Metallomicelle catalysis: Hydrolysis of p-nitrophenyl picolinate induced by schiff base Co(II) complexes in a gemini surfactant micellar solution

Article abstract of DOI:10.1002/kin.20284

Two Schiff base cobalt(II) complexes containing crowned substituents have been synthesized and employed to promote the hydrolysis of p-nitrophenyl picolinate (PNPP) in a buffered micellar solution formed by a cationic Gemini surfactant, bis(hexadecyldimet

Full text of DOI:10.1002/kin.20284

Metallomicelle Catalysis:
Hydrolysis of p-Nitrophenyl
Picolinate Induced by Schiff
Base Co(II) Complexes in a
Gemini Surfactant Micellar
Solution
WEIDONG JIANG,1−3 BIN XU,3 JIANZHANG LI,3 QI LIN,1,2 XIANCHENG ZENG,1 HUA CHEN1,2
1Key Laboratory of Green Chemistry and Technology (Sichuan University), Ministry of Education,
Sichuan Chengdu 610064, People’s Republic of China
2Organometal Homogeneous Catalytic Institute (Sichuan University), Sichuan Chengdu 610064, People’s Republic of China
3Department of Chemistry, Key Laboratory of Green Chemistry and Technology, Sichuan University of Science &
Engineering, Sichuan Zigong 643000, People’s Republic of China
Received 24 February 2007; revised 22 July 2007; accepted 23 July 2007
DOI 10.1002/kin.20284
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Two Schiff base cobalt(II) complexes containing crowned substituents have
been synthesized and employed to promote the hydrolysis of p-nitrophenyl picoli-
nate (PNPP) in a buffered micellar solution formed by a cationic Gemini surfactant,
bis(hexadecyldimethylammonium)hexane bromide (G(hex)C16, 2Br−) over a pH range of
6.50–8.50. In comparison, the reactivity of PNPP hydrolysis catalyzed by the same cata-
lysts in the other micellar system, formed by a conventional single-chain analogue, that is,
hexadecyltrimethylammonium bromide (CTAB), has also been evaluated under a selected
condition. The results clearly reveal that the two metallomicelles made of the aforemen-
tioned Co(II) complexes and the G(hex)C16 are both efficient for catalyzing PNPP hydroly-
sis with about 3 orders of magnitude in rate acceleration compared with the background
rate of PNPP spontaneous hydrolysis. Moreover, the rates of PNPP hydrolysis catalyzed by
Correspondence to: Hua Chen; e-mail: jwdxb@hotmail.com.
Contract grant sponsor: China National Natural Science Foun-
dation.
Contract grant numbers: 20072025 and 20173038.
Contract grant sponsor: Key Project of China Sichuan Province
Education Office.
Contract grant number: 2005D007.
2007 Wiley Periodicals, Inc.
c
ꢀ

METALLOMICELLE CATALYSIS: HYDROLYSIS OF p-NITROPHENYL PICOLINATE
673
the two cobalt(II) complexes in G(hex)C16 micelles are about 2 times higher than in CTAB
micelles, correspondingly. In addition, observations show that steric hindrance of substituents
of the two complexes is also one of the major influencing factors in the PNPP hydrolytic
reaction. ꢀ 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 672–680, 2007
C
INTRODUCTION
As we know, hydrolysis of carboxylic acid esters
plays an important role in chemistry and biochemistry
[1,2]. In the past two decades, many groups [3–9] have
focused on the hydrolysis of carboxylic acid esters,
promoted by a variety of catalytic systems, and these
researches provided highly important insights into the
catalytic reaction mechanism of carboxylic acid esters.
However, it is undoubtedly a big challenge that the
activity of an artificial hydrolase can approach to that
of natural enzymes. We have also devoted our attention
to study the hydrolysis of carboxylic acid esters and
found some satified catalytic systems [10–13]. In the
last 2 years, we began to explore the catalytic property
of Schiff base complexes in esters hydrolysis, and
surprisingly found that Schiff base complexes not
only accelerate the rate of some oxidation reactions
effectively [14,15] but also promote the hydrolytic
process of carboxylic acid esters and phosphate
diesteres notably [16,17].
