Conformational analysis of (Phenylenedimethylene)bis(n-octylammonium)dibromides in aqueous solution. Conformational change upon micellization

Article abstract of DOI:10.1021/jp9814202

Three gemini surfactants, in which two quaternary ammonium species (CH₃(CH₂)₇N⁺(CH₃) ₂) are linked at the polar headgroups by ο-, m-, or p-phenylenedimethylene spacers, have been synthesized. The critical micelle concentration (cmc) of these surfactants in aqueous solutions was determind by electrical conductivity. Selective-decoupling ¹³C NMR and ¹H NMR spectra were measured at various concentrations below and above the cmc. The selective-decoupling ¹³C NMR results revealed that the specific rotational isomers about the CH₂-aromatic carbon single bonds for the gemini surfactant having a m-phenylenedimethylene spacer are preferentially stabilized upon micellization, while for the gemini surfactant having an ο-phenylenedimethylene spacer, the presence of only the conformation in which the aromatic ring is sandwiched between two n-octyl chains was confirmed. Furthermore, it was found that variation in the stacking pattern of the aromatic rings after micellization of the surfactants is reflected in the ¹H spectral features of the aromatic protons. Comparison is made with the conformations of bis(quaternaryammonium) bromides with flexible spacer chains ((CH₂)_x) and of the corresponding n-alkylammonium bromide monomeric surfactants.

Full text of DOI:10.1021/jp9814202

J. Phys. Chem. B 1998, 102, 8965-8973
8965
Conformational Analysis of (Phenylenedimethylene)bis(n-octylammonium)dibromides in
Aqueous Solution. Conformational Change upon Micellization
†
,†
†
‡
Norikatsu Hattori, Akihiro Yoshino,* Hirofumi Okabayashi, and Charmian J. O’Connor
Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555,
Japan, and Department of Chemistry, The UniVersity of Auckland, PB 92019 Auckland, New Zealand
ReceiVed: March 10, 1998; In Final Form: July 13, 1998
+
Three gemini surfactants, in which two quaternary ammonium species (CH
3
(CH
2
)
7
N (CH
3 2
) ) are linked at
the polar headgroups by o-, m-, or p-phenylenedimethylene spacers, have been synthesized. The critical
micelle concentration (cmc) of these surfactants in aqueous solutions was determined by electrical conductivity.
Selective-decoupling 13C NMR and H NMR spectra were measured at various concentrations below and
1
1
3
above the cmc. The selective-decoupling C NMR results revealed that the specific rotational isomers about
the CH -aromatic carbon single bonds for the gemini surfactant having a m-phenylenedimethylene spacer are
2
preferentially stabilized upon micellization, while for the gemini surfactant having an o-phenylenedimethylene
spacer, the presence of only the conformation in which the aromatic ring is sandwiched between two n-octyl
chains was confirmed. Furthermore, it was found that variation in the stacking pattern of the aromatic rings
1
after micellization of the surfactants is reflected in the H spectral features of the aromatic protons. Comparison
2 s
is made with the conformations of bis(quaternaryammonium) bromides with flexible spacer chains ((CH ) )
and of the corresponding n-alkylammonium bromide monomeric surfactants.
Introduction
protein molecule also occur.14 The study of surfactant-protein
interactions has important biological implications with respect
to the lipid-membrane-protein interaction in biomembranes.
The interaction between biological macromolecules could
possibly result in a conformational change of the molecules.
The interaction of DNA with histones brings about a confor-
mational change in the DNA or the histones, and a conforma-
tional change due to a higher degree of association of the histone
The conformational change of molecules in a dissociation-
association system has fundamental significance, since it serves
as an important model for the relationship between the functional
appearance and the conformational change of biological sub-
stances. In particular, conformational studies of surfactant
molecules in an aggregated structure assist our understanding
of the physicochemical properties of phospholipid bilayers with
respect to their relationship to the structure of biomembranes.
We have already reported that one specific isomer, of the
possible rotational isomers about the CH2-CH2 single bonds
for the hydrocarbon moiety of simple surfactant molecules, is
1
5
octamer has been observed.
Such a conformational change should also affect the mor-
phology of aggregates, including biomembranes and their model
systems, since the type and structure of the self-assembly system
depend on their geometrical packing parameters,16 which are
determined from the conformational structure.
1
-6
preferentially stabilized upon the formation of micelles.
It
has been demonstrated for simple soap molecules (potassium
n-pentenoate and potassium n-hexenoate), by measurement of
the concentration dependence of the Raman scattering intensity
of the accordion vibrational mode, that the population of the
extended form (all-trans) of the hydrocarbon chain increases
17
Zana and Talmon have used cryogenic transmission electron
microscopy to investigate the microstructures of the aggregates
of the dimeric surfactants, bis(quaternary ammonium bromides),
in which two alkyldimethylammonium bromide chains are
linked by a polymethylene chain. They found that the dimeric
surfactants with a short spacer form long threadlike and
entangled micelles, while those with a longer spacer form
spherical micelles. These observations reveal that the length
of a spacer affects the morphology of spontaneous curvature in
molecular membranes. Thus, it is evident that the morphology
of an aggregate strongly depends on the structure of a dimeric
surfactant. In particular, variation in the length of a spacer chain
changes the packing parameters. Moreover, conformational
change of the dimeric surfactants probably occurs upon forma-
tion of the aggregates, leading to a further change of the packing
parameters. Accordingly, this process affects the morphology
of aggregates.
upon micellization.1 For potassium trans-3-hexenoate, potas-
sium trans-4-pentenoate, and potassium trans-5-hexenoate,
micellization leads to a conformational change about the C-C
single bond adjacent to the CdC double bond. These observa-
tions apply to surfactant molecules possessing longer hydro-
carbon chains. A conformational change due to micellization
,5
2
6
1
7-9
has also been observed in the H NMR
and Raman
spectra of the polar parts of surfactant molecules.
scattering4,10-13
The interaction between surfactant and proteins has been
extensively studied, and the secondary structural change caused
by surfactant binding of a protein has been observed. The
R-helical content of bovine serum albumin decreases with an
increase in the concentration of sodium dodecyl sulfate, and
conformational changes about the C-S-S-C bonds of the
Diamant and Andelman18 have presented a theoretical
explanation for the experimental results obtained by Zana and
1
7
Talmon for dimeric surfactants. In this theory, it was
suggested that the attractive and repulsive interactions of
*
Nagoya Institute of Technology.
The University of Auckland.
