (s, 4H), 3.63 (s, 12H), 3.88 (s, 4H), 5.92 (s, 4H), 6.88 (s, 4H); IR
-1
(KBr) cm : 2919, 2850, 1764, 1540, 1492, 1466, 1399, 1158, 1002, 983, 892; Anal.
Calcd. for C H O N Cl %: C, 68.88; H, 10.81; N, 3.49. Found %: C, 68.66; H,
4
6
86
4
2
2
1
2
2
0.86; N, 3.45.
.2 Methods
.2.1 Surface tension measurement
The surface tensions of the gemini surfactant solutions were measured with a
Wilhelmy plate technique (Krüss K20 Tensiometer, Germany) [17] at 298.15 ± 0.1 K,
and the surface tension values were measured at least three times. The surface tension
of pure water was obtained as being 71.99 ± 0.05 mN/m at 298.15 K. All solutions
were prepared with ultrapure water.
2
.2.2 Electrical conductivity measurement
The conductivity measurements were made by using a direct reading
conductometer, model S320 (Mettler-Toledo GmbH, Switzerland) at different
temperatures, KCl solution (0.1 M and 0.01 M) was used for calibration of
conductivity cell. The conductivity was recorded when its value fluctuated less than
1
% over a 5 min interval. The tested solution was added to a container kept in a
thermostat (THD 0506, NingBo TianHeng Instrument Factory, China) with the
temperature accuracy of ± 0.01℃. The concentration at the break point of the curve of
the conductivity versus concentration was the critical micelle concentration (CMC) of
gemini surfactants [18].
2
.2.3 Steady-state fluorescence measurement
Fluorescence measurements were performed using a F-4600 Fluorescence
spectrophotometer (Hitachi High-Technologies Corp, Tokyo Japan). Pyrene was used
as the fluorescence probe to investigate the micropolarity of gemini surfactant
solutions. The excitation wavelength was 336 nm, 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
4

ACCEPTED MANUSCRIPT
polarity of the pyrene-solubilizing medium (I /I ) [19]. All solutions were prepared by
1
3
-3
injecting the appropriate amount of ethanolic pyrene solution (1 × 10 M, 0.1 % of
the volume of the gemini surfactants solution) into the gemini surfactant solution,
followed by ultrasonication at 298.15 K for 2 h.
2
.2.4 Dye solubilization measurements
Batch tests for solubilization of methyl red (a water insoluble dye was used) in
various concentrations of gemini surfactant aqueous solutions were performed. Excess
amounts of methyl red were added to different concentration gemini surfactant
solutions which were stirred for 24 h at 298.15 K. Nondissolved methyl red was
filtered through 0.45 μm PVDF filters and the concentration of dissolved methyl red
was determined from absorption measurements at 520 nm (pH = 7) using a Shimadzu
UV-3600 spectrophotometer. A reference blank was prepared of the same surfactant
concentration to eliminate the effect of surfactant on UV absorbance. All
measurements were performed in triplicate and UV spectra of the methyl red in
gemini surfactant solutions were recorded in the wavelength range 250-600 nm.
3
. Results and discussion
3
.1 Synthesis
The general synthetic procedures for cationic gemini surfactants with diester and
rigid spacers (12-Ph-12, 14-Ph-14, 16-Ph-16) are outlined in Scheme 1. They were
easily prepared following the three-step procedure from the starting materials. First,
the commercially available hydroquinone was reacted with chloracetyl chloride in the
presence of pyridine to afford the key intermediate 1,4-phenylene bis(2-chloroacetate)
(1). Then alkylation of 1-bromoalkane (1-bromododecane, 1-bromotetradecane,
1
-bromohexadecane, 1 equiv) with an excess molar ratio of dimethylamine (5 equiv)
generated compounds 2, 3, and 4. Finally the gemini surfactants 12-Ph-12, 14-Ph-14
and 16-Ph-16 were prepared at a high yield of 85.05-88.76 % by quaternization of
1
,4-phenylene bis(2-chloroacetate) (1) with N,N-dimethylalkylamine (2, 3, 4),
respectively. The chemical structures of the synthesized gemini surfactants were
1
unambiguously characterized by H NMR spectroscopy, infrared spectroscopy and
elemental analysis.
3
.2 Surface activity of the gemini surfactants
The surface tension (γ) versus logarithm of concentration (C) profiles for the
gemini surfactant aqueous solutions were obtained at 298.15 K. As shown in Fig. 1,
the surface tension value decrease initially with the increase of gemini surfactants
concentration at the lower ranges and then achieve a relative stable value at higher
concentrations. The concentration at the break point is usually described as the CMC
of a gemini surfactant. The surface activity parameters of the gemini surfactants are
summarized in Table 1. The CMC values of 12-Ph-12, 14-Ph-14 and 16-Ph-16 are
1
.107, 0.2965 and 0.05530 mmol/L at 298.15 K, respectively, indicating excellent
micelle forming ability. Evidently, the CMC value gradually decreased with
5

