Role of surface potential in the catalytic action of micelles of cationic surfactants with a hydroxyalkyl fragment in the head group

Article abstract of DOI:10.1007/s11176-008-2002-1

The surface potential in aqueous micellar solutions of a series of cetyltrialkylammonium bromides containing a hydroxyalkyl fragment in the head group was determined with a spectral probe (p-nitrophenol). It was found that the catalytic effect exerted by these surfactants in solution on the base hydrolysis of carboxylic acid esters is determined by two factors. First, the charged nucleophile is concentrated at the micelle surface. Second, hydrogen bonding of the substrate with the hydroxyalkyl substituents results in its activation.

Full text of DOI:10.1007/s11176-008-2002-1

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 2, pp. 163 170.
Pleiades Publishing, Ltd., 2008.
Original Russian Text A.B. Mirgorodskaya, L.R. Bogdanova, L.A. Kudryavtseva, S.S. Lukashenko, A.I. Konovalov, 2008, published in Zhurnal
Obshchei Khimii, 2008, Vol. 78, No. 2, pp. 179 186.
Role of Surface Potential in the Catalytic Action of Micelles
of Cationic Surfactants with a Hydroxyalkyl Fragment
in the Head Group
A. B. Mirgorodskaya, L. R. Bogdanova, L. A. Kudryavtseva,
S. S. Lukashenko, and A. I. Konovalov
Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center,
Russian Academy of Sciences, ul. Akademika Arbuzova 8, Kazan, Tatarstan, 420088 Russia
e-mail: mirgorod@iopc.kcn.ru
Received June 17, 2007
Abstract The surface potential in aqueous micellar solutions of a series of cetyltrialkylammonium
bromides containing a hydroxyalkyl fragment in the head group was determined with a spectral probe ( p-nitro-
phenol). It was found that the catalytic effect exerted by these surfactants in solution on the base hydrolysis of
carboxylic acid esters is determined by two factors. First, the charged nucleophile is concentrated at the
micelle surface. Second, hydrogen bonding of the substrate with the hydroxyalkyl substituents results in its
activation.
DOI: 10.1134/S1070363208020023
Cationic surfactants are widely used in the labora-
tory practice and industry thanks to their capability to
solubilize organic compounds in water and to affect
the physicochemical properties and reactivity of the
solubilized substances [1 4].
trostatic, and specific (intermolecular hydrogen bond-
ing) interactions take place in micellar solutions of
these surfactants. The properties of hydroxyalkyl
surfactants were discussed in comparison with the
behavior of related cationic surfactants containing no
OH group. The structural formulas of the surfactants
examined in this study are given below.
The aggregation behavior of surfactants in solu-
tions is determined by a number of factors: nature of
surfactant, structural features of the head group, length
and extent of branching of the hydrocarbon radical,
salt background, solution acidity, and presence of
organic solvents and other organic species. The struc-
ture of surfactant ensembles determines such proper-
ties of the system as thermodynamic stability, visco-
elastic properties, solubilizing capacity for hydro-
philic and hydrophobic compounds, and other charac-
teristics [1, 5 8].
CH3
CH3
C H N+ CH3 C16H33 N+ CH2
1
6 33
Br
CH Br
CH3
3
I
II
CH3
CH3
C H N+ CH CH OH C H N+ CH CH CH OH
1
6
33
2
2
16 33
2
2
2
Br
Br
CH3
CH3
This study was aimed to elucidate the effect of the
head group of cationic surfactant molecules on their
aggregation behavior in aqueous solutions and on
their solubilizing properties and catalytic action in
hydrolysis of carboxylic acid esters. We examined
quaternary ammonium compounds with the same
III
IV
CH3
C16H33 N+ CH2CH2OH
CH CH OH
+
C H
16 33
N
Br
CH2CH2OH
2
2
Br
hydrophobic fragment (C H radical) and different
1
6
33
V
VI
head groups. We used surfactants containing one or
two hydroxyalkyl radicals in the hydrophilic part of
the molecule, differing in the distance between the
OH group and the cationic center. Hydrophobic, elec-
The hydrophilic lipophilic balance and structure of
head groups of the surfactants used determine the
microscopic properties of the interphase zone, and
163

1
64
MIRGORODSKAYA et al.
primarily the surface potential. The value and sign of
the surface potential are important characteristics for
ion molecule reactions in micellar media [5, 9 11].
