Behavior of wormlike micellar solutions formed without any additives from semi-fluorinated quaternary ammonium salts

Article abstract of DOI:10.1039/c3sm51591e

In the present work a series of fluorinated quaternary ammonium salts were synthesized in two steps. The physical properties of the obtained surfactants were characterized by DLS, zeta potential, viscosity, and Cryo-SEM. The surfactants with a long fluorinated chain form long and highly flexible aggregates that lead to highly viscous solutions and the formation of an entangled network made of flexible worm-like micelles. The viscoelastic properties of the surfactants that form hydrogels without any additives were characterized along with zeta potential and Cryo-SEM observations. They present a maximum viscosity as a function of worm-like micelles formation. The increase of the fluorinated and hydrocarbon chain lengths contributes to the enhancement of this viscoelastic behavior.

Full text of DOI:10.1039/c3sm51591e

Soft Matter
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PAPER
Behavior of wormlike micellar solutions formed without
any additives from semi-fluorinated quaternary
ammonium salts†
Cite this: Soft Matter, 2013, 9, 8992
ab
a
b
Gennifer Padoan, Elisabeth Taffin de Givenchy, Alessandro Zaggia,
a
a
b
a
Sonia Amigoni, Thierry Darmanin, Lino Conte and Fr e´ d e´ ric Guittard*
In the present work a series of fluorinated quaternary ammonium salts were synthesized in two steps. The
physical properties of the obtained surfactants were characterized by DLS, zeta potential, viscosity, and
Cryo-SEM. The surfactants with a long fluorinated chain form long and highly flexible aggregates that
lead to highly viscous solutions and the formation of an entangled network made of flexible worm-like
micelles. The viscoelastic properties of the surfactants that form hydrogels without any additives were
characterized along with zeta potential and Cryo-SEM observations. They present a maximum viscosity
as a function of worm-like micelles formation. The increase of the fluorinated and hydrocarbon chain
lengths contributes to the enhancement of this viscoelastic behavior.
Received 7th June 2013
Accepted 25th July 2013
DOI: 10.1039/c3sm51591e
www.rsc.org/softmatter
Wormlike micelles are dynamic systems and the aggregates
Introduction
are constantly breaking and recombining, making them ‘living
Surfactant solutions represent a well-documented class of self- polymers’ and proving an increase in viscosity with shear rate.1
0
assembled structures providing a powerful tool for several
The polymer analogy has been the catalyst for recent theo-
retical advances in understanding the properties of the aniso-
metric micellar solutions. By incorporating the dynamic nature
1
–5
applications in pharmaceutical and cosmetics industries.
6
7
The addition of an inorganic, a strongly binding organic salt,
or an oppositely charged surfactant leads to the growth of
11
of micelles into polymer solution theories, Cates has been able
aggregates. This growth generally leads to highly viscous to predict scaling laws for both equilibrium and dynamic
solutions and the formation of an entangled network made of properties of these systems. Because the micelles are in ther-

exible worm-like micelles, oen referred to as thread-like modynamic equilibrium with the surrounding solvent, they are
8
micelles. The length of these aggregates can reach up to 1 mm, not geometrically xed like polymer molecules. As a conse-
but the diameters are only a few times the length of the quence, the connecting or branching points of the micelles are
constituent molecules.
able to move along the primary axis of the micelles.
Prud'homme and Warr investigated the behavior of solu-
tions of tetradecyltrimethylammonium salicylate and they
These living polymers have one extra property that the normal
polymer system lacks (and hence their name): if either polymer
9
observed a rheological behavior of wormlike micelles in the chains or surfactant worm-like micelles are subjected to a high
surfactant solution similar to that of exible polymers. Yet, extensional force, they can be snapped in half. While polymer
above a critical concentration (c*), few weight percent, wormlike fragments always remain thereaer at this lower (halved)
micelles entangle into a transient network similar to a solution molecular weight, the broken surfactant micelles can reform
of exible polymers; they behave in exactly the same way as the under more quiescent conditions, since they are thermodynamic
equivalent macromolecules and entangle with each other to
form transient networks, which display remarkable viscoelastic
properties in a manner analogous to semi-dilute polymer
solutions.
objects held together by reversible bonds. If elastic liquids are
needed to withstand continual damage, and yet survive, then
these micellar systems are much more mechanically robust.
