Application of the pseudophase ion-exchange model to reactivity in quaternary water in oil microemulsions

Article abstract of DOI:10.1039/b618874e

The nucleophilic attack of a bromide ion on benzyl chloride (BCl) and the acid denitrosation of N-methyl-N-nitroso-para-toluenesulfonamide (MNTS) in quaternary microemulsions of TTABr and SDS containing 1-hexanol as cosurfactant were studied. The experimental results were examined in the light of a kinetic model which allows the quaternary microemulsion to be reduced to a pseudo-ternary microemulsion, by considering the cosurfactant partition between the continuous medium and the interface of the microemulsion to be determined by Shulman titration. The incorporation of the alcohol into the interface alters the interfacial volume to be used in constructing the kinetic model. The second step of the process involves determining how the reactants partition among the different microenvironments of the pseudo-ternary system, the interface being the sole reactive region in the microemulsion. Accounting for the results obtained in the acid denitrosation of MNTS required using the ion-exchange formalism. Based on the results, the product of the fraction of neutralized charge at the interface and the rate constant is independent of the water content of the system. This suggests that the degree of dissociation of the surfactant at the interface does not depend on the water content of the microemulsion. Under the assumption of a surfactant fraction of neutralized charge of β = 0.8, we calculated the rate constants at the interface for both processes. Such rate constants were smaller than in the aqueous medium and consistent with the effects found in nitroso group transfer reactions in microemulsions. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b618874e

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www.rsc.org/njc | New Journal of Chemistry
Application of the pseudophase ion-exchange model to reactivity in
quaternary water in oil microemulsions
Luis Garcıa-Rıo and Pablo Hervella
´ ´
Received (in Montpellier, France) 3rd January 2007, Accepted 14th February 2007
First published as an Advance Article on the web 14th March 2007
DOI: 10.1039/b618874e
The nucleophilic attack of a bromide ion on benzyl chloride (BCl) and the acid denitrosation of
N-methyl-N-nitroso-para-toluenesulfonamide (MNTS) in quaternary microemulsions of TTABr
and SDS containing 1-hexanol as cosurfactant were studied. The experimental results were
examined in the light of a kinetic model which allows the quaternary microemulsion to be
reduced to a pseudo-ternary microemulsion, by considering the cosurfactant partition between the
continuous medium and the interface of the microemulsion to be determined by Shulman
titration. The incorporation of the alcohol into the interface alters the interfacial volume to be
used in constructing the kinetic model. The second step of the process involves determining how
the reactants partition among the different microenvironments of the pseudo-ternary system, the
interface being the sole reactive region in the microemulsion. Accounting for the results obtained
in the acid denitrosation of MNTS required using the ion-exchange formalism. Based on the
results, the product of the fraction of neutralized charge at the interface and the rate constant is
independent of the water content of the system. This suggests that the degree of dissociation of
the surfactant at the interface does not depend on the water content of the microemulsion. Under
the assumption of a surfactant fraction of neutralized charge of b = 0.8, we calculated the rate
constants at the interface for both processes. Such rate constants were smaller than in the
aqueous medium and consistent with the effects found in nitroso group transfer reactions in
microemulsions.
Introduction
charge opposite to that of the interface, the pseudo-phase ion-
exchange formalism has been applied to aqueous ionic mi-
8
celles. The pseudo-phase ion-exchange model for micelles
Water-in-oil microemulsions are usually described as nano-
sized water droplets dispersed in a non-polar solvent with the
aid of a surfactant monolayer. Some surfactants can form no
microemulsions by themselves and require the presence of a
cosurfactant (usually an alcohol) to be stable; however, tern-
ary systems consisting of a surfactant, water and a non-polar
solvent are the most simple possible in this context and have
assumes reactions to take place in the two pseudo-phases
simultaneously. Micelles act as a separate phase and the
surfactant behaves like an ion-exchange resin, the partitioning
of ionic species being described by an ion-exchange constant.
9
The apparent failure of the pseudo-phase ion-exchange model
with reactive counterion surfactants and at high detergent or
salt concentrations in the presence of an excess of highly
1
thus been the most widely studied in this respect. The interest
2
of these complex fluids lies in their usefulness for oil recovery,
ꢀ
ꢀ
hydrophilic counterions (such as OH or F ) has been shown
to arise from the assumption that the degree of dissociation of
the counterion, a, remains constant.
3
4
solubilization of proteins and amino acids, in enzymatic
5
reactions, as alternatives to phase-transfer catalysts and in
6
7
the preparation of monodisperse colloid-size particles. This
The properties of water within drops and at the microemul-
sion interface are very strongly affected by the water content of
the microemulsion. The state of water within AOT reversed
array of applications of reversed micelles has been propitiated
by their aggregates possessing three solubilization sites,
namely: the non-polar continuum, the micellar interface and
the intramicellar water pool. Despite the vast scope of applica-
tion of microemulsions, their influence on the processes they
can host has scarcely been examined, particularly in those
cases involving a reaction between a hydrophobic substrate
and an ionic species bound to the interface. In bimolecular
reactions between an organic substrate and a univalent ion of
micelles and the influence of W (W = [H O]/[AOT]) on
2
micellar properties have been examined by using a variety of
10
techniques which have revealed that the properties of sur-
factant-trapped water differ from those of bulk water and
change strongly with the water content at W values below
about 10. The anomalous behavior of water at low W values
has been ascribed to local interactions of water molecules with
+
Na counterions and sulfosuccinate ion. Little is known
about the physicochemical properties of other surfactant
systems, particularly as regards the state of solubilized water.
The cationic surfactant cetyltrimethylammonium bromide has
for several years been known to form reversed micelles in
Departamento Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de
Santiago de Compostela. Avenida. De las Ciencias s/n, 15782
Santiago de Compostela, Spain. E-mail: qflgr3cn@usc.es; Fax: +34
981 595012
8
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various solvents including 1-hexanol, chloroform and dichlor-
11e,13
spectral
be determined. Based on the results, the reaction rate at the
interface of the quaternary microemulsion is smaller than in
water and aqueous micelles by effect of the decreased polarity
of the interface relative to micellar systems.
1
omethane. There is infrared and H-NMR
1
12
1
evidence for a change in the properties of water in these
1
3
cationic reversed micelles as W is raised; also, the C-NMR
chemical shifts for the surfactant are consistent with the
proposed change in local structure. These changes in water
properties in microemulsions can alter the amount of charge
that is neutralized at the interface.
Experimental
Tetradecyltrimethylammonium bromide (TTABr) and sodium
dodecyl sulfate (SDS) were supplied by Sigma and used with-
out further purification. 1-Hexanol and isooctane were Al-
drich products in the highest commercially available purity. N-
Methyl-N-nitroso-para-toluenesulfonamide (MNTS (Aldrich
99%)) and benzyl chloride (Merck 499%) were used as
received.