ACCEPTED MANUSCRIPT
increasing number of carbon atoms of the alkyl chains from 12 to 16, which was often
observed in common ionic surfactants with a homologous compound. In comparison
with the traditional monomeric surfactants CTAB (hexadecyltrimethylammonium
bromide) and DTAB (dodecyltrimethylammonium bromide) under the same
conditions, the CMC values for these gemini surfactants are much lower than
traditional monomeric surfactants with the same hydrophobic chain. It demonstrated
that the surface activity of synthesized cationic gemini surfactants is superior to that
of traditional monomeric surfactants.
The important physicochemical parameters including the surface tension value at
cmc (γCMC), the minimum surface area (A ) occupied by the gemini surfactant
min
molecule, and the surface excess concentration (Γmax), are listed in Table 1 for each
gemini surfactant. The surface area (A ) occupied by the gemini surfactant
min
molecules at the air-water interface reflect their packing density [20]. The surface
excess concentration (Γmax) and the area occupied by a surfactant molecule (A ) at
min
the air-water interface were calculated by the Gibbs adsorption Equation (1) and (2)
[
21,22]:


1
 d

Γmax  




(1)
2.303nRT  d logC


T
1
6
10
Amin

(2)
N Γ
A
max
Where R is the gas constant (8.314 J/mol K), T is the absolute temperature (K),
C is the concentration of gemini surfactant (mol/L), dγ/dlogC is the slope below the
CMC in the surface tension vs. concentration curves, N is Avogadro’s number
A
2
3
-1
(
6.02×10 mol ). The values of n = 3 (the number of species at the interface whose
concentration changes with the surfactant concentration) are used for calculating the
Γmax values [23,24]. As shown in Table1, the Γmax values of these gemini surfactants
decrease, but the Amin values increase with the increase of the hydrocarbon chain from
1
2 to 16, which suggest the gemini surfactant with the longer hydrophobic chain has
lower packing densities at the air-water interface. The possible explanation is that the
longer hydrophobic chain is more prone to bend at the air/water interface [25].
From the surface tension curves, two other important parameters, the adsorption
efficiency (pC ) and the effectiveness of surface tension reduction (πCMC) can be also
2
0
obtained in Table 1. The former parameter is calculated by using the Equation (3):
pC  logC
(3)
2
0
20
Where C is the molar concentration of gemini surfactant (mol/L) and C stands
2
0
for the surfactant concentration required to reduce the surface tension of pure water
by 20 mN/m. The pC value reflects the adsorption efficiency of surfactant molecules
2
0
at the air-water interface, the greater the pC20 value is, the more efficient the
surfactant molecule adsorbs at the air-water interface. The other parameter, πCMC is
the surface pressure at CMC, which is defined as the following Equation (4):
6

