influence of micellar propinquity on dynamics of ce(iv)-catalyzed bz oscillatory reaction under...

Influence of MicellarPropinquity on Dynamics ofCe(IV)-Catalyzed BZOscillatory Reaction underStirred ConditionsOYAIS AHMAD CHAT, MUZAFFAR HUSSAIN NAJAR, UZMA ASHRAF, SURAYA JABEEN,MASRAT MASWAL, RAIS AHMAD SHAH, ROHI MASRAT, AIJAZ AHMAD DAR

Department of Chemistry, University of Kashmir, Srinagar 190 006, India

Received 10 August 2013; revised 3 December 2013; accepted 18 February 2014

DOI 10.1002/kin.20851Published online 10 April 2014 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Oscillating reactions often employed to mimic and understand complex dynamicsin biological systems are known to be affected in aggregated host environments. The dynamicevolution of the oscillatory Belousov–Zhabotinsky (BZ) reaction upon addition of increasingamounts of anionic (sodium dodecylbenzenesulfonate; SDBS), cationic (hexadecyltrimethy-lammonium bromide; CTAB), nonionic (polyoxyethylene(20) cetyl ether; Brij58), and binarymixtures (CTAB + Brij58 and SDBS + Brij58) of surfactants was monitored using potentiom-etry at 25 and 35◦C under stirred batch conditions. The experimental results reveal that theoscillatory parameters of the Ce(IV)-catalyzed BZ reaction are significantly altered dependingon the concentration and nature of restricted micellar host environments. In the presence ofionic surfactants, it is proposed that the evolution of the oscillatory BZ system may be due toatypical proficiency (related to hydrophobic and electrostatic interactions) of such organizedself-assemblies to affect the reactivity by selectively confiscating some key reacting species.However, the response of the BZ system to nonionic Brij58 was attributed to the reaction amongthe alcoholic functional groups of the surfactant with some vital species of the BZ reaction.Moreover, the nonionic + ionic binary surfactant systems exhibited behaviors representativeof both the constitutive single surfactant systems. C© 2014 Wiley Periodicals, Inc. Int J ChemKinet 46: 351–362, 2014

INTRODUCTION

Oscillating reactions are ubiquitous in nature appear-ing from molecular to supracellular levels during bio-

Correspondence to: Aijaz Ahmad Dar; e-mail: aijaz [email protected].

C© 2014 Wiley Periodicals, Inc.

logical organization. While heart beating is the mostobvious of rhythms that sustain life, others include hor-mone cycles, circadian rhythms, and insulin secretionfrom β-cells in pancreas [1–6]. Owing to greater com-plexity of biological systems, studying in vitro oscil-latory chemical reactions plays a pivotal role in under-standing of such complex dynamics in the biologicalsystems [7]. Among the model systems, the

352 CHAT ET AL.

paradigmatic Belousov-Zhabotinsky (BZ) reaction isone of the most famous and extensively studied chem-ical oscillatory reactions [8–13] for understanding oftemporal, spatial, and spatio-temporal nonlinear dy-namics in nonequilibrium systems. The BZ reactionconsists of the bromination and oxidation of an or-ganic substrate (like malonic acid) in a strongly acidicmedium in the presence of a one electron–redox coupleas a catalyst such as Ce(IV)/Ce(III) and ferroin/ferrin[14,15]. Several models have been proposed [9,16,17]to explain a mechanism of BZ reactions and amongthem the Marburg–Budapest–Missoula (MBM) is con-sidered to be one of the most reliable [18].

A perturbation given to the system often bringsabout macroscopic changes in the spatiotemporal be-haviors of the BZ reaction [19]. Changes in the externalenvironment of an oscillatory chemical reaction by ap-plication of an electric field, a magnetic field, or lightcan be used to control the dynamics of the BZ oscilla-tory reaction [20–22]. Even an addition of nonreactingchemical species induces effective changes in the pat-tern dynamics internally. For example, self-assembledaggregates of surfactants have been employed to com-partmentalize crucial intermediates to affect the tem-poral dynamics of the BZ reaction [23–26]. The per-turbation experiments often give a new insight abouta reaction mechanism in addition to controllability ofthe pattern formation.

