Modification of reactivity by changing microemulsion composition. Basic hydrolysis of nitrophenyl acetate in AOT/isooctane/water systems

Article abstract of DOI:10.1039/b401226g

A study was carried out on the basic hydrolysis of p-nitrophenyl acetate (NPA) in AOT/isooctane/water microemulsions keeping constant the hydroxide ion concentration referred to the droplet water. The obtained results show that the pseudofirst order rate

Full text of DOI:10.1039/b401226g

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P A P E R
Modification of reactivity by changing microemulsion composition.
Basic hydrolysis of nitrophenyl acetate in AOT/isooctane/water
systems
a
b
b
Luis Garc ´ı a-R ´ı o,* Juan Carlos Mejuto and Mois e´ s P e´ rez-Lorenzo
a
Dpto. Qu ı´ mica F ı´ sica, Facultad de Qu ı´ mica, Universidad de Santiago, 15782, Santiago, Spain
Dpto. Qu ı´ mica F ı´ sica, Facultad de Ciencias, Universidad de Vigo, Campus de Ourense,
Ourense, Spain
b
Received (in Durham, UK) 27th January 2004, Accepted 21st April 2004
First published as an Advance Article on the web 25th June 2004
A study was carried out on the basic hydrolysis of p-nitrophenyl acetate (NPA) in AOT/isooctane/water
microemulsions keeping constant the hydroxide ion concentration referred to the droplet water. The obtained
results show that the pseudofirst order rate constant, kobs , is approximately 100 times lower than the
corresponding value in bulk water and increases together with the [AOT], keeping constant the molar ratio W,
W ¼ [H O]/[AOT]. On experiments varying W, keeping constant [AOT], it can be observed that k decreases
2
obs
from W ¼ 2 to W ¼ 10 as W increases, reaching a minimum value for W ffi 10, and then increasing again.
The observed behavior is a consequence of two factors: the variation of hydroxide ion concentration referred to
the total volume of the system, and the association of the NPA to the interface of the microemulsion. The
experimental results can be explained quantitatively considering that the NPA is distributed throughout
the three pseudophases of the system and that the reaction takes place solely in the aqueous microdroplet. The
w
second order rate constant in the aqueous microdroplet of the microemulsion, k , increase as W decreases,
w
2
showing a parallelism with the behavior observed in aqueous DMSO. The values of k2 increase approximately
35 times from W ¼ 40 to W ¼ 2 due fundamentally to the partial desolvation of the hydroxide ions as the water
content of the microemulsion decreases.
considered that the hydration of the tensioactives is the driving
force of the solubilization process of water in the micro-
emulsions.
Introduction
Microemulsions are pseudohomogeneous mixtures of water-
insoluble organic compounds, water and a surfactant. They
are often good solvents for both inorganic salts and nonpolar
organic molecules, and therefore are excellent media for reac-
tions between compounds with widely differing solubility char-
In spite of these peculiarities, the influence of this type of
aggregate on chemical reactivity has not undergone much
research. Certain organic reactions are faster in water than
9
in less polar solvents and therefore they should benefit from
1
,2
acteristics.
In anhydrous media, surfactants form small
the hydrophobic–hydrophilic equilibrium in microemulsions.
However, the changes in reactivity of the encased water com-
pared to that in network-organized bulk water, are far from
being well known and are even less well controlled. Recent stu-
dies carried out in our laboratory show that the changes in the
properties of the interfacial water can alter not just the reac-
3
polydisperse aggregates. Added water is solubilized in the
polar cores of the micelles, which grow accordingly. In dilute
systems, the structure of microemulsions is accounted for by
4
the ‘‘droplet model’’, which considers the disperse phase to
be formed by monodisperse spherical droplets, each one
5
surrounded by a layer of surfactant molecules. Amongst the
10
11
tion rate, but also the mechanism as the relationship W
varies, W ¼ [water]/[surfactant]. These results are a conse-
quence of the modification of the electrophilic and nucleophilic
character of the interfacial water. As W decreases the interfa-
cial water molecules are fundamentally implicated in the solva-
tion of the head group of the tensioactive. Consequently, a
decrease in the electrophilic character of the water occurs,
which in turns makes the solvation of anionic salient groups
more difficult. An increase in the nucleophilic character also
takes place. The equilibrium between these two factors is
tensioactive agents most widely used for the formation of
microemulsions we can cite the sodium salt of bis (2-ethyhexyl)-
sulfosuccinate, (AOT), which has the peculiarity of not
needing the addition of a co-surfactant in order for it to form
microemulsions.
Microemulsions are more and more frequently used as
microreactors, taking advantage of the reagent compartmenta-
lization and of the particular properties of the encased water.
The hydration of the tensioactives with the solubilized water
in the microemulsions has been the subject of a large body
1
2
responsible for the associative mechanisms being favored as
the W value decreases.
