Insights into the structure of the supramolecular amphiphile formed by a sulfonated calix[6]arene and alkyltrimethylammonium surfactants

Article abstract of DOI:10.1021/la3006794

In this work, we have studied the interactions between the water-soluble p-sulfonatocalix[6]arene and cationic surfactants octyltrimethylammonium bromide below the cmc and dodecyltrimethylammonium bromide above the cmc, by saturation transfer difference (STD) NMR spectroscopy. From the STD build-up curves, we have obtained the T1 independent cross relaxation rates, and the results show that the interactions established between the cationic headgroup of the surfactant and the OMe group of the macrocycle play an important role in the stabilization of the complex, both below and above the cmc.

Full text of DOI:10.1021/la3006794

Article
pubs.acs.org/Langmuir
Insights into the Structure of the Supramolecular Amphiphile
Formed by a Sulfonated Calix[6]arene and Alkyltrimethylammonium
Surfactants
†
‡
,†
Nuno Basilio, M. Martín-Pastor, and Luis García-Río*
†
Departamento de Química Física y Centro Singular de Investigacion
́
en Química Biolog
́
ica y Materiales Moleculares, Universidad de
Santiago, 15782 Santiago, Spain
‡
Unidade de Resonancia Magnet
S Supporting Information
́
ica, RIAIDT, Universidad Santiago de Compsostela, 15706 Santiago, Spain
*
ABSTRACT: In this work, we have studied the interactions
between the water-soluble p-sulfonatocalix[6]arene and
cationic surfactants octyltrimethylammonium bromide below
the cmc and dodecyltrimethylammonium bromide above the
cmc, by saturation transfer difference (STD) NMR spectros-
copy. From the STD build-up curves, we have obtained the T1
independent cross relaxation rates, and the results show that
the interactions established between the cationic headgroup of
the surfactant and the OMe group of the macrocycle play an
important role in the stabilization of the complex, both below
and above the cmc.
9
a
INTRODUCTION
Calixarenes are cyclic oligomers made of phenol units linked
by methylene bridges at the ortho-position. Because of their
tration (cmc) of pure CnTAB. For instance, it was found that
the addition of 5 mM of SC6HM reduces the cmc of
dodecyltrimethylammonium bromide (C12TAB) from 14 to
■
1
2
0
.2 mM. It was suggested that SC6HM forms a complex with
relatively facile chemical modification, calixarene derivatives
the ammonium surfactant that self-aggregates at lower
concentration than that of pure tensioactive. On the other
hand, in the course of our studies, we found that the most
common and less flexible p-sulfonatocalix[4]arene, bearing OH
groups at the lower rim, leads to the formation of vesicles in the
are often chosen as noncovalent building blocks for the
construction of supramolecular assemblies, such as self-
3
4
assembling capsules, rotaxanes and catenanes, micelles and
5
vesicles. Among the several assemblies that can be constructed
from calixarene building blocks, we are especially interested in
the study of micelles and vesicles formed from amphiphilic
9b
presence of cationic surfactants.
6
As a further step to understand at the molecular level the
structural effects that govern the micellization in this mixture of
compounds, in this Article we have employed saturation
transfer difference NMR spectroscopy (STD NMR) experi-
ments to determine the type of interactions involved and the
structural properties of the complexes formed between SC6HM
and CnTAB in the range of concentrations below and above
the cmc. STD NMR, together with transfer NOE, is one of the
most widespread NMR methods for the study of the
interactions between small ligands and macromolecular
calixarenes and related systems. Like other amphiphiles, those
constructed from the calixarene basic structure are obtained by
attaching hydrophilic and hydrophobic groups to selected
7
positions. Generally, hydrophobic or hydrophilic groups are
linked by means of covalent bonds to the calixarene framework
to form amphiphilic calixarenes. However, due to the
recognition properties of calixarenes, these macrocycles can
be used to construct supramolecular amphiphiles. In this case,
instead of covalent bonds, the functional segments required to
provide or improve the amphiphilic properties of the molecular
8
10
species are linked by means of noncovalent interactions. Using
receptors. The STD is aimed to map the interaction epitope
this approach, we and others have been able to construct
vesicles and micelles based on supramolecular amphiphiles
by determining the ligand regions in contact with the receptor.
