Control of the catalytic effect of cetyltrimethylammonium bromide micelles by the addition of background electrolyte and nonionic surfactant

Article abstract of DOI:10.1007/s11172-007-0312-9

The variation of the microscopic properties (surface potential, micropolarity, etc.) of the interface of cetyltrimethylammonium bromide micelles upon the addition of a background electrolyte or the nonionic surfactant Triton X-100 decreases the rates of i

Full text of DOI:10.1007/s11172-007-0312-9

Russian Chemical Bulletin, International Edition, Vol. 56, No. 10, pp. 2000—2007, October, 2007
2000
Control of the catalytic effect of cetyltrimethylammonium bromide micelles
by the addition of background electrolyte and nonionic surfactant
A. B. Mirgorodskaya,aꢀ L. Ya. Zakharova,a F. G. Valeeva,a A. V. Zakharov,b L. Z. Rizvanova,a L. A. Kudryavtseva,a
H. E. Harlampidi,b and A. I. Konovalova
aA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Scientific Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 273 2253. Eꢀmail: mirgorod@iopc.knc.ru
bKazan State Technological University,
68 ul. K. Marksa, 420015 Kazan, Russian Federation
The variation of the microscopic properties (surface potential, micropolarity, etc.) of the
interface of cetyltrimethylammonium bromide micelles upon the addition of a background
electrolyte or the nonionic surfactant Triton Xꢀ100 decreases the rates of ionꢀmolecular reacꢀ
tions, namely, alkaline hydrolysis of carboxylic acid esters and tetracoordinate phosphorus acid
esters, and results in the shift of acidꢀbase equilibria.
Key words: micelles, cetyltrimethylammonium bromide, background electrolyte,
Triton Xꢀ100, acidꢀbase properties, surface potential, esters, hydrolysis, kinetics.
Micellar solutions of ionic surfactants provide wide
possibilities for controlling the rate of ionꢀmolecular reꢀ
actions.1—5 In turn, the reactions themselves, in particuꢀ
lar, alkaline hydrolysis of carboxylic acid esters and phosꢀ
phorus acid esters, are sensitive indicators of the microꢀ
scopic properties of the medium in the reaction zone. As a
rule, similar processes occur in the Stern layer of ionic
micelles and depend, to a considerable extent, on the sign
of the micellar surface charge. For instance, alkaline
hydrolysis of substrates is accelerated in cationic micelles
due to the electrostatic attraction of the hydroxyl ions to
the micellar surface and is inhibited in anionic micelles
because of the electrostatic repulsion of the OH ions from
the likely charged surface. Thus, the surface potential
plays a key role in similar catalytic systems. The variation
of the potential value can be used for the directed control
of the rate of ionꢀmolecular reactions in a wide range
including the inhibition and catalytic effects.
surfactants. The insertion of oxyethylene fragments beꢀ
tween the charged ammonium groups decreases the surꢀ
face charge density, as in the case of the addition of elecꢀ
trolytes.10,11
In this work, we studied the catalytic effect of micellar
solutions of the cationic surfactant cetyltrimethylammoꢀ
nium bromide (CTAB) on the alkaline hydrolysis of carbꢀ
oxylic acid esters, namely, pꢀnitrophenyl acetate (PNPA),
pꢀnitrophenyl laurate (PNPL), and pꢀnitrophenyl
Oꢀethylchloromethylphosphonate (NPCP) (Scheme 1).
Scheme 1
An increase in the concentration of surfactant counterꢀ
ions achieved by the addition of a background electrolyte
neutralized partially the micellar surface charge, which,
in turn, decreases the destabilizing repulsion of the likely
charged head groups. This results in a drop of the absolute
value of the surface potential of the system, a decrease in
the critical micelle concentration (CMC), an increase in
the aggregation numbers, and a change in the micellar
effect on the physicochemical properties and reactivity of
solubilized substances.6—9 Other systems in which the surꢀ
face potential changes upon the variation of the composiꢀ
tion are binary systems containing ionic and nonionic
R = Me (PNPA), C11H23 (PNPL)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1933—1940, October, 2007.