6
of research, given that an understanding of this process is very
useful for understanding physicochemical processes and for
Previous studies on solvolytic reactions show that changes in
the properties of the interfacial water have repercussions on
the chemical reactivity. However existing mechanistic studies
do not reflect the changes in the properties of the water in
the water pool. In fact, a number of researchers consider that
for those reactions to take place in the water pool, the rate
constant must be equal to that which exists in conventional
aqueous media. This extrapolation is a consequence of a
7
finding applications in chemical and biological reactions.
Generally, three types of water are considered to exist: free
8
water, bound water and trapped water. However, the most
recent studies broaden this range to include four types, consid-
ering the different effects of the polar head groups of the ten-
sioactive and of the counterion. This interaction between the
water molecules and the surfactant is so strong that it has been
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
9
88
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5

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w
simplified vision of the structure of the microemulsions, where
the existence of only three types of water is considered: bulk
water, bound water and trapped water. In this simplified version
it is considered that the properties of the bulk water are equal
to those of conventional water, insofar as the difference
between the properties of the water of the microemulsions
and the conventional water is due to the different proportions
of bound and trapped water.
water and isooctane, K , by using different salt (NaClO
4
) con-
o
centrations in the water phase. The obtained results show that
w
w
À2
;
and
K is independent of the salt concentration: K ¼ 2.60 Â 10
o
o
À2
À2
À2
À2
À2
2.70 Â 10
;
2.94 Â 10
;
2.70 Â 10
;
2.67 Â 10
2.89 Â 10 for [NaClO
4
] ¼ 0; 1.00; 2.00; 3.25; 5.00 and 6.00
M. The absence of dependence of the distribution coefficient
w
on the salt concentration allows us to use Ko as a distribution
coefficient between the oil and water pseudophases of the
microemulsion irrespective of W values.
In order to clarify this, we have carried out a kinetic study
on the basic hydrolysis of p-nitrophenyl acetate (NPA)
(
Scheme 1) in microemulsions of AOT/isooctane/water. The
choice of this system is due to the fact that on account of elec-
trostatic considerations the basic hydrolysis of the NPA can
only occur in the water pool of the microemulsion. Likewise,
the rate constant of this reaction in an aqueous medium
Results
A kinetic study was carried out in which the hydroxide ions
concentration, as well as the microemulsion composition was
varied. In all experiments the nitrophenyl acetate concentra-
1
3
increases along with the percentage of DMSO. The obtained
results will show that the reaction rate in microemulsions is
sensitive to the water content, and increases as the W para-
meter decreases. In this sense it is possible to establish a corre-
lation between the values of W and the percentage of DMSO
in the reaction medium, so that the reaction rate in microemul-
sions of W ¼ 2 will be equivalent to aqueous media with 75%
À5
tion, [NPA] ffi 5.00 Â 10 M, was much lower than that of
the hydroxide ions. Fundamentally two types of experiment
were carried out, in which the concentration of the hydroxide
ions was varied while keeping constant the microemulsion
composition, and studies in which the microemulsion composi-
tion and the hydroxide ion concentration were varied simulta-
À
neously. In the latter case the OH concentration was kept
constant, referred to the aqueous volume of the system.
(
v/v) of DMSO.
À
Fig. 1 shows the influence of the concentration of OH on
obs for the basic hydrolysis of the NPA in AOT-based micro-
k
emulsions of W ¼ 2, [AOT] ¼ 0.40 M and [AOT] ¼ 0.70 M,
and W ¼ 50 and [AOT] ¼ 0.40 M. In all cases we observe a
clear linear dependence between the pseudo-first-order rate
Experimental section
AOT (Aldrich) was dried in a vacuum desiccator for two days
and then used without further purification. Microemulsions
were prepared by mixing isooctane (Aldrich), water and 1.00
À
w
À
w
constant and the [OH ] (eqn. (1)), where the [OH ]w
w
corresponds with the concentration of hydroxide ions in the
aqueous phase, referred to the volume of the aqueous
microdroplet of the microemulsion.
M
AOT/isooctane solution in appropriate proportions.
p-Nitrophenyl acetate (Aldrich) was of the highest available
purity and was used as supplied.
The hydrolysis reactions were followed by monitoring the
UV-Vis absorbance of the products of the reaction at
l ¼ 400 nm, using a HP 8453 spectrophotometer fitted with
thermostated cell holders. In all experiments the NPA concen-
app
w
w
À w
w
kobs ¼ ðk2 Þ ½OH Š
ð1Þ
As can be observed in Fig. 1 the slope of the representation
corresponds with the apparent second order rate constant,
app
(k2 ) . This rate constant should not be confused with the
w
À5
w
true second order rate constants, since (k2
the composition of the microemulsion.
tration, typically [NPA] ¼ 5.00 Â 10 M, was much smaller
app
w
w
)
depends on
than that of NaOH, and temperature was keep constant at
ꢀ
25 C. For the kinetic experiments the microemulsions were
prepared using an aqueous solution of NaOH,
A second type of experiment was carried out by studying the
influence of the composition of the microemulsion on the rate
of hydrolysis of the NPA. To this end, experiments were per-
formed in which the surfactant concentration was varied,
À
À2
[
stated to the contrary, the concentration of hydroxide ions
OH ] ¼ 5.00 Â 10 M. Thus, in all the experiments, unless
À2
referred to the aqueous phase is 5.00 Â 10 M. The kinetic
[
AOT] ¼ (0.10–0.60) M, while keeping constant the ratio W.