9
EXPERIMENTAL SECTION
All commercial reagents were of the highest purity available and were
used as received. p-Sulfonatocalix[6]arene (SC6) was prepared by
formed from p-sulfonato-calix[n]arenes and cationic guests.
■
The present work focuses on the structural characterization
of the complex formed from 5,11,17,23,29,35-hexasulfonato-
3
7,38,39,40,41,42-hexamethoxycalix[6]arene (SC6HM) and
alkyltrimethylammonium bromide surfactants (CnTAB). We
have recently reported that SC6HM promotes the formation of
micelles at concentrations below the critical micelle concen-
Received: February 19, 2012
Revised: March 21, 2012
Published: March 26, 2012
©
2012 American Chemical Society
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ipso-sulfonation of the correspondent p-tert-butylcalix[6]arene in
H SO at 80 °C and alkylated with CH I in basic media following
uations, the experiment permits one to determine the binding
epitope of a small ligand that is attached to a target
macromolecule. However, it has been found that when the
2
4
3
5a
10
literature procedures. NMR experiments were performed on a 17.6 T
1
Varian INOVA NMR spectrometer operating at a H frequency of 750
T1 relaxation time of individual protons is significantly different,
there are possibilities for misinterpretation of the STD
responses, and, consequently, the determination of the binding
MHz. The spectrometer control software was VNMR/VNMRJ 2.2D.
The spectra were referenced with an external standard reference of
TSP (3-(trimethylsilil)-propionic-d acid) dissolved in D O that was
12
epitope with this method may be erroneous, although some
4
2
13
solutions have been recently proposed.
inserted in the NMR tube. The NMR studies were conducted at 25
C. The temperature was calibrated with a standard reference of 4%
In this article, the association equilibrium between SC6HM
°
and CnTAB (Figure 1) is studied with STD NMR at 25 °C in
methanol-d in methanol sample. The spectra were processed and
4
D O at several concentrations (1:0, 1:1, 10:10, and 10:30 in
analyzed with MestRe-C v3.9 software (Mestrelab Inc.). Plots and
nonlinear data fitting of the data were performed with Origin v7.0
2
mM) covering the range of sample states corresponding to free
calixarene, complex, and micelle. In each case, a sample was
prepared at the required concentration of SC6HM and CnTAB
(Originlab Inc.).
9a
that was described in a previous study in our lab. While
sample 1:0 corresponds to free calixarene, in sample 1:1 both
free SC6HM and CnTAB exist in dynamic equilibrium with the
complex formed from the association of these two species. It is
worth noting that in our previous work we confirmed the
existence of a complex with a 1:1 stoichiometry. Samples 10:10
and 10:30 are above the cmc, and apart from the complex and
the free species, micellar aggregates are present in solution.
Indeed, judging by the cmc of this system (0.2 mM), it is likely
that a high fraction of SC6HM and CnTAB are in the
micellized state.
Figure 1. Molecules studied in this work: p-sulfonatocalix[6]arene
hexamethyl ether (SC6HM) and alkyltrimethylammonium surfactants
(
CnTAB). The labeling of the proton signals that were studied by
1
NMR is indicated.
Figure 2 shows H NMR reference spectrum (a) of a sample
containing 1 mM of calixarene SC6HM and 1 mM C TAB
8
NMR Sample Preparation. Mixtures of SC6HM and surfactant
CnTAB (Figure 1) were prepared in 5 mm standard NMR tubes. Four
samples were prepared with concentrations of SC6HM:CnTAB of 1:0,
(sample 1:1) along with the STD NMR appearing in the STD
spectra when the signals of C TAB are saturated and vice versa.
