Micelle catalyzed oxidative degradation of norfloxacin by chloramine-T

Article abstract of DOI:10.1016/j.molcata.2012.04.004

Present work reports influence of micellar media (CTAB) upon the oxidative degradation of fluoroquinolone family drug norfloxacin (NOR). The reaction has been studied at constant temperature (308 K). The stoichiometry of the reaction was found to be 1:4 a

Full text of DOI:10.1016/j.molcata.2012.04.004

Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Micelle catalyzed oxidative degradation of norfloxacin by chloramine-T
a
b,∗
g
b
c
d
Alpa Shrivastava , Ajaya Kumar Singh , Neerja Sachdev , Dilip Raj Shrivastava , Yokraj Katre ,
e
f
Surya P. Singh , Man Singh , J.C. Mejuto
Department of Applied Chemistry, Shri Shankaracharya College of Engineering & Technology, Bhilai, India
Department of Chemistry, Govt. V.Y.T. PG. Autonomous College Durg (C.G.), India
Dr. Khoobchand Baghel Govt. Arts & Science PG. Autonomous College, Bhilai-3, India
Department of Chemistry, Kalyan Mahavidyalaya, Bhilai Nagar Sect-7, Bhilai, India
Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 607, India
School of Chemical Sciences, Central University of Gujarat, Gandhinagar, India
a
b
c
d
e
f
g
Departamento de Química Física, Facultad de Ciencias, Universidad de Vigo, Ourense, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Present work reports influence of micellar media (CTAB) upon the oxidative degradation of fluoro-
quinolone family drug norfloxacin (NOR). The reaction has been studied at constant temperature (308 K).
The stoichiometry of the reaction was found to be 1:4 and oxidation products were identified by LC–MS
technique and other spectral studies. The reaction exhibited first-order-dependence on chloramine-T
Received 1 November 2011
Received in revised form 18 March 2012
Accepted 10 April 2012
Available online 3 May 2012
[CAT] and fractional-order dependence on [NOR]. The rate constant was independent of cationic micelles
of cetyltrimethylammonium bromide [CTAB] at lower concentrations followed by a rapid increase in rate
constant to a maximum and further gradual decrease in rate constant at higher CTAB was observed. In the
presence of CTAB, both the oxidant and substrate are distributed between the aqueous and the micellar
pseudo phases and then react to give the product. Compensation between water structure destruction
and substrate–micelle interaction plays an important role in the presence of CTAB. The rate constants
were obtained and the applicability of Piszkiewicz’s, Raghvan–Srinivasan’s and pseudo phase models
allowed us to obtain the reactivity constants in the aqueous pseudo phase and in the micellar pseudo
Keywords:
Oxidative degradation
Micellar catalysis
Critical micelle concentration (CMC)
Cetyltrimethylammonium bromide (CTAB)
Chloramine-T (CAT)
Norfloxacin (NOR)
w
m
phase (k2 and k2 ) and the binding constants of the reactants to the micellar phase (K1, K2). The effects
2
−
−
−
−
of inorganic salts i.e. [SO4 ], [HSO4 ], [Cl ] and [Br ] on reaction rate have also been studied.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
perusal of literature shows that not so many studies focusing on
the biodegradability and fate of FQs in the environment have been
reported [6–9]. However, strong binding can be expected to delay
degradation and may partly explain the apparent recalcitrance of
FQs [10]. Several recent studies have reported the environmental
presence of FQs in many countries such as Switzerland, the United
States, Australia and China [11]. The presence of broad spectrum
antibiotics like these in aquatic environment, even at low concen-
trations, may pose serious threats to the ecosystem and human
health by inducing proliferation of bacterial drug resistance [12].
Studies on the oxidation of few FQs [13–15] and only two cases
of oxidation of NOR [16,17] have been reported. However in natu-
ral water reservoirs already containing various surfactants as well
as oxidants, the fate of these drugs is a new emerging aspect for
researchers. This study reports the absolute kinetics and effects
of cationic surface active agent cetyltrimethylammonium bromide
Recently, presence and accumulation of pharmaceuticals and
personal care products (PPCPs) in the aquatic environment have
received increasing attention and putting a new challenge to drink-
ing water, waste water and water treatment systems [1–3]. Due to
their large quantities of uses, antimicrobial chemicals are continu-
ously introduced to the aquatic environment via treated, untreated
sewage, sludge, agricultural waste and runoffs. In this context,
fluoroquinolones (FQs) are probably among the most important
class of synthetic antibacterial agents as these are most commonly
used in human and veterinary medicines. In fact most of these
antibacterial agents are not fully metabolized in the body [4], there-
fore entered the environment through wastewater effluents [5]. A
(
CTAB) on the oxidative degradation of one of the FQs, norfloxacine
Abbreviations: NOR, norfloxacin; CAT, chloramine-T; CTAB, cetyltrimethyl
ammonium bromide; CMC, critical micelle concentration; PTS, p-toluene sulfon-
amide.
∗ Corresponding author. Tel.: +91 788 2223421; fax: +91 788 2212030.
E-mail address: ajayaksingh au@yahoo.co.in (A.K. Singh).
