Submicellar catalytic effect of cetyltrimethylammonium bromide in the oxidation of ethylenediaminetetraacetic acid by MnO 4 −

Colloids and Surfaces B: Biointerfaces 64 (2008) 42–48

Submicellar catalytic effect of cetyltrimethylammonium bromide in the oxidation of ethylenediaminetetraacetic acid by MnO ₄ ⁻

Maqsood Ahmad Malik, Zaheer Khan

^∗

Department of Chemistry, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi 110025, India Received 26 November 2007; received in revised form 1 January 2008; accepted 5 January 2008

Available online 16 January 2008

Abstract

The effects of cetyltrimethylammonium bromide (CTAB), sodiumdodecyl sulphate (SDS) and Triton X-100 (TX-100) on the oxidative degra- dation of ethylenediaminetetraacetic acid (EDTA) by MnO4−have been studied spectrophotometrically at 525 and 420 nm, respectively. It was found that cationic surfactant catalyse the reaction rate while anionic and non-ionic have no effect. The premicellar environment of CTAB strongly catalyses the reaction rate which may be due to the favorable electrostatic binding of both reactants (MnO₄⁻and EDTA) with the positive head groups of the CTAB aggregates. The influence of different parameters such as [MnO4−], [EDTA], [H⁺] and [surfactants] were also considered.

The reaction follows the first- and fractional-order kinetics with respect to [MnO4−] and [EDTA]. The proposed mechanism and the derived rate law are consistent with the observed kinetics.

© 2008 Elsevier B.V. All rights reserved.

Keywords: Premicellar catalysis; CTAB; SDS; EDTA; MnO4−

1. Introduction

Surfactants are amphipathic molecules having both hydrophobic and hydrophilic properties. Surfactants properties have attracted growing attention for use in biochemistry, bio- logical and chemical research applications[1]. Researches on surfactant behavior are completely multidisciplinary in nature.

The investigation of electron transfer processes in organized molecular assemblies (e.g., micelles) has added a new dimension to biochemical research[2–5]. Micellar catalysis has received considerable attention in view of analogies drawn between micellar and enzyme catalysis[6–8]. The salient properties of the surfactants that affect electron transfer reactions are localiza- tion and compartmentalization effect, pre-orientational polarity and counter-ion effect and the effect of charged interfaces [9–11].

The chemical literature contains abundant reports aimed towards understanding the mechanism of manganese(VII) oxi- dation of S-, O- and N-containing inorganic and organic

∗Corresponding author. Present address: Department of Chemistry, Faculty of Science, King Abdul Aziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia.

E-mail address:drkhanchem@yahoo.co.in(Z. Khan).

reductants in acidic neutral and alkaline media[12–16]. Kinetic and mechanistic studies of the oxidation of biomolecules, such as hydroxy acids (lactic, oxalic, malic) [16–18], ascorbic acid [19], amino acids (cysteine, glycine, methionine)[20], fructose [21] and paracetamol [22]) by water-soluble colloidal MnO2

have carried out to understand the role of MnO2sols.

Ethylenediaminetetraacetic acid (EDTA) (multifunctional␣- amino acid) forms complexes with a large number of cations, including those of ions of the main-group metals. The complex formed by calcium with EDTA is used to treat lead poisoning.

Therefore, its susceptibility to biodegradation during waste- water treatment and in the aquatic environment is an important criterion for assessing its environmental impact and toxicity.

Oxidation of EDTA by permanganate under different exper- imental conditions (acidic and alkaline media) has been the subject of several investigators[23,24]. Surprisingly, despite a large body of information being available on the kinetic and mechanistic aspects of micellar catalysis, studies of their effects upon redox reactions of EDTA have not attracted due atten- tion. For this reason we have performed kinetic studies of the EDTA–MnO4− reaction in presence of three surfactants (cationic, anionic and non-anionic). These studies are useful when discussing the effects of micelles on electron transfer reac- tions. Therefore, in this paper we wish to report the results of the

0927-7765/$ – see front matter © 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.colsurfb.2008.01.003

M.A. Malik, Z. Khan / Colloids and Surfaces B: Biointerfaces 64 (2008) 42–48 43

oxidative degradation of EDTA by permanganate in presence of surfactants.

