Microheterogeneous solvation for aminolysis reactions in AOT-based water-in-oil microemulsions

Article abstract of DOI:10.1002/chem.200401067

A kinetic study was carried out on the aminolysis of p-nitrophenyl acetate (NPA) by n-decylamine (DEC), piperazine (PIP) and sarcosine (SAR) in AOT/isooctane/water (w/o) microemulsions. By using the pseudophase model both the rate constants at the interfa

Full text of DOI:10.1002/chem.200401067

FULL PAPER
Microheterogeneous Solvation for Aminolysis Reactions in AOT-Based
Water-in-Oil Microemulsions
[
a]
[b]
[a]
Luis García-Río,* Juan Carlos Mejuto, and MoisØs PØrez-Lorenzo
Abstract: A kinetic study was carried
out on the aminolysis of p-nitrophenyl
acetate (NPA) by n-decylamine
creases. In the aqueous microdroplet
croemulsion the predominant interac-
+
ꢀ
the predominant interaction Na ···OH
tion SO ···HOH causes an increase in
2
3
causes a decrease in the strength of the
hydrogen bonds and therefore facili-
tates the desolvation of the reagents as
W decreases. This desolvation of the
the electronic density on the water
molecules and the consequent decrease
in their efficiency in the solvation of
the partial negative charge, which de-
velops on the carbonyl oxygen atom in
(
(
DEC), piperazine (PIP) and sarcosine
SAR) in AOT/isooctane/water (w/o)
microemulsions. By using the pseudo-
w
phase model both the rate constants at
reagents causes the increase of k2 as W
i
the interface, k , and the water micro-
decreases. In the interface of the mi-
the transition state of the reaction. This
2
w
i
2
droplet, k , can be obtained. The ob-
decrease in the solvation causes k to
2
i
2
tained results show that k increases to-
decrease together with the water con-
tent of the system.
Keywords: aminolysis · kinetics ·
reaction mechanism · microemul-
sions · reactivity
gether with the water content of the
w
microemulsion, whereas k2 increases
as the water content of the system de-
Introduction
In order to improve the applications of microemulsions as
new reaction media it is necessary to have models, which ex-
plain the kinetic behavior of simple reactions in these
media. These models should take into account the distribu-
tion of different reagents throughout the various pseudo-
phases of the microemulsion and the possibility of the reac-
tion-taking place simultaneously in one or many phases. It
will thus be possible to design experiments in which a varia-
tion of the composition of the microemulsion causes the re-
action to occur in different microenvironments of the
system. An additional factor, which complicates the kinetic
studies carried out on microemulsions, are the changes in
their physical properties as the water content varies. The
most used surfactant is sodium bis(2-ethylhexyl)sulfosucci-
nate, also known as AOT. The phase diagram of AOT/
A microemulsion is defined as a thermodynamically stable,
isotropic dispersion of two relatively immiscible liquids, con-
sisting of microdomains of one or both liquids stabilized by
[
1,2]
an interfacial film or surface-active molecules.
In prepa-
rative organic chemistry microemulsions have been used to
overcome reagent solubility problems because the ability of
microemulsions to solubilize both polar and non-polar sub-
stances and to compartmentalize and concentrate reagents
Microemulsions are also very widely used to the design and
synthesis of novel materials that requires a precise control
[3]
[4]
of the shape and size of the precursor materials.
water/isooctane shows a very large domain of water-in-oil
[
a] Prof. L. García-Río, Dr. M. PØrez-Lorenzo
Departamento Química Física
Facultade de Química, Universidade de Santiago
[5,6]
droplets.
As the parameter W varies, W=[H O]/[AOT],
2
changes occur in the microviscosity of the system, polarity,
and so on. In this sense, the changes observed by various
1
5782 Santiago (Spain)
[7–24]
Fax : (+34)981-595-012
E-mail: qflgr3cn@usc.es
techniques
are especially important when studying the
properties of the solubilized water in AOT-based microe-
mulsions. These changes in the physical properties of the
[
b] Prof. J. C. Mejuto
Departamento Química Física
Facultade de Ciencias, Universidade de Vigo
Campus de Ourense, Ourense (Spain)
[25]
water can modify the reactivity and even the mechanism
[26]
by which the reactions occur.
The present study intends to extend the kinetic models to
the study of reactions that can occur simultaneously in vari-
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2005, 11, 4361 – 4373
DOI: 10.1002/chem.200401067
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4361

ous microenvironments of microemulsions. In order to carry
out the study a reaction has been chosen which is very well
known in water and in various organic solvents, namely the
aminolysis of the 4-nitrophenylacetate. Three different
amines have been chosen because of their different degrees
of hydrophobicity: n-decylamine, piperazine and sarcosine
(
Scheme 1). The use of sarcosine and piperazine will allow
us to verify whether the reactivity sequence observed in an
aqueous medium is transferred to the microdroplet and the
interface of the microemulsion on using amines with differ-
ent pK values.
a
The results obtained show that the aminolysis of the NPA
by piperazine and sarcosine takes place simultaneously in
the interface and in the aqueous microdroplet. As the water
content of the system decreases, there is an increase in the
percentage of the reaction, which occurs in the interface of
the microemulsion. Aminolysis by n-decylamine takes place
only at the interface of AOT-based microemulsions.
DEC; aminolysis by PIP and finally the aminolysis of the
NPA by SAR.
Aminolysis of NPA by decyl-
amine: A study has been carried
out on the influence of the mi-
croemulsion composition on
kobs in the aminolysis of NPA
by decylamine, keeping the
total amine concentration con-
stant with regard to the total
volume of the system, [DEC] =
Scheme 1.
ꢀ
2
2
.50”10 m, and varying the
composition of the microemul-
sion. Figure 1 shows by way of
Results
example the values of kobs obtained for different composi-
tions of the system. These results show that kobs increases to-
gether with the AOT concentration, and then decreases.
The results shown in Figure 1 can be explained as a conse-
quence of the DEC and NPA incorporation to the interface
of the microemulsions when increasing the [AOT], which in-
creases the local concentration of the reagents and thus the
A study was carried out on the influence of the composition
of the microemulsion on the aminolysis rate constant of 4-
nitrophenylacetate by amino acids, piperazine and n-decyla-
mine. The compositions of the w/o microemulsions, which
have been used to carry out this study, is shown in the phase
diagram below. The compositions were selected with the
purpose of covering a wide interval: the content of AOT
varies between 6–30% (w/w); that of water between 0.5–
4
(
[
3% (w/w) and that of the isooctane between 29–93%
w/w). These composition intervals allow us to vary W, W=
H O]/[AOT], between W=2–40. Hence we used microe-
2
mulsions which display a wide variation in properties such
as polarity, viscosity, hydrogen bond donation capacity, and
saline content.
