Star-shaped trimeric quaternary ammonium bromide surfactants: Adsorption and aggregation properties

Article abstract of DOI:10.1021/la301220y

Novel star-shaped trimeric surfactants consisting of three quaternary ammonium surfactants linked to a tris(2-aminoethyl)amine core were synthesized. Each ammonium had two methyls and a straight alkyl chain of 8, 10, 12, or 14 carbons. The adsorption and aggregation properties of these tris(N-alkyl-N,N- dimethyl-2-ammoniumethyl)amine bromides (3C_ntrisQ, in which n represents alkyl chain carbon number) were characterized by equilibrium and dynamic surface tension, rheology, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) techniques. 3C _ntrisQ showed critical micelle concentrations (CMC) 1 order of magnitude lower than that of the corresponding gemini surfactants with an ethylene spacer and the corresponding monomeric surfactants. The logarithm of the CMC decreased linearly with increasing hydrocarbon chain length for 3C _ntrisQ. The slope of the line, which is well-known as Klevens equation, was larger than those of the monomeric and gemini surfactants; however, considering the total carbon number in the chains, the slope was shallower than the monomeric and was close to the gemini. Through the results such as surface tensions at the CMC (32-34 mN m^-1) and the parameters of standard free energy, CMC/C₂₀ and pC₂₀, it was found that 3C_ntrisQ could adsorb densely at the air/water interface despite the strong electrostatic repulsion between multiple quaternary ammonium headgroups. Moreover, dynamic surface tension measurements showed that the kinetics of adsorption for 3C_ntrisQ to the air/water interface was slow because of their bulky structures. Furthermore, the results of rheology, SANS, and cryo-TEM determined that 3C_ntrisQ with n = 10 and 12 formed ellipsoidal micelles at low concentrations in solution and the structures transformed to threadlike micelles with very few branches for n = 12 as the concentration increased, but for n = 14 threadlike micelles formed at relatively low concentrations.

Full text of DOI:10.1021/la301220y

Article
pubs.acs.org/Langmuir
Star-Shaped Trimeric Quaternary Ammonium Bromide Surfactants:
Adsorption and Aggregation Properties
,†
‡
§
,‡
⊥
Tomokazu Yoshimura,* Takumi Kusano, Hiroki Iwase, Mitsuhiro Shibayama,* Tetsuya Ogawa,
and Hiroki Kurata⊥
†
Research Group of Chemistry, Division of Natural Sciences, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan
Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
Comprehensive Research Organization for Science (CROSS), 162-1 Shirakata, Tokai, Ibaraki 319-1106, Japan
‡
§
⊥Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
*
S Supporting Information
ABSTRACT: Novel star-shaped trimeric surfactants consisting of three quaternary ammonium surfactants linked to a tris(2-
aminoethyl)amine core were synthesized. Each ammonium had two methyls and a straight alkyl chain of 8, 10, 12, or 14 carbons.
The adsorption and aggregation properties of these tris(N-alkyl-N,N-dimethyl-2-ammoniumethyl)amine bromides (3C trisQ, in
n
which n represents alkyl chain carbon number) were characterized by equilibrium and dynamic surface tension, rheology, small-
angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) techniques. 3C trisQ showed
n
critical micelle concentrations (CMC) 1 order of magnitude lower than that of the corresponding gemini surfactants with an
ethylene spacer and the corresponding monomeric surfactants. The logarithm of the CMC decreased linearly with increasing
hydrocarbon chain length for 3C trisQ. The slope of the line, which is well-known as Klevens equation, was larger than those of
n
the monomeric and gemini surfactants; however, considering the total carbon number in the chains, the slope was shallower than
−1
the monomeric and was close to the gemini. Through the results such as surface tensions at the CMC (32−34 mN m ) and the
parameters of standard free energy, CMC/C and pC , it was found that 3C trisQ could adsorb densely at the air/water
2
0
20
n
interface despite the strong electrostatic repulsion between multiple quaternary ammonium headgroups. Moreover, dynamic
surface tension measurements showed that the kinetics of adsorption for 3C trisQ to the air/water interface was slow because of
n
their bulky structures. Furthermore, the results of rheology, SANS, and cryo-TEM determined that 3C trisQ with n = 10 and 12
n
formed ellipsoidal micelles at low concentrations in solution and the structures transformed to threadlike micelles with very few
branches for n = 12 as the concentration increased, but for n = 14 threadlike micelles formed at relatively low concentrations.
INTRODUCTION
and tetrameric surfactants. In the first report on trimeric
surfactants, Zana et al. synthesized quaternary ammonium-type
surfactants with three dodecyl chains and three ammonium
headgroups connected by two propylene spacer groups. By
changing the counterions (Br vs Cl), they investigated the
surfactants’ critical micelle concentrations (CMC), micelle
ionization degrees, and micelle aggregation numbers. Esumi,
■
Recently, oligomeric surfactants with multiple hydrophobic
groups and multiple hydrophilic groups in a molecule have
become attractive as a new class of surfactants. Gemini or
dimeric surfactants, the simplest oligomeric surfactants, contain
two hydrophobic and two hydrophilic groups, and they have
unusual and unique properties that merit synthetic, phys-
icochemical, and applied investigations. Gemini surfactants
consist of conventional monomeric surfactant fragments
connected together with spacer group near hydrophilic
headgroups. Extending gemini surfactants furnishes trimeric
3
,4
1
,2
Received: March 22, 2012
Revised: May 26, 2012
Published: May 29, 2012
©
2012 American Chemical Society
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Ikeda, and Zana prepared similar trimeric surfactants with
hydrochloric acid (35%), and sodium hydroxide were obtained from
Kanto Chemicals Co., Inc. (Tokyo, Japan). All chemicals were reagent-
grade commercial materials and used without further purification.
