Cationic mixed micelles as reaction medium for hydrolysis reactions

Article abstract of DOI:10.1007/s10953-015-0383-4

The influence of cationic mixed micelles composed of quartenary ammonium surfactants on hydrolysis reactions has been studied in detail. The basic hydrolysis of N-methyl-N-nitroso-p-toluene sulphonamide has been chosen as the reaction probe, while mixed micelles composed of lauryl trimethyl ammonium chloride and octadecyl trimethyl ammonium chloride with different molar ratios were studied as the reaction medium. The ion-exchange pseudophase model was used to fit the experimental results to obtain the kinetic and thermodynamic parameters of the reaction. The result show that the hydrophobic character of the mixed micelles drives the association of the substrate to them, leading to a local increase of reactant concentrations at the micellar interface and, therefore, to a catalytic effect. By tuning the molar ratio of the mixed micelles it is possible to control substrate binding affinity and thus the catalytic efficiency of the reaction medium.

Full text of DOI:10.1007/s10953-015-0383-4

J Solution Chem (2015) 44:1866–1874
DOI 10.1007/s10953-015-0383-4
Cationic Mixed Micelles as Reaction Medium
for Hydrolysis Reactions
1
Isabel Fernandez Jorge Perez-Juste Pablo Herves
1
1
•
•
´
´
´
Received: 3 February 2015 / Accepted: 25 April 2015 / Published online: 19 September 2015
Ó Springer Science+Business Media New York 2015
Abstract The influence of cationic mixed micelles composed of quartenary ammonium
surfactants on hydrolysis reactions has been studied in detail. The basic hydrolysis of N-
methyl-N-nitroso-p-toluene sulphonamide has been chosen as the reaction probe, while
mixed micelles composed of lauryl trimethyl ammonium chloride and octadecyl trimethyl
ammonium chloride with different molar ratios were studied as the reaction medium. The
ion-exchange pseudophase model was used to fit the experimental results to obtain the
kinetic and thermodynamic parameters of the reaction. The result show that the
hydrophobic character of the mixed micelles drives the association of the substrate to them,
leading to a local increase of reactant concentrations at the micellar interface and, there-
fore, to a catalytic effect. By tuning the molar ratio of the mixed micelles it is possible to
control substrate binding affinity and thus the catalytic efficiency of the reaction medium.
Keywords Kinetics Á Mixed micelles Á Catalysis Á Microheterogeneous media
1 Introduction
Micellar surfactants are amphiphilic materials which contain both apolar, hydrophobic and
polar, hydrophilic groups. Micelles are highly dynamic, often polydisperse aggregates
formed from surfactants above the critical micelle concentration, cmc [1]. Micellization is
primarily driven by bulk hydrophobic interactions between the alkyl chains of the sur-
factant monomers and usually results from a favorable entropy change [2]. Micellar
Electronic supplementary material The online version of this article (doi:10.1007/s10953-015-0383-4)
contains supplementary material, which is available to authorized users.
´
Pablo Herves
jherves@uvigo.es
&
1
Department of Physical Chemistry, Faculty of Chemistry, University of Vigo, 36310 Vigo, Spain
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systems have the ability to alter chemical reactivity. Reaction rates and equilibrium con-
stants in micellar media can differ from the values in bulk water. Most hydrolysis reactions
provide an excellent opportunity for comparisons of micellar medium effects. Based on the
mechanisms of these hydrolytic processes, one expects that the properties of the micelles,
including the lower local water concentration in the micellar Stern region, the polarity of
the micelles and the local charge in the micellar Stern region resulting from incomplete
counterion binding, play an important role in the catalysis or inhibition of these processes
[3, 4]. Kinetic studies in these systems may be interpreted on the basis of the pseudophase
model [5, 6], which allowed us to explain a large number of kinetic results [7–9], with the
simple assumption of reaction distribution between the two pseudophases.
