Influence of the strong electrolyte on the aggregation behavior and catalytic properties of dicationic surfactants

Article abstract of DOI:10.1007/s11172-011-0398-y

The introduction of potassium chloride into an aqueous solution of dicationic (gemini) surfactants results in the partial neutralization of the surface charge of micelles, which is accompanied by an increase in their size, a decrease in the critical conce

Full text of DOI:10.1007/s11172-011-0398-y

Russian Chemical Bulletin, International Edition, Vol. 60, No. 12, pp. 2597—2601, December, 2011
2597
Influence of the strong electrolyte on the aggregation behavior
and catalytic properties of dicationic surfactants
E. I. Yackevich, A. B. Mirgorodskaya, L. Ya. Zakharova, and A. I. Konovalov
A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences,
8
ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 273 2253. Eꢀmail: mirgorod@iopc.ru
The introduction of potassium chloride into an aqueous solution of dicationic (gemini)
surfactants results in the partial neutralization of the surface charge of micelles, which is
accompanied by an increase in their size, a decrease in the critical concentration of micelle
formation, a decrease in the surface potential of the system, and a change in the micellar
influence on the hydrolytic stability of solubilized carboxylic esters.
Key words: dicationic surfactants, electrolyte, aggregation behavior, surface potential, micelꢀ
lar catalytic effect.
Dicationic (gemini) surfactants contain two hydroꢀ
phobic radicals and two head groups bound by spacers
with different degrees of rigidity. These compounds differ
from amphiphilic analogs containing one head group and
one hydrocarbon chain by lower (by an order of magniꢀ
tude) values of critical micelle concentration (CMC) and
high surface activity and wetting effect. They are characꢀ
terized by the unusual morphological behavior, high soluꢀ
bilizing effect, and a possibility to considerably decrease
the range of used concentrations.1 The promising use of
gemini surfactants as a medium for chemical reactions
the CMC and increases the aggregation numbers. At cerꢀ
tain concentrations, the electrolyte can induce micellar
8
,18
transitions "sphere"—"cylinder."
All these factors inꢀ
duce a change in the micellar influence on the physicoꢀ
chemical properties and reactivity of solubilized subꢀ
1
7,19
stances.
In the present work, the properties of hexamethyleneꢀ
1,6ꢀbis(dimethylcetylammonium) bromide (Gem 6ꢀ16) in
the presence of additive of the strong electrolyte (potassiꢀ
um chloride) were studied in order to reveal the possibility
of directed control of the rate of chemical processes in
solutions of dicationic surfactants. The influence of KCl
on the aggregation behavior of this surfactant was considꢀ
ered. The micellar catalytic effect of solutions of Gem 6ꢀ16
was estimated for the alkaline hydrolysis of carboxylic
esters: pꢀnitrophenyl acetate (PNPA) and pꢀnitrophenyl
caprinate (PNPC).
—7
6—11
was shown.
of gemini surfactants1
plexes with DNA and play a role of vector in genetic engiꢀ
There are data on high bactericidal activity
2,13
and their ability to form comꢀ
neering.1
4,15
Many publications devoted to the structural
properties of systems based on gemini surfactants have
appeared in the recent years; however, the number of works
on studying their influence on the rate of chemical proꢀ
cesses and establishing the factors determining their cataꢀ
lytic effect is substantially smaller.
Experimental
It was shown for classical monocationic surfactants
that the modification of their micellar solutions by addiꢀ
tives of the background electrolyte substantially affects
their aggregation behavior. This is related to the fact that
an increase in the concentration of counterions of the
surfactant attained by the introduction of an electrolyte
results in the partial neutralization of the surface charge of
Samples of gemini surfactant Gem 6ꢀ16 were synthesized by
the reaction of N,N,N´,N´ꢀtetramethylꢀ1,6ꢀhexamethylenediꢀ
amine with cetyl bromide in acetone followed by double recrysꢀ
tallization from EtOH using a known procedure. The strucꢀ
tures of the synthesized compounds were confirmed by elemenꢀ
tal analysis and IR and NMR spectroscopy data. Commercially
available TritonꢀХꢀ100 (Sigma) and carboxylic esters (Fluka)
containing 99% of the major substance were used.
The surface properties were studied by the ring detachment
method on a Du Nouy tensiometer (Kruss). The data on the
aggregate sizes were obtained by dynamic light scattering on
a Malvern Instruments´ Zetasizer Nano spectrometer (HeꢀNe
laser, 633 nm). Prior to measurements, the studied solutions
were filtered using the Millipore filters with the pore diameter
2
0
micelles. The latter, in turn, decreases the destabilizing
repulsion of likely charged head groups.1
6,17
This decreasꢀ
es the absolute value of surface potential of the system and
*
Dedicated to Academician of the Russian Academy of Sciences
R. Z. Sagdeev on the occasion of his 70th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2546—2550, December, 2011.
066ꢀ5285/11/6012ꢀ2597 © 2011 Springer Science+Business Media, Inc.
1