1066ꢀ5285/07/5610ꢀ2000 © 2007 Springer Science+Business Media, Inc.

Control of catalytic effect in micelles
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
2001
γ/Dyne cm–1
The objects of the study were CTAB—electrolyte and
CTAB—Triton Xꢀ100 systems for which the monotonic
decrease in the surface potential with an increase in the
concentration of electrolytes or nonionic surfactants has
been found previously.7,11 One of indicators of the denꢀ
sity of the micellar surface charge is the shift of acidꢀbase
equilibria in ionic micelles. Therefore, to test the influꢀ
ence of electrolytes and nonionic surfactants on the surꢀ
face potential, we studied the change in pK of nꢀoctylꢀ
amine in micellar solutions.
1
2
3
4
5
6
7
60
50
40
Experimental
Commercial preparations of CTAB and Triton Xꢀ100
(Sigma) and nꢀoctylamine and pꢀnitrophenyl esters of carboxyꢀ
lic acids (Fluka) containing ~99% of the major substance were
used. pꢀNitrophenyl ester of phosphorus acid was synthesized
according to an earlier described procedure.12
10–5
10–4
10–3
СSurf/mol L–1
Fig. 1. Isotherms of surface tension (γ) for CTAB—Triton Xꢀ100
mixed micellar solutions at the molar ratio of the ionic surfacꢀ
The pKa values of nꢀoctylamine were determined by potenꢀ
tiometric titration with a 0.1 M solution of HCl using a pHꢀ340
instrument. The kinetics of the reactions were studied spectroꢀ
photometrically on a Specord UV—Vis instrument at 25 °С by
monitoring a change in the absorbance of solutions at a waveꢀ
length of 400 nm (formation of the pꢀnitrophenoxide anion).
tant α = 0.9 (1), 0.7 (2), 0.5 (3), 0.3 (4), 0.1 (5), 1.0 (6),
and 0 (7); 25 °C.
1
surfactant binary systems is determined to a great extent
by their structure: nature of head groups and the length of
hydrocarbon radicals. In the case of mixed micelles of
ionic and nonionic surfactants, their composition can difꢀ
fer substantially from the composition of the solution.
A series of thermodynamic theories and semiempirical
models was developed for the quantitative description of
the properties of mixed micellar solutions. For nonꢀideal
mixing, some quantitative characteristics of surfactant soꢀ
lutions (CMC, composition of micellar aggregates, conꢀ
centration of the monomeric form of diphilic molecules)
can be predicted in the framework of the semiempirical
model based on the regular solution theory13 in which the
activity coefficients of the surfactant ( f1 and f2)
The initial concentration of the substrate was 5•10–5 mol L–1
,
and the conversion was higher than 90%. The apparent rate
constants of the pseudoꢀfirst order (kapp) were determined from
the dependence log(A∞ – Aτ) = –0.434kappτ + const, where Aτ
and A∞ are the absorbances of solutions at the moment τ and
after the reaction completion. The kapp values were calculated
by the leastꢀsquares method.
The kinetic curves obtained were analyzed in the framework
of the pseudoꢀphase approach using the equation1,2
kapp = (kmKSCSurf + k0)/(1 + KSCSurf),
(1)
(2)
,
1/C* = α /( f1C1) + α /( f2C2)
(3)
1
2
where k0 and km (s–1) are the rate constants of the first order in
an aqueous medium and in the micellar phase, respectively;
k´app (L mol–1 s–1) is the secondꢀorder rate constant obtained
by the division of kapp into the total concentration of the hydroxyl
ion; k2,0 and k2,m (L mol–1 s–1) are the secondꢀorder rate conꢀ
stants in the aqueous and micellar phases, respectively; KS and
КОН (L mol–1) are the binding constants of the substrate
and hydroxyl ions; V (L mol–1) is the molar volume of the surꢀ
factant; C (mol L–1) is the total concentration of the surfactant
minus CMC.