2. Experimental section 2.1. Materials

Potassium permanganate (Merck, India, 99%) and disodium salt of ethylenediaminetetraacetic acid (Merck, India, 99%) were used as received. Surfactants used were cetyltrimethylam- monium bromide, sodiumdodecyl sulphate and Triton X-100, purchased from Merck (India). CO2-free, deionized and distilled water was used for the preparation of stock solutions of all the reagents. Permanganate and EDTA solutions were stored in a dark glass and polythene bottles, respectively, because EDTA solution gradually leaches metal ions from glass containers, resulting in a change in the effective [EDTA]. An ELICO LI-120 digital pH meter fitted with a CH-41 combination electrode was used for pH measurements.

2.2. Rate measurements

The reactions were started in glass-stoppered two-necked flask fitted with double walled condenser to check evapora- tion. A mixture containing required amount of permanganate, CTAB, water and other reagents (whenever necessary) was ther- mally equilibrated at desired temperature (25±0.1^◦C) and to this was added a measured amount of EDTA solution, pre- equilibrated at the same temperature. The reaction volume was always 50 cm³. Over the entire range of this study, reactions were carried out underpseudo-first-order conditions using an excess of [EDTA] over [MnO4−]. The rate of disappearance of permanganate ion was monitored at 525 nm using Spectronic- 21D Spectrophotometer with cell of path length 1 cm. A value of 2320 dm³mol⁻¹cm⁻¹was calculated for the molar absorp- tion coefficient of MnO4−. The pseudo-first-order rate constants (kobs1, s⁻¹) were determined from the initial part of the plots of log (absorbance) versus time with a fixed-time method (vide infra). Other details of the kinetic measurements were the same as described elsewhere[19,21]. The pH of the reaction mixture was also measured at the end of each kinetic run and observed that pH drift during the reaction is very small (within 0.05 unit).

3. Critical micelle concentration (CMC) determination To determine the CMC, the conductivity measurements of the surfactants (CTAB and SDS) solutions were made with con- ductivity bridge (305, Systronic, India) using conductivity cell (CM 82T; cell constant = 1.02 cm⁻¹), the cmc values of these surfactants were determined from plots of the specific conduc- tivity versus [surfactant] in the absence and presence of MnO4−

and EDTA. The break point of nearly two striaght-line portions in the plot are taken as an indication of micelle formation and this corresponds to the CMC of surfactant [25]. The experi- ments were carried out at 25^◦C under varying conditions, that is, water + EDTA, water + MnO4−and water + MnO⁻4+ EDTA.

The results are given inTable 1.

Table 1

Critical micelle concentration (cmc) values of CTAB and SDS surfactants in the absence and presence of reactants^a

Solution 10⁴cmc (mol dm⁻³)

CTAB SDS

Water 10.1 (9.8) 80.0 (80.1)^b

EDTA 8.8 80.0

MnO4− 8.7 80.0

a [MnO4−] = 2.0×10⁻⁴mol dm⁻³, EDTA = 2.0×10⁻³mol dm⁻³, tempera- ture = 25^◦C.

b The literature values are quoted in parentheses at 25^◦C (Ref.[25]).

4. Results and discussion

4.1. Effects of [reactants] in presence of [surfactant]

It is well established that surfactant can alter reaction mecha- nism, molecularities and orders by virtue of their medium effect;

and that they can be utilized as mechanistic probes for reac- tion mechanism[26]. Preliminary observations showed that the solution of CTAB became turbid in presence of HClO4. There- fore, H2SO4was used to maintain the acidic strength constant.

The most interesting features of the present observations are the very fast decrease in the absorbance of the reaction mixture containing MnO4−+ EDTA + CTAB at 525 nm in the presence of H2SO4. Therefore, the choice of the best conditions for the kinetic experiments is a crucial problem that we address first. In order to examine the effect of variables, experiments were tried at [MnO4−] (=1.0×10⁻⁴to 3.0×10⁻⁴mol dm⁻³) and [EDTA] (=1.0×10⁻³to 5.0×10⁻³mol dm⁻³) in presence of [CTAB] (=0.5×10⁻⁴ to 8.0×10⁻⁴mol dm⁻³), [TX- 100] (=8.0×10⁻⁴mol dm⁻³) and [SDS] (=2.0×10⁻³ to 8.0×10⁻³mol dm⁻³). It should be emphasized here that reac- tions were studied without adding HClO4and H2SO4.

