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Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) 6–13

Micellar effect on the kinetics of oxidation ofmalic acid by N-bromophthalimide

Sangeeta Patil a, Y.R. Katre a,∗, Ajaya Kumar Singh b

a Department of Chemistry, Kalyan Mahavidyalaya, Bhilai Nagar, Durg, Chhattisgarh 490006, Indiab Department of Chemistry, Government Vishwanath Yadav Tamaskar Post Graduate Autonomous College, Durg, Chhattisgarh, India

Received 27 January 2007; received in revised form 10 April 2007; accepted 4 May 2007Available online 16 May 2007

bstract

The effect of cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium lauryl sulphate (SDS) on the oxidationf malic acid (MA) by N-bromophthalimide (NBP) has been studied at 40 ◦C. The reaction exhibits first order dependence on [NBP], fractionalnd inverse fractional order dependence on [malic acid] and [HClO4], respectively. The effect of sodium perchlorate (NaClO4), phthalimide andercuric acetate has also been done. The rate of reaction increased with an increase in dielectric constant of the medium. CTAB strongly catalyzes

he reaction and typical kobs and [CTAB] profile was observed, i.e., with a progressive increase in [CTAB], the reaction rate increased, at higheroncentration constancy in kobs was observed, where as SDS slightly retards the reaction. The results are treated quantitatively in terms of Menger

nd Portnoy and Piszkiewicz models. The various activation parameters in presence of CTAB and SDS have been also evaluated. A suitableechanism consistent with the experimental finding has been proposed. The rate constants kobs, cooperativity index (n), binding constants (KS)ith surfactants has been evaluated. 2007 Elsevier B.V. All rights reserved.

auryl

aEtpmmsa

NNbll

eywords: N-bromophthalimide; Cetyltrimethylammonium bromide; Sodium l

. Introduction

The presence of micelles can have marked effects on ther-odynamic favourability and reaction kinetics [1] as well

s on many physical properties. Organic reactions involv-ng ionic, polar and neutral reactants in micellar solution areenerally believed to occur in the Stern layer of a micellef an ionic surfactant. The catalysis and inhibition by ionicicelles is due to ionic micellar incorporation of both the

eactants [2]. Due to these facts a significant amount of sys-ematic kinetic results have been reported on the effect oficelles on various organic reactions [3–18] during past few

ecades.Malic acid is an alpha-hydroxy organic acid. It is a key inter-

ediate in the major biochemical energy-producing cycle inells known as Kreb cycle that takes place in the cells mito-hondria in most living organisms. The body synthesizes malic

∗ Corresponding author.E-mail address: yokrajkatre@gmail.com (Y.R. Katre).

qoasitT

927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.05.015

sulphate; Malic acid; Oxidation; Kinetics

cid during the process of converting carbohydrates to energy.xtremely preliminary evidences suggest that individuals with

he disease fibromyalgia (a disorder that involves fatigue andain in the muscles) might have difficulty in creating or utilizingalic acid [19]. Such a deficiency could interface with normaluscle function. So it is very interesting to study the effect of

urfactants on biologically important hydroxy acid, i.e., maliccid.

A perusal of literature [20] shows that the reactivity of-bromophthalimide (NBP) could be compared with other-bromoimide such as N-bromosuccinimide (NBS) and N-romosaccharin (NBSA). Since NBP is more stable than theatter. It is extremely stable in solid state when kept out ofight and moisture. Its standard solution has excellent keepingualities. There are several reports available in the literaturen the oxidation of alpha-hydroxy acids by oxidants suchs N-bromosuccinimide [21], N-bromoacetamide [22], potas-

ium bromate [23], N-bromobenzenesuphonamide [24], andodate [25]. However, the details of micellar effect on oxida-ion of malic acid by N-bromophthalimide are yet unknown.his prompted us to study the micellar effect on the kinet-

Physicochem. Eng. Aspects 308 (2007) 6–13 7

im

2

2

rSiTSMaa2dSp

2

cvfmibmTta

3

[sutt

tw

Fig. 1. Plot of log[NBP] vs. time at 40 ◦C. 20% (v/v) acetica[[

4

cWio

5

cwap

aotsw

t

S. Patil et al. / Colloids and Surfaces A:

cs of the oxidation of the malic acid by NBP in the acidicedium.

