Influence of temperature on the reactivity of phosphorus acid esters in reverse micellar systems based on sodium bis(2-ethylhexyl)sulfosuccinate

Article abstract of DOI:10.1007/s11172-005-0426-x

A change in the reactivity of ethyl p-nitrophenyl chloromethylphosphonate in the sodium bis(2-ethylhexyl)sulfosuccinate-n-nonane-water system around the percolation threshold was found. Study of location sites of the reactants by NMR self-diffusion and optical spectroscopy and modeling of the kinetic data in terms of the pseudophase approach demonstrated that below the percolation threshold, the reaction occurs in the surface layer. The observed rate constant for substrate hydrolysis in a microemulsion below the percolation threshold is described by the Arrhenius equation, like that in aqueous solutions. Above the percolation threshold, the slope of the Arrhenius plot sharply changes, which is apparently due to a change in the reactant location pattern and, hence, the microscopic properties of the medium in the region of their solubilization.

Full text of DOI:10.1007/s11172-005-0426-x

Russian Chemical Bulletin, International Edition, Vol. 54, No. 6, pp. 1449—1457, June, 2005
1449
Influence of temperature on the reactivity of
phosphorus acid esters in reverse micellar systems based on
sodium bis(2ꢀethylhexyl)sulfosuccinate
a
a
a
L. Ya. Zakharova,a A. R. Ibragimova, F. G. Valeeva, V. M. Zakharov,
ꢀ
L. A. Kudryavtseva,a A. I. Konovalov,a N. L. Zakharchenko,a and Yu. F. Zuevb
aA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 73 2253. Eꢀmail: lucia@iopc.knc.ru
bKazan Institute of Biochemistry and Biophysics,
Kazan Research Center of the Russian Academy of Sciences,
p.o. 30, 420111 Kazan, Russian Federation
A change in the reactivity of ethyl pꢀnitrophenyl chloromethylphosphonate in the sodium
bis(2ꢀethylhexyl)sulfosuccinate—nꢀnonane—water system around the percolation threshold
was found. Study of location sites of the reactants by NMR selfꢀdiffusion and optical spectrosꢀ
copy and modeling of the kinetic data in terms of the pseudophase approach demonstrated that
below the percolation threshold, the reaction occurs in the surface layer. The observed rate
constant for substrate hydrolysis in a microemulsion below the percolation threshold is deꢀ
scribed by the Arrhenius equation, like that in aqueous solutions. Above the percolation
threshold, the slope of the Arrhenius plot sharply changes, which is apparently due to a change
in the reactant location pattern and, hence, the microscopic properties of the medium in the
region of their solubilization.
Key words: reverse micelles, microemulsions, kinetics, hydrolysis, phosphorus acid esters,
percolation, pseudophase model.
The reverse micellar systems are thermodynamically
stable dispersions of nanoꢀsized water drops in an organic
liquid (oil) stabilized by a surfactant monolayer. A freꢀ
quently used surfactant is sodium bis(2ꢀethylhexyl)sulꢀ
fosuccinate (Aerosol OT, AOT). The branched molꢀ
ecules of this surfactant allow the formation of reverse
micelles (region L2 in the phase diagram of microꢀ
emulsion) without the addition of other surfactants over a
broad range of temperatures and compositions.1 The reꢀ
verse micellar systems offer broad possibilities for the deꢀ
liberate control of the rates and mechanisms of chemical
reactions.2
The continuous phase in these systems is formed by
nonpolar organic liquids; hence, its conductivity is relaꢀ
tively low and determined by the exchange of ions formed
upon dissociation of the head groups of ionic surfactants
between the reverse micelles during their thermal motion
and random collisions. Under certain conditions (for exꢀ
ample, on temperature rise), the conductivity of such
systems sharply increases by four to five orders of magniꢀ
tude.3,4 This phenomenon, which has been called electric
percolation, is due5 to clusterization of reverse micelles
and charge migration within extended clusters (Scheme 1).