Fig. 1shows examples of some of the kinetic curves of the oxidation of EDTA by MnO4− from which the values of rate constants were calculated. From the inspection of the plots of Fig. 1, it is clear that oxidation proceed in two stages. The time up to the linearity can be considered to be the non-catalytic path (first stage). After this point, the deviation in linear plot could be called the autocatalytic path (second stage). The time at which the deviation commenced was found to be decrease with the [EDTA]. These observations are in good agreement with our previous results[16,19]. Thepseudo-first-order rate constants (kobs1) were obtained from the measurements of slopes of the initial tangents to the plots of log(absorbance) versus time.

It was observed that the rate constants decreased as the initial [MnO4−] increased at fixed [EDTA] (=2.0×10⁻³mol dm⁻³), [CTAB] (=2.0×10⁻⁴mol dm⁻³) and temperature (=25^◦C) (Table 2). The basic trend in chemical kinetics is that the pseudo-first-order rate constants are independent of the initial concentration of the reactant in defect. The same type of defect has been obtained in many MnO4−reactions and especially in those having an autocatalytic character[14,15].

At constant [MnO4−] (=2.0×10⁻⁴mol dm⁻³), [CTAB]

(=2.0×10⁻⁴mol dm⁻³) and temperature (=25^◦C), the reac-

Fig. 1. Plots of log(absorbance) versus time at 525 nm for the oxida- tion of EDTA by MnO4− in presence of CTAB. Reaction conditions:

[MnO4−] (=2.0×10⁻⁴mol dm⁻³); [EDTA] (=2.0×10⁻³mol dm⁻³); tem- perature = 25^◦C; [CTAB] = (A) 1.0, (B) 2.0, (C) 3.0, (D) 4.0 and (E) 5.0×10⁻⁴mol dm⁻³.

tion rate increased with an increase in [EDTA] (=1.0×10⁻³to 5.0×10⁻³mol dm⁻³). The plot of logkobs1versus log [EDTA]

is linear with slopeca.0.58, indicating fractional-order depen- dence ofkobs1on [EDTA]. On the other hand, the plot of 1/kobs1

versus 1/[EDTA] is linear with a positive intercept and positive slope (Fig. 2). Such plot is indicative by Michacelis–Menten behavior (kinetic proof for complex formation between MnO4−

and EDTA). The same experiments were also performed in presence of [TX-100] (=8.0×10⁻⁴mol dm⁻³) and [SDS]

(=8.0×10⁻³mol dm⁻³). No appreciable change was observed in rate constants in presence of these surfactants.

In order to calculate the activation parameters, a series of kinetic runs performed at different temperatures at constant [MnO4−] (=2.0×10⁻⁴mol dm⁻³) and [EDTA]

(=2.0×10⁻³mol dm⁻³) in absence and presence of [CTAB]

(=2.0×10⁻⁴mol dm⁻³). The activation energy (Ea) and other parametersH^#andS^#) of this system were evaluated from Arrhenius (logkobs versus 1/T; Fig. 3) and Eyring (logkobs/T versus 1/T) plots. The results are summarized inTable 3. A com- parison between theEa values in aqueous and micellar media

Fig. 2. Plot of 1/kobs1 versus 1/[EDTA]. Reaction conditions: [MnO4−] (=2.0×10⁻⁴mol dm⁻³); [CTAB] (=2.0×10⁻⁴mol dm⁻³); temperature (=25^◦C).

Fig. 3. Arrhenius plots for the oxidation of EDTA by MnO⁻4in presence (A) and absence (B) of CTAB. Reaction conditions: [MnO4−] (=2.0×10⁻⁴mol dm⁻³);

[EDTA] (=2.0×10⁻³mol dm⁻³); [CTAB] (=2.0×10⁻⁴mol dm⁻³).