Experiments were carried out for W values between 2 and
0. In all cases the hydroxide ion concentration was kept con-
absorbance vs. time data always fit the first-order integrated
rate equation satisfactorily (r > 0.999); in what follows, kobs
denotes the pseudo-first-order rate constant. We were able to
reproduce the rate constants with an error margin of Æ5%.
In all cases we verified that the final spectrum of the product
of the reaction coincided with another obtained in pure water,
guaranteeing that the presence of the microemulsions did not
alter the product of the reaction.
4
stant, referred to the aqueous medium of the microemulsion,
The NPA distribution coefficient between water and isooc-
tane has been determined mixing equal volumes of both sol-
vents to which a constant amount of NPA was added. When
this had been vigorously shaken the mixture was separated
ꢀ
into phases at constant temperature (25 C) and a spect-
rophotometric analysis of the NPA content in the aqueous
and isooctane phase was carried out. A mean value of
w
À2
K ¼ 2.60 Â 10 was obtained, indicating that the solubility
o
of the NPA in isooctane is much greater than in bulk water.
Because salts may affect solubilities of organic compounds in
water, we have determined the distribution coefficient between
Fig. 1 Influence of the hydroxide ions concentration on kobs for the
basic hydrolysis of the NPA in AOT/isooctane/water microemulsions
ꢀ
at 25 C. (X) W ¼ 20; [AOT] ¼ 0.40 M; (S) W ¼ 20; [AOT] ¼ 0.70 M
and (L) W ¼ 50; [AOT] ¼ 0.40 M. The concentrations of hydroxide
Scheme 1
ions are referred to the volume of the aqueous microphase.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5
989

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À
w
À2
[
OH ] ¼ 5.00 Â 10 M. Fig. 2 shows the obtained results by
w
studying the influence of the surfactant concentration on k
for values of W ¼ 10; 20 and 40.
obs
As can be observed kobs increases together with the surfac-
tant concentration. When surfactant concentration increases
keeping constant the ratio W, the volume of the aqueous phase
À
should increase too. Therefore, the OH concentration
referred to the total volume of the microemulsion will increase.
Moreover because the basic hydrolysis reaction should take
place in the aqueous microdroplet (the only domain of the
microemulsion which is accesible for the hydroxide ions),
when the surfactant concentration increases so does the NPA
incorporated into the interface. Consequently the NPA con-
centration in the aqueous medium will decrease. So the experi-
mental results showed in Fig. 2 should be a result of the
À
balance between the increase of OH concentration and
decrease of NPA concentration in the aqueous microdroplet.
Fig. 3 Influence of W on kobs for the basic hydrolysis of the NPA in
À5
ꢀ
The reagent concentrations ([NPA] ¼ 5.00 Â 10 M and
AOT/isooctane/water microemulsions at 25 C, [AOT] ¼ 0.45 M and
À
w
w
À2
À
w
w
À
w
À2
[
OH ] ¼ 5.00 Â 10
M) guarantees that [OH ] keeps
[OH ]w ¼ 5.00 Â 10 M referred to the aqueous medium.
constant during the reaction time and allow us to neglect the
possibility of changes in the dynamical properties of the micro-
emulsion due to the reaction products. Moreover the basic
Discussion
1
4
Kinetic studies of reactions in water in oil (w/o) microemul-
sions can be interpreted in terms of reactivity, only if local
reagent concentrations and intrinsic rate constants in the var-
ious microphases of these organized media can be obtained
from the overall, apparent rate data. Our research group has
devised a kinetic model based on the formalism of the pseudo-
phase which can be applied to carry out a quantitative inter-
pretation of the influence of the composition of the
hydrolysis of AOT is a well known process that can affect
the kinetic behavior of basic hydrolysis process carried out
in microemulsions, as we have shown for the basic hydrolysis
1
4a
of crystal violet. Recently we have studied the basic hydro-
lysis of AOT by using sodium nitroprusside as a chemical
1
4b
probe
yielding a value for the second order rate constant
M
À4
À1 À1
of k ¼ 4.4 Â 10
s . The most unfavorable conditions
for AOT hydrolysis competition would be the higher AOT
concentration microemulsion. By using [AOT] ¼ 0.60 M we
can obtain an AOT half life time of 43.6 min. At this AOT
1
5,16
microemulsion on the chemical reactivity.
In order to
apply this formalism to the basic hydrolysis of the NPA we
need to consider the microemulsion formed by three strongly
differentiated pseudophases: an aqueous pseudophase (w), a
continuous medium formed fundamentally by the isooctane
concentration the lowest NPA hydrolysis rate constant is
À3 À1
s . This rate constant corresponds to a NPA half
3
.75 Â 10
life time of 3.1 min. As can be seen the rate for the basic hydro-
lysis of AOT is at least 15 times lower that the NPA one.
