Geminal alkylammonium surfactants: Aggregation properties and catalytic activity

Article abstract of DOI:10.1134/S1070363206100215

Geminal alkylammonium surfactants containing a hexamethylene spacer show a specific behavior, namely, a catalytic effect at low concentrations of the detergent in the system. The critical micellization concentrations of these surfactants are by an order o

Full text of DOI:10.1134/S1070363206100215

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 10, pp. 1625 1631.
Original Russian Text A.B. Mirgorodskaya, L.A. Kudryavtseva, V.A. Pankratov, S.S. Lukashenko, L.Z. Rizvanova, A.I. Konovalov, 2006, published in
Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 10, pp. 1696 1702.
Pleiades Publishing, Inc., 2006.
Geminal Alkylammonium Surfactants: Aggregation Properties
and Catalytic Activity
a
a
b
A. B. Mirgorodskaya , L. A. Kudryavtseva , V. A. Pankratov ,
a
a
a
S. S. Lukashenko , L. Z. Rizvanova , and A. I. Konovalov
a
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center,
Russian Academy of Sciences,
ul. Akad. Arbuzova 8, Kazan, Tatarstan, 420088 Russia
e-mail: mirgorod@iopc.kcn.ru
b
State University of Applied Biotechnology, Moscow, Russia
Received July 24, 2006
Abstract Geminal alkylammonium surfactants containing a hexamethylene spacer show a specific
behavior, namely, a catalytic effect at low concentrations of the detergent in the system. The critical
micellization concentrations of these surfactants are by an order of magnitude lower than those of the related
surfactants with one hydrophobic fragment. The effect exerted on the hydrolysis of carboxylic acid esters by
micellar structural rearrangements induced by addition of a supporting electrolyte is determined.
DOI: 10.1134/S1070363206100215
The interest in geminal surfactants and highly
organized systems based on them considerably in-
creased in the past decade. Geminal surfactants con-
tain two hydrophobic radicals and two head (as a rule,
charged) groups linked by rigid or flexible spacers.
These compounds differ from their amphiphilic ana-
logs containing one head group in higher (by an order
of magnitude) surface activity and lower critical mi-
cellization concentrations (CMC) [1 4]. They are
used as gelating substances and biologically active
additives, and also in synthesis of new mesoporous
materials.
surfactants makes them attractive in simulation of the
organization and functioning of a cell membrane
[5, 7]. Structural features of systems based on geminal
surfactants were considered in numerous papers pub-
lished recently; however, only a few papers concerned
the effect of these surfactants on the rates of chemical
reactions [8 10].
In this study we examined the aggregation behavior
and catalytic activity in cleavage of ester bonds of
geminal alkylammonium surfactants containing a
hexamethylene spacer and differing in the length of
the hydrophobic fragment and structure of the head
group:
The structural similarity of lipids and geminal
2Br
C H
+
N
(CH )
+
N
C H
12 25
1
2
25
2 6
+
+
[
R(CH ) N(CH ) N(CH ) R] 2Br ,
3 2 2 6 3 2
R = C H (Gem-1), C H (Gem-2)
1
2
25
16 33
(
Gem-3)
The properties of these compounds are compared to
those of the related cationic surfactants containing one
hydrophobic fragment.
form micelles in aqueous solutions at low concentra-
tions (10 4 10 M) [11, 12]. We studied the aggrega-
tion behavior of Gem-1 Gem-3 by conductometry.
The intersection point of the linear portions of the
concentration dependences of the specific electrical
conductivity corresponds to the CMC values, and the
6
It is known that geminal surfactants containing
a hexamethylene spacer between the charged centers
1625

1
626
MIRGORODSKAYA et al.
of the dimethyl fragment (Gem-1) by the piperidyl
S cm 1
moiety (Gem-3) leads to an increase in CMC, which
suggests that CMC is influenced by packing of the
surface layer. The curve for Gem-2 shows a second
bend at a concentration of 0.0017 M (Fig. 1), which
,
1
1
40
is identified as CMC and may correspond to micellar
2
20
structural transitions from spherical to ellipsoidal
aggregates. For example, a small-angle neutron scat-
tering study of Gem-2 [11] revealed at a concentration
of 0.0025 M micelles in the form of elongated ellip-
soids with semimajor and semiminor axes of 3.2 and
2.3 nm, respectively. The ratio between the semiaxis
lengths increases with the surfactant concentration.
