Exploring the charged nature of supramolecular micelles based on p-sulfonatocalix[6]arene and dodecyltrimethylammonium bromide

Article abstract of DOI:10.1039/c5cp03718b

The aggregation of supramolecular amphiphiles formed from hexamethylated p-sulfonatocalix[6]arene (SC6HM) and dodecyltrimethylammonium bromide (C₁₂TAB) was studied by capillary electrophoresis experiments and by kinetic probes. The hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC) was used to investigate the micropolarity of the micellar aggregates and their ability to solubilize and stabilize labile organic compounds against hydrolysis. Further insights were obtained using a more sophisticated kinetic probe: the basic hydrolysis of p-nitrophenylvalerate (NPV). This probe provides information on the ionic composition of the micellar interface and on the potential of the aggregates to be used as nanoreactors. The results obtained revealed that the charge of the micellar aggregates can be tuned from anionic to cationic through the adjustment of the C₁₂TAB : SC6HM molar ratio and confirmed that these micelles have good solubilization properties. On the other hand, the kinetics of the p-nitrophenylvalerate basic hydrolysis suggest that, in the concentration range comprised between the first and second CMCs, Br^- anions do not take part in the micellar structure.

Full text of DOI:10.1039/c5cp03718b

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Exploring the charged nature of supramolecular
micelles based on p-sulfonatocalix[6]arene and
dodecyltrimethylammonium bromide
Cite this: Phys.Chem.Chem.Phys.,
2
015, 17, 26378
a
b
b
b
Nuno Basılio,* Daniel Alfonso Spudeit, Juliana Bastos, Leandro Scorsin,
´
b
b
c
Haidi D. Fiedler, Faruk Nome and Luis Garcıa-Rıo*
´
´
The aggregation of supramolecular amphiphiles formed from hexamethylated p-sulfonatocalix[6]arene
SC6HM) and dodecyltrimethylammonium bromide (C12TAB) was studied by capillary electrophoresis
(
experiments and by kinetic probes. The hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC) was
used to investigate the micropolarity of the micellar aggregates and their ability to solubilize and
stabilize labile organic compounds against hydrolysis. Further insights were obtained using a more
sophisticated kinetic probe: the basic hydrolysis of p-nitrophenylvalerate (NPV). This probe provides
information on the ionic composition of the micellar interface and on the potential of the aggregates to
be used as nanoreactors. The results obtained revealed that the charge of the micellar aggregates can
be tuned from anionic to cationic through the adjustment of the C12TAB : SC6HM molar ratio and
confirmed that these micelles have good solubilization properties. On the other hand, the kinetics of the
p-nitrophenylvalerate basic hydrolysis suggest that, in the concentration range comprised between the
Received 27th June 2015,
Accepted 2nd September 2015
DOI: 10.1039/c5cp03718b
ꢀ
www.rsc.org/pccp
first and second CMCs, Br anions do not take part in the micellar structure.
3–5
not yet fully understood. In addition to conventional surfactants,
in recent years a new class of amphiphilic compounds emerged,
Introduction
Surfactants are amphiphiles, molecules that contain hydrophobic the so-called supramolecular amphiphiles. The main difference
1,2
and hydrophilic segments. Compounds of this sort are ubiquitous between conventional and supramolecular amphiphiles is that
in biological systems, as main components of membrane bilayers, in the former case the hydrophilic and hydrophobic segments
and also find use in numerous technological and industrial are connected via covalent bonds, while in the latter case these
applications: from common cleaning detergents to oil recovery, segments are held together by non-covalent interactions or
6
–8
pharmaceutical and cosmetic formulations, drug delivery systems, dynamic covalent bonds.
While, as discussed above, the
catalysis and synthesis of nanostructured advanced materials, to adsorption and aggregation properties of conventional surfactants
name a few. All of these applications arise from the ability of these are not fully understood, the field of supra-amphiphiles is in its
molecules to adsorb at surfaces and interfaces and to self-assemble infancy and the physico-chemical basis for the aggregation
into more or less defined aggregates of nanometric dimensions.
behavior of this special class of amphiphilic compounds is yet
Conventional surfactants consist of a polar or an ionic head to be established.
group connected to a hydrophobic alkyl chain. Despite their
The aggregation of supramolecular amphiphiles based on
relative structural simplicity when dissolved in water these water soluble p-sulfonatocalix[n]arenes and cationic organic
compounds can become involved in complex equilibria between guests has attracted the attention from some research groups
9–26
free monomers, interfacial adsorption and aggregation. These during the last few years.
