Critical micellar concentrations of quaternary ammonium surfactants with hydroxyethyl substituents on headgroups determined by isothermal titration calorimetry

Article abstract of DOI:10.1021/je100288k

The micelle formations in aqueous solutions of a set of quaternary ammonium surfactants, N-alkyl-N-2-hydroxyethyl-N,N-dimethyl ammonium bromide (C _nHDAB, n = 12, 14, and 16), N-alkyl-N,N-2-dihydroxyethyl-N-methyl ammonium bromide (C_nDHAB, n = 12, 14, and 16), along with N-dodecyl-N,N,N-trimethyl ammonium bromide (DTAB), and N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB), were investigated by the isothermal titration calorimetry (ITC). The critical micellar concentrations (CMCs) and the enthalpies of micellization at six different temperatures were determined. The entropies and Gibbs free energies of micellization were then calculated by using the pseudophase separation model. The effects on the thermodynamic data of the temperature, the length of the alkane chain, and the number of hydroxyethyl substituents on the surfactant headgroup were discussed. The CMC value of each surfactant increases slightly with the temperature increase. At a given temperature, both the increases of the length of the alkane chain and the number of hydroxyethyl substituents lead to the decreases of the CMC values.

Full text of DOI:10.1021/je100288k

3766
J. Chem. Eng. Data 2010, 55, 3766–3771
Critical Micellar Concentrations of Quaternary Ammonium Surfactants with
Hydroxyethyl Substituents on Headgroups Determined by Isothermal Titration
Calorimetry
Wei Tong,† Qing Zheng,† Shuang Shao,‡ Qunfang Lei,*,† and Wenjun Fang†
Department of Chemistry, Zhejiang University, Hangzhou 310027, China, and Department of Chemistry, Zhejiang Education
Institute, Hangzhou 310012, Zhejiang Province, China
The micelle formations in aqueous solutions of a set of quaternary ammonium surfactants, N-alkyl-N-2-
hydroxyethyl-N,N-dimethyl ammonium bromide (CnHDAB, n ) 12, 14, and 16), N-alkyl-N,N-2-
dihydroxyethyl-N-methyl ammonium bromide (CnDHAB, n ) 12, 14, and 16), along with N-dodecyl-N,N,N-
trimethyl ammonium bromide (DTAB), and N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB), were
investigated by the isothermal titration calorimetry (ITC). The critical micellar concentrations (CMCs) and
the enthalpies of micellization at six different temperatures were determined. The entropies and Gibbs free
energies of micellization were then calculated by using the pseudophase separation model. The effects on
the thermodynamic data of the temperature, the length of the alkane chain, and the number of hydroxyethyl
substituents on the surfactant headgroup were discussed. The CMC value of each surfactant increases slightly
with the temperature increase. At a given temperature, both the increases of the length of the alkane chain
and the number of hydroxyethyl substituents lead to the decreases of the CMC values.
group have been synthesized in this laboratory. To further
investigate the micellization properties of the surfactants, the
ITC method is used to determine the CMC values and the
enthalpies of micellization at different temperatures in the pres-
ent work. The corresponding changes in Gibbs free energy and
entropy of the micellization are calculated from the pseudophase
separation model. Furthermore, the influences on the thermo-
dynamic functions of the temperature, the length of the alkane
chain, and the substituent number on the headgroup are
discussed.
Introduction
Surfactants have widespread applications in fine industry,
medicine, biological systems, petroleum production, and soil
remediation techniques. Dispersed in water, surfactant molecules
can form different types of aggregations because of the
amphiphilic molecular structure, which usually consists of a
hydrophilic polar headgroup and a hydrophobic segment. When
forming micelles in aqueous solution, the hydrophilic head-
groups are located at the surface of the micelles, and the
hydrophobic segments are shielded from water. The thermo-
dynamics of the micellization process of surfactants in water
were widely studied with many different techniques, such as
light scattering,1-3 conductometry,4-6 surface tension measure-
ments,7-9 and calorimetry.10-12 More and more applications
of the isothermal titration calorimetry (ITC) are partly due to
the capability for the direct determination of the critical micelle
concentration (CMC) and the enthalpy of micellization (∆Hmic).