8
Figure 2 proves the binding interaction between SC6HM and
1
:1, 10:10, and 10:30. These concentrations are expressed in
C TAB. In addition, the differential STD signals permit a
8
millimolar concentration. In the sample 1:1, we used the cationic
surfactant octyltrimethylammonium bromide (n = 8), which allowed
us the use of higher concentration below the cmc (25 mM in the
presence of SC6HM ). In all other samples, we used dodecyl-
trimethylammonium bromide (n = 12).
qualitative characterization of the binding epitope of the ligand.
As can be observed, when the C TAB signals are saturated, the
8
9a
STD is more intense for the protons corresponding to the
OMe group of SC6HM. In addition, the STD intensity of the
calixarene protons is higher when the protons of the
trimethylammonium group are saturated (Figure 2d). This
set of observations seems to suggest that the ammonium
headgroup of the surfactant is closer to the OMe group of
SC6HM than to other protons. When the SC6HM signals are
saturated, the results point in the same direction: saturation of
Saturation Transferred-Difference (STD) Experiments. Signals
corresponding to protons Ha, Hb, and Hc of SC6HM and Ha′, Hc′, or
He′ of CnTAB (see Figure 1) were saturated in independent STD
10a
experiments. STD experiments were acquired by selective saturation
of the correspondingly proton signal with a train of 50 ms Gaussian
10
9
0° pulses separated by 1 ms. For each signal saturated, the STD
experiment was repeated with the following values of the saturation
time: 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, 2, 2.5, 3, and 3.5 s.
the aromatic (Figure 2g) and bridge CH protons (Figure 2f)
2
results mainly in intramolecular STD effects, while when the
OMe protons are saturated (Figure 2e), apart from the
intramolecular STD, the saturation is also transferred to the
The interscan relaxation delay (d ) and the acquisition time were set to
1
6
and 1 s, respectively, to achieve complete relaxation prior to any
scan. The STD spectra were processed, and the cross relaxation rates
σ
obs
were determined as described in the theory section (see the
protons of the N(CH ) group (STD effect is also observed for
I{S}
3 3
Supporting Information).
the b′ protons, but this result is not conclusive because the
signal is not sufficiently displaced from the signal that is being
saturated). This is quite a surprising result, because SC6HM
RESULTS AND DISCUSSION
■
14
Despite our efforts to apply methodology based on ROESY
experiments to determine the binding mode between SC6HM
and CnTAB, these experiments were unsuccessful. The
explanation for this result has to be found in the fact that for
concentrations above the cmc, the system calixarene−surfactant
is in the negative NOE regime typical of relative large size
molecular aggregates. Under this regime, NOE-based experi-
ments are always preferable over ROE experiments because
the former provide stronger measurable effects and are not
affected by the efficient transversal relaxation of large aggregates
that cause important sensitivity losses in ROESY.
lacks well-defined binding pockets due to its high flexibility,
and in consequence one might expect that its complexation
properties toward oppositely charged guest are dominated by
15
coulombic forces between the sulfonate groups and the
ammonium group. However, our results suggest that the
oxygen atoms in the OMe groups play an important role in the
complexation of cationic guests, presumably by charge−dipole
interactions between the electron lone pairs of the oxygen and
positive charge of the ammonium group. The formation of
complexes between calix[6]arenes derivatives and trimethy-
lammonium cations driven by cation−dipole interactions had
11
16
Among the different methods relying on NOE, the STD is
now a convenient and a well-established tool to study
association equilibria between molecules. In favorable sit-
been previously reported.
p-Sulfonatocalix[n]arenes (SCn) have aromatic cavities that
can induce large upfield chemical shifts (Δδ = δcomplex − δguest
)
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Figure 2. 1D STD spectra (STDon − STDoff) of the mixture SC6HM:C TAB in a ratio 1 mM:1 mM at 25 °C. (a) 1H reference spectrum; (b) 1D
8
STD with saturation of e′ signal of surfactant (0.7 ppm); (c) 1D STD with saturation of c′ signal of surfactant (1.58 ppm); (d) 1D STD with
saturation of the a′ signal of surfactant (2.89 ppm); (e) 1D STD with saturation of c signal of calixarene (3.21 ppm); (f) 1D STD with saturation of b
signal of calixarene (3.94 ppm); and (g) 1D STD with saturation of a signal of calixarene (7.33 ppm). The STD saturation time was 3 s, and the
signal saturated in each case is indicated with an asterisk below.
