Phase behaviors and self-assembly properties of two catanionic surfactant systems: C8F17COOH/TTAOH/H2O and C 8H17COOH/TTAOH/H2O

Article abstract of DOI:10.1021/jp104579h

Two fatty acids, perfluorononanoic acid (C₈F₁₇COOH) and nonanoic acid (C₈H₁₇COOH), were mixed with a cationic hydrocarbon surfactant, tetradecyltrimethylammonium hydroxide (TTAOH), in aqueous solutions for comparative investigation. Phase behaviors of the two systems are quite different because of the special properties of the fluorocarbon chains. For the C₈H₁₇COOH/TTAOH/H ₂O system, a single L_α phase region with phase transition from planar lamellar phase (L_αl phase) to vesicle phase (L_αv phase) was observed. For the C₈F ₁₇COOH/TTAOH/H₂O system, two single phases consisting of vesicles were obtained at room temperature. One is a high viscoelastic gel phase consisting of vesicles with crystalline state bialyers at the C ₈F₁₇COOH-rich side, which was confirmed by freeze-fracture transmission electron microscope (FF-TEM) and differential scanning calorimetry (DSC) measurements. With the increase of TTAOH proportion, another vesicle phase consisting of liquid state bilayers was observed after the two-phase region. The fluorosurfactant systems prefer to form vesicle bilayers than the corresponding hydrocarbon ones because of the rigid structure, the stronger hydrophobicity, and the larger volume of fluorocarbon chains.

Full text of DOI:10.1021/jp104579h

13128
J. Phys. Chem. B 2010, 114, 13128–13135
Phase Behaviors and Self-Assembly Properties of Two Catanionic Surfactant Systems:
C8F17COOH/TTAOH/H2O and C8H17COOH/TTAOH/H2O
Juan Zhang,† Aixin Song,† Zhibo Li,‡ Guiying Xu,† and Jingcheng Hao*,†
Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education,
Jinan 250100, P. R. China, and Beijing National Laboratory for Molecular Sciences, State Key Laboratory of
Physical and Chemistry Joint Laboratory Polymer Science and Materials, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100080, P. R. China
ReceiVed: May 19, 2010; ReVised Manuscript ReceiVed: September 5, 2010
Two fatty acids, perfluorononanoic acid (C8F17COOH) and nonanoic acid (C8H17COOH), were mixed with a
cationic hydrocarbon surfactant, tetradecyltrimethylammonium hydroxide (TTAOH), in aqueous solutions
for comparative investigation. Phase behaviors of the two systems are quite different because of the special
properties of the fluorocarbon chains. For the C8H17COOH/TTAOH/H2O system, a single LR phase region
with phase transition from planar lamellar phase (LRl phase) to vesicle phase (LRv phase) was observed. For
the C8F17COOH/TTAOH/H2O system, two single phases consisting of vesicles were obtained at room
temperature. One is a high viscoelastic gel phase consisting of vesicles with crystalline state bialyers at the
C8F17COOH-rich side, which was confirmed by freeze-fracture transmission electron microscope (FF-TEM)
and differential scanning calorimetry (DSC) measurements. With the increase of TTAOH proportion, another
vesicle phase consisting of liquid state bilayers was observed after the two-phase region. The fluorosurfactant
systems prefer to form vesicle bilayers than the corresponding hydrocarbon ones because of the rigid structure,
the stronger hydrophobicity, and the larger volume of fluorocarbon chains.