Synthesis of Tris(N,N-dimethyl-2-aminoethyl)amine. Tris(2-
aminoethyl)amine (48.6 g, 0.33 mol) was added slowly to a stirred
solution of formaldehyde (6.1 mol) and formic acid (5.6 mol) at room
temperature, and the mixture was refluxed for 12 h. Concentrated
hydrochloric acid was added to the mixture, and the solution was
heated on a water bath for 3 h. After the solution was concentrated on
a rotary vacuum evaporator, the residual solid was washed twice with
methanol and dried under reduced pressure to give tris(N,N-dimethyl-
2-aminoethyl)amine hydrochloride as a white solid in 65% yield. The
5
6
different spacer chain lengths (s = 2 and 6 ), spacers (2-
7
hydroxypropylene), and hydrocarbon chain lengths (C −
8
8
C ), and their adsorption and aggregation properties were
12
investigated in detail. For instance, trimeric surfactants have
much lower CMCs than those of the corresponding
monomeric and gemini surfactants, and CMC increases with
spacer length. The micelle aggregation number also decreases
with increasing spacer chain length in a trend similar to that of
gemini surfactants. In addition, cryo-TEM observations showed
that branched threadlike micelles form for s = 3, and spheroidal
micelles form for s = 6. Laschewsky et al. also synthesized
trimeric surfactants with rigid spacer groups such as trans-
butenylene, m-xylylene, and p-xylylene, and their surface
tension, viscosity, foaming, solubilizing capacity, and micelle
aggregation numbers were studied and compared to those of
1
data of H NMR and elemental analysis are shown in Supporting
Information.
Synthesis of Tris(N-alkyl-N,N-dimethyl-2-ammoniumethyl)-
amine Bromides (3C trisQ). Tris(N,N-dimethyl-2-aminoethyl)-
n
amine hydrochloride (10.0 g, 0.029 mol) was added slowly to 400
mL of methanol containing sodium hydroxide (16 g), and the solution
was stirred with heating for 2−3 h. After the solvent was removed by
evaporation, acetone was added to the residue, and the solution was
filtered to remove the inorganic precipitate. The evaporation of
acetone yielded tris(N,N-dimethyl-2-aminoethyl)amine as a brown
liquid. Next, n-octyl iodide (n = 8), n-decyl iodide (n = 10), n-dodecyl
iodide (n = 12), or n-tetradecyl bromide (n = 14) (0.174 mol) was
added to a stirred solution of tris(N,N-dimethyl-2-aminoethyl)amine
in about 200 mL of ethanol. The mixture was refluxed for over 40 h.
After the solvent was evaporated under reduced pressure, the residue
was washed several times first with ethyl acetate and then with hexane
and recrystallized from mixtures of ethyl acetate and ethanol to give
each tris(N-alkyl-N,N-dimethyl-2-ammoniumethyl)amine iodide or
bromide as white solids. The yields were 74%, 76%, 82%, and 45%
for n = 8, 10, 12, and 14, respectively.
9
,10
the corresponding gemini and tetrameric surfactants.
Furthermore, Wang et al. synthesized two types of trimeric
surfactants with amide groups in the spacers and reported their
fundamental properties and vesicle-to-micelle transitions in
1
1,12
solution.
anionic
There are also some reports concerning
1
3−15
16
and nonionic trimeric surfactants, in addition
to the cationic ones mentioned above. We also reported the
synthesis and properties of star-shaped anionic trimeric
surfactants, whose spacer groups radiate from a central tertiary
15
amine, derived from tris(2-aminoethyl)amine. However, there
are still relatively few reports on trimeric surfactants compared
with gemini. Trimeric surfactants are more difficult to
synthesize, and the starting materials are more expensive.
Additionally, the relationships between structures with varying
numbers of hydrocarbon chains and chain lengths are not yet
clear.
To prepare bromides for n = 8, 10, and 12, the iodide compounds
were dissolved in methanol and passed through a column of Dowex 1-
X8 anion-exchange resin (exchanged from Cl to Br form, 50−100
mesh). After the eluant was evaporated under reduced pressure, the
residue was washed with ethyl acetate, recrystallized from mixtures of
In this paper, we describe the synthesis of novel quaternary
ammonium star-shaped trimeric surfactants, tris(N-alkyl-N,N-
ethyl acetate and ethanol, and dried under reduced pressure, yielding
dimethyl-2-ammoniumethyl)amine bromides (3C trisQ, where
1
n
3C trisQ (n = 8, 10, and 12) as white solids. The data of H NMR and
n
n represents alkyl chain carbon number of 8, 10, 12, and 14).
elemental analysis are shown in Supporting Information.
3
C trisQ was derived from tris(2-aminoethyl)amine and is
Measurements. Except when used in the SANS experiment, the
surfactant solutions were prepared using Milli-Q Plus water (resistivity
18.2 MΩ cm) and the measurements were performed at 25 ± 0.5
C. The surfactant solutions for SANS measurement were prepared
n
shown in Figure 1. Several adsorption and aggregation
=
°
with deuterium oxide (D O), and the measurements were performed
2
at 28 ± 0.5 °C.
General Methods. The electrical conductivity measurements were
performed with a CM-30R TOA electrical conductivity meter to
determine Krafft temperature and CMC. The surface tensions of
aqueous solutions of the star-shaped trimeric surfactants were
̈
measured with a Kruss K122 tensiometer using the Wilhelmy plate
−2
technique. The surface excess concentration (Γ) in mol m and the
area occupied by each molecule (A) of each trimeric surfactant at the
air/solution interface were calculated using the classic Gibbs
1
7
adsorption isotherm equations, Γ = −(1/iRT)(d γ/d ln C) and A
Figure 1. Chemical structures of star-shaped quaternary ammonium
= 1/(NΓ). Here, γ is the surface tension, C the surfactant
−1
−1
bromide trimeric surfactants, 3C trisQ.
concentration, R the gas constant (8.31 J K mol ), T the absolute
temperature, and N Avogadro’s number. The value of i in the equation
for the trimeric surfactants is taken to be 4, which is the number of
possible species assuming complete dissociation in solution.
n
properties of the surfactants were evaluated including
equilibrium and dynamic surface tension, rheology, small-
angle neutron scattering (SANS), and cryogenic transmission
electron microscopy (cryo-TEM).
The fluorescence measurements of the star-shaped trimeric
surfactants were performed with a Hitachi 650-10S fluorescence
spectrophotometer. The dynamic surface tensions of the star-shaped
̈
trimeric surfactant solutions were measured with a Kruss BP2 bubble-
EXPERIMENTAL SECTION
■
pressure tensiometer. The rheological experiments were performed on
a stress-control rheometer (MCR-501, Anton Paar, Austria) in
viscosity measurement mode. The details of these measurements are
shown in Supporting Information.
Materials. Tris(2-aminoethyl)amine, n-octyl iodide, n-decyl iodide,
n-dodecyl iodide, and n-tetradecyl bromide were obtained from Tokyo
Chemical Industry Co., Ltd. (Tokyo, Japan). Formaldehyde (35%)
and formic acid (85%) solutions were purchased from Nacalai Tesque.