The change in Gibbs free energy (∆Gmic) can then be derived
by using different models. In the pseudophase separation model,
the micelle part is assumed to be treated as a separate phase,
and the value of ∆Gmic is calculated from the CMC data from
the following equation13,14
Experimental Section
Materials. Six quaternary ammonium surfactants, N-alkyl-
N-2-hydroxyethyl-N,N-dimethyl ammonium bromide (CnHDAB,
n ) 12, 14, and 16) and N-alkyl-N,N-2-dihydroxyethyl-N-methyl
ammonium bromide (CnDHAB, n ) 12, 14, and 16), were
synthesized from the reactions of 1-bromoalkane (1-bromo-
dodecane, 1-bromotetradecane, and 1-bromohexadecane) with
dimethylhydroxyethylamine and methyldihydroxyethylamine,
respectively, in acetone at about 60 °C for 6 h. They were
purified by using the recrystallization method with the mixed
solvents of acetone and methanol. The molecular structures were
examined by NMR (Bruker Advance 2B/400 Hz), IR spectrum
(NEXES 470), and elemental analysis (Carlabo EA1110).
N-Dodecyl-N,N,N-trimethyl ammonium bromide (DTAB) and
N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB) were
purchased from Sigma and used without further purification.
The chemical structures of the investigated surfactants are listed
in Table 1. All the surfactant solutions were prepared with
ultrapure water with the resistivity above 18.2 ·106 Ω·cm at
298.15 K produced by the Millipore Q3 system.
∆Gmic ) 2RT ln xCMC
(1)
where xCMC is the critical micelle concentration expressed as
mole fraction; T is the absolute temperature; and R is the gas
constant. ∆Smic can be evaluated by the Gibbs equation
∆Smic ) (∆Hmic - ∆Gmic)/T
(2)
Recently, several cationic surfactants with one or two
hydroxyethyl substituents on the quaternary ammonium head-
Calorimetric Method. The calorimetric experiments were
performed with a VP-ITC (Micro-Cal, USA), which has the
precision of 0.5 ncal ·s-1. The calorimeter has a sample cell
with the fixed volume of 1.45 mL and a gastight syringe with
* Corresponding author. Tel.: +86 571 87952371. Fax: +86 571 87951895.
E-mail: qflei@zju.edu.cn.
† Zhejiang University.
‡ Zhejiang Education Institute.
10.1021/je100288k  2010 American Chemical Society
Published on Web 06/10/2010

Journal of Chemical & Engineering Data, Vol. 55, No. 9, 2010 3767
Table 1. Nomenclature and Molecular Structures of the Cationic Surfactants
the maximum volume of 250 µL. The sample cell was filled
with ultrapure water. The surfactant solution as titrant was added
into the sample cell step by step through the syringe. The
number of injections, the volume of each injection, and the time
interval between each injection were controlled automatically
with software. The ITC experiments were carried out at
temperatures between (298.15 and 323.15) K in a 5 K interval.
Elemental analysis results for C19H42NO2Br in mass %: N, 3.51;
C, 58.11; H, 10.92. Calculated: N, 3.53; C, 57.56; H, 10.68.
C16DHAB. 1H NMR (CDCl3, 400 MHz, 298 K) δ: 0.86 (t, J
) 6.8 Hz, 3H), 1.22 to 1.3 (m, 26H), 1.66 to 1.80 (m, 2H),
3.31 (s, 3H), 3.52 (t, J ) 8.0 Hz, 2H), 3.69 to 3.79 (m, 4H),
4.04 to 4.10 (m, 4H), 4.93 to 5.04 (m, 2H). IR (KBr): ν (3293,
2919, 2851, 1465, 1380, 1086, 1051, 917, 728, 629) cm-1
.
Elemental analysis results for C21H46NO2Br in mass %: N, 3.56;
C, 60.04; H, 11.45. Calculated: N, 3.30; C, 59.42; H, 10.92.
Data from ITC Measurements. As an example, Figure 1
shows the representative isothermal titration curve obtained from
the dilution of a concentrated solution of CTAB into water at
298.15 K. The enthalpy of dilution of CTAB as a function of
the CTAB concentration is given in Figure 2, which can be
described in three different concentration regions. In the first
Results and Discussion
Structural Characterization of the Synthesized Surfactants. The
molecular structures of the synthesized surfactants were exam-
ined by NMR, IR spectrum, and elemental analysis with the
following results.