ACCEPTED MANUSCRIPT
πCMC γ -γ
(4)
0
CMC
Where γ and γ
are the surface tension of water and the surfactant solution at
0
CMC
CMC, respectively. πCMC represents the maximum reduction of surface tension caused
by the dissolution of surfactant molecules, becomes an indication for the effectiveness
of gemini surfactant to reduce the surface tension of water [20]. Hence, the larger the
πCMC value is, the higher the effectiveness of gemini surfactant is. From the values of
pC and π
in Table 1, the pC values of the gemini surfactants are 3.20, 3.62 and
20
2
0
CMC
4
.26 at 298.15 K, respectively. It is clear that the adsorption efficiency increases with
increasing length of the hydrophobic chain. The πCMC values of each gemini
surfactants are 39.68, 36.99 and 35.51 at 298.15 K, respectively, which indicate that
the πCMC value decreases with increasing alkyl chain length.
In addition, the value of CMC/C ratio can be taken as a measure of structural
2
0
factors in the adsorption and micellization process. The gemini surfactant with lower
CMC/C ratio has greater tendency to form micelles than to adsorb at the interface in
20
aqueous solution. As shown in Table 1, it is clear that the gemini surfactants with
longer hydrophobic chain length were easier to self-assemble than adsorb at the
air-water interface in aqueous solution. The γCMC values of the gemini surfactants are
3
2.29, 34.98 and 36.46 mN/m at 298.15 K, respectively. The results show that the
γCMC values increased from 32.29 to 36.46 mN/m with increasing hydrophobic chain
from 12 to 16.
1
1
1
2-Ph-12
4-Ph-14
6-Ph-16
6
6
5
5
4
4
3
3
5
0
5
0
5
0
5
0
1
E-5
1E-4
C (mol/L)
1E-3
Fig. 1. Surface tension (γ) vs the concentration (C) plots for the gemini surfactants at 298.15 K.
Table 1
Surface active properties and parameters of the gemini surfactants at 298.15 K.
γCMC
Γmax
Amin
πCMC
a
b
Compounds
CMC (mM)
CMC (mM)
pC20
CMC/C20
2
2
(
mN/m)
(μmol/m )
(nm )
(mN/m)
1
1
1
2-Ph-12
4-Ph-14
6-Ph-16
a
1.107 ± 0.04
0.2965 ± 0.02
1.215 ± 0.02
0.3097 ± 0.01
32.29
34.98
36.46
1.74
1.44
1.02
0.96
0.12
1.63
3.20
3.62
4.26
39.68
36.99
35.51
1.75
1.24
1.01
0.05530 ± 0.005 0.06840 ± 0.004
b
Measured by tensiometry, Measured by conductometry
7

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3
.3 Thermodynamic properties of micellization
In order to investigate the micellar behavior of the gemini surfactants in aqueous
solution, the thermodynamic properties of micellization have been investigated by
electrical conductivity measurements at four different temperatures (298.15, 303.15,
3
08.15 and 313.15 K). As shown in Fig. 2, the CMC values were determined as the
sharp break points in the plots of specific conductance (κ) versus gemini surfactants
concentration (C) [18], and the degree of counterion dissociation (α) was determined
from the ratio of the pre-micellar slope (S = dκ/dC) to the post-micellar slope (S =
1
2
dκ/dC), α = S /S [26,27]. The values of CMC for each gemini surfactants measured
2
1
by the electrical conductivity measurement are 1.215, 0.3097 and 0.06840 mmol/L at
98.15 K, respectively, which are close to the CMC values measured by surface
2
tension measurements. The data in Table 2 show that the CMC values increase with
the temperature. This character is in agreement with that of the other typical ionic
surfactants. In addition, the α values decrease significantly with increasing
hydrophobic tail length for the gemini surfactants. The long hydrophobic tails favor to
form micelles and result in a decrease in CMC values.
o
The standard Gibbs free energy of the aggregation (ΔG mic) can be calculated by
the following equation (5) [28]:
o
mic
ΔG  RT (3-2)ln XCMC
(5)
Where XCMC is the CMC in mole fraction, R is the gas constant and T is
temperature on the Kelvin scale, respectively. The XCMC is calculated by using the
equation of XCMC = CMC /( CMC + number of moles for the solvent).
As shown in Table 2, the calculated negative values indicate that the
o
micellization is a spontaneous process and the ΔG mic of the investigated gemini
surfactants have been found to be hydrophobically driven force for the formation of
micelles increases with respect to increasing alkyl chain length, which leads to the
o
most negative of ΔG
2
value for 16-Ph-16. On increasing the temperature from
mic
o
98.15 K to 313.15 K, the absolute values of ΔG
are increased due to the lower
mic
stability of the micellized gemini surfactant molecules than the reely dispersed
molecules in the aqueous phase. The standard enthalpy change for micellization
o
process (ΔH mic) can be determined using the Gibbs-Helmholtz equation (6) [28]:
o
mic
2