Fig. 3 shows the experimental results obtained on studying
the influence of W on the pseudo-first-order rate constant,
(
(
o) and an interface formed fundamentally by the surfactant
i). For electrostatic reasons, the hydroxide ions are to be
found solely within the aqueous pseudophase, while the NPA
is distributed throughout the three pseudophases of the system
k
obs , for the basic hydrolysis of the NPA. The experiments
(Scheme 2). The only convergence point of the reactants will
be, therefore, the aqueous pseudophase of the microemulsion.
have been carried out while keeping the surfactant concentra-
tion constant, [AOT] ¼ 0.45 M. As can be observed, kobs
decreases as W increases, reaching a minimum value for
W ffi 10, and, subsequently, it increases once again when W
increases. The increase in kobs as W changes is due fundamen-
tally to an increase in the volume of the aqueous microphase,
with the consequent increase of hydroxide ions referred to the
total volume of the microemulsion.
1
.
Determination of the true rate constants
The distribution of the NPA through the different pseudo-
phases is characterized by distribution constants from the aqu-
NPA
wi
medium to the interface, Koi , given by eqn. (2):
eous medium to the interface, K
NPA
, and from the continuous
½
NPAŠ
½NPAŠ
Z
NPA
wi
i
NPA
oi
i
K
¼
W
K
¼
ð2Þ
½
NPAŠ
½NPAŠo
w
where the parameter Z is defined as the molar ratio
Z ¼ [Isooctane]/[AOT], by analogy with parameter W.
Considering that the total NPA concentration will be the
sum of the concentrations in each of the three pseudophases
of the microemulsion, we can evaluate the concentration of
w
the NPA in the aqueous phase, [NPA] , thus:
NPA
oi
K
NPA NPA
W½NPAŠ
T
½
NPAŠ ¼
ð3Þ
w
NPA
wi
NPA
K
K
þ K
Z þ Koi
W
oi
wi
where the concentrations are referred to the total volume of
the system.
The reaction can only take place in the aqueous pseudo-
phase, since this is the only zone of the microemulsion where
1
8
the reactants can come into contact with each other. We
can obtain the following expression for the pseudo-first-order
rate constant
Fig. 2 Influence of surfactant concentration on kobs for the basic
hydrolysis of the NPA in AOT/isooctane/Water microemulsions
ꢀ
at (S) W ¼ 40; (X) W ¼ 20 and (L) W ¼ 10. T ¼ 25 C.
NPAW
K
À
w
À2
kobs ¼ k0
oi
[
emulsion.
OH ] ¼ 5.00 Â 10 M referred to the aqueous phase of the micro-
ð4Þ
w
NPA NPA
oi
þ K wN iPAZ þ K oN iPA
K
K
wi
W
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
9
90
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5

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Scheme 2
0
NPA
where k is the pseudo-first-order rate constant referred to the
aqueous medium. This rate constant can be expressed as a sec-
of the water content of the microemulsion. In this way, K
wi
NPA
and Koi should remain constant in spite of the changes in the
properties of the interfacial water and the aqueous microdro-
w
ond order rate constant in the aqueous microdroplet, k , as
À w
2
1
plet. The only term which can justify the variation of the
7
0
w
2
k ¼ k ½OH Š
ð5Þ
w
intercept and slope of eqn. (7) with W is the rate constant
w
where [OH ]w refers to the concentration of hydroxide ions in
the aqueous microdroplet referred to the volume of the
aqueous pseudophase of the microemulsion. We can write
the following expressions for kobs and (k2 )w ¼ kobs/[OH ] :
À
w
for NPA hydrolysis in the aqueous microdroplet, k . If this
2
1
9
rate constant increases as W decreases, it would be possible
to justify the increase in the intercept and slope as W increases.
It would be simpler to evaluate the dependency on W of the
quotient between the intercepts and slopes of Fig. 4, eqn. (7).
Given that the values of the distribution constants of NPA,
app w
À
w
w
w
2
À w
WK oN iPA
þ K wN iPAZ þ K oN iPA
kobs ¼ k ½OH Š
ð6Þ
w
NPA NPA
oi
K
K
W
wi
NPA
wi
NPA
K
and Koi , are independent of W, we should obtain a
NPA
wi
NPA
wi
linear dependency of the type:
Intercept
NPA
W
app
K
þ W
K
w
2
À
Á ¼
þ
Z
ð7Þ
NPA
oi
NPA
wi
w
w
NPA
oi
K
K
k2
k
k K
2
¼ Koi
þ
W
ð8Þ
w
Slope
Fig. 4 shows the good fit of eqn. (7) for series of experiments
carried out keeping constant the ratio W. Table 1 shows the
obtained values for the intercepts and slopes according to W.