Moreover, it is worthy of pointing out that metal-
lomicellar systems, which exhibit similar structural
and kinetic properties to natural enzymes, have been
extensively investigated as effective biomimetic
systems for the catalytic hydrolysis of p-nitrophenyl
picolinate (PNPP) [18–21]. Many insights have
been gained about the importance of a suitable
microenvironment provided by varied micelles for
the ester hydrolysis, for example, the solubilization
of substrate in micelles and electrostatic interaction.
Nevertheless, in previous studies, conventional sur-
factants frequently acted as reaction media for PNPP
hydrolysis. On the contrary, few Gemini surfactants
were applied in the hydrolysis of esters [22–29].
Gemini surfactants [30–35], which have lower crit-
ical micelle concentrations (CMC), greater propensity
of lowering the oil–water interfacial tension, better rhe-
ological behavior, and better wetting property, will dis-
place many conventional surfactants in a multitude of
scientific, industrial, and household applications [36–
37]. However, most previous studies on Gemini surfac-
tants have mainly focused on their specific aggregation
behavior and structural properties, with very limited
investigations on the effects of Gemini media on the
reaction rates [38]. As a result, a combination of the
crowned Schiff base complexes and Gemini surfactant
Figure 1 Schematic depiction of structures of Schiff base
Co(II) complexes (CoL12 and CoL22).
should provide us important information on the cat-
alytic hydrolysis of esters. To the best of our knowl-
edge, there is no description that crowned Schiff base
Co(II) complexes catalyze the hydrolysis of PNPP in
Gemini surfactant micellar solution so far.
On the basis of the above discussion, we have syn-
thesized two Co(II) complexes (Fig. 1) and elucidated
their reactivity toward PNPP hydrolysis in the pres-
ence of micelles formed by a cationic Gemini surfac-
tant G(hex)C16 (bis(hexadecyldimethylammonium)-
hexane bromide) over a pH range of 6.50–8.50. In
addition, with the goal of understanding the struc-
tural effects of Gemini surfactant G(hex)C16 on the
PNPP hydrolysis, control experiments were carried
out in other catalytic systems made up of Cobalt (II)
complexes mentioned above and CTAB (hexade-
cyltrimethylammonium bromide), a single-chained
analogue of G(hex)C16. The present observation in-
dicates that the cationic Gemini G(hex)C16 is a better
medium for PNPP hydrolysis compared with CTAB.
We believe this is a good attempt that the hydrolysis
of PNPP was catalyzed by metallomicellar systems,
which were formed by crowned Schiff base complexes
and Gemini surfactants.
EXPERIMENTAL
Instruments and Materials
The acidity of the buffers was measured at 25◦C us-
ing a radiometer PHM 26 pH meter fitted with G202C
glass and K4122 calomel electrodes. Kinetic runs were
conducted on a GBC 916 UV–vis spectrophotometer
International Journal of Chemical Kinetics DOI 10.1002/kin

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JIANG ET AL.
(GBC Co., Melbourne, Australia) equipped with a ther-
mostatic cell holder.
buffered mixture of Schiff base Mn(III) complex solu-
tion and Gemini surfactant G(hex)C16 with the desired
concentration at 25 0.1◦C. The final concentration
of the catalyst was 1.0 × 10−5 mol dm−3. The final
concentration of substrate was a series of values, that
All reagents were of analytical grade and used as re-
ceived unless otherwise noted. The buffer reagent Tris
(tris(hydroxymethyl)aminomethane) and cationic sur-
factant CTAB were the commercial products of Sigma-
Aldrich Co. (St. Louis, MO, USA). Potassium chloride
(KCl), ethanol, and methanol were purchased from the
Chengdu Kelong Chemical Co. (Sichuan Chengdu,
China). All buffers were made of standardized hy-
drochloric acid. Water used for kinetic measurements
was redistilled deionized water. Other reagents were
prepared by using the published methods: Schiff base
complexes CoL12 [39], CoL22 [15], p-nitrophenyl picol-
inate (PNPP) [40], and Gemini surfactant G(hex)C16
[41]. Ionic strength (I) was maintained at 0.1 M KCl.