 ln XCMC


(6)
ΔH  RT (3 2)
T

P
The values of lnxcmc in aqueous solutions were plotted against the temperature
and the slopes of these plots were taken as the values of dlnXCMC/dT. The calculated
o
ΔH
values at four temperatures in aqueous solutions are listed in Table 2. The
mic
o
values of standard enthalpy of micellization (ΔH mic) are negative for all the systems
studied. It means that the micellization process is exothermic in aqueous solutions.
o
mic
The negative value of the ΔH
is mostly becoming more negative with increase in
alkyl chain length of the gemini surfactants. It means that, the micellization process in
6-Ph-16 is more exothermic than 14-Ph-14 and 12-Ph-12 at the same temperature.
1
o
m
Furthermore, The standard entropy of micelle formation (ΔS ) can be calculated by
8

ACCEPTED MANUSCRIPT
the following equation (7) [28] and the values are summarized in Table 2.
o
mic
o
mic
(
H  G )
o
mic
ΔS

(7)
T
o
mic
From the observed data, one can see that the ΔS
values are positive in all
cases, suggesting that the entropy change is favorable to the formation of the micelles.
o
mic
The ΔS
tends to be less positive with the increasing temperature in range from
2
98.15 K to 313.15 K in aqueous solutions for the gemini surfactants, because the
surrounding iceberg structures of surfactant molecules diminished in size or became
less rigid as temperature increased. Thus the formation of micelles is difficult at
higher temperature.
1
1
1
400
200
000
500
(a)
(b)
2
3
3
98.15 K
03.15 K
08.15 K
298.15 K
303.15 K
308.15 K
313.15 K
4
3
00
00
313.15 K
8
6
4
2
00
00
00
00
0
200
1
00
0
0
1
2
3
4
0
.0
0.2
0.4
0.6
0.8
1.0
C (mmol/L)
C (mmol/L)
1
1
60 (c)
2
3
3
98.15 K
03.15 K
08.15 K
20
313.15 K
8
4
0
0
0
0
.00
0.05
0.10
C (mmol/L)
0.15
0.20
Fig. 2. Electrical conductivity (κ) versus the concentration (C) for (a) 12-Ph-12, (b) 14-Ph-14 and
c) 16-Ph-16 at various temperatures.
(
Table 2
Thermodynamic parameters for micellization of gemini surfactants in aqueous solutions at various
temperatures (T).
o
o
o
CMC
mM)
ΔG
ΔH
ΔS
mic
mic
mic
Compounds
T/K
α
(
(kJ/mol)
-42.34
(kJ/mol)
-15.72
(kJ/K/mol)
0.1948
2
3
3
3
98.15
03.15
08.15
13.15
1.215 ± 0.02
1.324 ± 0.02
1.397 ± 0.02
1.498 ± 0.02
0.3097 ± 0.01
0.3364 ± 0.01
0.7034
0.7368
0.7666
0.8004
0.5421
0.5763
1
2-Ph-12
-40.90
-39.79
-38.38
-57.40
-55.88
-15.57
-15.46
-15.23
-24.90
-25.45
0.1863
0.1793
0.1712
0.2629
0.2533
1
4-Ph-14
298.15
03.15
3
9