‡
1
0.1021/jp9814202 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/16/1998

8966 J. Phys. Chem. B, Vol. 102, No. 45, 1998
Hattori et al.
surfactant molecules and the conformational entropy of the
spacer chain are dominant factors in elucidation of the shapes
of aggregates formed in the aqueous solution. A conformational
change of surfactant molecules upon micellization may vary
the conformational entropy of the spacer chain.
To discuss quantitatively the relationship between the geo-
metrical parameters and morphological characteristics, it is
necessary to make a detailed investigation of the conformation
of a surfactant molecule in an aggregate and the conformational
change upon formation of its micelles in aqueous solution.
Stein and Gelman19 synthesized amphiphiles with a unique
headgroup topology, in which two carboxylates were rigidly
2
0
held on a dibenzobarrelane skeleton. Nussler and Engberts
investigated the relationship between amphiphilic structure and
aggregate morphology in 1,4-dialkylpyridinium salts, while
Menger and Yamazaki21 studied amphiphiles, in which the
hydrophobic moiety was a polynuclear aromatic ring system
or a hyperextended linear chain. In 1991, the name “gemini
surfactants” was assigned by Menger and Littau2 to a group
of amphiphilic molecules having a long hydrocarbon chain, an
ionic group, a rigid spacer, a second ionic group, and another
hydrocarbon tail. Furthermore, Menger and Littau2 reported
a new class of self-assembling molecules for “gemini surfac-
tants”. They measured the surface tension and surface pressure
for three series of gemini surfactants and discussed the molecular
orientation at the air/water interface using the film-balance
method. The remarkable properties of these “gemini surfactant”
solutions were attributed to distortion of the water-structure
2
3
Figure 1. Numbering scheme of gemini surfactants.
TABLE 1: Elemental Analysis of oxy8, mxy8, pxy8, sm 3-8,
sm 5-8 and sm 6-8
geminis
oxy8
C %
H %
N %
2
4,25
by the two hydrophobic chains in the molecules.
It is
calcd
found
calcd
found
calcd
found
calcda
found
calcdb
found
calcd
found
58.13
58.24
58.13
57.95
58.13
57.80
49.99
49.87
54.25
54.04
55.91
55.53
9.41
9.71
9.41
9.63
9.41
4.84
4.53
4.84
4.39
4.84
4.45
5.07
4.88
5.06
4.59
5.02
4.68
evident from these observations that the molecular orientation
at an interface is strongly related to the conformation of a
molecule. Therefore it is highly desirable to make detailed
studies of molecular conformations by use of spectroscopic
methods in order to investigate the orientation of molecules at
an interface.
mxy8
pxy8
9.78
sm 3-8
sm 5-8
sm 6-8
10.22
10.32
10.38
10.53
10.47
10.72
In this present study, the so-called “gemini surfactants”, in
which two n-alkyldimethylammonium bromide moieties are
connected by phenylenedimethylene spacer groups, have been
synthesized, and the conformational change of these surfactants
after micellization has been analyzed by use of the selective-
a
Calculated for dihydrated molecules. b Calculated for hemihydrated
1
3
26
molecules.
decoupling C NMR spectroscopic method. The molecular
conformations of bis(quaternaryammonium) bromides with
flexible spacer chains ((CH2)s) and of the corresponding
n-alkyltrimethylammonium bromide monomeric surfactants are
discussed and compared with those for the geminis with rigid
spacers.
three bonds, thereby providing direct information of the
conformations about the CH2-C (aromatic carbon) single bond.
Experimental Section
Takeuchi et al.26 used the selective decoupling 13C NMR
technique to analyze the structure of pentalenolactone and
showed that this technique is very powerful for reliable structural
elucidation of complex molecules. We briefly describe this
technique for the further application to determination of the
conformations of other gemini surfactants. In the completely
Materials. (Phenylenedimethylene)bis(n-alkylammonium)
dibromide surfactants and bis(quaternaryammonium bromides)
(Figure 1) were synthesized as follows.
For oxy8, mxy8, pxy8, and mxy10, reactions of n-octyldi-
methylamine and n-decyldimethylamine with the corresponding
2
7,28
phenylenedimethylene dibromide
were performed in dried
1
3
proton-decoupled C NMR spectrum, the resonance signals of
all carbon-13 nuclei are observed as a singlet and do not involve
any information on the coupling between the 13C-nucleus and
the proton. In contrast, in the nondecoupled spectrum, the
resonance signal of each carbon-13 nucleus is observed as
ethanol under reflux (T ) 353 K) for 24 h so as to ensure as
complete bisquarternization as possible. Bis(quaternaryammo-
nium) bromides (sm3-8 and sm5-8) were synthesized by
reactions of N,N′-tetramethylpropane and -pentane with n-
octylbromide in the same manner.
The two series of dimeric surfactants thus synthesized were
recrystallized in various solvent mixtures (ethanol-ether or
acetone-ether). Sample identification was confirmed by NMR
and elemental analysis (Table 1). The reactants were purchased
from Tokyo Kasei Co. and were purified before use. The
isotopic purity of the D2O (EURISO-TOP) used for NMR
measurements was 99.8%. The samples of the monomeric
13
1
multiplet as a consequence of C- H coupling (direct and long-
1
3
1
range C- H couplings), and, on the whole, the spectrum
becomes very complex. However, when only the resonance
13
signal of a specific proton interacting with a C-nucleus is
decoupled by using an appropriate weak power, one can
eliminate selectively the effect of either a long-range or a direct
1
3
1
13
1
C- H coupling and extract the C- H coupling through the

Conformational Analysis of Gemini Surfactants
J. Phys. Chem. B, Vol. 102, No. 45, 1998 8967
surfactants, n-octyltrimethylammonium bromide (OTAB, 98%)
and n-cetyltrimethylammonium bromide (CTAB, 98%) were
purchased from Tokyo Kasei Co. and were used without
purification.
Conductivity Measurements and CMC Determinations.
The electrical conductivity of the sample solutions was measured
with a Conductivity Meter CM-11P (TOA-Electronics Ltd.)
at 25.0 ( 0.1 °C. The critical micelle concentrations (cmc)
were determined from plots of specific electrical conductivity
against surfactant concentration.
1
3
C- and H-NMR Measurements. 13C NMR spectra were
recorded on a Varian Unity-400 plus spectrometer operating at
00.58 MHz at 30 °C, using an acquisition time of 2.560 s under
1
1
deuterium internal lock. Proton noise decoupling and selective
decoupling carbon-13 chemical shifts (δ, ppm) were determined
by use of 128 000 points in the time domain (sweep width
25 000 Hz). 3-(Trimethylsilyl)propanesulfonic acid sodium salt
(
DSS) was used as an external reference, and no susceptibility
13
correction was made. All C NMR measurements were made
in 5 mm NMR sample tubes at 30 °C. The estimated accuracy
1
3
of the observed C NMR chemical shifts is (0.004 ppm.
