Hydrolysis of phosphorus acid esters in the sodium bis(2-ethylhexyl) sulfosuccinate-n-nonane-polyethylene glycol-water supramolecular system before and after the percolation threshold

Article abstract of DOI:10.1007/s11172-007-0011-6

The kinetics of alkaline hydrolysis of O-ethyl O-p-nitrophenyl chloromethylphosphonate in a sodium bis(2-ethylhexyl)sulfosuccinate-n-nonane- polyethylene glycol-water reverse micellar system was studied in a wide range of concentrations of the surfactant

Full text of DOI:10.1007/s11172-007-0011-6

62
Russian Chemical Bulletin, International Edition, Vol. 56, No. 1, pp. 62—69, January, 2007
Hydrolysis of phosphorus acid esters in the sodium
bis(2ꢀethylhexyl)sulfosuccinate—nꢀnonane—polyethylene glycol—water
supramolecular system before and after the percolation threshold
aꢀ
a
a
a
L. Ya. Zakharova, F. G. Valeeva, A. R. Ibragimova, L. A. Kudryavtseva,
b
a
a
a
Yu. F. Zuev, L. V. Avvakumova, V. I. Kovalenko, and A. I. Konovalov
aA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center, Russian Academy of Sciences,
8
ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 73 2253. Eꢀmail: lucia@iopc.knc.ru
b
Kazan Institute of Biochemistry and Biophysics,
Kazan Research Center, Russian Academy of Sciences,
post box 30, 420111 Kazan, Russian Federation
The kinetics of alkaline hydrolysis of Oꢀethyl Oꢀpꢀnitrophenyl chloromethylphosphonate
in a sodium bis(2ꢀethylhexyl)sulfosuccinate—nꢀnonane—polyethylene glycol—water reverse
micellar system was studied in a wide range of concentrations of the surfactant and water. The
sign of the catalytic effect of micelles is inverted in the presence of the polymer. A sharp change
in the apparent rate constant of hydrolysis of the phosphonate was found in a region of the
temperature percolation threshold.
Key words: reverse micelles, microemulsions, polyethylene glycol, percolation, kinetics,
hydrolysis, phosphorus acid esters, pseudoꢀphase model.
Investigation of the reaction kinetics in selfꢀorganized
tural factor, i.e., change in the shape and size of the
systems is a rapidly developed scientific area.1 Wide posꢀ
,2
aggregates and, as a consequence, microenvironment of
the reactants and their distribution between different
microregions. Since the continuous phase in reverse miꢀ
celles is formed by the nonpolar organic liquid, its conꢀ
ductivity is low and determined by exchange of ions
formed due to the dissociation of the head groups of ionic
surfactants between reverse micelles during their thermal
motion. Under certain conditions, the conductivity of
such systems increases sharply by several orders of magniꢀ
sibilities for direct control of rates of various processes are
3
,4
provided by reverse micellar systems consisting of three
regions of different polarity: nanosized aqueous cores,
nonpolar macrophase of an organic solvent (oil), and
separating interfacial monolayer of a surfactant (Surf).5
A specific feature of the surface layer, whose thickness is
determined, on the average, by the hydrocarbon radical
length of Surf, is a sharp gradient of the medium polarity
on going from the aqueous core to the oil phase of the
organic solvent. The properties of water in the core of a
reverse micelle are also unusual, because they depend on
,6
9
tude. This phenomenon named electrical percolation is
caused by cluster formation of reverse micelles and charge
1
0
11
motion within the extended clusters. We have shown
7
,8
the mole ratio W = [H O]/[Surf]. Water in the core
that the reaction rates before and after the percolation
threshold can differ substantially: the processes can be
accelerated or retarded by inducing the cluster formation
of microemulsion drops.
2
exists, most likely, in two states. At W < 10—12 all H O
2
molecules are bound to the charged head groups of the
Surf and their counterions, and the properties of this
hydrateꢀstructured water differ strongly from those of norꢀ
mal water. At higher W water with the threeꢀdimensional
hydrogen bonding lattice appears in the center of the
core. Its properties are similar to those of bulk water. The
polarity gradient of the interfacial layer and two states of
water in reverse micelles make it possible to govern the
localization of the reactants and, hence, their reactivity
by changing the component ratio.