Surfactants are surface-acting agents that belong tothe wide class of the amphiphilic compounds that forma great variety of self-assembling structures in solu-tion, all of which share the characteristics of com-partmentalization between polar and nonpolar regions,with defined boundary interfaces [27,28]. Moreover,the compartmentalization due to the presence of mi-celles gives a biomimetic character to such systems andallows studying new features belonging to the biologi-cal realm (synchronization, communication, coupling,etc.).

The combination of a chemical oscillator withconfined reaction environments is a subject exten-sively studied during the past years. Surfactant-formingmicelles (direct and reverse) revealed to be veryfascinating and promising [23–25,29–34] due to theirability to selectively interact via electrostatic and/orhydrophobic interactions with the reaction substratesor products present in the medium [35,36]. The worksof Liveri et al. and others have exhaustively demon-strated the effects of different types of self-assembledadditives such as surfactants and/or polymers on theoscillatory parameters of the BZ oscillatory reactionsuch as an induction period and oscillation time pe-riod [23–26,29–31,37–43]. However, the literature has

scarcely highlighted the role of mixed surfactant sys-tems on the dynamics of the BZ oscillatory reaction.Pertinently, the surfactant mixtures have more naturalabundance due to their efficient solubilization, suspen-sion, dispersion, transportation, and catalytic proper-ties [44], so studying the influence of binary mixtureson complex BZ reactions deserves attention. Such stud-ies using self-assembled soft systems have interestedresearchers not only for elucidating a complex mecha-nism of oscillatory reactions but also for their relevanceto periodic phenomena observed in biochemical sys-tems.

Keeping in view the biomimetic nature of mi-celles in relation to biological membranes and as acontinuation of our previous work carried out undernonstirred conditions using spectrophotometry [45],we present the experimental observations on the ef-fect of different surfactant self-assemblies involv-ing cationic surfactant (hexadecyltrimethylammoniumbromide; CTAB), anionic surfactant (sodium dode-cylbenzenesulfonate; SDBS), and neutral surfactant(polyoxyethylene(20) cetyl ether; Brij58) and theirequimolar binary mixtures on the Ce(IV)-catalyzedBZ oscillatory reaction at two different temperatures25 and 35◦C under stirred closed conditions. Changesin the reaction kinetics were also found to be de-pendent on the nature of the surfactant and extentof the perturbation due to the self-assembling prop-erties of the surfactants. Such experimental observa-tions are expected to contribute to understanding ofthe nonlinear dynamic phenomena of BZ oscillatoryreactions.

MATERIALS AND METHODS

Materials

Malonicacid (MA), Ce(SO4)2, KBrO3, and H2SO4

were of commercial grade and analytical quality(Fluka, Steinheim, Germany) and were used withoutfurther purification. CTAB, SDBS, and Brij58 wereobtained from Aldrich (Steinheim, Germany). CTABwas used after recrystallization three times from anacetone–methanol mixture. SDBS was purified by re-crystallization from ethanol, and Brij58 was used asreceived.

Stock solutions of all the reagents were preparedin triple-distilled water. Equimolar CTAB + Brij58and SDBS + Brij58 binary mixtures were preparedby mixing appropriate volumes of surfactant solutions.Stock solutions of H2SO4 were standardized by acid–base titrations.

International Journal of Chemical Kinetics DOI 10.1002/kin.20851

DYNAMICS OF Ce(IV)-CATALYZED BZ OSCILLATORY REACTION UNDER STIRRED CONDITIONS 353

Methods

Potentiometric Measurements. The dynamics of theBZ system was followed under a constant stirred batchcondition by monitoring the variation in redox poten-tial of the solution as a function of time. The mea-surements were carried out in a thermostated cylin-drical glass reaction vessel (diameter 3.5 cm, height7.5 cm) closed with a rubber stopper. The poten-tiometric measurements of the oscillating solutionswere carried out under constantly stirred (50 rpm)batch conditions using a digital Cyberscan multimeter(model PC-5500) connected to a personal computerusing a commercial platinum electrode with calomelas a reference. The volume of the reaction mixturewas always 10 cm3. The solution was stirred mag-netically with a Teflon-coated stirrer (length 2.0 cm,diameter 0.8 cm). The temperature was maintainedwithin ±0.1◦C using a Thermo Scientific thermostaticcirculating water system assembled in a homemadedesign.