Some additives are known to dramatically alter the degree
1
4
of micelle dissociation in aqueous micellar systems. Thus, the
1
5
16
degrees of ionization of SDS and TTABr increase with
increasing alcohol concentration. The degree of micelle ioniza-
tion, a, reflects electrostatic interactions between charged
micelle surfaces and counterions, and provides a rough mea-
sure of the fraction of counterions in close proximity to micelle
Microemulsions of the desired compositions were prepared
from stock water–surfactant–isooctane–alcohol microemul-
sions by adding appropriate amounts of isooctane and water.
A constant alcohol : surfactant mole ratio of 4 : 1 for TTABr
and 5 : 1 for SDS was used throughout. Densities were
measured with a pycnometer.
1
7
surfaces with their solvation layer still intact that are in
contact with the micelle solvation layer. These interactions are
essentially determined by the charge density of the micelle
surface, which is strongly affected by surface solubilization of
the alcohol. Thus, they depend much less on the amount of
alcohol added, up to a fairly large content, as the micelle and
counterion solvation layers are likely to remain largely com-
posed of water molecules, even at fairly large alcohol concen-
Reactions were monitored at 25.0 1C on a Varian Cary 500
Scan UV-vis-NIR spectrophotometer equipped with thermo-
stated cell holders. The acid denitrosation of MNTS was
studied by monitoring the disappearance of the substrate at
260 nm. The conditions used in all cases were such that the
nitroso compound concentration was always much smaller
1
8
trations, in alcohol-water mixtures. As a result, changes in a
with the alcohol concentration are essentially due to the
alcohol penetrating micelles rather than to its presence in the
intermicellar region.
+
than the [H ]. Typically, such conditions included [MNTS] =
ꢀ4
ꢀ3
In this work, we examined two types of kinetic processes
between a hydrophobic substrate and an ionic species that
reacts by binding to the microemulsion interface. Specifically,
we studied the nucleophilic attack of bromide ions on benzyl
chloride (BCl) in TTABr–1-hexanol–isooctane–water micro-
emulsions and the acid denitrosation of N-methyl-N-nitroso-
para-toluenesulfonamide (MNTS) in SDS–1-hexanol–isoocta-
ne–water microemulsions (Scheme 1). All the experiments
were conducted at a constant [1-hexanol] : [TTABr] ratio of
1.91 ꢂ10 M and [HCl] = 6.67 ꢂ10 M. All kinetic data
fitted a first-order integrated equation with r 4 0.999. In what
follows, kobs is used to denote the pseudo first-order rate
constant. The nucleophilic attack of bromide ions on benzyl
chloride is a very slow reaction, so it was examined via the
initial rate method. Kinetic experiments involved monitoring
the absorbance change at 249 nm for (5–10)% of the reaction
time, using a substrate concentration [BCl] = 1.17 ꢂ10ꢀ M.
The slope of each absorbance-time plot was converted into a
rate constant via the overall absorbance change. The reaction
was also examined using the integration method in several
control tests that involved measuring the absorbance change
over at least 4 lifetimes and fitting absorbance-time data pairs
to a first-order integral equation. The kobs values obtained
with both methods were identical.
3
4
: 1 or a constant [1-hexanol] : [SDS] ratio of 5 : 1. The
surfactant counterion was the reactive species in the former
case; the latter, required applying the ion-exchange formalism
+
at the microemulsion interface to Na as counterion and H
+
as reactive ion.
The results obtained are explained in the light of a kinetic
model where the quaternary microemulsion is transformed
into a pseudo-ternary one and the change in interfacial volume
is assumed to result from incorporation of the cosurfactant
into the interface. Subsequently applying the pseudo-phase
ion-exchange model allowed the interfacial rate constants for
the target nucleophilic substitution and hydrolysis reactions to
Results and discussion
Alcohols can be regarded as cosolvents that partition between
the aqueous and oil-rich bulk phases on the one hand, and the
interfacial layer on the other, thereby decreasing the effective
hydrophilicity of the amphiphile and the effective hydropho-
bicity of the oil. In order to simplify the analysis of kinetic
data, a quaternary microemulsion can be transformed into a
pseudo-ternary one. Scheme 2 depicts the way the alcohol
partitions among the different pseudo-phases.
The alcohol, 1-hexanol, will initially partition among the
three microemulsion pseudo-phases in accordance with the
ROH
ROH
and Kwi , which are defined as:
partition constants Koi
½
ROHꢃ
½ROHꢃ
½ROHꢃ
ROH
i
ROH
i
Koi
¼
Z Kwi
¼
W
ð1Þ
½
ROHꢃo
Scheme 1
w
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Scheme 2
where [ROH]
o
, [ROH]
i
w
and [ROH] denote the alcohol con-
centrations in the continuous phase, interface and bulk water,
respectively, referred to the total microemulsion volume.
Parameters Z and W bear their usual meaning (i.e. Z =
t
Fig. 1 Plot of n
a
/n
s
vs. n /n
o s
according to eqn (3) for w/o TTABr–
[
Isooctane]/[Surfactant] and W = [H O]/[Surfactant]). By
2
isooctane–water–hexanol microemulsion systems at 25.0 1C with
different TTABr concentrations: (J) 0.0019; (K) 0.0030; (&)
0.0043; (’) 0.0060 and (D) 0.0078 M.
virtue of the very low solubility of 1-hexanol in water, the
quaternary microemulsion can be transformed into a pseudo-
ternary one consisting of isooctane and 1-hexanol as the
continuous phase, the surfactant (TTABr or SDS) and
t
1
-hexanol as the interface, and water as the aqueous phase.
are varied so as to get a series of n
a
/n
s
and n /n
o s
values whose
graphical plotting according to eqn (3) can yield the values of
o
Partitioning of the alcohol in the microemulsion
i
w
n
a
and n
a a
from the intercept and slope if n is negligible (see
Fig. 1 and Fig. 2).