In this work, we continued to study the factors deterꢀ
mining the catalytic effect of reverse microemulsions using
as an example a system sodium bis(2ꢀethylhexyl)sulfoꢀ
succinate (Aerosol OT, AOT)—nꢀnonane—water—polyꢀ
ethylene glycol (PEG) with molecular weight 10 000. The
introduction of the polymer allows a wider use of the
structural factor of catalysis, because it enhances the probꢀ
ability of formation of new catalytic complexes involving
reactants and components of the micellar system. A difꢀ
ferent influence of cluster formation of microemulsion
Additional possibilities for controlling reaction conꢀ
stants in water/oil microemulsions are given by the strucꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 60—67, January, 2007.
066ꢀ5285/07/5601ꢀ0062 © 2007 Springer Science+Business Media, Inc.
1

Hydrolysis of phosphorus acid esters
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
63
drops in the presence of the polymer on the reactivity of
compounds can be expected.
Scheme 1
Polymerꢀmodified reverse micellar systems are poorly
studied, and virtually no data on their catalytic properties
are available. It is known1
2—16
that, depending on the
nature and concentration of macromolecules, their localꢀ
ization and structure of combined Surf—polymer aggreꢀ
gates can differ considerably. A series of studies indicates
that the structural characteristics of reverse micelles reꢀ
main unchanged in the presence of hydrophilic polymers,
1
2
i.e., macromolecules are dissolved in the aqueous core.
A change in the micelle size in the presence of polymers
was found in some studies. In particular, an increase in
the size of reverse micelles upon the addition of small
polymer additives followed by a decrease in the particle
mechanism of inducing percolation transitions. In the
AOT—nꢀnonane—PEG—water system, the radius of the
aqueous core of micellar aggregates varies in an interval of
1
9
2—4 nm and the inertia radius of the polymer is ~3 nm.
1
3
radius with its increasing concentration has been shown.
These data show that the polymer sizes are close to those of
the aqueous core of micellar particles. The estimate of the
stoichiometric composition of the combined structures
indicates that the number of micelles per macromolecule
changes from 0.5 to 50 depending on the W value and
According to published data,1 the structure and properꢀ
ties of polymerꢀcolloidal complexes formed are deterꢀ
mined by the size ratio of the polymer to microemulsion
drop, i.e., by the molecular weight and concentration of
the polymer and the W parameter of the reverse systems.
We have previously1 obtained data characterizing
the physicochemical properties and catalytic effect of
Surf—polyelectrolyte compositions in reactions of nuꢀ
cleophilic substitution at the phosphorus atom. The
catalytic effect of a reverse micellar system AOT—deꢀ
cane—water was studied in the region of reverse systems
during alkaline hydrolysis of several esters of phosphorus
acids. The change in the reactivity of the compounds
above the percolation threshold has been found for
4
1
9
PEG concentration. Based on the aforesaid, one can
expect that in the AOT—nꢀnonane—PEG—water system
under study the forms of selfꢀorganization shown in Fig. 1
are preferential.
7
The task of this work is to study the effect of polymer
on the catalytic effect of the AOTꢀbased reverse micellar
system and cluster formation of micellar particles on the
kinetics of substrate hydrolysis in the absence and presꢀ
ence of PEG. For this purpose, we studied in parallel
experiments the conductivity of the AOT—nꢀnonane—
PEG—water system and kinetics of alkaline hydrolysis of
Oꢀethyl Oꢀpꢀnitrophenyl chloromethylphosphonate (1)
(Scheme 1) in the temperature interval covering the reꢀ
gion before and after the percolation threshold.
the first time.1
1,18
In addition, we studied structural
19
rearrangements of the AOT—nꢀnonane—water system
in the presence of PEG, determined temperatures of
the percolation threshold (T ), and discussed the
p
Fig. 1. Preferential forms of selfꢀorganization in the АОТ—nꢀnonane—PEG—water system under study.

64
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
Zakharova et al.