Micellar microstructures have a strong inuence on the
12
rheological properties of aqueous surfactant solutions. The
a
Univ. Nice Sophia Antipolis, CNRS, Laboratoire Physique Mati `e re Condens ´e e, UMR transition to a shear-thickened uid is commonly attributed to
7
336, Parc Valrose, 06100 Nice, France. E-mail: guittard@unice.fr; Fax: +33
the formation of shear-induced structures, where phase sepa-
ration takes place between a surfactant-rich and a surfactant-
poor phase. Their unique viscoelastic behavior is exploited to
tune the rheology in numerous applications with or without the
use of polymers or additives. For this reason they have been
4
92076156; Tel: +33 492076159
b
Dipartimento di Ingegneria Industriale, University of Padua, Via Marzolo, 9-35131,
Padua, Italy
†
Electronic supplementary information (ESI) available: Characterizations, yields
and cmc for the synthesized surfactants, complementary graphs of rheology, zeta
potential and Cryo-SEM images. See DOI: 10.1039/c3sm51591e
13
suggested for drag reducing uids in central-heating systems.
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Like all brous microstructures, the viscosity is high when the
A schematic representation of the synthesized uoro-
system is randomly dispersed in space and highly entangled, surfactants is given in Scheme 1. We report on the synthesis of
but once the ow has aligned the microstructures, the viscosity these surfactants, zeta potential and rheological measurements.
is quite low.
Moreover, viscoelastic properties of concentrated solutions of
This change from very high to very low viscosity takes place these hybrid surfactants as a function of temperature, stress
over a narrow range of stress or temperature. These self- and strain were studied along with Cryo-SEM observations.
assembled structures may reduce friction energy loss in
turbulent ow by up to 90% at relatively low surfactant
concentrations under appropriate ow and temperature
Materials and methods
conditions. This phenomenon is called drag reduction (DR) 3-Peruorobutyl-1,2-epoxypropane, 3-peruorohexyl-1,2-epoxy-
and it has signicant potential impacts on uid transport and propane and 3-peruorooctyl-1,2-epoxypropane were synthe-
1
4
20
on the environment. The self-assembly of surfactant DR sized according to the previous procedure. For the synthesis of
additives (DRAs) permits them to be used in recirculation hybrid surfactants, iodomethane and all chemicals were
15,16
applications such as in district heating/cooling systems.
purchased from Aldrich and used without further purication.
Cationic surfactants have been extensively investigated and it Unless specied, the solvents were used as received. Conr-
seems that they are the most effective surfactant drag reducers mation of the structures of the intermediates and products was
providing the proper conditions (such as the chemical struc- obtained by nuclear magnetic resonance (NMR) realized with a
tures and concentrations of surfactant and counter-ion). Some Bruker W-200 MHz instrument and mass spectrometry (MS) 80
cationic surfactants can spontaneously pack into cylindrical using a Thermo TRACEGC instrument from Thermoscher
structures without any additives at room temperature. Most of 81Corp. tted with an Automass III Multi spectrometer (elec-
the time, the addition of inorganic salts to ionic surfactants is tron ionization at 70 eV).
needed to promote micellar growth and viscoelastic proper-
17
ties. The mechanism involved is primarily the screening of
the electrostatic repulsion between the charged head-groups,
which results in reduction of the optimal molecular area at the
hydrocarbon–water interface, leading to an increase in the end-
Preparation of the semiuorinated tertiary amines (Anm
)
3
-Peruoroalkyl-1,2-epoxypropane (0.02 mol) [alkyl ¼ butyl (n ¼ 4),
hexyl (n ¼ 6), and octyl (n ¼ 8)] was added under stirring to
dialkylamine (0.022 mol) [alkyl ¼ methyl or ethyl] in methanol.
18
cap energy. Rather, little is known about uorosurfactants, in
particular cationic uorosurfactants, despite the technical
interest of their potential usefulness due to their strong
hydrophobicity and efficient surface activity. Solutions of these
surfactants have a number of peculiar properties because their
micellar structures are formed due to weak non-covalent
interactions and easily change in response to external actions.