The oscillating mixtures for the kinetic runs wereprepared by mixing freshly prepared aqueous stocksolutions of MA, KBrO3, and surfactants (at desiredconcentrations) with Ce(IV) in 0.5 mol dm−3 H2SO4.Appropriate volumes of the stock solutions were mixedin a 10-mL glass reactor to obtain the following initialconcentrations of reactants:

[MA] = 0.4 mol dm−3 [Ce (IV)] = 0.005 mol dm−3

[KBrO3] = 0.1 mol dm−3 [H2SO4] = 0.5 mol dm−3

The experiments were started by adding the Ce(IV)solution. Kinetic runs were carried at 25 and 35◦C instirred closed conditions.

Surface Tension Measurements. The presence of sta-ble micelles under reaction conditions was ensured us-ing tensiometry. Surface tension measurements weremade with a Kruss 9 tensiometer by the platinum ringdetachment method. The critical micelle concentra-tion (cmc) values in these reaction conditions weredetermined from the surface tension (γ ) versus thelogarithm of surfactant concentration (log C). Thesurfactant concentration was varied by adding con-centrated surfactant solution in small installments toa 30-mL solution containing 0.4 mol dm−3 MA,0.1 mol dm−3 KBrO3, and 0.5 mmol dm−3 H2SO4

using a Hamilton microsyringe, and readings weretaken after thorough mixing and temperature equili-bration. The temperature was maintained at 25 and35◦C (±0.1◦C) by circulating water from a Haake GHthermostat.

RESULTS AND DISCUSSION

Induction Period of the BZ Reaction in thePresence of Surfactant Systems

Figure 1 shows the temporal variation of potential dur-ing the BZ reaction under stirred batch conditions inthe absence of a perturbant (surfactant) at 25 and 35◦C.The results show that the potential of the solution de-creases monotonically until the so-called induction pe-riod (IP), where after the chemical oscillations beginwith an oscillation period (τ ). IP shows more thana twofold decrease on increasing the temperature by10◦C. Using surfactants of varied architectures as per-turbants to the BZ reaction, the oscillatory parameterssuch as IP and τ are affected by varying degrees, thechanges being characteristic of the type and concen-tration of the amphiphiles.

The influence of the surfactant concentration on theIP values of the BZ reaction using different single andbinary surfactant systems at two different temperaturesis shown in Figs. 2 and 3 along with the cmc valuesof each surfactant system obtained using surface ten-sion measurements. As observed from the figures, thevariation of IP is almost same at both the temperaturesin the presence of perturbants. However, the influenceis more pronounced at lower temperature (25◦C). Ingeneral, IP compared to the pure BZ system (in theabsence of the surfactant) increases by first addition ofthe surfactant in the pre-cmc range in the presence ofall the single surfactant systems. Furthermore, in thecase of CTAB and Brij58, the IP decreases with the in-crease in the concentration of the amphiphiles, whereasit first increases and then decreases in the presence ofanionic SDBS for the single surfactant systems. In thecase of binary mixtures, the IP decreases with an in-crease in the concentration of CTAB + Brij58, whereasit first increases and then decreases with an increase inthe concentration of the SDBS + Brij58 mixture. Thisreport on experimental observations on effects of dif-ferent micellar environments on dynamics of the BZoscillatory reaction is fairly in good agreement with ourprevious result [45] carried out under unstirred condi-tions in the presence of CTAB and SDBS. However,in the presence of Brij58 and its binary mixture withSDBS (SDBS + Brij58) instead of having a continuousincrease in IP under unstirred conditions, we observeda continuous decrease in IP with the addition of Brij58,whereas an increase is followed by a decrease withthe addition of the SDBS + Brij58 surfactant mixtureunder stirred conditions. This observation indicatesthat stirring affects the oscillatory behavior of the BZreaction in presence of the nonionic surfactant in


354 CHAT ET AL.

Figure 1 Time evolution of electromotive force (EMF) (in mV) in a closed, stirred typical BZ reaction at 25 and 35◦C.

either its single form or when in a mixed form withthe anionic surfactant.