1
H NMR spectra were also recorded on a Varian Unity-400
plus spectrometer operating at 399.96 MHz at 30 °C, using an
1
acquisition time of 10.924 s. Proton chemical shifts ( H, δ,
ppm), which are given relative to the signal of an external
reference, were determined by use of 128 000 points in the time
1
domain (sweep width: 5999.7 Hz). All H NMR measurements
were made in 5 mm NMR sample tubes at 30 °C, and the
1
estimated accuracy of the observed H NMR chemical shifts is
(
0.0002 ppm.
Power Optimization for Selective Decoupling 13C NMR
Measurements. Decoupling was carried out only for the
aromatic protons, and the pulse sequence is schematically shown
in Figure 2A. The magnitude of decoupling power was
optimized in order to obtain the true aromatic C-13 nucleus-
s
CH2-proton coupling constant. As is shown in Figure 2B, the
C NMR spectra of the 1,4-dioxane-benzene-d6 solution
dioxane 40%) were measured for the various proton decoupling
1
3
Figure 2. Pulse sequence ([A]) used for selective decoupling and 13
C
6
NMR spectra ([B]) of the 1,4-dioxane-benzene-d solution (dioxane
(
4
0 wt %) measured for various frequency settings of the proton-
powers in the range of 25-45 dB. In this power range, only
the protons of benzene-d6 which remained undeuterized were
completely decoupled. As a consequence, it was found that
the decoupling power 25 dB does not bring about a decrease of
the coupling constant in the dioxane-CH2 spin system. Thus,
it was concluded that a decoupling power of 25 dB is optimal
for selective decoupling of only aromatic protons in the present
gemini surfactant molecules.
decoupler to optimize decoupling power.
TABLE 2: Critical Micelle Concentrations at 25 °C
Gemini surfactants
CMC [wt %]
CMC [mol l-1]
Ra
oxy8
mxy8
pxy8
sm3-8
sm4-8
sm5-8
sm6-8
1.36
1.31
1.42
0.71
1.39
1.43
1.41
0.0235
0.0226
0.0245
0.0138
0.0262
0.0263
0.0252
0.616
0.618
0.538
0.702
0.674
0.693
0.672
In these experiments, therefore, aromatic protons were
s
completely decoupled, while the spin-coupling between the CH2
protons and aromatic carbon-13 nuclei remained constant, and
no reduction of the 13C- H coupling constant for the CH2
1
s
a
R: degree of ionization.
groups was found.
for the n-octyl and spacer chains, has been examined as a
measure of the flexibility of the polymethylene chains.
The specific electrical conductivity for the two series of
surfactants in aqueous solutions was measured at various
concentrations. Plots of conductivity against concentration
furnished two straight lines which intersected at the cmc. The
cmc values thus obtained are listed in Table 2 together with
the values (R) of degree of ionization obtained from the slopes
of the straight lines.
Results
This present study mainly concerned with the two series of
gemini surfactants with rigid spacers and flexible spacers. The
geminis with rigid spacers (o-, m-, and p-phenylenedimethylene)
were synthesized in order to examine how the relative geometric
disposition of the n-octyl chains affects the micellar behavior
of the gemini molecules. The dimeric surfactants with flexible
spacers (short polymethylene spacers ((CH2)s, s ) 3, 4, 5, and
The concentration dependence of the H- and 13C NMR
1
6)) were also synthesized to allow comparison with the micellar
spectra of these gemini surfactants in D2O solutions has been
1
behavior of the geminis. In particular, the conformation about
the CH2-CH2 single bond adjacent to the two nitrogen atoms,
investigated in detail. Figure 3 parts A and B shows the H-
13
and C NMR spectra, respectively, of mxy8 (as a representative

8968 J. Phys. Chem. B, Vol. 102, No. 45, 1998
Hattori et al.
Figure 3. The H NMR ([A]) and 13C NMR ([B]) spectra of mxy8
1
(
2
1.0 wt % in D O) as a representative of the gemini surfactants. Shift
s
s
assignments (Hph: aromatic protons, H: CH
H7 and H8, corresponding to the CH
2
protons, H1, H2, H3-
1
2
3
7
8
2
, CH
2
, CH
2
- CH
2
, and CH
2
1
Figure 4. Selective decoupling 13C NMR spectra of aromatic 13
C
protons, respectively) are shown. The H signals marked with an asterisk
show the resonance lines coming from the internal DSS reference. The
a
b
c
d
carbons ( C(a), C(b), C(c), and C(d)) for mxy8 in monomeric, 1.0
wt %, ([A]), and micellar, 3.0 wt %, ([B]) solutions.
13
13
C chemical shifts (δ ppm) of each C resonance signal for the 1.0
a
b
c
wt % mxy8 solution are 139.12 ( C), 130.58 ( C), 137.32 ( C), 132.38
d
s
1
2
3
(
C), 69.41 ( C), 51.96 (N-CH
3
), 66.74 ( C), 27.79 ( C), 24.23 ( C),
a
b
the resonance spectral features of the C and C carbons were
characteristic of a triplet signal, and this feature did not change
above and below the cmc. The selective-decoupling 13C NMR
spectrum does not provide any information on the effect of
micellization of pxy8, indicating either that the exchange among
coexistent conformers occurs faster than the NMR time scale
or that other magnetic nonequivalent conformers of pxy8 do
not coexist.
4
5
6
7
8
3
0.41 ( C, C), 33.23 ( C), 24.23 ( C), and 15.65 ppm ( C) relative to
the DSS reference.
of the gemini surfactant series) together with assignment of their
1
13
H resonance peaks. The C chemical shifts of mxy8 only are
listed in the legend of Figure 3. The extents (∆δ ) δ (3 wt
)-δ (1 wt %), ppm) of the 13C chemical shift change upon
%
micelle formation are 0.32-0.34 ppm for oxy8, 0.36-0.45 for
mxy8, and 0.33-0.40 ppm for pxy8, indicating that the ∆δ value
is almost independent of the relative geometric disposition of
the n-octyl chains.
