Influence of the oil on the properties of microemulsions as reaction media

Article abstract of DOI:10.1002/ejoc.200600086

The influence of the nature of the continuous medium on various properties of a wide range of water-in-oil (w/o) microemulsions was examined. ¹H NMR spectroscopy allowed to determine the properties of water in the studied systems and the way th

Full text of DOI:10.1002/ejoc.200600086

FULL PAPER
DOI: 10.1002/ejoc.200600086
Influence of the Oil on the Properties of Microemulsions as Reaction Media
[a]
[a]
[a]
Luis García-Río,* Ana Godoy, and Pedro Rodríguez-Dafonte
Keywords: Surfactants / Solvolysis / Kinetics / Microemulsion
The influence of the nature of the continuous medium on
various properties of a wide range of water-in-oil (w/o) mi-
croemulsions was examined. H NMR spectroscopy allowed
measure of polarity changes at the microemulsion interface.
The kinetic effects of the microemulsion composition on the
solvolysis of anisoyl chloride were studied, and the reagent
was found to react with water at the microemulsion interface
alone. Based on both kinetic and NMR results, the solvolysis
rate constants of anisoyl chloride decrease with increasing
penetration of the oil into the interface. Also, the resonance
signals for the water H atoms were found to change in paral-
lel with the solvolysis rate constant for anisoyl chloride.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
1
to determine the properties of water in the studied systems
and the way they are affected by the solvent to be elucidated.
Changes in interfacial polarity were examined from the 13
C
NMR signals of the surfactant molecule sodium bis(2-eth-
ylhexyl)sulfosuccinate (AOT). Chemical shifts were found to
vary with the water content of the microemulsion. The varia-
tion of the carbon chemical shift as a function of the water-
to-surfactant concentration ratio (W) was used as an indirect
Introduction
lets was assumed to be the major factor limiting microemul-
sion stability. An increase in molar mass of the hydrocarbon
oil was found to decrease the density of the continuous
phase and facilitate access of the interacting droplets for
mass transfer. An increase in droplet radius should also in-
crease the overlap volume between two droplets as they ap-
proach each other to form a cluster, thereby increasing the
interaction potential and facilitating percolation. While
droplet attraction plays a prominent role in percolation, the
nature of the droplet interface is also relevant to the process
because mass transfer is impossible unless the interface is
ruptured in some way.
Microemulsions are transparent, thermodynamically
stable solutions of low viscosity that form spontaneously
from mixtures of a surfactant, water, and oil.[1] A water-in-
oil (w/o) microemulsion is a solution of water droplets of
nanometer size dispersed in a nonpolar solvent with the aid
of a surfactant. This type of microemulsion has been widely
used as a microreactor for enzymatic reactions, tertiary oil
recovery, and nanomaterial production.[
2–5]
The surfactant
most widely used in microemulsions is sodium bis(2-eth-
ylhexyl)sulfosuccinate (AOT). Sulfonate head groups are
hydrophilic and tend to point to the microemulsion core,
whereas hydrocarbon chains are hydrophobic and spread
into the continuous phase. The properties of a microemul-
sion depend on the type of surfactant used, the water-to-
surfactant concentration ratio, W, and the properties of the
The characteristics of microemulsion aggregates, and
hence their ability to incorporate solutes, can be extremely
[25–30]
dependent on the particular external solvent.
The
most extensive comparative study in this context involved
AOT/hydrocarbon/water and AOT/benzene/water micro-
oil. Microemulsion properties have been widely studied in
[31]
emulsions
and showed clustering of benzene molecules
relation to various surfactant types;[
6–11]
however, the influ-
around AOT head groups to be seemingly more orderly
ence of the nonpolar continuous medium has scarcely been
examined, and the few studies on it have led to contradic-
tory conclusions.
[32]
1
[33]
than in saturated alkanes. However, H NMR studies
have shown AOT/benzene microemulsions to contain some
free water, even at low W values. The previous findings are
consistent with a specific interaction between the polar
head groups in AOT and the more polarizable solvents:
chlorobenzene, tetrachloromethane, and benzene. Based on
[
12–19]
Temperature-induced dynamic percolation of water-in-oil
microemulsions is known to depend on the properties of
the bulk solvent.[20] By using n-heptane, n-octane, and n-
decane as continuous media, Moulik and co-workers found
the percolation temperature to decrease with increasing
length of the alkyl chain in the hydrocarbon. In earlier re-
13
[12]
C NMR spectroscopic data,
benzene and tetrachloro-
methane were assumed to intercalate between AOT chains;
with cyclohexane, however, voids in the surfactant/oil inter-
face were assumed to be occupied by surfactant chains. Sol-
vent penetration was found to decrease in the following se-
[
21–24]
ports,
the strong attractive interaction between drop-
[
a] Departamento de Química Física, Facultade de Química, Uni-
quence: C H Ͼ CCl Ͼ C H . Similar conclusions were
versidade de Santiago de Compostela,
6
6
4
6
12
[
19]
15782 Santiago de Compostela
drawn regarding the solution enthalpies of water.