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Russ.Chem.Bull., Int.Ed., Vol. 60, No. 12, December, 2011
Yackevich et al.
0
.4 m. The measurements were carried out at least three times
for each sample.
The acid—base properties of pꢀnitrophenol were studied by
measuring the absorbance of its ionized form at different pH
values. The spectra were recorded on a Specord UVꢀVIS specꢀ
trophotometer in quartz cells with an absorbing layer thickness
I (%)
40
(
L) of 1 cm in the range from 250 to 600 nm. The molar absorpꢀ
3
0
tion coefficient () of the phenoxide form was determined from
the absorbance (D) measured at the wavelength corresponding
to the absorption maximum at pH > 10. The concentration of
2
0
the pꢀnitrophenoxide ion (C
mined from the equation CPhO
phenol (рКа,app) was calculated by the Henderson—Hasselbach
equation21
PhO–
) at a specified pH was deterꢀ
10
0
–
= D/L, and рК of pꢀnitroꢀ
а
10
4
8
3
6
2
,
(1)
4
1
2
where CPhOH is the concentration of pꢀnitrophenol.
dh/nm
The averaged values of рКа,app determined from three to five
independent experiments at different pH were used.
Fig. 1. Size of micelles Gem 6ꢀ16 in an aqueous solution in the
absence (1) and in the presence of potassium chloride in the
The kinetics of alkaline hydrolysis of pꢀnitrophenyl carboxylꢀ
ates was studied in 1 mM NaOH by spectrophotometry on
a Specord UVꢀVIS instrument in temperatureꢀcontrolled cells.
The occurrence of the process was monitored by a change in the
absorbance of the solutions at the wavelength 400 nm (formation
of the pꢀnitrophenoxide anion). The initial concentration of the
–
1
concentration 0.001 (2), 0.01 (3), and 0.1 mol L (4) (СSurf
=
=
0.005 mol L– , 25 С).
1
added electrolyte, resulting in an increase in the aggregaꢀ
tion numbers and, hence, the micelle size.
substrate was (2—8)•10– mol L , and the conversion was higher
5
–1
than 90%. The apparent rate constants of the pseudoꢀfirst order
To determine the values of surface potential of Gem 6ꢀ16
in the presence of the electrolyte, we used the accepted
approach to imply the study of the spectral properties of
probing molecules capable of incorporating into the interꢀ
(
kapp) were determined from the dependence
log(D — D ) = –0.434kapp + const,


2
3
where D and D are the absorbances of solutions at the moment 
facial layer and respond to the changes in its structure.


and after the completion of the reaction. The values of kapp were
calculated by the leastꢀsquares method.
pꢀNitrophenol, whose рК values were determined by specꢀ
а
trophotometry at the absorption band maximum of its
–
1
–1
anionic form ( = 400 nm,   18000 L mol cm ), was
Results and Discussion
chosen as such a probe. The apparent value of рК (рКа,app)
а
was calculated using Eq. (1) by the determination of
pꢀnitrophenol absorption at different pH. The changes in
рКа,app of pꢀnitrophenol at different concentrations of the
studied surfactants are shown in Fig. 2.
According to Ref. 23, the surface potential () was
estimated by Eq. (2)
The values of CMC in solutions of Gem 6ꢀ16 were
determined by the tensiometric method. It was shown that
an additive of potassium chloride facilitated the process of
micelle formation: at the concentrations С
equal to 0,
КСl
–
1
–5
0
2
.001, and 0.01 mol L the CMC value is 4.5•10 ,
–
5
–5
–1
.8•10 , and 1.7•10 mol L , respectively. The miꢀ
celle size increased, as it was detected by the dynamic
light scattering method. The increase in the hydrodynamꢀ
,
(2)
ic diameter (d ) caused by the addition of potassium chlorꢀ
where рKa,m is the negative logarithm of the dissociation
constant in the micellar phase (Ка,app = Ka,m at СSurf  );
pKa,n is the nonꢀelectrostatic component determined as
h
ide to a solution of Gem 6ꢀ16 with the concentration
0
.005 mol L–1 is illustrated in Fig. 1. As the electrolyte
–
1
concentration increases from 0 to 0.1 mol L , the value
рК in micellar solutions based on nonꢀionic surfactants
а
of d varies from 2 to 7 nm. The result obtained agrees
(in a solution of TritonꢀХꢀ100 рК of pꢀnitrophenol is
h
а
2
2
–1
with the data of the work in which the change in the
micellar size in an aqueous solution of Gem 6ꢀ16 (C
7.6); F = 96486 C mol is Faraday´s constant, and
–
1
–1
=
R = 8.314 J K mol is the gas constant.
Surf
=
0.03 mol L–1) under the action of potassium bromide
It follows from the data in Table 1 that the influence of
pꢀnitrophenol on рКа,m decreases with an increase in the
concentration of potassium chloride, which is explained
by a decrease in the surface potential. A similar decrease
in  upon the introduction of the strong electrolyte has
additives was observed by the dynamic light scattering
method. The aggregates become larger due to a decrease
in the repulsion of the head groups upon the neutralizaꢀ
tion of the surface charge of the micelles by anions of the

