Surface properties and aggregates in the mixed systems of bolaamphiphiles and their oppositely charged conventional surfactants

Article abstract of DOI:10.1021/jp0497920

Two transition points were observed in the surface tension curves of the mixed systems of cationic bolaamphiphile BPHTAB (biphenyl-4,4′- bis(oxyhexamethylenetrimethylammonium bromide)) and its oppositely charged conventional surfactants. Transmission electron microscopy, dynamic light scattering, and isothermal titration microcalorimetric experiments demonstrated that spherical vesicles or elongated aggregates formed at the lower surfactant concentration than the corresponding critical micelle concentration (cmc), which led to the existence of two transition points in surface tension curves. Addition of 10% formamide into the mixed system broke the vesicles and elongated aggregates and made the first breaking point disappear. The biphenyl group in the BPHTAB molecule and the appropriate attractive interaction between the cationic and anionic headgroups of the surfactants had a great influence on the ability of vesicle formation and determined whether the two transition points would exist in the surface tension curve.

Full text of DOI:10.1021/jp0497920

5256
J. Phys. Chem. B 2004, 108, 5256-5262
Surface Properties and Aggregates in the Mixed Systems of Bolaamphiphiles and Their
Oppositely Charged Conventional Surfactants
Feng Han, Xiao He, Jianbin Huang,* and Zichen Li
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and
Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China
Yilin Wang
Key Laboratory of Colloid and Interface Science, Center for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, P. R. China
Honglan Fu
College of Life Science, Peking UniVersity, Beijing 100871, P. R. China
ReceiVed: January 15, 2004; In Final Form: February 17, 2004
Two transition points were observed in the surface tension curves of the mixed systems of cationic
bolaamphiphile BPHTAB (biphenyl-4,4′-bis(oxyhexamethylenetrimethylammonium bromide)) and its op-
positely charged conventional surfactants. Transmission electron microscopy, dynamic light scattering, and
isothermal titration microcalorimetric experiments demonstrated that spherical vesicles or elongated aggregates
formed at the lower surfactant concentration than the corresponding critical micelle concentration (cmc),
which led to the existence of two transition points in surface tension curves. Addition of 10% formamide into
the mixed system broke the vesicles and elongated aggregates and made the first breaking point disappear.
The biphenyl group in the BPHTAB molecule and the appropriate attractive interaction between the cationic
and anionic headgroups of the surfactants had a great influence on the ability of vesicle formation and
determined whether the two transition points would exist in the surface tension curve.
Introduction
respective micelles in different concentrations; as a result, two
transition points in the surface tension curve are observed. In
the case of polymer and surfactant system, the surfactant
adsorption on the polymer is responsible for the first transition
point. This binding occurs abruptly in a similar fashion as
micelle formation.30 The second transition point indicates the
surfactant micelle formation as usual.
Bolaamphiphiles, molecules with two polar groups connected
by one or two hydrophobic chains, are receiving increased
attention as building blocks for structures such as membranes,
fibers, tubes, and so on.1-6 The aggregate morphologies of
bolaamphiphiles are dependent on their headgroups7-11 and the
structure of the hydrophobic chains.11-15 The surface properties
of bolaamphiphiles are also investigated and many new phe-
nomena are observed due to their reverse U-shape conformation
in surface phase.16-19 However, in contrast to many studies on
organized assembly formation20-23 and surface properties24,25
in conventional catanionic surfactant systems, the corresponding
research on mixed systems of bolaamphiphiles and their
oppositely charged conventional surfactants is rare.
There is one transition point in surface tension curve
indicating micelle formation in common surfactant systems.