As we can observe from the data in Table 1, both the slope
and the intercept increase along with W, tending to reach a
limiting value for W values higher than 15. In order to analyze
the dependence on W of the intercept and slope we should con-
sider the distribution constants of the NPA to be independent
However, the data in Table 1 show that the relationship
Intercept/Slope remains practically constant and independent
of W, with a mean value Intercept/Slope ¼ 25.9. To explain
the absence of this linear dependency we have recourse to
the previously determined distribution coefficient of the NPA
w
between bulk water and isooctane, K . This coefficient can
be written as follows:
o
ꢀ
w
w
w
NPA
X
X
½NPAŠ
n
nH O
½NPAŠw
Z
w
NPA
o
NPA
w
o
2
Ko
¼
ffi
ffi
ꢀ
ffi
ð9Þ
o
NPA
½NPAŠ
n
noil
½NPAŠ W
o
w
Table 1 Values of the intercepts and the slopes of the representations
of Fig. 4 in accordance with eqn. (7)
W
Intercept
Slope
Intercept/Slope
3
4
5
7
8.98 Æ 0.02
17.3 Æ 0.2
28.9 Æ 0.8
60 Æ 3
0.345 Æ 0.002
0.54 Æ 0.02
0.92 Æ 0.06
2.1 Æ 0.2
26.03
32.04
31.41
28.57
28.72
22.08
21.33
22.63
22.81
24.04
26.21
25.47
1
1
1
2
2
3
0
2
5
0
5
0
112 Æ 5
117 Æ 6
128 Æ 12
129 Æ 4
130 Æ 3
132 Æ 2
135 Æ 2
135 Æ 7
3.9 Æ 0.2
5.3 Æ 0.3
6.0 Æ 0.4
5.7 Æ 0.1
5.7 Æ 0.1
5.49 Æ 0.06
5.15 Æ 0.08
5.3 Æ 0.3
app
w
w
Fig. 4 Plot of the experimental data in the form W/(k2
accordance with eqn. (7). (X) W ¼ 40; (S) W ¼ 15; (L) W ¼ 7 and
N) W ¼ 4.
)
vs. Z in
35
4
0
(
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5
991

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4
Table 2 Influence of the ionic strength (NaClO ) on the observed rate
where Xw and Xo represent the molar fractions of NPA in
are
the number of NPA moles in the aqueous and isooctane phases
NPA NPA
the aqueous and isooctane phases respectively: nNPA, n
ꢀ
w
o
NPA
constant for the hydrolysis of the NPA at 25 C.
M
À2
[NaOH] ¼ 5.00 Â 10
and nH O and noil are the number of water and isooctane moles
2
À1
kobs/s
[
NaClO ]/M
4
w
in the respective phases. The concentrations [NPA]w and
o
[
[
NPA] refer to the volume of the phase, while [NPA]
NPA]
w
and
refer to the total volume of the system. On the basis
o
0
0
1
.05
.50
.00
0.554
0.422
0.344
0.285
0.234
0.199
0.169
o
of eqns. (2) and (9) we can write:
w
NPA
NPA
1.50
.25
3.00
.00
Ko ¼ Koi =Kwi
If we combine eqns. (8) and (10) we obtain
ð10Þ
ð11Þ
2
4
Intercept
Slope
À
Á
w
NPA
wi
¼
Ko
K
þ W
NPA
which can be simplified if K
ꢁ W, resulting in Intercept/
the increase of the reaction rate as the percentage of organic
co-solvent increases. In certain reactions, S 2 and S Ar, it
wi
. This relationship justifies the fact that the
w
Slope ¼ K K
o
NPA
wi
N
N
2
5
quotient Intercept/Slope remains constant and independent
of W. From the mean value of the relationship Intercept/
has been shown that the effect caused by the dipolar aprotic
solvents is a combination of the increase in the solvation of the
transition state and the desolvation of the nucleophile, espe-
cially in the case of the most basic nucleophiles.
w
o
À2
Slope ¼ 25.9 and the value of K ¼ 2.60 Â 10 we can obtain
NPA
oi
NPA
wi
and K
20
the values of K
When the values of K
¼ 25.9 and K
¼ 996.
NPA
oi
NPA
wi
26
are known we can
From a perusal of the previously obtained data, it is clear
w
obtain the values of k2 applying eqn. (7). The obtained results
are shown in Fig. 5. The second order rate constant in the
that the solvation of the non-electrolyte (ester molecule) is not
of major importance in causing rate enhancement in aprotic
solvents. Thus one can say that significant changes in the value
of kDMSO/kEtOH are not due to changes in the solvation of the
ester molecule. The structure of the transition state for the
ester hydrolysis reactions has a certain degree of negative
charge on the carbonyloxygen atom (Scheme 3), which confers
on this oxygen atom a good hydrogen bond acceptor capacity,
in such a way that it will be solvated more efficiently in protic
solvents. The transition state anion in the case of an aromatic
ester will be much more polarizable than that for an aliphatic
ester. Further, the phenyl ring attached to the carbonyl group
acts as an electron sink and thus lowers the electron density
around the carbonyl group, thereby decreasing the localization
of negative charge on the carbonyl oxygen atom with concur-
rent decrease in the hydrogen bond acceptor capacity of the
w
À1 À1
s
water droplet increases from a value of k ffi 7.5 M
for
2
w
2
À1 À1
s
This behavior should be attributed to the changes in the water
values of W > 10 to a value of k ¼ 257 M
for W ¼ 2.