Menger and Littau23 studied the molecular orientation of (p-
phenylenedimethylene) bis(stearylammonium) dibromide by the
film-balance method and showed that the gemini surfactant
molecules may take up an all-trans conformation at the air/water
interface and that the water molecules remain incorporated into
the hydrophilic portion of the monolayer, even in the condensed
state. For the pxy8 molecules with short hydrophobic chains,
we may assume that a conformation, similar to the all-trans
conformation, exists in the micellar state together with other
possible conformers.
In this present study, detection of the conformational change
of the phenylenedimethylene spacer portion, which is brought
about by micellization, is focused mainly on the use of selective
13
1
decoupling C NMR and H NMR spectra. Selective decou-
1
3
26
pling C NMR spectra of the sample solutions of the three
gemini surfactants were therefore measured below and above
1
3
the cmc. In these spectra, the splitting of the aromatic
C
1
resonance signal which is due to spin-coupling with the aromatic
protons disappears, and only splitting of the aromatic C-13
resonance line, which is caused by the interaction between the
aromatic C-13 nucleus and the four protons of the two CH2
groups, is observed.
pxy8. For the pxy8 ((p-phenylenedimethylene)bis(octyl-
ammonium) dibromide) sample solutions, a conformational
change should not be reflected in the selective-decoupling
mxy8 and mxy10. Figure 4 shows selective decoupling 3C
1
3
a
b
c
d
NMR spectra of the aromatic C-carbons ( C, C, C, and C)
coupled with the m-phenylenedimethylene CH2 protons for the
mxy8 gemini surfactant. It should be noted that for the
s
a
c
d
resonance signals of the C, C, and C carbon nuclei there exists
a marked difference in the spectral features between monomeric
and micellar solutions, although such a difference is not found
for the C resonance signal. This observation indicates that
selective decoupling C NMR spectra of aromatic carbon-13
1
3
b
C
a
13
NMR spectra, since the ipso-carbon ( C) is not influenced by
the conformation of the n-octyl chain and, moreover, the four
s
nuclei reflects a conformational change about the two CH2-
C single bonds, due to the formation of micelles.
b
b
C carbons are equivalent. Indeed, although selective decou-
13
pling C NMR spectra were measured at various concentrations
under the standardized decoupling conditions, it was found that
We may expect six skeletal structures for an mxy8 molecule
in solution, as shown in Figure 5A, since three rotational isomers

Conformational Analysis of Gemini Surfactants
J. Phys. Chem. B, Vol. 102, No. 45, 1998 8969
projection the two n-octyl-N segments are in the trans config-
a
b
uration with respect to the C- C bond. For type I′, both of
t
s b
the H C C planes are coplanar with the benzene plane and the
two n-octyl-N segments extend in the direction of the gauche
(
or gauche′) configuration. In the type II (or type II′) structure,
s
b
one of the two N C C planes is coplanar with the benzene-ring
plane, while the other plane is not. That is, one of the two
n-octyl-N segments is in the trans configuration with respect to
the C- C (or C- C) bond, while the other n-octyl-N segment
extends in the direction of the gauche (or gauche′) configuration.
In type III, the two n-octyl-N segments both extend in the
direction of the gauche (or gauche′) configuration. For type
III′, both of the N C C planes are coplanar with the benzene-
ring plane, and the two n-octyl-N segments are in the trans
configuration with respect to the C- C bond.
b
a
b
c
s
b
b
c
a
The spectral feature of the C carbon resonance depends
strongly upon the mode of spin-spin coupling through the three
a
bonds (vicinal coupling) between the C nucleus and the four
s
protons of the two CH2 groups, which varies with the
conformations (trans (t), gauche (g), and gauche′ (g′) with
b
a
s
b
respect to the C- C bond) about the CH2- C single bonds.
s
We note the presence of the four protons of the two CH2 groups
and express the configurations of these protons by use of a
combination of t, g, and g′, as seen in Figure 5. Thus, the quintet
lines for type I, the double-quartet lines for type II and the
triple-triplet lines for type III are all possible for the splitting
a
features of the C carbon resonance, due to coupling with the
four protons (Figure 5B).
b
For the aromatic C carbon, spin-spin coupling through the
s
two bonds (geminal coupling) between the CH2 protons and
b
C nucleus does not depend on the conformation about the
s
b
2
CH2- C single bond. Therefore, the JCH value, which is
b
obtained from the splitting feature of the C carbon signal, does
not provide any information for such a conformation.
c
The splitting pattern of the C carbon resonance signal, unlike
a
that of the C nucleus, reflects only the conformation about the
s
b
s
one CH2- C single bond. When the two protons of the CH2
b
c
group take the tg form with respect to the C- C bond (the
s
s
two CH2 groups of type I′ and one of the CH2 groups of type
c
II′), the C carbon resonance provides the double-doublet
signals (Figure 5B), while for the gg′ form of the two protons
s
(one of the CH2 groups of type II′ and both for type III′) the
c
signal pattern of the C carbon resonance becomes a triplet.
d
The C nucleus is very weakly coupled with the protons of
s
the two CH2 groups through the four bonds, and the value of
4
d
the coupling constant ( JCH) is very small. Therefore, the C
carbon resonance is observed as a singlet signal. When the
exchange between all the conformers occurs within the NMR
time scale or when an mxy8 molecule takes up only one
d
conformation, the signal of the C resonance should be observed
as a singlet. Conversely, when many conformations coexist and
the exchange between these conformations occurs more slowly
than with the NMR time scale, the number of resonance lines
should equal the number of all the conformations, and the ratio
of intensities of the resonance lines should also be equal to the
ratio of populations of these conformations.
Figure 5. Possible skeletal structures ([A]) of an mxy8 molecule (the
b
a
b
c
C- C bond is a criterion for types I, II, and III and the C- C bond
is a criterion for types I′, II′, and III′) and possible splitting patterns
We may therefore describe the conformational change of an
mxy8 molecule upon micellization, based on the results of the
1
3
a
c
[
B] for the C NMR signals of the C and C carbons.
s
b
13
(
trans, gauche, and gauche′) about the CH2- C single bond
selective decoupling C NMR spectra. For the micellar solution
b
a
a
are possible. For types I, II, and III, the C- C bond is a
criterion for the configurations of trans, gauche, and gauche′,
while for type I′, II′, and III′, the C- C bond is a criterion for
the three conformers. In the type I structure, both of the N C C
planes are coplanar with the benzene plane, and in the Newman
of mxy8, the splitting pattern of the C carbon resonance
spectrum is obviously characteristic of quintet-lines (gg′gg′)
(Figure 4B, indicating that the conformation of type I is
preferentially stabilized upon micellization. Conversely, for the
sample solution below the cmc, as shown in Figure 4A, a
b
c
s
b

8970 J. Phys. Chem. B, Vol. 102, No. 45, 1998
Hattori et al.