(α1 and α2 are the molar fractions of the ionic and nonꢀ
ionic surfactants in solution, respectively; С*, С1, and С2
are the CMC values for a mixed system, ionic surfactant,
and nonionic surfactant, respectively) and the interaction
parameter of the surfactant in mixed aggregates (β) were
introduced in order to take into account the nonꢀideal
character of the mixture
f1 = exp[β(1 – x1)2],
f2 = exp(βx12).
(4)
(5)
The surface tension was measured by the De Noüy ring
detachment method.
Here x1 is the molar fraction of the ionic surfactant in
mixed micelles, which can be calculated by the solution
of the equation
Results and Discussion
Selfꢀorganization in a CTAB—Triton Xꢀ100 binary sysꢀ
tem. The formation of mixed micelles in a CTAB—Triton
Xꢀ100 binary system was confirmed when studying the
surface tension of solutions with different fractions of the
surfactant (Fig. 1). Micelle formation in a solution of
(6)
using the iteration procedure.

2002
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
Mirgorodskaya et al.
The mathematical apparatus used in the calculations
has been described in detail.13 The β parameter can be
calculated by the equation
Shift of pKa of aliphatic amines. It is known that the
shift of acidꢀbase equilibria occurs in aqueous micellar
solutions of surfactants. The cationic surfactants exhibit
the enhancement and the anionic surfactants show the
weakening of the acidic properties of the compounds.
This is due to the selective solubilizing effect of micelles
with respect to the acidic and basic forms of the comꢀ
pounds. For ionic surfactant, this is mainly determined by
the surface potential of a micelle.16—18 Additives of a
background electrolyte decrease the surface potential, thus
affecting the value of the pKa shift.
The plots of pKa of nꢀoctylamine solubilized by CTAB
micelles vs surfactant and potassium bromide contents
are shown in Fig. 3. In the absence of electrolytes, the
surface potential of CTAB is 130—140 mV.7,19 Due to the
electrostatic repulsion, the protonated form of nꢀoctylꢀ
amine is bound by CTAB micelles less efficiently than the
neutral form. Therefore, an increase in the surfactant conꢀ
centration decreases the pKa value of amine (see Fig. 3,
curve 1). The addition of potassium bromide decreases
β = ln[(a1C*)/(x1C1)]/(1 – x1)2.
(7)
This parameter takes into account the degree of deviation
of the system from an ideal mixture and the character of
interaction of surfactant monomers in mixed micelles.
The negative value of the β parameter assumes that attracꢀ
tion forces between the surfactant molecules predomiꢀ
nate, whereas the positive value assumes the predominaꢀ
tion of repulsion forces. The increase in the absolute
β value indicates the more considerable deviation of the
micellar system from an ideal mixture.
An analysis of the tensiometric data using Eqs (3)—(7)
showed the negative deviation of the system from an ideal
mixture, the interaction parameter being β = –1.05. The
value obtained indicates the synergetic behavior of a
CTAB—Triton Xꢀ100 system, i.e., formation of aggreꢀ
gates of the mixed type. We have earlier14,15 found a simiꢀ
lar effect for binary solutions of CTAB with the nonionic
surfactants Brijꢀ97 and C14EO9. The CMC values deterꢀ
mined experimentally and calculated by Eq. (3) are given
in Fig. 2. The good agreement confirms that the model
and conclusions about mixed micelle formation are adꢀ
equate. The calculated molar fractions of CTAB in mixed
aggregates (х1) are lower than the stoichiometric fraction
of CTAB in the solution α1, i.e., mixed micelles are enꢀ
riched in the nonionic surfactant (see Fig. 2).
Ka,app•1011
pKa,app
3
10.7
10.6
10.5
10.4
10.3
10.2
10.1
10.0
2
1
2
–log(CKBr/mol L–1
)
СМС•103/mol L–1
1.1
x1
1.0
0.8
0.6
0.4
0.2
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
3
2
2
1
1
2
4
6
CCTAB•103/mol L–1
Fig. 3. Plots of рКа of nꢀoctylamine (0.005 mol L–1) vs CTAB
concentration at a KBr content of 0 (1), 0.01 (2), and
0.02 mol L–1 (3). The dependence of the apparent rate of
the acidꢀbase equilibrium of nꢀoctylamine in a micellar soluꢀ
tion of CTAB (0.0025 mol L–1) on the logarithm of the potasꢀ
0.2
0.4
0.6
0.8
α
1
Fig. 2. Plots of the CMC of a CTAB—Triton Xꢀ100 mixed
micellar system (1) (points are experimental data, and line is the
calculation by Eq. (3)) and the molar fraction of CTAB in
mixed aggregates (х1) (2) vs surfactant fraction in the soluꢀ
sium bromide concentration is shown in inset; 25 °С (Ccr
=
tion (α ).