However, in some systems, two transition points exist in the
surface tension curve. This phenomenon usually can be observed
in the following two series of mixed systems: one is the mixture
of fluorocarbon and hydrocarbon surfactants,26-29 and the other
is the mixed system of polymer and conventional surfactants.30,31
After Klevens and Raison’s report26 on the fluorocarbon and
hydrocarbon mixed system, several other reports27-29 prove that
existence of the two transition points in this system is due to
the mutual phobicity of the hydrocarbon and fluorocarbon
chains. The hydrocarbon and fluorocarbon surfactants form
It is easy to find that in both series of the mixed systems
mentioned above there is a great structure difference between
the two components: hydrocarbon and fluorocarbon chains in
the first series, polymers and small conventional surfactant
molecules in the second series. We have reported that the two
transition points in one surface tension curve can also be
observed in the mixed system of a cationic bolaamphiphile
(biphenyl-4,4′-bis(oxyhexamethylenetrimethylammonium bro-
mide), BPHTAB) and sodium dodecyl sulfate (SDS).32 It is
interesting to note that there is no great structure difference
between the two hydrocarbon compounds in this mixed system.
In fact, strong electrostatic attractive interaction between the
polar headgroups of BPHTAB and SDS exists. To make a
further study on the relationship between molecule structure and
molecule properties in this kind of system, in this paper, three
bolaamphiphiles with biphenyl groups in the middle of their
molecules are synthesized. They are mixed with their oppositely
charged conventional surfactants. Combined the results in the
mixed systems of bolaamphiphiles having monophenyl group
and their oppositely charged conventional surfactants, we try
to explore the course of the existence of the two transition points
in our systems. On the basis of the results of transmission
* To whom correspondence should be addressed. Fax: (86) 10-6275-
1708. E-mail: JBHuang@chem.pku.edu.cn.
10.1021/jp0497920 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/02/2004

Mixed Systems of Bolaamphiphiles
J. Phys. Chem. B, Vol. 108, No. 17, 2004 5257
SCHEME 1: Synthesis of BPHTAB and BPHEAB
CHART 1: Structural Formula of PHTAB
SCHEME 2: Synthesis of BPSPC
Figure 1. γ-log c curves of the BPHTAB/SDS mixed systems at
30.0 °C.
was baked at 500 °C for 6 h. Formamide (AR grade) and
Na2B4O4‚10H2O (AR grade) were from Beijing Chemical Co.
and used as received. Water was distilled from the KMnO4-
containing deionized water to remove traces of organic com-
pounds.
Sample Preparation. The surfactants were mixed in molar
ratios and the solvents (ethanol/water and formamide/water)
were mixed in volume ratios. XBola stood for the molar content
of bolaamphiphile in the mixture. The pH values were adjusted
to 9.2 by addition of Na2B4O4‚10H2O in the systems containing
carboxylate surfactants. The temperature of all experiments was
kept at 30.0 ( 0.5 °C.
electron microscopy, dynamic light scattering and isothermal
titration microcalorimetric experiments, we find that the forma-
tion of various kinds of organized assemblies such as vesicles
or micelles at different surfactant concentrations will be
responsible for the existence of two transition points in the
surface tension curves.
Experimental Section
Surface Tension. The surface tension measurement was
conducted using the drop volume method.33 The values of
concentration and the surface tension at the two transition points
were determined from the surface tension curves (γ-log c
curves).
Transmission Electron Microscopy (TEM). Micrographs
were obtained with a JEM-100CX II transmission electron
microscope by the freeze fracture technique and negative-
staining methods. (1) Freeze-fracture technique: Fracturing and
replication were carried out in an EE-FED.B freeze-fracture
device equipped with a JEE-4X vacuum evaporator. (2) Nega-
tive-stained method: Uranyl acetate solution (1%) was used as
the staining agent. One drop of the solution was placed onto a
carbon Formvar-coated copper grid (230 mesh). Filter paper
was employed to suck away the excess liquid.
Dynamic Light Scattering (DLS). Dynamic light scattering
measurements were carried out by a spectrometer of standard
design (ALV-5000/E/WIN Multiple Tau Digital Correlator) with
a Spectra-Physics 2017 200 mW Ar laser (514.5 nm wave-
length). The scattering angle was 90°, and the intensity
autocorrelation functions were analyzed using the methods of
Cumulant and Contin.
Isothermal Titration Microcalorimetry. The calorimetric
measurements were performed with the commercial microcalo-
rimetric-measuring channel of the 2277 TAM (Thermometric
AB, Jarfalla, Sweden). Raw data curves were integrated using
Digitam 4.1 software.