The PNPP stock solution for kinetics was prepared in
distilled acetonitrile. Owing to the small solubility of
the title complexes in water, we first dissolved them
in ethanol, and then mixed the Co(II) complex ethanol
solution and buffered micellar solution, according to
a calculated dosage to obtain the buffered metallomi-
celle.
is, 2.00 × 10−4 mol dm−3, 2.67 × 10−4 mol dm−3
,
3.33 × 10−4 mol dm−3, 4.00 × 10−4 mol dm−3, and
4.67 × 10−4 mol dm−3. The CMC of Gemini surfac-
tant G(hex)C16 at 25 0.1◦C is about 4.3 × 10−5 mol
dm−3 [41], and the CMC of CTAB is 9.20 × 10−4 mol
dm−3 [10]. The ionic strength of the catalytic system
was 0.1 M KCl in overall experiments. The release of
p-nitrophenolate ions was measured at 400 nm on a
GBC 916 UV–vis spectrophotometer under the condi-
tions of the about 20–47-fold excess of substrate over
the concentration of complex. The listed data are the
average of two or three runs with uncertainty of less
than 3%.
RESULTS AND DISCUSSION
Pseudo-First-Order Rate Constants (kobs
)
of PNPP Catalytic Hydrolysis
Kinetic Measurements
To study the reactive properties of PNPP hydrolysis in
G(hex)C16 and CTAB micellar solutions, the concen-
tration of each surfactant solution for kinetic runs must
be higher than its CMC to ensure formation of micelles
in Tris–TrisH+ buffer.
All kobs obtained values are presented in Table I.
From the values given in Table I, we can obtain impor-
tant correlative information. First, the CoL2 (L = L1,
L2)/G(hex)C16 metallomicelle is a predominant sys-
tem for promoting PNPP hydrolysis compared to the
metallomicellar systems formed by CoL2 (L = L1, L2)
Pseudo-first-order rate constants (kobs) of PNPP hy-
drolysis reaction were obtained based on the initial rate
method, that is, according to the following equations:
(rate)0 = −(dC/dt)0 = −(dA/dt)0/ε and (rate)0 = kobs
[ML]0, where (rate)0 is the initial rate of PNPP hy-
drolysis and [ML]0 is the initial concentration of the
complex. Hence, kobs was obtained from the slopes of
the linear plots of (rate)0 against [ML]0.
Each kinetic run was started by injecting the desired
PNPP stock solution into a 1-cm cuvette filling 3 mL
Table I Pseudo-First-Order Rate Constants (kobs) for PNPP Hydrolysis Promoted by CoL2 (L = L1, L2) in Gemini
Surfactant G(hex)C16 and CTABa
103[S] (mol dm−3
0.267 0.333
103k (s−1) for CoL12/G(hex)C16b
)
103[S] (mol dm−3
0.333 0.400
103k (s−1) for CoL22/G(hex)C16b
)
0.200
0.400
0.467
0.200
0.267
0.467
pH
obs
obs
6.50
7.00
7.50
8.00
8.50
7.00c
7.00d
3.617
13.93
14.24
4.131
16.38
17.64
53.13
80.02
5.287
19.47
20.40
60.84
99.88
5.760
21.08
22.60
71.63
114.6
6.334
23.67
28.07
82.43
126.3
1.681
7.675
8.396
23.93
52.18
2.210
9.230
9.978
30.91
63.24
2.398
10.25
11.84
37.09
78.17
2.550
11.09
13.21
45.13
95.89
3.058
13.10
15.35
49.32
111.7
37.90
60.06
7.8 × 10−3
7.370
8.809
9.614
10.40
11.71
4.137
4.969
5.468
5.665
6.161
a 25 0.1◦C, [G(hex)C16] = 1.0 × 10−4 mol dm−3, [CTAB] = 1.0 × 10−2 mol dm−3, [CoL2] = 1.0 × 10−5 mol dm−3
.