Table 2

Dependence of pseudo-first-order rate constants on [MnO4−] and [EDTA] for the reduction of MnO4−by EDTA at 25^◦C in presence of CTAB (=2.0×10⁻⁴mol dm⁻³) 10⁴[MnO4−] (mol dm⁻³) 10³[EDTA] (mol dm⁻³) pH 10⁴kobs1(s⁻¹) 10⁴kcal1(s⁻¹) (kobs1−kcal1)/kobs1

0.6 2.0 5.26 3.5 (1.7)^a

1.0 2.9 (1.6)

1.4 2.6 (1.4)

2.0 2.2 (1.2)

2.4 1.3 (0.9)

2.0 1.0 5.50 1.4 1.4 0.00

2.0 5.26 2.2 2.2 0.00

3.0 5.17 2.5 2.7 −0.08

4.0 5.10 2.8 3.0 −0.07

5.0 5.07 3.4 3.4 0.00

aThekobs1values in the absence of CTAB are given in parentheses.

M.A. Malik, Z. Khan / Colloids and Surfaces B: Biointerfaces 64 (2008) 42–48 45 Table 3

Values of pseudo-first-order rate constants (kobs1) and activation parameters for the reduction of MnO4−by EDTA in presence of CTAB (=2.0×10⁻⁴mol dm⁻³)

Temperature (^◦C) 10³[EDTA] (mol dm⁻³) 10⁴[MnO⁻4] (mol dm⁻³) 10⁴kobs1(s⁻¹)

25 2.0 2.0 2.2 (1.8)^a

35 4.5 (3.8)

45 9.3 (8.3)

Activation parameters Aqueous Micellar

Ea(kJ mol⁻¹) 60 56

H^#(kJ mol⁻¹) 57 54

S^#(J K⁻¹mol⁻¹) −517 −503

aThekobs1values in the absence of CTAB are given in parentheses.

indicates that the CTAB surfactant act as catalyst and provide a new reaction path with a lower value ofEa.

To study the effect of pH, a series of kinetic runs were also performed in presence of H2SO4 (range: 1.0×10⁻⁴ to 5.0×10⁻⁴mol dm⁻³) at constant concentrations of other reagents (EDTA, MnO4− and CTAB). The rate constants, obtained as a function [H2SO4] was found to increase with increasing amounts of [H2SO4] (kobs1×10⁴= 1.5, 1.8, 2.1, 7.7 and 9.2 s⁻¹ at [H2SO4] = 1.0, 2.0, 3.0, 4.0 and 5.0×10⁻⁴mol dm⁻³, respectively). We may thus safely con- clude that the oxidation of EDTA depends upon the acidity of the medium.

4.2. Rate constants—[surfactants] profile for oxidation of EDTA by MnO4−

In order to see the role of surfactants on the oxida- tion of EDTA by MnO4−, the effects of [CTAB], [SDS]

and [TX-100] were studied at 2.0×10⁻³mol dm⁻³ EDTA and 2.0×10⁻⁴mol dm⁻³ MnO4−. Preliminary observations showed that the oxidation of EDTA by permanganate is very fast in presence of CTAB (>6.0×10⁻⁴mol dm⁻³). Therefore, the kinetic studies were limited in the [CTAB] range of 0.5×10⁻⁴ to 5.0×10⁻⁴mol dm⁻³.The plot ofkobs1against [CTAB] shows gradual increase of rates of nearly 10-fold with the increase in [CTAB] (Fig. 4) which clearly demonstrate the CTAB cat- alytic effect not only above but even below CMC, i.e., micellar as well as submicellar catalysis are observed. These observa- tions are most interesting instead of kobs1, attained constant values (for unimolecular reactions) or pass through a maximum (for bimolecular reactions) with [CTAB],[7,27]. The observed catalytic effect may, therefore, be due to (i) presence of pre- micelles and/or (ii) preponement of micellization by reactants [28](as is also confirmed by CMC decreases at reaction con- ditions,Table 1). The anionic SDS surfactant neither catalysed nor inhibited the oxidation reaction (Fig. 4). It is not surprising because there is an electrostatic repulsion between the reactants (MnO4−and EDTA) (vide infra) and the negative head groups of SDS micelles. As a result both the reactants are located in the bulk aqueous medium and the rate remains unaffected. On the other hand, reaction mixture containing MnO4−, EDTA and non-ionic TX-100 surfactant became turbid after some time due to the instability of polyoxyethylene chain of TX-100.