Cationic surfactants impart the positive charge to the
phase boundary. As a result, anions present in the
system are concentrated in the Stern layer, and acid
properties of compounds are enhanced. The selective
solubilizing power of micelles of ionic surfactnats
toward acid and base forms of compounds is mainly
determined by the surface potential of the micelle ( ).
Therefore, one of approaches to determination of the
surface potential is based on evaluation of the acid
base properties of probe molecules that can be incor-
porated in the interphase layer and can respond to
changes in its structure [9, 10]. In this study, as such
1
2
3
D
0
.9
4
00 nm
max
0
0
.6
.3
4
5
6
0
.0
3
0
25
20
a probe we used p-nitrophenol whose pK values were
3
1
a
10 , cm
determined spectrophotometrically. High molar ex-
tinction coefficients of this compound allow its use in
Fig. 1. Spectra of p-nitrophenol in an aqueous micellar
0.05 M). pH: (1) 10.0, (2) 6.6, (3) 6.1,
4) 5.8, (5) 5.6, and (6) 5.3.
5
low concentrations [(2 5) 10 M]. At such concen-
solution (C
(
SA
trations, a decrease in the charge density of cationic
micelles upon solubilization of negatively charged
species is insignificant. Figure 1 shows as example
the spectra of p-nitrophenol in micellar solutions of
surfactant III, recorded at various pH values. From
the absorption of p-nitrophenolate at various pH
values, using the Henderson Hasselbach equation, we
pKa
1
calculated the observed pK value (pKa,obs) [12]:
a
7
.5
pKa,obs = pH + log[(phenol)/(phenolate).
(1)
The dependences of pKa,obs of p-nitrophenol on
surfactant concentrations are plotted in Fig. 2.
7
.0
The observed constant of acid base dissociation
Kobs) is related to surfactant concentration as follows
(
[
13]:
6
6
.5
.0
1
1
+ K (C
CMS)
CMC)
B
SA
Kobs
=
Ka,w
,
(2)
+ K (C
A
SA
where Ka,w is the acid base dissociation constant in
water, K and K are the binding constants of the
B
A
acid and base forms of a compound, CSA is the sur-
factant concentration, and CMC is the critical micelle
concentration.
2
3
4
5
.5
Least-squares treatment of the dependence of
(Ka,w Kobs)/(CSA CMC) on Kobs allows determi-
nation of the binding constants of both forms of a
5
6
7
compound, taking into account that, at CSA
,
5
.0
0
0.002 0.004 0.006 0.008 0.010
Kobs = Ka,wK /K = Ka,m, where Ka,m is the dissoci-
B A
CSA, M
ation constant of a compound in the micellar phase.
We determined the CMC values for I VI conducto-
metrically as points of intersection of the linear por-
tions in the concentration dependences of the specific
electrical conductivity (an example of such depen-
dence is shown in Fig. 3). The CMC values of the
Fig. 2. Dependence of pK of p-nitrophenol on the
surfactant concentration C
a
in aqueous solution. Sur-
SA
factants: (1) Triton X-100 (nonionic); (2) I, (3) III,
4) IV, (5) V, (6) VI, and (7) II (all cationic).
(
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

ROLE OF SURFACE POTENTIAL IN THE CATALYTIC ACTION OF MICELLES
165
micellar solutions of the surfactants under considera-
tion and the binding constants of the charged and
neutral forms of p-nitrophenol are given in Table 1.