b All runs were carried out in Gemini G(hex)C16 micelle over the pH range of 6.50–8.50.
c Pseudo-first-order rate constant (k0) of PNPP spontaneous hydrolysis under the conditions of pH 7.00 and 25 0.1◦C.
d Data listed in this line show kobs values of PNPP catalytic hydrolysis by the title complexes in the CTAB micellar solution at pH 7.00.
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METALLOMICELLE CATALYSIS: HYDROLYSIS OF p-NITROPHENYL PICOLINATE
675
and CTAB, correspondingly. Second, the catalytic ac-
tivity of CoL12/G(hex)C16 is about 2 times higher than
that of CoL22/G(hex)C16 system in a rate enhancement.
Comparing with the rate (k0) of PNPP spontaneous hy-
drolysis, the hydrolytic rates of PNPP mediated by the
two Schiff base cobalt(II) complexes in the Gemini
G(hex)C16 micellar solution increased by a factor of
about 1.8 × 103 for CoL21 and 9.9 × 102 for CoL22,
respectively, at pH 7.00 and [PNPP] = 2.0 × 10−4 mol
dm−3. Furthermore, under the same experimental con-
ditions, the reactivity of PNPP hydrolysis catalyzed by
the same catalyst in the two surfactant micelles follows
in the order of CTAB < G(hex)C16. This is possibly
due to the intrinsic difference in their structures. More-
over, these observations show that the rates of the PNPP
catalytic hydrolysis increase with the increase in both
pH values and the concentration of substrate ([S]).
in micellar solutions. Details are as follows. Step I:
The nitrogen of a pyridine ring in a substrate molecule
coordinates to the Co(II) ion in the hydrate com-
plex (CoL2(H2O)), which results in formation of
the catalyst–substrate complex [CoL2(H2O)S]. As we
know, the catalyst–substrate complex indeed bene-
fits the conversion of an intermolecular reaction to
a pseudo-intramolecular one [43,44]. This will re-
duce the activation energy of the reaction, and ul-
timately accelerate the hydrolysis of PNPP. Step II:
A metal-bound hydroxide acts as an actual nucle-
ophile to attack the positive carbon atom of the
carbonyl group easily and also promotes the depar-
ture of thep-nitrophenyl group from PNPP with the
first-order-rate constant (k). This step is the rate-
determining step of the total reaction concerning an
acid–base equilibrium (step II-1). Step III: Another
water molecule is coordinated with the Co(II) ion
to accelerate the release of picoline acid; eventually,
the next catalytic cycle is induced by a regenerated
catalyst.
Proposed Mechanism of PNPP Catalytic
Hydrolysis
Herein, a metal-hydroxide activation mechanism is
proposed to analyze the possible processes of PNPP
cleavage induced by the two Co(II) complexes in mi-
cellar solutions. The catalytic hydrolysis of the ma-
jority of common esters occurs through a nucleophilic
attack of the metal-coordinated hydroxide ion at the
carbonyl carbon of carboxylic acid esters [42].
pH Dependence of CoL2/G(hex)C16 Activity
The rates of all enzymatic reactions are sensitive to pH
[45]. To ascertain the pH dependence of PNPP catalytic
hydrolysis, kinetic experiments were performed over a
series of pH values, ranging from 6.50 to 8.50 in the
present study.
Scheme 1 shows the proposed pathway of PNPP
catalytic hydrolysis promoted by the title complexes
Scheme 1
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JIANG ET AL.
