Study of the bromide oxidation by bromate in zwitterionic micellar solutions

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Study of the BromideOxidation by Bromate inZwitterionic MicellarSolutionsAMALIA RODRIGUEZ, MARIA DEL MAR GRACIANI, MARIA MUNOZ, MARIA LUISA MOYA

Departamento de Quımica Fısica, Universidad de Sevilla, C/ Profesor Garcıa Gonzalez s/n, 41012 Sevilla, Spain

Received 1 October 1999; accepted 11 February 2000

ABSTRACT: The redox reaction Br– � BrO3– has been studied in aqueous zwitterionic micellar

solutions of N-tetradecyl-N, N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14, and N-hex-adecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-16. A simple expression for the ob-served rate constant, kobs, based on the pseudophase model, could explain the influences ofchanges in the surfactant concentration on kobs. The kinetic effect of added NaClO4 on thereaction rate in SB3-14 micellar solutions has also been studied. They were rationalized byconsidering the binding of the perchlorate anions to the sulfobetaine micelles and their com-petition with the reactive bromide ions for the micellar surface. � 2000 John Wiley & Sons, Inc.

Int J Chem Kinet 32: 388–394, 2000

INTRODUCTION

Bromine, although reasonably soluble in water, istoxic and unpleasant to work with; furthermore, itsready evaporation from aqueous solutions implies thatprecautions have to be taken against loss during re-actions/kinetic runs or in stored solutions. To avoidthese problems in bromination kinetic studies, themethod of generating brominein situ within the re-action cell is sometimes preferred. Oxidation of thebromate ion by bromide in acid media yields brominequantitatively (by using the primary standard KBrO3)

Correspondence to:M. L. Moya ([email protected])Contract grant sponsor: D.G.C.Y.T.Contract grant number: PB98-1110Contract grant sponsor: Consejerıá de Educacion y Ciencia de

la Junta de AndalucıáContract grant number: FQM-274

� 2000 John Wiley & Sons, Inc.

[1,2]. The stoichiometry of the reaction is:

� � �BrO � 5 Br � 6 H : 3 Br � 3 H O (1)3 2 2

This method of generating brominein situ has beenstudied in dilute [3–5] and concentrated [3,5,6–10]solutions of supporting electrolytes as well as in cat-ionic and anionic micellar solutions [11] in order toinvestigate kinetic salt effects and micellar effects onbromination reactions. The scope of this work is tocomplete the aforementioned studies by investigatingbromide oxidation by bromate in acid medium inzwitterionic micellar solutions. The sulfobetaine sur-factants N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14, and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-16,were chosen because no problems with acidity wereexpected in their micellar solutions. All experimentswere done at 298.2 K. This work is included in a gen-eral area of research on kinetic effects in surfactantsolutions developed by our group in the last few years[12–21].

BROMIDE OXIDATION BY BROMATE IN ZWITTERIONIC MICELLAR SOLUTIONS 389

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Table I Observed Rate Constant Values, for the Reaction in Acid Media at Various Bromide�1 � �k /s , Br � BrOobs 3

and Hydrogen Ions Concentrations mol NaClO4). T � 298.2 K,�3(� � 0.12 dm [HBr] � 0.06 M.

�[Br ]/MT 0.06 0.07 0.08 0.09 0.10 0.11 0.12a 4 �110 � k /sobs 6.8 8.1 9.4 10.4 11.6 12.8 14.3[H�

T]/M 0.06 0.07 0.08 0.09 0.10 0.11 0.12b 4 �110 � k /sobs 6.8 9.9 13.2 17.6 20 24 28

The bromide ion concentration was changed by adding NaBr.a

The hydrogen ion concentration was changed by adding HNO3.b

EXPERIMENTAL

Materials

KBrO3, NaBr, NaClO4, and HNO3were obtained fromMerck. HBr was from Aldrich. The concentration ofthe acids used (HBr and HNO3) was determined bytitration with standard NaOH solutions (Merck). N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesul-fonate, SB3-14, and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-16, were Aldrichproducts and used as received. The critical

n-C14H29N�Me2(CH2)3SO3�

SB3-14n-C16H33N�Me2(CH2)3SO3

�

SB3-16

micelle concentrations of the aqueous solutions ofthese two surfactants were obtained by tension sur-face measurements and were in good agreementwith literature values.