Introduction
Among the surfactants used to construct self-assembly
aggregates, fluorosurfactants have unique physicochemical
properties because of the fluorocarbon chains such as high
surface activity, hydrophobic and oleophobic properties, and
chemical stability, etc. Therefore, the aggregation behaviors of
fluorosurfactants in solutions often present differences from
those of hydrocarbon surfactants, which trigger a flame in the
study of fluorosurfactant systems.23-26 However, compared to
the systematic investigations of the corresponding hydrocarbon
surfactants, the reports of fluorocarbon surfactants have mostly
concentrated on the partial or perfluorocarboxylic acids and their
salts with monovalent ions as counterions,27-31 while research
on catanionic fluorocarbon/hydrocarbon surfactant mixtures is
comparatively rare.23,26
Vesicles that were early described in 19641 for lipids in
aqueous solution are interesting and important self-assembled
structures in scientific research, which have wide-ranging
industry applications.2 Since Kaler et al. reported the thermo-
dynamically stable vesicles in aqueous solutions of single-tailed
cationic and anionic (catanionic) surfactant mixtures in 1989,3
catanionic surfactant mixtures as typical model systems of
surfactants in solutions have attracted great attention in con-
structing self-assembled structures, especially the vesicles.4-12
However, an undesired phenomenon is often observed. Precipi-
tates often form at equimolecular mixtures because the elec-
trostatic interaction between the charged aggregates, in which
is screened by excess salts formed from the counterions of
cationic and anionic surfactants. In recent years, extensive
studies have been focused on salt-free catanionic surfactant
systems.13-19 Rich phase behaviors and novel aggregates have
been observed in these systems due to the absence of salts. To
obtain the salt-free catanionic surfactant systems, cationic
surfactants with OH- as counterions were often used to be mixed
with anionic ones with H+ as counterions and salts were avoided
because of the formation of H2O. In these salt-free catanionic
surfactant mixtures, vesicles of hydrocarbon mixtures16,18 have
been studied in detail and used to enhance the solubility of
fullerene C60 in aqueous solution.19 Recently, stable reverse
vesicles of salt-free catanionic surfactant mixtures in nonpolar
organic solvents like cyclohexane were prepared,20,21 which may
provide ideal micro- or nanoreaction centers for biological tests
to trap guest molecules such as enzymes.22
In recent years, our group has described the phase behaviors
of a series of salt-free catanionic surfactant systems composed
of alkyltrimethylammonium hydroxide and fatty acids with
different tails.32-34 Fascinating self-assembly aggregates, e.g.,
wormlike micelles, onion phase, and lamellar structures, with
remarkable viscoelasticity were observed. Herein, to perceive
the influence of fluorocarbon chains on aggregation behaviors
of catanionic surfactant systems, a perfluorononanoic acid
(C8F17COOH) was mixed with tetradecyltrimethylammonium
hydroxide (TTAOH) in aqueous solution to be compared with
the hydrocarbon catanionic system C8H17COOH/TTAOH/H2O.
The results show that both systems have rich phase behaviors,
but their aggregations are quite different. A phase transition from
the planar lamellar phase (LRl phase) to the vesicle phase (LRv
phase) in the C8H17COOH/TTAOH/H2O system was confirmed
by 2H NMR spectra. For the C8F17COOH/TTAOH/H2O system,
at the C8F17COOH-rich side, a gel phase consisting of vesicles
with crystalline state chains was demonstrated by freeze-fracture
transmission electron microscopy (FF-TEM) observations and
differential scanning calorimetry (DSC) measurements, which
* To whom correspondence should be addressed. E-mail: jhao@sdu.edu.cn.
Fax: +86-531-88564750(o).
† Shandong University.
‡ Chinese Academy of Sciences.
10.1021/jp104579h  2010 American Chemical Society
Published on Web 09/24/2010

C8F17COOH/TTAOH/H2O and C8H17COOH/TTAOH/H2O
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13129
was rarely reported previously. Moreover, an ordinary LRv phase
consisting of polydisperse uni- and multilamellar vesicles was
also observed. These results could provide further understanding
of the phase characteristics and the difference between of
hydrocarbon catanionic mixtures and hydro-/fluorocarbon ones,
which may be useful to design these mixed systems to fit various
requirements in academic research, material preparation, and
industry applications, etc.
Experimental Section
Chemicals and Materials. Tetradecyltrimethylammonium
bromide (TTABr) was purchased from Amresco Co. (Solon,
OH, U.S.A.). Nonanoic acid (C8H17COOH, >98%) was pur-
chased from Tokyo Chemical Industry CO., Ltd.. Perfluo-
rononanoic acid (C8F17COOH, >98%) was purchased from
Fluorochem Co. (Derbyshire, England). They were all directly
used without further purification. D2O (G99.9%) was from
Aldrich.