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3
3
08.15
13.15
0.3604 ± 0.01
0.3991 ± 0.01
0.6019
0.6404
0.6931
0.7236
0.7477
0.798
-54.96
-53.08
-54.39
-52.74
-51.68
-48.34
-25.08
-24.44
-28.70
-28.55
-28.58
-27.54
0.2465
0.2358
0.2787
0.2681
0.2605
0.2423
1
6-Ph-16
298.15
0.06840 ± 0.004
0.07690± 0.004
0.08330 ± 0.004
0.10190 ± 0.004
3
3
3
03.15
08.15
13.15
3
.4 Micropolarity
Fluorescence measurement is often employed to monitor the self-aggregation for
gemini surfactant in aqueous solution by using the emission of pyrene [3,29,30]. The
shape and intensity of the photoluminescence (PL) spectra for pyene is sensitive to its
microenvironment at the site of solubilization of the fluorophore. Once the gemini
surfactants begin to form micelles in aqueous solutions, the extreme hydrophobic
pyrene molecules are transferred from the water environment to the hydrophobic core
of micelles. Therefore, the fluorescence intensity ratio of the first to third band (I /I )
1
3
is commonly used to analyze the micropolarity of the micelle system in polar media,
reflecting the compactness of aggregates. Fig. 3 shows the variations of the pyrene
polarity ratio I /I with the change of the surfactant concentration. With increasing the
1
3
gemini surfactant concentration close to the CMC value, there is an obvious decrease
in I /I . Then the I /I value keeps constant as the concentration of surfactant increases
1
3
1 3
in aqueous solutions, which indicate that the aggregate structures are compact and the
microenvironment does not change. Moreover, the final I /I ratio for all three gemini
1
3
surfactants is almost the same values as that of a conventional anionic surfactant,
which indicates that the micropolarity of surfactant micelles is very low.
1
1
1
1
1
1
1
1
.38
.35
.32
.29
.26
.23
.20
.17
12-Ph-12
1
1
4-Ph-14
6-Ph-16
1
E-6
1E-5
1E-4
C (mol/L)
1E-3
Fig. 3. Variation of the pyrene intensity ratio I /I vs the concentrations (C) for the gemini
1
3
surfactants at 298.15 K.
3
.5 Dye solubilization
Dye solubilization of methyl red in various concentrations of gemini surfactant
aqueous solutions were performed by using the UV-visible spectroscopy. As shown by
the correlation of absorbance of methyl red at 520 nm in various concentrations of
gemini surfactant aqueous solutions in Fig. 4, the absorbance of methyl red at 520 nm
1
0

ACCEPTED MANUSCRIPT
increases with increasing the gemini surfactant concentration, which can be attributed
to the increased solubility of methyl red in aqueous solutions. The absorbance of
methyl red increases rapidly when the gemini surfactant concentration reaches the
CMC, and finally achieves a relatively stable value as the concentration of surfactant
increases in aqueous solutions. In addition, the dye solubilization rate of methyl red
for all three gemini surfactants decrease with increasing length of the hydrocarbon
chains, indicating that hydrophobic tail length has a certain effect on the
microenvironment. Based on the solubilization experiments, these gemini surfactants
can really enhance the solubility of water insoluble dyes, which may further extend
the potential application of dyes.
1
2-Ph-12
1
1
1
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
1
1
4-Ph-14
6-Ph-16
1
E-5
1E-4
1E-3
C (mol/L)
Fig. 4. Plot showing correlation of absorbance of methyl red in various concentrations of gemini
surfactant aqueous solutions (at λmax = 520 nm) at 298.15 K.
4
. Conclusions
The physicochemical properties of three cationic gemini surfactants with diester
and rigid spacers in aqueous solutions have been investigated. The surface activity of
gemini surfactants increases with increasing the hydrophobic part: the γCMC values
increased from 32.29 to 36.46 mN/m with increasing hydrophobic chain from 12 to
1
6. All the evaluated thermodynamic parameters show that the process of
micellization is spontaneous and exothermic in environment, and the micellization
process becomes less favorable with the decrease of alkyl chain length and increase in
temperature. These investigated micellar systems show low micropolarity of
surfactant micelles and solubilizing power toward hydrophobic compounds.
Acknowledgments
We thank the Natural Science Foundation of Anhui Province (KJ2016A184,
1
508085QB41), the Talent Foundation of Education Department of Anhui
(ZD2016176) and the Key Discipline Construction of Anhui Science and Technology
University (AKZDXK2015A01) for financial support of this work.
References
11