a
spectral pattern of the C carbon resonance is found in which
the quartet lines (a part of tggg′) are superimposed upon the
quintet lines (gg′gg′), implying that type I and type II conforma-
tions coexist below the cmc.
c
For the spectral feature of the C-13 resonance, it is evident
that the splitting pattern characteristic of the gg′ form becomes
predominant upon micellization, although the two splitting
patterns characteristic of the different species of two gg′ forms
are superimposed upon each other. This observation indicates
the absence of type I′ and II′ conformations and is consistent
a
with the splitting pattern obtained from a C carbon resonance.
d
The C carbon resonance of the monomeric and micellar
solutions of mxy8 consists of two resonance peaks. Below the
cmc, the intensity of the resonance peak at low field is almost
equal to that of the high field peak, while above the cmc the
high field peak increases in intensity. Such a variation of the
Figure 6. Configurations of the two n-octyl-N segments in an oxy8
molecule.
d
C carbon resonance peaks also reflects a conformational
feature to one in which the two triplet signals arising from two
different conformers are superimposed. This observation
indicates that these conformers are magnetically nonequivalent,
change, which is expected from analysis of the splitting pattern
a
a
of the C carbon resonance. From consideration of the C and
d
d
C carbon intensities, the peak of the high field C resonance
s
a
due to the conformational change about the two CH2- C single
bonds. Moreover, exchange between these conformers occurs
slowly compared with the NMR time scale. The observed
difference in chemical shift may be caused by a stacking of the
benzene planes due to micelle formation.
comes from a type I structure and that of the low field resonance
d
comes from a type II. The difference in the C carbon chemical
shift between the two resonance peaks may be caused by the
d
magnetic environmental difference of the C carbon in solution,
d
in addition to the difference in charge density of the C carbon,
c
For the C carbon nucleus, the spectral features of the selective
s
b
due to the conformational change about the two CH2- C single
13
decoupling C resonance are found to be singlet for both the
bonds mentioned above.
monomeric and micellar solutions, since the spin coupling
For mxy10 with n-decyl chains, selective decoupling 13C
NMR spectra of the aromatic 13C-carbons coupled with the
m-phenylenedimethylene CH2 protons were also measured. The
results showed that the spectral features both below and above
cmc were very similar to those for mxy8 (spectra not shown),
revealing that a conformational change, similar to that of mxy8,
occurs for the mxy10 molecule upon micellization.
s
c
through the four bonds between the CH2 proton and C carbon
b
nuclei is very weak. At the C carbon, two resonance peaks
caused by the two conformers are observed. Conversely, the
c
C carbon of oxy8 provides only a singlet signal.
H NMR Spectral Variation of Aromatic Protons upon
1
1
Micellization. Figure 7 shows the aromatic region of the H
NMR spectra of mxy8 and oxy8 in D2O solution at different
1
oxy8. For the oxy8 sample solutions, selective decoupling
C NMR spectra were also measured under the same selective
concentrations. The assignment of the H signals, which were
1
3
made from the splitting patterns, are also shown in the legends
decoupling conditions as were used for the pxy8 and mxy8
samples.
The dominant feature of the C-13 resonance was a quintet,
to this figure. For the mxy8-D O solutions (Figure 7A), the
spectral feature of the aromatic protons depends strongly upon
2
a
concentration. Below the cmc, this feature does not change,
while above the cmc it varies rapidly with an increase in
caused by spin-spin coupling through both two and three bonds
a
a
between the C-13 nuclei and o-phenylenedimethylene CH2
concentration. The CH proton resonance is a singlet, and the
c
protons. The quintet-splitting feature is characteristic of the
gg′ form. The values of two coupling constants obtained from
the observed spectra are J1 ) 4.19 Hz and J2 ) 3.82 Hz for the
monomeric solution (1 wt %) and J1 ) 3.82 Hz and J2 ) 3.81
Hz for the micellar solution (3 wt %), indicating that there is
no marked difference in the J values between the monomeric
and micellar solutions. Since the observed J value is a weighted
average of the J values in the monomer and micellar states, for
the sample solutions of low micellar concentration the J value
in the monomeric state will make a major contribution to the
coupling constant and will dominate the observed value.
Therefore, when the original gg′ form is stabilized in both the
monomeric and micellar states, then a marked difference in the
J values will not be observed. Consequently, micellization of
the oxy8 surfactant does not result in a conformational change
downfield shift upon micellization is about 0.05 ppm. The CH
proton resonance is a doublet and micellization does not bring
1
d
about any H chemical shift change. The CH proton resonance
is a double-doublet, and a very small upfield shift is observed
upon micellization. No change is observed in spin-spin
coupling, despite the change in chemical shift upon micelliza-
tion. Such a variation in the spectral feature reflects a change
in the nature of stacking about the aromatic rings of the nearest
neighboring molecules.
For the oxy8-D O solutions (Figure 7B), the spectral features
2
of the aromatic protons also depend on concentration. Below
the cmc, the spectral feature is a singlet, while above the cmc
it changes rapidly to the AA′BB′ pattern with an increase in
concentration. The difference in the chemical shifts between
b
c
the CH and CH proton resonance peaks is ca. 0.07 ppm. In
the oxy8 molecule, the stacking effect of this benzene ring may
be larger than that for the mxy8 molecule.
s
a
about the CH2- C single bonds. The latter case implies that
s
the protons of CH2 do not take up the trans position with respect
a
b
to the C- C bond and that the two n-octyl-N segments are in
Conformation of the N-Terminal CH2-CH2 Segment for
the n-Octyl Chain. For the oxy8-, mxy8-, pxy8-, and mxy10-
a
a
a trans configuration with respect to the C- C bond, as shown
schematically in Figure 6. We suggest that the latter case may
predominate in the oxy8 solution.
1
1
D2O solutions, H NMR spectra of the CH2 protons for the
n-octyl and n-decyl chains were used to investigate the
b
conformation about the CH2- CH2 single bond. The 1
H
1
2
The spectral feature of the C carbon resonance is charac-
teristic of a triplet signal. However, micellization changes this
resonance lines which are observed at 3.25-3.35 ppm are

Conformational Analysis of Gemini Surfactants
J. Phys. Chem. B, Vol. 102, No. 45, 1998 8971
1
1
set of correct values for vicinal H- H coupling constants for
CH2-CH2 segments in the gauche and trans configurations. The
calculated populations of the gauche and trans isomers for the
three gemini surfactant solutions are also listed in Table 3. It is
seen that there is no marked difference in population between
the two solutions, indicating that the trans form is preferentially
stabilized both below and above the cmc.