The dynamic response of the BZ oscillatory sys-tem to the addition of amphiphilic perturbants can beexplained by taking into account the interaction be-tween micellar assemblies and key reaction steps of theBZ reaction. It is conceivable that the ionic/polar headgroups of micelles may act as a surface where reactantsand solvents are adsorbed in a characteristic manner,dramatically affecting the reaction events of the non-linear BZ reaction [46]. Moreover, the hydrophobiccore of the micelles may act as regions where non-polar/nonionic reactants may preferentially partition,altering their effective concentration and hence thecourse of the BZ reaction. Such compartmentaliza-tion of reactants in microemulsions has been shownto markedly affect oscillations and pattern formationin BZ reactions [23]. In addition, the presence of mi-celles in a BZ reaction system affects transport prop-erties of the reactants due to changes in the diffusioncoefficient of the reactants and/or changes in viscosityand density of the medium [31,32,46]. Bromomalonic

acid (BrMA)/Br2 and MA/HOBr (key species of thisBZ reaction) are, respectively, known [47] for beingorganophilic and hydrophilic in nature. Therefore, it isreasonable to assume that hydrophilic compounds suchas MA and HOBr could be adsorbed on the micellarsurface, whereas hydrophobic species such as BrMAand Br2 would have ability to be solubilized into the mi-celles [48,49] affecting the reaction pathway and henceoscillatory parameters. Thus, bearing in mind all theseconsiderations, the variation in IP in the presence ofmicelles can be explained using the MBM model [18].According to this model, the induction period dependson the concentration of BrMA species. For initiatingthe oscillations in the BZ reaction, a certain minimumconcentration of BrMA species, [BrMA]crit, is requiredin the reaction mixture. The formation of BrMA takesplace through the following two reactions [18] as perthe MBM model:

HOBr+ BrMA + H2O (MBM31)

MA(enol) + Br2 BrMA + Br− (MBM30)+ H +

MA(enol)



0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

60

70

80

90

100

110

120

25oC 35 oC

[CTAB] (mM)

Ind

uct

ion

per

iod

(s)

42434445464748495051525354555657585960

cmc = 0.009 mM

Ind

uct

ion

per

iod

(s)

Cationic surfactant-CTAB

cmc = 0.005 mM

0 1 2 3 4 5 6 770

80

90

100

110

120

[Brij58] (mM)

Ind

uct

ion

per

iod

(s)

cmc = 0.009 mM

cmc= 0.006 mM

35

40

45

50

55

60

Nonionic surfactant-Brij58 25 oC 35 oC

In

du

ctio

n p

erio

d (

s)

0 1 2 3 4 5 6 7 8

96

98

100

102

104

106

108

110

112

114

116

[SDBS] (mM)

Ind

uct

ion

per

iod

(s)

42

44

46

48

50

52

54 25 oC 35 oC

Ind

uct

ion

per

iod

(s)

Anionic surfactant-SDBS

cmc = 0.019 mM

cmc = 0.030 mM

Figure 2 Variation in induction period with surfactant con-centration in single surfactant systems at 25 and 35◦C.

In addition to reactions (MBM30) and (MBM31),the changes in IP could also be addressed by takinginto account reaction (MBM3) [18]:

Br− + BrO 3− + 2H + HOBr + HBrO2 (MBM3)

The increase in IP upon first addition (below cmc)might be either related to alteration of water structuredue to their hydrophobic interactions or due to theinteraction of surfactant monomers with key species

0.0 0.2 0.4 0.6 0.8 1.040

50

60

70

80

90

100

110

120

25 C 35 C

[CTAB+Brij58 ] (mM)

Ind

uct

ion

per

iod

(s)

Cationic-nonionic surfactant system- CTAB+Brij58

cmc = 0.002 mM

cmc = 0.001 mM

40

50

60

70

In

du

ctio

n p

erio

d (

s)

0.0 0.2 0.4 0.6 0.8 1.060

80

100

120

140

160

180

25 °

°

°

°

C

35 C

[SDBS+Brij58 ](mM)

Ind

uct

ion

per

iod

(s)

cmc = 0.002 mM

cmc = 0.003 mM

Anionic-nonionic surfactant system-SDBS-Brij58

50

60

70

80

90

In

du

ctio

n p

erio

d (

s)

Figure 3 Variation in induction period with surfactant con-centration in binary surfactant systems at 25 and 35◦C.

responsible for BrMA formation. Since the BZ systemconsists of different species such as radicals, positiveions, negative ions, and neutral species present in theaqueous medium, it is expected that surfactants wouldinduce different effects depending on the charge ofthe surfactant in solution. In addition, the variationin medium properties like surface tension, viscosity,density, and ionic strength owing to the presence ofsurface active agents could also affect the course of theBZ reaction. Moreover, above cmc the hydrophobiccore of the micelles may entrap nonpolar species, thusinfluencing the kinetics of such a reaction [23,50].