2
1
properties of the microemulsion as W varies.
2
. Second order rate constant dependence on W
Fig. 5 shows that the rate constant in the aqueous microdro-
plet increases approximately 35 times as W decreases from
W ¼ 40 to W ¼ 2, remaining constant for values of W > 12.
The obtained value for high water contents is slightly lower
than that previously obtained in bulk water in our laboratory,
H2O
À1 À1
w
À1 À1
k2 ¼ 11:3 M
s
. This difference between k ffi 7.5 M
s
can be a consequence of the effect
2
H2O
À1 À1
and k2 ¼ 11:3 M
s
transition state. This will result in an increase of the kDMSO
/
exerted by the ionic strength on the reaction rate (see
Table 2). More important is the decrease in the rate constant
of the reaction as W increases. It is well known that ester sapo-
nification rates are subject to large rate accelerations on going
2
7
kEtOH value.
Therefore, it seems that the greatest effect on the rate of the
reaction in aqueous DMSO must be the desolvation of
the hydroxide ion. The repercussion of the desolvation on
the reaction rate or on reactivity–structure correlations is
far-reaching. In fact, there exist numerous examples that show
that the rate of certain nucleophile attacks decreases as the
basicity of the nucleophile increases, entailing negative
Brønsted exponents. This behavior has been found for some
2
2
from protic to dipolar aprotic solvents. Generally two expla-
nations have been considered for this phenomenon: the first is
that the effect is a consequence of the desolvation of the
hydroxide ion in the dipolar aprotic solvent. An alternative
theory attributes the effect to the greater capacity of the dipo-
lar aprotic solvent to solvate the transition state. The results
obtained in our laboratory from studying the influence of the
percentage of DMSO on the hydrolysis rate of NPA confirm
2
3
2
4
2
8
phosphoryl transfer reactions to amines, for reactions of
highly reactive carbocations with amines and for reactions
2
9
3
0
of thiolate ions with Fischer carbene complexes. Likewise
values of the Brønsted exponent near to zero have been found
3
1
for reactions of diphenylketene with amines. Studies carried
2
8
out by Jencks indicate that these anomalous Brønsted expo-
nents are a consequence of the requirement of partial desolva-
tion of the nucleophile prior to the reaction. Normally it is
considered that the desolvation is a pre-equilibrium which
occurs at a separate stage, in such a way that for a nucleophile
attack a two-step model can be adopted, like that shown in
Scheme 4.
Fig. 5 shows that as the water content in the microemulsion
decreases, there occurs a change in its properties which is able
to alter chemical reactivity. The observed effect will be a
Fig. 5 Influence of W on kw for the basic hydrolysis of the NPA in
2
ꢀ
À
w
w
AOT/isooctane/water microemulsions at 25 C. [OH ] ¼ 5.00 Â
À2
10
M referred to the aqueous medium.
Scheme 3
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
9
92
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5

View Article Online
consequence of the desolvation of the hydroxide ion, which
entails an increase in reactivity, and the lower degree of solva-
tion of the transition state, with a consequent decrease in the
reaction rate. The balance of both factors will allow us to
explain the experimental results. From a quantitative point of
view the desolvation of the hydroxide ion seems to be the great-
est contribution to the decrease in activation energy of
the reaction.
Scheme 5
8
The available studies in the bibliography show that the
the water content of the microemulsion increases, as a
consequence of the variation of the physical properties of the
water. However, this change in the physical properties is not
reflected in the distribution constants, Koi and Kwi , since
these remain independent of W. The reason for this different
sensibility must be found in the enthalpies of solvent transfer
of the hydroxide ion, of the ester and of the transition state.
For the purpose of comparison, Haberfield et al. have stu-
died the basic hydrolysis of the ethyl acetate in 60% aqueous
DMSO and 60% aqueous ethanol. The activation enthalpies
properties of the water are modified as the water content in
the microemulsion varies. For example, the frequency of the
vibration of the bond O–H of the water changes from 3493
NPA
NPA
À1
À1
cm
to 3416 cm
when W increases from W ¼ 1 to
W ¼ 20, reaching a value near to that obtained in pure bulk
water for high values of W in microemulsions formed by
NaOT/heptane/water. When the H-NMR technique is used
1
2
2
to investigate the properties of the water it is found that he
chemical shift of the protons of the water increases from
d ¼ 4.030 ppm for W ¼ 4 to d ¼ 4.430 ppm for W ¼ 50 in
microemulsions of NaOT/isooctane/water.
6
À1
for the alkaline hydrolysis are DH ¼ 10.9 kcal mol and
6
À1
DH ¼ 14.9 kcal mol
for aqueous DMSO and aqueous
ethanol respectively. It can be noted that a factor of
The results obtained for the basic hydrolysis of the NPA are
consistent with the existence of four types of water: trapped
water, bound water (bound to the counterion, or bound to
the headgroup of the surfactant), and normal water. These four
types of water should coexist and interchange rapidly. The
trapped water is found dispersed amongst the hydrocarbon
chains of the surfactant molecules, existing as monomers and
dimers, and does not hydrogen bond with its surroundings.