. Experimental procedures

.1. Materials

N-bromophthalimide (Aldrich, Germany, 99%) was used aseceived. Melting point of the sample was found to be 481 K.olution of NBP was prepared in 80% acetic acid and stored

n a black coated flask to prevent photochemical deterioration.he prepared solution was standardized iodometrically [26].tandard solutions of SDS (GR) and Malic acid (Loba chem,umbai, India, 99%) were prepared in double distilled water

nd standard aqueous solutions of CTAB (GR) and mercuriccetate (Loba chem, Mumbai, India) was acidified with 40 and0% acetic acid, respectively. Perchloric acid (GR), diluted withouble distilled water, was standardized by acid–base titration.tandard solutions of KCl, KBr, NaClO4 and phthalimide wererepared with double distilled water.

.2. Kinetic measurements

The required solutions of malic acid, mercuric acetate, per-hloric acid (HClO4) and surfactant were taken in a reactionessel. The reaction vessel was covered with amber colored clothrom out side to avoid photochemical reaction [27]. The reactionixture was thermostated at the 40 ± 0.1 ◦C. The reaction was

nitiated with addition of required volume of thermally equili-rated N-bromophthalimide. Pseudo-first order conditions wereaintained in each kinetic run by keeping the [MA] � [NBP].he progress of reaction was followed titrimetrically. Aliquots of

he reaction mixture were pipetted out at different time intervalsnd unreacted NBP was estimated iodometrically.

. Stoichiometry and product analysis

The reaction mixture containing a known excess ofNBP] � [MA] was kept in presence of HClO4, Hg(OAc)2 andurfactant at 40 ◦C for 72 h. After completion of the reaction, thenconsumed NBP was estimated iodometrically. It was foundhat 1 mol of malic acid consumed 1 mol of NBP. Accordingly,he following stoichiometric equation can be formulated:

The presence of carbonyl compound (2-aldoethanoic acid) ashe main product of oxidation was detected by spot test [28] andith 2:4 dinitrophenyl hydrazine method [24].

5[

cid, [NBP] = 2 × 10−4 mol dm−3, [MA] = 3 × 10−2 mol dm−3,HClO4] = 5 × 10−3 mol dm−3, [Hg(OAc)2] = 7.5 × 10−4 mol dm−3, andCTAB] = 9 × 10−4 mol dm−3.

. Test for free radicals

To test for the presence of free radicals, a reaction mixtureontaining acrylamide was kept for 24 h in an inert atmosphere.

hen the reaction mixture was diluted with methanol, no precip-tate formation was seen. This suggests that there is no possibilityf formation of free radicals in the reaction.

. Results and discussion

Kinetic studies were conducted under the conditions whereoncentration of [Hg(OAc)2] � [NBP]. All the investigationsere made in the presence of Hg(OAc)2. Mercuric acetate obvi-

tes molecular bromine oxidation and ensures that the oxidationroceeds purely through NBP [29–32].

The kinetics was followed in binary solvent mixtures of aceticcid and water in presence of surfactants. The reaction is of firstrder in [NBP] as indicated by the linear plot of log[NBP] versusime (Fig. 1). The first order rate constants, evaluated from thelope of these plots, are reported in Table 1. It is observed thatith the increase in initial log[NBP], the value of kobs decreases.The rate constants were determined at different ini-

ial malic acid concentrations ranging from 5 × 10−3 to

0 × 10−3 mol dm−3, at constant values of [NBP], [CTAB],SDS], [Hg(OAc)2] and [HClO4] at 40 ◦C. The kobs increases

8 S. Patil et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) 6–13

Table 1Effect of [NBP], [MA] and [HClO4] on the oxidation of malic acid by NBP

104 [NBP]mol dm−3

102 [MA]mol dm−3

103 [HClO4]mol dm−3

% (v/v)acetic acid

105kobsa (s−1) Exp. 105kobs

a (s−1) Calc. 105kobsb (s−1) Exp. 105kobs

b (s−1) Calc.