It could be easily assumed that compartmentaliza-tion of different species necessary for BrMA formationpresent in the BZ system would affect the IP values.Partitioning of hydrophobic reaction species such asBrMA and Br2 in the interior of micelles is expected toresult in inhibition of reaction (MBM30) irrespective


356 CHAT ET AL.

of nature of micellar charge and stirring in the sys-tem, a factor resulting in retardation of formation ofBrMA and hence a consequent increase in IP as ittakes longer time to reach [BrMA]crit. However, on theother hand, owing to their hydrophilic nature MA andHOBr could share same hydrophilic solubilization sitewithin micelles and hence could expectedly result inrate enhancement of reaction (MBM31) contributingto a decrease in values of IP. Considering these factorsresponsible for variation of IP values in the presenceof different micelles, the probable explanation couldbe furnished as follows:

In the case of cationic CTAB micelles, the higherconcentration of Br− ions would lead to an increase inthe rate of reaction (MBM3) due to simultaneous lo-calization of negatively charged BrO3

− on the micellarsurface aided by favorable electrostatic interactions.Moreover, it is expected that there would be significantsolubilization of slightly negatively charged substrateMA in the micellar palisade layer which would reactwith HOBr as per reaction (MBM31) produced duringreaction (MBM3) at the micellar surface. This formsthe positive feedback loop for the rapid productionof BrMA, justifying the decrease in IP with the con-centration of CTAB within the medium (Fig. 2). Theenhancement of the reaction between BrO3

− and Br−

in cationic micelles has been reported in the literatureand explained on similar grounds [51]. The similarityin the result obtained under nonstirred conditions [45]indicates that stirring does not affect the results as elec-trostatic interactions have a major role to play.

In the considered range of Brij58 concentrations, theIP is quantitatively affected (Fig. 2). The effect on IPin the presence of polyoxyethylene perturbent (Brij58)can be explained by taking into account alteration ofdifferent reaction paths in the complex BZ mechanismdue to participation of the terminal OH functionalityof the head group in the Brij58 surfactant. The experi-mental observations reveal that the presence of the OHgroup containing perturbants in general leads to vari-ation of oscillation parameters by active participationin a reaction network [51,52]. The involvement of OHgroups in the BZ reaction has been confirmed by theaddition of methanol and ethylene glycol to the BZsystem [38,53,54]. Therefore, the impact of Brij58 ondynamics of the BZ reaction could be explained bytaking into account reactions of terminal OH groups.

The terminal OH reacts to produce bromous acid asper the following processes [37]:

OH + +BrO3 H+ CHO + +HBrO2 H2O (R1)−

COOH+ +BrO3− H+

CHO + +HBrO2 H2O (R2)

According to the MBM model, the bromous acidproduced in these reactions can participate in two dif-ferent reaction pathways:

I. Autocatalytic cycle that reoxidizes the catalyst:

BrO3− HBrO2

HBrO2

Br2O4

+ H+ Br2O4 ++

+ + + +

H2O

H2O

(MBM6)

(MBM7)

(MBM8)

2 BrO2..

BrO2.

Ce(III) H+ Ce(IV)

II. Formation of hypobromous acid by the reactionwith bromide ion:

HBrO2 HOBrBr+ (MBM2)− 2

The impact of these two reactions on IP would bethe opposite. BrO2

• is the autocatalytic species thatis crucial for the positive feedback to the loop. Anincrease in its concentration would lead to an enhance-ment of the loop itself, which would mean an increasein the IP length due to its minimal use in reaction(MBM2) and hence a decrease in reaction (MBM31).On the other hand, the overproduction of HOBr wouldpromote the bromination of malonic acid via reaction(MBM31), leading to the decrease in IP. Moreover, itis also possible that the reaction of HOBr with bromidewould produce bromine (MBM1) that, in turn, wouldalso produce bromomalonic acid (MBM30), again con-tributing to a decrease in IP:

BrHOBr Br2H+ + + H2O (MBM1)+−

The fate of IP is, therefore, decided by a com-promise between production of HOBr (MBM2)and pathways involving reactions (MBM6) and(MBM7). It is apropos to mention that at higherCe(IV) concentration (5 × 10−3 mol dm−3), re-action (MBM2) predominates, whereas at lowerconcentrations (8 × 10−4 mol dm−3) reactions(MBM6) and (MBM7) predominate [53]. Since inthe present study, the concentration of Ce(IV) used is5 × 10−3 mol dm−3, the production of HOBr via reac-tion (MBM2) is expected to predominate. It is reportedthat at higher Ce(IV) concentrations, the production ofmalonyl radicals that react with bromate is enhanced((MBM27) and (MBM48)). This leads to a decrease inthe concentration of bromate and hence hampers posi-tive feedback to the loop. Moreover, the BrO2

. radicalis rapidly scavenged by the malonyl radical (MBM32),



again leading to the reduced contribution to positivefeedback to the loop. Therefore, keeping in view thepossible involvement of hydroxyl end groups in the re-action together with the higher concentration of Ce(IV)used, a decrease in IP in the presence of Brij58 can beattributed to the larger production of HOBr and hencebromomalonic acid:

Ce(IV) + MA MA.+ Ce(III) (MBM27)+ H+

MA. + BrO 3− + TA (MBM48)+ H+ 2 BrO2

.

MA. + BrO2.

BrO2MA (MBM32)

However, in the presence of SDBS, the decreasein rates of both reactions (MBM30) and (MBM31)results in an overall decrease in the concentration ofBrMA relative to that in the pure BZ reaction andhence an increase in IP. It is observed that IP decreasesabove 3 mmol dm−3 SDBS. This decrease may beattributed to clouding that was observed above thisconcentration. Although ionic surfactants rarely showclouding [55,56] in the presence of ions of certain elec-trolytes, they may also show this phenomenon [57,58].Beyond the cloud point, the surfactant properties arediminished to a large extent, thus reducing the effect ofSDBS on reactions (MBM30) and (MBM31) at highconcentrations with a consequent decrease in the IP.Also, in the presence of SDBS, a reaction betweenBrO3

−and Br− would preferentially occur in a bulkaqueous phase due to repulsive interactions betweenthese ions and negatively charged head groups of sur-factants. Therefore, IP tends to increase in the presenceof SDBS concentrations before the clouding of SDBS.Similar micellar effects have been observed in the re-action between BrO3

− and Br− in the acidic mediumin the presence of cationic and anionic micelles [51].

In view of the advanced surface activity of mixedmicelles, an attempt was made to study the effectof binary mixtures of Brij58 with anionic SDBS andcationic CTAB in their 1:1 binary combination on theabove-mentioned BZ oscillatory reaction. In equimo-lar binary surfactant systems, the IP is again influenceddepending on the nature of the binary mixture, whichcan also be justified in terms of relative changes inthe reaction rate of some key reactions as stated abovein the case of single surfactant systems. It is observedfrom the data in Fig. 3 that in the presence of the CTAB+ Brij58 mixture there is a decrease in the IP, whereasin the SDBS + Brij58 mixture IP shows an increasefollowed by a decrease with the increase in the totalsurfactant concentration.

In the case of the CTAB + Brij58 mixture, owing tothe presence of small positive charge density on the mi-cellar surface because of a small micellar mole fraction

of CTAB compared to pure CTAB, the concentrationof BrO3

− and Br− would be enhanced, although tosmaller extent, facilitated by the slight positive charge.In addition, decreased electrostatic repulsion betweencharged the head group of CTAB (small fraction inmixed micelles) and H+ ions may possibly play part inaugmenting reaction (MBM3). Moreover, the involve-ment of terminal OH groups of Brij58 in different reac-tions and thereof its impact on oscillatory parameterscannot be ruled out. In fact, since mixed micelles arepredominantly composed of nonionic counterpart [59],it is safe to argue that IP would be influenced more bythe presence of terminal OH groups of Brij58 at higherconcentrations of the CTAB + Brij58 mixture. As men-tioned above, the participation of terminal OH groupsin the reaction under the experimental conditions ofthis study would lead to the decrease in IP characteris-tics in the presence of Brij58 micelles. Moreover, thesynergistic stabilization of substrate (MA) and its fa-vorable enolization in polar regions of mixed micellesmay together be responsible for a large decrease inthe length of IP than in the presence of correspondingsingle surfactant systems.