The parameter estimated for Gem-2 by small-angle
neutron scattering for limiting concentrations of
, S cm 1
1
00
7
0
6
5
4
0
0
0
8
0
6
0
3
0
0
.03 M is 0.25 0.27 [11], which is consistent with
2
0
our conductometric data (Table 1). It is logical to
expect for Gem-2 a transition to cylindrical micelles at
a certain high concentration; even at a concentration
of 0.05 M, the ratio of the ellipsoid semiaxes in-
creases to 2.5, and decreases to 0.16 [11]. However,
in our conductometric studies we have not reached the
Gem-2 concentration at which the formation of cy-
lindrical aggregates could be detected.
4
0
10
0
2
0
0
0
.0000 0.0004 0.0008
CGem-2, M
0
.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030
CGem-2, M
Micellar solutions of the surfactants were used as
reaction media for base hydrolysis of carboxylic acid
esters: p-nitrophenyl acetate and p-nitrophenyl laurate.
The general equation of the reaction is as follows:
Fig. 1. Concentration dependence of the specific elec-
trical conductivity of aqueous solutions of Gem-2. The
insert shows the range of low concentrations.
ratio of the slopes above and below CMC corresponds
to the degree of ionization of the micelle ( ). Table 1
shows that, as compared to one-tail analogs (pub-
lished conductometric data are given), the CMC val-
ues of geminal surfactants are lower by an order of
magnitude. Also, it can be seen that an increase in the
hydrophobicity of the alkyl radical in geminal surfact-
ants facilitates the micelle formation. Replacement
RC(O)OC H NO + 2OH
CH C(O)O
3
6
4
2
+
OC H NO ,
6 4 2
R = CH , C H .
3
11 23
The catalytic effect expected for this reaction in
solutions of cationic surfactants is primarily associ-
ated with accumulation of hydroxide ions at the posi-
tively charged interphase surface, which increases the
probability of the reaction contact of the hydrophilic
nucleophile with the substrate solubilized by the
micelle. The dependences of the observed rate con-
Table 1. CMC values and degrees of binding of counterions
for the surfactants studied (conductometry, water, 30 C)
Surfactants
CMC , M
stant (k ) of the base hydrolysis on the concentration
1
obs
of the cationic surfactant are shown in Figs. 2 and 3.
These data show that the geminal surfactants in hand
exert on the cleavage of ester bonds a strong catalytic
effect manifested at lower concentrations compared to
one-tail analogs.
It should be noted that the esters we chose differ in
the hydrophilic lipophilic properties (the distribution
ratio between oil and water is 10.8 for p-nitrophenyl
acetate and 205.4 for p-nitrophenyl laurate [15]),
which should be reflected in the effect of surfactants
on the rate of their hydrolysis. Figures 2 and 3 show
that the micellar catalytic effect for these substrates
differs essentially. Whereas the rate constant for the
Gem-1
Gem-2
0.00125
0.000045
CMC 0.0012 M)
0.36
0.23
(
2
a
Gem-3
0.0017
0.29
a
0.22a [13]
0.25 [14]
0.23a [13]
0.27 [14]
0.25a [13]
Cetyltrimethylammonium 0.00092 [13]
bromide
Tetradecyltrimethyl-
ammonium bromide
Dodecyltrimethyl-
ammonium bromide
0.0092 [14]
a
0.0036 [13]
0.00394 [14]
a
0.0145 [13]
0.0153 [14]
a
At 25 C.
