First anionic micelle with unusually long lifetime: Self-assembly of fluorocarbon-hydrocarbon hybrid surfactant

Article abstract of DOI:10.1021/ja0178564

The exchange of a fluorocarbon-hydrocarbon hybrid surfactant between monomer and micelle states in deuterium oxide has been investigated through ¹⁹F NMR and ¹H NMR experiments. The CF₃ group in the surfactant gives two kin

Full text of DOI:10.1021/ja0178564

Published on Web 05/14/2002
First Anionic Micelle with Unusually Long Lifetime: Self-Assembly of
Fluorocarbon-Hydrocarbon Hybrid Surfactant
Yukishige Kondo, Haruhiko Miyazawa, Hideki Sakai, Masahiko Abe, and Norio Yoshino*
Department of Industrial Chemistry, Faculty of Engineering, Tokyo UniVersity of Science, and
Institute of Colloid and Interface Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku,
Tokyo 162-8601, Japan, and Department of Engineering Chemistry, Faculty of Science and Technology,
Tokyo UniVersity of Science, Yamazaki, Noda, Chiba 278-8501, Japan
Received December 21, 2001
In the mixture of a fluorocarbon surfactant (FCS) and a
hydrocarbon surfactant (HCS), FCS-rich and HCS-rich micelles are
generally formed because of immiscibility between the fluorocarbon
and hydrocarbon chains.1 To solve this problem, new surfactants
having a fluorocarbon chain and a hydrocarbon chain in a molecule,
namely a hybrid surfactant, have been synthesized.2 These surfac-
tants formed aqueous micelles, and their critical micelle concentra-
tions (cmc’s) followed Kleven’s equation, although being unstable
to hydrolysis. We have also prepared sulfonate-type fluorocarbon
hybrid surfactants (CmF2m+1C6H4COCH(SO3Na)CnH2n+1; m ) 4,6,8;
n ) 4,6,8).3 These surfactants were highly stable to hydrolysis and
showed an emulsification ability for the ternary mixture of n-octane,
(hydrocarbon oil)/perfluoropolyether (fluorocarbon oil)/water, and
thus were expected to make it possible to prepare water-based fluoro
paints. Further, two specific properties of the hybrid surfactants
have been found: one is very high viscosity of the 10 wt % aqueous
solution of the surfactant having m ) 6 and n ) 4 at 37 °C,4 and
the other is an intramicellar phase separation between the fluoro-
carbon chain and the hydrocarbon chain.5 Neither homogeneously
double-chained analogues nor general surfactants have such proper-
ties.6 This fact suggests that hybridity of the hydrophobic part in
Figure 1. 19F NMR spectra of ω-CF3 group of F6H5OS in D2O at different
concentrations.
the hybrid surfactant molecule plays an important role in the unique
properties. Here we will report another interesting property of a
hybrid surfactant (F6H5OS). To our knowledge, this is the first
neighboring fluorine nuclei. As the surfactant concentration in-
report concerning anionic surfactants on the unusually slow
creased, an additional signal appeared at the upper magnetic field
(-86.91 ppm), and then both signals were broadened. The integrated
exchange of surfactant between micelle and monomer states on the
NMR time scale.
intensity (Iup) of the upfield signal increased rapidly with concentra-
tion increasing above 0.30 mM, whereas the downfield signal
intensity (Idown) almost leveled off (Supporting Information). The
threshold value agreed well with the cmc (0.34 mM) obtained by
surface tension measurements (Wilhelmy method) for aqueous
solutions of F6H5OS. It is thought that the upfield signal comes
from the F6H5OS micelles and that the downfield signal corre-
sponds to the surfactant monomer. The lifetime (τmic) of surfactant
micelles is generally in the order of 10-5 to 10-6 s, and the exchange
of surfactant molecules between monomer and micelle states is fast
on the NMR time scale,10 and accordingly a single weight-average
signal for the surfactants in both states is observed.11 The appearance
of an original signal of F6H5OS micelle must indicate the slow
exchange of the surfactant molecule between the monomer and
micelle states on the NMR time scale.
F6H5OS was synthesized from a ketone having a fluorocarbon
chain and a hydrocarbon chain. Details of the synthesis will be
described elsewhere.7 The NMR experiments were all performed
at 30 °C on a Bruker Avance DPX-400 spectrometer operating at
1
400 and 376 MHz for H and 19F nuclei, respectively. Samples
were prepared in D2O (purity 99.8%, Merck). Trifluoroacetic acid
in D2O was used as an external reference (-79.45 ppm) for 19F
NMR measurement.
Figure 1 shows the 19F NMR spectra of the ω-CF3 group in
F6H5OS at different concentrations. At 0.22 mM, F6H5OS
exhibited one sharp signal that was split into a triplet due to the
The τmic value of F6H5OS, estimated from the half width of
NMR signals, slightly decreased with increases in the surfactant
concentration, indicating that surfactant exchange between the two
states became faster (Figure 2). The τmic value at the cmc, obtained
by extrapolating the τmic-c curve to 0.30 mM, was 2.0 ms. This
* To whom correspondence should be addressed. E-mail: yoshino@
ci.kagu.sut.ac.jp.
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J. AM. CHEM. SOC. 2002, 124, 6516-6517
10.1021/ja0178564 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