The conditions for micelle adhesion, shortꢀterm coalesꢀ
cence, and return to the initial state are determined, first
of all, by the properties of the hydrophobic surface of
reverse micelles. A necessary condition for the temporal
coalescence of particles is the mutual penetration of the
hydrocarbon radicals of the interacting micelles, which
depends on the structural organization of the surfactant
surface layer.6
Clusterization of AOTꢀbased reverse micelles is obꢀ
served over a broad range of concentrations of the disꢀ
perse phase and reflects the changes taking place in the
structure of the micelle hydrophobic shells on temperaꢀ
ture rise. This induces a change in the physicochemical
parameters of the compound microenvironment (viscosꢀ
ity, polarity, etc.), which can influence the kinetics of
chemical reactions in these media.
As we have shown previously,7 the structural rearꢀ
rangement of reverse micelles affects the reactivity of phosꢀ
phorus acid esters. To elucidate the causes and the mechaꢀ
nism of this relationship, we continued the studies of the
catalytic effect of an AOTꢀbased reverse micellar system.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1407—1414, June, 2005.
1066ꢀ5285/05/5406ꢀ1449 © 2005 Springer Science+Business Media, Inc.

1450
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Zakharova et al.
Scheme 1
Na+
Na+
OH– OH–
Na+
Na+
OH–
Na+
OH–
Na+
OH–
OH–
Na+
Na+
Na+
Na+
OH–
Na+
OH–
OH–
Na+
Na+
Na+
Na+
Na+
Na+
OH–
Na+
OH–
Na+
Na+
OH–
Na+
OH–
OH–
Na+
Na+
OH–
OH–
Na+
OH–
Na+
OH–
Na+
Na+
Na+
OH–
Na+
OH–
Na+
Na+
OH–
Na+
OH–
OH–
Na+
OH–
Na+
Na+
Na+
Na+
OH–
Na+
OH–
Na+
OH–
Na+
OH– Na+
Na+
OH–
Na+
OH–
Na+
Na+
OH–
Na+
OH–
OH–
OH–
Na+
Na+
Na+
The selfꢀdiffusion coefficients (D) were measured by Fourier
This work is devoted to the kinetics of alkaline hydrolysis
of ethyl pꢀnitrophenyl chloromethylphosphonate (1) in
the AOT—nꢀnonane—water system (Scheme 2) at temꢀ
peratures below and above the spontaneous clusterization
threshold. In addition, location sites of the substrate in
the reverse micelles were determined by the NMR selfꢀ
diffusion technique.
Transform 1H NMR spectroscopy with a pulse magnetic field
gradient10 using a modified Tesla BS 587 A NMR spectrometer
1
operating at a frequency of 80 MHz (for H) and equipped with
a gradient block designed for producing a magnetic field gradiꢀ
ent of up to 0.5 T m–1. The selfꢀdiffusion coefficients were
determined from the individual proton signals of system compoꢀ
nents using a standard twoꢀpulse Hahn sequence. The error of
determination of the coefficients did not exceed 5%.
The kinetics of hydrolysis was studied by spectrophotometry
on a Specord Мꢀ400 instrument based on the variation of abꢀ
sorption of the leavingꢀgroup anion under the pseudoꢀfirst order
conditions. The observed rate constants (kobs) were found from
the dependence
Scheme 2
ln(A∞ – A) = –kobst + const,
where A and A∞ are the solution absorbances at time instant t
and after completion of the reaction, respectively. They were
found using the weighed leastꢀsquares method taking into acꢀ
count the arithmetic mean values of the three measurements
differing by at most 5%.