1
on the H NMR signals of their included guests molecules.
Carefully analysis of the magnitude of the Δδ for the different
protons of the guests has been used to elucidate the structure of
Values of the binding constants of organic ammonium
19
4
5
−1
cations with SCn are in the range of 10 −10 M due
principally to the cooperation of π-stacking, CH−π, and
cation−π interactions inside the cavity of the host and
additional coulombic interactions provided by the presence of
the sulfonate groups at the portal of the macrocycle. In the case
of the complex formed between SC6HM and C12TAB, we
17
18
several inclusion complexes. Moreover, in the study of the
complexation of bicyclic azoalkanes with p-sulfonatocalix[4]-
arene (SC4), it was found that the structural assignments based
on the complexation induced Δδ were fully in line with 2D
ROESY NMR experiments. The low Δδ obtained for the
trimethylammonium protons of CnTAB in the presence of
SC6HM (∼ −0.13 ppm) is in agreement with results
presented here, suggesting that surfactant guest is not included
in the pseudocavity of SC6HM. On the other hand, in a recent
study, we observed values of Δδ ≈ −1.86 ppm for the
ammonium headgroup of tetradecyltrimethylammonium bro-
9a
−1
obtained an association constant of approximately 1500 M ,
which is 1 or 2 orders of magnitude lower than the majority of
the reported binding constants for the formation of complexes
9a
20
between SCn and ammonium cations. This decrease in
binding ability could be due to the lack of cooperative
interactions in the case of SC6HM, which could be explained
by the flexibility of host that leads to a nondefined cavity, which
is not able to promote, for example, cation−π interactions.
Alkylation of the phenolic oxygens seems to play an important
role in the ability of the SCn to form complexes with organic
cations. It had been shown that the 5,11,17,23-tetrasulfonato-
7
mide (TTABr) in the presence of SC4, indicating that, in this
case, the ammonium group is incorporated into the aromatic
cavity of the host, probably with cation−π interactions
contributing to the stabilization of the complex. In the case
of the present study, the results indicate that the most
important interaction between SC6HM and CnTAB is
probably of the ion−dipole type.
2
5,26,27,28-tetrakis(n-butyl)-calix[4]arene (SC4TB) derivative
blocked in the cone conformation presents weaker binding
affinities (2−3 orders of magnitude) for organic cations, with
21
respect to SC4. This observation had been attributed to the
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Figure 3. Normalized STD build-up curves of the mixture of SC6HM and CnTAB, with saturation of signal a of SC6HM. The concentrations used
are: (a) 1 mM:1 mM (below the cmc), (b) 10 mM:10 mM, and (c) 10 mM:30 mM (with b and c above the cmc). (○) STDb(a), (●) STDc(a), (□)
STDa′(a), (▲) STDe′(a)
.
fact that SC4TB presents a weaker π-electron density (due to
the alkylation of the phenolic oxygens, which, in contrary to
SC4, are no longer ionizable), the absence of complexed high-
energy water molecules, and its tighter cavity. Because of this
combination of factors, it had been proposed that the organic
cations interact predominantly with the oppositely charged
sulfonate groups of SC4TB rather than with the aryl rings using
π-interactions. Conversely, the organic cations are placed
outside the aromatic cavity at the level of the sulfonate groups
when complexed with SC4TB, while in the case of SC4 this
kind of cationic guests penetrates more deeply inside the
aromatic cavity.
positive charge of TMA should play an important role. In
addition, the absence of aliphatic chain in TMA seems to not
alter the binding mode significantly.
The differences in the binding modes of SC6HM and SC4 in
the complexes formed between trimethylammonium surfactants
and these two calixarenes could help to explain the differences
in the type of aggregates formed (micelles in the case of
9
SC6HM and vesicles for SC4), but the high conformational
flexibility of SC6HM in comparison with SC4 could also play a
significant role in the aggregation process.