3364
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Influence of the Oil on the Properties of Microemulsions as Reaction Media
FULL PAPER
A variety of experimental techniques have confirmed NMR time scale. The position of the NMR signal may be
that the bulk solvent influences the size and structure of of help with a view to estimating the contribution of each
[
34,35]
microemulsions.
In this work, we conducted a system- type of water to the overall signal.
At low W values, most of the water present interacts with
atic study of the influence of the continuous medium on the
+
properties of AOT-based microemulsions involving various Na ions and the polar groups of AOT. Hydration of the
solvents, including alkanes (n-heptane, isooctane, and n-do- anionic head groups in AOT molecules increases the elec-
decane), cycloalkanes (cyclopentane, cyclohexane, and cy- tron density on the H atoms in water molecules and shifts
cloheptane), aromatic hydrocarbons (toluene as well as o-, the water signal upfield. As the water content increases, the
m- and p-xylene), trichloromethane and tetrachlorometh- system contains a gradually larger amount of free water in
ane. The microheterogeneous systems were characterized the microemulsion droplets that are available for hydrogen
from kinetic and NMR spectroscopic data. H NMR and bonding. This decreases the electron density on the H
C NMR spectroscopy allowed us to elucidate the proper- atoms and shifts the signal for free water downfield (i.e. to
ties of water and the AOT molecule,
the microemulsions. The latter technique provided infor-
1
1
3
[
36]
[19,37]
respectively, in higher δ values) relative to bonded water.
As the water content of the microemulsion continues to
mation about the local environment of each individual nu- grow, the number of bonded molecules increases and so
cleus. The polarity of the interface was determined by using does, especially markedly, that of free water. On the other
a chemical reaction as a probe; the reaction concerned was hand, the amount of trapped water at the interface remains
[
15]
the solvolysis of anisoyl chloride (Scheme 1), for which the constant.
rate constant is strongly dependent on the solvent polarity. changes from δ Ϸ 4.0 at low W values to levels close to that
As a result, the chemical shift for water
water
By virtue of its hydrophobic nature, anisoyl chloride can be for water (δ
Ϸ 4.8) for high W values.
expected to partition between the continuous medium and All solvents exhibited a similar behaviour, but also sub-
the microemulsion interface where the reaction takes place. stantial differences among them. Thus, the solvent mole-
The influence of the continuous medium on the reaction cules that penetrated deeply into the interface established
rate can be used as a measure of the degree of oil penetra- interactions with the head groups of AOT and decreased
[
17]
tion into the interface. The use of this reaction as a chemi- droplet size. As droplet size decreased, so did the number
cal probe instead of a typical spectroscopic probe has the of free water molecules in the microemulsions. Thus, at a
advantage that the probe is small enough to avoid substan- given W value, the deeper the oil penetrated into the inter-
1
tial alteration of the interface.
face, the lower was the H NMR chemical shift for the
water molecules. The results shown in Figure 1 (top) sug-
gest that the degree of oil penetration into the interface in-
creased in the following sequence: n-heptane Ͻ n-dodecane
Ϸ isooctane for aliphatic hydrocarbons and cycloheptane
Ͻ cyclohexane Ͻ cyclopentane [see Figure 1 (centre)] for
cyclic hydrocarbons.
These results are consistent with those for the interdrop-
let attractive interaction in water/AOT/oil microemulsions,
which was found to increase with increasing chain length
of the oil. An increased molecular mass of the hydrocarbon
decreased the density of the continuous phase, thereby facil-
itating access of the interacting droplets and hence mass
Scheme 1.
[
38]
transfer. Robinson et al.
studied the rate of micellar ex-
change, k , in water/AOT/alkane systems and found it to
ex
increase slightly with increasing number of carbon atoms
from n-pentane to n-dodecane; also, they found kex to be
significantly smaller for the corresponding cyclic alkanes
[
39]
(
particularly cyclohexane). Binks et al. discussed the ex-
Results and Discussion
tent of oil penetration into the surfactant chain region and
found micelle rigidity to increase with decreasing solvent
1
[40]
1
. Influence of the Oil on the H NMR Resonance Signal
chain length from C14 to C . Theoretical studies account
7
for the Water H Atoms
for the dependence of absorption on alkane chain length.