In all cases it was found that the observed rate constant
presents a linear dependency on the nucleophilic concentra-
tion when the reaction takes place in the microemulsion.
This behavior is consistent with that found in water and in
[27]
other polar solvents. The different solubilities of SAR, pi-
perazine and n-decylamine mean that the kinetic behavior
in the microemulsions is different. Therefore, we will pres-
ent separately the results in order, so that the hydrophilic
character of the amine increases: aminolysis of NPA by
Figure 1. Influence of the microemulsion composition on kobs for the ami-
nolysis of NPA by n-decylamine. [DEC]=2.50”10 m referred to the
total volume of the system; T=258C; *: W=3, *: W=7, &: W=20, and
ꢀ2
&
: W=35.
4362
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Chem. Eur. J. 2005, 11, 4361 – 4373

Water-in-Oil Microemulsions
FULL PAPER
reaction rate. Once all the NPA and DEC have been associ-
ated with the interface, as the [AOT] increases there is also
an increase in the interfacial volume and therefore a dilu-
tion of the reagents in the interface with the consequent re-
duction of kobs. When the [AOT] remains constant and W in-
creases, there is an increase in the size of the droplets and,
therefore, in the fraction of the disperse phase. This increase
in the fraction of the disperse phase causes a reduction in
the volume occupied by the continuous medium, so that the
quantity of NPA and DEC associated with the interface in-
creases. In this way the increase of kobs together with W can
be explained.
Reaction of SAR with NPA: A kinetic study has been car-
ried out in which we have varied the concentration of amino
acid, SAR, as well as the composition of the microemulsion.
In all cases the concentration of nitrophenyl acetate,
ꢀ
5
[NPA]=(2–4)”10 m, was much lower than the concentra-
tion of amino acid. On varying the concentration of amino
acid while keeping the composition of the microemulsion
constant, the observed rate constant, kobs, presents a linear
dependency on the amino acid concentration (not shown).
In other experiments the composition of the microemulsion
and the concentration of amino acid have been varied simul-
taneously, while keeping constant the concentration of
amino acid referred to the aqueous volume of the microe-
Reaction of piperazine with NPA: To investigate the influ-
ence of the composition of the microemulsion on the ami-
nolysis of the NPA by piperazine experiments were carried
out varying the composition of the microemulsion using as
an aqueous medium a 0.10m solution of piperazine. There-
fore, on varying the composition of the microemulsion, the
amine concentration also varies, so that in order to analyze
the results obtained it is more convenient to use a second
[28]
w
w
ꢀ2
mulsion, [SAR] = 5.00”10 m. In all cases a quantity of
sodium hydroxide was added to the amino acid solution, suf-
ficient to guarantee that the amino acid would take an
anionic form. Given that on varying the microemulsion com-
position also varies the amino acid concentration, it is more
simple to carry out an analysis of the results in terms of a
app
second order rate pseudoconstant, k2 , which includes the
app
app
order rate pseudoconstant, k2 , which includes the pipera-
concentration of amino acid: kobs = k2 [SAR] . The term
T
app
zine concentration: kobs = k2 [PIP] . The term [PIP] corre-
[SAR] corresponds to the total concentration of sarcosine
T
T
T
sponds to the total concentration of piperazine in the micro-
emulsion referred to the total volume of the system. The re-
sults are shown in Figure 2. The rate constant k2 decreases
as the surfactant concentration increases and as the water
content of the system increases.
in the microemulsion referred to the total volume of the
app
system. The value of k2 is always lower than that which is
app
H2O
ꢀ1 ꢀ1
found in pure water, k2 =2.01m s .
app
Figure 3 shows that k2 decreases as the surfactant con-
centration and the water content of the system increase as
was observed in the aminolysis by piperazine.
app
Figure 2. Influence of the microemulsion composition on k2 in the ami-
nolysis of NPA by PIP. [PIP]T = 0.10m referred to the aqueous phase of
app
2
Figure 3. Influence of the microemulsion composition on k
for the ami-
ꢀ
2
the system; T=258C; *: W=7, *: W=15,
&
: W=30, and &: W=40.
nolysis of NPA by SAR [SAR]=5.00”10 m referred to the aqueous
phase of the system; T=258C; *: W=4, *: W=7, &: W=15, and
: W=
0.
&
3
This behavior is contrary to that observed in the aminoly-
sis of the NPA by DEC (see Figure 1) and must be a conse-
quence of the different distribution of the reagents and to
an increase of the volume of the interface on increasing
Discussion
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
various microphases of these organized media can be ob-
tained from the overall, apparent rate data. In our research
group a kinetic model has been developed, based on the for-
malism of the pseudophase which can be applied to inter-
[
AOT]. It is important to note that in all cases the rate con-
app
stant k2 is lower than the aminolysis rate constant of NPA
by piperazine in bulk water, k2
H2O
ꢀ1 ꢀ1
= 6.09m s . This de-
crease of the rate constant is due to the distribution of the
reagents throughout the different pseudophases of the
system.
Chem. Eur. J. 2005, 11, 4361 – 4373
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L. García-Río et al.
NPA
pret quantitatively the influence of the composition of the
Koi W½NPAŠT
½
NPAŠw ¼ K
ð2Þ
[29,30]
NPA
oi
NPA
wi
NPA
NPA
microemulsion on the chemical reactivity.
As is usual on
K
þ Kwi Z þ Koi
W
studying reactivity in colloidal systems (micelles, vesicles
and microemulsions) the activity coefficients of the compo-
nents in these systems are independent of their concentra-
tions. To apply this formalism to the aminolysis of the NPA
we must consider the microemulsion formed by three
NPA
oi
NPA
K
NPA
K
½NPAŠT
wi
NPA
½NPAŠ ¼
ð3Þ
i
NPA
NPA
K
K
þ Kwi Z þ Koi
W
oi
wi
[31]
strongly differentiated pseudophases: an aqueous pseudo-
phase (w), a continuous medium formed fundamentally by
the isooctane (o) and an interface formed fundamentally by
the surfactant (i). The different kinetic behavior makes it
necessary for us to propose different kinetic models for ami-
nolysis by n-decylamine, piperazine and sarcosine.
where the concentrations are referred to the total volume of
the system.