The kinetic data were analyzed using a pseudophase model
according to which the reactants are distributed among the phases
and the reaction proceeds in one or several pseudophases. For
reactions that proceed in the surface layer, while one reactant
(substrate) is distributed between the oil phase and the surfacꢀ
tant monolayer and the other reactant (hydroxide ion) is distribꢀ
uted between the surface layer and water, the equation for the
observed rate constant has the form11
Experimental
Compound 1 was prepared by a known procedure;8 comꢀ
mercial AOT (Sigma) was used as received. The reverse micellar
systems were prepared by adding a calculated amount of water
to a solution of AOT in nꢀnonane of a specified concentration
followed by vigorous shaking of the mixture until a transparent
solution formed. The molar ratios of the components, namely,
W = [H2O]/[AOT], characterizing the size of the water cores
of the micelles, and Z = [nꢀnonane]/[AOT], characterizing
the micelle concentration, were varied in the 9.8—37.6 and
5—22 ranges, respectively, corresponding to the region L2 in the
system phase diagram.9
kobs = (kiKSKOH[OH]tot)/{(KS + Z)(KOH + W)[AOT]},
(1)
where ki/s–1 is the rate constant for the bimolecular reaction
provided that the nucleophile concentration is expressed as the
[OH]/[AOT] molar ratio; KS is the partition constant of the
substrate (S) between the oil phase and the surfactant; KOH is
the partition constant of the hydroxide ion between the aqueous
phase and the surfactant; [OH]tot/mol L–1 is the total concenꢀ
tration of the OH– ions. The constant ki is related to the tradiꢀ
The conductivity was measured by an OK 102/1 conductoꢀ
meter (Radelkis, Hungary) at frequencies of 80 Hz and 3 kHz in
a parallelꢀsided cell maintained in a thermostat. The temperaꢀ
ture was controlled to an accuracy of 0.2 °C.

Reverse micellar systems based on aerosol OT
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
1451
tional pseudoꢀfirst (ki´/s–1) and second (k2,i/L mol–1 s–1) order
Conductivity/µS cm–1
rate constants by the following relations:
3
105
ki´ = ki([OH]/[AOT]),
k2,i = kiV,
2
104
1
where V is the molar volume of the surfactant. The constants KS
and KOH are given by
103
102
101
KS = ([S]i[nꢀC9H20])/([S]o[AOT]),
KOH = ([OH]i[H2O])/([OH]w[AOT]).
Here, the molar concentrations of the substrate (S), nꢀnonane,
and surfactant are given in brackets and the subscripts i, o,
and w refer to the surface layer, oil, and water pseudophases,
respectively. The main assumptions and approximations used in
the pseudophase model have been considered previously.12 The
linearized form of the equation (1)
20
40
60
T/°C
Fig. 1. Conductivity of the AOT—nꢀnonane—water system
vs. temperature at W = 9.8 (1), 15.1 (2), and 20.0 (3) (CAOT
0.4 mol L–1).
=
1/(kobs[AOT]) = (KOH + W)/(kiKOH[OH]) +
+ {(KOH + W)/(kiKSKOH[OH])}Z
(2)
D•1011/m2 s–1
allows one to calculate the KS, KOH, ki, and k2,i. It can be seen
from Eq. (2) that the binding constant of the substrate KS can be
determined graphically using the plot 1/(kobs[AOT])—Z from
the ratio of the yꢀintercept to the slope of the plot. This model
implies that the KS value does not change upon the variation of W.
3
102
2
Results and Discussion
4
According to the conductometric data shown in
Fig. 1, the percolation threshold temperature (Tp) in the
AOT—nꢀnonane—water system is 26—38 °C depending
on the parameter W. Previously,7 we discovered some
increase in the percolation threshold temperature with
an increase in the alkali concentration; however, for
[NaOH] = 0.01 mol L–1, which was maintained in the
experiment, this effect is insignificant. The results of conꢀ
ductivity measurements were used to choose conditions
for the kinetic measurements before and after the micelle
clusterization threshold, which was defined as the temꢀ
perature of the sign change of the second derivative of the
conductivity with respect to temperature.