Water was obtained from a Millipore Milli-Qwater system; its conductivity being less than 10�6

S cm�1.

Kinetics

Rates of the redox reaction BrO3� � Br� were deter-mined by following the appearance of bromine at 390nm. The rate measurements were performed using aUnicam UV-2 spectrophotometer. The KBrO3concen-tration (5� 10�4 mol dm�3) was always much smallerthan that of the other reagents in order to work underpseudo-first-order conditions. Observed first-orderrate constants were obtained from the slopes of theln(A�A t) against time plots, where At and A� are theabsorbances at timet and at the end of the reaction,respectively. First-order kinetic plots were linear forat least three half-lives. Rate constants were reproduc-ible within 5%.

We used HNO3 acid and NaClO4 salt to acidify thereaction medium and to control the ionic strength, re-

spectively, because the presence of the anions NO3�

and ClO4� does not affect the bromate-bromide redoxreaction since the bromate anion is a stronger oxidiz-ing agent [22]. Even in the absence of BrO3

�, and inspite of the oxidation of Br� (to Br2) by NO3

� andClO4

� being thermodynamically favorable, the reac-tion rates must be extremely slow.

RESULTS AND DISCUSSION1

Kinetics of the oxidation of the bromide ion by bro-mate in acid media has a rate equation of the form [3]:

�� d[BrO ]3 � � � 2� v � k [BrO ][Br ][H ] (2)3 3dt

therefore, the observed first-order rate constant can bewritten:

� � 2k � k [Br ][H ] (3)obs 3

Figure 1 shows the dependence of the logarithm of theobserved first-order rate constant,kobs/s�1, on the log-arithm of the bromide ion concentration (keeping theH� ion concentration constant) as well as on the hy-drogen ion concentration (keeping the Br� ion con-centration constant) in pure water. In all cases, 0.06mol dm�3 of HBr was present in the reaction mediumand the bromide ion concentration was changed byadding NaBr, whereas the hydrogen ion concentrationwas changed by adding HNO3. The ionic strength wasmaintained equal to 0.12 mol dm�3 by adding the ad-equate amount of NaClO4 in each case. From the val-ues of the slopes of the two straight lines shown inFigure 1, one can see that the reaction is first-orderwith respect to bromide ions and second-order withrespect to hydrogen ions, in agreement with Eq. (3).

Once the reaction was studied in homogeneus aque-

1 The observed rate constant values for the reaction Br� � BrO3�

in acid media in pure water as well as in zwitterionic micellar so-lutions are available to the readers in Tables I to V.

390 RODRIGUEZ, DEL MAR GRACIANI, MUNOZ, AND MOYA


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Table II Observed Rate Constant Values, for the Reaction in Sulfobetaine Micellar Solutions�1 � �k /s , Br � BrOobs 3

in the Presence of Various SB3-14 abd SB3-16 Concentrations. T � 298.2 K, [HBr] � 0.06 M.

410 � [SB3-14]/M 4 �110 � k /sobs410 � [SB3-16]/M 4 �110 � k /sobs

0.003.007.0010.020.030.050.080.0100120150170200

6.87.47.88.08.69.410.010.610.911.311.311.611.6

0.003.005.0010.015.020.025.030.040.050.060.070.080.090.0100125150175200

6.86.97.07.47.67.88.08.38.68.78.98.99.09.29.29.39.49.49.4

Figure 2 Influence of changes in the sulfobetaine surfac-tants concentration onkobs for the reaction Br� � BrO3

� inaqueous micellar solutions. [HBr]� 0.06 mol dm�3. T �298.2 K. (�) SB3-14; (�) SB3-16.