Inc. (Kyoto, Japan). Acetone, ethanol, ethyl acetate, hexane, methanol,
Small-Angle Neutron Scattering (SANS). SANS experiments were
performed using the SANS-U instrument owned by the Institute for
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Solid State Physics at the University of Tokyo placed at research
reactor JRR-3 of the Japan Atomic Energy Agency, Tokai, Japan. The
neutron wavelength was 7.0 Å with a wavelength distribution (Δλ/λ)
of ca. 0.1 and sample-to-detector distances of 1 and 8 m. The
scattering profiles I(q) were obtained in the q-range 0.005 to 0.3 Å .
To determine the shape of the micelles quantitatively, the SANS
data were fit to a model. In general, the scattering function for an
assembly of particles is given by
Krafft Temperature and CMC. A clear aqueous solution of
star-type trimeric surfactants (0.20 wt %) was prepared by
dissolving in hot water and placed in a refrigerator at ∼5 °C for
at least 24 h. The temperature of the cooled surfactant solution
was then raised gradually under constant stirring, and the
conductance (κ) was measured by raising range of 0.5 °C to 1.0
−1
°
C. The abrupt change in the κ versus temperature plots was
not observed because the Krafft temperatures of star-type
2
2
I(q) = n(Δρ) V P(q)S(q)
(1)
trimeric surfactants 3C trisQ are below 5 °C. The surfactant
n
solutions were clear and showed no precipitates in visual
observations. Therefore, the three quaternary ammonium
where n is the number of particles per unit volume in solution, Δρ the
scattering contrast, and V the volume of a single particle. P(q) and
S(q) are the form factor and the interparticle structure factor,
respectively. Here, the form factor P(q) for an ellipsoidal shape is given
by
headgroups gave 3C trisQ good water solubilities at 25 °C
n
despite the three long hydrocarbon chains, even where n = 14.
The surface tensions as functions of the concentration for
3
C trisQ with n = 8, 10, 12, and 14 at 25 °C were determined
2
n
1
⎛ J (qR )⎞
1
s
and plotted in Figure 2. The surface tensions for n = 10, 12, and
P(q) = 9 ∫0
⎜
⎟ dx
⎝ qR
⎠
s
(2)
where J (x) is the first-order Bessel function of x. The R is defined as
1
s
2
2
1/2
R = R [1 + x (u − 1)]
(3)
s
1
where R and u are the minor axis and the axial ratio, respectively.
1
Note that for spherical micelles, u = 1. The interparticle structure
factor (SHP(q)) was calculated by using a model proposed by Hayter
18
and Penford. This structure factor, taking into consideration the
particle-shape (S′(q)), is given by
2
2
S′(q) = 1 + |⟨F(q)⟩| /⟨|F(q)|⟩ (SHP(q) − 1)
where F(q) is the scattering amplitude.
(4)
The form factor P(q) for rod particles with a radius of R and length
c
of L is given by
2
Figure 2. Variation in surface tension with surfactant concentration for
3C trisQ at 25 °C: ▼
(green), n = 8; ■ (blue), 10; ◆ (red), 12; ●
n
(black), 14.
2
/π ⎡ 2J (qR sin α)
⎤
c
sin(qLcos α/2)
qLcos α/2
1
P(q) = ∫0
⎢
⎣
⎥ sin αdα
qR sin α
⎦
c
(5)
Cryogenic Transmission Electron Microscopy (cryo-TEM). After a
small amount (3−5 μL) of the sample solution was placed on a TEM
copper grid covered by a porous carbon film, the excess solution on
the grid was blotted with filter paper to form a thin liquid film, and the
grid was immediately plunged into liquid propane (−180 °C) cooled
by liquid nitrogen in a cryofixation apparatus (LEICA, Reichert KF-
1
4 decreased with increasing concentration, reaching clear
breakpoints that were taken to be the CMCs. Unfortunately, for
the shortest chain length n = 8, the CMC could not be obtained
from the plot. In this case, micelles did not form in solution
even at the highest concentration studied due to the short
chains, and the solution simply became turbid. The surface
tension and CMC data were compared to that of the
corresponding monomeric, gemini, and linear trimeric
surfactants, as listed in Table 1. The data include the CMC,
surface tension at the CMC (γCMC), surface excess concen-
tration (Γ), and area occupied by each surfactant molecule (A).
8
0) to fix liquid water as vitreous ice. Then, the ice was transferred into
the specimen stage of a cryo-TEM (JEOL, JEM-2100F(G5)) operated
at an acceleration voltage of 200 kV through its cryotransfer apparatus
cooled by liquid nitrogen. The specimen stage was cooled by liquid
helium, and the specimen was kept at −269 °C during observation.
RESULTS AND DISCUSSION
■
Synthesis. We reacted tris(N,N-dimethyl-2-aminoethyl)-
amine with n-alkyl iodides (except for n = 14) to obtain the
star-type cationic trimeric surfactants in high yields and
substitute all three amines. Reactions with n-alkyl iodides
produced the trimeric surfactants in high yields and less time
compared with n-alkyl bromides. We tried to measure the
surface tension using these trimeric surfactants with iodide
counterions; however, they did not dissolve well enough in
water, and no breakpoints could be obtained in the surface
tension vs concentration plots. Therefore, to improve their
water solubility, the counterions of the trimeric surfactants with
n = 8, 10, and 12 were exchanged with bromide by Dowex 1-
The 3C trisQ series was compared against monomeric
(C TAB; dodecyltrimethylammonium bromide),
(12-2-12; 1,2-bis(dodecyldimethylammonium)ethane bro-
n
5
,19
gemini
12
5
,20
mide),
spacer chain lengths of 2, 3, and 6) surfactants.
12 series, 3C trisQ exhibited a CMC value lower than that of
the corresponding monomeric and gemini surfactants. It is
noteworthy that the CMC decreased by 1 order of magnitude
for each additional hydrocarbon chain−hydrophilic headgroup.