101
1
100
3.0
3.2
3.4
T –1•103/K–1
The selfꢀdiffusion coefficients D of the system comꢀ
ponents found by NMR10 spectroscopy are presented in
Fig. 2. Owing to the specific features of AOTꢀstabilized
reverse micellar systems, it was possible to use micelle
clusterization for determining the substrate location sites
in the reverse micelles. Although compound 1 is rather
hydrophobic, its structure implies accumulation not only
in the oil pseudophase but also in other microregions of
the system. It is reasonable to suggest that if compound 1
is completely dissolved in nꢀnonane, the temperature deꢀ
pendence of its selfꢀdiffusion coefficient plotted in the
Arrhenius coordinates would be parallel to the depenꢀ
dence of the selfꢀdiffusion coefficient of nꢀnonane. The
absolute D values for phosphonate 1 dissolved in the oil
Fig. 2. Temperature dependences of the selfꢀdiffusion coeffiꢀ
cients (D) of components of the AOT—nꢀnonane—water sysꢀ
tem: AOT (1), water (2), nꢀnonane (3), pꢀnitrophenyl ethyl
chloromethylphosphonate (4) (W = 20.0, CAOT = 0.4 mol L–1).
phase should not differ much from the D value for
nꢀnonane as the sizes of their molecules are similar. In
reality, the coefficient D found experimentally for comꢀ
pound 1 in this system is intermediate between the values
for AOT (micelle diffusion) and nꢀnonane. The exponenꢀ
tial pattern of the diffusion decay of the resonance signal
from the protons of compound 1 suggests that this comꢀ
pound is distributed between the disperse phase and the
dispersion medium. These two states undergo fast (on

1452
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Zakharova et al.
kobs/s–1
the NMR time scale) exchange. Using the twoꢀstate
model,13,14 we found that 52% of the substrate is disꢀ
solved in the dispersion medium (nꢀnonane), while the
rest is located in the reverse micelles. The site of location
of compound 1 in the reverse micelles is determined by
analyzing the temperature dependence of the selfꢀdiffuꢀ
sion coefficient. Provided that phosphonate 1 is located
in the aqueous cores of the micelles, the temperature
variations of its diffusion are expected to reproduce the
temperature variations of water diffusion. It follows from
the data presented in Fig. 2 that the temperature depenꢀ
dence of the selfꢀdiffusion coefficient of compound 1
reflects the temperature variation of the selfꢀdiffusion coꢀ
efficient of AOT, which is due to the substrate accumulaꢀ
tion in the surfactant monolayer. Thus, phosphonate 1 is
distributed between the nꢀnonane bulk and the micelles
in which it is located in the surface layer formed by AOT
molecules.
4
0.05
1
2
1
2
0.01
We studied the kinetics of hydrolysis of phosphonate 1
at different temperatures covering a range below and above
the percolation threshold. The plots logkobs—1/T for variꢀ
ous W (CAOT = 0.4 mol L–1, CNaOH = 0.01 mol L–1) are
presented in Fig. 3. Unlike aqueous solutions, in reverse
micellar systems, the usual Arrhenius dependence holds
only up to a certain, critical temperature (Tcr). Compariꢀ
son of the data presented in Figs 1 and 3 shows that the
Tcr and Tp values are almost equal for equal W values
(Table 1). We assume that the change in the slope of the
Arrhenius plot is related to those temperatureꢀinduced
structural changes in reverse micelles that induce clusꢀ
terization. Evidently, this may be accompanied by changes
in some characteristics of the micellar microenvironment
of the reactants (micropolarity, surface potential, etc.),
which affect the reactivity.
The temperature dependence of the rate constant for
chemical reactions in solutions is known15 to be described
by the Arrhenius equation. Unlike molecular solutions,
micellar media are microheterogeneous. The kinetics of
these systems is often described using the pseudophase
approach according to which the reaction may occur in
three microregions: in the oil and aqueous pseudophases
and in the surface layer. In this case, the equation for the
observed rate constant has three constituents.16 In this
3
3
3.1
3.2
3.3
3.4 T –1•103/K–1
Fig. 3. Arrhenius dependence of the observed rate constant
for the alkaline hydrolysis of phosphonate 1 (kobs) in the
AOT—nꢀnonane—water reverse micellar system at W = 9.8 (1),
15.1 (2), and 20.0 (3) and in water (4) (CNaOH = 0.01 mol L–1
CAOT = 0.4 mol L–1).