In microemulsions, penetration expands the spontaneous
1
The H NMR spectrum for the water/AOT/solvent sys- curvature of the surfactant layer to reduce the micelle ra-
tem was used to elucidate the types of water present in the dius. The limit of penetration of the solvent molecules into
microemulsion: water trapped between the surfactant alkyl the surfactant layer has been estimated to be in the region
chains, water bonded to head groups, free water, and water of 9.5 carbon atoms, above which nonpolar solvent mole-
bonded to the counterion (sodium). The NMR spectro- cules can hardly be expected to penetrate surfactant lay-
[
17]
[41]
scopic data provided an average value for the four types of ers.
However, Keh and Valeur
have shown that the
water as their rate of exchange is much higher than the characteristics of AOT micelles appear not to be altered in
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L. García-Río, A. Godoy, P. Rodríguez-Dafonte
FULL PAPER
the water H atoms suggest that penetration is slightly
deeper with o-xylene than with the other aromatic solvents.
Penetration is also more marked in CCl and CHCl . This
4
3
results contradict the increased percolation temperature
found in xylenes and toluene as the continuous media for
[
20]
AOT-based microemulsions.
2
. Influence of the Oil on the 13C NMR Signals for AOT
In characterizing microemulsions, the 13C NMR tech-
nique has the advantage that it requires the use of no probe
molecule, and hence avoids potential alteration of the target
system. Also, the carbon spectrum provides valuable infor-
[37]
mation about the environment of the AOT molecules.
By analyzing the 13C NMR spectrum as a function of
the water content of the microemulsion one can assess envi-
ronmental changes (i.e. changes at the interface) as a func-
tion of the microemulsion droplet size. The addition of
water has a slight effect on the chemical shifts for the car-
bon atoms C4, C4Ј, C10, and C10Ј (see Scheme 2 for num-
bering). The result is always an upfield shift. In any case,
the most substantial changes are those in the region of the
head group. Thus, C1 is influenced by the hydration of the
head group: once completed, δC1 levels off. The carbonyl
group at C2Ј is highly sensitive to the nature of the local
environment. Thus, the chemical shift for C2Ј decreases
with increasing W, which suggests an increased polarity of
the medium. Figure 2 shows the variation of the chemical
shift with W.
Figure 1. Variation of the chemical shift for the water H atoms as
a function of W in water/AOT/oil microemulsions at 25.0 °C.
the n-alkane series from n-hexane to n-dodecane, and only
slightly altered in various nonpolar solvents. On the other
hand, studies based on synchrotron radiation small-angle
X-ray scattering spectroscopy have shown that the penetra-
tion of solvent molecules into the micelle surfactant layer
depends strongly on the length of the linear hydrocarbon
Scheme 2.
Maitra et al. examined the aggregational behaviour of
AOT in nonpolar solvents and the uses of these systems in
[
17]
chain for n-hexane, n-heptane, n-octane, and isooctane.
[
42]
Highly polarizable solvents such as benzene, toluene, and various areas. The AOT molecule can occur in two major
tetrachloromethane penetrate into the amphiphilic layer conformations, namely: trans (II) and gauche (I) (see
presumably up to the water core boundary. As can be seen Scheme 3). As the water content increases, so does the pro-
from Figure 1 (bottom), the resonance signals for the water portion of the trans isomer, leading to an increase in the
H atoms in toluene, m-xylene, and p-xylene microemulsions surfactant–water contact area at the interface. Conforma-
are indistinguishable. The δ values are all smaller than those tional changes in AOT are a result of polarity changes.
for n-heptane microemulsions, which is consistent with in-
The influence of organic molecules on the properties of
creasing penetration into the oil. The resonance signals for the interface can be ascribed to two different effects: (a)
3366
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Influence of the Oil on the Properties of Microemulsions as Reaction Media
FULL PAPER
A comparison of the results for n-heptane, isooctane, and
n-dodecane microemulsions reveals that the proportion of
trans isomer increases with increasing length of the hydro-
carbon chain. This is a result of increased penetration of
the oil into the interface and hence of an increased distance
between AOT molecules (see Scheme 4). The increased in-
terfacial volume results in closer contact between water
molecules and AOT head groups, which favours the trans
conformer. In fact, oil penetration increased in the sequence
n-heptane Ͻ isooctane Ͻ n-dodecane. Also, the branched
nature of the isooctane molecule results in a large molecular
volume and hence in substantially low δC2Ј values relative
to n-heptane microemulsions.