In the present paper, we investigated the surface composition
of the w/o droplets by dilution method, in which TTABr or
SDS is used as the surfactant, n-hexanol as the cosurfactant,
and isooctane as the oil with addition of different amounts of
water. For a stable w/o microemulsion, a critical amount of
n-hexanol (cosurfactant) distributes between the bulk oil
phase and the droplet surface, whereas the surfactant almost
solely resides on the droplet surface since it is insoluble in
ROH
The alcohol distribution constant, Koi , between the con-
tinuous medium and the interface of the microemulsions can
be obtained as the ratio between the intercept/slope of plots
like those showed in Fig. 1 and Fig. 2 for TTABr and
SDS-based microemulsions:
i
a
½
ROHꢃ
n no
ROH
Koi
i
¼
Z ¼
¼Intercept=Slope
ð4Þ
1
9
o
isooctane. Supposing the number of moles of n-hexanol at
½ROHꢃ
n ns
a
o
i
a
o
the interface and in bulk oil phase being n
tively, the total number of moles of n-hexanol n
sum of both, where the number of moles of n-hexanol in water,
and n
a
, respec-
a
equals to the
The alcohol distribution constants thus obtained for TTABr
ROH
and SDS microemulsions, viz. (K
)TTABr = 35 ꢄ1 and
oi
w
ROH
n
a
, is negligible because of its very small solubility:
(Koi )SDS = 19 ꢄ1, were subsequently used to transform a
quaternary microemulsion into a pseudo-ternary one. Alco-
hols behave highly non-ideally in solution in alkanes due to
self-aggregation, however their observed partition coefficients
are concentration independent. The reason for this behavior
should reside in their incorporation to the microemulsion
i
a
o
a
n = n + n
a
(2)
Other conditions remaining the same, the ratios between the
o
a
number of moles of alcohol in oil (n ) and the number of moles of
oil (n ), and between the mole fraction of alcohol at the interface
o
i
o
o
i
o
(
X
a
) and that in the oil (n
a a o d a a
) are fixed: K = n /n and K = X /X ,
where K and K are the appropriate constants, the latter being the
d
distribution constant. Addition of oil in the system would change
K and K by abstracting the alcohol in it and making the system
d
unstable. Since the stability of the microdispersion of water is
affected by a threshold population of alcohol at the interface and
consequently in the bulk oil, the addition of alcohol can reestab-
lish the threshold stable condition of the system by restoring the
required magnitudes of both K and K
d
.
Representing the number of moles of alcohol in water and at
o
w
i
the interface by n
a a a o
and n , respectively, by using K = n /n , the
total number of moles of alcohol in the microemulsion system,
t
20,21
a
n per mole of added surfactant can be represented as:
t
w
i
a
n
n þn
no
a
a
¼
þK ns
ð3Þ
t
ns
ns
Fig. 2 Plot of n /n vs. n /n according to eqn (3) for w/o SDS–
o
a
s
s
isooctane–water–hexanol microemulsion systems at 25.0 1C with
different SDS concentrations: (K) 0.0037; (&) 0.0049; (’) 0.0054
and (D) 0.0073 M.
where n is the number of moles of surfactant present in the
s
t
system. In the dilution experiment at a constant n , n and n
s
a
o
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Fig. 3 Influence of W and surfactant concentration upon Vtot /V
i
: (left) TTABr–hexanol–isooctane–water microemulsions. [Hexanol]/[TTABr] =
4, (right) SDS–hexanol–isooctane–water microemulsions. [Hexanol]/[TTABr] = 5.
interface in such a way that the percent of alcohol in the
continuous medium is very small.
Because the percentage of trapped water at the interface is
lower than 3% of total water and because the small molar
volume of water, we can neglect its contribution to the total
volume of the interface with an error smaller than 10%. We
Variation of the interfacial volume
ꢀ
ꢀ
used a molar volume of surfactant VTTABr = 0.326 M for
1
Transforming a quaternary microemulsion into a pseudo-
ternary one entails redefining the composition-related para-
meters for the system concerned. Since the alcohol can be
taken to be a solute at the interface, we used the Stilbs
2
5
ꢀ1
26–28
ꢀ
TTABr and VSDS = 0.246 M for SDS,
which corre-
sponded to the overall volume of the micelles. The molar
ꢀ1
ꢀ
volume of 1-hexanol, VROH = 0.126 M , was calculated
2
2
approximation under the assumption that the surfactant
was the sole component determining the properties of the
interface. Therefore, the composition parameter W (W =
from its density.
Fig. 3 shows how the relationship Vtot/V
according to the eqn (7)) decreases as the surfactant concen-
tration increases. The decrease in Vtot/V with the TTABr or
(values calculated
i
[
H
2
O]/[Surfactant]) remained unchanged. However, the com-
i
position of the continuous medium is altered by the presence
SDS concentration is a consequence of the alcohol incorpora-
tion into the interface and its consequent increase in volume.
of the alcohol, so parameter Z, initially defined as Z =
[
Isooctane]/[Surfactant], was redefined in the following form:
i
Hence, the calculated values for Vtot/V are independent of the
W parameter. This behavior is due to the quantity of solubi-
lized alcohol in the aqueous pseudophase of the microemul-
sion being negligible.
½
Isooctaneꢃþ½ROHꢃ
ꢅ
o
Z ¼
½
Surfactantꢃ
Z½ROHꢃ
tot
¼
Z þ
ð5Þ
The kinetic results obtained should be explained by taking
into account the alcohol incorporation into the interface of the
microemulsion and consequently the variation of the volume
of the interface with the surfactant concentration and with the
molar relationship [ROH]tot/[TTABr].
ROH
ðKoi þZÞ½Surfactantꢃ
where [ROH]tot denotes the total concentration of 1-hexanol
added to the microemulsion. In our experimental conditions,
[
ROH]tot /[TTABr] = 4 and [ROH]tot /[SDS] = 5.
For this formalism to be applicable, the total number of
moles of alcohol that are incorporated into the interface must
Nucleophilic substitution by bromide ion in benzyl chloride
2
3
increase with increasing concentration of surfactant, and
hence increasing the interfacial volume. In ternary microemul-
sions (e.g. AOT–isooctane–water microemulsions), the interfa-
cial volume can be assumed to remain constant. In quaternary
microemulsions, however, the incorporation of large amounts
of cosurfactant into the interface cause the interfacial volume to
change with the surfactant concentration. This situation is
similar to that in micellar systems when a high concentration
This reaction was studied in TTABr–1-hexanol–isooctane–
water microemulsions at a constant [1-Hexanol] : [TTABr]
ratio of 4 : 1. The aim was twofold, namely: (a) to construct a
kinetic model accounting for the reactivity of benzyl chloride
ꢀ
towards Br ion irrespective of the microemulsion composi-
tion and (b) to use the model to determine the fraction of
neutralized charge at the interface of the microemulsion.
2
4
of alcohol is added to the reaction medium.
The interfacial volume can be assumed to consist of the
volumes of surfactant and alcohol incorporated into the inter-
face, namely:
Influence of the microemulsion composition. We examined
the influence of the microemulsion composition on the ob-
served rate constant, kobs, for the nucleophilic attack of
bromide ion on benzyl chloride at a variable surfactant con-
centration, [TTABr], from 0.10 to 0.70 M and a constant mole
ꢀSurfactant
= V
ꢀ
V
i
[Surfactant]
i
+ VROH[ROH]
i
(6)
ROH
oi
Vtot
1
K
þZ
ratio W = [H O]/[TTABr]. By way of example, Fig. 4 shows
2
¼
þ
ð7Þ
V
ꢀSurfactant½Surfactantꢃ
ꢀ
V
ROHK oRi OH½ROHꢃtot
Vi
selected results. As can be seen, kobs increases on increasing the
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BCl
The partition constant for benzyl chloride, Koi , can be
defined in classical terms as:
½
BCliꢃ
Z
BCl
ꢅ
Koi
¼
ð8Þ
½
BCloꢃ
where Z* = ([Isooctane] + [1-Hexanol])/[TTABr]; subscripts
i and o denote the interface and continuous medium, respec-
tively; and concentrations are referred to the total volume of
the microemulsion.