The N-nitroso compounds are of great interest from a biological point of view, because
a wide variety of structurally related compounds possessing the N-nitroso-N-alkyl func-
tionality have demonstrated cancer chemotherapeutic potential [10, 11]. Besides, the dis-
covery of the bioregulatory roles of nitric oxide [12–16], and its anti-carcinogen properties
[17, 18], has greatly increased the interest in nitrosation reactions because nitroso com-
pounds are able to delivery nitric oxide in a controlled manner [19]. Due to this widespread
significance of nitroso compounds, knowledge of their reactions, mechanisms and kinetics
is of much importance [20].
In this work, we present a study on the influence of cationic mixed micelles, composed
of lauryl trimethyl ammonium chloride (LTACl) and octadecyl trimethyl ammonium
chloride (OTACl) with different molar ratios, on the basic hydrolysis of N-methyl-N-
nitroso-p-toluenesulfonamide (MNTS). We choose these surfactants because they are
similar, being the only difference the length of their hydrophobic chains. The mechanism
for the basic hydrolysis of MNTS in water is well known [21], the nucleophilic attack of
OH- on the sulfur atom being the rate determining step (see Scheme 1). The purpose of the
present work was to extend our studies on hydrolysis reactions to different microhetero-
geneous media, viz., mixed micelles of cationic surfactants.
2 Experimental
The micellar surfactants OTACl and LTACl were obtained from Sigma (purities C95.0
and 98.0 % respectively) and used without further purification. NaOH (Sigma, 98 %)
solutions were titrated against potassium hydrogen phthalate and were prepared weekly.
All aqueous solutions were prepared in doubly distilled water. The low solubility of MNTS
(Sigma, 99 %) in water made it necessary to prepare its solutions in acetonitrile (from
Merck, ACS grade). The percentage of acetonitrile in the reaction mixture was always 1 %
by volume. The reactions were followed by recording the decrease in absorbance at
250 nm due to the disappearance of MNTS using a Hewlett Packard 8453 spectropho-
tometer with a cell holder thermostated at (25 0.1) °C. All kinetic experiments were
performed with the MNTS concentration (approximately 6 9 10-5 molÁL-1) much smaller
-OH
O
O
S N
O
CH3
NO
-
+
S O-
O
+ CH2N2
H3C
H3C
Scheme 1 Mechanism for the basic hydrolysis of MNTS
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J Solution Chem (2015) 44:1866–1874
than those of the other reagents used. The absorbance–time data of all kinetic experiments
were fitted by the first-order integrated equation, and the values of the pseudo-first order
rate constants, kobs, were reproducible to within 3 %.
3 Results and Discussion
The influence of cationic mixed micelles on the basic hydrolysis of MNTS was studied at
seven different LTACl:OTACl molar ratios (1:0, 1:0.25, 1:0.5, 1:1, 0.5:1, 0.25:1 and 0:1).
Although the hydrolysis of MNTS in the presence of pure cationic micelles has been
previously studied by our group [22], here we examined the influence of LTACl and
OTACl concentration on the hydrolysis reaction to ensure good consistency in the eval-
uations of the experimental results. All the experiments were carried out at
[NaOH] = 0.085 molÁL-1 and the surfactant concentrations were varied between 0 and
0.18 molÁL-1. Figure 1 shows the results for three molar ratios (1:0.25, 1:1 and 0.25:1;
Fig. S1 in the Supporting Information shows the other molar ratios and the pure surfactants
for comparison). It can be observed that initially, at surfactant concentrations below the
cmc, the ratio between the observed reaction rate and the rate in absence of micelles (kobs
/
kobs,w) remains unchanged. A further increase in the surfactant concentration above the cmc
lead to an increase in (kobs/kobs,w) up to a maximum, followed by a decrease (see Fig. 1).
The kinetic data were quantitatively explained by using the ion-exchange pseudophase
model, assuming that the cationic nature of the surfactants favored the presence of OH- at
the micellar pseudophase. The overall reaction rate will be, therefore, equal to the sum of
the rates at the micellar and aqueous pseudophases (see Scheme 2, where subscripts w and
m denote the aqueous and micellar pseudophases, and kw and km are the rate constants in
the respective pseudophases).