For the micellar solutions of the dimeric surfactants (sm3-8
1
2
and sm5-8), the conformations about the n-alkyl CH2- CH2
1
2
and about the spacer CsH2- CsH2 single bonds were investi-
1
gated by using the H NMR spectral analysis mentioned above.
The Pt and Pg values thus calculated are also listed in Table 3.
It is found that although the trans form is predominant for these
conformations, there exists a difference in population between
1
2
1
2
the CH2- CH2 and the CsH2- CsH2 segments. That is, for
the n-octyl chain segment the Pt value tends to increase,
compared with that for the spacer segment.
1
For the monomeric surfactants OTAB and CTAB, H NMR
1
2
spectral analyses for the conformation of the CH2- CH2
segment were made, leading to the conclusion of predominant
stabilization of the trans form both below and above the cmc:
1
H NMR parameters for the OTAB micellar solution (8 wt %)
were 12.4 Hz for JAX, 4.7 Hz for JAX′, -12.5 Hz for JAA, and
-
12.3 Hz for JXX′ and the estimated Pt value ) 91%, while
1
Figure 7. Concentration dependence of the H NMR spectra of mxy8
those for the OTAB micellar solution (0.3 wt %) were 12.4 Hz
(
[A], a: 5.0, b: 3.0, and c: 1.0 wt %) and oxy8 ([B], a: 4.0, b: 3.0,
for JAX, 4.7 Hz for JAX′, -13.6 Hz for JAA, and -12.5 Hz for
and c: 1.0 wt %) in D
2
O. For the spectrum (a) of mxy8 the resonance
a
JXX′ and the Pt value ) 91%. This result shows that the -
signals at 7.71, 7.74, and 7.67 ppm are assigned to the Ha, Hc, and Hd
aromatic protons, respectively, and for the spectrum (a) of oxy8 the
signals at 7.81 and 7.74 ppm are assigned to the Hb and Hc aromatic
protons, respectively.
b
CH2- CH2 segment adjacent to the polar group is in an
extremely restricted state because of its proximity to the bulky
+
(CH3)3N group.
TABLE 3: 1H NMR Vicinal Coupling Constants (Hz) and
Estimated Populations (%) of Trans and Gauche Forms
Discussion
Menger et al.23 used surface tension, film balance, and other
methods to investigate the aggregation behavior of the geminis
pxy8, pxy12, pxy16, and pxy18, (which correspond to com-
pounds C-8, C-12, C-16, and C-18, respectively, in the ab-
breviations adopted by Menger et al.) and discussed possible
orientations for these geminis at an air/water interface. They
concluded that a conformational change of pxy18 (from the
conformation lying flat on the water surface to that (the so-
called “horseshoe” type structure) standing up vertically to the
p-phenylenedimethylene plane) is made possible by compressing
the monolayer.
One can estimate the cross section per molecule from the
pressure-area isotherms. In particular, the value of the cross
section, which is obtained as the process varies dynamically
from the expanded state to the condensed state, provides
significant information about the molecular orientation. It may
also be expected that a conformational change should occur
during the process of such a phase transition. Elucidation of
such a conformational change on the water surface may lead to
an understanding of the physicochemical property of the thin
film.
The determination of a reliable molecular structure which is
stabilized in the micellar state may be useful for estimating the
molecular orientation of the geminis at the air/water interface
and for determining any structural variation caused by this
orientation. For example, when the gemini molecules oxy8 and
pxy8 are adsorbed at the air-water interface, it may be estimated
that the N- C- C- C- C-N and N- C- C- C- C- C-N
skeletons containing the benzene ring are oriented vertically to
the air/water interface, since these planes are rigid and planar,
as discussed below.
Gemini
surfactants
a
JAX JAX′
JAA
JXX′ Pg Pt
1
2
oxy8
C- C
(1 wt %)mon 12.2
4.8 -12.4 -12.6 11 89
(1 wt %)mon 12.2
4.7 -12.2 -12.4 11 89
(
3 wt %)mic 12.2
1
2
mxy8
mxy10
pxy8
sm 3-8
sm 5-8
a
C- C
4.9 -12.8 -13.0 11 89
(
3 wt %)mic 12.4
4.6 -12.9 -12.7
4.7 -12.8 -12.5 9 91
4.6 -12.5 -11.0 92
4.7 -12.5 -12.5 10 90
4.7 -12.7 -12.9 91
12.4
11.8
12.2
11.8
9
91
1
2
C- C
(0.4 wt %)mon 12.4
(
2 wt %)mic 12.5
8
1
2
C- C
(1 wt %)mon 12.3
(
3 wt %)mic 12.4
9
1
2
C- C
(2 wt %)mic
4.5 -10.3 -10.2 9 91
4.9 -9.8 -9.8 16 84
4.6 -12.9 -12.7 11 89
5.1 -9.8 -9.8 16 84
1
1
1
2
CS- CS
(2 wt %)mic
(3 wt %)mic
(3 wt %)mic
2
C- C
2
CS- CS
The symbols mon and mic show monomeric and micellar states,
respectively.
assigned to the CH2 protons adjacent to the nitrogen atom in
1
the n-octyl chain. In the H NMR spectrum the four protons
1
2
of the CH2- CH2 segment provide a splitting pattern which is
typical of an AA′XX′ system: the resonance signals of the two
1
CH2 protons correspond to the AA′ part. Thus the spectral
feature reflects the conformations of the N-terminal CH2-CH2
segments. To examine the conformations about the CH2-CH2
29
single bonds, the LAOCOON III program was used to
1
calculate the H NMR spectrum of the N-terminal CH2-CH2
protons. Vicinal coupling constants (JAX, JAX′, JAA, and JXX′),
obtained by comparing the observed spectrum with the calcu-
lated best fit spectrum, are listed in Table 3.
s
a
a
s
s
a
b
a
s
The populations of the trans and gauche isomers are
expressed30-32
by eq 1
P ) 1 - P ) (J - J )/(J - J )
(1)
where the Jg and Jt values Jg ) 4.14 and Jt ) 13.22 Hz32 are a
g
t
t
AX′
t
g
Comparison of the hydrophobicity of the spacers suggests
that the three spacers (o-, m-, and p-phenylenedimethylenes)

8
972 J. Phys. Chem. B, Vol. 102, No. 45, 1998
Hattori et al.
of the geminis should correspond to the polymethylene spacer
compared with the case of oxy8. This dense packing may result
(CH2)8. However, comparison of the distance between the two
in an increase in the trans population (Pt).
polar groups shows that the N‚‚‚N distances for oxy8, mxy8,
and pxy8 correspond approximately to those for sm3-8, sm4-8,
and sm5-8, respectively, since geometrical calculation of the
N‚‚‚N distances for the extended polymethylene spacers of these
compounds are 5.83 Å for oxy8, 7.25 Å for mxy8, 7.31 Å for
pxy8, 4.96 Å for sm3-8, 6.26 Å for sm4-8, and 7.44 Å for
sm5-8. Therefore, the series of these gemini molecules cor-
respond to the dimeric surfactants with shorter polymethylene
spacers ((CH2)s, s ) 2-6).