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[
[
[
1] R. Zana, Y. Talmon, Nature 362 (1993) 228-230.
2] P. Patial, A. Shaheen, I. Ahmad, J. Surfactant. Deterg. 16 (2013) 49-56.
3] D.Q. Xu, B.H. Qi, D. Fang, X.M. Zhang, T. Zhang, J. Surfactant. Deterg. 19
(
2016) 91-99.
4] B. Li, Q. Zhang, Y. Xi, Z.N. Gao, Colloids Surf. A Physicochem. Eng. Asp. 470
2015) 211-217.
[
(
[
[
[
5] D.H. Xie, J.X. Zhao, Langmuir 29 (2013) 545-553.
6] M.Q. Ao, G.Y. Xu, Y. Zhu,Y. Bai, J. Colloid Interface Sci. 326: (2008) 490-495.
7] H.C. Zhang, K. Li, H.J. Liang, J.J. Wang, Colloids Surf. A Physicochem. Eng. Asp.
3
29:75-81.
[
[
[
[
8] D.Q. Xu, Z.W. Pan, Chin. Chem. Lett. 25 (2014) 1169-1173.
9] T. Zhang, Z.W. Pan, H.X. Gao, J. Surfactant. Deterg. 18 (2015)1003-1009.
10] A.R. Tehrani-Bagha, K. Holmberg, Langmuir 26 (2010) 9276-9275.
11] M. Akram, I.A. Bhat, W.F. Bhat, Kabir-ud-Din, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy 150 (2015) 440-450.
[
[
12] S.M. Tawfik, J. Ind. and Eng. Chem. 28 (2015) 171-183.
13] H.L. Zhu, Z.Y. Hu, D. Liang, J.L. Wang, D.L. Cao, J. Mol. Liq. 216 (2016)
5
65-570.
[
[
[
14] D.H. Hu, X.F. Guo, L.H. Jia, J. Surfactant. Deterg. 16 (2013) 913-919.
15] D.H. Xie, J.X. Zhao, Langmuir 29 (2013) 545-553.
16] D.Y. Zhu, F. Cheng, Y. Chen, J.S. Chun, Colloids Surf. A Physicochem. Eng. Asp.
3
97 (2012) 1-7.
[
17] C. Faustinoa, C. Serafima, I. Ferreiraa, L. Pinheiro, Colloids Surf. A
Physicochem. Eng. Asp. 480 (2015) 426-432.
[
[
[
18] R.J. Williams, J.N. Phillips, K.J. Mysels, Trans. Faraday Soc. 51 (1955)728-737.
19] K. Kalyanasundaram, J.K. Thomas, J. Am. Chem. Soc. 99 (1977) 2039-2044
20] A. Laschewsky, L. Wattebled, M. Arotcareìna, J.L. Habib-Jiwan, R.H. Rakotoaly,
Langmuir 21 (2005) 7170-7179
[
[
[
21] E. Alami, G. Beinert, P. Marie, R. Zana, Langmuir 9 (1993) 1465-1467.
22] L.D. Song, M.J. Rosen, Langmuir 12 (1996) 1149-1153.
23] F. Devinsky, I. Lacko, F. Bittererova, L. Tomeckova, J. Colloid Interface Sci. 114
(1986) 314-322.
[
[
24] Z.X. Li, C.C. Don, R.K. Thomas, Langmuir 15 (1999) 4392-4396.
25] L.M. Zhou, X.H. Jiang, Y.T. Li, Z. Chen, X.Q. Hu, Langmuir 23 (2007)
11404-11408.
[
[
[
[
[
26] R. Zana, R. Benraou, R. Rueff, Langmuir 7 (1991) 1072-1075
27] R. Zana, Langmuir 12 (1996) 1208-1211.
28] A.K. Tiwari, Sonu, S.K. Saha, et al, J. Chem. Thermodyn. 70 (2014) 24-32.
29] S. Bhattacharya, J. Haldar, Langmuir 20 (2004) 7940-7947.
30] C.C. Ren, F. Wang, Z.Q. Zhang, H.H. Nie, N. Li, M, Cui, Colloids Surf. A
Physicochem. Eng. Asp. 467:(2015) 1-8.
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Highlights



A group of cationic gemini surfactants with diester and rigid spacers were
synthesized and characterized
The surface activity and thermodynamic properties of the micellization of
gemini surfactants were investigated.
The dye solubilization power of gemini surfactants was investigated by using
UV-visible spectroscopy
1
3