Figure 2 Plots of 1/kobs versus 1/[PNPP] for PNPP hydrolysis promoted by CoL2 (L = L1, L2) in Gemini G(hex)C16 micellar
solutions over a pH range of 6.50–8.50. Conditions: pH 7.00, 25 0.1◦C, [G(hex)C16] = 1.0 × 10−4 mol dm−3, [CoL2] =
1.0 × 10−5 mol dm−3
.
Combining the previous report [11] with the theory
of initial rate method, we can have
plots in Fig. 2 show a good linear relationship (r ≥
0.98), which suggests that the proposed mechanism
for PNPP hydrolysis in this paper is reasonable. Each
k of PNPP catalytic hydrolysis in CoL2/G(hex)C16 and
CoL2/CTAB systems is shown in Table II.
1
kobs
1
1
=
+
(1)
k
Kk[S]
As shown in Tables I and II, both kobs and k val-
ues increase as a function of pH from 6.50 to 8.50.
This hints that the hydrolytic reaction possibly under-
goes a deprotonated process (step II-1 in Scheme 1) at
the determining-rate step, and the hydrolysis of PNPP
catalyzed by Co(II) complexes is characteristic of the
acid–base catalysis. In addition, the k values of PNPP
catalytic hydrolysis in the two buffered micellar solu-
tions follow an increasing order of CTAB < G(hex)C16
at pH 7.00.
In Eq. (1), [S] denotes the free PNPP concentration
and can be displaced by the initial concentration of
PNPP on the basis of the initial rate method, K is the
association constant between the substrate and CoL2 in
the micellar solution, k is the apparent first-order rate
constant for the intracomplex nucleophilic reaction in
the catalyst–substrate compound and is pH dependent.
The values of k can be evaluated from the in-
tercepts of the plots of 1/kobs versus 1/[S] for the
CoL2/G(hex)C16 system by the linear regression
method, as shown in Fig. 2. For comparison, 1/kobs
1/[S] plots for CoL2/CTAB are given in Fig. 3. All
In step II-1, Ka signifies the acidic ionization con-
stant of H2O molecule coordinated to Co(II) ion,
and k1 displays the pH independent first-order rate
Figure 3 Plots of 1/k versus 1/[PNPP] for PNPP catalytic hydrolysis by Schiff base Co(II) complexes in G(hex)C16 and
obs
CTAB micellar solution. Conditions: pH 7.00, 25 0.1◦C, [G(hex)C16] = 1.0 × 10−4 mol dm−3, [CoL2] = 1.0 × 10−5 mol
dm−3. Symbols: ꢀ, G(hex)C16; , CTAB.
ꢁ
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METALLOMICELLE CATALYSIS: HYDROLYSIS OF p-NITROPHENYL PICOLINATE
677
Table II The k Values of PNPP Catalytic Hydrolysis by CoL2/G(hex)C16 and CoL2/CTAB at 25 0.1◦Ca
pH
6.50
7.00
7.50
8.00
8.50
103k(CoL12/CTAB)
19.19
9.524
47.62
23.47
103k(CoL22/CTAB)
103k(CoL12/G(hex)C16)
103k(CoL22/G(hex)C16)
15.11
6.489
73.53
35.59
625.0
270.3
1000
625.0
a Conditions are the same as in Table I.
constant of the hydrolysis reaction in the metallomi-
cellar system. The deprotonation of the intermediate
CoL2(H2O)S first gives a more catalytically active
species (CoL2(OH)S)−, and p-nitrophenol is subse-
quently released from the PNPP molecule by k1 step.
According to the principles of the chemical equilibra-
tion, it is obvious that the rate of PNPP hydrolysis
increases with the increase in pH values in the current
study.