This signified that the trimeric surfactants had excellent micelle-
forming ability at low concentrations because of the increased
hydrophobicity provided by three hydrocarbon chains. The
and linear-type trimeric (12-s-12-s-12; s represents
5,7,8
For the n =
12
X8, and the bromo 3C trisQs were used in the measurements
CMC of starlike structured 3C trisQ was slightly higher than
n
12
described below. These star-type trimeric surfactants 3C trisQ
that of linear 12-2-12-2-12. This was in line with our
assumption that 3C trisQ, based on tris(2-aminoethyl)amine,
n
could be obtained in higher yields and more simply than the
12
linear trimeric surfactants (m-2-m-2-m) that we reported
previously.
would have greater hydrophilicity because of its tertiary amine.
3C trisQ also exhibited a CMC lower than that of the linear
8
12
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Table 1. The Critical Micelle Concentration, the Surface Tension at the CMC, the Surface Excess Concentration, the Area
Occupied per Molecule at the Air/Solution Interface, and the Standard Free Energy of Micellization and Adsorption at the Air/
Solution Interface for the Trimeric Surfactants 3C trisQ as well as for Monomeric and Oligomeric Quaternary Ammonium
n
Bromides at 25 °C
cmc (mmol dm−3
a
)
cmc (mmol dm−3
b
)
γcmc (mN m−1
)
10 Γ (mol m−2) A (nm /molecule) ΔG°mic (kJ mol−1
6
2
)
ΔG°ads (kJ mol−1
)
surfactant
3
3
3
3
C trisQ
−
1.17
−
0.875
0.883
0.820
0.970
1.90
1.88
2.03
1.70
0.49
−
−
8
C trisQ
1.60
33.4
32.3
32.1
−30.0
−36.3
−33.7
−18.3
−74.0
−85.0
−75.0
10
C trisQ
0.139
0.00647
0.177
0.384
12
C trisQ
14
c
d
c
d
c
c
C TAB
14 , 16
0.9, 0.967
38.6 , 39
3.42
12
c
e
c
e
c
c
c
f
1
1
1
1
2-2-12
31.4, 32.4
2.31
0.72, 1.00
c
f
f
c
f
c
2-2-12-2-12
2-3-12-3-12
2-6-12-6-12
0.08, 0.065
36.4
1.29, 1.11
1.28, 1.49
g
g
g
g
g
0.14
1.75
g
1.47
−64.5
−57.0
g
g
0.28
0.7
2.49
a
b
c
d
e
f
g
Surface tension method. Electrical conductivity method. From ref 5. From ref 19. From ref 20. From ref 8. From ref 7.
cationic trimeric surfactants with spacers such as trans-1,4-
buten-2-ylene, m-xylylene, and p-xylylene (0.36, 0.28, and 0.29
surfactants was similar to that on gemini, and smaller than that
for monomeric ones. The number of chains was plotted against
−3
9
mmol dm , respectively) and two star-type cationic trimeric
surfactants with three amide groups in the spacers (0.20 and
the logarithm of the CMC of each surfactant (C TAB, n-2-n, n-
n
2-n-2-n, and 3C trisQ) for hydrocarbon chain lengths n = 8, 10,
n
−3
12
0
.33 mmol dm ), although their counterions were chloride.
12, and 14 in Figure 4. The relationships between the chain
The plots of hydrocarbon chain length against the logarithm of
the CMC of the star-type trimeric surfactants 3C trisQ are
n
1
7
21
compared with monomeric, gemini, and linear-type
trimeric surfactants in Figure 3. It is generally known that
8
Figure 4. Relationships between the CMC and the hydrocarbon chain
1
7
length for star-type trimeric 3C trisQ, C TAB (monomeric), n-2-n
(
n
n
2
1
8
gemini), and n-2-n-2-n (linear-type trimeric): ▼ (green), hydro-
carbon chain length n = 8; ■ (blue), 10; ◆ (red), 12; ● (black), 14.
Figure 3. Relationships between the CMC and the hydrocarbon chain
1
7
length for star-type trimeric 3C trisQ, C TAB (monomeric), n-2-n
number and the logarithm of CMC gave straight lines for the
n
n
2
1
8
(
gemini), and n-2-n-2-n (linear-type trimeric): ■, 3C trisQ; △,
C TAB-(n-2-n)-(n-2-n-2-n) series that have two unit spacers, a
n
n
8
C TAB; ○, n-2-n; ●, n-2-n-2-n.
n
phenomenon not described in the previous paper. That is, it
can be expressed by the equation
the CMC decreases logarithmically as the carbon number (n)
in the hydrocarbon chain of a homologous series increases, and
the relation can be expressed by the so-called Klevens equation
as
log CMC = C − Dm
where C and D are constants, and m is the number of
hydrocarbon chains for analogous surfactants. The D values
were 0.51, 0.88, 1.02, and 1.34 for hydrocarbon chain lengths n
(7)
log CMC = A − Bn
(6)
=
8, 10, 12, and 14, respectively. The change in CMC as more
where A and B are constants specific to the homologous series
under constant conditions of temperature, pressure, and other
chains were added (slope) increased with chain length. This
result was consistent with the fact that micelles form more
easily with more hydrophobic surfactants. On the other hand,
parameters. The relationship for star-type 3C trisQ was linear
n
for hydrocarbon chain lengths from 10 to 14, and its slope was
quite close to that for the linear-type trimeric series in our
previous work. That is, the B values were 0.31, 0.40, 0.58, and
star-type trimeric surfactants 3C trisQ with n = 10 and 12
deviated from these straight lines. This was probably because
n
3C trisQ contained one hydrophilic tertiary amine in addition
n
0
.56 for C TAB, n-2-n, n-2-n-2-n, and 3C trisQ, respectively.
to three quaternary ammonium headgroups in a molecule as
described above. This result may also come from the change in
n
n
One can see that the B values increased with more chains,
indicating that the variation of CMC with the chain length was
much larger for trimeric surfactants. However, when the total
carbon number in the hydrocarbon chains was taken into
account, the effect of chain length on the CMC of the trimeric
the arrangement of three hydrocarbon chains of 3C trisQ
n
molecule at the air/water interface at some critical hydrocarbon
chain length which could be 12−14 carbon atoms. This feature
is absent in the case of linear-type trimeric surfactants where
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the straight orientation of hydrocarbon chains into the
hydrophobic phase is present at all hydrocarbon chains. From
these data, it was found that the longer the chain length and the
more the chain number of surfactants, the lower the CMC.