0.028 mol L–1).
1

Control of catalytic effect in micelles
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
2003
pKa
the surface potential of CTAB due to an additional bindꢀ
ing of the counterions with the head groups. For instance,
when the KBr concentration increases in solution to
0.02 mol L–1, the surface potential decreases7 to 118 mV.
When the micellar charge is compensated by the counterꢀ
ions, the influence of CTAB on pKa of nꢀoctylamine deꢀ
creases (see Fig. 3, curves 2 and 3).
10.6
1
In some cases, the introduction of an electrolyte into
surfactant solutions results in structural rearrangements
in the system, in particular, induces sphere—cylinder miꢀ
cellar transitions.19—22 The violation in stationarity of the
ionꢀexchange processes that occur in the systems can reꢀ
flect the character of dependences of the apparent acidꢀ
base equilibrium constants on the logarithm of the salt
concentration (see Fig. 3, inset), which gain the shape of
broken straight lines with breaks in the points related to
qualitative changes in the state of micelles (critical
points, Сcr). The Сcr values obtained when studying the
effect of potassium bromide on pKa of nꢀoctylamine (Сcr =
0.028 mol L–1 at СCTAB = 0.0025 mol L–1) are close to
those obtained earlier.19
Another way to exert influence upon the surface poꢀ
tential of a micelle is the addition of a nonionic detergent
to a solution of a cationic surfactant. In mixed composiꢀ
tions, the surface potential depends on the molar fraction
of each component of the mixture. The decrease in the
potential observed11 upon the addition of Triton Xꢀ100
to a solution of CTAB reflects on the pKa values of nꢀoctylꢀ
amine (Fig. 4). The shift of pKa related to a change in the
basicity of nꢀoctylamine on going from an aqueous meꢀ
dium to the micellar phase (∆рК) decreases with an
increase in the fraction of Triton Xꢀ100 (α2) in a
CTAB—Triton Xꢀ100 system (see Fig. 4, inset). In the
case when the contents of the surfactant and nꢀoctylamine
in the solution are comparable, the neutral form of the
latter can also play the role of a nonionic amphiphile,
which can be incorporated into the interfacial layer and
induce a decrease in the charge density in the layer.
Catalytic effect of CTAB—electolyte systems. Micellar
solutions of CTAB containing KBr were used as reaction
media for the alkaline hydrolysis of carboxylic acid esters,
viz., PNPA and PNPL.
10.5
10.4
10.3
10.2
10.1
10.0
2
3
∆pKa
0.6
0.4
0.2
4
5
0.2
0.6
α
2
2
4
6
CSurf•103/mol L–1
Fig. 4. Plots of рКа of nꢀoctylamine vs total surfactant concenꢀ
tration in a CTAB—Triton Xꢀ100 mixed micellar solution at the
molar fraction of Triton Xꢀ100 α = 1.0 (1), 0.5 (2), 0.2 (3),
0.1 (4), and 0 (5). The dependence of the shift of рКа of nꢀoctylꢀ
amine on the molar fraction of Triton Xꢀ100 in the mixed comꢀ
position is shown in inset.
2
and in the presence of 0.01 M KBr) are presented in
Fig. 5. The experimental kinetic data were analyzed in the
framework of the pseudoꢀphase model of micellar catalyꢀ
sis according to Eq. (1). The results are given in Table 1.