Materials. Biphenyl-4,4′-bis(oxyhexamethylenetrimethyl-
ammonium bromide) (BPHTAB) and biphenyl-4,4′-bis(oxy-
hexamethylenetriethylammonium bromide) (BPHEAB) were
synthesized according to Scheme 1. First, 4,4′-bis((bromohexa-
methylene)oxy)biphenyl was synthesized by refluxing 4,4′-
dihydroxybiphenyl, 1,6-dibromohexane, and K2CO3 in acetone
for 3 days. The product was recrystallized from acetone. Second,
BPHTAB and BPHEAB were obtained from 4,4′-bis((bromo-
hexamethylene)oxy)biphenyl and the corresponding tri-
alkylamine. The products were recrystallized from ethanol.
Phenyl-4,4′-bis(oxyhexamethylenetrimethylammonium bromide)
(PHTAB) (Chart 1) was prepared by the same procedure as
BPHTAB except that hydroquinone was used instead of 4,4′-
dihydroxybiphenyl. Biphenyl-4,4′-bis(sodium oxypentamethyl-
enecarboxylate) (BPSPC) was synthesized from 4,4′-dihydroxy-
biphenyl and 6-bromohexoic acid in NaOH solution (Scheme
2). The product was recrystallized from an ethanol-water
1
mixture. All the compounds were characterized by H NMR
spectra and elemental analysis.
Dodecyltrimethylammonium bromide (C12NMe3Br) and dode-
cyltriethylammonium bromide (C12NEt3Br) were synthesized
from n-dodecyl bromide and the corresponding trialkylamine.
The products were recrystallized from an ethanol-acetone
mixture. Sodium decanoate (C9COONa) was prepared by
neutralizing the decanoic acid (C9COOH) with NaOH in ethanol.
C9COOH was distilled twice before use. Sodium dodecyl sulfate
(SDS) of AR grade was purchased from Sigma Co. and used
as received. The purity of all the surfactants was examined by
surface tension measurement, and no minimum was found in
all the surface tension curves.
Results and Discussion
Surface Properties. The γ-log c curves of the BPHTAB/
SDS mixed systems of two surfactant proportions are shown in
Figure 1. There are two transition points in each γ-log c curve,
Ethanol was refluxed with magnesium scraps and iodine for
more than 2 h and then distilled. Sodium bromide of AR grade

5258 J. Phys. Chem. B, Vol. 108, No. 17, 2004
Han et al.
Figure 2. γ-log c curves of the BPHTAB/SDS mixed systems in 0.1
mol/L NaBr at 30.0 °C.
Figure 4. γ-log c curve of the BPHTAB/C9COONa mixed systems
at 30.0 °C (XBola ) 0.60, pH ) 9.2).
TABLE 2: C1, C2, γ1, and γ2 Values of the BPHTAB/
C9COONa Mixed Systems at 30.0 °C (XBola ) 0.60, pH )
9.2)
XBola C1 (10-4 mol/L) C2 (10-3 mol/L) γ1 (mN/m) γ2 (mN/m)
0.60
4.47
5.00
50.9
42.6
values of C1 and C2 are higher in the BPHTAB/C9COONa
system than those in the BPHTAB/SDS system.
The above results indicate that it is not a unique phenomenon
that the two transition points exist in one γ-log c curve in this
bolaamphiphile and its opposite charged surfactant mixed
system. Obviously the explanation for two transition points
phenomenon in the fluorocarbon/hydrocarbon mixed surfactant
systems and polymer/conventional mixed surfactant systems is
not suitable for our systems. There is no mutual phobicity of
the hydrocarbon and fluorocarbon chains or the surfactant’s
adsorption on the polymer. In fact, strong attractive interaction
of cationic and anionic surfactants exists in the BPHTAB/SDS
system and the BPHTAB/C9COONa system. A new explanation
for this new phenomenon should be provided.
Figure 3. γ-log c curves of the BPHTAB/SDS mixed systems in
0.10 mol/L NaBr with ethanol addition at 30.0 °C (XBola ) 0.33).