Preparation of Tetradecyltrimethylammonium Hydroxide
(TTAOH). TTAOH stock solution was prepared from TTABr
aqueous solution using a strong base anion exchanger (Ion
exchanger III, Merck) at about 40 °C. TTAOH solution was
detected by AgNO3 solution with excess HNO3 to confirm if
Figure 1. Phase behavior of the C8H17COOH/TTAOH/H2O system
observed without (up) and with (below) crossed polarizers. The total
surfactant concentration is 100.0 mmol·L-1, rTTAOH(CH) from left to right
is 0, 0.1, 0.25, 0.3, 0.4, 0.45, 0.5, and 0.6. T ) 25.0 ( 0.1 °C.
sample was placed on a 0.1 mm thick copper disk covered with
a second copper disk. Then the copper sandwich with the sample
was plunged into liquid propane that had been cooled by liquid
nitrogen to freeze. Fracturing and replication were carried out
on a Balzers BAF-400D equipment at -140 °C at the Institute
of Biophysics, CAS. Pt/C was deposited at an angle of 45°.
The replicas were examined in a JEOL JEM-1400 electron
microscope operated at 120 kV.
Cryo-Transmission Electron Microscope (Cryo-TEM)
Observations. The samples were prepared in a controlled
environment vitrification system (CEVS) at room temperature.
First, about 5 µL sample solution was loaded onto a lacey
support TEM grid held by a tweezer. The excess liquid was
removed with a piece of blotting paper, leaving a thin film
stretched over the holes. After about 10 s to relax any stress
induced by the blotting, the samples were instantly frozen by
plunging them into liquid ethane at its melting temperature
(-183 °C). Then the vitrified specimens were inserted into liquid
nitrogen and transferred to a cryogenic sample holder (Gatan
626). The observations of were carried out on a JEM 2200FS
TEM (200 keV) at about -174 °C.
Rheological Measurements. Steady and dynamic rheological
experiments were performed on a HAAKE RheoStress 6000
rheometer with a coaxial cylinder sensor system (Z41°Ti) for
low viscous samples and cone-plate system (C35/1°Ti L07116)
for samples with high viscosity. In oscillatory measurements,
an amplitude sweep at a fixed frequency of 1.0 Hz was
performed prior to the following frequency sweep to ensure the
selected stress was in the linear viscoelastic region. Then the
magnitudes of the complex viscosity |η*|, the storage modulus
G′, and the loss modulus G′′ were measured in the frequency
range 0.01-10 Hz. The samples were measured at 25.0 ( 0.1
°C by using a Circulator HAAKE DC10 cyclic water bath
(Karlsruhe, Germany).
Br- ions were exchanged by OH- absolutely (Ag+ + Br-
f
AgBr V). The stock TTAOH solution was determined by
acid-base titration with 0.10 mol ·L-1 HCl to be 221.4
mmol·L-1
.
Phase Behavior Study. The total surfactant concentration
was fixed to be 100.0 mmol ·L-1 and the molar ratio of TTAOH,
rTTAOH (rTTAOH ) nTTAOH/(nfatty
+ nTTAOH)), is changeable.
acid
All samples were first ultrasonicated at 60 °C to accelerate the
dissolution of surfactants. Then they were stored for at least 4
weeks at T ) 25.0 ( 0.1 °C to reach equilibrium. The phase
diagrams were established on the basis of visual inspection with
the help of crossed polarizers.
pH Measurements. The pH measurements were performed
on a PHS-3C precision pH meter (China) at 25.0 ( 0.1 °C.
Two-phase samples were equably stirred during the measurements.
Conductivity Measurements. The conductivity measure-
ments were performed on a DDSJ-308A conductivity meter
(China) at 25.0 ( 0.1 °C. Two-phase samples were equably
stirred during the conductivity measurements. The conductivity
value for each sample is the average of three measurements.
Textures of the Birefringent Lr phase Solutions. The
birefringent LR phase solutions were characterized on a Carl
Zeiss Axioskop 40 light microscope (Jena, Germany) at 25.0
( 0.1 °C.
2
2H and 19F NMR Measurements. All samples for H and
19F NMR measurements were first prepared in water and then
were dried in a LGJ-10C freeze drier. After that, they were
redissolved in D2O with their original concentrations unchanged.