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GEMINAL ALKYLAMMONIUM SURFACTANTS: AGGREGATION PROPERTIES
1627
kobs, s 1
kobs, s 1
1
0
0
.008
0
0
0
.006
.005
.004
1
2
3
4
.006
2
0
.004
.002
5
4
3
0
0
.003
.002
5
0
6
0
.000
0
.001
0
.000
0.005
0.010
0.015
surfactant, M
C
0
.000 0.005
0.010 0.015 0.020
Csurfactant, M
Fig. 2. Observed rate constant of base hydrolysis of
p-nitrophenyl acetate as a function of surfactant con-
centration (pH 10.0, 25 C): (1) Gem-1, (2) cetyltri-
methylammonium bromide, (3) Gem-2, (4) Gem-3,
Fig. 3. Observed rate constant of base hydrolysis of
p-nitrophenyl laurate as a function of surfactant con-
centration (pH 10.0, 25 C): (1) cetyltrimethylammonium
bromide, (2) Gem-1, (3) Gem-3, (4) Gem-2, and (5) tetra-
decyltrimethylammonium bromide.
(
(
5)
tetradecyltrimethylammonium
bromide,
and
6) dodecyltrimethylammonium bromide.
acetate increases by a factor of 4 8, for the laurate
the reaction is accelerated by two orders of magnitude.
The amphiphilic structure of the laurate in aqueous
solutions promotes its self-association, which leads to
shielding of the active centers of the substrate and, as
a consequence, to abnormally low reactivity of the
laurate, compared to acetate, in base hydrolysis in
aqueous solutions [16]. In micellar solutions, incor-
poration of the hydrophobic laurate into a surfactant
micelle is accompanied by unrolling of its globular
associates, which makes the substrate accessible to the
attack of a nucleophile and results in considerable
acceleration of the hydrolysis (Fig. 3).
and k are the rate constants in aqueous solution and
m
in the micellar phase, respectively; and K is the
s
substrate binding constant. The calculation results are
given in Table 2.
These data show that, for surfactants with one
hydrophobic fragment, the p-nitrophenyl acetate bind-
ing constants are low, and the catalytic effect strongly
depends on the number of carbon atoms in the alkyl
radical. The strongest acceleration of the hydrolysis
of the ester is observed with cetyltrimethylammonium
bromide, whereas the effect of dodecyltrimethylam-
monium bromide is so weak that, e.g., curve 6 in
Fig. 2 is not described by the equation of the pseudo-
phase model. Geminal surfactants, compared to those
with one hydrophobic fragment, are characterized by
better binding of the substrate (Table 2). For Gem-1
and Gem-3 containing the same dodecyl fragment, the
Quantitative characteristics reflecting the interac-
tion of a substrate with a micelle can be derived from
the experimental kinetic data using a pseudophase
model of micellar catalysis [17], Eq. (1):
k K C + k0
K values are close, and for the cetyl derivative they
m
s
det
s
kobs
=
,
(1)
are higher. However, the extent of the reaction accel-
eration is similar for all the three surfactants. This fact
suggests that, for geminal surfactants, the contribution
of the electrostatic interactions providing accumula-
1
+ K C
s det
where Cdet is the surfactant concentration corrected
for the critical micellization concentration (CMC); k0
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

1628
MIRGORODSKAYA et al.
Table 2. Parameters of micellar-catalyzed base hydrolysis of p-nitrophenyl acetate and p-nitrophenyl laurate in solutions of
various surfactants (pH 10.0, 25 C)
Surfactant
Substrate
k , s 1
K , l mol 1
CMC, M
kobs(max)/k a
m
s
0
Cetyltrimethylammonium bromide
Acetate
Laurate
Acetate
Laurate
Acetate
Laurate
Acetate
Laurate
Acetate
Laurate
0.0073
0.011
450
1100
360
710
820
4000
2300
8000
850
0.0004
0.00044
0.0007
0.0006
0.00008
0.0006
0.00004
0.00008
0.00005
0.00008
7
275
3
95
8
113
6
110
6
Tetradecyltrimethylammonium bromide
0.0043
0.0038
0.0065
0.0045
0.0053
0.0044
0.0052
0.0053
Gem-1
Gem-2
Gem-3
1100
125
a
The k (max)/k ratio characterizes the micellar catalytic effect (without surfactant at pH 10.0, for the hydrolysis of the acetate,
obs
0
1
1
k
= 0.0008 s , and for the laurate, k = 0.00004 s ).