C12HDAB. 1H NMR (CDCl3, 400 MHz, 298 K) δ: 0.86 (t, J
) 6.8 Hz, 3H), 1.23 to 1.33 (m, 18H), 1.68 to 1.78 (m, 2H),
3.36 (s, 6H), 3.52 (t, J ) 8.4 Hz, 2H), 3.75 (t, J ) 4.6 Hz, 2H),
4.08 to 4.14 (m, 2H), 4.29 to 4.50 (m, 1H). IR (KBr): ν (3242,
region, at surfactant concentrations up to 0.8 ·10-3 mol·L-1
,
below the CMC value, the relatively large and constant
enthalpies are due to the dilution of micelles, demicellization
of micelles, and dilution of the resultant monomers. The second
region, in which the endothermic effect decreases sharply,
indicates that the CMC value of the surfactant is reached. The
enthalpic effect in the third concentration range from ap-
proximately 1.2 ·10-3 mol·L-1 becomes small and keeps almost
constant. It is because the system is concentrated enough and
the surfactant molecules mainly exist in the form of micelles.
The small enthalpic effects result from the dilution of micelles
and no demicellization of micelles. The difference in enthalpy
between the first and the third concentration region should be
2918, 2850, 1477, 1383, 1077, 1053, 965, 911, 728, 716) cm-1
.
Elemental analysis results for C16H36NOBr in mass %: N, 4.15;
C, 56.70; H, 10.75. Calculated: N, 4.14; C, 56.79; H, 10.72.
C14HDAB. 1H NMR (CDCl3, 400 MHz, 298 K) δ: 0.85 (t, J
) 6.8 Hz, 3H), 1.22 to 1.31 (m, 22H), 1.71 to 1.74 (m, 2H),
3.35 (s, 6H), 3.52 (t, J ) 8.4 Hz, 2H), 3.79 (t, J ) 4.6 Hz, 2H),
4.09 to 4.13 (m, 2H), 4.63 to 4.68 (m, 1H). IR (KBr): ν (3243,
2918, 2850, 1476, 1383, 1080, 1056, 917, 731, 602) cm-1
.
Elemental analysis results for C18H40NOBr in mass %: N, 3.86;
C, 59.32; H, 11.22. Calculated: N, 3.81; C, 59.00; H, 11.00.
C16HDAB. 1H NMR (CDCl3, 400 MHz, 298 K) δ: 0.85 (t, J
) 6.6 Hz, 3H), 1.22 to 1.32 (m, 26H), 1.70 to 1.75 (m, 2H),
3.35 (s, 6H), 3.52 (t, J ) 8.2 Hz, 2H), 3.73 (t, J ) 4.6 Hz, 2H),
4.08 to 4.12 (m, 2H), 4.30 to 4.54 (m, 1H). IR (KBr): ν (3244,
2917, 2849, 1470, 1383, 1081, 1053, 965, 720) cm-1. Elemental
analysis results for C20H44NOBr in mass %: N, 3.52; C, 61.27;
H, 11.47. Calculated: N, 3.55; C, 60.90; H, 11.24.
C12DHAB. 1H NMR (CDCl3, 400 MHz, 298 K) δ: 0.84 (t, J
) 6.8 Hz, 3H), 1.21 to 1.35 (m, 18H), 1.64 to 1.79 (m, 2H),
3.37 (s, 3H), 3.52 (t, J ) 8.0 Hz, 2H), 3.66 to 3.80 (m, 4H),
4.03 to 4.15 (m, 4H), 4.59 to 4.77 (m, 2H). IR (KBr): ν (3260,
2918, 2850, 1469, 1384, 1057, 964, 721) cm-1. Elemental
analysis results for C17H38NO2Br in mass %: N, 3.87; C, 55.50;
H, 10.54. Calculated: N, 3.80; C, 55.43; H, 10.40.
C14DHAB. 1H NMR (CDCl3, 400 MHz, 298 K) δ: 0.86 (t, J
) 6.8 Hz, 3H), 1.23 to 1.36 (m, 22H), 1.67 to 1.79 (m, 2H),
3.34 (s, 3H), 3.54 (t, J ) 7.6 Hz, 2H), 3.68 to 3.82 (m, 4H),
4.11 to 4.18 (m, 4H), 4.46 to 4.57 (m, 2H). IR (KBr): ν (3294,
2920, 2852, 1466, 1444, 1085, 1050, 911, 728, 630) cm-1
;
Figure 1. Curve of heat flow as a function of CTAB at 298.15 K.