(NOR) by oxidant chloramine-T (CAT). This may be explaining how
NOR gets transformed in the presence of an oxidant (CAT) and
surfactant (CTAB) in aqueous media. During the water chlorina-
tion process behaviors of some FQs have been reported [18]. Our
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.04.004

2
A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
investigation is based upon the influence of cationic surfactant on
the transformation of NOR by CAT. The CAT itself is used as an anti-
septic drug [19] and also most commonly used for various water
treatment processes, disinfectant, deodorant, etc. [20].
purified by distillation. The NOR solution was also stored in dark
colored bottle to prevent any photochemical reactions. To recrys-
tallize CTAB (Merck), it was dissolved in minimum quantity of luke
◦
warm MeOH (Loba Chem.) at about 40 C. Then Et O was then
2
The chemistry of aromatic sulfonyl haloamines (N-haloamines)
is of interest due to its diverse behavior. N-haloamines are a group
of mild oxidizing agents, which have been used extensively for the
oxidation of several organic compounds. The versatile nature of
N-haloamines is due to halonium cations and nitrogen anions in
their structure, which can act as both a base and a nucleophile
added with constant stirring until a permanent white precipitate
was obtained. The content was cooled to room temperature and
kept in the refrigerator for further cooling. Crystalline white solid
so obtained was filtered in a Buchner funnel. Finally crystals were
washed with excess cold ether and air dried.
All the kinetic measurements were carried out in a black-coated
vessel at constant temperature (308 K) and experiments were car-
ried out under pseudo first-order condition with [NOR] ꢀ [CAT].
The reaction was initiated by rapid addition of known amounts of
oxidants to the reaction mixture containing the required amounts
of NOR, sulfuric acid, CTAB and water in glass stopper Pyrex boiling
tubes, thermostated at the same temperature. Progress of reaction
was monitored by iodometric determination of unconsumed [CAT]
in known aliquots of the reaction mixtures at different time inter-
vals. Each kinetic run was studied for ∼80% reaction. Runs were
reported twice and had <4% standard deviation. The rate constants
of reactions in each kinetic run were determined by the slope of
tangent drawn by plots of remaining log [CAT] against time data.
[
21–25]. Thus, these compounds can react with a wide range of
functional groups to cause a variety of molecular changes. The
prominent member of this class of compounds is sodium-N-chloro-
4
-methylbenzensulfonamide, generally known as chloramine-T
(
CAT). Several reviews have been published on the oxidizing behav-
ior of CAT [26–28] and a number of publications focus on the
mechanistic aspects of the redox reactions in acidic media. In most
of the studies one of the species, RNHCl (R = CH C H SO ), HOCl, or
3
6
4
2
+
H OCl , has been considered as the reactive species [29,30]. Similar
2
information on CAT in micellar medium is limited.
Surfactants of cationic nature like CTAB are well known antibac-
terial compounds.
N
Br
Cetyltrimethylammonium bromide
They are also used as environmental cleaning products, in
cosmetic formulations and pharmaceuticals [31,32]. Surfactant
properties have attracted growing attention in biochemistry, bio-
logical and chemical research applications. Surfactants as reaction
medium, affect reaction rates, position of the chemical equilib-
rium, products, and in some cases stereochemistry of the reactions
Excess of CAT over NOR was allowed to react in the presence
of sulfuric acid. The residual oxidant in each case was determined
iodometrically after 24 h. The results showed that 4 mol of CAT
was required for each mole of NOR. On the basis of analysis of
the reaction products, the stoichiometric equation is proposed in
Scheme 1.
[
33,34]. Surfactants are having growing number of scientific and
The reduced product of oxidant CAT i.e. p-toluene sulfonamide
technological applications as these are found to control the size of
synthesized microparticles and play an important role in solubiliza-
tion and extraction. These surfactants are able to enhance reactions
between non-hydro soluble organic substrates and hydro-soluble
reagents, the kinetics of numerous reactions in microemulsions
have been well studied [35]. In the presence of cationic micelles of
CTAB, catalysis is observed when reactants are taken into the micel-
lar pseudo phase. In fact, a greater reactivity has been reported in
micellar pseudo phase as compared to the bulk solution. Analo-
gies between enzyme and micellar catalysis have been widely
discussed [36], but are discounted [37] because micellar catalysis
generally lacks the substrate specificity and high catalytic activity of
enzymes. Micellar effects on the rates are sensitive to the nature of
counterions and to head group bulk [38]. Transition of spherical to
larger micelles may be caused by salt or surfactant additions [39] or
solute solubilization [40,41]. It is well known that the addition of a
salt decreases the electrostatic interactions between monomers in
the micelle and also affects the partitioning of organic compounds
between micelles and bulk solvent [42]. Micellar catalysis can be
enhanced by adding some salts indicating that the rate enhance-
ment is sensitive to changes in micellar morphology and charge
density [43].