From the results presented in Tables I and II, it is
found that the kobs and k values in the presence of
CoL2/G(hex)C16 increased sharply at pH >7.50. The
pH–k curve (Fig. 4) displays a sigmoid tendency even
if the k values do not level off obviously at the higher
pH range. The sigmoidal pH–k profile can be fitted
to a single-proton ionization process by considering
that the deprotonated form is the active species. The
pKa value of Co-bound H2O can be determined with
the inflection points, that is, 7.9 for CoL12 and 8.3 for
CoL22, respectively. In the same way, we attempt to
obtain the pKa values of the metal-coordinated water
by the following method.
According to the literature [10,45], we cite a typical
equation as follows:
1
1
1
k1Ka
=
+
[H+]
(2)
k
k1
On the basis of Eq. (2), we can calculate the k1 and Ka
values through the slope and the intercept of the plot
of 1/k versus [H+] (see the insert in Fig. 4). The re-
sults show that k1 and pKa values are 0.53 mol−1 dm3
s−1 and 8.02 for CoL21, whereas they are 0.48 mol−1
dm3 s−1 and 8.38 for CoL22, respectively. The obtained
pKa values by this method are consistent with those
estimated for pKa values on the basis of pH–k pro-
files as shown in Fig. 4. The smaller pKa value of
CoL12 indicates that the generation of the active Co-
OH− moiety species in CoL12/16-6-16 system is easier
than that in CoL22/G(hex)C16 system, and then the nu-
cleophilic attack of a negative metal-bound hydroxide
on the substrate takes place handily. Thus, CoL12 with
the lower pKa (8.02) holds the predominance of speed-
ing up the process of PNPP hydrolysis under the same
experimental conditions.
Structural Effects of the Complexes on the
Hydrolytic Rate of PNPP
Usually, the catalytic activity of enzyme is correlative
with the enzymatic structure [46,47]. From Table I, it
can be seen that both CoL2 (L = L1, L2)/G(hex)C16
(or CTAB) system exhibit higher activity for the PNPP
hydrolysis reaction at pH 7.00, [PNPP] = 2.0 × 10−4
mol dm−3. However, the rates of PNPP hydrolysis cat-
alyzed by CoL12 are 1.8 times greater than that of CoL22
in the same micellar solution, that is, in G(hex)C16
or CTAB system. The above-mentioned results ade-
quately demonstrate that the difference in their activ-
ities is dependent on the intrinsic structure of the two
Co(II) complexes.
Figure 4 Plots of 103k versus pH for the PNPP hydrolytic
reaction at various pH in Gemini G(hex)C16 micellar solu-
tion. Conditions: pH 7.00, 25 0.1◦C, [G(hex)C16] = 1.0 ×
10−4 mol dm−3, [CoL2] = 1.0 × 10−5 mol dm−3. Symbols:
, CoL1; , CoL2; (insert) , CoL1; , CoL2.
A variety of studies [48–50] confirmed that the nu-
cleophilic attack is very sensitive to the size of both
the reactants and catalyst. In the present case, the ge-
ometrical size of the two Co(II) complexes may be
ꢂ
ꢂ
ꢀ
ꢀ
2
2
2
2
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678
JIANG ET AL.
one important factor influencing their catalytic activ-
ity. An opening catalytic center is required to benefit
the coordination between the PNPP molecule and the
catalytic center. The active site is blocked primarily by
outer bulky substituents due to the closeness of local
rigid framework to the Co(II) center in CoL2, and this
results in the hindrance of the PNPP molecule binding
to the Co(II); therefore, it limits the rate acceleration
of the reaction. In CoL12, no big para-substituent con-
nects to the benzene ring linked to the nitrogen atom
of C N bond, which constructs an open environment
for the PNPP molecule approaching the Co(II) ion.
This will be favorable for the formation of the produc-
tive catalyst–substrate complex [43,44], and then the
hydrolytic rate of PNPP is accelerated notably. In con-
trast, the active site of CoL22 is very steric hindered due
to the link of the aza-15-crown-5 as a larger substituent
to the benzene ring in CoL22. Consequently, CoL22 does
not provide more coordinated directions for the PNPP
molecule to the Co(II) center, which results in a smaller
concentration of catalyst–substrate complex. For this
reason, the catalytic activity of CoL22 is lower than that
of CoL12.