This may be attributed to the thermodynamics, as there was
interface and oriented themselves so as to cause high surface
activity, due to interactions among the multiple hydrocarbon
chains. The star-type 3C trisQ occupied an area slightly more
n
than that of the linear-type trimeric surfactants, because of the
differences in the skeletons of tris(2-aminoethyl)amine and
diethylenetriamine. Thus, it was found that the star-type
trimeric surfactants could adsorb densely at the interface by
interactions of the multiple hydrocarbon chains despite the
strong electrostatic repulsion between multiple quaternary
ammonium headgroups.
9
,22
less entropic loss in the formation of micelles of surfactants.
Thus, we found a relationship between the CMC and the
number of chains for a class of cationic surfactants in addition
to the Klevens equation that deals with chain length. There has
been previous research on the relationship between log CMC
and the number of chains for the C TAB-(12-3-12)-(12-3-12-
These results were also supported by pC and the CMC/C
1
2
20
20
3
3
-12) series with dodecyl chains and spacer chain length of 3
ratio, which were proposed by Rosen, and the Gibbs free
and cationic oligomeric surfactants with dodecyl chains and9
trans-1,4-buten-2-ylene, m-xylylene, or p-xylylene spacers;
however, the linearity was not emphasized. Further studies
with other surfactant series will be needed to confirm this
relationship.
The CMC obtained by the surface tension method was
almost in agreement with that by the electrical conductivity
energy of micellization and adsorption (ΔG°mic, ΔG°ads,
respectively). Here, C20 is the bulk liquid phase concentration
of surfactant required to depress the surface tension of water by
−1
20 mN m . The larger the pC20 value, the greater the tendency
of the surfactant to adsorb at the interface vs forming a micelle
in solution. For larger CMC/C20 ratios, the adsorption to the
surface is easier than forming micelles. For 3C trisQ, the pC20
n
method for 3C trisQ with n = 10 and 12. However, for n = 14,
the CMC values obtained using these two methods were not
values were 3.9, 5.0, 6.2, and the CMC/C20 ratio values were
9.5, 12.4, 9.1 for n = 10, 12, and 14, respectively. The results
showed that the pC20 and the CMC/C20 ratio values of
n
the same. Then, we investigated the CMC of 3C trisQ by
14
using the pyrene fluorescence technique. The result showed
that the CMC obtained by the fluorescence method was higher
than that by the surface tension method and was lower than
that by the conductivity method. That is, 3C trisQ showed the
3C
n
trisQ were much larger than those of most ionic
monomeric surfactants where pC20 is below 3 and the CMC/
17
C
20 ratio ranges from 2 to 4. This suggested that adsorption
of the present star-type trimeric surfactants is easier than
1
4
difference in CMC values by the methods of measurement.
One of the reasons may be that the star-type trimeric
surfactants with long chains form loose micelles or a premicelle
such as a dimer or trimer at the concentrations above the cmc
obtained by the surface tension method. This unusual aggregate
behavior at low concentration is often observed for the gemini
surfactants with long chains. To clarify these properties, further
details using small-angle X-ray scattering (SAXS) equipped
with powerful X-rays, etc., will be needed.
micellization. The ΔG°mic and ΔG°ads of the trimeric surfactants
2
3,24
could be obtained from the following equations:
⎛
⎜
1
3
⎞ ⎛ CMC ⎞ ⎛RT ⎞
ΔG°mic = RT
+ β
⎟
ln
⎜
⎟
⎠
−
⎜
⎝
⎟
ln 3
⎝
⎠ ⎝
⎠
(8)
55.3
3
πCMC
ΔG°ads = ΔG°mic
−
(9)
Γ
where πCMC denotes the surface pressure at the CMC (πCMC
γ − γ , and where γ and γ
=
Adsorption to the Air/Water Interface. The star-type
trimeric surfactants in solution required a long time to
equilibrium for the surface tension, especially at the
concentration below the cmc, and long chain length. The
surface tensions at the cmc of star-type trimeric surfactants
are the surface tensions of
CMC
0
CMC
0
water and the surfactant solution at the CMC, respectively). β
is the apparent degree of counterion binding at the micelle/
solution interface, calculated from β = 1 − α (α is the micelle
ionization degree). Here, α is taken to be the ratio of the values
of d κ/d C (κ denotes the conductivity) above and below the
CMC obtained from the electrical conductivity measurements
(Figure 1S in Supporting Information). The α values of
−
1
3
C trisQ with n = 10−14 were 32−34 mN m , and they were
n
not dependent on hydrocarbon chain length. 3C trisQ
n
exhibited γCMC values smaller than those of the corresponding
monomeric surfactants, and almost the same γCMC values as
3C trisQ were estimated to be 0.241, 0.224, and 0.504 for n =
n
those of the gemini surfactants. The γCMC values of 3C trisQ
10, 12, and 14, respectively. The ΔG° and ΔG° values of
n
mic
ads
were also smaller or almost the same as those of linear-type
cationic trimeric surfactants with three dodecyl chains and two
3C trisQ are also summarized in Table 1. The absolute values
n
of ΔG° are significantly greater than those of ΔG° for all of
ads
mic
−1
8
spacers such as ethylene (36.4 mN m ), 2-hydroxypropylene
the hydrocarbon chain lengths, suggesting that in comparison
to micellization, the adsorption of star-type trimeric surfactants
is higher.