The possibility of the reaction taking place in the continu-
ous medium can be discarded due to the second order de-
pendency which exists between kobs and the amine concen-
tration in isooctane (data not shown). Likewise, the value of
kobs obtained for the aminolysis of the NPA by n-decylamine
NPA aminolysis by n-decylamine: The experimental results
obtained in the aminolysis of NPA by decylamine can be in-
terpreted from Figure 4. The NPA is found distributed be-
tween the three pseudophases of the system. The low solu-
bility of the n-decylamine in water allows us to exclude the
possibility of it being distributed throughout the three pseu-
dophases of the microemulsion. In this way the NPA and
the decylamine will only be able to contact the continuous
medium and the interface of the microemulsion.
ꢀ
5
ꢀ1
in pure isooctane, k
1
ffi4.5”10
s
for [DEC]=2.50”
obs
ꢀ
2
0 m, is approximately 250 times lower than the value ob-
tained in AOT-based microemulsions for the same amine
concentration. Therefore, the reaction will take place solely
in the interface of the microemulsion.
Equation (3) can be simplified taking into account the
[32]
NPA
NPA
NPA
values previously obtained
for Koi and Kwi (Koi
=
NPA
2
5.9 and Kwi = 996):
NPA
Koi ½NPAŠT
½NPAŠi
ffi
ð4Þ
NPA
oi
K
þ Z
The following expression for the observed rate constant,
kobs, can be obtained:
NPA
oi
K
NPA
oi
kobs ¼ k0
ð5Þ
i
K
þ Z
Figure 4.
0
where k is the first order rate pseudoconstant referred to
i
the interface. This rate constant can be expressed as a bimo-
The distribution of the NPA throughout the different
pseudophases of the microemulsion (Figure 4) is governed
by the following equilibrium constants:
i
lecular rate constant in the interface, k , thus:
2
Vtot
½DECŠi
0
i
2
i
i
i
2
i
2
k ¼ k ½DECŠ ¼ k
½DECŠi ¼ k
ð6Þ
i
Vi
V
AOT½AOTŠ
½
NPAŠ
½NPAŠ
½NPAŠ
NPA
wi
i
NPA
oi
i
K
¼
W
K
¼
Z
ð1Þ
½NPAŠ
w
o
i
i
where [DEC] refers to the n-decylamine concentration in
where [NPA] , [NPA] and [NPA] correspond to the NPA
the interface of the microemulsion referred to the volume of
w
i
o
concentrations in the aqueous microdroplet, the interface
and the continuous medium in the microemulsion, referred
to the total volume of the system. The parameter Z has
been defined as the molar relationship Z = [isoooctane]/
the interface, whereas [DEC] corresponds to the amine con-
i
centration in the interface of the microemulsion referred to
the total volume of the system. As is usual in microemul-
sions and direct micelles, the interface has been considered
to be consisting solely of the surfactant, in such a way that
the volume of the interface is given by the volume occupied
[
AOT], by analogy with the parameter W. We can evaluate
the concentrations of NPA in the aqueous microdroplet,
NPA] , and in the interface of the microemulsion, [NPA] ,
¯
by the AOT. In this way V /V = 1/V [AOT] where V
tot i AOT
¯
[
w
i
AOT
¯
considering that the total concentration of NPA will be the
sum of the concentration in each of the three pseudophases
of the system:
represents the molar volume of the surfactant, V
=
AOT
ꢀ
1
0.34m and [AOT] represents the total concentration of
AOT referred to the total volume of the microemulsion.
4364
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Water-in-Oil Microemulsions
FULL PAPER
On the basis of the distribution equilibrium of the n-de-
cylamine between the continuous medium and the interface
of the microemulsion, we can obtain:
This decrease has generally been attributed to an insuffi-
cient hydration of the interface of the microemulsion as the
water content of the system decreases.
DEC
oi
K
½
DECŠ ¼
½DECŠT
ð7Þ
ð8Þ
ð9Þ
i
DEC
K
þ Z
oi
By combining Equations (5)–(7) we obtain:
i
2
NPA
oi
NPA
DEC
oi
k
K
K
½DECŠT
DEC
kobs
¼
VAOT½AOTŠ ðKoi þ ZÞ ðKoi þ ZÞ
The rate equation can be reordered thus:
NPA
DEC
oi
½
DECŠ
ðKoi þ ZÞðK
þ ZÞ
DEC
T
i
2
¼
Figure 6. Influence of W on logk for the aminolysis of NPA by n-decyl-
ꢀ
i
2
NPA
Koi
oi
kobs
V
AOT½AOTŠ
k K
amine in AOT/isooctane/water microemulsions at 258C; [DEC]=2.50”
0 m referred to the total volume of the system.
ꢀ
2
1
which predicts the existence of a linear and quadratic de-
¯
VAOT[AOT] on the parameter Z of
obs
pendency of [DEC] /k
T
NPA aminolysis by piperazine: The piperazine is a hydro-
philic species, insoluble in isooctane, which is found between
the aqueous microdroplet and the interface of the microe-
mulsion. To explain the kinetic behavior that we observed in
the aminolysis of the NPA by piperazine we must consider
the reactive scheme showed in Figure 7, where the aminoly-
sis reaction can take place simultaneously in the interface
and in the microdroplet of the microemulsion.
microemulsion composition. Figure 5 shows by way of exam-
ple the good fit of previous equation to the experimental re-
sults.
Figure 5. Experimental results obtained for the aminolysis of NPA by n-
decylamine in AOT-based microemulsions according to Equation (9).
ꢀ
2
[
DEC]
T
=2.50”10 m; T=258C; *: W=3, *: W=7, &: W=12, and
&
:
W=25.
Figure 7.
From representations such as those of Figure 5, and using
NPA
the value of Koi = 25.9, we can obtain the values of the
Using the same procedure as we used to obtain the NPA
concentration in the different microenvironments of the mi-
croemulsion, we can obtain the concentrations of pipera-
zine:
distribution constant of the n-decylamine between the con-
tinuous medium and the interface, K , and the aminolysis
rate constant at the interface, k . The obtained values for
DEC
oi
i
2
DEC
Koi are independent of W, allowing us to calculate a mean
DEC
i
2
PIP
wi
PIP
wi
value for Koi =48.4. However, the values obtained for k
K
½PIPŠ
T
W½PIPŠ
T
½
PIPŠ ¼
½PIPŠw ¼ K
ð10Þ
i
PIP
wi
present a clear variation with W, as shown in Figure 6. As
the water content of the system increases, there is also an in-
crease in the aminolysis rate constant at the interface. This
behavior is consistent with the decrease in the nitroso group
transfer rate constant in the interface of AOT-based micro-
K
þ W
þ W
In accordance with the mechanism of Figure 7, the reaction
will take place simultaneously in the interface and in the
aqueous microdroplet of the microemulsion. Therefore, on
[29]
emulsions, as the water content of the system decreases.