(
PTS) was initially characterized by TLC [45]. Further confirmation
◦
was determined by its melting point, 139 C, which was within
close proximity to the reported temperature of 137–140 C in
◦
previous works [46]. PTS was quantitatively determined by its
reaction with xanthydrol to yield the corresponding N-xanthyl-p-
toluenesulfonamide [47]. In a typical experiment, equal quantities
of separated PTS and xanthydrol (0.20 g) were dissolved in 10 mL
of glacial acetic acid. The reaction mixture was stirred for 3 min
at room temperature and allowed to stand for 90 min. The deriva-
tive was filtered, recrystallized with dioxane/water (3:1), and dried
at room temperature. The mass of PTS was obtained with 80–85%
recovery in this case. Finally, the main oxidation product of NOR and
other products were isolated and confirmed by GC–MS (JEOL-JMS,
Mate-MS system, Japan) (Supporting information, Figs. S-1 and S-
2
). Carbon dioxide was identified by bubbling N2 gas through the
reaction mixture and passing liberated gas through a U-shaped tube
containing a saturated Ba(OH)2 solution. The oxidation product of
NOR was also confirmed by FT-IR spectral studies (IR (KBr, ␥ cm ):
−1
1
730s, C O; carboxylic, 1621s, C O; 3059s, NH; 3500s, OH).
3
. Results
To study the effects of concentration of various reactants on
2
. Materials and methods
rate of oxidation reaction and with a view to ultimately propose
a suitable reaction mechanism, concentration of each reactant was
varied keeping concentration of other reactants constant. Firstly,
the rate of reaction in the absence of cationic surfactant (CTAB)
was determined and then the role of CTAB on the degradation
of NOR by common chlorinating agent (during water treatment
process) CAT, in acidic medium was analysed. The values of rate
constants in the absence and presence of CTAB were found to be
Analytical grade reagents were used without further purifica-
tion and solutions were prepared in triple distilled water. The stock
solution of CAT (Loba Chem.) was stored in dark colored bottle and
the strength of CAT was checked iodometrically [44]. The NOR (USP
powder, plama, m.p. 215–216 C, 99.9%) was used as received and
stock solution was prepared in 10% acetic acid which was further
◦

A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
3
Scheme 1. Proposed reaction scheme showing stoichiometry and products of the oxidative degradation of NOR by CAT.
−4 −1
−4
0.90 × 10
and 2.72 × 10
s
respectively, which clearly indi-
the homogeneous aqueous medium with all possible intermediary
situations [48].
cates the catalytic role of surfactant in the title reaction. The k
obs
value was found to be independent of the initial concentration of
−
3
−3
oxidant CAT in the range (0.2–1.5) × 10 mol dm , whereas with
3.1. Effect of [CAT] and [NOR] on kobs
−
2
−3
an increase in [NOR] from (0.25–4.00) × 10 mol dm , the kobs
was found to increase nearly 2.5-fold (in both the cases the [CTAB]
Under the experimental conditions, the rate of disappearance
−4
−3
−3
was fixed at 5.5 × 10 mol dm ). The nearly similar value of Gibb’s
free energy of activation in both media (aqueous and CTAB) sup-
ports the operation of same reaction mechanism in aqueous or in
surfactant media (Supporting documents, Table ST 1). The follow-
ing results and observations of product formation in an aqueous
medium [16,17] lead us to safely conclude that the reaction mecha-
nism in the presence of cationic micelles remains the same as that in
of [CAT] was determined at various [CAT] from 0.2 × 10
to
−
3
−3
1.5 × 10 mol dm , which showed a first order dependenceon the
reaction rate (Table 1). The pseudo-first-order rate constants (kobs)
have been evaluated from a linear plot of log [CAT] versus time
(Supporting information, Fig. S-3). The dependence of the rate con-
−
2
stant on [NOR] was determined at different [NOR] from 0.25 × 10
−
2
−3
to 4 × 10 mol dm at constant concentration of other reactants
Table 1
Effect of [CAT], [NOR] and [H2SO4] on the rate of oxidation of norfloxacine at 308 K.
CAT] (×10 mol dm−3
3
)
[NOR] (×10 mol dm
2
−3
)
[H2SO4] (×10 mol dm
2
−3
)
−dc/dt (×10 mol dm
8
−3
)
kobs (×10
4
−1
s )
[
Without surfactant
With surfactant
Without surfactant
With surfactant
0.20
0.40
0.50
0.80
1.00
1.50
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.25
0.75
1.00
1.50
3.00
4.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.05
0.10
0.20
0.30
0.50
0.80
0.18
0.36
0.46
0.74
0.91
1.38
5.03
7.80
9.00
10.50
13.20
14.40
9.42
9.03
7.43
7.16
6.89
6.54
5.30
10.72
13.45
21.60
27.30
40.50
14.97
24.18
27.25
31.08
37.17
42.67
34.37
33.40
27.50
26.78
25.48
24.18
0.90
0.91
0.92
0.92
0.91
0.92
0.50
0.78
0.90
1.05
1.32
1.44
0.93
0.90
0.74
0.71
0.68
0.65
2.65
2.68
2.69
2.70
2.73
2.70
1.49
2.42
2.73
3.11
3.72
4.27
3.42
3.34
2.75
2.65
2.55
2.42

4
A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
4
4
4
3
3
3
.2
4.7
Without Surfactant
With Surfactant
4
4
4
4
4
4
.6
.5
.4
.3
.2
.1
.1
.0
.9
.8
.7
1
.4 1.6 1.8 2.0 2.2 2.4 2.6
1.4 1.6 1.8 2.0 2.2 2.4 2.6
2
+ log [NOR]
2 + log [NOR]
Fig. 1. Effect of variation of [NOR] on the observed rate constants. Experimental conditions: at 308 K, [CAT] = 1 × 10− mol dm
3
−3
,
[NOR] = 1 × 10−2 mol dm−3
,
+
−3
−4
−3
.