1.0 3.0 5 20 28.9 28.6 4.24 4.201.5 3.0 5 20 19.6 19.5 6.85 6.802.0 3.0 5 20 15.6 15.1 3.71 3.752.5 3.0 5 20 11.5 11.7 2.94 2.983.0 3.0 5 20 10.4 10.2 2.56 2.504.0 3.0 5 20 7.28 7.23 1.94 1.905.0 3.0 5 20 – – 1.63 1.602.0 0.5 5 20 4.46 4.40 1.94 1.902.0 1.0 5 20 7.09 7.02 2.41 2.452.0 2.0 5 20 11.4 11.4 3.48 3.432.0 3.0 5 20 15.6 15.1 3.71 3.712.0 4.0 5 20 19.6 19.6 4.81 4.802.0 5.0 5 20 23.3 23.1 5.23 5.202.0 3.0 5 20 15.6 15.1 3.71 3.752.0 3.0 10 20 13.1 13.5 2.91 2.952.0 3.0 15 20 11.9 11.7 2.67 2.612.0 3.0 25 20 10.9 10.6 2.24 2.212.0 3.0 30 20 – – 2.1 2.02.0 3.0 40 20 9.2 9.26 – –2.0 3.0 50 20 8.52 8.50 1.71 1.752.0 3.0 5 20 15.6 15.1 3.71 3.752.0 3.0 5 25 14.6 14.3 3.21 3.262.0 3.0 5 30 13.1 13.5 2.95 2.952.0 3.0 5 40 11.1 11.4 2.45 2.472.0 3.0 5 50 9.3 9.28 1.91 1.98

k 3, [H

wosFcm

t(

F2m

lTk

t

obsa for [CTAB] = 9 × 10−4 mol dm−3 and kobs

b for [SDS] = 8 × 10−3 mol dm−

ith increase in concentration of malic acid (Table 1). The plotf log kobs versus log[MA] was linear with slope less than unityhowing fractional orders dependence on the malic acid (Fig. 2).urther, a plot of 1/kobs versus 1/[MA] was linear with an inter-ept on y-axis, confirming the fractional order dependence on

alic acid (Fig. 3).As the concentration of HClO4 increases (from 5 × 10−3

o 50 × 10−3 mol dm−3) the value of rate constant decreasesTable 1). This may be due to protonation of substrate. The plot of

ig. 2. Plot of log[MA] vs. log kobs. 40 ◦C, 20% (v/v) acetic acid, [NBP] =× 10−4 mol dm−3, [HClO4] = 5 × 10−3 mol dm−3, [Hg(OAc)2] = 7.5 × 10−4

ol dm−3, [CTAB] = 9 × 10−4 mol dm−3, and [SDS] = 8 × 10−3 mol dm−3.

esms

F2m

g(OAc)2] = 7.5 × 10−4 mol dm−3, and temperature = 40 ◦C.

og kobs versus log[HClO4] is linear with negative slope (Fig. 4).he slope being less than unity indicates inverse fractional orderinetics in HClO4.

Change in NaClO4 (to study the effect of ionic strength ofhe medium) to the reaction mixture showed an insignificantffect. Addition of mercuric acetate in the reaction mixture also

howed an insignificant effect on the rate of reaction, suggestingercuric acetate acting as bromide ion scavenger only. Succes-

ive addition of phthalimide (as one of the oxidation products of

ig. 3. Plot of 1/kobs vs. 1/MA. 40 ◦C, 20% (v/v) acetic acid, [NBP] =× 10−4 mol dm−3, [HClO4] = 5 × 10−3 mol dm−3, [Hg(OAc)2] = 7.5 × 10−4

ol dm−3, [CTAB] = 9 × 10−4 mol dm−3, and [SDS] = 8 × 10−3 mol dm−3.

S. Patil et al. / Colloids and Surfaces A: Physi

Fig. 4. Plot of log[HClO4] vs. log kobs at 40 ◦C. 20% (v/v) acetica[[

Nr

om(tottrsw[i

6

ipaiN

N

N

N

H

teotectioesoae

cid, [NBP] = 2 × 10−4 mol dm−3, [MA] = 3 × 10−2 mol dm−3,Hg(OAc)2] = 7.5 × 10−4 mol dm−3, [CTAB] = 9 × 10−4 mol dm−3, andSDS] = 8 × 10−3 mol dm−3.

BP) to the reaction mixture showed decreasing effect on theate.