STD Build-Up Curves. As was commented on above, the
size of the observed STD signal is not only dependent on the
distance between the ligand and the macromolecule. Saturation
of a signal in the bound state is counteracted by their
To validate the results presented here, 1D STD experiments
were acquired for samples made of mixtures of SC4 with
tetramethylammonium chloride (TMA), SC4TB with TMA,
and also SC6HM with TMA (see the Supporting Information).
10
longitudinal relaxation times T1 in the free state. To prevent
possible misinterpretations due to T1 effects, the construction
In the case of SC4 and SC4TB, saturation of the CH protons
3
12b
of STD build-up curves was proposed, and the slope of the
of TMA leads to a STD response almost exclusively on the ArH
groups of the corresponding calixarene. This indicates that in
both cases the TMA cation is located inside the aromatic cavity
STD build-up curves at 0 saturation time can be used to
eliminate T1 bias at long saturation times.
Figure 3 shows some of the STD build-up curves obtained
when the aromatic protons of the calixarene (a) are saturated,
and, in the same way, Figure 4 shows selected STD build-up
curves obtained from saturation of the methyl protons in the
ammonium headgroup of the surfactant. In these plots, the
STD normalized intensity of a given proton signal is
represented against the saturation time. In what follows, we
will use the notation STDx{y} to refer to the STD curve
obtained for proton x upon saturation of proton y.
(
(
SC4) or in the upper rim interacting with the sulfonate groups
SC4TB), and the magnetization is, as expected, mostly
transferred to the ArH protons. On the other hand, in the
case of SC6HM (Figure S3), the only appreciable STD
response is observed for the OMe groups when the signal of the
TMA cation is saturated. This result suggests that in the
complex formed between SC6HM and TMA, the binding mode
should be similar to that observed for CnTAB and ion−dipole
interactions between oxygen atoms in the OMe group, and the
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Figure 4. Normalized STD build-up curves of the mixture of 1 and 2, with saturation of signal a′ of surfactant 2. The concentrations used are: (a) 1
mM:1 mM (below the cmc), (b) 10 mM:10 mM, and (c) 10 mM:30 mM (with b and c above the cmc). (□) STDa(a′), (●) STDb(a′), (○) STDc(a′)
△) STDe′(a′)
,
(
.
Figure 5. Aggregate evolution on increasing the concentration of CnTAB. Below the cmc, the complex exists in dynamic equilibrium with free
SC6HM and CnTAB. When the concentration of CnTAB is increased to values above the cmc, micellar aggregates composed of SC6HM and
CnTAB are formed. When the concentration of CnTAB is further increased from 10 to 30 mM, most of the added surfactant is incorporated into the
micelles, leading to micellar growth due to surface charge neutralization and, eventually, to complexation of free SC6HM.
The plots of Figure 3 show the curves obtained for the
saturation of proton a of SC6HM, corresponding to the
mixtures of SC6HM and CnTAB below and above the cmc. In
each plot, the intramolecular STDb{a} and STDc{a} have the
strongest STD effect. In the three plots, the shortest saturation
time for which the curves STDb{a} and STDc{a} reach their
maximum follows the order 10 mM:30 mM ≅ 10 mM:10 mM
correlation time that SC6HM feels for the different states
considered (i.e., association complex and micellar aggregates).
The maximum intensity reached by STDb{a} in each of the plots
of Figure 3 suggests that the correlation time follows the order
10 mM:30 mM > 10 mM:10 mM > 1 mM:1 mM. Utterly, the
molecular tumbling correlation time of the calixarene is
consistent with the expected larger average size of the micellar
aggregates, in which SC6HM is forming part, in comparison
with the association complex. The fact that the correlation time
for the sample 10 mM:30 mM is higher than for the sample 10
mM:10 mM can indicate that the micelles grown when the
concentration of CnTAB increases with respect to that of
SC6HM or that a higher fraction of SC6HM is incorporated in
the micelles or both (Figure 5). This is in line with the
≪
1 mM:1 mM. On the other hand, the maximum normalized
intensity of STDb{a} in the three curves follows the order 10
mM:30 mM > 10 mM:10 mM > 1 mM:1 mM. The fact that the
intramolecular STDb{a} is not very dependent on the
conformation adopted by the calixarene (molecular modeling
calculations performed by us showed that the intramolecular
NOE distances of proton a to the two methylene protons b are
9a
2.7 ± 0.3 and 3.6 ± 0.2 Å; the possibility of these NOEs
conclusions presented in our previous study of this system.