Kinetic model for the reaction in microemulsions. Based on
the reactant partitioning of Scheme 3, the interface is the sole
microemulsion region where the reactants can come into
contact and hence the sole reactive region. The observed rate
constant for the reaction will therefore be given by:
Fig. 4 Influence of the TTABr concentration on kobs in the nucleo-
philic attack of bromide ion on benzyl chloride in TTABr–1-hexano-
l–isooctane–water microemulsions at 25.0 1C. W = (J) 5, (K) 7, (&)
BCl
kBr
ꢀ
K
oi
ðKoi þZꢅÞ
BCl
10 and (’) 15.
kobs ¼
ð9Þ
0
where k Brꢀbeing is the pseudo first-order rate constant at the
surfactant concentration, but was virtually independent of the
water content of the microemulsion, W.
interface and can be expressed as a bimolecular rate constant
2
at the interface as:
9
The behavior of this system can be interpreted in qualitative
terms as a function of the partitioning of benzyl chloride
between the continuous medium and the interface. Raising
the surfactant concentration increases the amount of benzyl
chloride that binds to the interface. The reaction between
bromide ions and benzyl chloride can only take place at the
microemulsion interface owing to the strongly hydrophobic
character of the substrate; also, if the fraction of micellar
charge that is neutralized does not change with the water
content of the microemulsion, the concentration of bromide
ions bound to the interface must be independent of W.
VT
0
i
Br
ꢀi
i
½Brꢀꢃ
k
ꢀ
¼k
ꢀ
ꢀ
½Br ꢃ¼k
ꢀ
Br
Br
i
i
Vi
V
T
i
Br
¼
k
b½TTABrꢃ
ð10Þ
Vi
ꢀ
i
i
[
Br ]
being the concentration of bromide ion at the interface
ꢀ
relative to the interfacial volume, and [Br ]
i
the concentration
of bromide ion referred to the total micro-emulsion volume. In
the absence of bromide ions other than those resulting from
dissociation of the surfactant, the total concentration of
bromide at the interface will be equal to the product of the
fraction of neutralized charge at the interface, b, and the total
concentration of surfactant. Based on the foregoing, one can
derive the following expression for the pseudo first-order rate
constant:
Partitioning of the reactants. The quaternary microemulsion
can be simplified to a pseudo-ternary microemulsion (Scheme
BCl
VT
K
oi
BCl
i
Br
2
) with isooctane and 1-hexanol as the continuous medium,
kobs ¼Vi b½TTABrꢃk
ꢀ
ð11Þ
ðKoi þZꢅÞ
TTABr and 1-hexanol as the interface, and water as the
aqueous medium. Accurately interpreting the kinetic results
obtained in the nucleophilic substitution by bromide ion in
benzyl chloride entails considering the way the reactants
partition among the different pseudo-phases of the microemul-
sion. Scheme 3 shows the partitioning of benzyl chloride
between the continuous medium and the microemulsion inter-
face. The low solubility of this reactant in water allows its
partitioning between the aqueous medium and the interface to
be assumed negligible.
which can be rearranged to:
VT ½TTABrꢃ
1
1
ꢅ
Z
¼bki þbki
ð12Þ
BCl
Vi
kobs
ꢀ
ꢀ
K
oi
Br
Br
This equation predicts
[TTABr])/(V
a
linear relationship between
(V
T
i
k
obs) and the microemulsion composition
*
parameter, Z . By way of example, Fig. 5 illustrates the good
fit of the results to eqn (12).
Table 1 lists the slopes and intercepts obtained by fitting the
experimental kobs values to eqn (12) at different W values. As
can be seen, the intercept was in many cases similar in
magnitude to its own error owing to the extrapolation re-
quired to obtain it. Based on eqn (12), the slope-to-intercept
ratio should be constant and independent of W (Intercept/
BCl
Slope = Koi ). The results of Table 1 suggest that, in fact,
such a ratio remains constant throughout. If all W values are
BCl
considered, then Koi = 2.1 ꢄ0.8, which is consistent with the
results obtained by using the W values 10, 15, 35 and 45 alone
(
i.e. those where the intercept can be accurately determined). If
BCl
Scheme 3
only such W values are considered, then Koi = 2.7 ꢄ0.5. In
8
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BCl
k
i
Fig. 6 Variation of the product bKoi
Br
ꢀ
as a function of W for
Fig. 5 Fitting of the data in Fig. 4 to eqn (12). W = (J) 5, (K) 7,
&) 10 and (’) 15.
the nucleophilic attack of bromide ion on benzyl chloride in TTABr–1-
hexanol–isooctane–water microemulsions.
(
what follows, we shall use this value for the partition constant
of benzyl chloride between the different phases in the micro-
emulsion.
The problem here lies in quantifying the sensitivity of the
rate of nucleophilic substitution by bromide ion in benzyl
chloride to the solvent polarity. Based on the results of Jaeger
As can be seen from Fig. 6, the reciprocal of the slopes in
Table 1, which were obtained from eqn (12), were W-indepen-
3
2
ꢀ3
ꢀ1 ꢀ1
et al., the rate constant is k = 3.0 ꢂ10
M
min in 25%
ꢀ1
min
ꢀ
6
ꢀ1
ꢀ3
ꢀ1
dent and averaged at 1/Intercept = (8.7 ꢄ0.1) ꢂ10
M
(v/v) water–ethanol mixtures and k = 2.9 ꢂ10
M
ꢀ
1
BCl
i
s
. This suggests that the product bKoi
of the water content of the microemulsion and hence that so is
k
Br
ꢀ
is independent
in 10% (w/w) CTABr–water mixtures at 40.0 1C. These results
suggest that the reaction is scarcely sensitive to the polarity of
i
i
ꢀ6
ꢀ1 ꢀ1
i
bkBr
ꢀ
, which averaged at bkBr
ꢀ
= 3.22 ꢂ10
M
s
.
the medium, so kBr
ꢀ
should possess a unique value irrespective
Therefore, the proposed kinetic model provides an accurate
explanation for the kinetic results obtained under rather
variable microemulsion compositions. The model satisfacto-
rily explains the reactivity of benzyl chloride irrespective of the
of the particular microemulsion composition provided 5 o W
i
ꢀ is independent of W, then the fraction of
o 45. If k
Br
neutralized charge at the interface, b, should remain constant
throughout the previous range of water content in the micro-
emulsion. This was subsequently confirmed in studying the
acid denitrosation of N-methyl-N-nitroso-para-toluenesulfo-
namide.