The introduction of uorinated chains in surfactants is very
promising, thanks to their high hydrophobicity and stiffness,
for the formation of structures with relatively little curvature,
ꢀ
The mixture was heated at 60 C for 6 to 15 h. Purication of
the red liquid by column chromatography on silica gel (dichlor-
omethane : methanol¼ 99 : 1)gaveaminesAnm asyellowliquids.
The experiments were reproduced several times. In all cases, the
yields (see details in the ESI†) were reproducible (ꢁ3%).
General procedure for the synthesis of the hybrid surfactants
(
F H )
n m
A mixture of a tertiary amine (Anm) and an excess of iodo-
14,19
ꢀ
such as cylindrical micelles.
methane was reuxed in acetonitrile at 60 C for over 24 h.
Scheme 1 Synthetic route towards the studied surfactants F
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n m
H .
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Iodomethane and acetonitrile were removed by evaporation, the potential, eqn (2) can be used to determine the zeta potential of
residue was washed several times with anhydrous diethyl ether a particle. Then, k is related to the ionic strength of electrolyte
and recrystallized several times from a mixture of methanol and solution according to
diethyl ether, and then the residue was washed several times
2
1/2 1/2
I
k ¼ (2000F )/(3
0
3
r
RT )
(4)
n m
with petroleum ether to give F H in the form of white solids
(see yields, chemical characterizations, critical micellar
where F is the Faraday constant, 3
the relative permittivity, R the gas constant, T the tempera-
ture, and I the ionic strength of the electrolyte solution.
0
the permittivity of a vacuum,
S
concentration (cmc) and surface tensions (g ) in the ESI†).
3
r
22
Dynamic light scattering experiments
The Z-average diameter is the mean hydrodynamic diameter and
the polydispersity index is an estimate of the width of distribu-
tion. Both of these parameters were calculated according to the
International Standard on dynamic light scattering (ISO13321).
Rheological measurements
The rheological properties of the samples were measured using
an Anton Parr Physica MCR-301 rheometer. Cone geometry was
used in each case (50 mm diameter, 1 angle). The gap was
ꢀ
All the measurements were done at a temperature of 25 C,
0.05 mm. A solvent trap was used to minimize water evaporation.
which was controlled by means of a thermostat using a Zetasizer
Nano-ZS model 3600 (Malvern Instruments Ltd) equipped with a
He–Ne laser (l ¼ 633 nm, 4.0 mW). Measurements of the size of
the surfactant aggregates were performed by DLS to examine the
state of aggregation in aqueous solution. The aqueous solutions
were prepared using deionizated water and ltered through a
ꢀ
Experiments were performed at 20, 30, 40, and 50 ꢁ 1 C. The
viscosity of the sample was obtained from steady-shear
ꢃ1
measurements with the shear ranging from 0.3 to 500 s
Frequency sweep measurements were performed at a given stress
in the frequency region varying from 0.01 to 100 rad s . All the
rheology/viscosity data given in this paper are average values of
.
ꢃ1
0.45 mm lter. The time-dependent correlation function of the
two measurements;thedeviationswereintherangeofꢁ0.01Pas.
scattered light intensity was measured at a scattering angle of
ꢀ
9
0 relative to the laser source (red-scattering detection).
Microscopic observations
Measurements were repeated several weeks aer prepara-
tion, and no modication of the sizes was visualized. DLS
diffusion coefficients (D) were evaluated by tting the equation
of Stokes–Einstein:
Cryogenic scanning electron microscopy (Cryo-SEM) images
were obtained with an FESEM6700F JEOL microscope (Japan).
One drop of the sample was rapidly frozen in nitrogen slush at
ꢀ
ꢃ220 C and transferred under vacuum into the cryofracture
D ¼ (k
B
T )/6phr
(1)
apparatus (Alto 2500 GATAN UK) chamber where it was frac-
ꢀ
tured at ꢃ100 C and maintained at this temperature for 7 min
where kB is Boltzmann's constant, T is the absolute tempera-
ture, and h is the viscosity of the solvent. The radius (r) of the
particles was estimated from their diffusion coefficient (D).
for sublimation. It was then metallized with AuPd and intro-
duced into the microscope chamber where it was maintained at
ꢀ
ꢃ100 C during the observation. To ensure reproducibility, the
micrographs presented in this article were chosen from a large
number of negatives.