For the mxy10 micelles, it was also found that the Pt value
1
2
for the CH2- CH2 segments of the main chain tends to
increase. This tendency may indicate that the increased length
of the main chain brings about a densely packed state of the
n-decyl chains in the micelle due to an increase in aggregation
number, a tendency which may promote a further restricted state
1
2
for the CH2- CH2 segment.
In fact, it is evident that there exists a difference in the
position of the phenylenedimethylene spacer between the type
I structure and the stabilized oxy8 structure. This difference
should change the balance of the hydrophobic and hydrophilic
portions of the micelles and may affect the micellar structure.
Thus, detailed studies of the micellar structures for these geminis
are strongly desirable.
In our previous small-angle neutron scattering study of the
33
dimeric surfactant system with m ) 10 and shorter (CH2)s
spacers (s ) 2-6), we showed that the aggregation number of
a minimum micelle formed by the dimeric surfactants depends
to only a small extent on the length of a spacer methylene chain,
implying that the n-decyl group of the dimeric surfactants plays
an important role in formation of a minimum micelle. There-
fore, for oxy8, mxy8, pxy8, and mxy10, we may assume that
two n-octyl chains strongly contribute to the micellar behavior.
1H NMR spectral variation of aromatic protons upon micel-
lization (Figure 7) provides information on the environment of
the benzene ring for mxy8 and oxy8. For mxy8, a singlet signal
of the Ha proton tends to shift downfield (from 7.67 to 7.71
ppm) with an increase in concentration. The doublet signal of
the Hc proton at 7.74 ppm is not dependent on concentration.
The Hd signal, which is observed as a double-doublet, shifts
upfield (from 7.69 to 7.67 ppm) as the concentration increases.
We may use the Pt (or Pg) value as a measure of order in the
n-octyl chain and in the polymethylene spacer. For oxy8, mxy8,
1
2
and pxy8, it may be assumed that the CH2- CH2 segments of
the n-octyl chains, on the whole, are in the highly ordered state
both below and above the cmc. The reason is probably due to
presence of the bulky polar groups, which hinder the free
rotation about the CH2-CH2 single bond. For the dimeric
surfactants (sm3-8 and sm5-8) with flexible spacers, it was found
1
The reason the H chemical shift change of the Hc protons does
not depend on concentration may be due to the reorientational
1
effect of the benzene plane. The H signals of the aromatic
protons are sensitive to the variation in the environment. In
fact, it has already been reported that they shift downfield in a
hydrophilic environment but upfield in a hydrophobic environ-
1
2
that the CH2- CH2 segments of two n-octyl chains are in an
extremely restricted state, while the extent of ordering for the
1
2
CsH2- CsH2 segment of the flexible spacer tends to decrease.
34
ment. Accordingly, we may use the direction of the change
This fact may indicate that it is the n-octyl chain rather than
the spacer which plays an important role in determining the
micellar behavior of these surfactants as well as that of the
geminis.
1
in the H chemical shift upon micellization as a measure of the
environment. Thus, it may be assumed that the Hd proton is
placed on the side of the hydrophobic core, and the Hc protons
may be close to the border region between the hydrophilic layer
1
Depending on the relative geometric disposition of the two
CH2 groups in the spacers, such a restricted state should not
and the hydrophobic core. For the oxy8 molecules, the H
s
signals of the Hc aromatic protons do not change with increasing
concentration, while the signals of the Hb protons shift
downfield (from 7.75 to 7.81 ppm) upon micellization. This
observation shows that the Hb protons at least of a benzene
ring may be located in the aqueous region. Thus, we may
estimate approximately the environment of a benzene moiety
in the micellar state.
be neglected, even if it is small in extent, since the Pt values
actually tend to increase in the order oxy8 < mxy8 < pxy8.
This trend may be related to the stabilization of a specific isomer
for the gemini molecules upon micellization.
For the conformation of an oxy8 molecule stabilized in the
s
a
a
s
micellar state, it should be noted that the N- C- C- C- C-N
skeleton of o-phenylenedimethylene is planar and the conforma-
The molecular conformations of the oxy8 and mxy8 mol-
ecules which are stabilized upon micellization, which have been
a
s
tion about the rigid C- C single bond is gauche. Moreover,
as mentioned in the Results the two n-octyl-N segments are in
elucidated in this present study, can be used to calculate the
a
a
16
a trans configuration with respect to the C- C double bond.
Accordingly, when two n-octyl-N segments take up a fully
extended form, we may assume that the two n-octyl chains are
not parallel to each other and fan out toward the methyl
terminals of the n-octyl chains. Furthermore, in this molecular
conformation, a phenylene ring of the spacer may be placed in
the hydrophobic region rather than the hydrophilic region.
Therefore, when the oxy8 molecules taking up such a confor-
mation form micelles, the packing feature of n-octyl chains must
become loose, since there exists a vacant space between the
n-octyl chains and the o-phenylene ring, leading to the tendency
for an increase in the gauche-population. That is, this loosely
packed state may bring about a decrease in the Pt value.
packing parameters for these molecules in micelles.
For
surfactant molecules of optimal area a0, hydrocarbon volume
v, and critical chain length lc, the packing parameter is defined
as v/a0lc. The relationship between the packing parameters of
a surfactant and micellar shape has been shown by Israelachvili
1
6
et al. Accordingly, the packing parameters calculated for the
oxy8 and mxy8 molecules in the micellar state are useful for
estimation of micellar shape. For calculation of the packing
parameter, the optimal area a0 and critical chain length lc can
be determined from the molecular model. However, the
hydrocarbon volume v depends on the extent of hydration of
the hydrophobic portion.