TABLE 1: C1, C2, γ1, and γ2 Values of the BPHTAB/SDS
Mixed Systems at 30.0 °C
XBola C1 (10-5 mol/L) C2 (10-5 mol/L) γ1 (mN/m) γ2 (mN/m)
Organized Assembly Formation. Organized assemblies are
also studied in both the BPHTAB/SDS and BPHTAB/C9COONa
systems. TEM measurements show that spherical vesicles or
elongated aggregates exist when the total surfactant concentra-
tion is higher than C1 in the BPHTAB/SDS system (Figure 5a)
and the BPHTAB/C9COONa system (Figure 5b). Predictably,
no aggregate is observed in the solution when the total surfactant
concentration is lower than C1 in these systems.
0.33
0.60
1.54
1.25
4.59
5.00
50.7
51.7
43.1
43.3
which is different from the case in the usual surfactant system.
The transition point at lower concentrations is designated as
the first point, and the transition point at higher concentration
is designated as the second point (see Figure 1). The total
surfactant concentrations at the two transition points (C1, C2)
and surface tension values of the solution at the two transition
points (γ1, γ2) are listed in Table 1.
The surface properties in the BPHTAB/SDS mixed systems
in 0.10 mol/L NaBr were also investigated,32 and the γ-log c
curves are shown in Figure 2. It can be seen that the two
transition points also exist, but the total surfactant concentration
and surface tension at the transition point are different.
Moreover, the two transition points also exist in the γ-log c
curve with the addition of some ethanol (see Figure 3).
A similar result is also found in the BPHTAB/C9COONa
systems, especially when XBola is 0.60 and the pH is 9.2 (see
Figure 4 and Table 2). In Figure 4, the two transition points
are clearly shown in the γ-log c curve, and the concentration
range between C1 and C2 is wider in the BPHTAB/C9COONa
system than that in the BPHTAB/SDS system. Meanwhile, the
The DLS results of the BPHTAB/C9COONa system in the
total surfactant concentration of 0.002 mol/L (between C1 and
C2) and 0.01 mol/L (above C2) are shown in Figure 6. It can be
seen that the aggregates with average hydrodynamic radius of
100 nm exist at 0.002 mol/L, which may be designated as
vesicles and elongated aggregates. When the total surfactant
concentration is increased to 0.01 mol/L, a new kind of
aggregate with the average hydrodynamic radius of 2 and 17
nm can be observed, which usually indicates micelle formation.
It should be noted that at this concentration vesicles (with the
average hydrodynamic radius of 100 nm) still coexist in the
system. TEM measurements also demonstrate that spherical
vesicles or elongated aggregates exist at concentrations higher
than C2, in all the above systems. Combined with the TEM and
DLS results, it can be concluded that vesicles and elongated
aggregates exist while the total surfactant concentration is

Mixed Systems of Bolaamphiphiles
J. Phys. Chem. B, Vol. 108, No. 17, 2004 5259
Figure 5. TEM micrographs for the mixed systems: (a) BPHTAB/SDS (Ctotal ) 3.0 × 10-5 mol/L, XBola ) 0.60, negative staining); (b) BPHTAB/
C9COONa (Ctotal ) 2.0 × 10-3 mol/L, XBola ) 0.60, pH ) 9.2, negative staining); (c) BPHTAB/SDS (Ctotal ) 6.0 × 10-5 mol/L, XBola ) 0.60, 0.1
mol/L NaBr, freeze fracture); (d) BPHTAB/SDS (Ctotal ) 2.0 × 10-4 mol/L, XBola ) 0.33, EtOH:H2O ) 1:4, freeze fracture).
SDS and BPHTAB/C9COONa systems, spherical vesicles and
elongated aggregates form below the cmc, which influences the
concentration of free surfactant molecule in the bulk solution,
followed by the variation of the adsorption of surfactants. Thus
it is reasonable that the existence of two transition points comes
from the formation of different kinds of surfactant aggregates
in different concentration regions. That is to say, the existence
of two transition points is mainly due to the fact that spherical
vesicle and elongated aggregate form at the lower surfactant
concentration than the concentration of micelle formation. When
the concentration is lower than C1, no organized assemblies exist
in the solution. Thus surface tension decreases with the
increasing surfactant concentration according to the Gibbs
adsorption equation. At the concentration of C1, spherical
vesicles or elongated aggregates begin to form, which influences
the surface adsorption and induces the first transition point to
appear. At the concentration of C2, great amount of micelles
begin to form, which induces the second transition point. That
is to say, C2 is the cmc in these kinds of systems.