Zeta potential measurements
Zeta potential measurements of the surfactant complex were
carried out using a Malvern Instruments Ltd NanoZs by taking Results and discussion
the average of three measurements at the stationary level. The
Dynamic light scattering discussion
cell used was a 5 mm ꢂ 2 mm rectangular quartz capillary. The
Fig. 1 shows the diameter (D) of the population of aggregates as
a function of the solution concentrations for F and F . D
increases as the concentration and the value of D was higher for
temperature of the experiments was 298.15 ꢁ 0.01 K controlled
by a proportional temperature controller HETO. The zeta
potential z were calculated from the electrophoretic mobilities
4
H
1
4 2
H
+
21
4 2 3 4 1
F H suggesting the inuence of the head size (N Me for F H
(m
E
) using the Henry equation:
+
against N MeEt for F H ) on larger aggregates.
2
4
2
3
m h
E
1
Fig. 2 shows the diameter D of the aggregates in solution (at
ve times the cmc) as a function of the uorinated chain length:
z ¼
(2)
2
3
0
3
r
f ðkaÞ

it can be observed that higher values of D are reached with
longer uorinated chains. It reects the ability of uorinated
surfactants to form aggregates with lengths as long as several
micrometers. These ndings are consistent with the earlier
where 3 is the permittivity of vacuum, 3 and h are the relative
0
r
permittivity and viscosity of the medium, respectively, a is the
particle radius, and k is the Debye length. The function f (ka)
depends on the particle shape and for our systems it was
determined by
10
observations by Zana and co-workers for hybrid surfactants
having a very strong propensity for micellar growth and
(3) formation of micelles of very low curvature.
9
75
330
f ðkaÞ ¼ 1:5 ꢃ 2
þ
ꢃ
ka 2k2a2 k3r3
ꢃ
Moreover, in our case, the presence of big counter-ions I
The Henry function is appraised to be 1 for small kr and 1.5 near to the polar head-groups of the surfactant molecules
for large kr. Hence, for larger aggregates of relatively small decreases the repulsion force between the head-groups. This
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organizationinlarge aggregates (vesicles orcylinders)for allofthe
synthesized surfactants.
Zeta potential discussion
Zeta potential (z) measurements for F
8
H
1
and F
8
H
2
as a func-
) times the
tion of concentration for 20 (C ), 30 (C
1
2
), and 40 (C
3
cmc were carried out; these concentrations were chosen to
observe the behavior until the maximal solubility of the
synthesized surfactants was reached. The concentration over 40
8 1
times the cmc of F H was not realized because the surfactant is
insoluble at this concentration. The main peak was found to be
positive and it corresponds to the positive change of head
25
groups in these quaternary ammonium surfactants.
The main positive peak is probably attributed to large
aggregates with a low degree of curvature and a weak adsorption
of I counter-ions at the micelle surface.
4 1 4 2
Fig. 1 Aggregate diameters as a function of concentration for F H and F H .
ꢃ
Here, the positive zeta potential of F H and F H decreases
8
1
8
2
with increasing surfactant concentration due to the formation
of entangled networks of wormlike micelles caused by higher
hydrophobic interactions.
Also, zeta potential measurements were carried out at a
concentration twice the cmc of F
n
H
m
with n ¼ 4, 6, 8 and m ¼ 1,
2
in order to study the inuence of the uorinated chains. The
measured values for the synthesized surfactants are summa-
rized in Table 1. All values were positive, due to the head group,
and the values increased on increasing the uorinated chain
length. The uorinated chain enhances the hydrophobic
interactions leading to an increase in the value of the zeta
potential.