3768 Journal of Chemical & Engineering Data, Vol. 55, No. 9, 2010
Figure 4. Enthalpies of dilution of DTAB as a function of surfactant
concentration at different temperatures.
Figure 2. Enthalpy of dilution of CTAB in water at 298.15 K.
Figure 3. First derivative of enthalpy curve calculated from interpolated
values for CTAB at 298.15 K.
Figure 5. Enthalpies of dilution of C12HDAB as a function of surfactant
concentration at different temperatures.
15
the enthalpy of micellization, ∆Hmic
.
The CMC value is
obtained as the midpoint of the second range and determined
from the first-order differential of the enthalpy profile curve as
shown in Figure 3.
chains in aqueous medium are destroyed when the hydrocarbon
chains are removed from the aqueous medium to the interior
of the micelles.
The enthalpies of dilution at different temperatures versus
concentration for eight surfactants are shown in detail in Figures
4 to 11. It can be seen that all the dilution processes of the
surfactant solutions are endothermic and the dilution enthalpies
have positive values. Not all of the enthalpy plateaus before
the CMC values stay flat because of the complicated interactions
in the surfactant solutions.16,17 Table 2 presents the determined
CMC and ∆Hmic values at different temperatures for eight
surfactants, along with the literature values of DTAB and
CTAB.17 The deviations for CMC are less than 5 %, and those
for ∆Hmic are less than 8 % from the corresponding literature
values. It follows that they are in satisfactory agreement. The
calculated values of ∆Gmic and ∆Smic from the pseudo-
phase separation model are also summarized in Table 2. In all
the cases, negative values of ∆Hmic and ∆Gmic and positive
values of ∆Smic are observed, which indicates that the formation
of the micelles has been driven by both the increase in entropy
and the loss in enthalpy. The positive ∆Smic values result from
that the ordered water molecules are excluded from the micellar
interior during the micelle formation from monomers. The
structures of the water molecules surrounding the hydrocarbon
Effects of the Temperature. Table 2 indicates that the
increase in temperature causes a slight increase of the CMC
value. This phenomenon has already been reported in the
literature.10 However, ∆Hmic changes clearly when the temper-
ature increases. In the case of C12HDAB, ∆Hmic gradually
becomes more negative from (-2.723 to -7.955) kJ ·mol-1 with
the temperature increasing from (298.15 to 323.15) K. It is
because the increasing temperature causes the decrease in the
amount of water molecules bound to the surfactant headgroups,
which makes the dehydration heat decrease when the micelles
form. ∆Smic for C12HDAB is 129.6 J·mol-1 ·K-1 at 298.15 K
and decreases to 119.4 J · mol-1 · K-1 at 308.15 K and 111.5
J·mol-1 ·K-1 at 323.15 K. The positive values of ∆Smic result
from the destruction of the iceberg structure of the ordered water
molecules around the hydrophobic alkane chains during the
formation process of the micelles. The temperature-dependent
trend is ascribed to the decrease in the amount of ordered water
molecules around the hydrophobic alkane chains and water
molecules bound to the surfactant headgroups when increasing
the temperature. More negative values of ∆Gmic can be observed
as the temperature increases, for example, -41.37 kJ ·mol-1 at

Journal of Chemical & Engineering Data, Vol. 55, No. 9, 2010 3769
Figure 9. Enthalpies of dilution of CTAB as a function of surfactant
concentration at different temperatures.
Figure 6. Enthalpies of dilution of C12DHAB as a function of surfactant
concentration at different temperatures.
Figure 7. Enthalpies of dilution of C14HDAB as a function of surfactant
concentration at different temperatures.
Figure 10. Enthalpies of dilution of C16HDAB as a function of surfactant
concentration at different temperatures.
the increase of the temperature. The decrease in ∆Smic has a
smaller effect on ∆Gmic than the change in ∆Hmic
.