The stability and final architecture of
phenomena have been the subject of numerous studies, but are the self-assembled aggregate depend strongly on the concentration
ratio and structural features of both the guest and host. For
a
example, highly flexible p-sulfonatocalix[n]arenes methylated at
the lower rim form micelle-like aggregates in the presence of
cationic surfactants while native p-sulfonatocalix[n]arenes,
which are conformationally less mobile due to intramolecular
hydrogen bonding between the hydroxyl groups, self-assemble
into unilamellar vesicles and multilamellar nanoparticles in the
Laborat o´ rio Associado para a Qu ´ı mica Verde (LAQV), REQUIMTE, Departamento
de Qu ´ı mica, Faculdade de Ci ˆe ncias e Tecnologia, Universidade Nova de Lisboa,
2
829-516 Monte de Caparica, Portugal. E-mail: nuno.basilio@fct.unl.pt
b
c
INCT-Cat a´ lise, Departamento de Qu ´ı mica, Universidade Federal de Santa
Catarina, 88040-900 Florian o´ polis, Santa Catarina, Brazil
Departamento de Qu ´ı mica F ´ı sica, Centro de Investigaci o´ n en Qu ´ı mica Biol o´ gica y
Materiales Moleculares (CIQUS), Universidad de Santiago de Compostela,
9
–26
1
5782 Santiago de Compostela, Spain. E-mail: luis.garcia@usc.es
presence of positively charged amphiphiles.
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The micellar aggregation of hexamethylated p-sulfonatocalix- Experimental section
[
6]arene (SC6HM) in the presence of cationic surfactants displays
All chemicals used were of the highest commercially available
purity and none required further purification. Hexamethylated
very interesting behavior and shares some features with oppositely
charged polyelectrolyte/surfactant systems that deserve to be
9
,12,13
9
,12,13
p-sulfonatocalix[6]arene was available from previous studies.
-Methoxybenzenesulfonyl chloride (MBSC) and p-nitrophenyl-
investigated in detail.
CMC ) of pure dodecyltrimethylammonium bromide (C12TAB)
is CMC = 14 mM and in the presence of SC6HM is shifted to
CMC = 0.2 mM. The magnitude of the shift seems to be rather
insensitive to the calixarene concentration (at least from 0.1 to
mM). Similarly, below the charge neutralization point, the
The critical micelle concentration
4
(
0
valerate (NPV) stock solutions were prepared in acetonitrile, due
to their instability in water. The final acetonitrile concentration in
the reaction medium was 1% (v/v).
Kinetic runs were initiated by injecting 30 mL of MBSC or
NPV stock solution into a 1 cm path length cuvette containing
all other compounds dissolved in 2970 mL of water. Reaction
kinetics were recorded by measuring the absorbance changes at
0
1
5
aggregation number (N) also seems to be fairly independent of
the SC6HM and C TAB concentrations (N E 20–25). For a constant
12
concentration of SC6HM and above the charge neutralization point,
N increases linearly with the concentration of C12TAB. Despite this
increase, it was also observed that only a small fraction of the added
surfactant is absorbed by the micelles and the main fraction
remains free in the bulk solution. As a consequence, when the
concentration of a free surfactant reaches a value equal to the
295 nm in the case of MBSC or at 400 nm for the basic
hydrolysis of NPV in a Cary 50 UV-Vis spectrophotometer
equipped with a cell holder thermostated at (25.0 ꢁ 0.1) 1C.
ꢀ
4
The MBSC and NPV concentrations were always 1.0 ꢂ 10
M
ꢀ
5
and 2.6 ꢂ 10 M, respectively. The absorbance–time data of all
kinetic experiments were fitted by first-order integrated equations,
and the values of the pseudo-first-order rate constants (kobs) were
reproducible to within 3%.
CMC0 a second aggregation process is observed (CMC ). In
2
previous work, we were able to estimate the concentration of
free C TAB and the micellized C TAB : SC6HM ratio above the
1
2
12
3
D
Electrophoretic analysis was carried out on an Agilent CE
CMC
experiments. The results illustrate that the concentration of
free C12TAB is approximately constant and equal to the CMC
for total concentrations of surfactants slightly below the charge
neutralization point. This means that above the CMC , all
1
from self-diffusion coefficients obtained by DOSY NMR
1
2
capillary electrophoresis system, with on-column diode-array
detection at 25 ꢁ 0.5 1C and electropherograms were monitored
at 200 nm for SC6HM and 235 nm for thiourea using data
treatment software (HP Chemstation). Fused-silica capillaries
1
1
(Microtube, Araraquara, Brazil) with a total length of 48.5 cm,
surfactant molecules added to the solution are transformed into
micelles, as a consequence of the highly cooperative micellization
process. Above the charge neutralization point the concentration
of free C12TAB increases linearly with the total surfactant
concentration until a second aggregation process is observed
and a new plateau is reached. This behavior suggests that the
aggregation mechanism switches from cooperative to non-
cooperative around the charge neutralization point, switching
again to cooperative above the second aggregation process. The
dependence of the micellized C TAB : SC6HM ratio on the total
effective lengths of 40.0 cm and 50 mm i.d. were used in all
experiments. The capillaries were conditioned by flushing with
1
M NaOH (5 min), deionized water (5 min) and electrolyte
solution (10 min). Between experiments, the capillary was
reconditioned by flushing 2 min with the background electro-
lyte (BGE) containing 5 mM sodium tetraborate (TBS), pH = 9.2.