Phosphorus acid esters were chosen, because they are
and the reaction occurs in one or several pseudoꢀphases. For the
reactions that occur in the surface layer, when one of the reacꢀ
tants (substrate) is distributed between the oil phase and surfacꢀ
tant monolayer and the second reactant (hydroxide ion) is disꢀ
tributed between the surface layer and water (see Scheme 2), the
biologically active and widely used as pesticides, medical
products, etc. Therefore, the problem of their hydrolytic
stability is practically significant. The phosphoryl group
transfer plays an important role in biocycles of highest
organisms. In addition, this reaction is widely used in aqueꢀ
ous media in the absence of surfactants and in individual
2
2
equation for the apparent rate constant (kapp) has the form
kapp = (kiKSKOH[OH]tot)/{(KS + Z )(KOH + W )[AOT]}, (1)
2
micellar solutions, which allows correct estimation of the
where k /s– is the rate constants in terms of the nucleophile
1
i
contribution of different components to the catalytic efꢀ
fect of more complicated micellar systems to be made.
concentration expressed as the mole ratio [OH]/[AOT]; KS is
the partition constant of the substrate (S) between the oil phase
and surfactant; KOH is the partition constant of the hydroxide
Experimental
ion between the aqueous phase and surfactant; [OH] /mol L–1
tot
is the total concentration of the OH– ions. The k constant is
i
Compound 1 was synthesized according to a known proceꢀ
–1
related to the traditional constants of the pseudoꢀfirst (k ´/s
)
i
2
0
dure ; AOT (Sigma) was used without preliminary purification.
Reverse micellar systems were prepared by the addition of a
specified amount of water to an AOT solution in nꢀnonane of
specified concentration and vigorous shaking of the mixture
until a transparent solution was formed. The mole ratios of
–1 –1
and second (k2,i/L mol
s ) orders through the following corꢀ
relations:
k ´ = k ([OH]/[AOT]),
i
i
k2,i = k V,
i
the components of the system, viz., W = [H O]/[AOT] and
2
Z = [nꢀnonane]/[AOT], were varied in the intervals 9.8—37.6
where V is the molar volume of the surfactant. The K and K
S
OH
and 5—22, respectively. Micellar solutions were always optically
homogeneous and, according to the phase diagram of AOT,
corresponded to the region of reverse micellar systems.
constants are represented by
2
1
K = ([S] [nꢀC H ])/([S] [AOT]),
S
i
9
20
o
Conductivity was measured with an OK 102/1 conductoꢀ
meter (Radelkis, Hungary) on a frequencies of 80 Hz and 3 kHz
in a temperatureꢀcontrolled planarꢀparallel cell. The temperaꢀ
ture was maintained constant with an accuracy of ±0.2 °C.
IR spectra were recorded on a Vector 22 FTꢀIR spectroꢀ
meter (Bruker) with a resolution of 4 cm–1. The spectra of
KOH = ([OH] [H O])/([OH] [AOT]).
i
2
w
The main assumptions and approximations used in the
pseudoꢀphase model were considered earlier. The linearized
form of Eq. (1)
2
3
1
/(kapp[AOT]) = (KOH + W )/(k KOH[OH]) +
(2)
–
1
i
solutions were detected in a region of 1000—4000 cm in a
+
{(KOH + W )/(k K KOH[OH])}Z
i S
fluorite cell with a thickness of 0.07 mm at 20 °C. The AOT
–
1
concentration was maintained at a level of 0.07 mol L , and the
PEG concentration was 0.1 mol L–1
makes it possible to calculate the K , K , and k values.
S
OH
i
.
As can be seen from Eq. (2), the binding constant of the
The hydrolysis kinetics was studied spectrophotometrically
on a Specord Мꢀ400 instrument from a change in the absorption
of the leaving group anion in the pseudoꢀfirst order regime. We
have previously shown that the alkaline hydrolysis of phosphonic
esters occurs in the surface layer of the water/oil microemulsions
substrate K can easily be determined from the 1/(kapp[AOT])—Z
plot as the ratio of the section cut in the ordinate to its slope.
S
This model assumes that the K value remains unchanged when
S
2
W is varied.
(
Scheme 2; indices "o," "w," and "i" indicate the oil phase, aqueꢀ
Results and Discussion
ous phase, and interface, respectively), whose amphiphilic propꢀ
erties provide the affinity to both reactants: hydrophobic subꢀ
strate and hydrophilic nucleophile.
It can be assumed that the effect of the polymer on the
rate of the model reaction depends on the polymer localꢀ
ization. Probably, the effect of PEG is maximum when it
is solubilized near the surface layer, because this can reꢀ
sult in a change in the physical properties of the interfaꢀ
cial zone where hydrolysis occurs and affect the binding
of the reactants by the nanoaggregates.