,
study, we consider a simpler situation where the reaction
zone is assumed to be limited to the surface layer and the
observed rate constant is described by Eq. (1). According
to the known data,11,12 the rate of a chemical reaction
should not depend on the micelle formation dynamics or
reactant exchange between the pseudophases. The fulfillꢀ
ment of the Arrhenius equation then depends on the deꢀ
gree to which the binding constants are temperatureꢀdeꢀ
pendent. In micellar systems in the absence of phase tranꢀ
sitions, the binding constants of the reactants depend on
the temperature; however, the contribution of these
changes to the observed rate constant can be neglected
compared with the case of exponential variation of the
rate constant, especially in a narrow temperature range.
When the logkobs—1/T plot is linear, analysis of the kiꢀ
netic data in micellar solutions allows one to calculate the
activation energy.
Table 1. Values of Tp and Tcr and Arrhenius equation parameters
below the percolation threshold (activation energy (Еа) and preꢀ
exponential factor (logA)) for alkaline hydrolysis of phosphoꢀ
nate 1 in the AOT—nꢀnonane—water reverse micellar system
The properties of the microdrops in which the chemiꢀ
cal reaction occurs are also subject to the influence of the
temperature. For the system under study, this influence is
most pronounced around the temperature percolation
threshold. In view of the complex structure of the microꢀ
emulsion and the presence of the polarity gradient in the
surface layer, not only a quantitative change in the partiꢀ
W
Tp
Tcr
Еа
logA
/kJ mol–1
°C
09.8
15.1
20.0
26
35
37
25
32
36
31.5
31.2
26.7
3.7
3.5
2.7

Reverse micellar systems based on aerosol OT
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
1453
kobs/s–1
tion coefficients of the reactants is possible under temꢀ
perature perturbation conditions, but also qualitative
changes, resulting in the migration of the reaction zone.
In this case, additional influence of the temperature on
the observed rate constant is expected, due to the change
of the microscopic properties in the reaction zone. In a
simplified form, this effect can be compared with change
of the solvent, and is not necessarily described by the
Arrhenius equation.
0.025
a
0.020
0.015
0.010
3
2
1
Therefore, the effect of temperature on the reaction
kinetics in reverse micellar systems can be separated into
two components. One is related to the usual thermal actiꢀ
vation of chemical processes, while the other is due to the
change in the reactant location sites and partition coeffiꢀ
cients in the system due to modification of the properties
of aggregates. The usual course of the Arrhenius depenꢀ
dence (see Fig. 3) at T < Tcr indicates that the former
component makes a predominant contribution. In this
case, the slope of the logkobs—1/T plot is described by
the activation energy (Еa) of the alkaline hydrolysis of
substrate 1. Above the critical temperature, the competiꢀ
tive trend predominates, the pattern of the dependence
being determined by the change in the local properties of
the reactant microenvironment with temperature. In the
region T > Tcr, kobs decreases following an increase in T.
Although the linearity in the logkobs—1/T coordinates
holds, it is hardly reasonable to speak of the fulfillment of
the Arrhenius equation. With this data interpretation,
which is shown in Fig. 3, it becomes clear that Tcr coinꢀ
cides with the region of the percolation threshold when
sharp changes take place in the structure of microꢀ
emulsions.
We studied in detail the reaction kinetics at various
temperatures above and below the percolation threshold.