In order to find the effect of dielectric constant (polarity)f the medium on the rate, experiments in aqueous acetic acidixtures of varying compositions, were performed. The data

Table 1) clearly reveal that the rate increases with decrease inhe percentage of acetic acid, i.e., increasing dielectric constantr polarity of the medium leads to a charge development inhe transition state involving a more polar activated complexhan the reactant [33]. The effect of added salts on the rate ofeaction was also explored because salts as additives, in micellar

ystems, acquire a special ability to induce structural changeshich may, in turn, modify the substrate–surfactant interaction

1]. In the present case, KCl has no effect whereas with thencreasing concentration of KBr, rate of reaction increased.

it

a

Scheme 1

cochem. Eng. Aspects 308 (2007) 6–13 9

. Mechanism

It has been reported earlier by several workers [34] that NBPs a stable oxidizing and brominating agent because of largeolarity of N Br bond. NBP is capable of producing Br− reason-bly shows that NBP, like other similar N-halo imides, may existn various forms in acidic medium, that is, free NBP, protonatedBP, Br+, HOBr, (H2OBr)+, as per the following equilibria:

BP + H2O � HOBr + NHP (1)

BP + H+ � NHP + Br+ (2)

BP + H+ � (NBPH)+ (3)

OBr + H+ � (H2OBr)+ (4)

Addition of phthalimide in the reaction mixture decreaseshe rate of oxidation in acidic media suggesting that the pre-quilibrium step involves a process in which phthalimide is onef the products. When NBP or (NBPH)+ is assumed as the reac-ive species, the derived rate laws fail to explain the negativeffect of phthalimide. Hence neither of these species can beonsidered as the reactive species. When (H2OBr)+ is taken ashe reactive species, the rate law obtained shows first order kinet-cs with respect to hydrogen ion concentrations, contrary to ourbserved negative fractional order in [HClO4]. Although it fullyxplains the negative effect of phthalimide, therefore, the pos-ibility of cationic bromine (Br+) as reactive species is ruledut. Thus, the only choice left is HOBr, which, when considereds the reactive species of NBP, leads to a rate law capable ofxplaining all the kinetic observations and other effects. Hence,

n the light of kinetic observations, HOBr can safely be assumedo be the main reactive species of NBP for the present reaction.

On the basis of above experimental findings, taking HOBrs the most reactive species of NBP, Scheme 1 can be proposed

.

1 : Physicochem. Eng. Aspects 308 (2007) 6–13

fN

−

w

t

7

7

cmiiwrtsiSmamcitCatSmmCtrf

TCe

S

1234

Lf2m

Table 3Effect of [Surfactant] on reaction rate

103 [Surfactant]mol dm−3

For CTAB 105

kobs (s−1)For SDS 105

kobs (s−1)

0 5.43 5.432.0 – 4.762.5 – 4.504.0 – 4.435.0 – 4.296.0 – 3.968.0 – 3.759.0 – 3.25

20.0 6.29 –30.0 8.16 –40.0 9.63 –50.0 11.9 –60.0 12.2 –70.0 13.0 –80.0 14.8 –90.0 15.1 –

100.0 14.8 –110.0 14.1 –130.0 14.5 –

[7a

o[

7

acMimto

0 S. Patil et al. / Colloids and Surfaces A

or micellar effect on the kinetics of oxidation of malic acid byBP in acidic medium.The rate of disappearance of NBP is given as

d[NBP]

dt= kK1K2[MA][NBP]T

[NHP][H+] + K1K2[MA]

here [NBP]T = [NBP] [HOBr] [X−].The above rate law is in good agreement with the experimen-

al results in absence of surfactant.

. In presence of surfactant

.1. CMC determination

Surfactants spontaneously aggregate above a certain con-entration called critical micelle concentration (CMC) to formicelle [35], whose determination has considerable practical

mportance, normally to understand the self-organizing behav-or of surfactants in exact ways. Micelles act as microreactors,hich both speed or inhibit the rate of uni- and bimolecular

eactions [36–38]. Micelle aggregates affect chemical reac-ivity primarily by binding or excluding reactants and onlyecondarily by changing the free energy of activation. The crit-cal micelle concentration values of the surfactant (CTAB andDS) were determined conductometrically (Digital conductivityeter, model 611E, Electronic India Company) in the presence

nd absence of reactants at 40 ◦C. The CMC values were deter-ined from plots of the specific conductivity versus surfactant

oncentration. The breakpoints of nearly straight-line portionsn the plot are taken as an indication of micelle formation, andhis correspond to the CMC of surfactant. The CMC values ofTAB and SDS in different experimental conditions at 40 ◦Cre summarized in Table 2. The CMC values are lower thanhose given in the literature for aqueous solutions of CTAB andDS without added electrolyte, which were found to be approxi-ately about 2.0 × 10−4 and 3.16 × 10−3 mol dm−3 in reactionixture for malic acid, respectively. The difference among the