[
H ] = 0.1 mol dm , [CTAB] = 5.5 × 10 mol dm
at 308 K (Table 1). The plot of log kobs versus log [NOR] was found
to be a straight line with positive slope (0.53 and 0.42) indicating
that reaction with respect to [NOR] is fractional order (Fig. 1).
Determination of CMC has considerable practical importance to
understand the self organizing behavior of surfactants. Micelle
aggregates affect chemical reactivity either by binding/excluding
reactants or by changing the free energy of activation. The CMC val-
ues of CTAB were determined conductometrically in the presence
of reactants at 308 K. The CMC values were determined from plots
of equivalent conductivity versus [CTAB]. The breakpoint of nearly
straight-line portions in the plots is taken as an indication of micelle
formation and corresponds to the CMC of surfactant. The micelle
formation in an aqueous solution is known to be affected by organic
additives. Many studies reported decrease in the CMC values in
presence of additives [49], as observed in the present investigation.
The values of CMC of surfactant in water, in the presence of oxidant
+
3
.2. Effect of [H ] and dielectric constant on k
obs
It was observed that the aqueous solution of CTAB becomes tur-
bid in the presence of HClO . The turbidity increases with increase
4
in [HClO ] at constant [CTAB], whereas in the presence of H SO ,
4
2
4
such turbidity was not observed. The rate constant was found
−
4
−4 −1
−3
to decrease slightly from 3.42 × 10
to 2.42 × 10
s
with an
+
−1
−1
increasing [H ] from 0.05 × 10 to 0.8 × 10 mol dm (Table 1).
The effect of changing solvent composition on the reaction rate
was studied by varying the concentration of acetic acid from 5 to
−
3
−3
−2
−3
)
(
1 × 10 mol dm ), in presence of reductant (1 × 10 mol dm
−
4
and in presence of acid were found to be 9.0 × 10 (quite close
3
0% by volume. The rate constants listed in Table 2 suggest that
−
4
−4
−4
and
to the literature value, i.e. 9.2 × 10 ), 8.0 × 10 , 5 × 10
the rate constant decreased with increasing acetic acid content in
the reaction mixture, i.e. with the rate of reaction decreased with
decreasing dielectric constant of the medium. Addition of PTS (a
reduced product of the CAT) did not affect the rate of oxidation
−
4
−3
2
.5 × 10 mol dm at 308 K (Table 3). The micelle formation in an
aqueous solution is well known to be affected by organic additives.
(
Table 2).
3.4. Test for free radical
3
.3. Determination of CMC
During the course of oxidation no free radical was gener-
ated which was confirmed by using acrylonitrile monomer. To
−
3
−3
Surfactants spontaneously aggregate above a certain concen-
the reaction mixture (containing [CAT] = 1 × 10 mol dm
,
−
2
−3
+
−1
−3
tration called critical micelle concentration (CMC) to form micelle.
[NOR] = 1 × 10 mol dm
,
[H ] = 1 × 10 mol dm
and
−
4
−3
[
CTAB] = 5.5 × 10 mol dm ),
a known amount of acryloni-
3
trile (20 cm ) was added. Addition of acrylonitrile to the reaction
mixture in an inert atmosphere (under N2 atmosphere) did not
initiate polymerization showing the absence of free radicals.
Table 2
Effect of acetic acid and PTS on the oxidation of norfloxacine at 308 K.
4
−3
4
−1
s )
[CH3COOH] (%)
[PTS] (×10 mol dm
)
kobs (×10
0
5
0
0
0
–
–
–
–
–
–
–
0.0
0.5
2.72
2.67
2.30
1.92
1.54
2.70
2.72
2.69
2.70
2.70
Table 3
CMC values of CTAB in different experimental conditions.
1
2
3
CMC (×10−4 mol dm−3
Solution
)
Water + CTAB + water
Water + CTAB + CAT
Water + CTAB + NRF
Water + CTAB + acid
9.0a
8.0
5.0
2.5
1
2
4
.0
.0
.0
CMC values in terms of equivalent conductivity are measured at 308 K.
−3
−3
[NOR] = 1 × 10−2 mol dm−3
a
−4
−3
Experimental conditions: [CAT] = 1 × 10 mol dm
,
,
Literature value of CTAB at 298 K = 9.2 × 10 mol dm
.
+
−3
−4
−3
.
[
H ] = 0.2 mol dm , [CTAB] = 5.5 × 10 mol dm
Figures are enclosed in Supporting documents, Fig. 3.

A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
5
Scheme 2. Reaction steps, involved in the oxidative transformation of NOR by CAT.