They show (see Fig. 5 and Table 1) that the micellar
catalytic effect for the esters under study differs substanꢀ
tially. In the systems containing no potassium bromide,
the maximum km/k0 ratio for PNPA is about four, whereas
for PNPL it is by two orders of magnitude higher. It is
known that the laurate selfꢀassociates in aqueous soluꢀ
tions, resulting in the shielding of the reaction center and
an anomalously low rate of alkaline hydrolysis compared
to the acetate (the secondꢀorder rate constant is 8.4 and
0.024 L mol–1 s–1 for PNPA and PNPL, respectively). In
micellar solutions of cationic surfactants, molecules beꢀ
come separated and globular associates of hydrophobic
PNPL are untwisted when the PNPL is incorporated into
the CTABꢀbased micelle. This makes the substrate accesꢀ
sible for the attack by the nucleophile and accelerates
considerably the process. The presence of the electrolyte
The application of a cationic surfactant provides a
positive charge on the micellar surface and induces the
concentrating of hydroxyl ions in the Stern layer. In this
case, the ester solubilized by the micelle is brought toꢀ
gether with the highly hydrophilic nucleophile, providing
hydrolysis acceleration. The introduction of a background
electrolyte into a CTAB solution decreases the surface
potential of the system, decreases the attraction of the
hydroxyl ion to the interface, and should decrease the
micellar catalytic effect. The plots of the apparent rate
constant of alkaline hydrolysis for two esters differed in
hydrophilic—lipophilic balance vs cationic surfactant conꢀ
centration (in the absence of the background electrolyte

2004
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
Mirgorodskaya et al.
kapp/s–1
Table 1. Quantitative analysis according to Eq. (1) of the kinetic
data for the alkaline hydrolysis of carboxylic acid esters in a
CTAB micellar solution in the absence of a background electroꢀ
lyte in the presence of potassium bromide (see Fig. 5)
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
1
a
Subꢀ
strate
CKBr
k0
km
Ks*
CMC
km
/mol L–1
/mol L–1 /k0**
s–1
PNPA
PNPL
0
0.042
0.036
0.032
0.00012 0.063 7000
0.00017 0.053 2100
0.00019 0.047 2200
0.00017 0.031 2000
0.18
0.08
0.06
420
280
230
0.0005
0.0003
0.0002
0.0003 525
0.00017 317
0.00015 247
0.00025 163
4.3
2.2
1.9
2
0.005
0.010
0
0.005
0.010
0.015
3
* In L mol–1
.
** The km/k0 ratio is a measure of the catalytic effect.
sium bromide decreases the binding constant of the both
substrates. In micellar solutions of CTAB, differences in
the reaction behavior of the substrates under study are
aligned and their km values become close, whereas the
micellar effect for PNPL is substantially higher (acceleraꢀ
tion by more than 500 times). As already assumed, the
decrease in the surface potential due to the addition of
potassium bromide weakens the attraction of the hydroxyl
ion to the micellar surface and thus decreases the catalytic
effect of the system.
The apparent rate constant decreases nonmonotoniꢀ
cally with a decrease in the potassium bromide content,
and the plot of kapp vs logCKBr (Fig. 6), as for the acidꢀ
base equilibrium constants, has a critical point caused by
structural rearrangements in the system. Since the Ccr
value characterizes the properties of the micellar system
itself, its value is virtually independent of the type of the
process and equals the total concentration all bromide
ions that are present in the solution (~0.03 mol L–1),
which virtually coincides with the value obtained by the
shift of pKa of nꢀoctylamine (0.028 mol L–1).