Chem. Eur. J. 2005, 11, 4361 – 4373
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L. García-Río et al.
the basis of Equations (2) and (3), we can propose the fol-
lowing expression for the observed rate constant, kobs
:
0
NPA
oi
0
NPA NPA
k K W þ k K
K
w
i
oi
wi
NPA
kobs ¼ K
ð11Þ
NPA NPA
NPA
K
þ K Z þ Koi
W
oi
wi
wi
0
0
where k and k are the rate pseudoconstants of the first
i
w
order referred to the interface and the aqueous microdrop-
let, respectively. These rate constants can be expressed as bi-
i
2
molecular rate constants in the interface, k (see previous
discussion of the aminolysis of the NPA by n-decylamine),
w
and in the aqueous microdroplet, k , thus:
2
Figure 8. Fit of Equation (15) for the aminolysis of the NPA by PIP.
[
PIP]=0.10m referred to the aqueous phase of the system; T=258C; *:
i
2
PIP
wi
PIP
Vtot
Vi
k
K
½PIPŠT
W=7, *: W=15,
&
: W=30, and &: W=40.
0
i
2
i
i
i
2
k ¼ k ½PIPŠ ¼ k
½PIPŠi
¼
i
Vꢀ AOT½AOTŠ ðKwi þ WÞ
ð12Þ
w
Vtot
w
k2
½PIPŠT
PIP
0
w
2
w
w
k
¼ k ½PIPŠ ¼ k2
½PIPŠw ¼ Vꢀ
w
VH2O
½AOTŠ ðKwi þ WÞ
ð13Þ
H O
2
i
i
w
w
where [PIP] and [PIP] correspond to the concentrations of
PIP in the interface and in the aqueous microdroplet, refer-
red to the volumes of these pseudophases. These concentra-
tions are transformed into concentrations referred to the
total volume of the system by taking into account just the
phase volumes. By combining Equations (11)–(13) we can
obtain the following rate Equation (14):
Figure 9. Influence of W on 1/slope (from plots like those of Figure 8) for
the aminolysis of NPA by piperazine in AOT/isooctane/water microemul-
sions at 258C; [PIP]=0.10m, referred to the aqueous phase of the
system.
NPA NPA PIP
NPA
W
oi
K
K
K
K
i
2
oi
wi
wi
w
2 Vꢀ
k
þ k
Vꢀ AOT½AOTŠ
½AOTŠ
½PIPŠT
PIP
ð14Þ
H2O
kobs
¼
NPA NPA
NPA
NPA
Koi
K
þ Kwi Z þ Koi W ðKwi þ WÞ
wi
creases, until a maximum value for W is reached, W ffi 10. A
subsequent reduction in W causes a reduction in 1/slope.
The results can be interpreted considering the following
Equation (16) that relates 1/slope with the water content of
the microemulsion.
This Equation can be rewritten:
½
PIPŠ
T
ꢀ
ꢀ
kobs½AOTŠV
V
H2O AOT
ð15Þ
PIP
NPA NPA
NPA
NPA
wi
ðKwi þ WÞ½K
K
þ K W þ K ZŠ
oi
NPA NPA PIP
wi
oi
w
w
2
NPA
oi
Vꢀ AOT
¼
i
2
NPA
ꢀ
2
oi H O
k
K
i
2
ꢀ
NPA
oi
ꢀ
k K
V
þ
W
NPA PIP
k K
K
K
V
þ k K
V
W
oi
wi
wi
H2O
2
AOT
1
K
K
wi
wi
ð16Þ
¼
1
slope
1
þ
W
PIP
wi
K
and predicts the existence of a linear dependency between
ꢀ
ꢀ
[
PIP] /(k [AOT]V
VAOT) and the Z parameter of micro-
T
obs
H2O
emulsion composition. Figure 8 shows the good fit of Equa-
tion (15) to the experimental results.
[29]
Previous studies have allowed us to determine the value
of the distribution constant of the piperazine between the
continuous medium and the interface of AOT-based micro-
The ordinates and slopes of Figure 8 show a clear depend-
ency on W, as predicted by Equation (15). To carry out a
quantitative analysis it is fitting to use the slopes, since they
can be determined more accurately. Figure 9 shows the de-
pendency of 1/slope (from Figure 8) on W for the aminolysis
of the NPA by piperazine. As can be observed, the term 1/
slope increases as the water content of the system (W) de-
PIP
emulsions Kwi = 9.5 at 258C. Using this value, we can re-
write Equation (16) as:
w
2
NPA
ꢀ
oi AOT
1
þ 0:105 W
k K
V
i
2
NPA
oi
ꢀ
¼
k K
V
þ
W
ð17Þ
H2O
NPA PIP
slope
K
K
wi wi
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Water-in-Oil Microemulsions
FULL PAPER
which predicts the existence of a linear dependency between
by piperazine at the interface must be similar to that ob-
(
1+0.105W)/slope and the W parameter of microemulsion
served for the aminolysis by n-decylamine. Figure 6 shows
i
composition. Figure 10 shows the observed behavior in the
aminolysis of NPA by piperazine.
how k decreases for the aminolysis of NPA by DEC, as W
2
decreases, especially for values of W<10. It is important to
point out that in the case of the DEC the reaction occurs
solely in the interface of the microemulsion. For the aminol-
ysis reaction of NPA by piperazine we expect to find an
i
analogous behavior, that is, we expect that k will decrease
2
i
2
along with W. This decrease in k could explain the negative
deviation observed in Figure 8 when plotting (1+0.105W)/
slope against W. In this interpretation we are assuming that
the negative deviation of Figure 10 is due to the decrease of
i
2
w
2
k with W, and we ignore the influence of W on k . The val-
idity of this approximation lies in the fact that for W<10
more than 80% of the reaction takes place in the interface
of the microemulsion in such a way that the possible effects
w
on k2 must be of less importance. It is important to note
that the percentage of reaction in the interface increases as
W decreases.