The surface potential, in accordance with [9], can be
calculated by formula (3):
S cm 1
1
50
1
pKa,m = pKa,0
F /(2.303RT),
(3)
100
where pKa,0 is the nonelectrostatic term defined as
2
pK in micellar solutions based on nonionic surfact-
a
5
0
0
ants (Triton X-100, pK of p-nitrophenol 7.6) with the
a
same phase volumes as in the systems with cationic
1
surfactants; F = 96486 C mol is the Faraday con-
1
1
stant, and R = 8.314 J K mol is the gas constant.
0
0.001
0.002
0.003
0.004
CSA, M
The calculation results are given in Table 1.
It can be seen that the binding constant of the
anionic form of p-nitrophenol is approximately two
orders of magnitude higher than that of the neutral
Fig. 3. Concentration dependence of the specific elec-
trical conductivity of aqueous solutions of surfactants
(1) VI and (2) II.
form. As a result, pK of this compound in the micel-
a
lar phase of the surfactants is lower than that in water
by 1.8 2.2 log units. The surfactants containing a hy-
droxyalkyl fragment bind the phenolate more strongly
than does cetyltrimethylammonium bromide, but the
surface potentials of these compounds differ insignifi-
cantly. It can be expected that the radicals forming the
head group can affect the charge density at the quater-
nary nitrogen atom owing to the inductive effect,
which is similar for methyl, 2-hydroxyethyl, and, the
more so, 3-hydroxypropyl substituents. Compound VI
in which the ammonium group is incorporated in a
five-membered ring exhibits a higher surface potential.
surfactants on the rate of reactions involving these
esters. The presence of a hydroxy group in surfactant
molecules may lead to specific binding in the system
via hydrogen bonds, which should affect the rate of
base hydrolysis.
Note that surfactants containing a hydroxyalkyl
substituent in the head group exhibit acid properties
(
pK 12.4 12.6) and in strongly alkaline solutions can
a
act as zwitterionic compounds [16, 17]. However,
under our experimental conditions (pH 10.0, buffer
based on sodium tetraborate and sodium hydroxide),
the compounds are typical cationic surfactants. Kinet-
ic studies showed that, for both substrates, the shape
of the dependence of the observed rate constant (kobs)
When surfactant solutions are used as a medium for
chemical reactions involving ionic reactants, the sur-
face potential is responsible for their concentrating
near the micellar surface. In this study we examined
an ion molecule reaction of base hydrolysis of car-
boxylic acid esters, p-nitrophenyl acetate and p-nitro-
phenyl laurate. The reaction scheme is shown below.
Table 1. Values of CMC, surface potential, binding
constants, and pKa,ma of p-nitrophenol in micelles based
on cationic surfactants I VI (20 C)
The esters differ in their hydrophobic properties.
The distribution constant between oil and water is
Comp.
no.
CCM,
M
KHA
M
,
K ,
M
,
A
pKa,m
mV
1
0.8 for p-nitrophenyl acetate and 205.4 for p-nitro-
I
II
III
IV
V
0.0008
230
590
465
540
305
280
17300
51030
35290
51960
37400
49290
5.32
5.26
5.41
5.31
5.10
4.97
133
136
127
133
145
151
0.00045b
0.00075
0.0008
0.00065b
VI
0.0005
R C
O
NO2 + 2O H
a
pK of p-nitrophenol in water, as we determined spectropho-
a
b
tometrically, is 7.2. According to [27], CMC (M) of III is
0
(
0
.0072 (fluorimetry), 0.0078 (conductometry), or 0.0083
microcalorimetry), and that of V is 0.0072 (fluorimetry),
.0070 (conductometry), or 0.0077 (microcalorimetry).
RCOO + O
NO + H O,
2 2
4
00 nm
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

1
66
MIRGORODSKAYA et al.