corresponding to short intermolecular distances calixarene−
calixarene can be safely ruled out) suggests that the intensity of
STDb{a} primarily reflects the average molecular tumbling
The plots of Figure 4 show the curves correspondingly to the
1 mM:1 mM, 10 mM:10 mM, and 10 mM:30 mM samples
upon saturation of proton a′ of the surfactant. In each plot, the
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2
2
intermolecular curve STDc{a′} has the strongest STD effect as
compared to the intermolecular STDb{a′} and STDa{a′}. These
results strongly suggest that in the three samples considered for
the mixture of SC6HM and C12TAB, the molecules form a
complex in which the NMe3+ group of the surfactant is in close
proximity to the OMe pendant group of SC6HM (Figure 6). In
networks, and rebinding contributions. Those effects are
negligible in the determined values of CRR under the
extrapolation to zero saturation time used to determine them
(see Supporting Information, eqs 3−5).
obs
I{S}
The analysis of a STD curve such as those represented in
Figures 3 and 4 with the initial build-up approach permits one
of the proton pair I−S. In the
association equilibrium established between SC6HM and
obs
I{S}
to obtain the CRR σ
CnTAB, eq 6 (see the Supporting Information) applies, and
obs
the determined values of σ represent a combination of the
I{S}
free
bound
I{S}
.
individual CRR in the free and bound states, σ and σ
weighted by their respective molar fractions, χ and χ
,
I{S}
free
bound
obs
I{S}
The results of σ
obtained for selected proton pairs of
SC6HM and CnTAB in the three samples considered are given
in Tables S1−S3 (see the Supporting Information), respec-
obs
I{S}
tively. For comparison, the σ values obtained for a sample of
SC6HM in the absence of surfactant are also given in Table S4
(
Supporting Information). From Tables S1−S4, the plots of
obs
Figure 7 were built. In these figures, σ is represented as a
I{S}
Figure 6. Cartoon illustration of a possible structure of the complex
formed between SC6HM and C12TAB, with the ammonium
headgroup of the surfactant interacting with the oxygen atoms of
the calixarene. Because of the inherent flexibility, it should not be
assumed that SC6HM is in one fixed conformation.
function of the concentration of SC6HM and CnTAB in the
mixtures for selected intramolecular proton pairs.
As the concentration of SC6HM and/or CnTAB is raised
and the sample passes from free calixarene (1:0), complex
(
σ
1:1), to micelles (10:10) and micelles (10:30), the curves of
obs
in Figure 7 notoriously increase their magnitude. Such
I{S}
these three plots, the shortest saturation time for which the
curve STDc{a′} reaches its maximum follows the order 10
mM:30 mM > 10 mM:10 mM > 1 mM:1 mM. However, the
intensity of STDc{a′} is much stronger in 10 mM:10 mM sample
enhancement in CRR has to be primarily related to the
molecular tumbling correlation time, or in other words with the
average molecular size of the environment felt by molecules
SC6HM and C12TAB. The molecular environment is
considerably larger in the micellar states than in the complex
or free calixarene, as was commented on above for Figures 3
and 4.
(
Figure 4b) than in the 1 mM:1 mM (Figure 4a) and 10
mM:30 mM (Figure 4c) samples. These sets of observations
could reflect a combination of factors that lead to a closer
distance between protons a′−c in the micelles formed in sample
In Figure 8, we show the intermolecular σobs obtained from
1
0 mM:10 mM with respect to that of sample 10 mM:30 mM
I{S}
the irradiation of selected surfactant signals in sample 10:10. As
and for the association complex (sample 1 mM:1 mM), such as
changes in the composition of the micelles that might lead to
different packing, geometry, aggregation number, etc., and/or
different conformational rearrangement of the complex in the
three states.
obs
I{S}
can be observed, the large σ value obtained corresponds to
the c protons of the calixarene (OMe group) when the signal a′
of the surfactant (N(CH ) group) is irradiated. These results
3 3
clearly show that the ammonium headgroup of the surfactant is
closer to the OMe group than to the other protons of the
calixarene (aromatic and bridge), supporting the results
previously described in this Article. It must be pointed that
the same trend was observed in all other samples (1:1 and
10:30), suggesting that the binding mode between SC6HM and
C12TAB does not change substantially upon aggregation.