1
The influence of W and the oil on the H NMR chemical
shifts for the water H atoms and the 13C NMR shifts for
the carbon atom C2Ј are quite consistent. Thus, increasing
penetration of the oil into the interface resulted in increas-
ing separation between AOT molecules, which thus occu-
pied an increased volume and facilitated the penetration of
water molecules into the interface. The increased penetra-
tion resulted in stronger interactions between AOT head
–
groups and water molecules (SO ···HOH), and the
3
stronger interactions shifted δ for the water H atoms upfield
as oil penetration increased.
A comparison of the results obtained in cyclohexane and
cyclopentane (see Figure 2) as oils reveals an increased pre-
valence of the trans isomer relative to n-heptane microemul-
sions. This is seemingly abnormal, as cyclic hydrocarbons
such as cyclopentane and cyclohexane are known to pen-
etrate less deeply into the interface than do linear hydro-
1
carbons. This apparent anomaly is also observed in the H
NMR spectra for the water H atoms (Figure 1). Thus, the
δ values for the H atoms in cyclohexane microemulsions
were lower than those in n-heptane microemulsions. This
can be ascribed to the increased interfacial volume occupied
by cyclohexane relative to n-heptane.
The solvents most deeply penetrating into the interface,
viz. aromatic hydrocarbons and chlorinated hydrocarbons
(
CCl and CHCl ), interact with the ester group in AOT
4 3
Figure 2. Variation of the chemical shift for the carbon atom in the
carbonyl group (C2Ј) as a function of W in water/AOT/oil micro-
emulsions at 25.0 °C.
and increase the population of trans isomer. The resulting
δC2Ј values were always lower than those obtained in n-hep-
tane microemulsions. The results of Figure 2 are consistent
with the influence of the oil on the chemical shift for the
inclusion into the surfactant film and (b) replacement of water H atoms. Thus, all aromatic hydrocarbons exerted a
water molecules at the interface. Also, the incorporation of similar effect and halogenated hydrocarbons exhibited in-
the oil into the surfactant film alters its geometry in two creased penetration.
ways: (i) the additive acts as a spacer within the surfactant
monolayer and increases the effective area of the polar head
groups in AOT; and (ii) the binding of additives to the sur-
factant film increases disorder in the interfacial region,
3
. Influence of the Oil on the Solvolysis of Anisoyl Chloride
(AnCl)
thereby decreasing the rigidity of the AOT film and increas- The bulk solvent is known to affect various micellar proper-
[
44]
ing its deformability. The influence of additives on the re- ties, including the critical micelle concentration, the size
[
45]
placement of water molecules at the interface is probably a of w/o microemulsions, and solvent penetration into the
[
12]
result of their ability to displace those in the vicinity of interface.
As a result, the properties of water molecules
the polar area in the surfactant film. The results of some dissolved in the core can be influenced by the nature of the
[
43]
suggest that moderate concentrations of various solvent. Velázquez et al.[46] examined the influence of the
studies
organic molecules can “open up” the interface and facilitate solvent on the water-entrapped structure. Their IR spectro-
the penetration of water.
scopic results suggest that the water structure of AOT mi-
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L. García-Río, A. Godoy, P. Rodríguez-Dafonte
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Scheme 3.
function of their solubility. Anisoyl chloride is virtually in-
soluble in water, so it will only be present in the continuous
medium and the microemulsion interface (see Scheme 5),
where the reaction takes place. Figure 3 shows the variation
of the solvolysis rate constant with the AOT concentration
at constant W values of 18 and 40. As can be seen, kobs
increases with increasing AOT concentration as a result of
the incorporation of AnCl from the continuous medium
Scheme 4.
croemulsions dissolved in cyclohexane and isooctane with
variable water contents is quite similar. However, there are
significant differences from the water structure of micro-
emulsions in toluene.[ These results suggest that the water
interfacial layer is smaller in AOT microemulsions dissolved
in cyclohexane or isooctane than in those dissolved in tolu-
ene. This may be a result of toluene penetrating the interfa-
cial layer more readily[ than isooctane and cyclohexane.
Similar preferential penetration by an aromatic solvent has
been observed in Triton X-100 microemulsions.[ The kin-
etic implications of these changes in the properties of in-
terfacial water with the nature of the oil were examined by
studying the solvolysis of anisoyl chloride (AnCl). This re-
action involves a dissociation mechanism, and its rate is
47]
Scheme 5.