Effects of the Length of the Alkane Chain. As the tail length
in the surfactant molecule increases, the CMC value at a given
temperature decreases. The similar phenomenon was also
observed in some works reported in the literature.10,18 Taking
∆Hmic at 298.15 K as an example, it is -2.723 kJ ·mol-1 for
C12HDAB, -5.967 kJ · mol-1 for C14HDAB, and -9.678
kJ·mol-1 for C16HDAB. More negative values of ∆Hmic result
from the stronger hydrophobic interaction between the alkane
chains with the increase of the length of the alkane chains.
However, ∆Smic increases with the change of the hydrophobic
segment in surfactants from n-dodecyl to n-cetyl chain. It is
observed that ∆Smic at 298.15 K is (128.5, 140.3, and 153.2)
J · mol-1 · K-1 for C12DHAB, CH14DHAB, and C16DHAB,
respectively. This is the consequence of the increase in the
amount of ordered water surrounding the alkane chain. With
the increase in the length of the alkane chain, the changes in
∆Hmic and ∆Smic have the same effects on ∆Gmic. At constant
temperature, the ∆Gmic decreases of approximately 7.0 kJ ·mol-1
are observed for each addition of two -CH2- groups.
Figure 8. Enthalpies of dilution of C14DHAB as a function of surfactant
concentration at different temperatures.
298.15 K, -42.48 kJ ·mol-1 at 308.15 K, and -44.00 kJ ·mol-1
at 323.15 K for C12HDAB. It indicates that the decrease in ∆Smic
or in ∆Hmic has opposite effects on the change of ∆Gmic with
Effects of the Number of Hydroxyethyl Substituent. It is
noted that the CMC value decreases with the increase of the

3770 Journal of Chemical & Engineering Data, Vol. 55, No. 9, 2010
Table 2. CMC Values and Thermodynamic Parameters of
Micellization of the Surfactants, DTAB, CTAB, CnHDAB (n ) 12,
14, 16), and CnDHAB (n ) 12, 14, 16), in Aqueous Solution
Obtained from the ITC Measurementsa
T
CMC
∆Hmic
∆Gmic
∆Smic
compound
DTAB
K
10-3 mol·L-1 kJ·mol-1 kJ·mol-1 J·mol-1 ·K-1
298.15
303.15
15.00
15.07
15.51b
15.24
15.83b
15.67
15.98
16.92
13.20
13.43
13.92
14.22
14.83
15.43
12.46
12.62
13.09
13.36
13.89
14.56
3.318
3.356
3.505
3.636
3.742
3.998
3.118
3.165
3.269
3.376
3.580
3.731
0.888
0.907
0.950b
0.929
0.975b
0.977
1.041
1.104
0.797
0.839
0.879
0.925
0.975
1.055
0.738
0.786
0.799
0.838
0.909
0.974
-1.824
-3.709
-3.461b
-5.168
-5.321b
-6.172
-6.987
-7.563
-2.723
-4.195
-5.698
-6.726
-7.507
-7.955
-3.343
-4.934
-6.315
-7.507
-8.221
-8.935
-5.967
-8.645
-11.19
-13.58
-15.51
-17.22
-6.686
-9.366
-11.89
-14.04
-15.86
-17.57
-7.894
-11.08
-11.10b
-14.00
-14.37b
-16.78
-19.67
-22.15
-9.678
-12.44
-15.52
-18.01
-20.89
-22.88
-9.999
-12.91
-16.40
-19.29
-21.63
-23.77
-40.73
-41.39
130.5
124.3
308.15
-42.02
119.6
313.15
318.15
323.15
-42.56
-43.13
-43.50
-41.37
-41.97
-42.48
-43.06
-43.52
-44.00
-41.65
-42.29
-42.80
-43.39
-43.87
-44.31
-48.21
-48.96
-49.55
-50.16
-50.81
-51.25
-48.52
-49.26
-49.91
-50.55
-51.04
-51.62
-54.75
-55.56
116.2
113.6
111.2
129.6
124.6
119.4
116.0
113.2
111.5
128.5
123.2
118.4
114.6
112.1
109.5
141.7
133.0
124.5
116.8
111.0
105.3
140.3
131.6
123.4
116.6
110.6
105.4
157.1
146.7
C12HDAB 298.15
303.15
308.15
313.15
318.15
323.15
C12DHAB 298.15
303.15
Figure 11. Enthalpies of dilution of C16DHAB as a function of surfactant
concentration at different temperatures.