particular composition of the microemulsion.
i
The constancy of bkBr
namely: (a) both b and kBr
studied composition range or (b) their changes cancel each
ꢀ
has two possible implications,
i
ꢀ
are W-independent through the
The degree of dissociation of the surfactant in aqueous
micelles and oil-in-water microemulsions can be readily deter-
mined via conductivity measurements. Basically, conductivity
changes can be related to the degree of dissociation of the
surfactant that contributes with ionic components (e.g. sodium
ions from SDS) to the aqueous phase. There have been some
attempts at determining the degree of dissociation at the
other. Initially, one could expect the fraction of neutralized
charge at the interface to increase with decrease in W, which
i
would cause a slight increase in b and a decrease in kBr
the rate constants at the interface decrease on decreasing W
ꢀ
. That
3
0
was previously observed and ascribed to a decrease in the
ability of interfacial water to solvate the reactants and the
transition state upon decreasing the water content of the
microemulsion. However, when the rate of the reaction con-
cerned is scarcely sensitive to polarity changes in the medium
3
3
interface of SDS–1-butanol–water–toluene microemulsions
from conductivity measurements which have provided a values
from 0.52 to 0.05.
(
e.g. in nitroso group transfers), a single value of the rate
3
However, the conductivity of w/o microemulsions is closely
related to the rate of mass transfer between droplets, a
phenomenon that peaks at the onset of electrical percolation.
Research into the internal dynamics of w/o microemulsions
has largely focused on this intriguing phenomenon, which
causes an abrupt increase in electrical conductivity when either
the temperature or the volume fraction of the dispersed phase
reaches a critical value. Percolation therefore involves a sharp
change in electrical conductivity from very low levels typical of
small droplets dispersed in a nonconducting continuous med-
1
constant is obtained at any W value exceeding W 4 5–7.
Table 1 Intercepts and slopes obtained by fitting the experimental
obs values for the nucleophilic attack of bromide ion on benzyl
k
chloride in TTABr–1-hexanol–isooctane–water microemulsions to
eqn (12)
W
Intercept
Slope
5
5
5
5
5
5
5
7
(2 ꢄ1) ꢂ10
(1 ꢄ1) ꢂ10
(1.03 ꢄ0.09) ꢂ10
(1.40 ꢄ0.08) ꢂ10
(1.04 ꢄ0.01) ꢂ10
(1.07 ꢄ0.01) ꢂ10
3
4
ium to values higher by several powers of 10. Lang and
11d,35
5
5
1
1
2
3
3
4
4
0
5
0
0
5
0
5
(2.9 ꢄ0.1) ꢂ10
(2.4 ꢄ0.3) ꢂ10
coworkers
have related electrical percolation to the rate
constants governing the exchange of materials between dro-
plets (a process which in turn may have a decisive influence on
the rate of fast chemical reactions in w/o microemulsions).
Percolation does not occur as the result of the formation of
permanent bicontinuous structures in the medium, but rather
5
5
(2 ꢄ2) ꢂ10
(1.3 ꢄ0.1) ꢂ10
5
5
(2 ꢄ1) ꢂ10
(1.07 ꢄ0.08) ꢂ10
(1.07 ꢄ0.03) ꢂ10
5
5
(3.5 ꢄ0.3) ꢂ10
5
5
(1 ꢄ2) ꢂ10
(1.4 ꢄ0.1) ꢂ10
5
5
(2.6 ꢄ0.9) ꢂ10
(1.08 ꢄ0.06) ꢂ10
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of the structure of discrete droplets being preserved. One
plausible explanation assumes that the number of collisions
between droplets increases sharply in the vicinity of the
percolation threshold and leads to the formation of droplet
clusters. The opening of interdroplet channels in the clusters
facilitates the transport of materials (ions) between droplets
and gives rise to the abrupt increase in conductivity observed.
Therefore, the conductivity in microemulsions cannot be
related to changes in the number of ionic components. In fact,
the addition of salts to an AOT–isooctane–water w/o micro-
emulsion hinders electrical percolation. The absence of corre-
lation between conductivity and the number of ionic
components of the system precludes its use to determine the
degree of dissociation of the surfactant.
Acid denitrosation of N-methyl-N-nitroso-para-
toluenesulfonamide in SDS–1-hexanol–isooctane–water
microemulsions.
We studied the acid hydrolysis (denitrosation) of MNTS in
SDS–1-hexanol–isooctane–water microemulsions at a con-
stant [1-hexanol] : [SDS] ratio of 5 : 1. Kinetic experiments
were performed under conditions where HCl was in a very
large excess with respect to the N-nitroso compound. By way
of example, Fig. 7 shows the variation of the pseudo first-order
observed rate constant, kobs, with the microemulsion composi-
tion. Contrary to the nucleophilic attack of bromide ions on
benzyl chloride, kobs decreased with increasing concentration
of surfactant and was virtually independent of the water
content of the microemulsion, W.
The Poisson–Boltzmann equation has allowed some authors
to study the effect of W on the degree of counterion binding, b.
These results can be ascribed to a combination of two
factors. On the one hand, raising the surfactant concentration
increases that of MNTS at the microemulsion interface (the
3
6
Thus, Karpe and Ruckenstein found b to decrease with
increase in W; however, b changed by only 7.3% on raising
W from W = 6 (b E 0.86) to W = 50 (b E 0.80). Likewise,
the degree of dissociation of ionic microemulsions has been
+
41
sole region where MNTS and H O can come into contact).
3
The increased interfacial concentration of N-nitroso com-
pound should result in an increased reaction rate. However,
an increase in the concentration of SDS also increases that of
3
7
shown to increase with increasing droplet size. According to
3
8
Kozlovich and coworkers, the degree of counterion binding
of AOT head groups must be greater than 72%. These results
are consistent with the sole experimental study to the authors’
knowledge available where chemical trapping of bromide ions
+
unreactive Na ions. Competition between these unreactive
+
counterions and H
3
O
ions for binding to the microemulsion
interface must be the origin of the decrease in rate constant
observed as the surfactant concentration is raised. This in-
hibitory effect of an increased concentration of unreactive
counterions has frequently been encountered in micellar sys-
tems and interpreted in the light of the pseudo-phase ion-
3
9
in microemulsions has been used; b for CTAB microemul-
sions in dodecane/CHCl was found to be 0.8—which is very
3
similar to the values found in aqueous micelles—at W 4 15.