MC values arises from the well-known effect of added elec-

rolyte, which lowers the CMC by causing a decrease in theepulsion between the polar head groups at the micelle sur-ace. Our results of CMC decrease are in accord with the earlier

able 2ritical micelle concentration (CMC values of CTAB and SDS in differentxperimental conditions)

olutions 104 CMC(mol dm−3) CTAB

103 CMC(mol dm−3) SDS

. Water 10.0 8.5

. Water + NBP 9.3 7.8

. Water + malic acid 9.0 5.0

. Water + malic acid + NBPHg(OAc)2 + HClO4 + 20%(v/v) acetic acid

2.0 3.6

iterature value for CTAB at 25 ◦C = 9.2 × 10−4 mol dm−3; Literature valueor SDS at 25 ◦C = 8.1 × 10−3 mol dm−3. [MA] = 3 × 10−2 mol dm−3, [NBP] =× 10−4 mol dm−3, [Hg(OAc)2] = 7.5 × 10−4 mol dm−3, [HClO4] = 5 × 10−3

ol dm−3, 20% acetic acid, and temperature = 40 ◦C.

iaw4ticmiGkicpbtSko

NBP] = 2 × 10−4 mol dm−3, [MA] = 3 × 10−2 mol dm−3, [Hg(OAc)2] =.5 × 10−4 mol dm−3, [HClO4] = 5 × 10−3 mol dm−3, 20% (v/v) acetic acid,nd temperature = 40 ◦C.

bservations of micelle formation at relatively low surfactant39–41].

.2. Influence of CTAB on kobs

Surfactant micelles provide an unusual medium, which mayffect the rate of reaction. The fundamental process in micellaratalysis or inhibition is the counter ion binding with micelle.icelles can either attract the reactive ions or repel them depend-

ng upon the electrical charge on their head groups. Thus,icelles may bring the solubilized substrate and reactive ions

ogether or keep apart such that the reactions are catalyzedr inhibited. The effect of CTAB micelles on the rate wasnvestigated by performing the reaction in presence of varyingmounts of CTAB. The values listed in Table 3 show an increaseith increase in [CTAB] from nil to 10 × 10−4 mol dm−3 at0 ◦C. The catalytic effect of CTAB on kobs is explained inerms of pseudophase model of micelles. With a progressivencrease in [CTAB] the reaction rate increased, at higher con-entration a near constancy in kobs was observed. Most of theicellar mediated organic reactions are believed to occur either

nside the Stern layer or at the junction region of Stern orouy–Chapman layer [42]. The main factor involved in theinetic micellar effect that the electrostatic and hydrophobicnteraction between substrate and surfactant increases the con-entration of reactants into a small volume. In present case,ositively charged micellar surface absorb ionized malic acid

earing negative charge in the Stern layer. As the concen-ration of CTAB increases local molarity of reactants in thetern layer increased and therefore, the enhancement in theobs observed. At higher concentration constancy in kobs wasbserved.

S. Patil et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) 6–13 11

7

ti

ih

8

ac

8

btaomatocr

w(mKaoCf(

8

i

Table 4Other parameters for the oxidation of malic acid by NBP in presence of CTABand SDS

Parameters In presence of CTAB In presence of SDS

KS mol−1 dm3 2.47 × 103 5.31 × 102

km (s−1) 1.46 × 10−4 2.78 × 10−5

−log KD 3.4 0.27n 2.33 1.21

otOm[antttats

k

a

l

fsaiac

9

Scheme 2.

.3. Influence of SDS on kobs

A continuous decrease in the rate constant was observed withhe increase in [SDS] at constant [MA] and [NBP](Table 3). Thenhibition effect is due to the fact that N-bromophthalimide hasN Br bond which it binds to SDS micelles in Stern layer, whileonized malic acid, bearing negative charge is repelled by theead group of negatively charged SDS micelles.

