Solubilization and catalytic behavior of micellar system based on gemini surfactant with hydroxyalkylated head group

Article abstract of DOI:10.1016/j.molliq.2012.02.012

The correlation between aggregation, solubilization and catalytic properties has been found for series of cationic surfactants with hexadecyl radical of both monomeric and dimeric structures. The highest catalytic effect in the series, reaching three orde

Full text of DOI:10.1016/j.molliq.2012.02.012

Journal of Molecular Liquids 169 (2012) 106–109
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Solubilization and catalytic behavior of micellar system based on gemini surfactant
with hydroxyalkylated head group
Alla B. Mirgorodskaya, Ekaterina I. Yackevich, Svetlana S. Lukashenko,
Lucia Ya. Zakharova ⁎, Alexander I. Konovalov
A.E.Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of the Russian Academy of Sciences, Arbuzova 8, 420088 Kazan, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The correlation between aggregation, solubilization and catalytic properties has been found for series of cat-
ionic surfactants with hexadecyl radical of both monomeric and dimeric structures. The highest catalytic ef-
fect in the series, reaching three orders of magnitude is observed for a gemini surfactant with oxyethylated
head group in the case of hydrolysis of p-nitrophenyl caprate.
Received 27 November 2011
Received in revised form 20 February 2012
Accepted 21 February 2012
Available online 6 March 2012
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Gemini surfactant
Aggregation
Esters
Solubilization
Hydrolysis
Catalysis
1
. Introduction
between chemical structure, aggregation behavior and solution prop-
erties of gemini surfactants being revealed make it possible to synthe-
size purposely these compounds and to develop organized systems
showing target properties.
At the present time gemini (dimeric) surfactants have attracted an
increased attention. These surfactants are made up of two hydropho-
bic chains and two polar headgroups covalently linked through a
spacer group. These kinds of surfactants have a number of unique ag-
gregation properties in comparison with conventional single-chain
surfactants, such as much lower critical micelle concentration
In this paper aggregation behavior, solubilization capacity and cat-
alytic activity of gemini surfactant with hydroxyethylated head
group, i.e. hexanediyl-α,ω-bis[((2-hydroxyethyl)methylhexadecy-
lammonium)] bromide (Gem 6-16(OH)) toward the cleavage of
ester bonds have been investigated. Surfactants bearing hydroxyalkyl
fragments in their heads demonstrate strong ability to form specific
interactions in their solutions, i.e. hydrogen bonds along with electro-
static interactions and hydrophobic effect. This is assumed to be
reflected in their behavior. The results obtained are compared with
data for non-functional gemini surfactant hexanediyl-α,ω-bis(hexa-
decyldimethylammonium) bromide (Gem 6–16), as well as with sin-
gle head analogous, i.e. cetyltrimethyl ammonium bromide (CTAB)
and N-cetyl-N-(2-hydroxyethyl)-N,N-dimethylammonium bromide
(CHAB). p-Nitrophenyl esters of carboxylic acids were used as a
probe for estimation of solubilization capacity and catalytic activity
of micellar systems (See Scheme 1).
(
cmc), strong dependence on spacer structure, high surface and wet-
ting activity, special aggregate morphology, and so on. Gemini surfac-
tants are very attractive for catalysis and adsorption application,
analytical separation, solubilizing processes, nanoscale technology,
biotechnology, enhanced oil recovery and as synthetic vectors for
gene transfection. A numbers of papers concerning the aggregation
behavior and structural properties of gemini surfactants have been
published [1–8]. The catalytic effect of micellar surfactant solution
in nucleophilic substitution reactions have been reported [9–16].
In the case of cationic gemini surfactants the majority of studies
focuses on compounds with methyl substitutes in ammonium head
groups. It is however obvious that introducing other functional
groups can result in novel properties, therefore gemini surfactants
with alkylammonium head groups bearing polar fragments including
hydroxyethyl ones have been designed [17–19]. Correlations
2. Experimental
2.1. Chemicals
CTAB and the esters (acetate, caprilate, caprate, laurate) were from
Sigma-Aldrich (basic substance contents of higher than 99%). CHAB
⁎
Corresponding author.
E-mail address: lucia@iopc.ru (L.Y. Zakharova).
and Gem 6–16 were synthesized through reaction of cetylbromide
0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2012.02.012