In order to evaluate the interaction of C TAB with SC6HM, the
BGE containing 5 mM TBS was enriched with C12TAB in
concentrations ranging from 0.04 mM to 50 mM. Samples
1
2
1
2
ꢀ
1
containing SC6HM 0.5 mM and 140 mg L of thiourea (fixed;
electroosmotic flow marker) were diluted with BGE with different
contents of C12TAB. The conditions of pressure, analysis voltage
and time of injection for each sample were setted in accordance
with the concentration of C TAB. We used: i) +50 mBar, +25 kV,
surfactant concentration also provides important information
for the complete understanding of the system. Within the non-
cooperative range there is an increase in this parameter from
4
to 8 suggesting a transition from negatively charged to positively
12
charged micelles (SC6HM is a hexanion). Above the second
aggregation process the micellized C12TAB : SC6HM ratio shows
a significant increase due to the dilution of the SC6HM-rich
5
s for samples with [C TAB] between 0.04 and 0.12 mM;
12
ii) ꢀ50 mBar, +25 kV, 5 s for analysis of the SC6HM peak; iii)
ꢀ50 mBar, ꢀ25 kV, 5 s for EOF peak and [C12TAB] between 0.16
and 2 mM; iv) ꢀ50 mBar, +25 kV, 5 s for samples with [C12TAB]
between 2.2 and 50 mM. Electroosmotic mobility was calculated
from the migration time of thiourea (neutral marker). Electro-
pherograms were plotted on the effective mobility scale using
the following equation:
ꢀ
micelles within the new aggregates. While it is not clear if the Br
anions participate in the stabilization of the cationic SC6HM-rich
micelles it is obvious that they must be present in the aggregates
formed above the second aggregation process.
With the objective of gaining further insights into this intriguing
and complex behavior we decided to investigate this system using
the hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC) and
the basic hydrolysis of p-nitrophenylvalerate (NPV) as kinetic probes,
together with capillary electrophoresis experiments. Despite being
far less popular than fluorescent or colorimetric probes, reactivity
ꢀ
ꢁ
Leff
1
1
mep ¼ E
ꢀ
(1)
tcal
teof
probes provide a powerful methodology to investigate the where mep is the effective mobility, Leff is the total capillary
aggregation, counterion binding and interfacial properties of length, E the applied electric field, tcal is the apparent detection
2
7–31
self-organized systems based on amphiphilic compounds.
time, and teof is the detection time of the neutral marker. The
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effective mobility was calculated according to procedures
described previously.
As can be seen in Fig. 1A, when the surfactant concentrations
are in the range of 0.04 mM to 0.12 mM, the calixarene peak
3
2
(labeled as 2) shows up after the EOF marker (peak 1). The
results are consistent with expectations, since the CE analysis
was performed in the counter electroosmotic flow mode, and
the results indicate that the SC6HM : C12TAB aggregate is
Results and discussion
Electrophoresis
The quantification of the concentration of free C12TAB and of negatively charged. Subsequently, the concentration of C12TAB
the micellized C12TAB : SC6HM ratio from self-diffusion coefficients increases (values in the range of 0.16–2.0 mM C12TAB, Fig. 1B
required approximations regarding the diffusion coefficients of the and C), showing that additional surfactant monomers are
12
aggregates. In order to confirm the previous analysis, the charged incorporated into the SC6HM : C12TAB aggregate and, as a
nature of the aggregates was examined using capillary electro- consequence, there is a considerable decrease in electrophoretic
3
2
phoresis (CE). CE separations are based on measurements of mobility, which reaches null electrophoretic mobility values
the difference in mobility of charged compounds (or aggregates) with 3 mM of C12TAB (Fig. 1D). In experiments with surfactant
as a function of applied voltage. Thus, the interaction between concentration in the range of 3 to 18 mM, when the surfactant is
SC6HM and C12TAB was examined by CE, using the conditions in large excess in relation to the calixarene, the aggregate
described in the experimental section and the experimental formed between the surfactant and calixarene is positively
results are shown in Fig. 1A to E.