12]
48]
governed by the solvation capacity of the leaving group
–
(
Cl ), which makes it highly sensitive to the polarity of the
medium and hence a suitable probe for the electrophilicity
of water in microemulsions.
The experimental results were analysed in the light of the
micellar pseudophase model, which assumes a microemul-
sion to comprise three distinct domains: the aqueous pseu-
dophase, the AOT film, and the solvent.[49] The reactants
Figure 3. Influence of the AOT concentration on kobs for solvolysis
of anisoyl chloride in water/AOT/n-heptane microemulsions at W
and products will partition into the microemulsion as a = 18 (᭺) and W = 40 (᭹). T = 25.0 °C.
3
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Influence of the Oil on the Properties of Microemulsions as Reaction Media
FULL PAPER
into the interface. Also, it increased with increasing water pendent W values. In all cases, Koi values show that they
content due to an increase in the interfacial polarity.
Application of the pseudophase model (Scheme 5) leads
to Equation (1) for the observed rate constant.[
are W-independent.
K allows one to calculate the actual rate constant at the
oi
50]
microemulsion interface, using Equation (3).
(
1)
(3)
where Koi is the partition constant of AnCl between the
continuous medium and the interface, Koi = ([AnCl]i/ order to minimize potential sources of error arising from
The value of k was calculated from average K values in
i
oi
[
(
AnCl] )Z, with Z being the [oil]/[AOT] mol ratio. Equation uncertainty in the determination of the intercepts of Fig-
0
1) can be rewritten as Equation (2).
ure 4. The average Koi values used were 6.2 for cyclic and
alicyclic hydrocarbons, 1.4 for aromatic hydrocarbons, 5.0
for CCl , and 2.8 for CHCl . The interfacial rate constants
(
2)
4
3
thus obtained at variable W values are shown in Figure 5.
We examined the variation of k with the water content
i
As can be seen in Figure 4, the experimental data fit
equation (2) quite well. The ratio of the slope to the inter-
of the microemulsion in various continuous media. Anisoyl
chloride is a benzoyl chloride bearing an electron-releasing
substituent. This facilitates its heterolytic cleavage and the
formation of an acylium intermediate via a dissociation
pathway. The reaction rate must decrease with decreasing
water content in the microemulsion, i.e., with decreasing
content in interfacial water, which effects the solvation of
the AOT head groups and reduces the ability to solvate the
cept allowed us to determine K . Based on the intercepts
oi
and slopes of Figure 4, the obtained partition constants
were 4.3± 1.0 at W = 18 and 5.9± 0.8 at W = 40. As found
[
50]
in previous work, Koi should be independent of W.
In
fact, the partition constant was independent of W in all
studied media. The values obtained can be classified into
three large groups depending on the nature of the oil, na-
mely: (a) cyclic and alicyclic hydrocarbons, (b) aromatic hy-
–
Cl leaving group. This relates the interfacial rate constant
with the polarity: the higher is the electrophilicity of the
water, the higher will be the reaction rate.
drocarbons, and (c) halogenated hydrocarbons. The K val-
oi
ues obtained for the oils in group (a) (viz. 5.0, 5.9, 7.0, 6.0,
Based on the results of Figure 5, the reaction rate in the
alicyclic hydrocarbons decreases with increasing penetra-
tion of the hydrocarbon. An increased penetration of the
oil increases the distance between AOT molecules and facil-
itates interactions between water molecules at the interface
and the surfactant head groups as a result. The interactions
decrease the electrophilicity of the water and hence the reac-
tion rate. It should be noted that the rate of this reaction is
highly sensitive to the water content in the microemulsion;
thus, it decreased by more than 300 times from W = 50 to
W = 2 in water/AOT/n-heptane microemulsions. On the
other hand, it decreased by a factor less than 2 between n-
heptane, isooctane, and n-dodecane microemulsions. A
comparison of the n-heptane microemulsions with the cy-
clopentane, cyclohexane, and cycloheptane microemulsions
reveals a decreased reaction rate in the cyclic hydrocarbons.
6
.2, and 7.0 for n-heptane, isooctane, n-dodecane, cyclopen-
tane, cyclohexane, and cycloheptane, respectively) show
that AnCl partitions in a manner independent of the extent
of oil penetration into the interface. The values for the oils
in group (b) (viz. 2.5, 1.0, 0.9, and 1.2 for toluene, p-xylene,
m-xylene, and o-xylene, respectively) were lower than those
for the hydrocarbons in the previous group as a result of
their increased polarity and hence of the decreased polarity
difference between the continuous medium and the micro-
emulsion interface. Finally, the Koi values for CCl4 and
CHCl were 5.0 and 2.8, respectively. These partition coeffi-
cients are the mean values of those obtained at two inde-
3
1
This is consistent with the H NMR results and with de-
creased penetration of the oil, but also with an increased
volume resulting from the bulkiness of the cyclic hydro-
carbons relative to linear ones.