This model lead to the following equation for the observed rate constant:
Fig. 1 Influence of
LTACl:OTACl mixed surfactant
concentration on the basic
hydrolysis of MNTS.
[OH-] = 0.085 molÁL-1 at
different molar ratios (open
circle) 0.25:1 (filled circle) 1:1
and (open triangle) 1:0.25. The
lines represent the best fit of
Eq. 3 to the experimental data
(see text for details)
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1869
kw
OH-
+
Products
Products
MNTS
w
w
K MNTS
Water pseudophase
Micellar pseudophase
km
OH -
MNTS
+
m
m
Scheme 2 Ion-exchange pseudophase model
kw½OHÀŠwþkmKMNTS½OHÀŠ
1 þ KMNTS½DnŠ
m
kobs
¼
ð1Þ
where kw[OH-]w is the observed rate constant in pure water (kobs,w) and KMNTS is the
association constant or constant of substrate distribution between the two pseudophases
and Dn is the concentration of micellized surfactant ([surfactant]-cmc):
½MNTSŠ
m
KMNTS
¼
ð2Þ
½MNTSŠ½DnŠ
The cmc values were obtained from the kinetic data and calculated as the minimum
concentration of surfactant necessary to produce a change in the reaction rate. These
values, in the presence of OH- (see Table 1), are satisfactorily correlated with the
expected behavior of cmc values when the length of the surfactant chain increases, which is
a linear decrease of the logarithm of cmc with the number of carbons in the chain and then
with its hydrophobicity [23]. In our case, the cmcs increase with the percentage of the
surfactant with longer chain in the mixed micelles.
From Eq. 1 and taking into account that [OH-]m is the concentration of hydroxyl ions
on the micellar pseudophase we can derived the following equation:
kw½OHÀŠtotalþðkmKMNTS À kwÞmOH½DnŠ
kobs
¼
ð3Þ
1 þ KMNTS½DnŠ
where mOH, which denotes the [HO-]m/[Dn] ratio, satisfies the equation:
Table 1 Kinetic and thermodynamic parameters obtained for the basic hydrolysis of MNTS in the presence
of mixed micelles of LTACl:OTACl
km2 /LÁmol-1Ás-1
LTACl:OTACl
cmc/molÁL-1
KMNTS/molÁL-1
km/s-1
1:0
0.011 0.001
97 14
119 16
173 12
207 15
307 43
362 33
424 26
(6.3 0.3) 9 10-2
(5.3 0.3) 9 10-2
(5.2 0.2) 9 10-2
(6.0 0.2) 9 10-2
(5.2 0.3) 9 10-2
(5.7 0.3) 9 10-2
(7.0 0.2) 9 10-2
8.8 9 10-3
7.4 9 10-3
7.3 9 10-3
8.4 9 10-3
7.3 9 10-3
8.0 9 10-3
9.8 9 10-3
1:0.25
1:0.5
1:1
(7.5 0.3) 9 10-4
(4.0 0.3) 9 10-4
(2.5 0.4) 9 10-4
(2.0 0.5) 9 10-4
(1.5 0.4) 9 10-4
(6.5 1.0) 9 10-5
0.5:1
0.25:1
0:1
kw = 0.083 LÁmol-1Ás-1, K
= 10
OH
Cl
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"
#
"
#
½OHÀŠ þK ½X Š
b½OHÀŠ
Cl
À
OH
Cl
m2OH þ mOH
À b À
¼ 0
ð4Þ
total
OH
Cl
total
total
À
Á
À
Á
OH
K
À 1 ½DnŠ
K
À 1 ½DnŠ
where b is the fraction of charge neutralized by the counterion. This value has been set
constant an equal to 0.8, which is the value obtained by Chaimovich et al. [24], and it
OH
Cl
seems to be the most frequently used value. K
is the ion-exchange constant for the
equilibrium:
OHmÀ þ ClWÀ ꢀ OHÀW þ ClmÀ
ð5Þ
OH
Cl
The value of K can be obtained from the influence of salts on the observed rate constant
(vide infra).