Scheme 2
The AOT—nꢀnonane—PEG—water micellar system
was studied by FTꢀIR spectroscopy. The characteristic
frequencies of reverse micellar solutions at different water
contents and their assignment to functional groups are
given in Table 1. Special attention is given to the change
in the characteristic bands of AOT forming the interphase
layer and water that forms the core of the particles upon
the introduction of the polymer and variation of the
Kinetic data for the reactions in ternary reverse systems were
analyzed in the framework of the pseudoꢀphase model, acꢀ
cording to which the reactants are distributed between the phases
7
,8
2
2
W parameter (Fig. 2). As already mentioned, water in
reverse micelles and microemulsions can exist in different

Hydrolysis of phosphorus acid esters
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
65
Table 1. Vibrational frequencies (cm–1) of some functional
groups in the IR spectra of the AOT—nꢀnonane—water sysꢀ
tem in the absence and presence of PEG at different W =
In the presence of PEG, the characteristic frequenꢀ
cies of the functional groups of AOT change slightly
(
see Table 1). The main spectral changes concern the
[
H O]/[AOT]
2
ν(OH)—W plot. In particular, the range of changing the
ν(OH) values before reaching the plateau, covering the
region of bound water, becomes narrower (see Fig. 2).
Probably, this fact confirms that PEG is localized near
the head AOT groups, where mainly bound water exists.
These data agree with an assumption on PEG dissolution
Vibraꢀ
tion24
No PEG
CPEG = 0.1 mol L–1
W = 5 W = 10 W = 20 W = 5 W = 10 W = 20
ν(OH)
3474 3462
3424
1466
3451
1466
3448
1466
3423
1466
δ (CH ), 1466 1466
as
3
12
in the aqueous core of water/oil microemulsions.
δ(CH2)
δ (CH ) 1378 1378
ν(C=O) 1737 1737
δ(OH) 1640 1640
We also studied the kinetics of alkaline hydrolysis of
phosphonate 1 in a wide range of concentrations of AOT,
PEG, and water (parameter W controlling the size of the
aqueous core). Continuing investigation of the role of the
structural factor in micellar catalysis, we studied the efꢀ
fect of percolation transitions on the reactivity of the
substrate. The kinetic data for the alkaline hydrolysis of
phosphonate 1 in the AOT—nꢀnonane—PEG—water
system at different AOT concentrations are presented
in Fig. 3.
1378
1737
1639
1378
1737
1641
1378
1736
1643
1378
1736
1644
s
3
states: hydrate water (bound water) and bulk (free) water
localized in the center of the core. The bulk water is
accepted to appear at W > 12. As follows from the data
shown in Fig. 2, the frequency of the band of the OH group
(
ν(OH)) depends on the W value: it decreases from 3487
–
1
to 3419 cm with the W increase in the 1—20 interval,
and then the dependence reaches saturation. This shift
agrees with the thesis on different states of water in a
reverse microemulsion. At low W values water exists in
the bound state, and the ν(OH) values differ from those
for the absorption band of normal water. With an increase
in the content of the latter in the system, bulk water
appears in the center of the core, which is reflected by the
shift of the ν(OH) band and achievement of a plateau by
the ν(OH)—W plot.
The basic kinetic tendencies (decrease in kapp with an
increase in the concentrations of the surfactant and waꢀ
2
,8,25,26
ter) found earlier
for the kinetic dependences of
alkaline hydrolysis of phosphorus acid esters in the threeꢀ
component micellar systems are retained in the presence
of PEG. According to the data presented in Fig. 4, the
character of the effect depends on the polymer concenꢀ
tration: at small PEG additives (0.02—0.05 mol L– ) kapp
somewhat decreases, and the further increase in the PEG
1
2
–1
kapp•10 /s
ν(OH)/cm–1
4.0
3
3
3
3
3
3
480
470
460
450
440
430
3.5
3.0
2.5
2.0
1.5
1.0
0.5
1
1
2
2
3
0
.3
0.4
CAOT/mol L–1
1
0
20
Fig. 2. Plots of the frequency of stretching vibrations of water
ν(OH)) in the IR spectrum of the AOT—nꢀnonane—water reꢀ
30
W
Fig. 3. Plots of the apparent rate constant of alkaline hydrolysis
of phosphonate 1 (kapp) in the AOT—nꢀnonane—PEG—waꢀ
ter system vs. AOT concentration (CAOT) at W = 9.8 (1),
(
verse micellar system vs. W parameter in the absence (1) and
presence of PEG (2) (CPEG = 0.1 mol L–1, CAOT = 0.07 mol L ).