A comparison of the microemulsions formed by the aro-
matic and halogenated hydrocarbons with the n-heptane
microemulsions reveals an increased rate in the latter. This
is consistent with increased penetration of the oil (an aro-
matic hydrocarbon) into the interface and hence with
stronger interactions between interfacial water and the sur-
factant head groups. Such interactions hinder solvation of
the leaving group in the solvolysis of anisoyl chloride. It
should be noted that solvolysis in the microemulsions of
halogenated hydrocarbons was faster than expected by vir-
tue of their increased interfacial penetration, as confirmed
Figure 4. Plot of 1/kobs as a function of Z for solvolysis of anisoyl
chloride in water/AOT/n-heptane microemulsions at W = 18 (᭺)
and W = 40 (᭹). T = 25.0 °C.
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L. García-Río, A. Godoy, P. Rodríguez-Dafonte
FULL PAPER
of the ability of trichloromethane to stabilize the leaving
group in the solvolysis of anisoyl chloride through hydrogen
bonding. Note the good correlation for a variety of con-
tinuous media including alicyclic, cyclic, and aromatic hy-
drocarbons spanning a wide range of penetration into the
microemulsion interface.
Figure 6. Correlation between the solvolysis rate constants for ani-
1
soyl chloride in water/AOT/oil microemulsions and the H NMR
resonance signals for the water H atoms in (filled circle) microemul-
sions consisting of n-heptane, isooctane, n-dodecane, cyclopentane,
cyclohexane, cycloheptane, toluene, o-xylene, m-xylene, or p-xylene
as oils; (open circle) water/AOT/CCl
4
microemulsions; and (filled
triangle) water/AOT/CHCl
3
microemulsions.
Conclusions
Nuclear magnetic resonance spectroscopy is an effective
tool for studying the influence of the continuous medium
1
on the properties of microemulsions. In this work, H NMR
spectroscopy allowed the properties of water in microemul-
sions to be elucidated, and revealed that the amount of free
water decreases with increasing penetration of the hydro-
carbon. Also, 13C NMR spectroscopy allowed the polarity
at the microemulsion interface to be interpreted in the light
of the signals for the carbonyl C2Ј in AOT; in fact, the
polarity was found to increase with increasing penetration
of the oil into the interface. This was a result of the volume
available for AOT molecules−and hence interactions be-
tween water and the surfactant head groups−increasing
Figure 5. Variation of the solvolysis rate constant for anisoyl chlo-
ride as a function of W in water/AOT/oil microemulsions at
25.0 °C.
by the H NMR and 13C NMR results. The increased pene- with increasing penetration of the oil. The kinetic analysis
tration relative to aromatic hydrocarbons should have re- of the solvolysis of anisoyl chloride revealed that the reac-
sulted in a decreased reaction rate; however, the results of tion rate decreases with increasing the oil penetration into
Figure 5 show that the solvolysis rate was virtually identical the interface. This behaviour is a consequence of an in-
in xylene, CCl , and CHCl microemulsions. This anomaly crease in the electrophilicity of the interfacial water through
1
4
3
is a result of the increased polarity of halogenated hydro- interaction with the surfactant head groups.
carbons relative to n-heptane as shown by the values of the
[51]
polarity parameter E (30).
T
Figure 6 testifies to the good correlation between the sol- Experimental Section
volysis rate constant for anisoyl chloride and the chemical
shifts for the water H atoms. As can be clearly seen, the
solvolysis rate in the trichloromethane and tetrachloro-
AOT was obtained from Aldrich and used without further purifica-
tion following drying in a vacuum desiccator for two days. All sol-
vents were obtained from Aldrich in the highest commercially avail-
methane microemulsions was greater than expected from able purity. AOT solutions were prepared by direct weighing. Stock
the chemical shifts for the water H atoms. This was a result solutions containing a 0.5  concentration of the surfactant in tri-
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Eur. J. Org. Chem. 2006, 3364–3371

Influence of the Oil on the Properties of Microemulsions as Reaction Media
FULL PAPER
chloromethane and tetrachloromethane, and 1  in all other sol-
vents, were used.
[20] S. Ray, S. Paul, S. P. Moulik, J. Colloid Interface Sci. 1996, 183,
–12.