If a b value of 0.8, which is the most widely used for direct
8
exchange formalism.
i
40
micellar systems, is adopted, then kBr
ꢀ
is calculated to be
ꢀ1 ꢀ1
s , which is comparable with that
Providing a quantitative interpretation for the results entails
considering the way the reactants partition among the differ-
ent pseudo-phases in the microemulsions (see Scheme 4, where
MNTS is assumed to be exclusively present in the continuous
medium and the interface, which will thus constitute the sole
reactive region in the microemulsion).
i
ꢀ6
k
Br
ꢀ
= 4.02 ꢂ10
M
3
2
obtained by Jaeger et al. for the reaction in 10% (w/w)
ꢀ1
M
ꢀ5
CTABr-water mixtures at 40.0 1C (viz. k = 4.8 ꢂ10
ꢀ
1
s
). As can be seen, the reaction rate is roughly 10 times
smaller at the interface of TTABr–1-hexanol–isooctane–water
microemulsions than at the interface of aqueous micellar
systems; this is consistent with the decreased interfacial polar-
ity of microemulsions relative to simple micellar systems. As
shown later on Table 2, a similar behavior was observed in the
acid denitrosation of MNTS in single-component SDS micel-
lar systems and in SDS–1-hexanol–isooctane–water micro-
emulsions.
The equilibrium constant for MNTS distribution is defined
MNTS
as usual, Koi
i o
= ([MNTS ]Z*)/[MNTS ], with all concen-
i
H
trations referred to the total volume of the microemulsion, k
the bimolecular rate constant for the reaction and the inter-
H
face, and k the ion-exchange equilibrium constant between
Na
+
protons and the counterions (Na ). A mass balance assuming
Table 2 Bimolecular rate constant for the nitroso group transfer
from MNTS to piperazine (PIP) and N-methylbenzylamine
(MeBzAm) in various reaction media, and also for the denitrosation
of MNTS
ꢀ1
ꢀ1
s
Reaction
Reaction medium
k/M
a
ꢀ2
ꢀ3
ꢀ3
ꢀ2
MNTS + PIP
H
AOT–isooctane–H
2
O
2.98 ꢂ10
5.33 ꢂ10
4.69 ꢂ10
4.10 ꢂ10
b
2
O
c
c
TTABr–1-hexanol–isooctane–H
2
2
O
a
MNTS +
MeBzAm
2
H O
b
ꢀ3
AOT–isooctane–H
2
O
TTABr–1-hexanol–isooctane–H
3.1 ꢂ10
ꢀ4
ꢀ2
ꢀ3
O
6.99 ꢂ10
3.20 ꢂ10
3.50 ꢂ10
7.25 ꢂ10
+
d
MNTS + H
2
H O
e
Fig. 7 Influence of the SDS concentration on kobs for the acid
SDS
SDS–1-hexanol–isooctane–H
ꢀ4
denitrosation of MNTS in SDS–1-hexanol–isooctane–water micro-
2
O
ꢀ3
a
b
Ref. 45. Ref. 47. Ref. 31b. Ref. 44b. Ref. 49.
c
d
e
emulsions at 25.0 1C. [HCl] = 6.67 ꢂ10 M. W = (J) 7, (K) 10,
&) 15 and (’) 20.
(
8
66 | New J. Chem., 2007, 31, 860–870
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Scheme 4
the total MNTS concentration to be the combination of those
in the continuous medium and the interface can be used to
obtain the following expression for the pseudo first-order rate
constant:
Fig. 8 Fitting of the data of Fig. 4 to eqn (21). W = (J) 7, (K) 10,
(&) 15, and (’) 20.
K
MNTSk
0
oi
MNTS
H
kobs ¼
ð13Þ
and hence:
ðKoi
þZꢅÞ
þ
b½H ꢃ
Na
þ
T
0
½H ꢃ¼
ð19Þ
where k
H
is the pseudo first-order rate constant at the interface
i
H
aðK ꢀ1Þþ1
and can be expressed as a bimolecular rate constant at the
i
H
40
interface, k
, in such a way that:
where a is the degree of dissociation at the interface (a + b =
). Therefore, eqn (13) can be rewritten as the following
function of the total proton concentration:
1
MNTS
oi
MNTS
i
þi
VT K oMi NTSk ½H ꢃ
i
þ
K
k ½H ꢃ
H
i
H
i
kobs ¼
¼
ð14Þ
ꢅ
MNTS
ꢅ
ðKoi
þZ Þ
Vi ðKoi
þZ Þ
MNTS
oi
þ
VT
Vi
K
MNTS
b½H ꢃ
Na
i
H
T
+
i
kobs ¼
k
ð20Þ
[
i
H ] being the proton concentration at the interface relative
+
to the interfacial volume and [H ] that relative to the total
ꢅ
H
ðKoi
þZ ÞaðK ꢀ1Þþ1
i
which can in turn be rewritten as:
volume of the microemulsion. The proton concentration at the
interface can be determined by defining the ion-exchange
equilibrium constant:
H
Na
VT
1 aðK ꢀ1Þþ1
i
H
þ
Vikobs
¼k
b½H ꢃ
T
H
Na
H
K
1
aðK ꢀ1Þþ1
Na
ꢅ
Z
þ
þ
þ
þ
þ
k K
H
oi
ð21Þ
ꢀ
!
i
MNTS
þ
b½H ꢃ
T
H þNa
ꢀH þNa
ð15Þ
ð16Þ
i
w
w
i
as:
This equation predicts a linear relationship between the term
/(V obs) and Z*, which is confirmed by the results of Fig. 8.
þw
þi
þ
þ
V
T
k
i
½
½
H ꢃ½Na ꢃ
½H ꢃ½Na ꢃ
H
Na
w w
þi
i
i
w w
i
i
K
¼
¼
þw
w w
Table 3 lists the slopes and intercepts obtained by plotting
þ
þ
H ꢃ½Na ꢃ
½Hi ꢃi½Naw ꢃw
i
i
V /(V kobs) as a function of Z* at variable W values.
T i
+
w
w
w
+
w w
The slope/intercept allows one to calculate the partition
where [H ] and [H ] denote concentrations in a phase
constant of MNTS between the continuous medium and the
MNTS
referred to the phase volume and total microemulsion volume,
respectively. Converting values between the two forms entails
defining the volume of each phase.
interface. As can be seen from the data of Table 3, Koi
was
independent of W, which confirms the accuracy of the pro-
MNTS
posed model. Based on such data, an average Koi
MNTS
oi
value of
If one assumes the SDS concentrations in the continuous
medium and aqueous phase to be negligible (i.e. that SDS is
entirely present at the interface), the following expression for
the fraction of neutralized charge at the interface can be
derived:
K
= 13 ꢄ 3 was obtained that is consistent with
previously reported values for AOT and TTABr microemulsions.