4
. Mechanism and rate law
value became maximum would be the kinetic CMC of the surfac-
tant in the presence of substrate and from the plot, the kinetic CMC
−
4
A perusal of literature shows that CAT is a strong electrolyte in
value was found to be 5.8 × 10 M (Fig. 2).
aqueous solution which behaves as an oxidant. The possible oxidiz-
ing species in acidic solution of CAT are TsNH Cl, TsNHCl, TsNCl ,
and HOCl [50]. TsNHCl is expected to be the most probable oxidiz-
ing species involved in the reaction [19]. The pKa of CAT is 4.5, in
water containing 0.1 M [H ], and the CAT will be completely pro-
The increase in k was observed even at [CTAB] < CMC indicat-
obs
ing that reactants are included in the micelle formation (surfactant
molecules start aggregating below CMC). The catalysis below CMC
(i.e. submicellar catalysis) is not new, the feasibility can be sought
in the fact that small aggregates of the surfactants (dimers, trimers,
tetramers, etc.) exist below CMC. These small submicellar aggre-
gates can interact physically with the reactant molecules forming
catalytically active entities. The increase in kobs with [CTAB] may
be attributed to increasing solubilities/incorporation/association of
the reactant species with increase in [CTAB]. The observed higher
rate in the presence of CTAB could be attributed to the adsorption
of reactants on the positively charged micellar surface. Therefore,
it can be concluded that the reaction catalyzed by CTAB occurs in
the juncture region of Gouy–Chapman–Stern layer of the micelles
(Fig. 3).
The three important factors namely, electrostatic, hydrophobic
and hydrogen bonding seem to play an important role in bringing
the reactants (NOR and CAT) together in the small volume of the
micellar pseudo phase. The increase in rate with increasing [CTAB]
may be accounted for the fact that with increasing the concentra-
tion of the surfactant the structural arrangement of water increases,
thereby diminishing the availability of free water molecules for the
solubilization of NOR and CAT.
2
2
+
tonated in the form of TsNHCl. Under the present experimental
conditions it has been observed that the reaction is first-order with
respect to [CAT], fractional order with respect to [NOR] and zero-
order with respect to [PTS]. Considering the reactive species of CAT,
the reaction in Scheme 2 is proposed for the oxidation of NOR by
CAT.
The intermediates X and Y are shown in Scheme 3.
The mechanism in Scheme 2 is proposed in the light of the fact
that the electron flow during oxidation of NOR by CAT is in accor-
dance with the electrophilic attack by the oxidant (RNHCl of CAT)
through its chlorine, resulting in the formation of X which on fur-
ther hydrolysis, gives intermediate Y. This reacts stepwise with
other 3 mol of oxidant to yield the final product:
Considering Eqs. (a) and (b) of Scheme 2, we have equation (1)
and (2):
Rate = 4kobs[X]
kobsK[NOR][CAT]T
+ K[NOR]
(1)
(2)
4
Rate =
Consequently, the equilibrium represented in reaction (i.e.
Scheme 2), shifts toward right resulting in an increase in [X], thus
the rate increases until the formation of micelles occurs. Various
1
The above rate law is in good agreement with the experimental
results in absence of surfactant.
1
0
9
8
7
6
5
4
3
2
1
5
. In the presence of surfactant
5.1. Effect of CTAB on kobs
Micellar mediated reactions involving ionic, polar and neutral
reactants are generally believed to occur in the Stern layer of a
micelle of an ionic surfactant. Catalysis by micelles involves at least
three main steps: (i) binding of the substrate to the micelle, (ii) the
actual chemical change in or at the surface of the micelle and (iii)
release of products. To investigate the effect of ionic micelles on
the reaction rate, the kinetic experiments were performed in the
presence of varying [CTAB] at constant values of other reactants
at 308 K. It is obvious that the reaction is affected in some way by
micelle formation process. As the catalysis was observed at [CTAB]
below the CMC, it was necessary to measure the CMC under con-
ditions more closely to those used in the kinetic runs. Broxton [51]
method was used to determine the CMC of working CTAB solution.
In this method, the point of intersection of two linear parts of kobs
versus [CTAB] drawn just below and above CMC gives the value of
kinetic CMC. The CAT–NOR reaction is catalyzed by cationic surfac-
0
2
4
6
8
10
12
4
-3
[
CTAB] x 10 mol dm
Fig. 2. Pseudo first order rate constants against surfactant concentration at reac-
tant CTAB. A plot of k
versus [CTAB] showed a maximum rate at
−3
−3
−2
−3
,
obs
tion conditions: T = 308 K, [CAT] = 1 × 10 mol dm
,
[NOR] = 1 × 10 mol dm
−4
−3
+
−3
5
.8 × 10 mol dm . The concentration of CTAB at which the kobs
[H ] = 0.1 mol dm .

6
A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
Fig. 3. Oversimpilified two-dimensional schematic representation of the concentration effect of micelles.
models have been proposed to explain the micellar effects. Accord-
ing to Khan [52] the occurrence of maxima in the plots of kobs
versus [Dn] (Fig. 6) is a characteristic of micelle catalyzed bimolec-
ular reactions that do not involve cross-interface reactions. In
the present investigation, pseudo first order rate constant profile
against [CTAB], clearly indicates (Fig. 2) the acceleration effect of
For the micelle catalyzed reactions which show positive
cooperativity, the rate constant, as a function of the detergent con-
centration, is given by
n
km[D] + k K
w
n
D
kobs
=
(3)
KD − [D]
[
CTAB] on the reaction rate up to a maxima, thereafter decreasing
values of rate constants show the post micellar inhibition of reac-
tion rate. To explain the acceleration effect of [CTAB] at its lower
concentrations, Piszkiewicz’s, Raghvan–Srinivasan’s and pseudo
phase model have been applied. Further, to justify the post micellar
inhibition effect of [CTAB] Berezin’s model has also been applied.