charged. Finally, when [C12TAB] 4 CMC, micelles of C12TAB
Fig. 1 (A) Electropherogram profile of concentration of C12TAB varying from 0.04 mM to 0.12 mM (+50 mBar, +25 kV, 5 s); (B) 0.16 mM to 2.0 mM
(
ꢀ50 mBar, +25 kV, 5 s to SC6HM and ꢀ50 mBar, ꢀ25 kV, 5 s to EOF); (C) 2.2–2.8 mM (ꢀ50 mBar, +25 kV, 5 s); (D) EOF + SC6HM (neutral) at B3 mM
ꢀ50 mBar, +25 kV, 5 s); (E) electropherogram 4 3.4 mM to 50 mM of C12TAB (ꢀ50 mBar, +25 kV, 5 s). 1-EOF; 2-SC6HM.
(
2
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a
Table 1 Zeta potential values for each different aggregate formed
Point
CMC
Charge of aggregate Concentration (mM)
m
z (mV)
2
Cationic micelle
Neutral point Neutral point
18.0
3.0
50.40
0
CMC
1
Anionic micelle
0.2
ꢀ47.15
a
[
SC6HM] = 0.5 mM, T = 25 1C.
(
TBS = 0.005 M), it is possible to estimate a value of k = 2.3 ꢂ
8
ꢀ1
1
0 m , and accordingly, under our experimental conditions
of C12TAB micelles is 0.46, which allowed us to calculate the
value of Henry’s function f(kR ) = 0.69, following Ohshima’s
kR
m
m
34
approximation.
Fig. 2 Dependence of the electrophoretic mobility data on the concen-
tration of C12TAB in the presence of 0.5 mM of SC6HM.
Using the value of Henry’s function it was possible to
estimate the zeta potentials of the aggregates, and as shown in
Table 1, the zeta potential (z
a value which is consistent with those reported in the literature for
12TAB micelles. The mobility of SC6HM anionic calixarene, for
, is constant and probably reflects the mobility of
m 2
, in mV) above CMC is 50.4 millivolts,
are formed and the SC6HM molecules will be distributed
between the micelles, in a typical cationic aggregate (Fig. 1E).
The change in electrophoretic mobility as a function of surfactant
concentration can be observed in Fig. 2.
C
[C
12TAB] r CMC
1
the individual SC6HM molecules or of small aggregates. A crucial
The analysis of the mobility of the calixarene as a function of
point is the behavior at CMC (B0.2 mM), which corresponds to the
1
the concentration of C TAB shown in Fig. 2 allows observing
several critical points. The first one is around 0.2 mM, and
indicates the onset of the incorporation of surfactant molecules
1
2
formation of anionic aggregates of the calixarenes supramolecularly
complexed with C12TAB and we could calculate a zeta potential of
ꢀ
47.2 millivolts, using a micellar radius of 20 Å and the same
approximations described above to calculate Henry’s function.
Between CMC and CMC the change in Zeta potential follows
1
into the calixarenes and is denominated as (CMC ), where the
anionic species is predominant. After this point the electro-
phoretic mobility increases, crossing the null electrophoretic
mobility value around 3 mM, which corresponds to a 6 : 1 ratio
of C12TAB molecules for each SC6HM, in good agreement with
previous conclusions based on NMR data. Upon increasing the
concentration of the surfactant above 3.0 mM, the SC6HM :
1
2
linearly the changes in mobility, and it is interesting to observe
that the charge neutrality of the aggregate is attained when
[C TAB] = 3.0 mM, which corresponds exactly to a ratio of
12
([surfactant]/[calixarene]) = 6, which is the theoretically expected
neutralization point for an optimum effect of the complexation.
Above a ratio of 6 surfactant molecules for each calixarene, the
aggregate becomes increasingly positively charged, finally reaching
zeta potential values which are closely similar to that of a pure
C
12TAB aggregates become positively charged. These values are
consistent with those reported in a previous study performed
1
2
using conductivity analysis. The third inflexion point, at 18
mM, indicates CMC of the free surfactant in good agreement
with that previously obtained by conductivity. Overall, the results
35
2
C12TAB cationic micelle.
12
described here provide direct evidence for the transformation of Hydrolysis of MBSC
anionic into cationic micelles in this system.