(
a)
(b)
kobs, s 1
kobs, s 1
1
1
0
.25
0
.04
2
0
.20
2
0
.03
0
.15
3
0
0
.02
.01
3
0
.10
4
0
.05
4
5
5, 6
6
0
.00
0.00
0
0.002 0.004 0.006 0.008 0.01 CSA, M
0
0.002 0.004 0.006 0.008 0.01 CSA, M
Fig. 4. Observed rate constant of the base hydrolysis (pH 10.0, 25 C) of (a) p-nitrophenyl acetate and (b) p-nitrophenyl laurate
as a function of surfactant concentration: (1) VI, (2) V, (3) III, (4) IV, (5) I, and (6) II.
of hydrolysis on the surfactant concentration has a
shape typical of micelle-catalyzed processes with rela-
tively steep ascent followed by flattening out, but for
the laurate the range of the rate variation is consider-
ably wider. It is known that the rate constants of base
hydrolysis without surfactants for p-nitrophenyl ace-
tate are 20 times higher than for p-nitrophenyl laurate
owing to self-association of the latter and shielding of
the carbon atom subject to nucleophilic attack [18].
In micellar solutions, the structures of these associates
change, and differences in the reactivity of the two
esters are leveled off (Figs. 4a, 4b).
Table 2. Parameters of micelle-catalyzed base hydrolysis
of p-nitrophenyl acetate (PNPA) and p-nitrophenyl laurate
(
1
PNPL) in aqueous solutions of various surfactants (pH
0.0, 25 C)
Quantitative characteristics reflecting the substrate
micelle interaction can be derived from the experi-
mental kinetic data using the pseudo-phase model of
micellar catalysis [19]:
Comp.
no.
k ,
s
Ks,
M
CMC,
M
a
m
Substrate
k /k
m 0
1
1
I
PNPA
PNPL
PNPA
PNPL
PNPA
PNPL
PNPA
PNPL
PNPA
PNPL
PNPA
PNPL
0.0073 450
0.011 1100
0.0065 980
0.0063 460
0.0004
7
275
6.5
158
140
kmKs(CSA CMC) + k0
kobs
=
,
(4)
0.00044
0.00024
0.00027
0.00034
1
+ K (C
CMC)
s
SA
II
where k and k are the rate constants in the aqueous
medium and in the micellar phase, respectively, and
K is the substrate binding constant.
0
m
III
IV
V
0.14
380
750
300
420
230
120
230
250
s
0.019
0.053
0.014
0.28
0.069
0.38
0.000028 475
0.00017
0.00026
0.00034
0.00022 1725
0.000077 380
0.000027 1575
53
350
280
The results of calculations from the data plotted in
Figs. 4a and 4b are given in Table 2. The CMC values
found from the kinetic data are somewhat lower than
those determined conductometrically. This phenom-
enon is observed frequently and can be accounted for
by the fact that a substrate is bound by surfactant
molecules before the micellization is complete, and
in these premicellar associates the hydrolysis occurs
faster than in molecular solutions. The binding con-
stants of the esters under consideration are not very
VI
0.063
a
The k /k ratio characterizes the micellar-catalytic effect (for
m
0
1
the hydrolysis without surfactant at pH 10.0, k = 0.001 s
for PNPA and 0.00004 s for PNPL).
0
1
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ROLE OF SURFACE POTENTIAL IN THE CATALYTIC ACTION OF MICELLES
167
high, and in most cases a more hydrophobic substrate,
p-nitrophenyl laurate, is bound by the micelle some-
what better than p-nitrophenyl acetate. From the rate
constants of hydrolysis in the micellar phase, we can
evaluate the micellar-catalytic effect characterized by
kobs, s 1
0
.12
the ratio k /k , which is higher for laurate than for
m
0
acetate (Table 2). The highest rate constants are ob-
served with the systems based on surfactants V and
VI, which are characterized by the highest positive
surface potential and hence by the most pronounced
concentration of the nucleophile at the surface of the
micelle in which the substrate is solubiitzed. This fact,
however, does not account for the stronger accelerat-
ing effect exerted on the hydrolysis of both substrates
by the surfactants containing a hydroxyalkyl fragment,
compared to celyltrimethylammonium bromide I. For
p-nitrophenyl laurate, the rate constants of the hydrol-
ysis in solutions of III VI exceed those obtained with
I by a factor of 1.5 6, and for p-nitrophenyl acetate
the difference is still larger, by a factor of 7.5 55.