Here kobs represents the rate constant in the presence of surfactant.
On rearrangement and taking logarithm, above equation gives
kobs − kw
log
= n log [D] − log KD
(4)
km − k
obs
5
.2. Piszkiewicz’s model
where KD is dissociation constant of micellized detergent back to
its components, n is index of cooperativity, D is the total surfac-
tant, km and kw are rate constants in micellar and aqueous pseudo
phases respectively. To determine the kw and kobs for the oxida-
tion reaction of NOR by CAT in acidic medium, the reaction was
The observed higher rate in CTAB could be attributed to the
adsorption of substrate on the micellar surface, thereby, increasing
the local molarities of the complex in the Stern layer. The cataly-
sis of the reaction by CTAB can thus be explained by considering
the concentration effect of the reactants on or around the micellar
surface. CTAB concentrates both NOR and CAT onto the surface of
micelles and brings the reactants together in a close proximity, thus
enhancing the reaction rate. Both the reactants, NOR and CAT, are
fairly soluble in water, therefore, KD (binding constant) is evaluated
by kinetic method.
studied in absence of CTAB (that gave k ) and then in the pres-
w
ence of CTAB (that gave kobs). The correct value of km cannot be
determined experimentally, therefore its value is computed using
Menger–Portnoy’s approach [54]. Fig. 5 gives a straight line for
the plot of log [(kobs − k )/(k − k )] versus log [D] (Fig. 4). The
w
m
obs
−
4
3
−3
values of n (=4.672 × 10 ) and K (4.932 × 10 mol dm ) were
D
determined from the gradient and intercept (Table 4).
Reactions influenced by surfactants have been viewed as mod-
els on enzyme catalyzed reactions. A kinetic model analogous to
Hill model was used by Piszkiewicz [53] to explain the catalysis of
molecular reactions by surfactants. This model is applicable espe-
cially at lower surfactant concentration and the data may be treated
without reference to CMC. According to this model a substrate (S)
aggregates/associates with n number of detergent molecules (D)
to form critical micelle (DnS) which may react to yield product as
given in Scheme 3.
Fig. 4. Plots of log [(kobs − kw)/(km − kobs)] versus log [D] for the degradative oxi-
−
3
−3
,
Scheme 3. Formation of substrate detergent associate to yield products based on
Piszkiewicz assumption.
dation of NOR. Experimental conditions: at 308 K, [CAT] = 1 × 10 mol dm
−
2
−3
+
−3
.
[NOR] = 1 × 10 mol dm , [H ] = 0.1 mol dm

A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
7
Table 4
Parameters of surfactant and different constants calculated from models.
Parameters
Value
4
n
4.67 × 10−
−3.69
log KD
KD (mol dm−3)
2.03 × 10
4.98 × 10
3.69
−4
3
1
/KD
−
−
3
3
K1 (mol dm
K2 (mol dm
)
)
0.28
5.3. Raghvan and Srinivasan’s model
For bimolecular micellar catalyzed reactions Raghvan and Srini-
vasan [55] developed a model, which also predict consistency in
kobs values at high detergent concentrations and may be used for
evaluating constants of reactants. The distribution of the reac-
tant and the nucleophile in both phases, i.e. aqueous and micellar
phases has been considered in this model. The product formation is
assumed to result from decomposition of ternary complex involv-
ing substrate, nucleophile and micelle. The analysis of the data on
the basis of the model showed that almost all the nucleophile was
present in the bulk. A similar idea has also been proposed by Rom-
sted [56] and Reeves [57]. The model is represented in Scheme 4.
n
Fig. 5. Linear relationship between (kobs − kw)/kobs(1/[D] ) and (km/kobs) verifying
applicability of Raghvan and Srinivasan’s model. Experimental conditions were the
same as in Fig. 4.
and the nucleophile residing at the Stern layer of the micelle, with-
out being bound strongly at the micelle–water interface. Therefore,
the observed catalysis of reactions by cationic micelles might have
been the result of a proper orientation of the substrate in which
the polar portion residing at the Stern layer with non-polar portion
adsorbed into the cone for facile reaction with the nucleophile. This
conclusion is fairly supported by the data available.
n
kw + kmK K [D]
1
2
kobs = 1
(5)
+ K [D] {1 + K [S] }
1
n
2
T
This equation is identical to those derived by Martinek et al. [58],
5.4. Pseudo-phase kinetic model
Berezin [59] and Bunton et al. [60], where K , K2 are equilibrium
1
constants. Further, the above equation may be rearranged in the
form:
The catalytic behavior observed under the present experimental
conditions (Fig. 6) is in agreement with similar bimolecular reaction
which may be explained in terms of the pseudo-phase model of
micelles [61].