15.1 (2), and 20.0 (3) (C
= 0.02 mol L–1, CNaOH
=
PEG
–1
–1
0.01 mol L , 25 °C).

66
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
Zakharova et al.
2
–1
–1
CAO T– 1
kapp•10 /s
k
app
7
6
5
4
3
2
1
1
2
5
4
4
00
50
00
3
350
00
250
2
3
3
4
2
00
50
1
1
0
СPEG/mol L–1
100
0
.1
0.2
0.3
8
12
16
20
Z
Fig. 4. Plots of the apparent rate constant of alkaline hydrolysis
of phosphonate 1 (kapp) in the AOT—nꢀnonane—PEG—water
Fig. 5. Kinetic data presented in Fig. 3 linearized in the coordiꢀ
nates of Eq. (2) for W = 9.8 (1), 15.1 (2), and 20.0 (3).
system vs. PEG concentration (CPEG) at СAOT = 0.26 (1),
–1
0
.37 (2), 0.45 (3), and 0.50 mol L–1 (4) (CNaOH = 0.01 mol L
,
W = 15.1, 25 °C).
The kinetics of alkaline hydrolysis of phosphonate 1
was studied in a wide temperature range. The kinetic data
are shown in Fig. 6, a and b in the coordinates of the
Arrhenius equation in the AOT—nꢀnonane—PEG—waꢀ
ter system at different water contents and unchanged conꢀ
concentrations results in a considerable increase in the
rate constant. The sign of the effect changes: transition
from inhibition of the reaction to its catalysis occurs.
Structural organization of the system changes, most likely,
with an increase in the concentration of macromolecules,
which modifies the surface layer and affects the reactivity
of compounds. Perhaps, transitions between the strucꢀ
tures shown in Fig. 1 are observed in this case.
Additional information on the polymer effect on
the kinetics of the process was obtained by the graphiꢀ
cal analysis of the kinetic data presented in Fig. 3 using
Eq. (2) (Fig. 5). Although linearity is fulfilled in the
–
1
centrations of AOT (0.35—0.45 mol L ) and PEG. In
molecular solutions this plot is linear, and its slope deterꢀ
mines the activation energy of the process. As shown preꢀ
1
1,18
viously,
in the AOT—alkane—water reverse systems,
the slope of the Arrhenius dependence changes sharply at
some temperature Tcr dependent of the composition of
the system. When the temperature rises to T , the typical
cr
shape of the plots is observed in the absence of PEG:
linear increase in the rate constant with temperature.
A decrease in the slope or a negative slope of the plots is
1
/(kappСAOT)—Z coordinates, Eq. (1) does not obey in
the presence of the polymer. This is indicated by the
negative slope of the sections and different in absolute
values b/a ratios that characterize the binding constant of
the substrate (Table 2). The most probable explanation is
that the reaction zone changes in the presence of the
polymer and distribution of the reactants shown in Scheme
observed above T . The sharp change in the reactivity of
cr
phosphorus acid esters above the percolation threshold is
Table 2. Analysis of the kinetic data obtained at different temꢀ
peratures using Eqs (1) and (2)
2
is not invariant. In the presence of the polymer, the
W
T = 25 °C
T = 40 °C
reaction in the surface layer makes no determining conꢀ
tribution to the apparent rate constant, which is indicated
by unfulfilled Eq. (1). Inversion of the sign of the catalytic
effect (see Fig. 4) is also related, most likely, to a change
in the nature of aggregates in the presence of macroꢀ
molecules and, as a consequence, to a change in the
distribution of the reactants between different pseudoꢀ
phases.
a
b
KS
a
b
KS
9
1
2
.8
5.1
0.0
411.0 –14.7
–1.1 –297
642.6 –21.0
–28.0
3335 –163.4
246 10.8
2910 –147.2
–20.4
22.8
–19.8
326.8
–30.6
Note. a is the section cut in the ordinate; b is the slope of the
section (for the graphical solution of Eq. (2)).