The changes in electrophoretic mobility, shown in Fig. 2,
can be conveniently used to estimate zeta potentials (z ) by
means of eqn (2), which relates the charge of the micelle to its
mobility:
Another important feature to be investigated in the present
system is the capacity of these supramolecular micelles to
solubilize guest molecules or ions and test their potential to
be used as nanoreactors. Using simple reactions as kinetic
probes will allow these parameters to be investigated as well
as the micellar polarity and the effect of the micellar charge on
the compartmentalization of the reactants.
m
mZ
zm
¼
(2)
e0ef ðkRmÞ
where Z is the viscosity of the medium, f (kR
The rate of the solvolysis of MBSC is highly sensitive to the
m
) corresponds to polarity of the (micro)environment in which the probe is
Henry’s function, k is the Debye–H u¨ ckel shielding parameter located. Therefore this reaction has been recurrently used as
ꢀ1
30,36
(m
), R
m
is the radius of C12TAB micelle, and e
o
and e correspond a probe to investigate micellar and host–guest systems.
In
to the vacuum permittivity and the relative permittivity of the general, the reaction rate decreases as the probe is transferred
33,34
solvent.
from bulk water to more apolar medium such as the micellar
Initially, the zeta potential of the C TAB micelle was calculated interior/interface or the cavity of a macrocyclic container. The
12
above the CMC of the free surfactant, where the behavior should results depicted in Fig. 3-left suggest that MBSC does not
2
be closely related to a solution of C TAB without additives. In interact with SC6HM in this concentration range, or that the
12
order to proceed with the calculation of zeta potential, we adopted observed rate constant for the solvolysis does not vary significantly
35
m
the reported radius value (R ) of the C12TAB micelle of 20 Å.
from the bulk to the SC6HM pseudo-cavity: or both. SC6HM is
Considering the ionic strength of the supporting electrolyte highly flexible and is believed to exist in solution as a mixture of
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Fig. 3 (left) Influence of the SC6HM on the observed rate constant (kobs) for the hydrolysis of MBSC. (right) Variation of kobs with [C12TABr] in the absence
open squares) and in the presence of 5 mM of SC6HM (filled circles). All kinetic experiments were conducted at 25 1C in water with 1% acetonitrile (v/v).
(
several interconverting conformers, thus lacking a defined structure/ At this point it must be stressed that the approximation used to
37,38
cavity.
n
As a result of this poorly defined geometry, SC6HM obtain [D ] does not apply for low concentrations of SC6HM
usually displays weaker binding abilities comparatively to more because after neutralization of the calixarene charge, the
pre organized derivatives. The instability of MSBC in aqueous concentration of the free surfactant increases significantly
1
2
solution precludes the utilization of more conventional techniques before a second aggregation process takes place. Nevertheless,
to confirm or rule out the complexation of MSBC with SC6HM. for concentrations of SC6HM with a charge neutralization point
Fig. 3-right shows the changes observed for kobs as a function well above the CMC0 of the pure surfactant (14 mM), this
1
3
of the C TABr concentration in the absence and presence of 5 mM behavior is not observed, or insignificant. In the present work
12
of SC6HM. The plot can be divided into two regions: a first one that the concentration of SC6HM (5 mM) corresponds to a charge
corresponds to low surfactant concentration where the rate of the neutralization point of 30 mM, well above the CMC
hydrolysis of MBSC is essentially constant and a second one above a As can be observed in Fig. 3-right, the kobs data are satis-
certain critical concentration, that is identified by the point at which factorily fitted by eqn (3). Fitting was achieved with k
0
.
w
= (6.3 ꢁ
ꢀ
3
ꢀ1
ꢀ4 ꢀ1
m
S
k
obs starts decreasing. This point is identified as the onset of micelle 0.1) ꢂ 10
s
, k
m
= (1.6 ꢁ 0.4) ꢂ 10
s
and K
= (2.7 ꢁ 0.2) ꢂ
2
ꢀ1
ꢀ3 ꢀ1
formation (the critical micelle concentration, CMC) and the decrease 10 M in the absence of SC6HM and k = (6.1 ꢁ 0.1) ꢂ 10
s ,
w
ꢀ4
ꢀ1
m
2
ꢀ1
observed for kobs is attributed to the incorporation of MBSC within km = (1.7 ꢁ 0.2) ꢂ 10
the micellar aggregates, a lower polarity microenvironment that presence of 5 mM of SC6HM. The results obtained in the
inhibits the hydrolysis of the probe. The CMC and CMC values absence of SC6HM are in good agreement with those reported
obtained from the kinetic data, 13 mM and 0.25 mM in the absence in the literature. When both sets of results are compared it can
and presence of 5 mM of SC6HM, respectively, are in good be seen that k has the same value in the absence and in the
presence of calixarene. However it was shown previously that, in
s
and KS = (4.3 ꢁ 0.1) ꢂ 10 M in the
0
1
3
0
m
12,13
agreement with those previously obtained by other techniques.
The kinetic data can be quantitatively analyzed through the the case of MBSC, this parameter does not indicate a significant
application of the pseudophase formalism. This simple theory dependence on the hydrophobic character or the interfacial
3
0,40
considers that molecules and ions present in solutions are charge and polarity of the micellar aggregates.