Apparently, the less hydrophobic substrate is localized
near the head groups of the surfactants. with the for-
mation of a hydrogen-bonded complex in which the
carbonyl center acquires an additional positive charge
facilitating an attack by hydroxide ion:
1
kobs, s 1
1
0.10
0.05
.04
.03
2
0
0
0
.08
3
0
0
.02
.01
0
0
0.03 0.06 0.09
0.06
2
1
CSA, M
3
0
.04
0.02
0
.00
0
0.002 0.004 0.006 0.008 0.01
R
CSA, M
O=C O
NO2
HO
^
Fig. 5. Observed rate constant of base hydrolysis (pH
0.0, 25 C) of p-nitrophenyl acetate as a function of the
O H
1
CH2
concentration of surfactant III at KBr concentrations of
(1) 0, (2) 0.01, and (3) 0.02 M. The insert shows the cor-
responding plots for cetyltrimethylammonium bromide.
+
N CH
2
Such a type of substrate activation, leading to ac-
celeration of the base hydrolysis of phosphorus acid
esters by more than two orders of magnitude, was
observed with betaine forms of hydroxyethyl surfact-
ants [16]. p-Nitrophenyl laurate, tending to localize
in the micelle core, is disconnected from the hydroxy
group of the head fragment, which prevents hydrogen
bonding and activation of the carbonyl center. With
this substrate, the observed catalytic effect is mainly
due to the fact that the anionic nucleophile is concen-
trated at the positively charged micelle surface, i.e.,
process, the surface potential of the system decreases
in the absolute value, CMC decreases, the aggregation
numbers increase, and the micellar effect on physico-
chemical properties and reactivity of solubilized sub-
stances changes [23 25]. In this study, we added a
supporting electrolyte (KBr) to micellar solutions of
cetyltrimethylammonium bromide I and cetyldimeth-
yl(2-hydroxyethyl)ammonium bromide III, to decrease
the surface potential. In these systems, we studied the
kinetics of hydrolysis of p-nitrophenyl acetate. The
results are shown in Figs. 5 and 6. The potentials that
we determined spectrophotometrically were 120 mV
for I and 112 mV for III with 0.01 M KBr, and 107
and 102 mV, respectively, with 0.02 M KBr. The de-
pendence of the observed rate constant of hydrolysis
of p-nitrophenyl acetate on the surfactant concentra-
tion (Fig. 5) shows that the catalytic effect caused
by concentrating of hydroxide ion at the positively
charged interphase surface decreases, and its value
with cetyltrimethylammonium bromide does not ex-
ceed 2. However, for III the catalytic effect is also due
the effect is governed by
.
It is known [9, 20 22] that the surface potential in
micellar solutions of ionic surfactants can be con-
trolled by supporting electrolyte additions. This is due
to the fact that an increase in the concentration of
surfactant counterions with addition of an electrolyte
leads to partial neutralization of the surface charge of
the micelles, which, in turn, decreases the destabiliz-
ing repulsion of similarly charged head groups. In the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

1
68
MIRGORODSKAYA et al.
a characteristic shape with a break at CKBr = 0.04 M.