For the quantitative explanation of the influence of surfactants
on the reaction rate this model has been developed and is supported
by many workers [62–68]. For [CTAB] lower than its CMC, the rate
constant remains constant because it is the only reaction in water
and the concentrations of the two reactants keep constant. For
ꢀ
ꢁ
ꢀ
ꢁ
kobs − kw
1
km
kobs
a
=
(K K )
− K {1 + K [S] }
(6)
n
1
2
1
2
T
k
[D]
obs
The equation predicts
linear relationship between
n
(
kobs − kw)/k (1/[D] ) and (km/kobs). We have applied this model
obs
in our system using the value of n obtained from Piszkiewicz’s
cooperativity model (Table 4). The plots of K1 and K2 (binding
constants) have also been evaluated with the help of intercept and
slope of linear plot (Fig. 5). The value of intercept is considered
as K1 because K [S]T ꢁ 1. The values of intercept, K (3.6870) and
[
CTAB] to which the monomers start aggregating, we have observed
that at first a kinetic catalysis effect, as a consequence of local reac-
tive concentrations in the Stern layer. The observed rate constant
increases with the increase of the surfactant concentration till it
reaches the maximum value. When it reaches the maximum, the
constant decreases due to a dilution of the reactants in the micel-
lar pseudo phase, because when the [CTAB] increases, the number
and size of the micelles also increase. This behavior suggests the
use of the micellar pseudo phase model, considering the fact that
the reactions are taking place in both the aqueous and the micel-
lar pseudo phase. The different reactivities of the reactants present
in the two pseudo phases can be defined using the corresponding
2
1
K2 (0.2795) obtained from the model are reported in Table 4.
However, this model does not consider the effect of counter ions
nor the interaction of products with the micelles. The higher value
of K1 may be due to the very strong association of the substrates
with the micelles. The low value of K2 (K ꢁ K ) suggests that
2
1
[
N] = [N]
, and therefore, the nucleophile is present almost in
f
total
fill in the bulk phase. The consequence of the much lower value of
K2, i.e. the binding of nucleophile with micelles is negligible, which
conveys that only the substrate is distributed between micellar
and aqueous phases.
In the light of the above facts, we have inferred that the reac-
tion took place between the substrate solubilized into the micelle
Fig. 6. Fit of Eq. (12) to the experimental values of kobs for oxidative degradation
of NOR (1 × 10−2 mol −3) by CAT (1 × 10−3 mol dm−3) in varying concentrations of
Scheme 4. Formation of substrate detergent ternary associate to yield products
based on Raghvan–Srinivan’s assumption.
CTAB at 308 K.

8
A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
Scheme 5. Distribution of substrates between aqueous and micelle phases.
w
of kobs versus [Dn] if k 2m /V · K1 ꢀ kw. In the present investigation,
second order rate constants k2 (rate constant in aqueous pseudo
phase) and k 2m (rate constant in micellar pseudo phase) and are
portrayed in Scheme 5.
2
we observe there is a good relationship between the experimental
and theoretical values which predicts the micellar pseudo phase
model.
The phase-separation concept was used in order to calculate the
micelle concentration. It is assumed that the unassociated surfac-
tant concentration remains constant above the CMC of surfactant.
Thus, the [Dn] is defined as the micellized surfactant concentration:
5.5. Berezin’s model
The inhibition of rates at higher concentrations (beyond CMC)
of CTAB may be explained with the help of Berezin’s model [59],
which involves solubilization of both the reactants in the micellar
phase. The distribution of reactants can be shown as Scheme 6.
A quantitative rate expression for a bimolecular reaction occur-
ring in aqueous (kw path) and micellar (km path) phases for the
pseudo-first order rate constant is given by:
[
Dn] = [CTAB] − CMC
We know that the CMC depends on the additive concentra-
tion, therefore CMC was determined experimentally in each case
Table 3). In fact during the progress of the reaction, practically it
(7)
(
is difficult to determine the CMC of reaction mixture, therefore the
surfactant concentration from which we observed a kinetic effect
upon the reaction, was taken as the CMC calculated by Broxton [51]
ꢂ
kw + k K K (C
− CMC)
m
1
2
surf
−4
kobs = [
(13)
method (5.8 × 10 M) [69] in Eq. (7).
1 + K (C
− CMC)][1 + K (C
− CMC)]
1
surf
2
surf
According to the reaction in Scheme 5, the NOR is distributed
between the two pseudo phases, in a balanced process. In other
words, the NOR will be considered as a substrate, which is associ-
where K1 and K2 are the association constants of NOR and CAT
respectively with CTAB. C
is the analytical concentration of CTAB
surf
ꢂ
(k
= km/V), V being the molar volume of the micelle and kw and
ated with the micellar pseudo phase with a constant (K ) coincident
with the association CAT constant (K ). Sometimes, these con-
stants can easily be measured by different experimental techniques
m
1
km are the pseudo-first order rate constant in absence and pres-
ence of micelle, respectively. Since the oxidant is a neutral species
and the substrates are small molecules, the hydrophobic and elec-
trostatic interactions will not be very high. Since Csurf is very small
it may be possible that kw ꢀ kmK1K2(Csurf − CMC) so that Eq. (13)
takes the form as below
2
[
70,71], from Scheme 5 we have obtained the following equation
[
NOR]m
K1 = [
(8)
(9)
NOR] [Dn]
w
[
CAT]m
kw
K2 = [
kobs
=
(14)
CAT] [Dn]
2
w
1 + (K + K )(C
− CMC) + K K (C
− CMC)
1
2
surf
1
2
surf
The total NOR and CAT concentration is the sum of concentra-
tions in each pseudo phase
Again, since (C
− CMC) is very small, the terms containing
surf
Csurf − CMC)2 may be neglected and Eq. (14) may be rearranged
(
to:
[
NOR]total = [NOR]w + [NOR]m
CAT]total = [CAT]w + [CAT]m
(10)
(11)
1
kobs
1
kw
(K + K )
1
2
[
=
+
(Csurf − CMC)
(15)
kw
Based on Scheme 5 and according to the considerations above,
we have obtained the following expression for the observed rate
constant:
w
2
1
m
k
+ (k /V)K K [Dn]
1
2
2
kobs
=
[NOR]total
(12)
+ K [Dn] + K [Dn]
1
2
where V is the partial molar volume of the interfacial region in the
3
−1
micellar pseudo phase, determined by Bunton as 0.14 dm mol
[
w
72]. The reactivity constant in the aqueous phase, k2 has been cal-
culated kinetically and the association constants K , K2 have been
1
determined by the Raghvan and Srinivasan’s model. From the fit
of Eq. (12) to experimental data, the reactivity rate constant in the
m
micellar pseudo phase, k2 was calculated. Fig. 8 shows an example
of these fits, and Table 4 shows the obtained results for oxidative
degradation of NOR. Fig. 7 graphically represents the value of the
experimental rate constant versus the theoretical values that Eq.