A.B. Mirgorodskaya et al. / Journal of Molecular Liquids 169 (2012) 106–109
107
3 7 9
Scheme 1. R=CH (acetate), n-C H15 (caprilate), n-C H19 (caprate), n-C11H23 (laurate).
with 2-dimethylaminoethanole or N,N′-tetramethyl hexamethylene-
diamine, in analogy with ref. [15,20]. Gem 6-16(OH) was synthesized
by quaternization of hydroxyethylmethylcetylamine by hexamethylene
dibromide, in analogy with ref. [18].
2.4. Kinetic study
The reaction was controlled by monitoring the p-nitrophenolate-
anion absorption at 400 nm. A "Specord M-400" spectrophotometer
with temperature-controlled cell holders was employed. All runs
were performed at the substrate concentration of 5·10− mol·L
5
−1
.
2
.2. Micellization study
Required pH values were reached applying sodium tetraborate buffer
solutions.
Surface tension measurements were performed using the du Nouy
The observed rate constants (kobs) were determined from the
ring detachment methods using tensiometer K6 (Kruss). The experi-
mental details are described elsewhere [21].
∞ ∞
equation: ln (A −A)=−kobs t+const, where A and A are the ab-
sorbance of the micellar solutions at point t during and after comple-
tion of the reaction, respectively. The kobs values were calculated
using the weighed least-squares computing methods. Each value of
Dynamic light scattering (DLS) measurements were performed by
means of the PhotoCor Complex and Malvern Instruments’ Zetasizer
Nano. The measured autocorrelation functions were analyzed by
Malvern DTS software, the DynaLS program and the second-order
cumulant expansion methods. The diffusion coefficient was mea-
sured at least three times for each sample. The average error in
these experiments was approximately 4%. The solutions were filtered
with Millipore filters, to remove dust particles from the scattering vol-
ume. The experimental details are described elsewhere [22].
kobs is the average of at least three independent determinations dif-
fering by no more than 4%.
3. Results and discussion
3.1. Determination of the micellization characteristics
Surface tension study extracted the cmc value of Gem 6-16(OH)
−
6
equal to 9·10
mer surfactant CHAB (7·10
non-functional gemini analogue (4.0·10
M, which is ca. 80-fold lower than that of the mono-
−
4
2
.3. Solubilization study
M) and ca. 5-fold lower than value for
−
5
M) (Fig. 1). On the one
Solubilization properties of organized systems toward carboxylic
hand, the micellization of Gem 6-16(OH) is probably promoted by
the covalent fixation of two positively charged centers and on the
other, by the formation of hydrogen bonds between head groups.
By method of dynamic light scattering the size of the micelles
acid esters were determined according known procedure [23],
which involved the preparation of saturated solution of the esters fol-
lowed by their hydrolysis and spectrophotometry monitoring of
p-nitrophenol released. For this purpose, measured portion of freshly
purified ester was agitated by a stirrer for 6 h, whereupon undis-
solved residue was separated. Then an appointed volume of concen-
trated (1 M) NaOH solution was added to the aliquot taken from the
saturated ester solution, which resulted in the hydrolysis of the sub-
strate and formation of p-nitrophenol. Completion of the hydrolysis
was controlled by spectrophotometry, by means of reaching the pla-
teau value of the absorbency (D) at 400 nm corresponding to p-
nitrophenolate-anion. Concentration of p-nitrophenol coinciding
with concentration of the initial ester was calculated as follows:
h
was estimated. Hydrodynamic radius (R ) of gemini surfactants
are shown to equal 3.4 и 3.2 nm for Gem 6–16 and Gem 6-16(OH)
respectively at concentration of 0.01 M under neutral solution pH.
The higher values of hydrodynamic radius for the gemini surfactants
as compared to monomeric analogous at the same concentration
(2.2–2.5 nm for CTAB) are probably due to the asymmetry of aggre-
gates based on dimeric surfactants. It is noteworthy that under neu-
tral pH Gem 6–16 and Gem 6-16(OH) form micelles similar in size,
while in alkali solutions, hydrodynamic radiuses of their aggregates
considerably differ. Value of R
lution reaches 4.9 nm, while R
This may be due to the fact that hydroxyalkylated surfactants show
h
for Gem 6-16(OH) in 0.01 M alkali so-
C
PhO-=D/(ε×L), here L is a cell length, and ε is an molar extinction
h
for Gem 6–16 remains unchanged.
−
1
−1
coefficient of 18,000 L mol
cm
.