The kinetic dependences of the observed rate constant on
the surfactant concentration at 15—25 °C, i.e., at temꢀ
peratures below the clusterization threshold of the miꢀ
celles, are presented in Fig. 4. In the AOT—nꢀnoꢀ
nane—water system, the process is retarded by almost an
order of magnitude compared to the reaction in water. As
the surfactant and water concentrations increase, the obꢀ
served rate constant decreases. This trend is typical for
the case where the reaction occurs in the surface layer16—18
and is due to dilution of the reactants on the increase in
the volume of the aqueous phase and the interface surꢀ
face. Analysis of the kinetic data at different temperatures
(see Fig. 4) made it possible to calculate the activation
energy and the preꢀexponential factor, which are preꢀ
sented in Table 2. For any composition of the micellar
system, the Arrhenius equation holds, as indicated by
linearity of the logkobs—1/T plot with the correlation coꢀ
efficient R ≥ 0.9. The Еa values depend only slightly
on the AOT concentration and water and fall in the
29.0—34.5 kJ mol–1 range; the logA values also change
insignificantly (3.1—4.0) upon variation of the composiꢀ
0.3
0.4
CAOT/mol L–1
kobs/s–1
b
0.016
0.012
0.008
3
2
1
0.3
0.4
CAOT/mol L–1
kobs/s–1
c
0.005
0.004
0.003
0.002
3
2
1
0.3
0.4
CAOT/mol L–1
Fig. 4. Observed rate constant of the alkaline hydrolysis
of phosphonate 1 (kobs) vs. surfactant concentration in the
AOT—nꢀnonane—water reverse micellar system at W = 9.8 (a),
15.1 (b), and 20.0 (c) and T = 15 (1), 20 (2), and 25 °C (3)
(CNaOH = 0.01 mol L–1).
tion. The Arrhenius parameters for an aqueous solution in
the absence of a surfactant (Еа = 33.7 kJ mol–1, logA = 4.0)
fall in these ranges. These results provide the conclusion
that the reaction mechanism does not change when the
reaction is transferred from an aqueous solution to the
micellar pseudoꢀphase and on rather broad variation of
the micellar composition. The calculated activation enerꢀ
gies for the alkaline hydrolysis below the percolation
threshold given in Table 2 are in good agreement with the
values given in Table 1, which confirms the assumption
concerning the predominant contribution of the activaꢀ
tion of chemical reaction to the temperature dependence
of the rate constant below the percolation threshold.
As noted above, Eq. (1) implies distribution of the
substrate between the oil phase and the surface layer and

1454
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Zakharova et al.
–1
–1
–1
–1
Table 2. Arrhenius equation parameters (activation energy
(Еа/kJ mol–1) and preꢀexponential factor (logA)) for alkaline
hydrolysis of phosphonate 1 in the AOT—nꢀnonane—water reꢀ
verse micellar system
kobs
kobs
kobs
C
C
C
AOT
AOT
AOT
3
a
1000
800
600
400
200
СAOT
/mol L–1
W = 9.8
W = 15.1
W = 20.0
2
Ea
logA
Ea
logA
Ea
logA
1
0.24
0.31
0.40
0.45
0.50
30.3
30.3
33.5
33.5
34.5
3.7
3.7
4.1
4.1
4.0
31.0
33.3
33.0
33.3
32.9
3.7
4.0
3.9
3.9
3.8
33.0
30.6
29.0
33.1
33.3
3.5
3.0
3.1
3.4
3.4
10
–1
20
30
Z
1000
800
600
400
200
3
b
Note. In water without a surfactant Ea = 33.7 kJ mol–1
logA = 4.0.
,
distribution of the nucleophile (hydroxide ion) between
the aqueous core and the surface layer (Scheme 3). It is
necessary that the partition constants be independent of
the microemulsion composition. The graphical analysis
of the kinetic data at various temperatures below the perꢀ
colation threshold is shown in Fig. 5. The substrate bindꢀ
ing constants found by graphical solution of Eq. (2) are
presented in Table 3.
2
1
10
–1
20
30
Z
3
1200
1000
800
c
Scheme 3
600
2
400
1
200
10
20
30 Z
Fig. 5. Linearization of the kinetic data presented in Fig. 4 using
Eq. (2) for T = 15 (a), 20 (b), and 25 °C (c) and W = 9.8 (1),
15.1 (2), and 20.0 (3).