From a quantitative point of view the negative deviation
Figure 10. Influence of W on (1+0.105W)/slope [Eq. (17)] for the ami-
nolysis of NPA by piperazine in AOT/isooctane/water microemulsions at
2
58C; [PIP]=0.10m, referred to the aqueous phase of the system. The
line represents the fit of Equation (17) to the experimental results; for
the fit only the values obtained for W>10 have been considered.
observed in the representation of Figure 10 is compatible
i
2
with the decrease of k with W. The deviation between the
value predicted by Equation (17) and the value shown in
i
2
As can be observed there exists a clear linear dependency
for values of W > 10, while there is a negative deviation
from the linearity for W < 10. From the linear adjustment,
Figure 10 is 78%; this indicates that k has decreased in
value by approximately 78%. If we compare this extrapola-
tion with the results of Figure 6 we observe that the rate
constant of aminolysis of the NPA by decylamine in the in-
NPA
NPA
and considering the values of Koi =25.9 and Kwi =996
w
i
obtained previously we can obtain the rate constants k =
terface of AOT-based microemulsions, k , decreases by 79%
2
2
ꢀ
1
ꢀ1
i
2
ꢀ1 ꢀ1
w
2
6.64m
s
and k =0.39m s . The value of k obtained for
on passing from high values of W to W=2.
W > 10 is perfectly compatible with that which is deter-
H2O
2
ꢀ1 ꢀ1
mined in bulk water, k
= 6.09m s . Likewise the value
NPA aminolysis by sarcosine: To analyze the kinetic behav-
ior that is observed in the aminolysis of NPA by amino acids
it is necessary to consider a distribution model for these re-
agents. The amino acids in their ionic form are highly hydro-
philic species, which guarantees their presence in the aque-
ous microdroplet of the microemulsion. The size of the sar-
cosine also allows us to consider the possibility of its incor-
poration into the interface. Judging by the structures shown
in Scheme 1 it would be possible to consider the penetration
of the amino group of the sarcosine in the interface of the
microemulsions, while the carboxylate group can remain in
the aqueous medium. Therefore, to explain the kinetic be-
havior observed in the aminolysis of the NPA by sarcosine
we need to consider an analogous behavior to that proposed
previously for piperazine (Figure 7), where the aminolysis
reaction can take place simultaneously in the interface and
in the microdroplet of the microemulsion.
i
of k obtained for W > 10 shows a 17-fold decrease in the
2
rate constant when passing from bulk water to the interface
of the microemulsion. This reduction of the rate constant is
a consequence of the lower polarity of the interface of the
microemulsion by comparison with the aqueous microdrop-
let, and it is compatible with the decrease in the rate con-
stant in other processes such as nitroso group transfer. It
is important to point out that the ordinate of Figure 10 cor-
responds to the reaction in the interface. As can be ob-
served, in all cases the reaction in the interface is significant
[29]
against the total reaction percentage, due fundamentally to
PIP
the high value of Kwi
.
On the basis of Equation (17) and the results in Figure 10
we can calculate the reaction percentage which takes place
in the interface and in the aqueous microdroplet for each
value of W. For high W values, W=40, approximately 55%
of the reaction takes place in the aqueous microdroplet.
This percentage decreases sharply as the water content of
the system decreases, in such a way that for W=10 this per-
centage has been reduced to 22%. These results show that
for values of W<10 more than 80% of the reaction must
take place in the interface of the AOT-based microemul-
sion.
In the same way as previously, we can obtain the SAR
concentrations in the different microenvironments of the mi-
croemulsion:
SAR
K
½SARŠT
þ W
W½SARŠT
wi
SAR
½
SARŠ ¼
½SARŠw ¼ K
ð18Þ
i
SAR
wi
K
þ W
wi
The results of Figure 10 show a deviation from the behav-
ior predicted by Equation (17) for values of W<10. More-
over the behavior observed for the aminolysis of the NPA
In accordance with the mechanism of Figure 7 and with the
methodology devised previously for the aminolysis of the
Chem. Eur. J. 2005, 11, 4361 – 4373
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4367

L. García-Río et al.
ꢀ
3
w
2
NPA by piperazine, we can obtain a rate equation analogous
to Equation (15), which predicts the existence of a linear de-
sine, 1/slope = 7.50”10 , we can obtain the value of k =
ꢀ
1
ꢀ1
0.85m s ; the obtained value is slightly lower than that ob-
ꢀ ꢀ
pendency between [SAR] /(k [AOT]V V
H2O
H2O
ꢀ1 ꢀ1
AOT) and Z pa-
tained in bulk water, k2
= 1.73m s . This difference in
T
obs
rameter of microemulsions composition. Figure 11 shows the
good fit of Equation (15) to the experimental results.
reactivity may be a consequence of the high ionic concentra-
tion inside the aqueous microdroplet. For a microemulsion
of W=15, the saline concentration in the microdroplet
would be 3.7m if the surfactant were completely dissociated.
This decrease in the aminolysis rate constant with the saline
concentration is consistent with the experimental results ob-
tained in bulk water, where the addition of NaClO causes a
4
decrease in the reaction rate.
The existence of the limit value in the representation of
SAR
Figure 12 allows us to obtain a maximum value of Kwi . I n
order to be able to observe the limit value it is necessary to
SAR
verify the inequality: 1!W/Kwi . Given that this inequality
SAR
wi
is verified for values of W>10, it is necessary for K
ꢂ 1.
SAR
Henceforth we will use the value of Kwi =1, which will
allow us to obtain the corresponding minimum values of k .
Using this maximum value of Kwi we can write an equation
i
2
SAR
Figure 11. Fit of Equation (15) for the aminolysis of the NPA by SAR;
ꢀ
2
[
2
SAR]=5.00”10 m referred to the aqueous phase of the system; T=
58C; *: W=4, *: W=7, and
: W=30.
that is analogous to Equation (17) for the aminolysis of the
NPA by SAR.
&
w
2
NPA
ꢀ
oi AOT
Figure 12 shows the influence of W on 1/slope. As we can
1 þ W
i
2
NPA
oi
k
K
V
ꢀ
¼
k K
V
þ
W
NPA SAR
ð19Þ
H2O
observe the term 1/slope decreases as the water content of
the system increases, tending to reach a limiting value for
W > 10. This tendency to reach a limiting value is the main
qualitative difference between the behavior observed for the
aminolysis of the NPA by piperazine (Figure 9) and by sar-
cosine (Figure 12).
slope
K
K
wi wi
Figure 13 shows the representation of (1 + W)/slope for the
aminolysis NPA by SAR. As in the case of the aminolysis of
NPA by piperazine, a good fit of Equation (19) can be ob-
served for values of W>10. On the basis of the linear ad-
justment and considering the values of K
996, and Kwi = 1) obtained previously we can obtain
the rate constants k = 0.823m
NPA
oi
NPA
= 25.9, Kwi
SAR
=
w
2
ꢀ1 ꢀ1
i
2
s
and k = 2.59”
ꢀ
2
ꢀ1 ꢀ1
1
0 m s . Once again it can be seen that the rate constant
in the interface of the AOT-based microemulsion is approxi-
mately 30 times lower that in the aqueous microdroplet.