Figure 6. DLS measure results of the BPHTAB/C9COONa system at
30.0 °C (XBola ) 0.60, pH ) 9.2).
Effect of Additives. The organized assemblies in these mixed
systems with the addition of NaBr or ethanol are also investi-
gated. TEM results show that spherical vesicles and elongated
aggregates still exist when these additives are added into the
system (Figure 5c,d). It is consistent with the results that vesicles
can exist in 0.1 mol/L NaBr or at high volume ratio of ethanol/
water solution in conventional cationic and anionic mixed
surfactant systems.34 Hence it is understandable that the two
transition points phenomenon also exists in these systems after
the addition of NaBr or ethanol, regardless of the variation of
the total surfactant concentration and surface tension at the
transition point.
between C1 and C2. When the concentration is above C2,
micelles begin to form and micelles and vesicles coexist in the
solution.
It is well-known that the surface tension of surfactant solution
is determined by the nature of surfactant molecules and their
packing condition on the surface. Considering the equilibrium
among surfactant molecules in surface, free surfactant molecules
in the bulk phase and surfactant molecules in organized
assemblies (such as micelles and vesicles), it is possible that
the surface properties are influenced by the formation or
transformation of the organized assemblies in the solution.
In usual surfactant system, there is no aggregate in the
solution when the concentration is lower than the critical micelle
concentration (cmc), so there is one transition point in the γ-log
c curve indicating micelle formation. However, in the BPHTAB/
However, it is completely different in the presence of
formamide. The effect of formamide on the conventional
cationic and anionic surfactants mixed system is different from
that of ethanol based on our previous work.35-38 According to

5260 J. Phys. Chem. B, Vol. 108, No. 17, 2004
Han et al.
Figure 7. γ-log c curves of the BPHTAB/SDS mixed systems with
and without formamide at 30.0 °C (XBola ) 0.60).
TABLE 3: C2 Values in the BPHTAB/SDS Mixed Systems
with and without Formamide at 30.0 °C (XBola ) 0.60)
formamide ratio (%)
104C2 (mol/L)
0
5
10
0.50
1.26
1.86
Coulomb’s law, the relative dielectric constant of the solvent
can directly affect the electrostatic force between two ionic
surfactants. The dielectric constant of formamide is 111.0, which
is much higher than that of water. The attractive interaction
between the polar groups of cationic and anionic surfactants
and the solvophobic effect would both be reduced with
formamide addition. Therefore, formamide addition is disad-
vantageous to vesicle formation. On the other hand, surfactant
micelles can exist in formamide solution according to Ray’s
work.39 That is to say, if our explanation provided above is
reasonable, we will observe the surface tension curve with just
one transition point when formamide is added into the mixed
systems in suitable concentration.
Figure 8. Titration microcalorimetric curves of the BPHTAB/SDS
systems in water and in the 10% FA aqueous solution at 30 °C (XBola
) 0.60): (a) without FA; (b) with 10% FA.
The γ-log c curves and the surface properties of the
BPHTAB/SDS systems with and without formamide are shown
in Figure 7 and Table 3, respectively. In Figure 7, the γ-log c
curve of the system with 10% formamide is similar to the usual
γ-log c curve, which has only one transition point. There are
two transition points in the curve of the system with 5%
formamide, but the first point is not obvious. TEM results
demonstrate that neither the vesicle or elongated aggregate exists
in the system with 10% formamide. For the system with 5%
formamide, the density of vesicle or elongated aggregate is
greatly reduced compared with that in the system without
formamide addition.
The calorimetric measurement results also support the above
conclusion. The isothermal titration microcalorimetric curves
of the BPHTAB/SDS systems with and without10% FA addition
are shown in Figure 8. From Figure 8a it can be found that
there are two peaks in the curve of the BPHTAB/SDS system.