108
A.B. Mirgorodskaya et al. / Journal of Molecular Liquids 169 (2012) 106–109
As can be seen, an increase in the solubilizaion capacity occurs in
7
6
6
6
5
5
4
4
4
2
8
4
0
6
2
8
4
0
the series CTABbCHABbGem 6–16bGem 6-16(OH). Probably the latter
member of the series combines high micellization and solubilization
properties of gemini surfactants [25,26] with the contribution of hydro-
gen bonds between the substrate and head groups of surfactant. In
all the systems studied an increase in solubility of esters correlates
with their hydrophobicity, reaching a maximum in the case of
p-nitrophenylcaprate. The structure of this substrate is probably
most suitable for the embedding into the micelles. The change in the
substrate solubility and in their microenvironment in surfactant solu-
tions can result in changes in ester reactivity, in particular in hydrolytic
stability.
Figs. 3 and 4 show dependence of observed rate constants (kobs) of
alkaline hydrolysis of p-nitrophenyl esters of different structures on
the surfactant concentration. The reaction was studied at pH 9.2
under pseudo first order conditions, by monitoring the absorbance of
solutions at a wavelength of 400 nm (formation of p-nitrophenoxide-
anion). Catalytic action of cationic surfactants on hydrolysis of esters
is mainly connected with the concentration of hydroxide ion near the
positively charged micelles, where the substrate is solubilized. From
the kinetic data evident, that the maximum effect of gemini surfactants
is achieved at the concentrations lower than 0.001 M, i.e. substantially
lower as compared to their monomer analogues. Surfactants with
hydroxyalkylated head groups demonstrate higher (by order of magni-
tude) catalytic effect than their non-functional analogues; kobs values
for Gem 6-16(OH) is by 2–5 times higher than those for CHAB. The ki-
netic curves obtained were analyzed in terms of the pseudophase ap-
proach using the Eq. (1) [27]
1
2
3
4
1E-6
1E-5
1E-4
1E-3
0,01
Fig. 1. Surface tension isotherms of cationic surfactants; 1 — Gem 6–16 OH; 2 — Gem
–16; 3 — CHAB; 4 — CTAB, 25 °C.
6
properties of weak acids and under solution pH>12 may exist in a
betaine form [24]. In this case repulsion between head groups be-
comes lower, which allows to form micelles with higher numbers
of aggregation and larger size. Our further investigation were
carried out in neutral or slightly alkaline environments (pH 9.2),
when hydroxyalkylated surfactants demonstrate behavior typical for
cationic surfactants.
k0 þ kmKSC
kobs
¼
;
ð1Þ
1 þ KSC
0 m
и k
(s− ) are the first order rate constants in aqueous and
1
where k
micellar phase respectively, K
−
1
S
(M ) is the binding constant of sub-
strate; C is the total concentration of surfactant minus cmc. Calculated
parameters are summarized in Table 1.
3
.2. Solubilization and catalytic behavior
Micellar solutions of the above cationic surfactants are used as re-
action media for the alkaline hydrolysis of p-nitrophenyl esters of car-
boxylic acids with different lengths of hydrocarbon radical. The first
task was to compare solubilization capacity of the micelles towards
the substrates. The maximum concentration of p-nitrophenyl esters
in the surfactant solutions was determined by using the methodology
including the preparation of a saturated solution of this compound,
followed by its alkaline hydrolysis and spectroscopic monitoring of
p-nitrophenol liberation [23]. Data obtained reflect a change in the
substrate solubility, when transit from water to surfactant solution
0
0
0
,06
,04
,02
1
2
(
Fig. 2).
1
1
1
1
6
4
2
0
8
6
4
2
0
3
4
Acetate
Caprilate
Caprate
Laurate
1
4
2
0,00
,000
3
0
0,001
0,002
0,003
0,004
0,005
/
surf
CTAB
CHAB
6-16 Me 6-16OH
Fig. 3. The dependence of observed rate constant of hydrolysis of p-nitrophenyl esters of
carboxylic acids in micellar solution of gemini surfactants on their concentration (1 — caprate,
2 — caprilate, 3 — acetate, 4 — laurate); full symbol for Gem 6–16 OH; empty symbols for Gem
6–16; pH 9.2, 25 °C.
Fig. 2. Relative solubility of p-nitrophenyl esters of carboxylic acid when transit from
water to micellar solution of cationic surfactants.