On the other
m
distributed between two well-differentiated environments: bulk hand, the KS value shows a significant increase in the presence
water and a micellar pseudophase. For reactions taking place of SC6HM suggesting that the mixed micelles are more hydro-
on the sub-second and slower timescales the partition of the phobic than pure aggregates, with the absolute value being close
m
S
2
ꢀ1 30
reactants between the bulk and the micellar pseudophase can be to that observed for C16TAC (K
= [4.7 ꢁ 0.3] ꢂ 10 M ).
considered as a fast equilibrium so that the observed rate constant
is given by the molar fraction weighted average of the rate
constants corresponding to the reaction taking place in the two
environments. As shown elsewhere, kobs is given by eqn (3):
Basic hydrolysis of NPV
Because the hydrolysis of MBSC does not provide significant
information on the interfacial properties of the micellar aggregates,
another probe reaction was used to investigate it. Bimolecular
reactions of hydrophobic organic compounds and ionic reactants
are very convenient probes to gain insights into the ionic
39
m
kw þ kmK ½Dnꢃ
S
kobs ¼
(3)
m
S
1
þ K ½Dnꢃ
2
7
w
Where k is the rate constant for the hydrolysis reaction taking composition of micellar interfaces. The basic hydrolysis of
place in bulk water, km is the rate constant for the reaction taking nitrophenyl esters has become an iconic reaction in studies
m
place in the micellar pseudophase, KS is the binding constant concerning self-assembled amphiphilic systems based on cationic
m
of MBSC between the bulk and the micelles (defined as KS
=
surfactants. In this work the basic hydrolysis of p-nitrophenylvalerate
[
MBSC]
surfactant that can be approximated as [D
where [surfactant] is the total concentration of the surfactant. supra-amphiphile.
m
/[MBSC]
w
[D
n
]) and [D
n
] is the concentration of micellized was selected as a probe reaction (Scheme 1) to investigate the ionic
n
] = [surfactant]
0
ꢀ CMC; composition of micellar aggregates formed from SC6HM : C12TAB
0
2
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Scheme 1 Basic hydrolysis of p-nitrophenylvalerate.
As a basis for the study of the basic hydrolysis of p-nitro- respectively. By computing the values of mOH from eqn (5),
Br
phenylvalerate (NPV) in the complex system formed by SC6HM using previously determined values for b = 0.8 and K OH = 17,
and C12TAB, the influence of each component was investigated eqn (4) can be used to fit the kobs data represented in Fig. 4-right.
separately. Fig. 4-left shows the changes observed on the rate of As can be observed the theoretical curve fits reasonably well the
the reaction in the presence of increasing concentration of experimental data for concentrations of the surfactant below
SC6HM. The results suggest that NPV forms a complex with ca. 200 mM but for a higher concentration it starts to deviate from
ꢀ1
SC6HM (with K = 13 ꢁ 6 M obtained from the non-linear data the data points. This behavior is well documented and is related to
fit assuming a 1 : 1 complex) which stabilizes the guest under the known limitations of the pseudophase ion exchange model
2
7,39
basic conditions (kobs decreases) probably due to the repulsion for high concentrations of the surfactant.
From the fitting
= 6.5 ꢁ
ꢀ
of OH ions by the sulfonate groups of the host. Nevertheless, procedure the following values were obtained: k
w
ꢀ1
ꢀ1
m
S
2
ꢀ1
the association constant is very low, probably due to the lack of 0.1 M
s
, K
= (1.4 ꢁ 0.2) ꢂ 10 M and k
m
= 8.5 ꢁ 0.9
ꢀ
1
preorganization of the receptor, and for 5 mM of SC6HM the
s
. km is an apparent rate constant that is related to the
0
fraction of complexed NPV is very low (ca. 6%).
bimolecular rate constant (km ) for basic hydrolysis of the NPV
Fig. 4-right shows the dependence of kobs on the concentration located within the micellar aggregates through the relation
0
ꢀ
of C12TABr in the absence of SC6HM (open squares). The data
k
m
= k
m
/V
m
41–43
, where V
m
is the molar volume of the Stern layer
1
0
ꢀ1 ꢀ1
follow the expected trend for bimolecular reactions of organic (0.14 M ).
From this relation, k
m
= 1.2 M
s
can be
compounds with anionic reactants in the presence of cationic obtained. When compared with k
w
it is clear that the observed
obs is the constant for concentrations of the surfactant rate enhancement is due to an increase in the local concentra-
below the CMC (12 mM). It is worth noting that this value is tions of the reactants rather than to a true catalytic effect.