As can be seen, the slopes and intercepts obtained
from eqn (21) are independent of W (see Fig. 9). Based
on the experimental data, the following average values
þ
þ
i
i
½
H ꢃþ½Na ꢃ
b½Hþꢃ
Na
i
i
i
T
¼ð3:9 ꢄ0:8Þꢂ10ꢀ6
b ¼
ð17Þ
were obtained:
k
H
and
H aðK ꢀ1Þþ1
¼ð5:0 ꢄ0:8Þꢂ10ꢀ5. The results obtained
½
SDSꢃ
þ
i
MNTS
oi
b½H ꢃT
Na
k K
+
Also, if one assumes Na to be the dominant counterion in
H
aðKH ꢀ1Þþ1
4
2
+
i
in the acid denitrosation of MNTS are consistent with those
previously found in the nucleophilic attack of bromide ion on
benzyl chloride. Determining the slope and intercept in this
the microemulsion droplets, then, b D INa
Consequently, a mass balance for the total proton concentra-
+
tion under the assumption that the total concentration of Na
i
m /[SDS].
case requires knowing the ion-exchange equilibrium constant,
H
will be identical with that of SDS provides the following
equation:
k
Na. The most widely accepted value of such a constant for the
+
+
43
H
Na
Na /H ion exchange in micellar systems is k D 1, which
þ
þ
i
ð½H ꢃ ꢀ½H ꢃÞb½SDSꢃ
i
+
H
Na
T
allows the previous equations to be simplified to k b[H ] =
K
¼
ð18Þ
H
T
þ
½
H ꢃ½SDSꢃð1 ꢀbÞ
ꢀ6
i MNTS +
ꢀ5
.
i
(3.9 ꢄ0.8) ꢂ10 and k K
b[H ] = (5.0 ꢄ0.8) ꢂ10
H
oi
T
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Table 3 Slopes and intercepts of the plots obtained by fitting the
experimental kobs values for the acid denitrosation of MNTS in
SDS–1-hexanol–isooctane–water microemulsions at 25.0 1C to eqn
(21)
W
Intercept
Slope
5
4
7
(2.52 ꢄ0.07) ꢂ10
(2.23 ꢄ0.06) ꢂ10
5
4
1
1
2
2
0
5
0
5
(2.8 ꢄ0.1) ꢂ10
(2.0 ꢄ0.1) ꢂ10
Scheme 5
5
4
(2.6 ꢄ0.2) ꢂ10
(2.5 ꢄ0.2) ꢂ10
5
4
(3.4 ꢄ0.1) ꢂ10
(1.8 ꢄ0.2) ꢂ10
5
4
(2.0 ꢄ0.1) ꢂ10
(1.7 ꢄ0.2) ꢂ10
case. This result confirms that the charge fraction that is
neutralized at the interface remains constant throughout the
same W range.
Since the HCl concentration used in the kinetic experiments
i
Therefore, if one accepts that the fraction of neutralized
ꢀ3
ꢀ4
was [HCl] = 6.67 ꢂ10 M, k b = (5.8 ꢄ1.2) ꢂ10 and
H
charge at the interface and the rate constant for the acid
i
i
k K
H
MNTS
oi
ꢀ3
b = (7.5 ꢄ1.2) ꢂ10 . Based on the smaller error
denitrosation of MNTS are both independent of W, k can
H
i
made in determining slopes, in what follows we shall use
ꢀ3
be calculated simply by assuming b D 0.8. The k value thus
H
i
k K
H
MNTS
oi
i
ꢀ4
ꢀ1 ꢀ1
b = (7.5 ꢄ1.2) ꢂ10 . Also, since the partition
obtained is kH = 7.25 ꢂ10
M s , which is 48 times
constant of MNTS between the microemulsion pseudo-phases,
MNTS
oi
smaller than that in pure water at an ionic strength I = 0.50 M
ꢀ2
i
H
ꢀ4
ꢀ1 ꢀ1
ꢀ1 ꢀ1
M s ). The difference can be
K
, is 13 ꢄ3, then k
b = 5.8 ꢂ10
M
s
.
(HClO
4
) (k
H
= 3.5 ꢂ10
As in the previous reaction, the constancy of the product
i
H
ascribed to the decreased polarity of the interface in the
microemulsion relative to bulk water.
i
k
b with W may have resulted from either b and k
H
being
independent of W or their changes (viz. an increase in b and a
i
H
decrease in k with increasing W) cancelling each other. The
Comparison of reactivity in various media
acid hydrolysis of N-methyl-N-nitroso-para-toluenesulfona-
mide is a concerted process taking place via a transition state
such as that shown in Scheme 5. The formation of the N–H
bond and cleavage of the N–N bond are simultaneous, but not
Table 2 lists the rate constants obtained for the nitroso group
transfer from MNTS to piperazine (PIP) and N-methylbenzyl-
amine (MeBzAm), as well as for the acid denitrosation in
MNTS, in various reaction media. The nitroso group transfer
from alkyl nitrites to amines in water and direct micellar
4
4
synchronous.
The reaction is not very sensitive to the solvent polarity.
4
6
Thus, a comparison of the rate constant in water (k = 3.20 ꢂ
systems has previously been studied. The reactivity in the
micellar phase is known to be lower than in water owing to the
decreased polarity of the micellar interface. Thus, the reactiv-
ity of 1-phenylethylnitrite in the micellar phase is lower than
with 2-bromoethylnitrite when a particular amine is consid-
ered. This can be ascribed to the different sites of average
location in both substrates in the micellar interface. The more
hydrophilic 2-bromoethylnitrite will be located near the mi-
cellar surface, in a strongly hydrated zone, while 1-phenyl-
ethylnitrite will reside in a deeper zone of the Stern layer, the
properties of which are quite anisotropic, in a less polar
environment.
ꢀ
2
ꢀ1 ꢀ1
M s ) and in 65% (v/v) acetonitrile-water mixtures
1
0
ꢀ
3
ꢀ1 ꢀ1
M s ) reveals a reduction of only 28 times
(
k = 1.12 ꢂ10
in the reaction rate. This sensitivity level is similar to that of
nitroso group transfers from MNTS to secondary amines.
Thus, the nitrosation rate of N-methylbenzylamine by MNTS
4
5
decreases about 25 times from 10 to 66% (v/v) acetonitrile.
3
1
Based on previous results, the rate constants for the nitroso
group transfer in AOT–isooctane–water and TTABr–alcoho-
l–isooctane–water microemulsions are independent of W over
the range 5 o W o 45. Since the rate of MNTS denitrosation
is similarly sensitive to the polarity of the medium as its rate of
i
H
transnitrosation, k
should also be independent of W in this
A similar inhibitory effect is apparent when one compares
the rates of nitroso group transfer from MNTS to piperazine
4
7
and N-methylbenzylamine in water, ternary microemulsions
1
of AOT–isooctane–water and quaternary microemulsions
3b
of TTABr–1-hexanol–isooctane–water. Thus, the rate con-
4
8
stant is about 6 times smaller for piperazine and 60 times
smaller for N-methylbenzylamine.