This difference is greater than in the aminolysis by pipera-
zine and can be due in part to the statistical advantage that
the piperazine has, through possessing two equivalent nitro-
gen atoms. It is to be expected that this statistical advantage
will increase in the interface of the microemulsion, a region
of restricted mobility.
As in the case of the aminolysis of the NPA by piperazine,
the ordinate of Figure 13 corresponds to the reaction in the
interface. As it was possible to observe, its value is very
small with regard to (1+W)/slope, indicating that the reac-
tion takes place primarily in the aqueous microdroplet. In
this case the low percentage of the reaction in the interface
Figure 12. Influence of W on 1/slope for the aminolysis of NPA by sarco-
sine in AOT/isooctane/water microemulsions at 258C; [SAR]=5.00”
ꢀ
2
1
0 m referred to the aqueous phase of the system.
The existence of this limit value for W > 10 can be ex-
plained by using Equation (16) and considering that
SAR
is due to the low value of Kwi . On the basis of the values
w
2
NPA
oi
k K VAOT
1
SAR
wi
of the ordinate and the slope of Figure 13 we can calculate
the percentage of the reaction which takes place in the inter-
face and in the aqueous microdroplet of the microemulsion.
The percentage of the reaction which takes place in the
aqueous microdroplet for W=40 is 96%, and this value de-
creases to 86% for microemulsions of W=10. Therefore,
unlike what happened in the aminolysis of the NPA by pi-
perazine, the main part of the reaction takes place in the
i
2
NPA
oi
ꢀ
k K
V
H2Oꢁ
W and 1 ꢁ
K
W.
NPA SAR
K
K
wi
wi
w
2
NPA
ꢀ
oi AOT
1
k K
V
In this way it can be shown that
ffi
. I f
NPA
K
wi
slope
NPA
NPA
wi
we use the known values of Koi =25.9 and K
= 996,
ꢀ
3
w
2
we can obtain 1/slope ffi 8.84”10 k . By using the limiting
value of Figure 10 for the aminolysis of the NPA by sarco-
4368
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Water-in-Oil Microemulsions
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Figure 13. Influence of W on (1+W)/slope [Eq. (19)] for the aminolysis
of NPA by sarcosine in AOT/isooctane/water microemulsions at 258C;
w
Figure 14. Influence of W on k2 for the aminolysis of NPA by sarcosine
in AOT/isooctane/water microemulsions at 258C (*); [SAR]=5.00”
m referred to the volume of the aqueous microdroplet. Bimolecular
rate constant obtained in bulk water (*).
ꢀ
2
[
SAR]=5.00”10 m, referred to the aqueous phase of the system; the
ꢀ2
1
0
line represents the fit of Equation (19) to the experimental results. For
the fit only the values obtained for W>10 have been considered.
w
2
aqueous microdroplet. Likewise it is important to note that
for values of W<10 there is no observable negative devia-
tion in the representation of Figure 13, as occurred for the
aminolysis of the NPA by PIP (see Figure 10), but that a
positive deviation occurred. This positive deviation is a con-
the microemulsion, k , decrease as the value of W increases,
tending to reach a limit value for values of W>10. The limit
value reached for W>10 is lower than the value of the rate
constant of aminolysis in bulk water by sarcosine k
H2O
2
=
ꢀ
1
ꢀ1
1.73m s .
i
w
2
sequence of the variations of the rate constants k and k
2
with W. In the first place it is important to note that these
rate constants are independent of W for values of W>10.
This behavior is a consequence of the fact that the greater
changes undergone by the physical properties of the inter-
face and the aqueous microdroplet are observed for small
values of W, and that the aminolysis reaction of the NPA is
not particularly sensitive to the polarity of the medium if it
is compared with other processes such as the solvolysis reac-
tions.
Influence of W on the rate constants in the interface and in
the aqueous microdroplet: The rate limiting step for aminol-
ysis of substituted phenyl acetates in aqueous solution is
first order in amine concentrations and leads to the forma-
ꢃ
tion of a tetrahedral intermediate (T ). As the leaving abili-
ty decreases there is a change towards a rate limiting step
[
27,34]
which is second-order in amine concentration
and the
ꢃ
breakdown of T is the rate limiting step (Scheme 2). Since
the rate limiting step for the aminolysis of the 4-nitrophenyl
ꢃ
Previous studies on the aminolysis of the NPA by DEC
acetate is formation of T the steps subsequent to its forma-
i
2
and PIP show that k decreases together with the water con-
tion would be kinetically silent and thus no effect of crown
ether would be expected; this prediction was experimentally
confirmed for butylaminolysis of 4-nitrophenyl acetate in
tent of the system as a consequence of an inadequate hydra-
tion in the interface. Therefore the positive deviation ob-
served in Figure 13 can only be attributed to a variation of
[27]
acetonitrile.
w
the rate constant k2 with W. Contrary to what occurs with
i
2
w
2
k , our results show that the rate constant k must increase
as the water content of the system decreases. On the basis
of the results in Figure 13, and assuming a maximum value
i
2
ꢀ2 ꢀ1 ꢀ1
w
of k =2.59”10
m s , we can calculate the values of k2
for different values of W. In this approximation we consider
i
a maximum value of k . From the results obtained in the
2
Scheme 2.
aminolysis of the NPA by DEC and PIP it is to be expected
i
that k will decrease together with W. If we use a maximum
2
i
2
w
2
value of k we underestimate the value of k . However this
The existence of this mechanism for the aminolysis of 4-
nitrophenyl acetate in water and acetonitrile suggests that
this reaction mechanism must operate in the aqueous micro-
droplet and the interface of the microemulsion.