0
0
tion of hydroxide ions at the micellar surface appreci-
ably exceeds the contribution of hydrophobic inter-
actions.
The low value of CMC of Gem-2, estimated from
the kinetic data, is consistent with that determined
from the conductometric rata (Tables 1, 2). For gemi-
nal surfactants with the dodecyl fragment, however,
such agreement is not observed. Apparently, in this
case premicellar aggregation is possible, which favor-
ably affects the localization of the reactants in the
system but is not manifested in the electrical conduc-
tivity. The geminal compounds examined allow prep-
aration of micellar systems containing small amounts
of surfactants but exhibiting a strong catalytic effect.
kobs, s 1
0
0
.25
1
.20
As expected, the hydrolysis of the esters is accel-
erated in going to more alkaline solutions (Fig. 4).
However, an increase in the concentration of hydrox-
ide ions in going from the buffer system with pH 10.0
to 0.005 M NaOH does not cause significant changes
in the binding constants of the substrate with the
micelle and in the CMC values determined from the
kinetic data (Tables 2, 3). This fact suggests the ab-
sence of significant changes in the structure of the
micelles and accounts for the similar extent of accel-
eration of the p-nitrophenyl acetate hydrolysis.
2
0
0
.15
.10
3
4
0
0
.05
.00
Additions of supporting electrolytes are frequently
used to initiate structural transformations in micellar
systems and thus to alter their catalytic effect [18, 19].
An increase in the concentration of surfactant counter-
ions on adding an electrolyte leads to partial neutrali-
zation of the surface charge of the micelles, which, in
turn, decreases the destabilizing repulsion of the simi-
larly charged head groups. This leads to a decrease in
the absolute value of the surface potential of the sys-
tem, a decrease in CMC, an increase in the aggrega-
tion number, and also a change in the micellar effect
on the physicochemical properties and reactivity of
solubilized substances.
0
.000 0.002 0.004 0.006 0.008 0.010
Csurfactant, M
Fig. 4. Observed rate constant of base hydrolysis of
p-nitrophenyl acetate as a function of surfactant con-
centration with addition of KBr and without it (0.005 M
NaOH, 25 C): (1) Gem-2, C
ammonium bromide, C
monium bromide, C
= 0; (2) cetyltrimethyl-
= 0; (3) cetyltrimethylam-
= 0.01 M: and (4) Gem-2,
KBr
KBr
KBr
C
= 0.
KBr
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

GEMINAL ALKYLAMMONIUM SURFACTANTS: AGGREGATION PROPERTIES
1629
Table 3. Parameters of micellar-catalyzed base hydrolysis of p-nitrophenyl acetate without supporting electrolyte and in
the presence of potassium bromide (0.005 M NaOH, 25 C)
Surfactant
CKBr, M
k , s 1
k , s 1
K , l mol 1
CMC, M
k /k
0
m
s
m
0
Cetyltrimethylammonium
bromide
Gem-2
0
0.01
0
0.042
0.032
0.042
0.032
0.18
0.06
0.28
0.058
420
230
2500
3100
0.0005
0.0002
0.000047
0.00035
4.3
1.9
6.7
1.8
0
.01
Zakharova et al. [19 21] showed that addition of
KBr to a micellar solution of cetyltrimethylammoni-
um bromide decreases the amount of hydroxide ions
for Gem-2; the total concentration of bromide ions at
which the transition is observed is 0.03 M.
Presumably, in solutions of geminal surfactants,
addition of an electrolyte induces micellar rearrange-
ments similar to those occurring with an increase in
the surfactant concentration. Passing to cylindrical
systems leads to an increase in the density of packing
(
species inducing cleavage of ester bond) in the Stern
layer and, under certain conditions, can induce
sphere cylinder micellar transitions.