A.B. Mirgorodskaya et al. / Journal of Molecular Liquids 169 (2012) 106–109
109
thus favors the reaction, and stabilizes the transition state due to delo-
calization of electron density through hydrogen bonds. It is noteworthy
that for all the systems studied substrate specificity is observed, i.e. the
correlation between the rate of hydrolysis of substrates and their solu-
bility occurs. The highest kobs values are observed for p-nitrophenyl
caprate in the Gem 6-16(OH), for which the acceleration reaches
three orders of magnitude. It can be assumed that the localization of
this substrate is suitable for the nucleophilic attack.
1
4
. Conclusion
Thus, in micellar solution of gemini surfactant with hydroxyethy-
2
lated head group specific interactions are probable contributors along
with electrostatic interactions and hydrophobic effect. The hydrogen
bonds favor micellization in the system and the solubilization of re-
agents. This in turn results in the activation of the substrates and en-
hancement of the catalytic effect, occurring within extremely low
concentration range, which provides perspectives for the use of
these micellar systems for reaction media.
3
4
1
3
Acknowledgements
/
surf
Financial support from State Contract No. 02.740.11.0802 is great-
ly appreciated.
Fig. 4. The dependence of observed rate constant of hydrolysis of p-nitrophenyl esters of
carboxylic acids in micellar solution of cationic surfactants on their concentration (1 — cap-
rate, 2 — caprilate, 3 — acetate, 4 — laurate); full symbol for CTAB; empty symbols for CHAB;
pH 9.2, 25 °C.
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a
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/s−1
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c
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29
66
112
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Acetate
Caprate
Acetate
Caprylate
Caprate
Laurate
Acetate
Caprylate
Caprate
Laurate
Acetate
Caprylate
Caprate
Laurate
0.0019
0.00235
0.033
880
4440
230
−
−
−
−
−
−
−
−
−
−
−
−
−
4
4
4
4
4
5
5
5
5
5
5
5
5
1.86 10
2.40 10
1.98 10
3.25 10
1.97 10
2.05 10
2.43 10
2.17 10
2.63 10
2.36 10
2.48 10
5.61 10
1.03 10
CHAB
0.0225
0.0376
0.0168
0.0023
0.0053
0.0091
0.00051
0.0279
0.049
786
1750
1200
1460
2000
30900
3320
780
11000
6800
4300
Gem 6-16
Gem 6-16(OH)
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(
a
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c
−1
(acetate), 0.0002 s−1 (caprylate), 0.00008 s−1
k
0
values are equal to 0.0005 s
−
1
(
caprate), 0.00005 c
(laurate).