. Qualitative treatment of micellar catalysis

The data of kobs versus [CTAB] and [SDS] plots have beennalyzed in terms of Menger and Portnoy’s model for micellar-atalyzed reactions reported earlier.

.1. Menger and Portnoy’s model

Catalysis of micelles involves at least three main steps (i)inding of the substrate to the micelle, (ii) the actual chemicalransformation in the micelle (usually at the micellar surface),nd (iii) release of product(s). Menger and Portnoy [43] devel-ped a pseudophase model, which has been fitted to variousicellar-catalyzed reactions. According to this model the vari-

tion of the rate constant with surfactant is generally treated onhe assumption that substrate ‘S’ is distributed between the aque-us and micellar-phases as given in Scheme 2 where D is theoncentration of surfactant. This model leads to the followingelationship for micellar catalysis:

1

kw − kobs= 1

kw − km+ 1

(kw − km)Ks[Dn](8)

here Eq. (8) predicts that a plot of (kw − kobs)−1 versus [Dn]−1

Dn = [D] − CMC) should be linear. Menger and Portnoy’sodel allow us to determine kinetically the binding constants and the rate constant km in micellar phase. The value of KSnd km were calculated from the slope and intercept of the plotf (kw − kobs)−1 versus [Dn]−1.The Ks values of malic acid inTAB and SDS, under the present conditions of kinetics were

ound to be 2.47 × 103 and 5.31 × 102 mol−1 dm3, respectivelyTable 4).

.2. Piszkiewicz’s model

The micellar catalysis or inhibition could be applied theoret-cally by making certain simplifications and assuming that only

mti

Scheme 3.

ne substrate is incorporated into micelle and that the aggrega-ion number N of the micelle is independent of the substrate.n the basis of these assumptions Piszkiewicz [44] proposed aodel for micellar catalyzed reaction analogous to Hill model

45] of enzyme kinetics. This model is applicable especiallyt low surfactant concentration. This model assumes that ‘n’umber of surfactant molecules (D) and substrate (S) aggregateo yield the catalysis aggregate DnS which then reacts to yieldhe product (P). This is represented by Scheme 3 where KD ishe dissociation constant of micelle back to its free componentsnd km is the rate of reaction within the micelle. In Scheme 3he observed rate constant (kobs) is expressed as a function ofurfactant concentration D, by the equation:

obs = km[D]n + kwKD

KD + [D]n(9)

Following rate expression was obtained on rearrangementnd taking log of Eq. (9)

og

(kobs − kw

km − kobs

)= n log[D] − log KD (10)

The values of positive cooperativity (n) are 2.33 and 1.21or CTAB and SDS, respectively. The value of n is less than 6hows good agreement with earlier observations of Piszkiewicznd is viewed as indices of positive cooperativity that is inducednteraction of the additional substrate molecule due to the inter-ction of micelle with the first substrate molecule. log KD wasalculated from the intercept of the plot (Fig. 5; Table 4).

. Activation parameters

Activation parameters are believed to provide useful infor-ation regarding the environment in which chemical reactions

akes place. The effect of temperature on rate constant was stud-ed in the range 303–318 K. The results are shown in Table 5.

12 S. Patil et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 308 (2007) 6–13

Table 5Temperature effect and activation parameters

Parameters In absence of surfactant 105 kobs (s−1) In presence of CTAB 105 kobs (s−1) In presence of SDS 105 kobs (s−1)

303 – 7.08 –308 3.08 9.26 2.28313 5.43 15.1 3.75318 7.90 22.1 5.58323 10.2 – 7.90Ea (kJ mol−1) 71.8 67.0 76.5log Pz 7.72 7.37 8.4�H# (kJ mol−1) 69.2 64.5 73.9�S# (J K−1 mol−1) −105.8 −112.4 −93.9�G# (kJ mol−1) 102.4 9

[NBP] = 2 × 10−4 mol dm−3, [MA] = 3 × 10−2 mol dm−3, [Hg(OAc)2] = 7.5 × 10−[SDS] = 8 × 10−3 mol dm−3, and 20% (v/v) acetic acid.