2H and 19F NMR spectra were recorded on a Bruker Avance
400 spectrometer equipped with pulse field gradient module (Z
axis) using a 5 mm BBO probe. All the experiments were
operated at 25.0 ( 0.1 °C.
Results and Discussion
1. C8H17COOH/TTAOH/H2O System. Phase BehaWior.
The total surfactant concentration is fixed to be 100.0 mmol ·L-1
and the mixed ratio of C8H17COOH to TTAOH is variable,
which is marked by the mole fraction of TTAOH, rTTAOH(CH)
Differential Scanning Calorimetry (DSC) Measurements.
DSC measurements were operated on a DSC-Q10, TA Instru-
ments (New Castle, PA, U.S.A.). Each sample was analyzed in
an aluminum pan under a flow of nitrogen (20 mL/min) and
heated at 5 °C/min from 20 to 80 °C, an empty aluminum pan
was used as a reference.
Freeze-Fracture Transmission Electron Microscope (FF-
TEM) Observations. The microstructure of the gel phase was
characterized by FF-TEM observations. A small amount of
(rTTAOH(CH) ) nTTAOH/(nC H COOH + nTTAOH)). The phase behavior
17
and phase diagram are 8illustrated in Figures 1 and 2, respec-
tively. The conductivity and pH data are included in the phase
diagram. Nonanoic acid is an oily liquid and has very poor
solubility in water at room temperature. One can see that at
rTTAOH(CH) ) 0, nonanoic acid is on top of the aqueous solution
(L1 phase) due to its lower density than water. When
C8H17COOH and TTAOH are mixed together, with the increase

13130 J. Phys. Chem. B, Vol. 114, No. 41, 2010
Zhang et al.
between the deuteron quadrupole moment and the electric field
gradients around the nucleus. Moreover, the interaction is also
dominated by the molecular orientation in relation to the
magnetic field. For an isotropic solution, the properties are the
same in all directions, so the interaction is averaged to zero
due to the fast molecular motion and only a single sharp peak
can be observed in the spectrum. In anisotropic solutions such
as the lamellar or hexagonal phase, where the properties vary
with different directions, the deuteron should result in the
nonzero average of the interactions; therefore, doublet quadru-
2
pole splitting can be observed by H NMR operations. Thus,
2H NMR spectra show different properties in different self-
assembly solutions. One doublet splitting peak can be observed
in a lamellar or a hexangular liquid crystalline phase with
different splitting values. For the L3 phase (sponge phase) and
cubic liquid crystalline phase, there is no splitting but a single
peak in the 2H NMR spectra. For the LR phase aqueous solutions,
it is very different for the LRl phase (planar lamellar phase) and
LRv phase (vesicle phase). The LRl phase solutions are long-
range ordered macroscopically anisotropic systems. When H2O
is replaced by D2O to prepare the samples, D2O molecules at
the interface of the planar surfactant bilayers show macroscopic
anisotropy and doublet quadrupole splitting can be observed,
Figure 2. Phase diagram of the C8H17COOH/TTAOH/H2O system.
The conductivity (9) and pH (2) data are included in the phase
diagram. The total surfactant concentration is 100.0 mmol · L-1. T
) 25.0 ( 0.1 °C.
2
inducing the splitting peaks in H NMR spectra. However, for
the LRv phase, the solutions are composed of closed vesicle
bilayers and D2O molecules at the interface of bilayers exhibit
only local anisotropy, which induces the interneutralization
between the deuterium nuclei and the electric field. Thus, only
single peaks appear in 2H NMR spectra of LRv phase solutions.
Figure 4 shows the 2H NMR spectra of two LR phase samples
at rTTAOH(CH) ) 0.3 and 0.4, respectively. When rTTAOH(CH) ) 0.3,
one can see two equally splitting peaks, indicating the existence
of an anisotropic LRl phase. While when rTTAOH(CH) increases to
0.4, only a single sharp peak is observed, implying the transition
from LRl phase to LRv phase with the increase of TTAOH
proportion. The phase transition should be ascribed to the repulsion
between the bilayer charges. Nonanoic acid has an ionization
equilibrium in aqueous solution, i.e., C8H17COOH h C8H17COO-
Figure 3. Polarized textures of two birefringent LR phase samples at
rTTAOH(CH) ) 0.3 (a) and 0.4 (b).