Similar dependences are obtained in micellar solutions
of cetyltrimethylammonium bromide as a reaction
medium for the hydrolysis of p-nitrophenyl acetate
kobs, s 1
(
Fig. 5, insert). The value of C = 0.03 M, determined
cr
0
.06
for this surfactant, is consistent with the values given
in [20, 26]. Presumably, in solutions of the surfactants
under consideration, addition of an electrolyte causes
a sphere cylinder micelle transition. This is accompa-
nied by an increase in the packing density of surfact-
ant molecules in the aggregate, enhancement of hydro-
phobic interactions, changes in the degree of hydra-
tion, decrease in the degree of ionization of the head
groups, and weakening of the micellar catalytic effect
0
0
.05
.04
kobs, s 1
[
8]. Note that initiation of this transition in the case
0
.005
of hydroxyalkyl surfactant III requires adition of a
somewhat larger amount of the electrolyte than in the
case of cetyltrimethylammonium bromide. Further-
more, the possibility of the substrate activation by
intermolecular hydrogen bonding leads to the high
catalytic effect with both spherical and cylindrical
aggregates of surfactant III.
0
.004
0
.03
0
0
.003
.002
0
.02
0.001
1.0 1.5 2.0 2.5
Thus, the effect of the head group of the surfactant
on the catalytic properties of the system in ion mole-
cule processes, on the one hand, consists in changes
in the electron density at the quaternary nitrogen atom
and hence in the surface potential, which leads to
changes in the concentration gradient of an ionic
reagent in the micellar system. On the other hand,
the substrate can specifically interact (hydrogen bond-
ing) with the hydroxyalkyl substituent, which en-
hances the reactivity. Addition of a supporting electro-
lyte is an additional tool to control the catalytic effect
of the system by affecting the surface potential and
structure of micelles.
log C
KBr
1
.0
1.5
2.0
2.5
log CKBr
Fig. 6. Observed rate constant of base hydrolysis of
p-nitrophenyl acetate in micellar solutions of surfactant
III as a function of logarithm of KBr concentration
(
C
= 0.005 M, pH 10.0, 25 C). The insert shows
SA
the corresponding plots for cetyltrimethylammonium
bromide.
to substrate activation by hydrogen bonding with the
head group of the surfactant, and therefore the re-
sponse of this system to supporting electrolyte addi-
tion is weaker, and the value of the catalytic effect
reaches almost two orders of magnitude.
EXPERIMENTAL
Commercial cetyltrimethylammonium bromide and
Triton X-100 (Sigma) used in this study contained
about 99% main substance. Cationic surfactants with
a hydroxyalkyl substituent were prepared by quaterni-
zation of the corresponding amino alcohols with cetyl
bromide by the procedure given in [14, 27]. The struc-
ture of the compounds was proved by elemental anal-
Addition of an electrolyte to surfactant solutions in
some cases induces structural rearrangements in the
system, in particular, sphere cylinder micellar transi-
tions [8, 20, 22, 26]. Disturbance of the steady state of
ion-exchange processes occurring in these systems can
be reflected in the character of concentration depen-
dences of kinetic characteristics. Critical points (Ccr)
associated with qualitative changes in the micelle state
can apear in these dependences. A study of the kinet-
ics of base hydrolysis of p-nitrophenyl acetate in rela-
tion to the KBr concentration at a constant concen-
tration of hydroxyalkyl surfactant III in solution
1
ysis (Table 3) and H NMR spectroscopy. p-Nitro-
phenol and its carboxylic acid esters (Fluka) were
purified by standard procedures. Solutions were pre-
pared in double-distilled water.
The specific electrical conductivities of surfactant
solutions prepared in double-distilled water with the
conductivity no greater than 1.8 S cm 1 were meas-
ured with a CDM-2d device (Denmark).