Fig. 5. Graph showing positive cooperativity. 20% (v/v) acetic acid,[51

Fwfewi

F2mm

CTtis

1

tibnprlodrt

MA] = 3 × 10−2 mol dm−3, [NBP] = 2 × 10−4 mol dm−3, [HClO4] =× 10−3 mol dm−3, [Hg(OAc)2] = 7.5 × 10−4 mol dm−3, [CTAB] = 9 ×0−4 mol dm−3, and [SDS] = 8 × 10−3 mol dm−3.

rom Arrhenius plot (Fig. 6), the value of activation energy (Ea)as calculated and the values of �S# and �G# were computed

rom Eyring equation (Table 5). A higher value of Ea in pres-nce of SDS shows the inhibition effect on the rate of reactionhereas a low value in the presence of CTAB shows the increas-

ng effect. The large negative value of �S# in the presence of

ig. 6. Arrhenius plots for the oxidation of malic acid by N-bromophthalimide.0% (v/v) acetic acid, [NBP] = 2 × 10−4 mol dm−3, [MA] = 3 × 10−2

ol dm−3, [HClO4] = 5 × 10−3 mol dm−3, [Hg(OAc)2] = 7.5 × 10−4

ol dm−3, [CTAB] = 9 × 10−4 mol dm−3, and [SDS] = 8 × 10−3 mol dm−3.

omst

A

CWCv

R

9.7 103.7

4 mol dm−3, [HClO4] = 5 × 10−3 mol dm−3, [CTAB] = 9 × 10−4 mol dm−3,

TAB indicates that more ordered activated complex is formed.he fairly high positive values of �H# and �G# indicate that the

ransition state is highly solvated. The nearly same value of �G#

n absence and in presence of surfactants indicates operation ofimilar reaction mechanism in both the cases.

0. Conclusion

In the light of kinetic observations for the micellar effect onhe kinetics of oxidation of malic acid by N-bromophthalimiden the presence of perchloric acid, the following conclusions cane easily drawn: the reactive species of oxidant NBP is HOBrot NBP itself. Oxidation products were identified. Activationarameters were evaluated for both catalyzed and unanalyzedeactions. The critical micelle concentration values are muchower than those given in the literature for aqueous solutionsf CTAB and SDS without added electrolyte. The rate of oxi-ation increases with increasing concentration of CTAB whileate of oxidation slightly decreases with increasing concentra-ion of SDS. The micellar effect can be correlated with the naturef the reducing substrates and the reactions conditions. Theseicellar effects are quite important to understand and to sub-

tantiate the proposed mechanistic pathways. This may widenhe applicability of NBP oxidimetry in organic synthesis.

cknowledgements

One of the authors (YRK) thanks the University Grantsommission, New Delhi for the Minor Research Project grant.e also thank Professor Laurence S. Romsted, Department ofhemistry, Wright–Rieman Laboratories, New Jersey (USA) foraluable discussions.

eferences

[1] J.H. Fendler, E.J. Fendler, Catalysis in Micellar and Macromolecular Sys-tems, Academic Press, New York, 1975.

[2] E.H. Cordes, R.B. Dunlap, Acc. Chem. Res. 2 (1969) 329.

[3] M. Chiarini, G. Cerichelli, H.J. Foroudian, N.D. Gillitt, S.F. Yunes, C.A.

Bunton, Langmuir 20 (2004) 5201.[4] M. Chiarini, C.A. Bunton, Colloids Surf. A 245 (2004) 177.[5] M. Chiarini, N.D. Gillitt, C.A. Bunton, Langmuir 18 (2002) 3836.[6] M. Chiarini, C.A. Bunton, Langmuir 18 (2002) 8806.

Physi

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S. Patil et al. / Colloids and Surfaces A:

[7] Kabir-ud-Din, S. Kumar, Z. Khan, Colloids Surf. 168 (2000) 241.[8] Kabir-ud-Din, M.S. Ali, Z. Khan, Int. J. Chem. Kinet. 38 (2006) 18.[9] S. Pandey, S.K. Upadhyay, Ind. J. Chem. 44A (2005) 1822.10] H. Yao, D.E. Richarson, J. Am. Chem. Soc. 125 (2003) 6211.11] M. Kanthimathi, B.U. Nair, Int. J. Chem. Kinet. 36 (2004) 79.12] Kabir-ud-Din, A. Mohammad, A. Morshed, Z. Khan, Int. J. Chem. Kinet.