Dicationic surfactants
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 12, December, 2011
2599
pKa,app (arb. units)
Table 1. Influence of the electrolyte (KCl) on
рКа,m of pꢀnitrophenol and on the surface poꢀ
tential () micelles Gem 6ꢀ16
7
7
6
6
5
5
.5
.0
.5
.0
.5
.0
5
СKCl/mol L–1
рКа,m
/mV
0
5.00
5.45
6.15
6.40
154
127
091
071
0
0
0
.001
.01
.1
4
3
concentrating of hydroxide ions at the positively charged
interface, which increases the probability of the reaction
contact of the hydrophilic nucleophile with the substrate
solubilized by the micelle. A decrease in the surface potenꢀ
tial caused by the introduction of additives of the strong
electrolyte should decrease the concentrating effect and,
hence, the catalytic action of micelles. Another factor afꢀ
fecting the catalytic effect in the system is binding of the
substrate by the micelle, which determines the distribuꢀ
tion of the substrate between the voluminous medium and
dispersion phase. The results of kinetic experiments illusꢀ
trating the changes in the apparent rate constant of alkaꢀ
line hydrolysis of PNPA and PNPC at different concenꢀ
trations of Gem 6ꢀ16 are shown in Fig. 3. The data obꢀ
tained indicate a decrease in the catalytic effect of the
system upon the introduction of potassium chloride,
and PNPC is more sensitive to the electrolyte additive
than PNPA.
2
1
5
10
CSurf•10–4/mol L–1
Fig. 2. Dependence of рКа,app of pꢀnitrophenol on the concenꢀ
tration of the surfactants Gem 6ꢀ16 (1—4) and TritonꢀХꢀ100 (5)
in the absence (1 and 5) and in the presence of КСl in the
concentration 0.001 (2), 0.01 (3), and 0.1 mol L– (4).
1
The obtained kinetic data were analyzed in terms of
the pseudoꢀphase model of micellar catalysis using
Eq. (3)
been observed previously24 for cetyltrimethylammonium
bromide (CTAB); however, the changes in the case of this
monomeric surfactant are substantially lower than that of
the dimeric analog (for CTAB, the values of  are 133 and
25
,
(3)
–
1
1
20 mV at the KCl concentration 0 and 0.01 mol L
,
respectively).
where k and k (s–1) are the firstꢀorder rate constants in
0
m
Micellar solutions of Gem 6ꢀ16 were used as a reaction
medium for the alkaline hydrolysis of carboxylic esters,
pꢀnitrophenyl acetate (PNPA) and pꢀnitrophenyl capriꢀ
nate (PNPC) (Scheme 1).
an aqueous medium and in the micellar phase, respectiveꢀ
–1
ly; K (L mol ) is the binding constant of the substrate;
S
and CSurf is the total concentration of the surfactant minus
the CMC.
The quantitative characteristics determined by this
equation are given in Table 2. The obtained results reꢀ
vealed the decrease in the substrate binding by micelles of
Gem 6ꢀ16 upon the addition of potassium chloride to the
solution. The calculated values of the alkaline hydrolysis
rate constants in the micellar phase make it possible to
estimate the catalytic effect characterized by the ratio
k /k , which reaches one order of magnitude for PNPA
Scheme 1
m
0
PNPA, PNPC
and 530 for PNPC in the absence of electrolyte additives.
In the case of PNPA, the electrolyte additive insignifiꢀ
cantly decreases the catalytic effect. In the case of PNPC,
the electrolyte additive results in a sharp (by an order of
magnitude and more) decrease in the value of hydrolysis
acceleration. This distinction can be explained by differꢀ
ent hydrophobicities of the substrates and, hence, their
R = Me (PNPA), nꢀС Н (PMPC)
9
19
The catalytic effect expected for this process in soluꢀ
tions of cationic surfactants is explained, first of all, by