The total surfactant concentrations of the two peaks are 1.78 ×
10-5 and 5.08 × 10-5 mol/L, respectively. They are consistent
with the total surfactant concentration at the two transition points
in the γ-log c curve. Obviously, the first peak indicates the
formation of vesicles and elongated aggregates (∆H ) -15.5
kJ/mol), and the second peak indicates micelle formation (∆H
) -9.1 kJ/mol). On the other hand, from Figure 8b it can be
found that after the addition of 10% FA, there is only one abrupt
decrease in the titration microcalorimetric curve, which indicates
the micelle formation (∆H ) -10.9 kJ/mol). Corresponding to
the value of the transition point of the γ-log c curve, the total
surfactant concentration of the abrupt decrease in isothermal
titration microcalorimetric curve is 1.52 × 10-4 mol/L.
Effect of Surfactant Molecular Structure. The phenomenon
of the two transition points in the surface tension curve comes
from the fact that the formation of spherical vesicles and
elongated aggregates is prior to micelle formation. Usually in
the conventional cationic and anionic surfactant mixed systems,
vesicles form in the surfactant concentration near the cmc or
higher than the cmc. Therefore, the phenomenon of two
transition points is not common. Considering the fact that there
is only one transition point in the surface tension curve in the
PHTAB/SDS and PHTAB/C9COONa systems,19 we can assume
that the biphenyl group in the BPHTAB molecule may play an
important role in the priority formation of spherical vesicle and
elongated aggregate.
Like other bolaamphiphiles, the BPHTAB molecule would
take either of the two kinds of conformation in aggregates, line
conformation, and U shape conformation. Neither of these
conformations is in favor of micelles, as line conformation is
not beneficial to the moving of BPHTAB molecules between
micelle and solution (see Chart 2a), and the rigid biphenyl group
is disadvantageous to the formation of U shape conformation
in micelles (see Chart 2b). On the other hand, the line
conformation of BPHTAB in the vesicles is more favored (see

Mixed Systems of Bolaamphiphiles
J. Phys. Chem. B, Vol. 108, No. 17, 2004 5261
cannot form at a concentration lower than the cmc. That is to
say, bolaamphiphiles with a biphenyl group in the molecule and
suitable interaction between polar groups in the mixed cationic
and anionic surfactant system are the two basic determinants
to the existence of the two transition points in one surface
tension curve.
Conclusion
The surface properties and organized assemblies formation
are studied in the mixed systems of bolaamphiphiles with
biphenyl group and their oppositely charged conventional
surfactants. Two transition points are observed in the surface
tension curves of the BPHTAB/SDS and BPHTAB/C9COONa
mixed systems. Formation of different kinds of molecular
assemblies in different concentration region accounts for this
phenomenon, which is further explained by the structures of
bolaamphiphiles. We hope that this study would make some
advances in the field of the aggregate formation in the mixed
systems of bolaamphiphiles and their oppositely charged
conventional surfactants, then provide some useful information
on the relationship between surface properties and organized
assembly formation in solutions.
Figure 9. γ-log c curves of BPHEAB/SDS, BPHEAB/C9COONa,
and BPSPC/C12NEt3Br systems at 30 °C.
CHART 2: Conformation of BPHTAB Molecule in
Aggregates: (a) Line Conformation of BPHTAB in the
Micelle; (b) U Shape Conformation of BPHTAB in the
Micelle; (c) Line Conformation of BPHTAB in the
Vesicle
Acknowledgment. This work was supported by National
Natural Science Foundation (20233010, 20373003) of China.
References and Notes
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P ) Vc/a0lc
where Vc and lc are the volume and chain length of the
hydrophobic group, respectively, and a0 is the optimum area
per polar group. For vesicle formation, the proper value of P
falls in the range 0.5-1, whereas for micelle formation, P <
0.5. Considering that the triethyl group is bigger than the
trimethyl group, the values of a0 should be higher and P should
be lower in the BPHEAB/SDS, BPHEAB/C9COONa, and
BPSPC/C12NEt3Br systems than those in the BPHTAB/SDS and
BPHTAB/C9COONa systems. Thus the vesicle-forming capa-
bility in the former three systems will be weak, and vesicles
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