of the TTAOH proportion, nonanoic acid is dissolved gradually
and after a L1/LR phase region, a single LR phase is observed
from rTTAOH(CH) ) 0.25 to 0.43. The LR phase solutions are
slightly turbid and bluish with obvious birefringence. From
rTTAOH(CH) ) 0.43 to 0.49, the macroscopic phase separation with
a creamed LR phase on top and an isotropic L1 phase below is
observed, indicating that the density of the L1 phase is higher
than that of LR phase. After the two-phase region, from
rTTAOH(CH) ) 0.49 to 1.0, with TTAOH concentration increasing,
the system reaches a single isotropic L1 phase region with very
low viscosity.
In our previous work, the property and function of a salt-free
catanionic system, lauric acid (LA)/TTAOH/H2O, were studied and
the viscoelastic wormlike micellar phase and vesicle phase were
obtained.32,34 Compared with the LA/TTAOH/H2O system, the
C8H17COOH/TTAOH/H2O system consists of a fatty acid, nonano-
ic acid, which is a much shorter chain length to induce the different
phase property. In LA/TTAOH/H2O system, the single LR phase
exhibits at the TTAOH molar ratio from 0.43 to 0.46 with the same
total surfactant concentration, ctotal ) 100.0 mmol ·L-1. Thus, it
can be concluded that more amounts of fatty acids with shorter
tails are needed to construct the bilayers than those of fatty acids
with longer tails.
+ H+. With the increase of TTAOH amount, from rTTAOH(CH)
)
0.3 to 0.4, pH values of the LR phase solutions increase, as shown
in Figure 2, indicating that the ionization equilibrium moves to
form more C8H17COO-. Due to the participation of C8H17COO-
in bilayers, the charge density of the bilayers increases, which
results in the bilayers to bend and close to form vesicles under the
driving of electrostatic repulsions.
Rheological Properties. Dynamic rheological results of two
LR phase samples at rTTAOH(CH) ) 0.3 and 0.4 are shown in Figure
5a,b, respectively. For the two samples, both of the elastic
modulus G′ and the viscous modulus G′′ are very low and
increase with the oscillatory frequency. Moreover, |η*| values
are also very low. The results show that the two LR phase
samples do not present viscoelastic properties in the C8H17COOH/
TTAOH/H2O system. A trend of the intersection of G′ and G′′
can be observed at high oscillatory frequency (more than 10
Hz), which is too fast to be measured accurately, indicating a
very short stress relaxation time (τR). In our previous report of
the LA/TTAOH/H2O system, the LR phase and wormlike
micellar phase obtained were rather viscoelastic,32 which was
very different from the present system. These results of the
C8H17COOH/TTAOH/H2O and LA/TTAOH/H2O systems dem-
onstrate that rheological properties are affected by the chain
length of the surfactants.
Figure 3 shows the textures of two LR phase samples under
crossed polarizers. The sample at rTTAOH(CH) ) 0.3 has stripe
textures (Figure 3a), while the sample at rTTAOH(CH) ) 0.4 has cross-
like spherulite textures (Figure 3b). Both polarized textures of the
two samples demonstrate the existence of the lamellar phase.35
2
2H NMR Measurements of the Lr Phase. The H NMR
experiments are useful and convenient for determining the phase
2
behavior of surfactant solutions.31,36,37 When the H nucleus,
with spin quantum number I ) 1, is put in an external magnetic
2
field, the spectrum of H NMR depends on the interaction

C8F17COOH/TTAOH/H2O and C8H17COOH/TTAOH/H2O
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13131
Figure 4. 2H NMR spectra of the two LR phase samples at rTTAOH(CH) ) 0.3 and 0.4. T ) 25.0 ( 0.1 °C.
Figure 5. Oscillatory rheological data of two LR phase samples at rTTAOH(CH) ) 0.3 (a) and 0.4 (b). T ) 25.0 ( 0.1 °C.