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Russ.Chem.Bull., Int.Ed., Vol. 60, No. 12, December, 2011
Yackevich et al.
kapp•10–3/s–1
Table 2. Influence of KCl on the parameters of the alkaline
hydrolysis of PNPA and PNPC in micellar solutions of Gem 6ꢀ16
a
a
1
2
7
6
5
4
3
2
1
СKCl/mol L–1
b
k
KS
/L mol
CMC
/mol L
k /k
m
–1
m
0
–1
–1
/
s
PNPA
3
4
0
0.0079
0.0072
0.0064
0.0050
3010
2430
1650
1420
0.00010
0.00011
0.00012
0.00068
9
8
7
0
0
.001
.01
0.1
5.5
PNPC
0
0.0530
0.0110
0.0044
7290
3490
2510
0.000069
0.000042
0.000019
530
110
44
0
0
.001
.01
a
b
Hydrolysis conditions: 1 mM NaOH, 25 C.
–3
The values of k for PNPA and PNPC are 0.9•10 and
0
–
3 –1
0
.1•10
s , respectively.
0
1
2
CSurf•10–3/mol L–1
indicates that this substrate is localized in the micelle.
According to the pseudoꢀphase model of micellar catalyꢀ
sis, the apparent rate constant is an additive value and
describes the occurrence of the process in the both phases.
A decrease in the surface potential upon the introduction
of the electrolyte diminishes the ability of the micelle to
concentrate the nucleophile, which decreases the rate conꢀ
stant in the micellar phase. Therefore, the introduction of
the electrolyte should strongly affect the hydrolysis rate of
the hydrophobic substrate. For PNPA, whose portion
reacts in the voluminous aqueous phase, the decrease in
the surface potential upon the introduction of KCl is not
so substantial and the micellar catalytic effect decreases
insignificantly.
Thus, the influence of the strong electrolyte on the
aggregation behavior and catalytic properties of the diꢀ
cationic surfactants was examined using Gem 6ꢀ16 as
an example. The data obtained show that Gem 6ꢀ16 exerts
a high catalytic effect on the cleavage of carboxylic esters,
which is displayed at lower concentrations than that
for monomeric analogs. The introduction of a strong elecꢀ
trolyte into a solution of Gem 6ꢀ16 substantially decreasꢀ
es the surface potential and decreases the micellar cataꢀ
lytic effect. On the one hand, understanding of the
role of the electrolyte will allow one to control the rate
of chemical processes in solutions of the dicationic surfꢀ
actants. On the other hand, the possibility of the micelꢀ
lar catalytic effect in media with a certain amount of
salt additives containing the high concentration of buffer
components or background electrolytes can really be esꢀ
timated.
kapp/s–1
b
0
0
0
0
0
.05
.04
.03
.02
.01
0
1
2
3
4
0
1
2
CSurf•10–3/mol L–1
Fig. 3. Apparent rate constant of alkaline hydrolysis (kapp) of
PNPA (a) and PNPC (b) vs concentration of Gem 6ꢀ16 in the
absence (1) and in the presence of KCl in the concentration
.001 (2), 0.01 (3), and 0.1 mol L–1 (4) (NaOH, 25 C).
0
different localization in a micellar solution. More hydroꢀ
philic PNPA is characterized by a lower binding constant,
which results in its distribution between the voluminous
This work was financially supported by the Rusꢀ
sian Foundation for Basic Research (Project No. 09ꢀ03ꢀ
00778ꢀa).
phase and micelle, whereas the value of K for PNPC
S