Rheological discussion
Fig. 2 Aggregate diameters as a function of the fluorinated chain length (red
left scale for F and blue right scale for F surfactants).
n
H
1
n
H
2
The stiffness of the uorocarbon chain sometimes has an effect
17
on the packing leading uorinated surfactants, in aqueous
solutions, to form cylindrical micelles with lengths as long as
reduction in the repulsion allows the surfactant molecules to several micrometers, where analogous hydrocarbon would form
approach each other more closely and form larger aggregates.
spherical aggregates. The packing constraints of the per-
ꢃ
However, the electrostatic effect of the I counter-ion uoroalkyl chain are responsible for the formation of exible
binding on ionic micelles and the change in the hydrocarbon- aggregates in water at low concentrations and low tempera-
bonded structure of water cause hydrophobic interactions tures. Surfactant solutions commonly exhibit viscoelastic
23
24
between surfactant molecules. Also, due to Ostwald ripening,
properties only at low temperatures because heating enhances
aggregates of larger curvature realized themselves into single thermal vibrations in micelles and the chains are more
species due to their higher chemical potential and the liberated frequently breaking and their average length oen decreases.
single species adsorb to the aggregates of smaller curvature to Some surfactants reach a maximum viscosity at a specic
6
make them further larger in size. DLS analyses clearly show the temperature and then decrease with increasing temperature.
presence in the solution of different sizes of aggregates and the These surfactants are called thermo-responsive viscoelastic
transition from monomodal to bimodal structure with an surfactants.
increase in the uorinated chain length (see DLS curves in the
ESI†). The effect of a long uorinated chain on packing of hydrogels in pure water. Then, to investigate the importance of
surfactants is expected to affect strongly the curvature of a
8 1 8 2 6 2
The synthesized surfactants F H , F H and F H formed
25
surfactant layer and the incorporation of a uorinated chain
into the core provides a bimodal distribution for the long Table 1 Values of the zeta potential of surfactants F H
n m

uorinated chains (F Hm and F H ).
8 6 m
Surfactant
z (mV)
Surfactant
z (mV)
In fact, it may be assumed that with an increasing number of
carbon atoms in the uorinated chain, the hydrophobic interac-
tions become more intensive and larger aggregates can be F6H1
observed. These measurements suggest spontaneous
F
8
H
1
65.9
24.4
11.3
F
F6H2
F H
4 2
8
H
2
58.3
38.9
9.5
F
4
H
1
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
uorocarbon and hydrocarbon chains to give viscoelastic The increase of viscosity with temperature can be understood in
properties, rheological measurements were carried out. No terms of a decrease in the mean size of the micelles or the
7
peculiar hydrogel behavior was observed for the solutions of formation of cross-linked micellar networks.
11
FnHm with n ¼ 4, 6 and m ¼ 1, 2. Only F H , F H , and F H
Considering the reptation model ofCates, relaxation of chain
showed tendency to form hydrogels in pure water above their conformations occurs by the gradual disengagement of any given
8
1
8
2
6
2
26
cmc and only F
investigate concentrated solutions of F
ical measurements under different conditions were carried out. erties of exible surfactant micelles. In particular, the normalized
Viscosity as a function of temperature for F . Fig. 3 and 4 number density C(L) of micelles of length L is given by eqn (5):
show viscosity as a function of temperature for F H and F H at
8
H
1
and F
8
H
2
showed high viscosity. However, to chain, by curvilinear diffusion along its own contour from a
8
H
1
and F , rheolog- tube-like environment. Cates also deduced the rheological prop-
8 2
H
8
H
m
8
1
8
2
L
L
CðLÞ ¼ e
(5)
concentrations of 20 (C ), 30 (C ) and 40 (C ) times the cmc. We
1
2
3
can observe thermo-responsive viscoelasticity for all the
concentrations. Also, the value of the maximal viscosity and the
temperature at which the maximal viscosity appeared seem to
be dependent on the surfactant concentration and the hydro-
where C(L) is the normalized number density of micelles of
ꢀ
length L and the mean length L is related to the volume fraction
4
and to temperature T by eqn (6):
1
2
6
eðEbreak=2k
Þ
T
carbon chain length.
Lð4; TÞ ¼ 4
B
(6)
ꢀ
In particular, the maximum value was T ¼ 19 C at C
1
¼ 20
where Ebreak is the scission energy required to break a micelle
into two parts.