Cross Relaxation Rates (CRR). The STD normalized
−6
−3 2
intensities and the CRR are both sensitive to the r (or ⟨r ⟩ )
average distances of the protons involved. However, the direct
interpretation of the STD can be misleading due to the
pernicious effects that arise from the differences in T
longitudinal relaxation times, NOE spin diffusion thru dipolar
1
12
Figure 7. CCR σ oI {bS s} of selected proton pairs as a function of the concentration of SC6HM and CnTAB expressed in millimilar. Left: Intramolecular
obs
I{S}
obs
b{a}
obs
a{b}
obs
c{b}
obs
b{c}
obs
a′{c′}
or intraspecie σ of SC6HM, (black ) σ ; (red ) σ ; (green) σ ; and (blue) σ . Right: Intramolecular or intraspecie σ
of CnTAB.
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conclude that complex is formed by interaction between the
oxygen atoms in the OMe groups and the cationic head of the
surfactant. This is a surprising result because the interaction wa−s
supposed to be mainly between the oppositely charged SO3
groups of SC6HM. Moreover, we found that the binding mode
does not change upon the micellization of the complex. The
results obtained here also support the dynamic behavior of this
system, in particular, the variation in the micellar morphology
imposed by the changes in the SC6HM:CnTAB molar ratio.
ASSOCIATED CONTENT
Supporting Information
Theory, tables of cross relaxation rates, and CRR σ
■
*
S
obs
I{S}
determined for selected proton pairs in the different mixtures
of surfactants and calixarenes are reported. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 8. Intermolecular CCR σobs for SC6HM protons (a, b, c)
obtained from the STD build-up curve, corresponding to saturation of
the CnTAB signals (a′, b′, c′) in sample 10 mM:10 mM.
AUTHOR INFORMATION
Corresponding Author
Fax: +34 981 595012. E-mail: luis.garcia@usc.es.
■
*
The results in Figure 9 show the influence of the sample
composition on the intermolecular CCR for most relevant
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from Ministerio de Ciencia y Tecnologia
Project CTQ2011-22436) and Xunta de Galicia (PGIDIT10-
■
(
́
PXIB209113PR and 2007/085) is gratefully acknowledged.
N.B. acknowledges FCT for Ph.D. Grant SFRH/BD/29218/
2
006.
ABBREVIATIONS
■
CRR = cross relaxation rates
obs
σ
= observed cross relaxation rate for the pair of
I{S}
Figure 9. Intermolecular CCR σobs of selected proton pairs as a
protons I−S upon saturation of proton S
I{S}
free
σ
= cross relaxation rate for the pair of protons I−S in
I{S}
function of the concentration of SC6HM and C12TAB expressed in
obs
c{a′}
obs
c{c′}
obs
c{e′}
obs
a′{c}
the free state upon saturation of proton S
millimolar. (black) σ ; (red) σ ; (green) σ ; and (blue) σ .
bound
σ
= cross relaxation rate for the pair of protons I−S in
The values obtained for sample 10:30 upon irradiation of the
I{S}
obs
c{a′}
obs
c{c′}
obs
c{e′}
surfactant signals, σ , σ , and σ , were multiplied by 3 to
the bound state upon saturation of proton S
account for a higher fraction of free C12TAB.
STDI{S} = normalized saturation transfer difference intensity
for the pair of protons I−S upon saturation of proton S
proton pairs. When the surfactant protons are irradiated, it can
be observed that the CRR for the c protons of SC6HM
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c{a′}
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