As mentioned above, upon increasing the surfactant concentration the ratio kobs/kobs,w
reaches a maximum followed by a decrease (see Fig. 1). Such a decrease can be initially
ascribed to an increase in the concentration of the chloride ion, which competes with
hydroxyl ions for the micellar surface sites. To check this possibility, we have performed
series of kinetic runs with increasing amounts of NaCl at a fixed amount of surfactant
where the reaction rate reaches the maximum. Figure 2 shows the influence of sodium
chloride concentration on the observed rate constant for the basic hydrolysis of MNTS at a
constant concentration of OTACl (5.3 9 10-3 molÁL-1). It can be observed that as the
chloride concentration increases kobs decreases exponentially, confirming the assumption
of competition between hydroxyl and chloride ions at the micellar interface. This effect is
in quantitative agreement with the pseudophase ion exchange model (Eqs. 3, 4). The fitting
procedure was performed by means of a non-linear fitting program based on Marquardt’s
algorithm. The solid line in Fig. 2 represents the best fit of the experimental data to Eqs. 3
and 4. The fit was carried out using cmc values calculated from the kinetic data, and kw was
the experimental value in bulk water (0.083 LÁmol-1Ás-1) [25], to find the values of KMNTS
,
OH
Cl
k
m and KCOlH that best reproduce the kinetic data. A value of K = 10 was determined. It
should be mentioned that the pseudophase ion-exchange model predicts (Eqs. 4, 5 and
Fig. 2 Left influence of NaCl concentration upon the basic hydrolysis of MNTS, [NaOH] = 0.12 molÁL-1
and [OTACl] = 5.4 9 10-3 molÁL-1; right influence of the NaCl concentration on the chloride ion
Cl
OH
concentration in the micellar pseudophase according to Eq. 4 and assuming K = 10 (see text for details)
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J Solution Chem (2015) 44:1866–1874
OH
Cl
1871
assuming K = 10) an exponential increase of the chloride ion concentration in the
micellar pseudophase with the addition of extra unreactive chloride ions (see Fig. 2 right).
Now the biphasic pattern observed in Fig. 1 can be easily explained. On the one hand,
surfactant concentrations above the cmc lead to an increase in the relative concentrations of
MNTS and OH- in the Stern layer, which increase the observed rate constant, and the
ascending branch of the curve is observed. On the other hand, a further increase in the
surfactant concentration leads to an increased concentration of unreactive chloride ions and
a displacement of reactive ions (OH-) from the micellar surface, as predicted by Eq. 5.
Such competition leads to a decrease in the observed rate constant, as we have confirmed
by studying the effect of NaCl on the observed rate constant (see Fig. 2). The relative
contribution of both competing factors results in the experimental maximum observed.
Analogously, the solid lines in Fig. 1 represent the best fit of Eqs. 3 and 4 to the
OH
Cl
experimental data, but now using the value of K obtained from the influence of NaCl
OH
Cl
(K = 10). We can see in Figs. 1 and S1 that the model fits to the experimental data at
the seven molar ratios studied within the experimental error. This allows us to obtain again
values of KMNTS and km in good agreement with those found in the study of the influence of
NaCl. The obtained values for these constants for the different LTACl:OTACl mixed
systems are listed in Table 1.
Fig. 3 a Influence of the
percentage of OTACl in the
mixed micelles on the values of
KMNTS. b (open circle) Influence
of KMNTS on the catalytic
efficiency of the LTACl:OTACl
mixed micelles (filled circle).