−
1
6
(
32 mN m ), trans-1,4-buten-2-ylene, m-xylylene, or p-
−1
9
xylylene (40−41 mN m ). This indicated that star-shaped
C trisQ adsorbed strongly and oriented themselves at the air/
3
Kinetics of Adsorption to the Air/Water Interface. To
n
water interface. The three hydrocarbon chains, which exist in a
ring around tris(2-aminoethyl)amine, facilitated interaction, in
contrast with linear surfactants where the hydrocarbon chains
investigate the kinetics of adsorption of star-type trimeric
surfactants 3C trisQ to the air/water interface, dynamic surface
n
tension measurements were performed by the maximum bubble
pressure technique. These dynamic methods can measure the
surface tension in a few milliseconds and are well suited to
investigate the kinetics of surfactant adsorption. The variations
of ends separate. As a result, 3C trisQ efficiently lowered the
n
surface tension of water. The surfactants’ ability to reduce
surface tension is related to their Γ and A. The Γ values of
3
C trisQ were smaller and the A values were much larger than
of surface tension with surface age for 3C trisQ became small,
n
n
those of the corresponding monomeric and gemini surfactants,
and they were not independent of hydrocarbon chain length or
γCMC. However, the area occupied by one hydrocarbon chain of
as the hydrocarbon chain length increased from 8 to 12 (Figure
2S in Supporting Information). For n = 14, the dynamic surface
tension was close to that of water even for the longest
3
C trisQ was close to that of a monomeric surfactant. This
n
measurement time. This suggested that 3C trisQ did not
14
indicated that the trimeric surfactants adsorbed to the air/water
adsorb well for a short time because of steric hindrance caused
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by the long hydrocarbon chains in addition to micelle
formation in solution. Furthermore, we analyzed the results
of the dynamic surface tension with Ward and Tordai’s
of surfactant aggregates on shear behavior were investigated by
rheology measurements. We measured the shear-rate depend-
ence of the viscosity at two concentrations of star-type trimeric
25
diffusion-controlled adsorption model. The apparent mono-
mer diffusion coefficients (D) could be calculated from long-
time dynamic surface tension data by using the Hansen and
surfactants 3C trisQ with n = 10, 12, and 14 and plotted it in
n
Figure 6. Results for pure water were also shown as solid lines
2
6,27
Joos equation:
2
RTΓ
C0
π
4Dt
γ − γ =
e
(10)
where γ is the equilibrium surface tension at infinite time, γ the
e
dynamic surface tension, Γ the surface excess concentration,
which can be obtained from the equilibrium surface tension
measurements, C the constant surfactant concentration, and t
0
−1/2
the time. The plots of γ versus t
based on eq 10 should be
linear if the adsorption were diffusion-controlled, allowing the
evaluation of D from the slope of the straight line. We plotted
surfactant concentration against the D values for 3C trisQ with
n
n = 8, 10, and 12, along with the data from the corresponding
monomeric and gemini surfactants (C TAB, 12-2-12) in
1
2
Figure 5. The D values of conventional monomeric surfactants
Figure 6. Shear-rate dependence of viscosity for star-type trimeric
surfactants 3C trisQ (n = 10, 12, and 14) in aqueous solution: (a) n =
n
−
3
1
3
(
0 (■ (blue), 2.93; ● (red), 5.85 mmol dm ), (b) n = 12 (■ (blue),
.48; ● (red), 27.8 mmol dm ), (c) n = 14 (■ (blue), 0.162; ●
red), 1.62 mmol dm ).
−3
−
3
for reference. For 3C trisQ solutions at 2.93 and 5.85 mmol
10
−3
dm (2.5, 5.0 times the CMC, respectively), the shear-rate
dependence of viscosity was the same as that for water. This
was due to aggregation that did not affect solution viscosity. On
Figure 5. Relationships between diffusion coefficient and surfactant
the basis of the previous findings, we interpreted the shape of
concentration for star-type trimeric 3C trisQ (n = 8, 10, and 12) along
n
30,31
the aggregates to be spherical or ellipsoidal micelles.
For
with C TAB (monomeric) and 12-2-12 (gemini) (the diffusion
12
−3
3
C trisQ solutions at 3.48 mmol dm (25 times the CMC),
coefficients of C TAB and 12-2-12 were measured in this study): ▼
12
12
(
green), 3C trisQ;
■
(blue), 3C trisQ; ◆ (red), 3C trisQ; △,
the viscosity also agreed with that for water, indicating that
spherical micelles were formed in solution. As the concen-
tration increased to 27.8 mmol dm− (200 times the CMC), the
viscosity showed a decrease with increasing shear rate, which is
known as shear thinning and a typical behavior of rod- or
8
10
12
C TAB; ○, 12-2-12.
12
3
−10
2
−1
containing C TAB were on the order of 10
m s . The
12
3
2,33
34
diffusion coefficient became small by the order C TAB >
wormlike micelles
and chainlike polymers. These results
12
3
C trisQ > 3C trisQ > 12-2-12 > 3C trisQ. Essentially,
implied a spherical micelle-to-rodlike micelle transition as
concentration increased. This transition was also observed by
using SANS, cryo-TEM, and dynamic viscoelasticity for gemini
8
10
12
increasing the degree of oligomerization and the hydrocarbon
chain lengths led to a decrease in the kinetics of adsorption at
the air/water interface because of increased bulkiness. We
attributed this to the adsorption of the surfactant from the
subsurface to the interface. The diffusion coefficient of gemini
and trimeric surfactants also decreased with increasing
concentration, especially above the CMC because of aggregates
formed in solution. Miller proposed an adsorption kinetic
model that takes into account the formation and dissociation of
3
5,36
surfactants with dodecyl chains.
The details of the spherical
micelle-to-rodlike micelle transition for 3C trisQ will be
12
described later using SANS and cryo-TEM techniques. For
−3
3C trisQ solutions at 0.162 and 1.62 mmol dm (25 and 250
14
times the CMC, respectively), the viscosity was more viscous
than water and showed no change as the shear rate increased.
The viscosity of 3C trisQ was much lower than that of
14
2
8
micelles. Thus, it was found that the star-type trimeric
surfactants had slow kinetics of adsorption to the air/water
interface.
3C trisQ, because of the different concentrations used in
12
measurements for the two surfactants. As the concentration of
3C trisQ increases, higher viscosity may be obtained, although
14
Rheological Properties in Solution. It is known that
it does not dissolve in water.
Structures of Aggregates by SANS and cryo-TEM.
SANS is one of the most powerful techniques to quantitatively
rheological behavior depends on the shape of aggregates
2
9,30
formed by surfactants in solution.
The effects of the shape
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Article
3
investigate surfactant self-assembly systems in aqueous solution
on the nanoscale. SANS measurements were performed to
estimated. On the other hand, at 27.8 mmol dm− (200 times
the CMC), the rheological behavior suggested formation of
wormlike micelles in solution. Beyond the Guinier q-region, the
form factor P(q) for wormlike micelles obeyed the asymptotic
determine the structure of aggregates formed by 3C trisQ with
n
n = 10, 12, and 14 in solution (Figure 7). 3C trisQ and
1
0
−2
−1
behaviors of q at a low q-region, q at a medium q-region,
and q at highest q-region. However, the SANS results lacked
−4
−2
q
behavior at a lower q-region. Therefore, using the scattering
function of charged rod particles, a model-fitting analysis was
performed using eq 5. The structure factor S(q) was calculated
using eq 4. The result of the model fitting clearly showed the
formation of rodlike micelles with R = 2.00 nm and L = 16.61
c
−
3
nm for 3C trisQ at 27.8 mmol dm .