Figure 6. Typical sample photos of C8F17COOH/TTAOH/H2O mix-
tures without (up) and with (below) crossed polarizers. The total
surfactant concentration is 100.0 mmol ·L-1, rTTAOH(CF) from left to right
is 0, 0.1, 0.3, 0.4, 0.5, 0.55, 0.6, 0.7, and 1. T ) 25.0 ( 0.1 °C.
Figure 7. Phase diagram of the C8F17COOH/TTAOH/H2O system at
T ) 25.0 ( 0.1 °C. Conductivity (9) and pH (2) data are included in
the phase diagram.
2. C8F17COOH/TTAOH/H2O System. Phase BehaWior.
Compared with the C8H17COOH/TTAOH/H2O system, the
phase behavior of C8F17COOH/TTAOH/H2O, as shown in
Figure 6, is very different and interesting because of the different
properties of fluorocarbon and hydrocarbon chains. The phase
diagram including conductivity and pH data is shown in Figure
7. Without TTAOH, 100.0 mmol ·L-1 perfluorononanoic acid
solution is a gel phase with obvious birefringence and vis-
coelasticity, which will be discussed in a separate report in detail.
When perfluorononanoic acid is replaced by TTAOH gradually,
rTTAOH(CF) ) 0 to 0.16, the samples are single transparent
birefringent gel phase solutions consisting of packed vesicles
formed by crystalline chains, which is demonstrated by DSC
measurements and FF-TEM observation. Within the region from
rTTAOH(CF) ) 0.16 to 0.33, the macroscopic phase separation with
an isotropic L1 phase on top and the LR phase at bottom can be
observed, indicating that the density of the LR phase is higher
than that of the L1 phase. After the two-phase region, from
rTTAOH(CF) ) 0.33 to 0.60, a single birefringent LR phase region
appears, which is confirmed to be an LRv phase by cryo-TEM
at different ratios of rTTAOH(CF) (rTTAOH(CF) ) nTTAOH/(nC F
17COOH
+ nTTAOH)), different phase regions can be observed8. From

13132 J. Phys. Chem. B, Vol. 114, No. 41, 2010
Zhang et al.
because of the high Krafft point, 48.3 °C.27 However, when
heated to above 50 °C under stirring, a birefringent vesicle
solution can be obtained. The solution transforms into gel when
it is cooled to room temperature. Figure 8a shows that the
transition of fluorocarbon chains from crystalline state to liquid
state occurs at 47.9 °C, which is very close to the Krafft point.
When TTAOH is added, the transition temperature increases at
first, as shown in Figures 8a-d, and then decreases with the
further increase of TTAOH fraction, as shown in Figure 8d-f.
This phenomenon might be attributed to two factors: one is the
electrostatic interaction between C8F17COO- and TTA+, which
leads to the increase of chain-melting temperature; the other is
the “softer” TTA+ chains, having the chain-melting point below
room temperature. Thus, the transition temperature is the
integrative result of the two factors. In the LRv phase region,
one cannot observe the chain-melting process above 20 °C, as
shown in Figure 8g,h, indicating the liquid state of the ion-pair
surfactant chains at room temperature.
Figure 8. DSC curves of samples at rTTAOH(CF) ) 0 (a), 0.05 (b), 0.10
(c), 0.12 (d), 0.15 (e), 0.16 (f), 0.33 (g), and 0.45 (h). The total surfactant
concentration is 100.0 mmol ·L-1
.
19F and 2H NMR Measurements. Fontell et al. investigated
the phase equilibria of two-component systems of perfluo-
rononanoic acid and its salts in aqueous solutions between 20
and 100 °C.38 However, in their report only solutions below
0.2 wt % can be obtained. While in our study, for the
perfluorononanoic acid aqueous solution above cmc, 100.0
mmol·L-1 (4.4 wt %), vesicles form at high temperature (above
50 °C). When cooled to room temperature, the liquid state chains
freeze to crystalline state. Thus, the gel phase at the region from
rTTAOH(CF) ) 0 to 0.16 is composed of vesicles with the bilayers
formed by crystalline state chains. Compared with the corre-
sponding hydrocarbon ones, fluorocarbon chains are more
hydrophobic and rigid, which leads to the preference of the
aggregates with less curvature or elongation, i.e., vesicles,
lamellar structures, or wormlike micelles.39
2
determination and H NMR measurements. When rTTAOH(CF)
continues to increase, from rTTAOH(CF) ) 0.60 to 0.96, another
macroscopic phase separation occurs with a creamed LR phase
at bottom and an isotropic L1 phase on top. Finally, from
rTTAOH(CF) ) 0.96 to 1, a very narrow region of single isotropic
L1 phase is observed.