27
micelles.
k
0
!
slightly small than that determined above due to the higher salt
concentration (NaOH). Above this point the apparent reaction rate
increases due to the incorporation of NPV and OH in the micellar
ꢂ
ꢃ
ꢀ
m
S
kw½HO ꢃ þ kmK ꢀ kw mOH½Dnꢃ
0
kobs ¼
ꢂ
ꢃ
(4)
m
S
ꢀ
1 þ K ½D ꢃ
n
surface, increasing the local concentrations of both reactants. The
reaction rate reaches a maximum value and then decreases at a
higher concentration of the surfactant. This behavior is attributed
(
ꢂ
ꢃ
)
Br
OH
ꢀ
ꢀ
ꢀ
K
½Br ꢃ þ½HO ꢃ
b½HO ꢃ
OH
2
0
0
0
mOH þ
ꢂ
ꢃ
ꢀ b mOH ꢀ ꢂ
ꢃ
¼ 0
Br
OH
Br
K
ꢀ 1 ½Dnꢃ
K
ꢀ 1 ½Dnꢃ
ꢀ
to the competitive binding of inert Br anions that displace
(5)
ꢀ
reactive OH anions to the bulk aqueous solution.
The results depicted in Fig. 4-right can be quantitatively In the presence of 5 mM of SC6HM (Fig. 4-right, closed circles),
analyzed through the application of the pseudophase ion the kobs values follow a different trend from that observed in the
2
7
m
exchange model (eqn (4) and (5)). In eqn (4) and (5) k , K
absence of calixarene. At 0.2 mM (CMC ) the apparent rate
1
m
S
Br
and [D
n
] are the same as in eqn (3) while mOH, b and KOH are the constant starts dropping due to the incorporation of NPV into
concentration of OH ions in the micellar pseudophase the mixed SC6HM : C TAB micelles that form above this
ꢀ
1
2
ꢀ
(
n
defined as the mole ratio [ OH]/D ), the degree of counterion concentration. Drawing a parallel with the behavior observed
binding (which is assumed to be constant) and the ionic in the absence of SC6HM, the observed decrease in kobs suggests
Br
w M M w
exchange equilibrium constant (KOH = [OH] [Br] /[OH] [Br] ), that the micellar aggregates formed in the concentration range
Fig. 4 (left) Influence of the SC6HM on the observed rate constant (kobs) for the basic hydrolysis of NPV. (right) Variation of kobs with [C12TABr] in the
absence (open squares) and in the presence of 5 mM of SC6HM (filled circles). All kinetic experiments were conducted at 25 1C in water with [NaOH] = 5 mM
and 1% acetonitrile (v/v).
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ꢀ
ꢀ
between 0.2 and 30 mM do not contain exchangeable Br / OH
counterions because in this case a change in kobs should be
observed. This means that within this concentration range
the micelles are exclusively formed by SC6HM and C TAB. Above
3 F. M. Menger, L. Shi and S. A. A. Rizvi, J. Am. Chem. Soc.,
2009, 131, 10380–10381.
4 F. M. Menger and S. A. A. Rizvi, Langmuir, 2011, 27, 13975–13977.
5 F. A. Garc ´ı a Daza and A. D. Mackie, J. Phys. Chem. Lett., 2014,
5, 2027–2032.
12
3
0 mM the kobs values display a sharp increase, reach a maximum
value and drop again. Further, for high concentrations of C12TAB the
kinetic data attain the values obtained in the absence of SC6HM.
This behavior is typical of cationic micelles with exchangeable
counterions and indicates that the mixed micelles are gradually
transformed into ‘‘quasi like’’ micelles as the concentration of
SC6HM becomes more diluted within the aggregates.
6 Y. Kang, K. Liu and X. Zhang, Langmuir, 2014, 30, 5989–6001.
7 X. Zhang and C. Wang, Chem. Soc. Rev., 2011, 40, 94–101.
8 C. Wang, Z. Wang and X. Zhang, Acc. Chem. Res., 2012, 45,
608–618.
9 N. Basilio, M. Mart ´ı n-Pastor and L. Garc ´ı a-R ´ı o, Langmuir,
2012, 28, 6561–6568.
1
1
1
0 N. Basilio, V. Francisco and L. Garcia-Rio, Int. J. Mol. Sci.,
2013, 14, 3140–3157.
Conclusions
1 V. Francisco, N. Bas ´ı lio, L. Garcia-Rio, J. R. Leis, E. F. Marques
and C. V ´a zquez-V ´a zquez, Chem. Commun., 2010, 46, 6551–6553.
2 N. Basilio, B. G o´ mez, L. Garcia-Rio and V. Francisco, Chem.
Eur. J., 2013, 19, 4570–4576.