(
12) yields using the parameters shown in Table 4. According to
Fig. 7. Experimental and theoretical values of kobs for the degradative oxidation of
Khan [52] Eq. (12) predicts the occurrence of maxima in the plots
NOR by CAT in the presence of CTAB micelles.

A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
9
Scheme 6. Distribution of substrates between aqueous and micelle phases.
In bimolecular micellar catalyzed reactions, the rate of reaction
increases due to changes in environment of reactants which reduce
the free energy difference between the initial and transition states.
This can be attributed to an increase in the frequency of molecular
collisions as a result of the close association of the two reacting
species at the micellar interface.
6. Effect of inorganic electrolyte on kobs
With ionic surfactants, the addition of electrolytes such as NaBr
or NaCl to the surrounding solution at first acts primarily to mod-
ify interactions between micelles. However, at sufficiently elevated
salt concentrations, micelles grow above their size in pure water.
In the presence of strongly binding counter ions, such as salicylate,
CTAB forms greatly extended thread like micellar pseudo-lattice
Fig. 8. Graph between 1/kobs and (Csurf − CMC) verifying the applicability of Berezin
model explaining the inhibition effect of CTAB at its higher concentrations.
[
73]. The salt effect on micellar catalysis should be considered in
the light of its competition with the substrate molecule which inter-
acts with the micelle electrostatically and hydrophobically causing
structural changes [74]. All the factors affecting the size, shape, CMC
and other properties of the micelles may affect their effectiveness
in altering reaction rates and pathways. One of these factors is due
to the existence of salts or organic additives in the micellar media
[73]. By including changes in the properties of micelles, salts and
organic additives control the extent of substrate solubilization and
counter ion binding to micelles [75]. It has been suggested that for
If we plot a graph between 1/kobs and (Csurf − CMC), a straight
line would be obtained with positive intercept on y-axis. Straight
line with positive intercept on y-axis is obtained by plot of 1/kobs
versus (Csurf − CMC) (Fig. 8). From the values of positive intercept
and positive slope of the plot (vide Eq. (15)), which is in confor-
mity of the rate law, values of kw and (K + K ) were calculated as
1
2
−3
−1
3
−1
1
.03 × 10
s
and 4.13 × 10 L mol respectively.
Table 5
Effect of sodium bisulfate and sodium sulfate on the oxidation of norfloxacine at 308 K.
NaHSO4] (×10 mol dm−3
4
)
[Na2SO4] (×10 mol dm
4
−3
)
[KCl] (×10 mol dm
4
−3
)
[KBr] (×10 mol dm
4
−3
)
[KNO3] (×10 mol dm
4
−3
)
kobs (×10
4
−1
s )
[
0
0
1
2
4
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
.0
.5
.0
.0
.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.0
0.5
1.0
2.0
4.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.73
5.83
5.87
8.83
9.06
2.73
3.68
4.57
5.10
11.36
2.70
2.78
2.82
3.00
3.20
3.49
2.75
9.67
13.86
14.17
14.24
2.73
06.56
10.48
11.29
13.36
–
0.0
0.5
1.0
2.0
4.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.0
1.0
2.0
3.0
4.0
5.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.0
0.5
1.0
2.0
4.0
–
–
–
–
Experimental conditions: [CAT] = 1 × 10−3 mol dm−3, [NOR] = 1 × 10−2 mol dm , [H ] = 0.2 mol dm
−3
+
−3
.

1
0
A. Shrivastava et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 1–11
1
1
1
1
1
1
5
4
3
2
1
0
9
8
7
6
5
4
3
2
oxidimetry. The potential of oxidation by a mild oxidant in micel-
NaHSO4
lar environment may be a breakthrough in further research where
less hazardous oxidants may be used for oxidative decomposition
of drugs in clinical effluents.
Na SO4
2
KCl
KBr
KNO3
Acknowledgments
We thank Professor L. Romsted, Department of Chemistry,
Wright-Rieman Laboratories, New Jersey, USA, for valuable discus-
sions. We also wish to thank reviewers for the critical and useful
comments that refined the manuscript a lot.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2012.04.004.
0
1
2
3
4
5
6
4
-3
[
Salt] x 10 mol dm
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