DSC Measurements. Figure 8 shows DSC results of the gel
phase and LRv phase (i.e., LR phase in Figure 7 consisting of
vesicles) samples. The transition temperatures of the surfactant
chains from crystalline state to liquid state can be obtained.
Perfluorononanoic acid can dissociate completely into surfactant
anions and H+ below its cmc (2.75 mmol ·L-1) in aqueous
solutions.24 At concentrations above cmc, e.g., 100.0 mmol ·L-1,
perfluorononanoic acid cannot dissolve at room temperature
Figure 9. 19F NMR spectra of C8F17COOH in D2O at different concentrations (mmol · L-1): (a) 0.086 (below cmc), (b) 2.75 (at cmc), (c) 80.0
(gel phase), and (d) 100.0 (gel phase). Spectrum e is the sample at rTTAOH(CF) ) 0.45 with the total concentration of 100.0 mmol · L-1. T )
25.0 ( 0.1 °C.

C8F17COOH/TTAOH/H2O and C8H17COOH/TTAOH/H2O
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13133
Figure 10. 2H NMR spectra of samples at rTTAOH(CF) ) 0 and 0.45. T ) 25.0 ( 0.1 °C.
Figure 11. Polarized micrographs of three samples at rTTAOH(CF) ) 0 (a), 0.16 (b), and 0.45 (c). The total surfactant concentration is 100.0 mmol ·L-1
.
The 19F NMR spectra of C8F17COOH at different composi-
tions are shown in Figure 9a-d. In the monomer case, i.e.,
below its cmc, as shown in Figure 9a,b, the mobility of
field gradient. Thus, from 2H NMR spectra we can also conclude
the formation of the vesicle-formed gel phase and the LRv phase
(vesicle phase) of the two samples, which can be demonstrated
by FF-TEM and Cryo-TEM images below.
fluorocarbon chains is free, inducing the high resolution of 19
F
NMR signals of different groups. When the concentration of
C8F17COOH is above its cmc, for gel phase samples, as shown
in Figure 9c,d, the 19F NMR signals become broader and less
resolvable. However, the chemical shifts are unchangeable
except for the terminal methyl (ω-CF3) group. The reason
should be that the 19F NMR signals of crystalline chains
cannot be detected and we can only get the monomers’ signal.
Because the monomers’ mobility is prohibited in the gel
phase, the 19F NMR signals are less resolvable. For ω-CF3,
it is interesting that two separated 19F NMR peaks are
observed. The probable reason should be that, in crystalline
state chains, ω-CF3 is freer than other fluoromethylene groups
(-CF2-) and more sensitive to detect. Therefore, two signals
at -80.9 and -83.2 ppm for the ω-CF3 group are obtained,
belonging to monomers and crystalline state chains, respec-
tively. The separated NMR signals for one component with
different existent forms have been reported.28,40 For the LRv
phase sample with rTTAOH(CF) ) 0.45 (Figure 9e), only one
broad peak for the ω-CF3 group is detected. Similar behavior
in other fluorocarbon/hydrocarbon catanionic surfactant
systems was reported in our previous work.41 This may be
ascribed to two aspects: One is that in catanionic surfactant
systems, the monomer concentration is much lower than that
of individual surfactant due to the formation of surfactant
ion pairs.4 The other is that the mobility of fluorocarbon
chains is prohibited around hydrocarbon chains.
Images of the Gel Phase and LrW Phase. Figure 11 shows
the polarized micrographs of two gel phase and an LRv phase
samples, in which the cross-like spherulite textures are clearly
observed in all samples, indicating the existence of the lamellar
structures.35 Further characterizations are carried out by FF-
TEM and cryo-TEM images, as shown in Figure 12. Figure
12a,b show that the gel phase samples are composed of
polydisperse unilamellar vesicles. The LRv phase sample at
rTTAOH(CF) ) 0.45 consists of polydisperse vesicles including
ordinary and elongated ones, as shown in Figure 12c. Most of
the vesicles are unilamellar vesicles except very few bilamellar
ones. The elongated vesicles demonstrate the flexibility of the
vesicle bilayers.