Figure 1 Dependence of log(kobs/s�1) on log([Br�]/moldm�3), (�), and on log([H�/mol dm�3), ( �), for the reactionBr� � BrO3

� in pure water (I � 0.12 mol dm�3 NaClO4).T � 298.2 K.

ous solutions, the influence of changes in the sulfob-etaine surfactants’ concentrations on the reaction ratewas investigated. Figure 2 shows the kinetic resultsobtained. In SB3-14 and SB3-16 micellar solutions,kobs increases by increasing the surfactant concentra-tion at low [surfactant], and it tends to reach a plateauat high surfactant concentration. This behavior can beexplained by a treatment of first-order reactions basedon pseudophase models (see Eq. (4)) [23]. Two

basic assumptions are common to all pseudophasemodels:

k� � k� Ks[sulfobetaine ]w m nk � (4)obs 1 � Ks[sulfobetaine ]n

(i) micelles act as a separate phase from water; and (ii)changes in the observed rate constant due to added



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Table III Pseudofirst Rate Constant Values for theReaction at Various NaBr Concentrations.� �Br � BrO3

T � 298.2 K.[HBr] � 0.06 M,

[NaBr]/M a �1k� /swb �1k (SB3-14)/sobs

c �1k� /sm

0.00 6.8 10.4 12.00.01 8.1 11.3 12.60.02 9.4 12.2 13.40.03 10.4 13.1 14.20.04 11.6 13.9 14.90.05 12.8 14.7 15.40.06 14.3 15.6 16.2

Pure water.a

In the presence ofb [SB3-14]� 0.08 M.Estimated by using Eq. (4).c

Table V Pseudofirst Rate Constant Values for theReaction at Various NaClO4

� �Br � BrO3

Concentrations; T � 298.2 K[HBr] � 0.06 M,

[NaClO4]/M a �1k� /swb �1k (SB3-14)/sobs

c �1k� /sm

0.00 6.8 10.4 12.00.01 6.7 8.8 9.70.02 6.8 8.2 8.80.03 6.8 7.8 8.20.04 6.9 7.5 7.70.05 7.0 7.3 7.40.06 7.0 7.2 7.3

Pure water.a


Table IV Pseudofirst Rate Constant Values for theReaction at Various HNO3 Concentrations.� �Br � BrO3

T � 298.2 K.[HBr] � 0.06 M,

[HNO3]/M a �1k� /swb �1k (SB3-14)/sobs

c �1k� /sm

0.00 6.8 10.4 12.00.01 9.9 13.2 14.50.02 13.2 16.9 18.40.03 17.6 19.5 200.04 20 23 240.05 24 26 270.06 28 30 30

Pure water.a


surfactant and salt depend largely upon the distributionof the reagents between micelles and bulk water. InEq. (4), and are the pseudo-first-order rate con-k� k�w m

stants in the aqueous and micellar pseudophases, re-spectively,KS is the equilibrium binding constant ofthe bromate ions to the zwitterionic micelles, and [sul-fobetainen] is the concentration of micellized surfac-tant, equal to the total surfactant concentration minusthe critical micelle concentration. The binding of thebromate ions to the sulfobetaine micelles is describedin the form:

�[BrO ]3mKs � (5)�[BrO ][sulfobetaine ]3w n

where concentrations are molarities in terms of the to-tal volume of solution.

Solid lines in Figure 2 were obtained by fitting theexperimental kinetic data using Eq. (4). The value of

was measured experimentally by studying the re-k�w

action under the same working conditions but in theabsence of zwitterionic surfactants. Since the valuesof andKS estimated from the fittings are not sen-k�msitive to the cmc values considered (for example, inthe case of SB3-14 for cmc� 2.5� 10�4 mol dm�3

one obtains � (1.23 � 0.02) � 10�3 s�1 andk�mKS � (3.1� 0.4)� 102 mol�1 dm3, whereas for cmc� 10�5 mol dm�3 one obtains � (1.24� 0.02)�k�m10�3 s�1 andKS � (2.9� 0.3)� 102 mol�1 dm3), andtaking into account that the presence of 0.06 mol dm�3