12
To confirm the structures of the aggregates formed by
3
C trisQ, we performed cryo-TEM observations. Cryo-TEM
12
provides a direct visualization of the aggregates present in the
thin layers of a vitrified specimen of the solution. Cryo-TEM
micrographs for 3C trisQ at concentrations of 13.9 and 27.8
12
mmol dm− (100 and 200 times the CMC, respectively) are
3
given in Figure 8. We also tried cryo-TEM observations at 6.95
−3
mmol dm ; however, the micrograph did not clarify due to low
−3
concentration. At 13.9 mmol dm , threadlike micelles with
very few branches were clearly observed, and the length and the
number of the micelles increased with increasing concentration.
The rod length, L, of the threadlike micelles obtained by the
SANS analysis was expected to be approximately the same as
the persistence length or mesh size because the cryo-TEM
suggested that the micelles had longer contour lengths. Zana et
al. reported that linear-type quaternary ammonium salt trimeric
surfactants (12-s-12-s-12) form branched threadlike micelles for
4
,7
7
s = 3 and spheroidal micelles for s = 6 in solution. The
shapes of these micelles are also similar to those of the
Figure 7. SANS profiles and the fit curves for star-type trimeric
surfactants 3C trisQ (n = 10, 12, and 14) in aqueous solution: (a) n =
corresponding gemini surfactants 12-s-12 with spacer chain
n
35,37
−3
lengths s = 2−12.
The star-shaped oligomeric surfactants,
1
3
0 (■ (blue), 2.93; ● (red), 5.85 mmol dm ), (b) n = 12 (■ (blue),
−3
.48;
●
(red), 27.8 mmol dm ), (c) n = 14 (■ (blue), 0.162; ●
including 3C trisQ, were capable of forming such structures
12
−3
(red), 1.62 mmol dm ). The solid lines represent the best-fit
because the multiple hydrocarbon chains of these surfactants
could have different relative orientations in the micelles.
Furthermore, the strong coulomb repulsions between multiple
quaternary ammonium headgroups may have led to an increase
in the curvature of the surfactant film, which in turn favors
branched threadlike micelles.
For 3C14trisQ, the SANS profiles pointed to characteristics of
rodlike particles at both concentrations. In addition, peak
profiles were not observed (i.e., S(q) ≈ 1). Therefore, model
fitting was carried out using only the form factor of the rod
theoretical scattering functions.
−
1
3
0
C trisQ gave peak profiles in the q-range of 0.02 Å < q <
12
−1
.05 Å . These peaks were attributed to repulsive interparticle
interactions between the micelles because the micelles were
charged on these surfaces. With increasing surfactant
concentration, these peak positions (arrows in Figure 7) shifted
to higher q values. On the other hand, unlike the case of
3
C trisQ and 3C trisQ, the SANS profiles for 3C trisQ had
10 12 14
no peaks. That is, the SANS profiles at both concentrations
particles (eq 5). Through model-fitting analysis, the R values
c
−
1
−1
−3
showed I(q) ∼ q in the low q-range (q < 0.06 Å ) and I(q)
were estimated to be 2.26 nm for 0.162 mmol dm (25 times
−4
−1
−3
∼
q
in the high q-range (q > 0.06 Å ), indicating that
C trisQ formed rod- or wormlike micelles.
The scattering curves were fitted for 3C trisQ with n = 10,
2, and 14 (fit curves in Figure 7). As shown in Figure 6a, the
the CMC), and 2.13 nm for 1.62 mmol dm (250 times the
38
3
1
CMC). According to the literature, a scattering profile of
14
−1
rodlike particles indicates the crossover from q behavior to
n
0
constant (i.e., q ) at the lower q regions (Guinier q-region).
shear-rate dependence of the viscosity indicated that 3C trisQ
However, because this crossover was not observed in the actual
SANS results, the length (L) of rodlike micelles for 3C14trisQ
was expected to be more than 25 nm (= 1/qmin, where qmin is
the lowest accessible q limit). By cryo-TEM, it was confirmed
10
formed spherical or ellipsoidal micelles in solution. Therefore,
the theoretical scattering function for charged ellipsoidal
particles was fit to a model (eqs 2, 3, and 4). For 3C trisQ,
10
the R and u values were estimated to be R = 1.62 nm and u =
that the rodlike micelles for 3C trisQ were threadlike with few
branches at 0.324 and 0.648 mmol dm (50 and 100 times the
1
1
14
−3
−3
0
.55 at 2.93 mmol dm (2.5 times the CMC), and R = 1.71
1
−3
nm and u = 0.51 at 5.85 mmol dm (5 times the CMC). For
CMC, respectively), as shown in Figure 9. Cryo-TEM showed
that the branches of threadlike micelles were more for
3C trisQ than 3C trisQ, while the viscosity of 3C trisQ
3
C trisQ, according to the results of the rheological measure-
12
ments, spherical or ellipsoidal micelles were expected to be also
14
12
14
−3
formed at 3.48 mmol dm (25 times the CMC). Therefore,
the model fit was performed using the scattering function of
charged ellipsoidal particles calculated using eqs 2, 3, and 4. By
was very low as mentioned above. 3C trisQ may form, in
12
practice, threadlike micelles which have further branches,
because cryo-TEM is a qualitative method, which is the
observation of micelles that may be only a part of the whole
model-fitting analysis, R = 1.93 nm and u = 0.44 were
1
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Figure 9. Cryo-TEM images for 3C trisQ in solution: (a) 0.324
mmol dm , (b) 0.648 mmol dm .
14
Figure 8. Cryo-TEM images for 3C trisQ in solution: (a) 13.9 mmol
dm , (b) 27.8 mmol dm .
−3
−3
12
−3
−3
system. Thus, it was clarified that the aggregates that formed by
water despite having three hydrocarbon chains in each
molecule. The relationship between the logarithm of CMC
and hydrocarbon chain lengths was linear as with conventional
monomeric and gemini surfactants. We also found a linear
relationship between the logarithm of CMC and the number of
hydrocarbon chains for linear-type trimeric surfactants with
ethylene spacers. Although the adsorption kinetics of trimeric
surfactants to the air/water interface was slow, they strongly
adsorbed and oriented themselves at the interface, indicating
that they lowered the surface tension of water efficiently. The
aggregation state of trimeric surfactants in solution was
significantly influenced by their hydrocarbon chain lengths
and concentrations; that is, for n = 10, ellipsoidal micelle
formed, for n = 12, the ellipsoidal micelle changed to threadlike
micelles with increasing concentration, and for n = 14,
the star-type trimeric surfactants 3C trisQ depended on both
n
hydrocarbon chain length and concentration; when they
increased, different sequence shapes were observed, from
ellipsoidal micelles to threadlike micelles. That is, it was
possible to control the shape of the aggregates by changing
their factors.