higher NMR frequency, faster surfactant exchange between mono-
mer and micelle states is detectable as two separate NMR signals
because the NMR time scale becomes shorter. Previously, the
exchange rate of hybrid surfactants, very analogous to F6H5OS
but having no phenylene group, was investigated using a 19F NMR
spectrometer operating at 470 MHz, and consequently no slow
surfactant exchange on the NMR time scale was observed even
for the hydrophobic hybrid surfactant (abbreviated F7H6, see ref
17) whose cmc is lower than that of F6H5OS.17 On the other hand,
we have confirmed that F6H3OS (which is the hybrid surfactant
with a hydrocarbon chain shorter than that of F6H5OS by two
carbon atoms) also exhibits two separate 19F NMR signals corre-
sponding to the ω-CF3 groups in monomer and micelle states above
cmc. F6H3OS is much less hydrophobic compared to F7H6 and
F6H5OS because the cmc of F6H3OS is 0.81 mM. It has been
believed that the surfactant exchange rate depends primarily on the
hydrophobicity of the surfactants.10b,13a However, no one has
discussed where the unusually slow behavior of surfactant exchange
comes from. We suggest that surfactant interdigitation in micelles
could contribute to prolongation of micelle lifetime. Further studies
are in progress on the details of the micelle structure of F6H5OS
and interactions stabilizing its structure. Such intramicellar interac-
tions may also contribute to the long lifetime.
Figure 2. Relationship between the micelle lifetime (τmic) and the F6H5OS
concentration.
indicates that F6H5OS micelles have a lifetime that is about 102 to
103 times longer than that of general surfactant micelles.
Slow surfactant exchange on the NMR time scale has thus far
been found only for cationic gemini surfactants.13 The gemini
surfactants with long alkyl chains such as tetradecyl or octadecyl
groups have τmic of 40-100 ms, and a hybrid gemini cationic
surfactant having a dodecyl group and a perfluorooctyl group also
yields slow exchange. Although fluorinated nonionic surfactants
with an amide bonding also show slow surfactant exchange,14 they
form large aggregates of ca. 220 nm diameter, different from
micelles.10a Pulsed-gradient spin-echo (PGSE) experiments indi-
cated that F6H5OS forms significantly small micelles with a
hydrodynamic radius of 0.6 nm in the concentration range where
slow surfactant exchange is observed on the NMR time scale
(Supporting Information). Our finding is believed to be the first
example of unusually slow exchange of anionic surfactants between
monomer and micelle states on the NMR time scale.
Supporting Information Available: Procedures of PGSE experi-
ment and micelle lifetime estimation, a figure showing integrated
intensity ratio vs surfactant concentration, and a table containing
diffusion coefficients and hydrodynamic radii of micelles (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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On the basis of molecular modeling, the two hydrophobic chains
in F6H5OS, tridecafluorohexylphenyl and pentyl chains, are 1.2
and 0.7 nm long, respectively. Surfactant generally forms spherical
micelles having a hydrodynamic radius about same as its hydro-
phobic chain length in dilute aqueous solutions.15 However, the
hydrodynamic radius of F6H5OS micelle was much smaller than
the fluoroalkylphenyl chain length. F6H5OS micelles seem to be
composed of interdigitated structures of surfactant molecules. Such
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from the micelle, resulting in prolongation of micelle lifetime. The
19F NMR signal for the micelle appeared at ca. 2.3 ppm upfield
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(7) The synthesis of F6H5OS is briefly outlined as follows: the ketone having
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according to the previous paper (ref 3) and it was reduced to the
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by the reaction of the alcohol with SO3/pyridine complex in pyridine
solvent, followed by neutralization with NaHCO3 aqueous solution.
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1
from that for the monomer, as can be seen in Figure 1. The H
NMR experiment (not shown) also yielded a signal of the ω-CH3
group in F6H5OS, (a signal which was a weight-average for the
surfactants in both states of monomer and micelle) shifted upfield
with the surfactant concentration above the cmc. These facts suggest
that the CF3 and CH3 groups are shielded more strongly in micelle
state than in monomer state, and thus these terminal groups are
surrounded by the fluorocarbon chain2,11the benzene ring16 or both
in F6H5OS micelles. Such a situation is conceivable in the micelle
composed of the interdigitated surfactant structure. On the other
hand, the hydrodynamic radius of the F6H5OS micelle at a
concentration of 1.30 mM, at which the surfactant showed a fast
exchange on the NMR time scale, was 1.1 nm, approximately equal
to the length of the fluoroalkylphenyl group (Supporting Informa-
tion). The looseness of the interdigitation may bring about a
shortening of the micelle lifetime, increasing the micelle size. At
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