of HBr is not expected to decrease the cmcs of the twosulfobetaine micellar solutions importantly [24]. Thefittings were done considering the cmcs of the zwit-terionic surfactants solutions with no added species.These are 2.75� 10�4 mol dm�3 and 2.80� 10�5 moldm�3 for SB3-14 and SB3-16, respectively. The ad-justable parameters obtained were� (1.24� 0.03)k�m� 10�3 s�1 andKS � (3.1 � 0.4) � 102 mol�1 dm3

for SB3-14 micellar solutions and � (0.99� 0.01)k�m� 10�3 s�1 andKS � (3.1 � 0.2) � 103 mol�1 dm3

for SB3-16 micellar solutions. The differences in thecmc and aggregation numbers of SB3-14 and SB3-16micellar solutions could account for the difference be-tween the equilibrium binding constants estimated forthe two micellar solutions [25,26]. The similarity ofthe values obtained for the two sulfobetaine surfac-k�mtants can be explained as follows. depends on thek�mlocal bromide and hydrogen ion concentrations in themicellar pseudophase. Because the charge density dueto the cationic ammonium centers is higher than thatat the anionic sulfonate centers [27], anions bind morestrongly than cations to sulfobetaine surfactants [27–31]. At first, no substantial difference between the hy-drogen ion concentration in the two pseudophases ofthe micellar solutions is expected. On the other hand,the local bromide ion concentration at themicellar sur-face is expected to be higher than that in the aqueous



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Figure 4 Dependence of the observed first-order rate con-stants for the reaction Br� � BrO3

� on the total hydrogenion concentration: (�) pure water; (�) [SB3-14]� 0.008mol dm�3; (�) estimated by using Eq. (4) (see text).k�m[SB3-14]� 0.008 mol dm�3 and [HBr] � 0.06 mol dm�3.T � 298.2 K.


� on the total bromide ionconcentration: (�) pure water; (�) [SB3-14]� 0.008 moldm�3; (�) estimated by using Eq. (4) (see text). [SB3-k�m14] � 0.008 mol dm�3 and [HBr] � 0.06 mol dm�3. T �298.2 K.

bulk phase [25]. On this basis, will be larger thank�m(� 6.79� 10�4 s�1), as was found. Because the twok�w

surfactants studied are sulfobetaines, the excess in thebromide ion concentration at themicellar surface com-pared to that in the aqueous phase is expected to besimilar in both cases. If this is so, similar valuesk�mhave to be found, in agreement with the estimated val-ues.

It is interesting to study the influence of changes inthe bromide and hydrogen ion concentrations on thereaction rate in sulfobetainemicelles, at a constant sur-factant concentration. The reaction under study takesplace in the aqueous phase as well as in the micellarpseudophase and the observed pseudofirst-order rateconstant is expressed by Eq. (4). In order to investigatethe dependence of on [Br�

T] and [H�T], the reactionk�m

was studied in pure water and in SB3-14 micelles un-der the same working conditions. Then, by using Eq.(4), the experimental andkobs values and the esti-k�wmated value ofKS, it was possible to obtain valuesk�mfor the different bromide and hydrogen ion concentra-tions. In all the experiments, [SB3-14]� 0.008 moldm�3 and [HBr] � 0.06 mol dm�3. The bromide ionand hydrogen ion concentrations were increased byadding NaBr and HNO3, respectively. Figure 3 showsthe dependence ofkobs (SB3-14), and (estimatedk� k�w m

by using Eq. (4)) on [Br�T]. Figure 4 shows the de-pendence ofkobs (SB3-14), and (estimated byk� k�w m

using Eq. (4)) on [H�T]. In the two figures, one cansee that changes less than upon changing [Br�

T]k� k�m w

and [H�T]. This can be explained by considering the

binding of anions to the zwitterionic micelles, if we

assume that the mechanism of the reaction does notchange in the micellar solutions. In this regard, Dom-ınguez et al. [11] found that the reaction under studyfollows the same mechanism in SDS, TTAB, andTTANO3 aqueous micellar solutions as that in purewater. Therefore, it seems reasonable to expect thesame in sulfobetaine micellar solutions. Quantitativeestimations of the selectivities of sulfobetainemicellestoward different anions have been done by using con-ductivity measurements and expressing the ion distri-bution as a Langmuir isotherm [23,32]:

�[BrO ]3mK�s� (6)� �[BrO ]([sulfobetaine ]� [BrO ])3w n 3m

It has been found that for bromide and nitrate anions,� 4.3 mol�1 dm3 [25,33] and � 7.1 mol�1K� K�Br NO3

dm3 [33]). On the basis of a Langmuir isotherm, andassuming that the partial molar volume of the reactionregion at the micellar-water interface does not changesubstantially by adding NaBr or HNO3, one would ex-pect the local concentration of bromide ions to becomesaturated at high total bromide ion concentrations.That is, the local concentration of bromide ions at thewater-micellar interface will increase less than theaqueous bromide ion concentration when the addedNaBr concentration increases and, therefore, willk�malso change less than by increasing added [NaBr],k�was is seen in Figure 3. In regard to the dependence of

on [H�T], and because the cation binding to thek�m

sulfobetaine micelles is weak, the observed results canbe rationalized taking into account the competition be-



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� on the added NaClO4concentration: (�) pure water; (�) [SB3-14]� 0.008 moldm�3; (�) estimated by using Eq. (4) (see text). [SB3-k�m14] � 0.008 mol dm�3 and [HBr] � 0.06 mol dm�3. T �298.2 K.

tween the reactive bromide ions and the nitrate ionsfor the micellar surface. By adding HNO3, the hydro-gen ion concentration is increased, but at the sametime, the increasing nitrate ion concentration will re-sult in a decrease of the local bromide ion concentra-tion in the reaction region, the micellar-water inter-face. Therefore, will increase less than byk� k�m w

increasing added HNO3 in agreement with the exper-imental results.

In order to shed more light on this point, the influ-ence of added NaClO4 on was investigated. Figurek�m5 shows the values of ,kobs (SB3-14), and (esti-k� k�w m

mated by using Eq. (4)) in the presence of [SB3-14] � 0.008 mol dm�3 and [HBr] � 0.06 mol dm�3.One can see that the kinetic salt effects in pure waterfor these added NaClO4 concentrations are close tonegligible. However, decreases substantially by in-k�mcreasing the added electrolyte concentration. The saltNaClO4 was chosen as background electrolyte for tworeasons. The first one is that the presence of perchlo-rate ions does not affect the redox reaction under study(mentioned in the experimental section). The secondis that � 40 mol�1 dm3 in SB3-14 micelles [25]K�CLO4

and, therefore, large kinetic salt effects were expectedin sulfobetaine micelles. In fact, these negative salteffects are observed and they can be rationalized byconsidering the competition between the reactive bro-mide ions and the added perchlorate ions for the mi-cellar surface. An increase in the perchlorate ion con-centration will result in a decrease in the local bromideconcentration in the reaction region, themicellar-waterinterface, and then in a decrease in , as was found.k�w

We conclude that the redox reaction Br� � BrO3�

takes place in the micellar pseudophase as well as inthe aqueous phase of sulfobetaine micellar solutions.The influence of changes in the surfactant concentra-tion on the observed rate constant is consistent with apseudophase model. The effect of added NaClO4 canbe rationalized by considering the affinity of the bro-mide anions for the sulfobetaine micelles and theircompetition with the added perchlorate anions for themicellar surface.

This work was financed by D.G.C.Y.T. (Grant PB98-1110)and Consejerıá de Educacioń y Ciencia de la Junta de An-dalucıa (FQM-274).

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Documents

Study of the bromide oxidation by bromate in zwitterionic micellar solutions