CONCLUSIONS
■
In this article, we designed and synthesized novel star-type
cationic trimeric surfactants consisting of three hydrocarbon
chains, three quaternary ammonium headgroups, and a tris(2-
aminoethyl)amine spacer. We investigated in detail their
adsorption and aggregation properties by using various
analytical techniques. These surfactants were very soluble in
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threadlike micelles formed at low concentration and no
transitions were observed as it increased.
(7) In, M.; Bec, V.; Aguerre-Chariol, O.; Zana, R. Quaternary
ammonium bromide surfactant oligomers in aqueous solution: self-
association and microstructure. Langmuir 2000, 16, 141−148.
Until now, a large number of gemini surfactants have been
designed and synthesized by many researchers and their
adsorption and aggregation properties investigated. However,
there are very few reports for trimeric and tetrameric
surfactants, which represent a natural extension of gemini
(8) Yoshimura, T.; Yoshida, H.; Ohno, A.; Esumi, K. Physicochemical
properties of quaternary ammonium bromide-type trimeric surfactants.
J. Colloid Interface Sci. 2003, 267, 167−172.
(
́
9) Laschewsky, A.; Wattebled, L.; Arotcarena, M.; Habib-Jiwan, J.-L.;
Rakotoaly, R. H. Synthesis and properties of cationic oligomeric
surfactants. Langmuir 2005, 21, 7170−7179.
3
−16
surfactants.
This is probably because the design and
synthesis of oligomeric surfactants such as 3C trisQ are very
n
(10) Wattebled, L.; Laschewsky, A.; Moussa, A.; Habib-Jiwan, J.-L.
Aggregation numbers of cationic oligomeric surfactants: a time-
resolved fluorescence quenching study. Langmuir 2006, 22, 2551−
2557.
difficult unlike gemini surfactants, which can be prepared
relatively easily. The results in this work may aid future studies
of structure−performance relationships for new oligomeric
surfactants with novel structures. They will also help to explain
the adsorption and aggregation behavior of different types of
trimeric surfactants, which have already been reported. We
expect to produce further studies on these star-type trimeric
surfactants, which were easier to synthesize than expected.
These trimeric surfactants should form a new category of
surfactants to go along with gemini surfactants.
(11) Hou, Y.; Cao, M.; Deng, M.; Wang, Y. Highly-ordered selective
self-assembly of a trimeric cationic surfactant on a mica surface.
Langmuir 2008, 24, 10572−10574.
(12) Wu, C.; Hou, Y.; Deng, M.; Huang, X.; Yu, D.; Xiang, J.; Liu, Y.;
Li, Z.; Wang, Y. Molecular conformation-controlled vesicle/micelle
transition of cationic trimeric surfactants in aqueous solution.
Langmuir 2010, 26, 7922−7927.
(13) Yoshimura, T.; Esumi, K. Physicochemical properties of ring-
type trimeric surfactants from cyanuric chloride. Langmuir 2003, 19,
3535−3538.
ASSOCIATED CONTENT
■
(
14) Yoshimura, T.; Esumi, K. Physicochemical properties of anionic
*
S
Supporting Information
H NMR and elemental analysis data of synthesized
compounds, details of experimental methods, electrical
conductivity and dynamic surface tension data of the star-
1
triple-chain surfactants in alkaline solutions. J. Colloid Interface Sci.
004, 276, 450−455.
15) Yoshimura, T.; Kimura, N.; Onitsuka, E.; Shosenji, H.; Esumi, K.
2
(
Synthesis and surface-active properties of trimeric-type anionic
surfactants derived from tris(2-aminoethyl)amine. J. Surfactants Deterg.
type trimeric surfactants 3C trisQ. This material is available free
n
of charge via the Internet at http://pubs.acs.org.
2
(
004, 7, 67−74.
16) Abdul-Raouf, M. E.-S.; Abdul-Raheim, A.-R. M.; Maysour, N. E.-
AUTHOR INFORMATION
S.; Mohamed, H. Synthesis, surface-active properties, and emulsifica-
tion efficiency of trimeric-type nonionic surfactants derived from
tris(2-aminoethyl)amine. J. Surfactants Deterg. 2011, 14, 185−193.
■
*
Corresponding Author
(T.Y.) E-mail: yoshimura@cc.nara-wu.ac.jp, fax: +81 742 20
(17) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John
3
7
393; (M.S.) E-mail: sibayama@issp.u-tokyo.ac.jp, fax: +81 4
314 6069.
Wiley and Sons: New York, 2004.
(18) Hayter, J. B.; Penfold, J. An analytic structure factor for
macroion solutions. Mol. Phys. 1981, 42, 109−118.
Notes
(19) Rosen, M. J.; Mathias, J. H.; Davenport, L. Aberrant aggregation
The authors declare no competing financial interest.
behavior in cationic gemini surfactants investigated by surface tension,
interfacial tension, and fluorescence methods. Langmuir 1999, 15,
7340−7346.
ACKNOWLEDGMENTS
■
(20) Menger, F. M.; Keiper, J. S.; Mbadugha, B. N. A.; Caran, K. L.;
We are grateful to Prof. Hideto Shosenji in the Department of
Applied Chemistry and Biochemistry of Kumamoto University
for advice of synthesis of surfactants. We also thank Prof. Kunio
Esumi in the Department of Applied Chemistry of Tokyo
University of Science for many useful suggestions.
Romsted, L. S. Interfacial composition of gemini surfactant micelles
determined by chemical trapping. Langmuir 2000, 16, 9095−9098.
́
(21) Zana, R.; Levy, H. Alkanediyl-α,ω-bis(dimethylalkylammonium
bromide) surfactants (dimeric surfactants). Part 6. CMC of the
ethanediyl-1,2-bis(dimethylalkylammonium bromide) series. Colloids
Surf., A 1997, 127, 229−232.
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