Dicationic surfactants
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 12, December, 2011
2601
References
. F. M. Menger, J. S. Keiper, Angew. Chem., 2000, 112, 1980.
. S. K. Hait, S. P. Moulik, Curr. Sci., 2002, 82, 1101.
. C. Groth, M. Nyden, R. Holmberg, J. R. Kanicky, D. O.
Shah, J. Surfactants Deterg., 2004, 7, 247.
. R. Zana, Adv. Colloid Interface Sci., 2002, 97, 205.
. R. Jiang, J. Zhao, X. Hu, X. Pei, L. Zhang, J. Colloid Interꢀ
face Sci., 2009, 340, 98.
. A. B. Mirgorodskaya, L. A. Kudryavtseva, N. N. Vylegzhanꢀ
ina, B. Z. Idiyatullin, Yu. F. Zuev, Izv. Akad. Nauk, Ser.
Khim., 2010, 768 [Russ. Chem. Bull., Int. Ed., 2010, 59, 790].
. A. Rodríguez, M. M. Graciani, K. Bittermann, A. T. Carꢀ
mona, M. L. Moyá, J. Colloid Interface Sci., 2007, 313, 542.
. W. Jiang, B. Xu, Q. Lin, J. Li, F. Liu, X. Zeng, H. Chen,
Colloids Surf., A., 2007, 315, 103.
17. L. Ya. Zakharova, D. B. Kudryavtsev, L. A. Kudryavtseva,
Yu. F. Zuev, N. L. Zakharchenko, N. N. Vylegzhanina,
Z. Sh. Idiyatullin, V. D. Fedotov, Zh. Obshch. Khim., 2002,
1
2
3
7
2, 458 [Russ. J. Gen. Chem. (Engl. Transl.), 2002, 72, 426].
1
8. A. I. Serdyuk, R. V. Kucher, Mitsellyarnye perekhody v rastꢀ
vorakh poverkhnostnoꢀaktivnykh veshchestv [Micellar Transiꢀ
tions in Solutions of Surfactants], Naukova Dumka, Kiev,
4
5
1
987, 208 pp. (in Russian).
1
9. L. Ya. Zakharova, D. B. Kudryavtsev, F. G. Valeeva, L. A.
Kudryavtseva, Zh. Obshch. Khim., 2002, 72, 1296 [Russ. J.
Gen. Chem. (Engl. Transl.), 2002, 72, 1215].
6
2
2
0. B. S. Sekhon, Reson. J. Sci. Educ., 2004, 9, No. 3, 42.
1. A. Albert, E. P. Sargeant, Ionization Constants of Acids and
Bases. A Laboratory Manual, Methuen Press, London, 1962.
2. U. S. Siddiqui, G. Ghosh, KabirꢀudꢀDin, Langmuir, 2006,
7
8
9
2
2
2
2, 9874.
3. N. O. MchedlovꢀPetrosyan, Differentsirovanie sily organꢀ
icheskikh kislot v istinnykh i organizovannykh rastvorakh [Difꢀ
ferentiation of the Strength of Organic Acids in True and Orgaꢀ
nized Solutions], Izdꢀvo Khar´k. Nats. unꢀta im. V. N. Karaziꢀ
na, Kharkov, 2004, 326 pp. (in Russian).
. S. Bhattacharya, V. P. Kumar, Langmuir, 2005, 21, 71.
0. L.ꢀG. Qiu, X. Jiang, L.ꢀN. Gu, G. Hu, J. Mol. Catal. A:
Chem., 2007, 277, 15.
1
1
1. A. B. Mirgorodskaya, L. A. Kudryavtseva, Zh. Obshch. Khim.,
2
7
009, 79, 44 [Russ. J. Gen. Chem. (Engl. Transl.), 2009,
9, 42].
2
2
4. A. B. Mirgorodskaya, L. A. Kudryavtseva, V. A. Pankratov,
S. S. Lukashenko, L. Z. Rizvanova, A. I. Konovalov,
Zh. Obshch. Khim., 2006, 76, 1696 [Russ. J. Gen. Chem.
1
1
1
1
1
2. F. Devinsky, L. Masarova, I. Lacko, D. Mlynarchik, J. Bioꢀ
pharm. Sci., 1991, 2, 1.
3. A. Laatiris, M. E. Achouri, M. R. Iufante, Y. Bensouda,
Microbiol. Res., 2008, 163, 645.
4. N. Jiang, P. Li, Y. Wang, J. Wang, H. Yan, R. K. Thomas,
J. Phys. Chem. B, 2004, 108, 15385.
(
Engl. Transl.), 2006, 76, 1625].
5. I. V. Berezin, K. Martinek, A. K. Yatsimirskii, Usp. Khim.,
1
4
973, 42, 1729 [Russ. Chem. Rev. (Engl. Transl.), 1973,
2, 787].
5. Sh. D. Wettig, I. Badea, M. Donkuru, R. E. Verrall, M. Foldꢀ
vari, J. Gene Med., 2007, 9, 649.
6. K. Shinoda, B. Tamamushi, T. Nakagava, T. Isemura, in
Colloidal Surfactants. Some Physicochemical Properties, Acad.
Press, New York—London, 1963, 310 pp.
Received May 19, 2011