and 20 °C are almost equal, being both 45 3 (see Table 3),
i.e., >50% of the substrate is located in the surface layer at
any composition of the micellar system. This is in good
agreement with NMR data for selfꢀdiffusion. At 25 °C,
the binding constant of the substrate is twice lower, which
is apparently due to reactant redistribution taking place as
the temperature approaches the percolation threshold. The
other parameters of Eq. (1) at 25 °C are as follows:
KOH = 1.9, ki = 3.9 s–1. The k2,1 value was calculated from
It was found that Eqs (1) and (2) hold for all temperaꢀ
tures, as indicated by the linearity of the 1/kobsCAOT—Z
plot (see Fig. 5) and by similarity of the KS values obꢀ
tained upon variation of W (see Table 3). This means that
below the percolation threshold, the reaction proceeds
only in the surface layer. Typically, the KS values at 15
Table 3. Results of quantitative analysis of the kinetic data at
different temperatures using graphical solution of Eq. (2)
the relation k2,i = kiV. Assuming that V = 0.37 L mol–1 11
,
one gets k2,i = 1.44 L mol–1 s–1. The results of calculaꢀ
tions allow one to draw conclusions on what factors deꢀ
termine the effect of microemulsions on the reactivity of
phosphonates. According to known data,19 changes in the
microenvironment properties (micellar microenvironment
factor) and in the local reactant concentration (concenꢀ
trating factor) upon transfer of the reaction from a moꢀ
lecular solution to micelles are the main factors responꢀ
W
T = 15 °C
KS
T = 20 °C
KS
T = 25 °C
KS
a
b
a
b
a
b
9.8
188.3 4.2 45.0 143.1 3.4 42.1 97.2 4.5 21.3
15.1 269.9 5.7 47.6 214.4 4.5 47.6 143.4 6.9 20.8
20.0 530.1 19.2 43.7 656.3 15.1 43.5 474.4 22.1 21.4
Note. a is the yꢀintercept; b is the slope of the intercept

Reverse micellar systems based on aerosol OT
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
1455
sible for the catalytic action of micellar systems. One
component of the microenvironment factor can be comꢀ
pared with the solvent effect: this includes the medium
polarity, the efficiency of solvation of the reactants and
the transition state, etc. This contribution can be convenꢀ
tionally designated as the enthalpy constituent of the miꢀ
croenvironment factor. In addition, the transfer of the
reaction to the micellar pseudophase may also change the
entropy contribution, which takes into account the deꢀ
gree of mobility and the mutual orientation of the reacꢀ
tants (configuration of the activated complex). The miꢀ
croenvironment factor can be described quantitatively by
the k2,i/k2,w value (k2,w = 4.0 L mol–1 s–1), which is equal
to 0.36. From this, it follows that the inhibitory effect of
the AOT—nꢀnonane—water system in the hydrolysis of
phosphonate 1 is mainly due to the adverse influence of
the reactant microenvironment in the surface layer. The
factor of reactant concentrating, which plays the crucial
role in normal micelles, does not exert a significant effect
in reverse micellar systems. This is apparently due to the
low partition constant of hydroxide ions.
increases with an increase in the water content, while the
tendency of the rate constant to decrease with an increase
in the surfactant concentration is retained. Second, for
W = 9.8 and 15.1 above the percolation threshold, kobs
substantially decreases (by up to an order of magnitude)
as the temperature rises from 25 to 40 °C. For W = 20.0
and high AOT concentrations, the kobs values at 25 and
40 °C are almost equal, while for [AOT] < 0.38 mol L–1
,
the rate constant at 40 °C is somewhat higher than that
at 25 °C. At 40 °C, the variations of kobs upon the variaꢀ
tion of the system composition are more pronounced than
at low temperatures (cf. Fig. 4). For instance, an increase
in the surfactant concentration at an invariable water conꢀ
tent induces a 3 to 4ꢀfold decrease in kobs, while the
variation of W at a constant surfactant concentration reꢀ
sults in a 7ꢀfold change in the rate constant. Below the
percolation threshold, variation of the surfactant concenꢀ
tration changes kobs at most 1.5ꢀfold, and upon the variaꢀ
tion of W, the kobs value changes 5ꢀfold.