The normal water is found in the center of the aqueous micro-
droplet and has strong hydrogen bond interactions. Besides
these two types of water, there exist other bound water mole-
cules in the vicinity of the ionic tensioactives. The local inter-
actions of the water molecules with the counterions and the
headgroups of the tensioactive (Scheme 5) cause opposite
effects on the structure of the water. The hydration of the anio-
nic head groups of the tensioactives increases the electronic
density on the hydrogen atoms in the water molecules, with
the consequent rupture of the hydrogen bonds of the normal
water. The intensity of the O–H bonds increases and this factor
means that the chemical displacement of the water protons
bonded to the headgroups is found in higher fields than the
normal water, and the vibration frequency of the O–H bond
is found at higher frequencies.
À1
dDH ¼ 4.0 kcal mol favors the aqueous DMSO. This fac-
À
tor should be compared to the desolvation of the OH by the
DMSO, finding that the enthalpy of solvent transfer,
À
dDH
if the desolvation of the base were the only factor causing a
¼ À14:24 kcal mol . It thus appears that
6
solvent effect on rate and on DH in this reaction, then a far
larger effect would be expected than is actually observed.
Haberfeld’s results show that the enthalpy of solvent transfer
from 60% aqueous DMSO to 60% aqueous ethanol for ethyl
Ethylacetate
À1
acetate is dDH
¼ 0.23 kcal mol . As might be
DMSO!Ethanol
expected, the enthalpies of transfer of the esters do not contri-
bute significantly to the differences in the enthalpies of the
reactants in these solvents. This smaller degree of sensitivity
of the enthalpy of transfer of the ester in comparison with
OH or the transition state is the reason why the distribution
NPA
NPA
constants Koi and Kwi are not modified when the reactivity
is altered.
4
of NPA
.
Influence of dynamical phenomena on the basic hydrolysis
Dynamical measurements have been used to characterize the
intramicellar water in w/o microemulsions. These studies all
find that the water inside the micelles moves differently from
bulk water. Using viscosity sensitive probes, Hasegawa
et al. have measured the microviscosity inside reverse
micelles of varying size. Their results indicate that the water
inside micelles with W < 10 has a higher viscosity than that
of the bulk, a fact that they ascribe to water bound to the
AOT polar headgroups. For microemulsions larger than
W ¼ 10, the viscosity decreases slowly as the size of the micro-
emulsions increases. Solvent relaxation with fluorescence
probes reveals two different solvation rates inside the micro-
emulsions that authors attribute to water bound to the polar
headgroups of AOT and bulk-like water. The results from
these and other studies indicate that water in AOT microe-
mulsions moves more slowly than bulk water. While ordinary
water molecules relax in the subpicosecond time scale, inside
microemulsions the solvation dynamics becomes several thou-
sand times slower and occurs in the nanosecond time sca-
On the other hand, the counterions accumulated in the inter-
ior of the aqueous microdroplets can polarize the water mole-
cules, giving rise to a lower electronic density around the
protons and consequently a decrease in the strength of the O–H
bond of the water. This effect causes a displacement to lower
3
2
1
fields of the resonance signals of H-NMR of the protons of
the water and the frequency of the vibration of the O–H bond
is displaced to lower frequencies. This weakening of the strength
of the hydrogen bond will contribute to the increase in the-
desolvation of the hydroxide ion as the water content of the
microemulsion decreases and consequently the reaction rate
increases. Moreover changes in polarity, pH, viscosity and
altered interfacial rigidity may concertedly act to modify the
chemical reactivity in w/o microemulsions when W decreases.
3
3
3
4
3.
Influence of W on the distribution constants of the NPA
Previous results show that the second order rate constant of
the basic hydrolysis of the NPA decreases about 35 times as
3
4a,35
le.
Water motion in the low water content
microemulsions was essentially frozen while in higher ones
additional bulk-like relaxation components developed. How-
ever the time scale for water relaxation remains below that
for the basic hydrolysis of NPA. Due to this, during the reac-
tion time we observe an average of the dynamical properties of
water. The observation of an average value of the physical
1
properties is also consistent with the H-NMR observed beha-
vior. There is a large body of evidence showing the existence of
Scheme 4
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5
993

View Article Online
This behavior means that the reactivity in the aqueous micro-
several water states inside AOT-based microemulsions. How-
ever the rate of exchange between these water states is too fast
for the NMR time scale so that only one average signal is
observed. This signal is W-dependent because of the different
percentages of these water states.
droplet of a microemulsion of W ¼ 2 is approximately equal
to that found in 75% aqueous DMSO.