ꢀ
times cmc and T ¼ 26 C at C ¼ 30 times cmc for F H , and
2
8
1
ꢀ
ꢀ
ꢀ
T ¼ 32 C at C ¼ 20 times cmc, T ¼ 42 C at 40 times cmc, and
1
This temperature dependence of the micellar length
produces a decrease in micellar size and thus a greater resis-
tance to the ow. However, networks have to be, in some way,
similar to exible polymers with their stress relaxation partly
controlled by the motion of micelles along its own contour and
by a reversible scission of the micelles. The formation of an
inter-micellar junction may be viewed as a fusion reaction
between the end cap of one cylindrical micelle and the main
T ¼ 37 C times cmc at 40 times cmc for F H .
8
2
As schematized in Fig. 5, there is a thermodynamic expla-
nation usually proposed for this increase–decrease in viscosity.
10
body of another, forming a Y-like joint.
Also, Fig. 3 and 4 show that the trend of the viscosity vs.
temperature curve is similar to the increasing surfactant
concentration, but the temperature at which the maximum
viscosity is reached gradually shis to lower values. Then, they
show that highly viscoelastic solutions are formed at a low
temperature and at a high surfactant concentration. In fact,
Fig. 4 shows that the value of maximal viscosity for F
8 2 3
H at C is
higher than the value at C and C because the viscosity
1
2
increases with the surfactant concentration due to an increase
in the mean micellar length. As the contour length decreases,
for the viscosity is predicted to decrease because of a reduction in
the extent of entanglement of micelles. Therefore, changes in
solution concentrations can lead to the formation of other self-
assembled structures and the formation of a micellar network
takes place at a lower temperature. Also, as shown in Fig. 4 the
1 2 3
Fig. 3 Viscosity as a function of temperature at concentrations C , C , and C
8 2
F H .
8 2
maximal viscosity is higher for F H for all concentrations,
1 2
Fig. 4 Viscosity as a function of temperature at concentrations C and C for
F
8
H
1
.
Fig. 5 Change of micelles and viscosity as a function of temperature.
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indicating that an increase of hydrophobicity leads to the
formation of larger molecular assemblies in solution; in fact, if
the aggregates are located in the vicinity of the micelle surface
they are likely to actively participate in the hydrophobic inter-
micellar interaction, thereby forming large molecular assem-
blies through the gathering of small molecular assemblies. For
a saturated network the reptation model breaks down, which
8 1 3
suggests that the system becomes very uid and so F H at C
shows a low viscosity and no hydrogel behavior.
Viscoelastic measurements
0
Temperature sweep. The storage modulus (G ) and the loss
0
0
modulus (G ) as a function of temperature at a xed frequency of
Hz and 20% strain for the concentrations C , C , and C were
also analyzed. The results at low frequencies are somewhat scat-
1
1
2
3
teredbecausetherheometerwasoutofitsmeasuringrange.Inthe Fig. 7 Correlation between viscosity and relaxation time of F H2 at C3.
8
ꢀ
range of temperatures used (T ¼ 10–70 C) the plots are linear and
0
00
the value of G is always larger than G , then the storage modulus
ꢀ
shows no frequency dependency in the range of 10–70 C. In this
range, analyzed surfactants F H and F H present a viscoelastic
8
1
8
2
behavior. In the ESI,† examples of measurements are given.
0
Frequency sweep. The storage modulus (G ) and the loss
0
0
ꢀ
modulus (G ) as a function of frequency at 20, 30, 40 and 50 C
and 20% xed strain were analyzed to calculate the evolution of
relaxation times as a function of temperature. Fig. 6 shows an
example of measurements for F
gathered in Fig. 7 and 8.
8 1 1
H at C . All the values are
Reptation and breaking time
The relaxation time can be correlated with reptation time srep
and breaking time s of Cates's model. In fact, the relaxation
b
time is the geometric mean of two characteristic times, for
micellar breaking–recombination and for micellar reptation, as
proposed by the reptation-reaction model. However, the repta-
tion time (srep) is dened in eqn (7):
8 2 1
Fig. 8 Correlation between viscosity and relaxation time of F H at C .
creation of an interface between its hydrophobic domain and
water. The surfactant head-groups are brought to the aggregate
surface, giving rise to steric repulsion between them.
Then, the breaking time is dened in eqn (8)
3
/2
s
rep ¼ Lꢀ 4
(7)
it is the time required to form a worm-micelle of contour length
3
/2
ꢀ
L to pass through a hypothetical tube, with 4 volume fraction.