21] M. J. Hou, M. Kim, D. O. Shah, J. Colloid Interface Sci. 1988,
23, 398–412.
22] A. Jada, J. Lang, R. Zana, J. Phys. Chem. 1989, 93, 10–12.
A. Jada, J. Lang, R. Zana, J. Phys. Chem. 1990, 94, 381–387.
[24] P. Alexandridis, J. F. Holzwarth, T. A. Hatton, J. Phys. Chem.
6
[
[
NMR spectra were recorded with the aid of a coaxial tube filled
1
with [D
tetramethylsilane were used as H NMR references, and those of
]DMSO as reference for the 13C NMR spectra. All spectra were
6
]DMSO to lock on the deuterium signal. The signals of
1
[23]
[D
6
recorded with a Bruker AM 500 MHz spectrometer.
1995, 99, 8222–8232.
[
[
[
25] E. Abuin, E. Lissi, R. Duarte, J. J. Silber, M. A. Biasutti, Lang-
Anisoyl chloride was obtained from Aldrich in 99% purity and
used to prepare 5×10–3  stock solutions by direct weighing that
were used in the kinetic study. The concentration of anisoyl chlo-
ride in the microemulsion used was 1.0×10–4 . The reaction was
monitored by measuring the UV–Vis absorbance of the products
at 295 nm on a Cary 500 Scan UV–Vis–NIR spectrophotometer.
Absorbance-time data were fitted to a first-order integral equation
in order to obtain the pseudo-first-order rate constant, kobs, with a
reproducibility of 5%.
muir 2002, 18, 8340–8344.
26] N. M. Correa, E. N. Durantini, J. J. Silber, J. Colloid Interface
Sci. 2001, 240, 573–580.
27] R. D. Falcone, N. M. Correa, M. A. Biasutti, J. J. Silber, Lang-
muir 2000, 16, 3070–3076.
[28] P. Lopez-Cornejo, S. M. B. Costa, Langmuir 1998, 14, 2042–
2049.
[
29] R. D. Falcone, N. M. Correa, M. A. Biasutti, J. J. Silber, Mole-
cules 2000, 5, 553–554.
[
[
30] N. Z. Atay, B. H. Robinson, Langmuir 1999, 15, 5056–5064.
31] N. M. Correa, M. A. Biasutti, J. J. Silber, J. Colloid Interface
Sci. 1995, 172, 71–76.
Acknowledgments
[
32] K. Mukherjee, S. P. Moulik, D. C. Mukherjee, Langmuir 1993,
9, 1727–1730.
Funding by Xunta de Galicia (PGIDT03-PXIC20905PN and PGI-
DIT04TMT209003PR) and Spain’s Ministry of Science and Tech-
nology (Project BQU2002–01184) is gratefully acknowledged. A.
G. thanks Xunta de Galicia for a predoctoral fellowship.
[
[
33] F. Heatley, J. Chem. Soc., Faraday Trans. 1 1988, 84, 343–354.
34] R. A. Day, B. H. Robinson, J. H. R. Clarke, J. V. Doherty, J.
Chem. Soc., Faraday Trans. 1 1979, 75, 132–139.
35] M. Ueda, Z. A. Schelly, Langmuir 1988, 4, 653–655.
36] a) A. N. Maitra, H. F. Eicke, J. Phys. Chem. 1981, 85, 2687–
2691; b) A. Maitra, J. Phys. Chem. 1984, 88, 5122–5125; c) A.
Llor, P. Rigny, J. Am. Chem. Soc. 1986, 108, 7533–7541; d) M.
Wong, J. K. Thomas, T. Nowak, J. Am. Chem. Soc. 1977, 99,
4730–4736.
[37] a) M. Ueno, H. Kishimoto, Y. Kyogoku, Bull. Chem. Soc. Jpn.
1976, 49, 1776–1779; b) M. Ueno, H. Kishimoto, Y. Kyogoku,
J. Colloid Interface Sci. 1978, 63, 113–119; c) M. Ueno, H.
Kishimoto, J. Phys. Chem. 1983, 87, 850–856.
[38] P. D. I. Fletcher, A. M. Howe, B. H. Robinson, J. Chem. Soc.,
Faraday Trans. 1 1987, 83, 985–1006.
[
[
[
[
1] M. P. Pileni, Editor. Studies in Physical and Theoretical Chemis-
try 65: Structure and Reactivity in Reverse Micelles, Elsevier,
Amsterdam, 1989.
2] N. Krieger, M. A. Taipa, E. H. M. Melo, J. L. Lima-Filho,
M. R. Aires-Barros, J. M. S. Cabral, Appl. Biochem. Biotechnol.
1
997, 67, 87–95.