308.15
313.15
318.15
323.15
number of the hydroxyethyl substituents on the quaternary
ammonium headgroup. The formation of hydrogen bonds
between hydroxyethyl groups in micelles weakens the
repulsion between the charged headgroups. As a result, the
micellization process occurs at a relatively low surfactant
concentration. With the increase in the hydroxyethyl sub-
stituent number, ∆Hmic becomes more and more negative from
DTAB to C12DHAB (-1.824 kJ · mol-1 for DTAB, -2.723
kJ · mol-1 for C12HDAB, and -3.343 kJ · mol-1 for C12DHAB
at 298.15 K, respectively). In these systems, more hydrogen
bonds form among the hydroxyl groups in the micelles with
the increase of the hydroxyethyl substituent number, and more
energies are released. In the case of ∆Smic at 298.15 K, the
values are 130.5 J · mol-1 · K-1 for DTAB, 129.6 J · mol-1 · K-1
for C12HDAB, and 128.5 J · mol-1 · K-1 for C12DHAB, re-
spectively. ∆Smic progressively decreases with the increase
in the hydroxyethyl substituent number. As mentioned above,
more hydroxyethyl substituents on a surfactant headgroup
cause more water molecules to be excluded from the micellar
interior. It leads to an increase in the entropy of the system.
However, the hydrogen bonds formed between hydroxyl
groups and water molecules cause the decrease of the disorder
degree. From the obtained results, the conclusion can be
drawn that the formation of hydrogen bonds makes a larger
contribution to the change in ∆Smic. With increasing number
of hydroxyethyl substituents, the decreases in both ∆Hmic and
∆Smic make opposite contributions to ∆Gmic. It is noted that
∆Gmic at 298.15 K becomes more and more negative from
-40.73 kJ · mol-1 for DTAB and -41.37 kJ · mol-1 for
C12HDAB to -41.65 kJ · mol-1 for C12DHAB. Thus, the
influence results on ∆Gmic of the increasing amount of the
hydroxyethyl substituents are similar to those of the increas-
ing temperature.
C14HDAB 298.15
303.15
308.15
313.15
318.15
323.15
C14DHAB 298.15
303.15
308.15
313.15
318.15
323.15
298.15
303.15
CTAB
308.15
-56.35
137.4
313.15
318.15
323.15
-57.00
-57.58
-58.17
-55.28
-55.95
-56.64
-57.29
-57.93
-58.41
-55.66
-56.28
-57.12
-57.80
-58.30
-58.84
128.5
119.2
111.5
153.0
143.5
133.4
125.4
116.4
110.0
153.2
143.1
132.2
123.0
115.2
108.5
C16HDAB 298.15
303.15
308.15
313.15
318.15
323.15
C16DHAB 298.15
303.15
308.15
313.15
318.15
323.15
a Experimental precisions for CMC and ∆Hmic are estimated to be (
4 % and ( 5 %, respectively. b Ref 17.
(CMC) and the enthalpies of micellization, ∆Hmic, of the
surfactants at different temperatures were determined directly
by using isothermal titration calorimetry (ITC), and the
corresponding Gibbs free energy, ∆Gmic, and entropy, ∆Smic
,
Conclusions
of the micellization were calculated. The CMC value
decreases as the amount of the hydroxyethyl substituent on
the surfactant headgroup increases and the alkane chain
lengthens. The increase in temperature causes a slight increase
in the CMC value. The increase in temperature, length of
the hydrophobic alkane chain, and the decrease in the number
of hydroxyethyl substituents make the ∆Hmic values more
negative, respectively. Both the increases in temperature and
the number of hydroxyethyl substituents make ∆Smic decrease.
The thermodynamics of the micellization process for eight
quaternary ammonium surfactants, N-alkyl-N-2-hydroxyethyl-
N,N-dimethyl ammonium bromide (CnHDAB, n ) 12, 14,
and 16), N-alkyl-N,N-2-dihydroxyethyl-N-methyl ammonium
bromide (CnDHAB, n ) 12, 14, and 16), along with
N-dodecyl-N,N,N-trimethyl ammonium bromide (DTAB) and
N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB), were
investigated. The values of the critical micellar concentration

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Received for review March 25, 2010. Accepted May 28, 2010. The
authors are grateful for the financial support from the National Natural
Science Foundation of China (No. 20673098) and the Natural Science
Foundation of Zhejiang Province, China (No. Y4080201).
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