Hydrolysis of phosphorus acid esters
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
67
logkapp
logkapp
a
b
–
–
–
2.0
2.2
2.4
–
1.6
1
´
1
–1.8
–
2.0
T –1•10 /K
3
–1
3.2
logkapp
1.4
3.3
3.4
T –1•10 /K
3
–1
3
.1
3.2
3.3
3.4
logkapp
–
–
–
–
–
–
–
1.8
2.0
2.2
2.4
1.6
1.8
2
2
´
3
.1
logkapp
1.6
3.2
3.3
T –1•10 /K
3
–1
3.2
logkapp
1.45
3.3
3.4
T –1•10 /K
3
–1
–
–
–
–
–
–
1.8
2.0
2.2
2.4
–
1.50
3´
3
–1.55
–1.60
T –1•10 /K
3
–1
3.2
3.3
3.4
T –1•10 /K
3
–1
3
.1
3.2
3.3
Fig. 6. Arrhenius plots of the apparent rate constant of alkaline hydrolysis of phosphonate 1 (k ) in the AOT—nꢀnonane—
app
PEG—water system at CPEG = 0.02 (a) and 0.5 mol L– (b) and W = 9.8 (1, 1´), 15.1 (2, 2´), and 20.0 (3, 3´) (CNaOH = 0.01 mol L ).
1
–1
caused by a change in localization of the reactants due to
the cluster formation of microemulsion drops, in particuꢀ
lar, by the migration of the reaction zone to the region
with a decreased microscopic polarity. Similarly, a break
at Tcr is observed in the logkapp—1/T plots (see Fig. 6);
however, only at W = 9.8 the run of the plots coincides
with that observed earlier in the absence of PEG. At
W = 15.1 and 20.0 the typical Arrhenius dependence is
close. Identical character of the T and T values has
cr p
1
8
been found previously for the AOT—alkane—water sysꢀ
tems, which suggested a relationship between the reactivꢀ
ity of the compounds and cluster formation of reverse
particles that causes the percolation phenomenon. The
results obtained in the present work confirm this concluꢀ
1
3
sion. Moreover, the quantitative data on the percolaꢀ
tion transition in the water/oil microemulsions suggest
that the preferential formation of combined structures
according to the type presented in Fig. 1 (bottom) can
induce cluster formation and the percolation transition in
the system in the presence of the polymer. In other words,
PEG can affect significantly the catalytic effect of reverse
micelles due to both the formation of new Surf—polymer
combined structures and promotion of cluster formation
of microemulsion drops.
fulfilled only above T , and below this value the rate
cr
constant decreases with the temperature rise. Compariꢀ
son of the T values (see Fig. 6, a and b) and the temperaꢀ
cr
ture of percolation threshold (Table 3) shows that they are
Table 3. Values of Tp (conductometric data) and Tcr for the
alkaline hydrolysis of phosphonate 1 in the AOT—nꢀnonane—
PEG—water reverse micellar system (for experimental condiꢀ
tions, see Fig. 6, a)
Analysis of the data presented in Fig. 6, a and b, shows
that in the presence of the polymer at W > 10 tendencies
for changing kapp with temperature change for opposite
tendencies, i.e., additives of macromolecules exert a subꢀ
stantial effect on the kinetics of hydrolysis of phosꢀ
phonate 1 in the AOTꢀbased reverse systems. This conꢀ
firms the formation of combined structures in which the
redistribution of reactants between pseudoꢀphases differs
W
Tp/°С at C
/mol L–1
0.5
Tcr/°С at C
/mol L–1
0.5
PEG
PEG
0
0.02
0
0.02
9
1
2
.8
5.1
0.0
26
35
37
25
33.5
37
25
28
32
25
32
36
26
33
36.5
26
28.5
32

68
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
Zakharova et al.
3
–1
from that in individual microemulsions drops. Taking into
account that the apparent rate constant kapp in reverse
micelles depends, generally in a complicated way, on the
contribution of the reaction in different microscopic reꢀ
gions, i.e., on the partition of compounds and their reacꢀ
tivity in this microenvironment, the plots in Fig. 6, a and b,
can be explained as follows. When the temperature rises,
the reactants reallocate between different nanoregions due
to cluster formation. This change in localization decreases
the apparent rate constant of substrate hydrolysis due to a
change in the microenvironment or a decrease in the
partition coefficients of the reactants.1 This inhibition
effect competes with the tendency for increasing the reacꢀ
tivity with the temperature rise typical of molecular soluꢀ
tions. Probably, at W = 9.8 and temperatures below Tcr
the normal temperature effect of reaction acceleration
predominates (see Fig. 6, a and b), and above this temꢀ
perature coinciding with the percolation threshold preꢀ
vailing is the inhibition effect caused by cluster formaꢀ
tion. To the contrary, at W = 15.1 and 20.0 before the
percolation threshold the change in the structural characꢀ
teristics of the system and related relocation of the reacꢀ
tants exert the decisive effect on their reactivity, and above
Tcr the normal increase in kapp with temperature is obꢀ
served. The specific behavior of the AOT—nꢀnonane—
PEG—water system at W = 9.8 is caused, most likely, by
the fact that for this composition normal bulk water is
absent from the reverse micelles and only water in hydrate
shells of the head AOT groups is observed. Therefore, the
localization of the polymer and reactants and their redisꢀ
tribution upon cluster formation can substantially differ
from those occurring at higher W values, when the
microemulsion particles are formed.