Rheological Properties. Figure 13a,b shows the dynamic
rheological properties of gel phase and LRv phase samples.
The gel phase samples at rTTAOH(CF) ) 0 and 0.16 have similar
rheological properties and show high viscoelasticity. The
elastic modulus G′ of the both samples are higher than the
viscous modulus G′′, showing the elasticity dominating
property, which are the typical characteristics of gels.42,43 In
the LRv phase region, the rheological property is very different
from that of the gel phase. At rTTAOH(CF) ) 0.4, as shown in
Figure 13c, G′ is lower than G′′ at low oscillatory frequency
and they intersect at a certain frequency. At rTTAOH(CF) ) 0.45,
as shown in Figure 13d, both G′ and G′′ are much lower
compared with those at rTTAOH(CF) ) 0.4, and the intersection
of G′ and G′′ cannot be found within the frequency measured,
implying a very weak viscoelasticity and a very fast relaxation
process. From further study, one can find that G′, G′′, and
|η*| all decrease with the increase of rTTAOH(CF), namely, the
2
Figure 10 shows the H NMR spectra of two samples at
2
rTTAOH(CF) ) 0 and 0.45. Only a single peak appears in the H
NMR spectraum for each sample, indicating a zero average of
the quadrupole couplings between the 2H nuclei and the electric

13134 J. Phys. Chem. B, Vol. 114, No. 41, 2010
Zhang et al.
Figure 12. FF-TEM images of two gel phase samples at rTTAOH(CF) ) 0 (a) and 0.16 (b) and the cryo-TEM image (c) of sample at rTTAOH(CF)
0.45. The total surfactant concentration is 100.0 mmol ·L-1
)
.
Figure 13. Oscillatory shear rheograms of four samples at rTTAOH(CF) ) 0 (a), 0.16 (b), 0.4 (c), and 0.45 (d). The total concentration of surfactants
is 100.0 mmol ·L-1. T ) 25.0 ( 0.1 °C.
Conclusions
In summary, the phase behaviors of two catanionic surfactant
systems, C8F17COOH/TTAOH/H2O and C8H17COOH/TTAOH/
H2O, were studied and showed remarkable differences because
of the contrast between the fluorocarbon and hydrocarbon
chains. The C8H17COOH/TTAOH/H2O system exhibits phase
behavior similar to that of other fatty acid/TTAOH/H2O systems,
while the phase regions are dependent on the chain length of
the fatty acids. Within the LR phase region, the transition from
the LRl phase to the LRv phase appears due to the electrostatic
repulsion between the bilayer charges, which increases with the
increase of TTAOH proportion. The C8F17COOH/TTAOH/H2O
system exhibits much higher capability of forming vesicle
bilayers than that of the corresponding C8H17COOH/TTAOH/
H2O system. At the C8F17COOH-rich side, a high viscoelastic
gel phase consisting of crystalline vesicle bilayers is obtained
at room temperature. With the increase of TTAOH proportion,
Figure 14. Steady shear rheograms of the selected samples at rTTAOH(CF)
) 0, 0.16, 0.40, 0.45, and 0.50. T ) 25.0 ( 0.1 °C.
viscoelasticity decreases with the decrease of the C8F17COOH
the crystalline state vesicle bilayers melt to form liquid state
proportion. A similar phenomenon can also be observed in
the steady shear results in Figure 14. With the increase of
TTAOH proportion, the apparent viscosity decrease, which
is in accordance with visual observations.
vesicle bilayers. This phenomenon should be ascribed to the
rigid structure and stronger hydrophobicity of the fluorocarbon
chains, which prefer the formation of less curved aggregates
and a high melting temperature. Considering that fluorinated

C8F17COOH/TTAOH/H2O and C8H17COOH/TTAOH/H2O
J. Phys. Chem. B, Vol. 114, No. 41, 2010 13135
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Acknowledgment. This work is financially supported by the
NSFC (Grant Nos. 20833010 and 20803045) and National Basic
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