Figure 5 Proposed hydrolytic process of PNPP promoted
by the title complex in the Stern layer of G(hex)C16 micelles
at 25 0.1◦C.
Furthermore, Choi et al. [51] have once mentioned
that smaller molecule easily solubilizes in micellar so-
lutions. As a result, more small CoL12 molecules pen-
etrate into Gemini G(hex)C16 micelles expediently,
which are in favor of the hydrolytic process of PNPP.
In a word, the difference in catalytic behavior of the
two complexes highlights that the presence of aza-
15-crown-5 moiety in CoL22 is an essential structural
element, influencing the hydrolytic rate of PNPP in the
same micelles. That is to say, bulky substituents closer
to the active site of catalyst may play a negative role in
catalyzing the hydrolysis of PNPP.
cal micelle) [31]. On the one hand, Gemini micelles
are superior to conventional single-tailed surfactants
in solubilizing hydrophobic reactants in water [53].
In our study, there is a supereminent solubilization
of the hydrophobic reactants (including substrate and
catalyst) in the Stem layer of G(hex)C16 micelles, in
which the dimethylammonium headgroup exists, asso-
ciated with bromide counterions and water molecules,
spacer methylene groups, and the hydrated portion of
the n-hexadecyl chain [54]. Therefore, the increase in
local concentrations of PNPP and the catalyst in the
G(hex)C16 micelle phase enhances the collision fre-
quency of the reactants, and this will remarkably ac-
celerate the rate of PNPP hydrolytic reaction compared
with in CTAB micelles. On the other hand, a coopera-
tive effect of positive Co(II) ion [13] and headgroups
of Gemini G(hex)C16 markedly stabilized the nega-
tive transition state, which gives the additional rate en-
hancement of the PNPP hydrolysis reaction. By com-
parison, a similar function of CTAB is weaker than that
of G(hex)C16 due to the small charge density of head-
groups of CTAB. Consequently, the synergic effect of
aforementioned factors leads to the higher hydrolytic
rate of PNPP in G(hex)C16 micelles than that in CTAB.
Roles of G(hex)C16 Micelle in the PNPP
Hydrolysis Reaction
Gemini surfactant [52] is a new class of amphiphilic
molecule, containing two headgroups and two aliphatic
chains. They have physicochemical properties that are
different from those of comparable conventional sur-
factants (single chain, single headgroup) [30].
Figure 5 shows the metal-promoted PNPP hy-
drolysis in the interfacial region of G(hex)C16 mi-
celles. From the obtained results, the hydrolytic rate of
PNPP mediated by the two Schiff base Co(II) com-
plexes in G(hex)C16 micellar solutions is approxi-
mately twofold greater than that in CTAB micellar
solutions. This confirms that Gemini surfactant has
more surface activity over its conventional counterpart
does, though the shape of the G(hex)C16 micelle is
closer to that assumed by a CTAB micelle (spheri-
CONCLUSIONS
In this paper, the hydrolysis of PNPP promoted by
the crowned Schiff base Co(II) complexes in Gemini
surfactant G(hex)C16 and CTAB micellar solution has
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METALLOMICELLE CATALYSIS: HYDROLYSIS OF p-NITROPHENYL PICOLINATE
679
been studied. The results obtained show that the hy-
drolytic rates of PNPP, induced by each Co(II) complex
in G(hex)C16 micellar solution, are greater than that in
the CTAB micellar solution. This may be contributed to
the unique surface property of Gemini G(hex)C16. In
addition, CoL12 possesses better catalytic activity when
compared with CoL22 in the identical micelles, which
is most probably due to the smaller steric hindrance
of substituent groups residing in the former complex.
Experimental results also imply that Gemini surfactant
will likely be a class of potential and excellent substi-
tutes for conventional surfactants in artificial enzyme
research fields.
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