2
4
6
CCTAB•103/mol L–1
kapp/s–1
0.06
0.05
0.04
0.03
0.02
0.01
1
b
2
3
4
We have earlier studied the salt effect in the alkaline
hydrolysis of phosphorus acid esters in micellar solutions
of cationic surfactants with the pyridinium head group8
and found a sharp change in the reactivity of the phosꢀ
phonates in the region of sphere—rod micellar transiꢀ
tions. Continuing these studies, we measured the kinetics
of alkaline hydrolysis of phosphonate 1 in micellar soluꢀ
tions of CTAB (0.001 mol L–1) in the presence of several
sodium salts (Fig. 7). When choosing counterions, their
hydrophilicity was taken into account. In addition to inꢀ
organic salts, we studied sodium salicylate (NaSal), which
is characterized23 by high affinity to micelles of cationic
surfactants. As in the case of hydrolysis of carboxylic
acid esters, the apparent rate constant decreases considꢀ
erably. The inhibiting salt effect increases in the order
Cl– < Br– < Sal–. It is seen that for the organic salicylate
0
2
4
6
CCTAB•103/mol L–1
Fig. 5. Plots of the apparent rate constant of alkaline hydrolysis
of PNPA (a) and PNPL (b) vs CTAB concentration (0.005 M
NaOH, 25 °C) in the absence of an electrolyte (1) and in the
presence of KBr in a concentration of 0.005 (2), 0.01 (3), and
0.015 mol L–1 (4).
does not basically change the situation; however, the cataꢀ
lytic effect value is somewhat lower in this case (see Fig. 5,
Table 1).
The results in Table 1 show that PNPL is much strongly
bound by a micelle than PNPA and the addition of potasꢀ

Control of catalytic effect in micelles
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
2005
kapp/s–1
kapp/s–1
0.12
0.14
kapp/s–1
0.12
0.10
0.08
0.06
0.12
0.10
0.08
0.10
0.06
0.04
0.08
1
0.04
0.01
0.1
CNaBr/mol L–1
0.06
0.04
0.02
2
0.02
1
2
3
4
0
1.0
1.5
2.0 –log(CKBr/mol L–1
)
Fig. 6. Plots of the apparent rate constant of alkaline hydrolysis
of carboxylic acid esters in micellar solutions of CTAB vs logaꢀ
rithm of the potassium bromide concentration (0.005 M NaOH,
СCTAB = 0.0025 mol L–1, 25 °C): 1, PNPA (Ccr = 0.022 mol L–1);
2, PNPL (Ccr = 0.027 mol L–1).
0.2
0.4
0.6
0.8
Csalt/mol L–1
ion the reaction is significantly retarded (by more than an
order of magnitude) at much smaller salt concentrations.
It can be assumed that the surface charge of the micelles is
efficiently compensated and also the substrate is displaced
from the reaction zone (Stern layer) by the hydrophobic
salicylate ions.
Fig. 7. Plots of the apparent rate constant of alkaline hydrolysis
of NPCP vs electrolyte concentration: 1, NaCl; 2, NaBr;
3, 4, NaSal; 0.005 (1—3) and 0.01 M NaOH (4); 0.001 М CTAB,
25 °С. In inset the dependence of kapp for alkaline hydrolysis of
NPCP on the NaBr concentration (logarithmic scale) under the
same conditions.
We analyzed the kinetic data for the hydrolysis of
NPCP in the presence of NaBr in the logarithmic coordiꢀ
nates (see Fig. 7, inset). The presence of the break in the
line in the kapp—logCNaBr coordinates indicates a sharp
change in the reactivity of the phosphonate. The value
Ccr = 0.028 mol L–1 coincides with analogous values in
Figs 3 and 6 and with the data obtained earlier.21 Thus,
the Ccr value is independent of the type of the process
under study, i.e., Ccr is an indicator of the critical state of
micellar aggregates, particularly, of the rearrangement of
spherical micelles into cylindrical ones.19 These results
confirm the conclusions about the role of the structural
factor in micellar catalysis.7,8,19,21
Thus, the addition of a background electrolyte, on the
one hand, decreases the surface potential of the micellar
system and, on the other hand, favors structural rearꢀ
rangements in the system, which reflects the reactivity of
the substances solubilized by micelles.