(
0.005 M) shows that, with increasing KBr concentra-
tion, the observed rate constant decreases nonuniform-
ly. The dependences kobs logCKBr (Fig. 6) acquire
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

ROLE OF SURFACE POTENTIAL IN THE CATALYTIC ACTION OF MICELLES
169
Table 3. Characteristics of the synthesized samples of cationic surfactants
C, %
calculated
H, %
calculated
N, %
calculated
Br, %
Surfactant
T , C
m
found
found
found
found
calculated
II
68 69
203 205
89 90
116 118
212 214
68.09
60.84
61.72
59.39
62.75
68.16
60.89
61.74
59.42
62.84
10.58
11.32
11.40
10.94
11.05
10.52
11.24
11.35
10.92
11.03
3.21
3.52
3.46
3.38
3.40
3.18
3.52
3.43
3.30
3.33
18.09
20.17
19.55
18.80
19.23
18.04
20.25
19.56
18.82
19.00
III
IV
V
VI
Acid base properties of p-nitrophenol were studied
by measuring the absorption of the ionized form at
various pH values. The spectra were recorded with a
Specord UV0VIS spectrophotometer in quartz cells
with the absorbing layer thickness L = 1 cm in the
range from 250 to 400 nm. The extinction coefficient
of the phenolate form was determined from the opti-
cal density D in the absorption maximum. The con-
centration of p-nitrophenol CPhO at a given pH was
determined from the relationship CPhO = D/( L).
Three to five replicate measurements at each pH value
were made, and their results were averaged. Their
reproducibility was within 0.05.
2. Morales-Rojas, H. and Moss, R.A., Chem. Rev., 2002,
vol. 102, no. 7, p. 2497.
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. Zana, R. and Xia, J., Gemini Surfactants Synthesis,
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The reaction kinetics were studied spectrophotomet-
rically on a Specord UV-VIS device in temperature-
controlled cells. The reaction progress was monitored
by variation of the optical density of the solutions at
a wavelength of 400 nm (formation of p-nitropheno-
late anion). The initial concentration of the substrate
7. Basu Ray, G., Chakraborty, I., Gosh, S., Moulik, S.P.,
and Palepu, R., Langmuir, 2005, vol. 21, no. 24,
p. 10958.
8
. Serdyuk, A.I. and Kucher, R.V., Mitsellyarnye pere-
khody v rastvorakh poverkhnostno-aktivnykh ve-
shchestv (Micellar Transitions in Surfactant Solutions),
Kiev: Naukova Dumka, 1987.
5
was (2 8) 10 M, and the degree of conversion was
more than 90%. The observed pseudo-first-order rate
constants kobs were determined from the relationship
log(D
D ) = 0.434kobs + const (D and D are
9
. Mchedlov-Petrosyan, N.O., Differentsirovanie sily
organicheskikh kislot v istinnykh i organizovannykh
rastvorakh (Differentiation of the Strength of Organic
Acids in True and Organized Solutions), Kharkov:
Khar’k. Nats. Univ., 2004.
the optical densities of the solutions at time and
after the reaction completion, respectively). The quan-
tities kobs were calculated by the least-squares meth-
od. The experimental data were described by a first-
order equation with the correlation coefficient higher
than 0.99. The values of kobs obtained from two repli-
cate experiments were averaged, and the differences
between them did not exceed 4%.
1
1
0. Mirgorodskaya, A.B., Kudryavtseva, L.A., Zakharo-
va, L.Ya., and Bel’skii, V.E., Izv. Ross. Akad. Nauk,
Ser. Khim., 1998, no. 7, p. 1333.
1. Solntsev, K.M., Al-Ainain, S.A., Il’ichev, Y.V., and
Kuzmin, M.G., J. Phys. Chem. A, 2004, vol. 108,
no. 40, p. 8212.
ACKNOWLEDGMENTS
The study was financially supported by the Russian
Foundation for Basic Research (project no. 05-03-
12. Albert, A. and Serjeant, E., Ionization Constants of
Acids and Bases. A Laboratory Manual, New York:
Wiley, 1962.
3
3110).
1
3. Berezin, I.V., Martinek, K., and Yatsimirskii, A.K.,
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