When studying the aminolysis of the NPA in AOT-based
microemulsions we found two types of strongly differentiat-
ed behavior: the rate constant of the aminolysis in the aque-
ous microdroplet increases as W decreases, Figure 14, while
the rate constant in the interface increases along with W,
Figure 6. This change in behavior cannot be attributed to
discrepancy must be small given that the greater percentage
of the reaction takes place in the aqueous microdroplet of
the microemulsion. The results obtained are shown in
w
Figure 14. It can be seen that the value of k2 increases as
the water content of the system decreases. It is important to
point out that this variation cannot be justified as a conse-
quence of the reaction in the interface of the microemulsion,
since the reaction constitutes just 5% of the rate observed
for W=4. The rate constants in the aqueous microdroplet of
Chem. Eur. J. 2005, 11, 4361 – 4373
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4369

L. García-Río et al.
different mechanisms, since it has been shown that the ami-
nolysis mechanism of the NPA in water and in the interface
must take place through the formation of an addition inter-
ring attached to the carbonyl group acts as an electron hole
and thus lowers the electron density around the carbonyl
group, thereby decreasing the localization of negative
charge on the carbonyl oxygen atom with concurrent de-
crease in the hydrogen bond acceptor capacity of the transi-
tion state. This will result in an increase of the kDMSO/kEtOH
ꢃ
mediate, T . Therefore, this difference in behavior must be
a consequence of the different properties of the water in the
[9,36]
aqueous and interfacial microdroplet,
and of its different
[39]
capacity for solvation of the reagents and transition states of
the reaction.
value.
Therefore, in the basic hydrolysis of the NPA it seems
that the greatest effect on the reaction rate in aqueous
DMSO must be the desolvation of the hydroxyl ion. The re-
percussion of the desolvation on the reaction or on reactivi-
ty–structure correlations is very wide. In fact, there exist nu-
merous examples in which it has been found that the rate of
certain nucleophile attacks decreases as the basicity of the
nucleophile increases, leading to negative Brønsted expo-
nents. This behavior has been found for some phosphoryl
Aminolysis of NPA by SAR in the aqueous microdroplet of
AOT-based microemulsions: The results obtained on study-
ing the aminolysis of NPA by SAR show that the aminolysis
w
rate constant decreases until it reaches a limit value of k =
2
ꢀ
1
ꢀ1
0
.85m
s
for W>10. This value is lower than the value ob-
H2O
ꢀ1 ꢀ1
tained in pure water, k2
=1.73m s . This deviation of
the value of the rate constant obtained in the aqueous mi-
crodroplet of the AOT-based microemulsions with regard to
bulk water can be explained as a consequence of the high
saline content of the microdroplet of the microemulsion. Ex-
periments carried out in bulk water show that the rate con-
stants of aminolysis of NPA by SAR decrease approximately
[40]
transfer reactions to amines, and for reactions of highly
[41]
reactive carbocations with amines and for reactions of thi-
[42]
olate ions with Fischer carbene complexes.
Likewise
Brønsted exponent values of around zero have been found
[43]
for diphenylketene reactions with amines. The studies car-
[40]
by 30% when the ionic strength (NaClO ) increases to 3m.
ried out by Jencks indicate that these anomalous Brønsted
exponents are a consequence of the partial desolvation re-
quirement of the nucleophile prior to the reaction. It is usu-
ally considered that the desolvation is a preequilibrium
which occurs in a separate stage, in such a way as to adopt a
two-step model for a nucleophilic attack like that which is
shown in Scheme 3.
4
More important is the decrease in the rate constant of the
reaction as W increases. The results obtained in our labora-
tory on studying the influence of the percentage of DMSO
on the aminolysis rate of the NPA by SAR show that the
rate constant of the reaction increases approximately eight
times, when the percentage of DMSO increases, up to 80%.
[37]
In certain reactions S 2 and S Ar it has been shown that
N
N
the effect caused by the aprotic dipolar solvents is a combi-
nation of the solvation of the transition state and the desol-
vation of the nucleophile, especially in the case of the most
basic nucleophiles. A correlation has been observed be-
w
2
w
2
tween k for the aminolysis of NPA by SAR and k for the
[32]
ꢀ
basic hydrolysis of the NPA by OH (see supplementary
material). In both cases the rate constant in the aqueous mi-
crodroplet of AOT-based microemulsions increases as the
water content of the system decreases.
Scheme 3.
[38]
From a perusal of the data obtained previously,
it is
So that the observed effect showen in Figure 14 must be a
consequence of the desolvation of the amine groups of the
amino acid, with the consequent increase in reactivity, and
the lower degree of solvation in the transition state, with the
consequent decrease in the reaction rate. The balance of
both factors will allow us to explain the behavior observed.
From a quantitative point of view the desolvation of the nu-
cleophile seems to be the greater contribution to the de-
crease in the activation energy of the reaction.
clear that the solvation of the non-electrolyte (ester mole-
cule) is not of major importance in causing rate enhance-
ment 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 hydrolysis reactions of esters has a
certain degree of negative charge on the carbonyl oxygen
atom, which confers on this oxygen atom the capacity to be
a good acceptor of the hydrogen bond, in such a way that it
will be solvated better in protic solvents. The transition state
anion in the case of an aromatic ester will be much more po-
larizable than that for an aliphatic ester. Further, the phenyl
Aminolysis of NPA by DEC in the interface of AOT-based
microemulsions: Figure 6 shows the variation of the rate
constant of aminolysis of the NPA by n-decylamine in the
interface of AOT-based microemulsions. This behavior can
be explained as a consequence of the changes in the proper-
ties of the water as W varies. As mentioned previously the
rate determining step of the reaction will be the formation
ꢃ
of the intermediate T . In the transition state of the rate de-
termining step there must be a certain degree of partial neg-
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Chem. Eur. J. 2005, 11, 4361 – 4373

Water-in-Oil Microemulsions
FULL PAPER
ative charge on the oxygen atom of the carbonyl group. The
solvation of this partial negative charge will depend on the
electrophilic character of the water molecules: as the elec-
trophilicity decreases, so too does the reaction rate. This be-
havior has also been shown in the study of the solvolysis re-
1) The aminolysis rate constant of NPA by n-decylamine in
the interface of AOT-based microemulsions increases to-
gether with W. This behavior is parallel with that which
1
is presented by the resonance signals of H NMR of the
protons of water and is similar to that which is observed
for the solvolysis reaction of the diphenylmethyl chlo-
ride. This increase in the reactivity when W increases has
been attributed to an increase in the solvation capacity
of the partial negative charge which is generated in the
transition state leading to the formation of the addition
[25]
action of the diphenylmethyl chloride in AOT-based mi-
croemulsions. The solvolysis rate constant in the interface
decreases together with W due to the fact that the solvation
ꢀ
of the salient Cl also decreases. A correlation have been
i
2
observed between logk for the aminolysis of the NPA by
ꢃ
decylamine and logk for the solvolysis of the diphenylmeth-
intermediate, T .
i
yl chloride (see Supporting Information). The existence of
this correlation shows that the decrease in the aminolysis
rate constant of the NPA by decylamine as W decreases is a
consequence of the small capacity of the interfacial water to
solvate the development of the partial negative charge on
the carbonylic oxygen atom in the transition state for the
2) The aminolysis reaction of the NPA by SAR occurs pre-
dominantly in the aqueous microdroplet of the system
and the rate constant increases when the water content
of the system decreases. This increase is probably a con-
sequence of the desolvation of the nucleophile as the
water content of the system decreases and it is consistent
with the behavior obtained on studying the reaction in
ꢃ
formation of T .