3. The comparison of the enthalpy of solvent transfer from
60% aqueous DMSO to 60% aqueous ethanol for ethyl acetate,
OH and transition state shows that the contribution of the
À
The internal dynamics of microemulsions is a well-known
process that is related to percolation phenomena. Several
research groups have studied this process in the presence and
enthalpy of solvent transfer for the ester to the activation
enthalpy of the reaction is negligible. The value of
3
6
Ethylacetate
DMSO!Ethanol
À1
À1
absence of additive. This process occur as the result of inter-
droplet channels opening in droplet clusters that allows trans-
port of materials between them as has been showed by Moulik
dDH
dDH
¼ 0.23 kcal mol
¼ À14:24 kcal mol
in comparison with
allows us to explain
À
OH
DMSO!Ethanol
the variation of the rate of reaction as W decreases while the
3
7
and coworkers. Lang, Zana and coworkers have related elec-
trical percolation to the rate constant governing the exchange
distribution constants of the NPA do not alter.
3
8
of materials between droplets. For AOT/isooctane/water
microemulsions the rate of exchange of materials between dro-
9
À1 À1
plets has been obtained as ffi10 M
s . It means that this
Acknowledgements
phenomenon will be relevant only to fast reactions, whose rate
may totally or partially depend on material exchange rates.
Our kinetic results for nitrophenyl acetate hydrolysis are far
away from a diffusion controlled reaction (by 10 times) so
that changes in the internal dynamics of the microemulsion
are not relevant in our case.
Financial support from Ministerio de Ciencia y Tecnolog ´ı a
(
Project BQU2002-01184) and Xunta de Galicia (PGIDT00P-
1
2
XI20907PR and PGIDT03-PXIC20905PN) is gratefully
acknowledged.
The lack of influence of electrical percolation phenomena
upon chemical controlled reactions has been probed by our
research group by studying the nitroso group transfer from
N-methyl-N-nitroso-p-toluenesulfonamide to several second-
References
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ꢀ
ꢀ
ary amines at 25 and 35 C. Under the experimental condi-
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2
3
1
5
parameters. We have also studied the influence of electrical
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V. Degiorgio, Physics of Amphiphiles: Micelles, Vesicles and
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1
0
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8
9
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3
9
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1
0
1
L. Garc ´ı a-R ´ı o, J. R. Leis and E. Iglesias, J. Phys. Chem., 1995, 99,
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2 318.
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12
13
14
L. Garc ´ı a-R ´ı o and J. R. Leis, Chem. Commun., 2000, 455.
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The results obtained on studying the basic hydrolysis of the
NPA in microemulsions of AOT/isooctane/water can be sum-
marized as follows:
8
J. Perez-Juste, J. Phys. Org. Chem., 2002, 15, 576.
89; (b) L. Garc ´ı a-R ´ı o, P. Herv e´ s, J. R. Leis, J. C. Mejuto and
´
1
. When studying the basic hydrolysis of the NPA in AOT-
15 L. Garc ´ı a-R ´ı o, E. Iglesias, J. R. Leis and M. E. Pe n˜ a, J. Phys.
Chem., 1993, 97, 3437.
based microemulsions it is found that the rate constant
observed is approximately 100 times lower than in bulk water
and depends on the composition of the microemulsion. To
interpret these results the formalism of the pseudophase has
been applied, considering the distribution of the NPA through-
out the three pseudophases of the microemulsion and reaction
in the aqueous microdroplet. The model proposed has allowed
us to obtain the distribution constants of the NPA throughout
the three pseudophases of the microemulsion and the second
1
6
(a) L. Garc ´ı a-R ´ı o, J. R. Leis and J. C. Mejuto, J. Phys. Chem.,
996, 100, 10 981; (b) L. Garcia-Rio, P. Herves, J. C. Mejuto,
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7, 372.
As will be shown, the experimental results obtained will be consis-
1
2
1
7
NPA
NPA
tent with the fact that Kwi and Koi do not vary along with the
composition of the microemulsion. This behavior is in accordance
with the fact that the enthalpy of solvent transfer between aqu-
eous DMSO and aqueous ethanol is very small by comparison
with the enthalpy of solvent transfer for the transition state or
the enthalpy of solvent transfer for the hydroxide ions.
w
order rate constant in the aqueous microdroplet, k .
2
2
. The second order rate constant in the aqueous microdro-
18 There is a large body of studies showing the validity of the droplet
model as a good description of the microheterogeneous structure
of microemulsions. The existence of this microstructure guaran-
tees the compartmentalization of the reagents and hence the valid-
ity of the kinetic model. See for example the photon correlation
spectroscopy studies showing the existence of a good correlation
between the Stokes radii of AOT/isooctane/water ternary systems
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480). SAXS and SANS studies have also shown the existence of
plet increases as the water content of the system decreases.
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of the water of the microemulsion. As W decreases the struc-
ture of the hydrogen bond is broken, and consequently the sol-
vation capacity of the transition state decreases. However, the
w
effect which has the greatest contribution to the increase of k2
is the partial desolvation of the hydroxide ion as W decreases.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
9
94
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5

View Article Online
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1
9
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The value obtained for K
NPA
wi
2
0
1
¼ 996 perfectly satisfies the previous
NPA
supposition that Kwi ꢁ W for all the studied W values, given
that W varies between 2 < W < 40.
2
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T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 8 8 – 9 9 5
995