At srep, the formation of the aggregate is associated with the
ꢃ1
ꢀ
r
s ¼ (k L)
(8)
b
it is the average time necessary for a chain of average length to
break into two pieces by a reversible scission characterized by a
temperature-dependent rate constant k
r
per unit time and per
unit arc length, which is the same for all elongated micelles and
27
is independent of time and of volume fraction.
Fig. 7 and 8 show a correlation between the relaxation time
and change of viscosity as a function of temperature. At low
temperatures either not all of the surfactants in the solution
form wormlike micelles, or not all of the wormlike micelles were
large enough to entangle. Therefore, the relaxation time was
very low at low temperatures because surfactant micelles
exchange slowly at lower temperatures and they have strong
hydrophobic interactions with single micelles. Then, with an
increase of the temperature a very small reduction of repulsion
between the head-groups and the hydrophobic micellar aggre-
gates (interactions between the hydrophobic chains located
0
00
8 1 1
Fig. 6 G and G as a function of frequency for F H at C .
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Fig. 9 Representative Cryo-SEM micrographs of F
8
H
1
(20ꢂ cmc); (a and b) scale bar 100 nm; (c) scale bar 1 mm.
near to the micelle surface) determines the increase of molec-
ular aggregates. Then, the aggregates accomplish cylinder to
worm-like micelles increasing the viscosity and allowing the
28
system to release the stress quickly. Similarly, an increase of
the micelle length oen leads to lower viscosity due to faster
relaxation, because the triggering tricks consist essentially of
reducing the preferred curvature of the micelles.
Moreover, for an entangled network of wormlike micelles,
junctions provide a mean relax stress to increase the viscosity.
Hence, at the maximum viscosity, uorocarbon chains are
removed from contact with water and are subjected to packing
constraints because of the polar head group requirements that
should remain at the aggregate–water interface and the micelle
core should have the hydrocarbon chains in the liquid.
Consequently, they are incorporated into the micelle core
and the surfactant forms compact micelles with increasing
relaxation time. Then, an increase of temperature determines
the degradation process of micelles leading to a decrease of
viscosity and relaxation time.
Fig. 10 Representative Cryo-SEM micrographs of F
8
H
2
(5ꢂ cmc); scale bar 1 mm.
length, are linked together at junctions that range from simple
-fold unions to larger, at, multifunctional connections. The
same results were found for F as shown in Fig. 11 (scale bar
3
Cryo-SEM discussion
8 1
H
Cryo-SEM selected images were presented to establish the
morphologies associated with the surfactant solutions. Fig. 9a
shows a Cryo-SEM image (scale bar 100 nm) of F H solution in
8 1
water at 20 times the cmc. The predominant structural feature in
this imageissphericalnodules, withlittleevidence ofbranchesor
cylindrical aggregates. Fig. 9b shows a giant cylindrical micelle in
the medium of a small spherical nodule. Fig. 9c (scale bar 1 mm)
shows small cylindrical aggregates linked together at junctions.
Fig. 10 (scale bar 1 mm) shows small branched cylindrical
8 2
aggregates observed for F H at a concentration of about 5 times
the cmc. The basic structural element of the random network
appears to be approximately cylindrical, with a diameter of 50–
100 nm, consistent with the dimensions extracted from the
form factor scattering. Although it is impossible to determine
the aspect ratio with any certainty, these cylinders appear to be
rather long, with length-to-radius exceeding at least 30–40 nm.
This image reveals a densely connected, random, network
morphology. Strands of surfactants, roughly 40–50 nm in Fig. 11 Representative Cryo-SEM micrographs of F
8
H
1
(7ꢂ cmc); scale bar 10 mm.
8
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0 mm). In contrast to spheres, cylinders, and vesicles, there is
no way to continuously expand (dilate) a three-dimensional
network formed from bulk (hydrophobic) cylindrical struts and
multifunctional branches. The preferred local structure is
dominated by Y-junctions, thus leading to the formation of a
dense network under equilibrium conditions.
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Acknowledgements
2
The authors thank Jean-Pierre Laugier of the Centre Commun
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Antipolis) for the Cryo-SEM images and the team of Fluides et
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