The parameters were calculated for oxy8, mxy8, and pxy8,
assuming that the two n-octyl chains and the benzene ring
belong to the hydrophobic core and are unhydrated. The
packing parameters, which were calculated for the stabilized
conformation of an oxy8 molecule and for the type I structure
of a mxy8 molecule, are 0.39-0.40 and 0.35, respectively. For
For the mxy8 molecules in the micellar state, as mentioned
above, the type I structure is predominantly stabilized, implying
s
b
a
b
s
that the N- C- C- C- C- C-N skeleton takes up the all-
trans form. Therefore, when the mxy8 molecules form micelles,
the n-octyl chains within these micelles may be densely packed,

Conformational Analysis of Gemini Surfactants
J. Phys. Chem. B, Vol. 102, No. 45, 1998 8973
the molecular model of pxy8, in which two n-octyl segments
are perpendicular to the p-phenylenedimethylene plane contain-
ing two nitrogen atoms, the calculated parameter is 0.33. On
the basis of these parameters, we assume that the micellar shape
for oxy8 may be cylindrical and that for mxy8 and pxy8 may
be spherical.
atoms of the rigid spacer is coplanar with the plane of the
benzene ring.
Preferential stabilization of a specific conformation probably
occurs on the water surface when these molecules are adsorbed
at the air/water interface.
If we assume that the aromatic carbons as well as some of
the methylene groups of the n-octyl chain are also hydrated by
their location in the water phase, the parameters (0.32-0.33)
which are characteristic of a spherical shape are obtained. Thus,
the position (environment) of a benzene ring in the micelle is
important for estimation of the shape of the micelle.
References and Notes
(
1) Okabayashi, H.; Okuyama, M.; Kitagawa, T. Bull. Chem. Soc. Jpn.
1
975, 48, 2264.
(2) Okabayashi, H.; Abe, M. J. Phys. Chem. 1980, 84, 999.
(
3) Okabayashi, H.; Yoshida, T.; Terada, Y.; Ikeda, T.; Matsushita,
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(
1
Consideration of the results of the change in the H NMR
T. J. Am. Chem. Soc. 1982, 104, 5399.
chemical shift suggests that the packing parameters of the mxy8
and oxy8 molecules may become smaller (ca. 0.33) and leads
us to the assumption that both mxy8 and oxy8 adopt a spherical
shape.
(5) Okabayashi, H.; Taga, K.; Tsukamoto, K.; Tamaoki, H.; Yoshida,
T.; Matsuura, H. Chem. Scr. 1985, 25, 153.
(6) (a) Tsukamoto, K.; Ohshima, K.; Taga, K.; Okabayashi, H.;
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(
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Conclusion
(
Three gemini surfactants (oxy8, mxy8, and pxy8) with o-,
m-, and p-phenylenedimethylene spacers have been synthesized.
The conformational changes caused by micellization of these
surfactants, and their related compounds in D2O were investi-
gated by selective-decoupling 13C NMR and 1H NMR methods.
It was found that introduction of rigid spacers into the gemini
surfactants promotes stabilization of the distance between the
two hydrophilic groups, since the specific conformation of the
spacer portion is preferentially stabilized upon micellization.
For the two series with rigid and flexible spacers, the conforma-
tion of the two n-octyl chains tends to take up an all-trans
conformation, implying that the two n-octyl chains play a
significant role in micellar behavior. For the dimeric surfactants
with flexible spacers, the population of the trans form of the
(
11) Taga, K.; Ohshima, K.; Matsuoka, H.; Yoshida, T.; Okabayashi,
H. Colloids and Surfaces A 1993, 81, 59.
12) Okabayashi, H.; Hirata, H.; Suzuki, Y.; Taga, K.; Mathew, C.
Vibrational Spectroscopy 1996, 10, 239.
13) Etori, H.; Yamada, Y.; Taga, K.; Okabayashi, H.; Ohshima K.;
O’Connor, C. J. Vibrational Spectroscopy 1997, 14, 133.
14) Aoki, K.; Okabayashi, H.; Maegawa, S.; Mizuno, T.; Murata, M.;
Hiramatsu, K. Biochim. Biophys. Acta 1982, 703, 11.
15) Adler, A. J.; Ross, D. G.; Chen, K.; Stafford, P. A.; Woiszwillo,
(
(
(
(
M. J.; Fasman, G. D. Biochemistry 1974, 13, 616.
(16) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc.,
Faraday Trans. 1 1976, 72, 1525.
1
2
CsH2- CsH2 segment of the spacer tends to decrease upon
(
17) Zana, R.; Talmon, Y. Nature 1993, 362, 228.
micellization, indicating that the steric compressive strain of
the polymethylene spacer, caused by aggregation of the two
n-octyl chains, may bring about an increase in the gauche-
population. Conversely, for the dimeric surfactants with
(18) Diamant, H.; Andelman, D. Langmuir 1994, 10, 2910.
(19) Stein, T. M.; Gelman, S. H. J. Am. Chem. Soc. 1992, 114, 3943.
20) Nusselder, J.-J. H.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1989,
11, 5000.
(
1
(
21) Menger, F. M.; Yamasaki, Y. J. Am. Chem. Soc. 1993, 115, 3840.
1
aromatic spacers, the population of the trans form of the CH2-
(22) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451.
(23) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083.
2
CH2 segments of the two n-octyl chains tends to decrease upon
(
(
24) Rosen, M. J. CHEMTECH 1993, 23, 30.
micellization.
25) Rosen, M. J.; Zhu, Z. H.; Hua, X. Y. J. Am. Oil Chem. Soc. 1992,
For the oxy8 molecules, the micelles are in a loosely packed
state owing to the presence of gauche forms in the skeleton of
the rigid spacer, because of preferential stabilization of a specific
conformation upon micelle formation.
6
9, 30.
(26) Takeuchi, S.; Uzawa, J.; Seto, H.; Yonehara, H. Tetrahedron Lett.
977, 34, 2943.
(
(
1
27) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072.
28) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9,
Since the difference in energy between the rotational isomers
for the mxy8 molecules in D2O may be small, we may speculate
that the hydrophobic interaction between the two n-octyl chains
and the aromatic ring plays a critical role in preferential
stabilization of type I in the micellar state. We may therefore
conclude that for oxy8 and mxy8 the benzene ring of the spacer
is directed toward the hydrophobic core and that the plane
containing the two nitrogen atoms and the two methylene-carbon
940.
(
(
(
29) Castellano, S.; Bothner-By, A. A. J. Chem. Phys. 1964, 41, 3863.
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(
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