Unlike the plots presented in Fig. 4, the kinetic data
for 40 °C cannot be linearized in the coordinates of Eq. (2),
as indicated by the scatter of points and the negative slope
of the plots (Fig. 7). Apparently, due to the system perturꢀ
bation near and above the clusterization temperature, the
distribution of the reactants is not described by Scheme 3
and the reaction zone is not confined to the surface layer.
According to the data presented in Fig. 1, the temperaꢀ
tureꢀinduced variations of the microemulsion state are
substantially different depending on its composition, i.e.,
at the same temperature, the reactants may occur under
different conditions depending on the surfactant concenꢀ
tration and water content, and the application of Eq. (1)
becomes illegitimate. The fact that Eq. (1) does not hold
above the percolation threshold is convincing evidence
confirming the change in the distribution and location of
the reactants and, hence, in the nature of their microenꢀ
vironment.
Study of the absorption spectra of pꢀnitrophenol, a
product of the reaction under consideration used to moniꢀ
tor the reaction kinetics, has shown (Fig. 8) that raising
the temperature in the 25—40 °C range entails a decrease
in the intensity of the absorption band at λ = 400 nm, i.e.,
the proportion of the dissociated form of the compound
decreases. This fact indicates that the acid properties of
pꢀnitrophenol decrease on temperature rise. The decrease
in the absorbance (see Fig. 8) is much more pronounced
than the expected change caused by the temperature deꢀ
pendence of the extinction coefficient, which is observed
for an aqueous solution of pꢀnitrophenol or in the
AOT—nꢀnonane—water reverse system beyond the perꢀ
colation transition area. The presence of a substantial
polarity gradient in reverse micelles (dielectric permeꢀ
ability changes from 2—4 in the bulk pseudophase to 80 in
the aqueous core at a high degree of hydration) provides
the conclusion that the observed shift of the compound
The dependences of kobs on the medium composition
at 40 °C, i.e., above the percolation threshold, are preꢀ
sented in Fig. 6. When comparing the data presented in
Figs 4 and 6, one can note several essential differences.
First, above the percolation threshold, kobs shows the opꢀ
posite dependence on W, in particular, the rate constant
kobs•103/s–1
12
10
8
3
2
6
4
1
2
0.3
0.4
0.5 CAOT/mol L–1
Fig. 6. Observed rate constant of the alkaline hydrolysis of
phosphonate 1 (kobs) vs. surfactant concentration (CAOT) in the
AOT—nꢀnonane—water reverse micellar system at W = 9.8 (1),
15.1 (2), and 20.0 (3) (T = 40 °C; CNaOH = 0.01 mol L–1).

1456
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 6, June, 2005
Zakharova et al.
–1
–1
kobs
C
A
AOT
1
0.8
0.6
0.4
0.2
8000
a
6000
4000
2000
2
3
12
14
16
18
20
Z
b
–1
–1
kobs
C
AOT
1000
800
600
300
400
500 λ/nm
Fig. 8. Absorption spectra of pꢀnitrophenol in the visible region
at 25 (1), 30 (2), and 40 °C (3) (CAOT = 0.4 mol L–1, CNaOH
=
8
10
12
14
16
18
Z
0.01 mol L–1).
–1
–1
kobs
C
AOT
800
600
400
c
solution composition. Above the percolation threshold,
the reaction zone is not confined to the surface layer. In
this case, the reactivity of phosphonate 1 does not obey
the Arrhenius equation, the pattern of temperature deꢀ
pendence of the observed rate constant for hydrolysis subꢀ
strate 1 being determined by the change in the local propꢀ
erties of the reactant microenvironment on temperaꢀ
ture rise.
8
10
12
14
16
Z
Fig. 7. Linearization of the kinetic data presented in Fig. 6 using
Eq. (2): W = 9.8 (a), 15.1 (b), and 20.0 (c).
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 05ꢀ03ꢀ
08086).
рKа is caused by the change in its location site following
temperature rise and by the transfer of the reaction to a
zone with a reduced micropolarity.
References
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Arrhenius equation; the activation energy and logA change
slightly on the transfer of the reaction from the aqueous
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Received July 22, 2004;
in revised form November 26, 2004