The dependences of the observed rate constant of
base hydrolysis (0.005 M NaOH) of p-nitrophenyl
acetate in micellar solutions of cetyltrimethylammo-
nium bromide and Gem-2 on the cationic surfactant
concentration (Fig. 4) show that addition of KBr
kobs, s 1
0
.15
(
CKBr = 0.01 M) decreases the catalytic effect. The
1
kobs, s 0.10
system based on the geminal surfactant is more sensi-
tive to the addition of the supporting electrolyte.
However, analysis of our data using Eq. (1) shows
that this concentration of KBr is insufficient to induce
structural rearrangements in the system (Tables 2, 3).
A decrease in the rate of the base hydrolysis is caused
by a decrease in the surface potential of the micelle
0.18
1
0.16 0.05
2
0.14
0.00
1
2
log C
KBr
0
.12
.10
.08
.06
(
by analogy with [21]), leading to a decrease in the
capability of the system to concentrate hydroxide ions
at the interphase layer. As a result, the catalytic effect
decreases.
0
3
0
A kinetic study of the base hydrolysis of p-nitro-
phenyl esters in relation to the KBr concentration at
a constant content of Gem-2 in solution shows that,
with increasing KBr content, the observed rate con-
stant decreases nonuniformly. The dependences of
kobs on logCKBr (Fig. 5) are broken lines with a bend
0
1
2
0.04
0.02
0.00
in a point C . At the same surfactant concentration,
cr
the Ccr values virtually coincide for both substrates.
The fact that the value obtained is independent of the
structure of the ester means that it characterizes the
properties of the micellar system itself. With an in-
crease in the surfactant content in the system, smaller
amounts of the electrolyte are required to induce the
rearrangement: The transition is observed at a total
concentration of bromide ions of about 0.023 M. Sim-
ilar dependences were obtained with micellar solu-
tions of cetyltrimethylammonium bromide as media
for the hydrolysis of carboxylic acid esters (Fig. 5,
1
2
3
4
log CKBr [M]
Fig. 5. Observed rate constant of base hydrolysis of
carboxylic acid esters (25 C) in micellar solutions of
Gem-2 as a function of the logarithm of the KBr con-
centration: (1, 2) C
= 0.0025 M, (1) p-nitro-
surfactant
phenyl acetate and (2) p-nitrophenyl laurate;
(3) C = 0.005 M, p-nitrophenyl acetate. Insert:
surfactant
similar dependence for cetyltrimethylammonium bro-
mide: (1) p-nitrophenyl acetate, C
(2) p-nitrophenyl laurate, C
= 0.005 M;
surfactant
insert). The C value for this surfactant is higher than
= 0.0025 M.
cr
surfactant
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 10 2006

1
630
Table 4. Characteristics of the synthesized samples of geminal surfactants
C, % H, %
calculated calculated
MIRGORODSKAYA et al.
N, %
calculated
Br, %
calculated
Surfactant Tm,
C
found
found
found
found
Gem-1
Gem-2
Gem-3
232 234
203 204
209 210
60.8
64.3
64.2
60.92
64.42
64.00
11.91
12.13
11.15
11.13
11.58
10.93
4.24
3.56
3.80
4.18
3.57
3.73
24.2
21.0
21.65
23.84
20.41
21.33
1
of surfactant molecules in the aggregate, enhancement
of hydrophobic interactions, changes in the degree of
hydration, amd decrease in the degree of ionization of
the head groups. As a result, in the range of existence
of cylindrical micelles, the catalytic effect of solutions
of geminal alkylammonium surfactants on the base
hydrolysis of carboxylic acid esters is weak, like that
of cylindrical micelles of cetyltrimethylammonium
bromide. The strongest acceleration of the process is
observed at low concentrations of the detergent in
solution, in the range of existence of spheroidal aggre-
gates, which makes systems based on geminal surfact-
ants attractive for efficient cleavage of ester bonds.
tivity 1.8 S cm ) was determined with a CDM-2d
device (Denmark).
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