In the presence of the equimolar SDBS + Brij58system, the variation of IP is a blend of behavior ob-served in its individual counterparts. It increases ini-tially similar to pure SDBS followed by a decreasesimilar to that in pure Brij58. At lower SDBS +Brij58concentrations (Fig. 3), the ionic effect, i.e., repul-sion between negative charge on mixed micelles andBrO3

−/Br− at the micellar surface, predominates, re-sulting in reduction of reaction (MBM3) and hence asubsequent increase in IP of the BZ reaction. However,at higher SDBS + Brij58 concentrations (above 0.1mmol dm−3) the influence of terminal OH participa-tion is expected to supersede the ionic effect becausethe mixed micelles of SDBS + Brij58 is predominantlymade of Brij58 [59] and hence presents a similar in-fluence on IP above certain concentration (0.1 mmoldm−3).

Therefore, changes in IP with the concentration ofvarious single surfactant and their equimolar binarysurfactant systems could be justified through electro-static interactions as well as solubilization tendenciesof such amphiphiles toward some important speciesprevalent in the solution in addition to their possibleinvolvement in some reactions as per the MBM mech-anism of the BZ reaction.

Oscillation Period of the BZ Reaction in thePresence Surfactant Systems

The time taken to complete one cycle of oscillationis called the oscillation period (τ ). The justifications


358 CHAT ET AL.

provided above for the observed trends in IP are alsosupported by the trends in other oscillatory parameterssuch as the oscillation period. The average oscillationperiod calculated for the pure BZ reaction is about 44 sat 25◦C and 19 s at 35◦C. For comparison between theeffects of different surfactants on the oscillation period,the average oscillation periods for the BZ reaction car-ried in the presence of various single/mixed surfactantsystems at concentrations about 10 times of their cmcvalues were chosen. The average oscillation periodsfor the BZ reaction at 25◦C in the presence of CTAB(37 s), Brij58 (38 s), and CTAB + Brij58 (37 s) arelower compared to that of without the perturbant. How-ever, an increase is observed in the presence of SDBS(42 s) and SDBS + Brij58 (67 s). Similar behavior ofthe effect of various self-assemblies on τ was observedat 35◦C. These observations can be explained on thebasis of justifications provided for IP variation becausethe oscillation period also depends on the concentra-tion of the autocatalytic BrMA. As already mentioned,in the presence of CTAB, Brij58, and CTAB + Brij58micelles, the concentration of BrMA takes less time toreach the [BrMA]crit value because of an increase inits rate of formation, due to which it takes less time toswitch on the autocatalytic phase of the oscillations andhence decreases the oscillation period. On the contrary,since the presence of micelles of SDBS and SDBS +Brij58 reduces the rate of formation of BrMA, theoscillation period increases. The lesser magnitude ofvariation at higher temperature may be attributed to thedominant role of thermal motions of different speciesinvolved which overshadows the role of different self-assemblies in compartmentalization of these moieties.

Overall Behavior of the BZ Reaction in thePresence of Surfactant Systems

The potentiometric time series of the BZ system undera stirred batch in the presence of different surfactantsat concentrations 10 times their cmc (addition of 0.1mM CTAB, 0.05 mM Brij58, 0.3 mM SDBS, 0.01 mMCTAB + Brij58, and 0.01 mM SDBS + Brij58) val-ues at 25◦C are shown in Fig. 4. As is evident fromthe figure, the presence of self-assemblies of differentcharge, hydrophobicity, and size in the BZ reactionmixture affects the behavior of oscillations in severalways. When the magnitude of amplitude of oscilla-tions in the presence of these surfactants is comparedwith that for pure BZ reaction, it is observed that itis enhanced in all the systems with slight differences.Single surfactant systems increase the amplitude ofoscillations to lesser extents than the binary mixtures.From the plots in Fig. 4 it can be inferred that this en-hancement is not regular throughout the whole region

but shows somewhat an irregular trend with respectto persistence of the high amplitude oscillations. Inthe presence of CTAB, SDBS, and SDBS + Brij58,the amplitude of oscillations is always greater than inthe pure BZ reaction, whereas for the BZ reaction inthe presence of Brij58 and CTAB+Brij58 mixture, theamplitude of oscillations initially shows a larger in-crease but after certain time interval these show lessermagnitude than the corresponding regions in the pureBZ reaction. When the nature of oscillation maximaand minima is compared for the pure BZ reaction withthat in the presence of different surfactants, it has beenobserved that up to about 3000 s, maxima show al-most an irregular trend in the pure BZ reaction with anoverall decrease, whereas after this time, these showalmost a constant value. A similar trend has been ob-served in the presence of CTAB, Brij58, and CTAB +Brij58 with the difference in the borderline time. Con-trary to this, a complete disorganized/irregular trend interms of oscillation maxima has been observed in thepresence of SDBS and in both oscillation maxima andminima in the case of the SDBS + Brij58 surfactantsystem. Such irregular oscillatory behavior is more ev-ident in the mixed SDBS + Brij58 system than in thepure SDBS system.