[
[
3] M. P. Pileni, J. Phys. Chem. 1993, 97, 6961–6973.
4] F. Debuigne, L. Jeunieau, M. Wiame, J. B. Nagy, Langmuir
2
000, 16, 7605–7611.
[
[
[
5] P. Walde, Q. Mao, R. Bru, P. L. Luisi, R. Kuboi, Pure Appl.
Chem. 1992, 64, 1771–1775.
6] E. Caponetti, A. Lizzio, R. Triolo, W. L. Griffith, J. S. Johnson,
Langmuir 1992, 8, 1554–1562.
[39] B. P. Binks, H. Kellay, J. Meunier, Europhys. Lett. 1991, 16, 53–
58.
[40] D. W. R. Gruen, D. A. Haydon, Pure Appl. Chem. 1980, 52,
7] A. L. Compere, W. L. Griffith, J. S. Johnson Jr, E. Caponetti,
D. Chillura-Martino, R. Triolo, J. Phys. Chem. B 1997, 101,
1229–1240.
[41] E. Keh, B. Valeur, J. Colloid Interface Sci. 1981, 79, 465–478.
[42] T. K. De, A. Maitra, Adv. Colloid Interface Sci. 1995, 59, 95–
193.
7
139–7146.
[8] S. M. Andrade, S. M. B. Costa, Photochem. Photobiol. Sci.
2
002, 1, 500–506.
[43] Y. S. Kang, H. J. D. McManus, L. Kevan, J. Phys. Chem. 1992,
[
[
9] T. C. Jao, W. S. Joyce, Langmuir 1990, 6, 944–948.
96, 10055–10060.
10] P. Menassa, T. Lussier, C. Sandorfy, Can. J. Chem. 1986, 64,
[44] S. Muto, K. Meguro, Bull. Chem. Soc. Jpn. 1973, 46, 1316–
1320.
6
33–634.
[
11] O. A. El Seoud, M. J. Da Silva, L. P. Barbur, A. Martins, J.
[45] M. Zulauf, H. F. Eicke, J. Phys. Chem. 1979, 83, 480–486.
[46] C. Gonzalez-Blanco, L. J. Rodriguez, M. M. Velásquez, J. Col-
loid Interface Sci. 1999, 211, 380–386.
Chem. Soc., Perkin Trans. 2 1978, 331–335.
[
[
12] C. A. Martin, L. J. Magid, J. Phys. Chem. 1981, 85, 3938–3944.
13] L. J. Magid, K. Konno, C. A. Martin, J. Phys. Chem. 1981, 85,
[47] C. Gonzalez-Blanco, L. J. Rodríguez, M. M. Velázquez, Lang-
muir 1997, 13, 1938–1945.
1434–1439.
[
[
[
[
[
[
14] N. M. Correa, E. N. Durantini, J. J. Silber, J. Colloid Interface
[48] D. M. Zhu, X. Wu, Z. A. Schelly, J. Phys. Chem. 1992, 96,
Sci. 1998, 208, 96–103.
7121–7126.
15] G.-W. Zhou, G.-Z. Li, W.-J. Chen, Langmuir 2002, 18, 4566–
[49] L. García-Río, J. R. Leis, M. E. Peña, E. Iglesias, J. Phys.
Chem. 1993, 97, 3437–3442.
[50] a) L. García-Río, J. R. Leis, Chem. Commun. 2000, 455–456;
b) L. García-Río, J. R. Leis, J. A. Moreira, J. Am. Chem. Soc.
2000, 122, 10325–10334; c) L. García-Río, J. R. Leis, J. C. Me-
juto, Langmuir 2003, 19, 3190–3197.
4571.
16] P. R. Majhi, S. P. Moulik, J. Phys. Chem. B 1999, 103, 5977–
5983.
17] M. Hirai, R. Kawai-Hirai, M. Sanada, H. Iwase, S. Mitsuya,
J. Phys. Chem. B 1999, 103, 9658–9662.
18] S. K. Hait, A. Sanyal, S. P. Moulik, J. Phys. Chem. B 2002,
[51] C. Reichardt, Solvents and Solvent Effects in Organic Chemis-
try, 3rd ed., Wiley-VCH, Weinheim, 2003.
106, 12642–12650.
19] A. Goto, H. Yoshioka, H. Kishimoto, T. Fujita, Langmuir
992, 8, 441–445.
Received: February 1, 2006
Published Online: May 22, 2006
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Eur. J. Org. Chem. 2006, 3364–3371
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3371