The rate constant for the acid hydrolysis of MNTS in
49
4
water is about 10 times greater than that in SDS micelles,
4
which can be ascribed to a decreased polarity of the micellar
interface relative to the aqueous medium. The constant in
SDS–1-hexanol–isooctane–water microemulsions is about 45
times smaller than in water. This is consistent with the
influence of microemulsions and micelles on the rate of
transnitrosation and hydrolysis of MNTS. It should be noted
that the results for both the acid denitrosation of MNTS and
the nucleophilic attack of bromide ion on benzyl chloride
suggest that the microemulsion interface is less polar than in
Fig. 9 Variation of the slopes and intercepts of Table 3 with the water
content of the microemulsion. (J) 1/Intercept; (K) 1/Slope.
8
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single-component micellar systems; this is a consequence of
the alcohol acting as a cosurfactant and displacing hydration
water molecules from the microemulsion interface.
Harwell, Marcel Dekker, Inc., New York, 1989, and references
therein.
E. B. Leodidis and T. A. Hatton, J. Phys. Chem., 1990, 94, 6400.
F. Yang and A. J. Russell, Biotechnol. Bioeng., 1994, 43, 232.
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¨
4
5
6
(
2004, 20, 409.
b) N. Ohtani, T. Ohta, Y. Hosoda and T. Yamashita, Langmuir,
Conclusions
7
8
B. H. Robinson, A. N. Khan-Lodhi and T. Towey, in Structure
and Reactivity in Reversed Micelles, ed. M. P. Pileni, Elsevier,
Amsterdam, 1989.
(a) J. H. Fendler and E. J. Fendler, Catalysis in Micellar and
Macromolecular Systems, Academic Press, New York, 1975; (b)
L. S. Romsted, J. Phys. Chem., 1985, 89, 5107; (c) L. S. Romsted,
J. Phys. Chem., 1985, 89, 5113.
C. A. Bunton and L. S. Romsted, in Solution Behavior of
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and E. J. Fendler, Plenum, New York, 1982, pp. 975.
Using the pseudo-phase formalism to explain the influence of
the composition of the microemulsion on the nucleophilic
attack of bromide ions on benzyl chloride and the acid
denitrosation of N-methyl-N-nitroso-para-toluenesulfonamide
allowed us to
9
(a) Simplify a quaternary microemulsion (TTABr–1-hexa-
nol–isooctane–water or SDS–1-hexanol–isooctane–water) to
one consisting of only three components, namely: an aqueous
phase, an interface formed by the surfactant and 1-hexanol,
and a continuous medium composed of isooctane and
10 (a) H. F. Eicke, Top. Curr. Chem., 1980, 87, 85; (b) P. L. Luisi and
L. J. Magid, CRC Crit. Rev. Biochem., 1986, 20, 409; (c) Y.
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1
-hexanol. The incorporation of the alcohol into the interface
11, 4873; (e) T. K. De and A. Maitra, Adv. Colloid Interface Sci.,
1995, 59, 95, and references therein.
increases the interfacial volume, which is now the combination
of those occupied by the surfactant and alcohol.
1
1 (a) P. Ekwall, L. Mandell and K. Fontel, J. Colloid Interface Sci.,
1
969, 29, 639; (b) M. Seno, K. Sawada, K. Araki, K. Iwamoto
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147, 152.
bromide ion to benzyl chloride to partition solely between
the continuous medium and the interface, and hence that the
reaction takes place at the microemulsion interface only. The
pseudo-phase formalism allowed us to determine the partition
constant of the substrate between the continuous medium and
1
2 (a) P. D. Profio, R. Germani, G. Onori, A. Santucci, G. Savelli
and C. A. Bunton, Langmuir, 1998, 14, 768; (b) J. Sunamoto, K.
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the interface, as well as the product of the rate constant and
i
the charge fraction neutralized at the interface (bk
ꢀ
=
Br
ꢀ
6
ꢀ1 ꢀ1
i
ꢀ is independent of
3
.22 ꢂ10
M
s
). The fact that bkBr
W suggests that the degree of dissociation of the surfactant at
the microemulsion interface is independent of the water con-
tent of the microemulsion.
1
(c) Obtain similar results for the acid denitrosation of
J. Phys. Org. Chem., 1989, 2, 553.
14 (a) R. Zana, Adv. Colloid Interface Sci., 1995, 57, 1; (b) J. Van
Nieuwkoop and G. Snoei, J. Colloid Interface Sci., 1985, 103, 417.
MNTS in SDS–1-hexanol–isooctane–water microemulsions.
+
H
+
On the assumption that kNa D 1 for the Na /H ion-
i
H
i
H
exchange equilibrium, an average k
ꢀ
b value of k
b = 5.8 ꢂ
1
5 A. Jain and R. P. B. Singh, J. Colloid Interface Sci., 1981, 81, 536.
4
ꢀ1 ꢀ1
1
0
M s
was obtained over the range 7 o W o 25. This
result confirms that the fraction of neutralized charge at the
16 R. Zana, S. Yiv, C. Strazielle and P. Lianos, J. Colloid Interface
Sci., 1981, 80, 208.
17 P. Mukerjee, J. Phys. Chem., 1962, 66, 1733.
18 M. Dollet, J. Juillard and R. Zana, J. Solution Chem., 1980, 9,
interface is independent of the water content of the micro-
i
emulsion. Under the assumption that b = 0.8, a kH value was
obtained by analogy with micellar systems that is roughly 45
times smaller than in pure water; this is consistent with other
results for nitroso group transfers in microemulsions.
827.
19 G. Palazzo, F. Lopez, M. Giustini, G. Colafemmina and A.
Ceglie, J. Phys. Chem. B, 2003, 107, 1924.
0 J. E. Bowcott and J. H. Schulman, Z. Elektrochem., 1955, 59, 283.
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2
2
4
4, 242; (b) V. K. Bansal, D. O. Shah and J. P. O’Conel, J. Colloid
Interface Sci., 1980, 75, 462; (c) K. S. Birdi, Colloid Polym. Sci.,
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Acknowledgements
Financial support from Spain’s Ministerio de Ciencia y Tec-
´
a (Project CTQ2005-04779) and Xunta de Galicia
2
1
2
000, 16, 3101; (f) S. K. Hait and S. P. Moulik, Langmuir, 2002,
8, 6736; (g) M. Giustini, S. Murgia and G. Palazzo, Langmuir,
004, 20, 7381.
nologı
(
PGIDT03-PXIC20905PN and PGIDT04TMT209003PR) is
gratefully acknowledged.
22 P. Stilbs, J. Colloid Interface Sci., 1982, 87, 385.
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3 L. Garcı
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4 C. Bravo, J. R. Leis and M. E. Pen
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70 | New J. Chem., 2007, 31, 860–870
This journal is ꢁc the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007