Literature values for TTACl and
CTACl
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The km values obtained are almost constant (0.070–0.052 s-1) and without any ten-
dency within the different cationinc mixed micelles. These values cannot be directly
compared to kw (the bimolecular rate constant in water), because both constants are defined
in different terms (km in terms of mole per mole concentration in surfactant; kw in terms of
mole per volume concentration). However, assuming the molar volume of the Stern layer
of (ca 0.14 LÁmol-1) [5, 6], we have calculated the bimolecular reactivity constant in the
micellar interphase, km2 , to be (0.0098–0.0073 LÁmol-1Ás-1). It is about one-tenth that in
pure water (kw = 0.083 LÁmol-1Ás-1) [25]. This suggests that the hydrolysis rate in the
Stern layer is intrinsically one-tenth that in the aqueous pseudophase, which confirms that
the origin of the catalytic effect observed in the presence of micelles (Fig. 1) does not
correspond to an increase in reactivity but, rather, to a local increase in the reagent
concentration in the Stern layer. This intrinsically decreased reactivity may be attributed to
the polarity at the micellar interphase, which is lower than that in water. In fact, the Stern
layer has a dielectric constant of ca. 35 [26], markedly lower than that of water. The
influence of the lowering of the dielectric constant upon the reaction rate was investigated
by our group in dioxane–water [27] mixtures and acetonitrile–water mixture [28]. The
results obtained showed that, in fact, the effect of decreasing polarity of the solvent is a
decrease in the reaction rate.
We can see in Table 1 that the association constant of MNTS to the pure cationic
micelles is higher for OTACl (424 LÁmol-1) than for LTACl (97 LÁmol-1), while it
increases as the percentage of OTACl in the mixed micelles increases; in fact a sigmoidal
dependence is observed when we plot the values of KMNTS versus the percentage of
LTACl:OTACl (see Fig. 3a). This result indicates that the association constant of the
organic substrate to the micelles is driven mainly by hydrophobic forces.
The catalytic efficiency (viz., the ratio between the maximum observed rate constant at
a given OH- concentration and the rate constant in pure water, kobs,max/kobs,w), increases
with increasing the percentage of OTACl in the LTACl:OTACl mixed micelles. In pure
OTACl micelles it is more than two times higher than that for LTACl and the efficiency of
the mixed micelles LTACl:OTACl with a molar ratio of 0.25:1 is almost 1.7 times higher
than that with molar ratio 1:0.25. A linear relationship between the catalytic efficiency,
kobs,max/kobs,w, and the values of KMNTS is also observed (see Fig. 3b). The observed
tendency can be understood considering that OTACl micelles are more hydrophobic than
LTACl. The increase of the percentage of OTACl in the mixed micelles leads to more
hydrophobic micelles with higher values of the association constant (see Fig. 3a) with
higher capacity to dissolve organic substrates. Then, the increase of the values of KMNTS
due to the increase of the percentage of OTACl leads finally to a higher catalytic efficiency
of the mixed micelles. We have also included in Fig. 3b the bibliographic values of the
catalytic efficiency of TTACl and CTACl micelles whose chain length are between OTACl
and TTACl [22]. We can see in Fig. 3b that their catalytic efficiencies match perfectly with
those of the mixed micelles. Interestingly, the catalytic efficiency of OTACl micelles is
about 20 times lower than that for the same reaction in the presence of DODAC vesicles
[29]. The higher catalytic efficiency of vesicles is a consequence of the high substrate
dissolving capacity of the vesicles.
In conclusion, we found that cationic mixed micelles of LTACl:OTACl catalyze the
basic hydrolysis of MNTS. The kinetic behavior was satisfactorily explained using the
pseudophase ion-exchange model, which let us to obtain the thermodynamic and kinetic
parameters of the reaction. We found that the hydrophobic character of the mixed micelles
drives the association of the substrate to the micellar pseudophase and the catalytic
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J Solution Chem (2015) 44:1866–1874
1873
efficiency of the micelles. It can also be addressed that the catalysis observed in these
systems is due to the increase of local reagents concentrations at the micellar interface
rather than to the decrease in the energy of activation required for the OH- attack to take
place.
Acknowledgments Financial support from the Spanish MINECO (MAT 2013-45168-R) is gratefully
acknowledged.
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