kapp•10 /s
16
14
12
10
8
2
8
6
3
4
1
2
0
.3
0.4
CAOT/mol L–1
Fig. 7. Plots of the apparent rate constant of alkaline hydrolysis
of phosphonate 1 (kapp) in the AOT—nꢀnonane—PEG—waꢀ
ter system vs. AOT concentration (CAOT) at W = 9.8 (1),
5.1 (2), and 20.0 (3) (C
.01 mol L– , 40 °C).
1
0
= 0.02 mol L–1, CNaOH
=
PEG
1
high surfactant concentrations at W = 9.8 for which lower
kapp values than those for W = 15.1 and 20.0 are observed.
Additional information on the effect of the polymer
on the kinetics of the process was obtained by the graphiꢀ
cal analysis of the kinetic data presented in Fig. 7 using
Eqs (1) and (2) (Fig. 8). Although linearity is fulfilled in
the 1/(kapp[AOT])—Z coordinates, in the presence of the
polymer Eq. (1) obeys neither before nor after the percoꢀ
lation threshold. This is indicated by the negative slope of
the sections and different in absolute values b/a ratios that
characterize the binding constant of the substrate (see
Table 2). The most probable explanation is a change in
the distribution of the reactants between the pseudoꢀ
phases in the presence of the polymer. Perhaps, in this
case the substrate is distributed between all pseudoꢀphases
and the contribution of the reactions in the aqueous core
increases, i.e., the results of analysis confirm the change
in the properties of the nanoaggregates caused by the
formation of Surf—polymer complexes.
Thus, our study demonstrated a possibility of direct
controlling the rate of chemical reactions using reverse
micellar systems as nanoreactors. The concentration and
temperature regimes of the reaction should be mentioned
among tools controlling the process rate. Varying conꢀ
centrations of all components of the reverse system, we
found a wide range of changes in the catalytic effect from
inhibition of the reaction to its acceleration. An addiꢀ
tional factor affecting the reaction rate is related to the
induction of cluster formation in the system when the
activation energy of substrate hydrolysis changes sharply
in the point of the temperature percolation threshold. In
More detailed investigation of the hydrolysis kinetics
–
1
of phosphonate 1 at CPEG = 0.02 mol L in the form of
the kapp—СAOT plot at 40 °C, i.e., above the percolation
threshold, is presented in Fig. 7. Comparison with the
kinetic data at 25 °C (see Fig. 3) suggests the following. At
W = 9.8, despite the temperature increase from 25
to 40 °C, the reaction rate decreases by 2.5—5 times deꢀ
pending on the surfactant concentration. The point corꢀ
responding to 40 °C (see Fig. 6, a) lies in the descending
line, i.e., in this case the structural factor of the temperaꢀ
ture dependence preventing unfavorable changes in the
microenvironment of the reactants is predominant. At
W = 15.1 and 20.0 the apparent rate constant decreases
slightly or remains constant when the temperature inꢀ
creases to 40 °C. These cases concern the ascending line
(
see Fig. 6, a), and mutual compensation of two effects is
observed: temperature activation of the chemical reaction
and unfavorable structural changes of the system in the
microenvironment of the reactants. As a whole, the kiꢀ
netic tendencies are retained with the temperature rise:
kapp decreases with an increase in the surfactant concenꢀ
tration and parameter W. The exception is the region of

Hydrolysis of phosphorus acid esters
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
69
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16
20 Z
Fig. 8. Kinetic data presented in Fig. 7 linearized in the coordiꢀ
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pseudoꢀphase model.
It should be emphasized in conclusion that investigaꢀ
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tions are simulated by their resemblance to enzymatic
systems. In the present work, we showed that the use of
polycomponent supramolecular ensembles as catalytic sysꢀ
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characterized by a complicated multivariant matched
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2
2
2
2
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 05ꢀ03ꢀ
0
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2
2
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in revised form August 8, 2006