35 (2003) 543.13] B. Pare, P. Kaur, V.W. Bhagwat, C. Fragliani, J. Korean Chem. Soc. 48

(2004) 195.14] Kabir-ud-Din, K. Hartani, Z. Khan, Int. J. Chem. Kinet. 33 (2001) 377.15] S. Patil, Y.R. Katre, Int. J. Chem. Sci. 4 (2006) 311.16] G.K. Joshi, Y.R. Katre, A.K. Singh, J. Surf. Deter. 9 (3) (2006) 231.17] Y.R. Katre, A.K. Singh, S. Patil, G.K. Joshi, Oxid. Commun. 29 (2006)

129.18] Y.R. Katre, A.K. Singh, G.K. Joshi, S. Patil, Oxid. Commun. 29 (2006)

137.19] G.E. Abraham, J.D. Flechas, J. Nutr. Med. 3 (1992) 49.20] O. Abou, M.I. Walash, M. El-Kerdawy, El-Ashry, J. Drug Res. 12 (1980)

77.21] R. Natarajan, N. Venkatasubramanium, Ind. J. Chem. 13 (1975) 261.22] M.L. Bishnoi, K.K. Banerji, Ind. J. Chem. 25A (1986) 660.23] R. Natarajan, N. Venkatasubramanian, Int. J. Chem. Kinet. 8 (2004)

205.24] A. Mathur, V. Sharma, K.K. Banerji, Ind. J. Chem. 27A (1988) 123.25] A. Brahmaiah, P. Manikyambo, Ind. J. Chem. 34A (1995) 900.26] M.Z. Barkat, M.F. Abd. El-Wahab, Anal. Chem. 26 (1954) 1973.27] V. Thiagarajan, S. Ramakrishanan, Ind. J. Chem. 37B (1998) 113.

[

[[[

cochem. Eng. Aspects 308 (2007) 6–13 13

28] F. Feigl, Spot Tests in Organic Analysis, Elsevier Scientific PublishingCompany, New York, 1975, p. 483.

29] A. Zachariah, Asian J. Chem. 15 (2003) 1567.30] T.D. Nair, R.A. Zachariah, Asian J. Chem. 9 (1997) 297.31] R. Natarajan, N. Venkatasubramanian, Ind. J. Chem. 13 (1975) 261.32] Ch.S. Reddy, T.V. Kumar, Ind. J. Chem. 34A (1995) 871.33] K.J. Laidler, Pearson Education, New Delhi, 2004, pp. 197.34] S.F.A. Jabbar, V.S. Rao, Ind. J. Chem. 33A (1994) 69.35] J. Israelachvili, Intermolecular and Surface Forces, second ed., Academic

Press, London, 1991.36] L.S. Romsted, K.L. Mittal, B. Lindman (Eds.), Surfactants in Solution, vol.

II, Plenum Press, New York, 1984, pp. 1015.37] H. Chaimovich, F.M.V. Aleixo, I.M. Cuccovia, D. Zanette, F.H. Quina, in:

K.L. Mittal, E.J. Fendler (Eds.), Solution Behavior of Surfactants: Theo-retical and Applied Aspects, vol. II, Plenum Press, New York, 1982, pp.949.

38] C.A. Bunton, M. Gratzel, K. Kalyanasundaram (Eds.), Kinetics and Catal-ysis in Microheterogeneous Systems, Marcel Dekker, New York, 1991, p.13.

39] J. Steigmam, N. Shane, J. Phys. Chem. 69 (1965) 968.40] F.M. Menger, J.M. Jerkunica, J. Am. Chem. Soc. 101 (1979) 1896.41] B.E. Gillespie, M.J. Smith, P.A.H. Wyatt, J. Chem. Soc. (1969) 1896.

42] L.S. Romsted, K.L. Mittal (Eds.), Micellization, Solubilization and

Microemulsion, vol. II, Plenum Press, New York, 1971.43] F.M. Menger, C.E. Portnoy, J. Am. Chem. Soc. 89 (1967) 4698.44] D.J. Piszkiewicz, Am. Chem. Soc. 99 (1977) 7695.45] A.V. Hill, J. Physiol. 40 (1910) 4.