Figure 5 shows the time evolution of EMF for theBZ reaction in the presence of surfactants at 35◦C. Itis clear from the plots that the effect of surfactants onthe sustained oscillations is different at 35◦C as com-pared to at 25◦C. The oscillations sustain upto 3000 sat 25◦C in every surfactant system, but the oscilla-tions decay exponentially in the presence of Brij58and SDBS, whereas decay is linear in the presence ofCTAB and CTAB + Brij58 surfactants. We observedthat the oscillations sustain without appreciable decayupto 3000 s in the presence of the SDBS + Brij58mixture in contrast to observation of a wavy natureof the oscillation pattern at 25◦C. It is interesting tonote that oscillations that sustain in single and mixedsurfactant systems at 25◦C show appreciable decay at35◦C, whereas a chaotic pattern of oscillations in theSDBS + Brij58 system at 25◦C is converted into a sus-tained and homogeneous pattern at 35◦C. Thus fromthe above results, it may be concluded that the over-all behavior of the BZ oscillatory reaction in terms ofamplitude of oscillations, maxima and minima, showa complicated change in the presence of different self-assemblies and at various temperatures. This behaviormay be attributed to very complex nature of this os-cillatory reaction involving a large number of neutral,cationic, anionic, and free radical species. The simi-larity in the arbitrariness of the oscillatory behavior atthe two temperatures selected further points towardits complex dependence on the medium properties,



Figure 4 Time evolution of EMF (in mV) in a closed stirred typical Ce(IV)-catalyzed BZ reaction system at 25◦C with initialconcentrations of reactants [MA] = 0.4 M, [Ce(IV)] = 0.005 M, [KBrO3] = 0.1 M, and [H2SO4] = 0.5 M in the presence ofdifferent surfactants above their cmc values in a time range of (A) 0–3000 s and (B) 0–1000 s.

because decreasing the temperature may also affectthe movement of various species in or out from thesurfactant micelles to different extents. Decreasingtemperature may result in a decrease in thermal mo-tions of all the species present in the reaction mix-

ture, which may increase their persistence time indifferent micelles. Thus as mentioned above, the un-even compartmentalization of intermediates may leadto their complex influence on the overall oscillatorybehavior.


360 CHAT ET AL.

Figure 5 Time evolution of EMF (in mV) in a closed stirred typical Ce(IV)-catalyzed BZ reaction system at 35◦C with initialconcentrations of reactants [MA] = 0.4 M, [Ce(IV)] = 0.005 M, [KBrO3] = 0.1 M, and [H2SO4] = 0.5 M in the presence ofdifferent surfactants above their cmc values in a time range of (A) 0–3000 s and (B) 0–1000 s.

CONCLUSIONS

In the present article, we reported how the aqueousCe(IV)-catalyzed BZ system responds to perturbationsinduced by the addition of the surfactants of varying

architectures. The values of the induction period IPand oscillation period τ of the BZ patterns are af-fected quantitatively. IP decreases with an increase inthe surfactant concentration in the presence of CTABand Brij58, whereas it first increases followed by a



decrease in the presence of SDBS among single sur-factant systems. IP in the case of CTAB + Brij58decreases unremittingly, whereas it increases initiallyfollowed by a decrease in the case of SDBS + Brij58mixed micelles. The observed trends have been as-cribed to the hydrophobic/electrostatic interactions be-tween amphiphilic perturbants and/or involvement ofalcoholic end groups of nonionic Brij58 in some vitalreactions of the BZ system as per the MBM model. Inaddition, the study highlights the impact of surfactantaggregates on the overall pattern of BZ oscillations thatdepends on the combination of nature of purturbantsand temperature.

OAC (SRF-UGC) acknowledges the financial support fromthe University Grants Commission, India, under the ResearchFellowship Scheme in Science for Meritorious Students(RFSMS).

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