Catalytic effect of a CTAB—Triton Xꢀ100 system. The
results of studying the kinetics of alkaline hydrolysis of
the phosphonate in a CTAB—Triton Xꢀ100 mixed micelꢀ
lar system with the variation of the ionic/nonionic surfacꢀ
tant ratio are presented in Fig. 8. It turned out that the
catalytic effect of this system differs significantly from
those of the earlier14,15 studied CTAB—Brijꢀ97 and
CTAB—С14ЕО9 binary solutions for which the nonꢀ
monotonic change in the rate constant was observed at
small molar fractions of the nonionic surfactant. In parꢀ
ticular, at α1 = 0.6—0.9 the reaction rate constant inꢀ
creases with an increase in the content of the nonionic
surfactant, i.e., with a decrease in the surface potential. In
the system under study (see Fig. 8), even a minor additive
of the nonionic surfactant (α1 = 0.9) decreases considerꢀ
ably the apparent rate constant, which continues to deꢀ
crease smoothly with an increase in the molar fraction of
Triton Xꢀ100. At α1 = 0.1 the sign of the catalytic effect
changes, i.e., transition from the acceleration of the hydroꢀ
lysis to its inhibition occurs.
The data obtained were analyzed in the framework of
the pseudoꢀphase approach using Eq. (2). A possibility of
using this equation for mixed systems was shown,14,15 and
examples for the calculation of the binding constants and
molar volume of the surfactant at different ratios were

2006
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 10, October, 2007
Mirgorodskaya et al.
kapp•103/s–1
Table 2. Quantitative analysis according to Eq. (2) of the kinetic
data for the alkaline hydrolysis of NPCP in a CTAB—Triton
Xꢀ100 mixed micellar system (see Fig. 8)a
1
2
3
4
5
6
100
b
c
c
α
(kapp/k0)max KS
KOH k2,m
Fc
Fm
FcFm
1
L mol–1
1.0
0.9
0.7
0.5
20.3
14.0
9.5
593
69
51
90
85
0.62
127.5 0.16 20.1
80
60
40
20
2828
1600
1300
0.40
0.19
0.15
132
200
180
0.10 13.8
0.05 9.7
0.04 7.0
6.2
a k2,0 = 4.0 L mol–1 s–1
.
b In L mol–1 s–1
.
c
Calculated
using
the
equation
. The maximum acꢀ
celeration of the reaction equal to the ratio of the apparent rate
constant (kapp) to the rate constant of the pseudoꢀfirst order in
water (k0) is given in the right part. The first cofactor in the right
part takes into account the influence of the micellar microenviꢀ
ronment on the reactivity (Fm), and the second cofactor takes
into account the effect of reactant concentrating in micelles (Fc).
0
0.5
1.0
CSurf•102/mol L–1
as a tool of directed control of the rate of ionꢀmolecular
reactions in micellar solutions of ionic surfactants due to
a change in the microscopic properties of the interface,
such as surface potential, micropolarity, etc. This makes it
possible to consider surfactant—electrolyte micellar sysꢀ
tems as efficient catalytic systems with a wide range of
action, including inhibition and catalysis, for the hydroꢀ
lytic cleavage of ester bonds.
Fig. 8. Plots of the apparent rate constant of alkaline hydrolysis
of NPCP vs total surfactant concentration at the CTAB molar
fraction α = 1.0 (1), 0.9 (2), 0.7 (3), 0.5 (4), 0.3 (5), and 0.1 (6);
1
0.001 М NaOH, 25 °С.
presented. As can be seen from the data in Table 2, the
main contribution to the catalytic effect is made by the
factor of reactant concentrating and the micellar microenꢀ
vironment exerts a negative effect on the reactivity of the
substrate. A decrease in the molar fraction of CTAB in the
system results in the further decrease in the Fm factor,
which causes the resulting diminishing of the catalytic
effect. Probably, the change in the microscopic properties
of the interface and, first of all, of the micropolarity upon
the insertion of the oxyethylene fragments between the
cationic head groups destabilizes the transition state of
the reaction. Despite the decrease in the surface potential
of the mixed micelles from 130 to 70 mV in the α1 interval
from 1.0 to 0.3,11 the КОН binding constants change inꢀ
significantly and the concentrating factor somewhat inꢀ
creases. The same regularity has been found earlier14,15
for other binary systems based on CTAB. Probably, in
ionic—nonionic surfactant binary systems the surface poꢀ
tential plays the secondary role in controlling the reactivꢀ
ity of compounds, and the major effect is made by the
change in the microscopic properties of the interface with
the variation of the surfactant ratio.
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Received June 25, 2007