The results obtained are the first kinetic evidence of the
existence of four types of water in the AOT-based micro-
emulsions. In this study we have always used the same reac-
tion: aminolysis of NPA but in such conditions that the reac-
tion occurs in different microenvironments. As the water
mixtures of DMSO/H O.
2
3) The aminolysis of the NPA by piperazine presents an in-
termediate behavior: the reaction occurs simultaneously
in the aqueous microdroplet and the interface, and the
percentage of interfacial reaction increases as W de-
creases. For values of W<10 the reaction occurs mainly
in the interface and the interfacial rate constant decreas-
es along with W, in parallel with the behavior observed
in the aminolysis of the NPA by DEC.
content of the system decreases the strength of the interac-
+
tion Na ···OH increases. This increase causes a decrease in
2
the partial negative charge on the oxygen atom, and, conse-
quently, the strength of the structure of the hydrogen bond
of the water is reduced. This reduction is shown through a
decrease in the solvation of the reagents (with the conse-
quent increase in the reaction rate) and of the transition
state (with the consequent reduction in the reaction rate).
The obtained results show that the solvation of the reagents
exerts a greater effect on the reaction rate that the decreas-
es in the solvation of the partial negative charge on the car-
bonylic oxygen atom in the transition state. When the reac-
4) The diversity of observed behaviors is consistent with
the existence of four types of water in the AOT-based
microemulsions. The water present in the microdroplet
+
is found mainly solvating the cations Na . This solva-
+
tion, Na ···OH , causes a decrease in the negative
2
charge on the oxygen atom and consequently weakens
the hydrogen bond. This weaking in the structure of the
hydrogen bonds causes a decrease in the solvation of the
reagents. The interfacial water is involved fundamentally
tion takes place in the interface of the microemulsion the in-
ꢀ
teraction SO ···HOH causes an increase in the electronic
in the solvation of the head groups of the tensioactives
3
ꢀ
density in the water molecules. This increase in the electron-
ic density causes a decrease in the electrophilic character of
the water molecules as W decreases. This decrease translates
into a smaller capacity for solvation in the interface as the
water content decreases and causes an increase in rate as a
consequence of the solvation of the reagents and a decrease
in rate by the solvation of the transition state of the reaction
as W decreases. The experimental results show that the sol-
vation of the transition state of the reaction is the predomi-
nant factor. The smaller repercussion of the solvation of the
reagents is a consequence of the fact that the insufficient
solvation of the interface causes the reagents to be partially
desolvated even for very high W values.
SO ···HOH. This solvation causes an increase in the
3
electronic density on the hydrogen atoms with the conse-
quent decrease in their efficiency in the solvation of the
partial positive charge that is developed in the transition
state of the reaction.
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 1.00m AOT/isooctane solution in
appropriate proportions. p-Nitrophenyl acetate, sarcosine, piperazine and
n-decylamine (Aldrich) were of the highest available purity and were
used as supplied.
Conclusion
The aminolysis reactions were followed by monitoring the UV/Vis ab-
sorbance of the products of the reaction at l=400 nm, using a HP8453
spectrophotometer fitted with thermostated cell holders. In all experi-
The present study allows us to propose the following conclu-
sions.
ꢀ5
ments the NPA concentration, typically [NPA]=(2–4)”10 m, was much
smaller than that of the amine. The reaction between the NPA and SAR
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L. García-Río et al.
ꢀ
2
was studied using a concentration of amino acid of 5.0”10 m referred to
the volume of the droplet. For these experiments the microemulsions
were prepared using an aqueous solution of amino acid with a concentra-
[24] a) C. Gonzµlez-Blanco, L. J. Rodríguez, M. M. Velµzquez, Langmuir
1997, 13, 1938–1945; b) G.-W. Zhou, G.-Z. Li, W.-J. Chen, Langmuir
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183.
ꢀ
2
tion of 5.00”10 m. An equal concentration of NaOH was added to this
solution to guarantee that the carboxyl group would be deprotonated.
The piperazine solutions were prepared in an aqueous medium, so that
the concentration of piperazine referred to the aqueous microdroplet was
0
.10m. The solutions of DEC were prepared in isooctane and were added
ꢀ
2
to the reaction mixture so that [DEC]=2.50”10 m. In all the experi-
ments the temperature remained constant at (25.0ꢃ0.1)8C. The kinetic
absorbance versus time data always fit the first-order integrated rate
equation satisfactorily (r>0.999); in what follows,
k
obs denotes the
[28] Given the microheterogeneous character of the reaction medium it
is necessary to differentiate between concentrations referred to the
total volume of the system and concentrations referred to the
volume of the phase. We will always use a nomenclature in which
pseudo-first-order rate constant. We were able to reproduce the rate con-
stants with an error margin of ꢃ5%. In all cases we verified that the
final spectrum of the product of the reaction coincided with another ob-
tained in pure water, guaranteeing that the presence of the microemul-
sions did not alter the product of the reaction. A compilation of the ki-
netic data is supplied as Supporting Information.
w
i
o
[S] , [S] and [S] denote the concentrations of the substrate in the
w
i
o
aqueous microdroplet referred to the volume of the aqueous micro-
droplet; the substrate concentration in the interface referred to the
volume of the interface and substrate concentration in the continu-
ous medium referred to the volume of the continuous medium. The
concentrations written with just a subindex will always be referred
to the total volume of the system: [S] , [S] and [S] correspond to
w
i
o
the substrate concentrations in the aqueous microdroplet, the inter-
face and the continuous medium respectively, referred in all cases to
the total volume of the system.
Acknowledgements
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[
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Received: October 20, 2004
Revised: February 23, 2005
Published online: May 10, 2005
Chem. Eur. J. 2005, 11, 4361 – 4373
www.chemeurj.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4373