This work demonstrates that supra-amphiphiles formed from
SC6HM and C12TAB aggregate into micellar aggregates with
tunable degrees of ionization. For low C12TAB : SC6HM molar
ratios the micelles are negatively charged. As the concentration
of C12TAB increases the negative micellar charge decreases and
reaches neutrality at C12TAB : SC6HM E 6. Above this point the
micelles become positively charged and their ability to solubilize
more C TAB decreases substantially. As a result the fraction of
1
3 N. Basilio and L. Garc ´ı a-R ´ı o, Chem. – Eur. J., 2009, 15, 9315–9319.
14 D.-S. Guo and Y. Liu, Acc. Chem. Res., 2014, 47, 1925–1934.
15 B.-P. Jiang, D.-S. Guo, Y.-C. Liu, K.-P. Wang and Y. Liu, ACS
Nano, 2014, 8, 1609–1618.
1
1
1
1
2
2
6 Z. Qin, D.-S. Guo, X.-N. Gao and Y. Liu, Soft Matter, 2014, 10,
1
2
2
253–2263.
7 K. Wang, D.-S. Guo, M.-Y. Zhao and Y. Liu, Chem. – Eur. J.,
014, DOI: 10.1002/chem.201303963.
8 Y. Cao, Y. Wang, D. Guo and Y. Liu, Sci. China Chem., 2013,
7, 371–378.
9 D.-S. Guo, B.-P. Jiang, X. Wang and Y. Liu, Org. Biomol.
Chem., 2012, 10, 720–723.
0 D.-S. Guo, K. Wang, Y.-X. Wang and Y. Liu, J. Am. Chem. Soc.,
free C TAB also increases until it reaches a new critical
1
2
concentration leading to a second aggregation process.
2
The kinetics of the MBSC hydrolysis and NPV basic hydrolysis
show that C12TAB : SC6HM micelles have the ability to solubilize
hydrophobic compounds within their structure. This result together
with their reduced CMC demonstrate the higher potential of
C TAB : SC6HM based supra-amphiphiles compared with conven-
5
12
tional surfactants. The basic hydrolysis of NPV was shown to be a
very effective probe to investigate the ionic composition of the
micellar interface in this complex system. The results suggest that
2012, 134, 10244–10250.
1 Z. Li, C. Hu, Y. Cheng, H. Xu, X. Cao, X. Song, H. Zhang and
Y. Liu, Sci. China Chem., 2012, 55, 2063–2068.
ꢀ
Br anions do not participate in the micelles formed between the
2
2
2 K. Wang, D. Guo and Y. Liu, Chem. – Eur. J., 2012, 18, 8758–8764.
3 K. Wang, D.-S. Guo, X. Wang and Y. Liu, ACS Nano, 2011, 5,
first and second CMCs. Above this last point, the micellized
C12TAB : SC6HM ratio increases significantly and the counteranions
2
880–2894.
4 K. Wang, D.-S. Guo and Y. Liu, Chem. – Eur. J., 2010, 16,
006–8011.
are transferred into the micellar interface screening the electrostatic
repulsion between positively charged C TAB molecules.
2
2
12
8
5 V. Wintgens, C. Le Coeur, C. Amiel, J.-M. Guigner, J. G.
Acknowledgements
Harangoz o´ , Z. Miskolczy and L. Bicz ´o k, Langmuir, 2013, 29,
7682–7688.
N.B. acknowledges the Fundaç ˜a opara a Ci ˆe ncia e Tecnologia
FCT, Portugal) for a postdoctoral grant (SFRH/BPD/84805/
012). We are grateful to INCT-Cat ´a lise/ CNPq, PRONEX,
2
2
2
6 G. Gattuso, A. Notti, A. Pappalardo, S. Pappalardo, M. F. Parisi
and F. Puntoriero, Tetrahedron Lett., 2013, 54, 188–191.
7 C. A. Bunton, F. Nome, F. H. Quina and L. S. Romsted, Acc.
Chem. Res., 1991, 24, 357–364.
(
2
FAPESC, CSF/CNPq and CAPES for their support of this work.
We acknowledge the financial support from the Ministerio de
Economia y Competitividad of Spain (project CTQ2014-55208-P)
and Xunta de Galicia (2007/085).
8 Y. Geng, L. S. Romsted and F. Menger, J. Am. Chem. Soc.,
2006, 128, 492–501.
2
3
9 L. S. Romsted, Langmuir, 2007, 23, 414–424.
0 M. Cepeda, R. Davi n˜ a, L. Garc ´ı a-R ´ı o, M. Paraj o´ , P. Rodr ´ı guez-
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H. D. Fiedler and F. Nome, Langmuir, 2012, 28, 1758–1764.
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