4.3. Analysis of kobs1—[CTAB] data

It is well established that an aqueous surfactant solution has three components: surfactant monomers in the aqueous solu- tion, micellar aggregates, and monomers absorbed as a film at the interface. The surfactant is in dynamic equilibrium among all these components [29]. Surfactant monomers rapidly join and leave micelles (as micelles have a transient character), and the aggregation number represents only an average over time.

The premicellar catalytic effect can be brought in the fact that small aggregates of CTAB (dimers, trimers, tetramers, etc.) exit below the CMC, these small submicellar aggregates can interact physically with the reactants forming active entities.

On the basis of the above description, the possible cause of rate enhancement may be discussed. The presence of nega- tive charge on the EDTA and MnO4− must be considered. It is certainly possible that the negative charge on MnO4−forms an ion pair (Q⁺MnO4−) with the positive quaternary ammo- nium (Q⁺; –N⁺(CH3)3) head group of CTAB molecules which

Fig. 4. Effect of [surfactants] (CTAB () and SDS (䊉)) on the rate constants of the oxidation of EDTA by MnO4−. Reaction conditions:

[MnO4−] (=2.0×10⁻⁴mol dm⁻³); [EDTA] (=2.0×10⁻³mol dm⁻³); temper- ature (=25^◦C).

Scheme 1. Proposed mechanism for the oxidative degradation of EDTA by MnO4−.

brings the reactants together through electrostatic interactions.

Coordination of a Q⁺cation by the permanganate anion would decrease electron density of the MnO4−which, in turn, increases the oxidizing power of the permanganate. This is a reason- able explanation for the submicellar catalysis in the present work.

Table 1 suggesting that the MnO4− and EDTA are inter- acts with the CTAB surfactant, and submicellar aggregates are formed [30,31]. Both the reactants will be preferentially located at the positively charged CTAB aggregated molecules and, therefore, the kinetic CMC of CTAB is lower than in water.

5. Mechanism

In aqueous solution, various EDTA species like H6Y²⁺, H5Y⁺, H4Y, H3Y⁻, H2Y²⁻, HY³⁻and Y⁴⁻(Y = EDTA) exist in equilibrium and nature of these species depends upon the pH of the solution [32]. The first two species (H6Y²⁺

and H5Y⁺) are relatively strong acids and normally are not of importance in evaluation of dissociation constants. There- fore, EDTA has only four values of dissociation constants, i.e., Ka3= 1.02×10⁻²; Ka4= 2.14×10⁻³; Ka5= 6.92×10⁻⁷;

Ka6= 5.5×10⁻¹¹. Under the experimental conditions used in this work ([HClO4] = 0.0 mol dm⁻³), H4Y species exists in significant concentration and this species is reactive towards complexation with permanganate. The most satisfac- tory mechanism to fit the experimental data is represented by Scheme 1.

InScheme 1, the reactive species of EDTA (H4Y) and per- manganate (MnO4−) readily form complex (complex 1; Eq.

(2)). By analogy with previous results[12,13,15,17]we assume that complex1 decomposes in a rate-determining one-step one- electron oxidation–reduction mechanism to give free radical and Mn(VI) (Eq. (3)). After the slow step, the radical reacts with a molecule of MnO4−to yield the imine cation and Mn(VI) (Eq.

(5)). In the next step, imine cation gives HCHO and ethylenedi- aminetriacetic acid (oxidation products of EDTA) (Eq. (6)) after hydrolysis. It is well known that in the MnO⁻4reduction, various species of manganese (Mn(VI), Mn(V), Mn(IV), and Mn(III)) are formed as an intermediate(s). The presence of Mn(VI) and Mn(V) is ruled out by the fact that they are highly unstable in an aqueous acidic neutral media[12]. The intermediate(s), Mn(VI) and Mn(V) may decompose to Mn(IV) species. This oxidation state is commonly involved in the MnO4−oxidation of organic compounds[12,13]. A rate law consistent withScheme 1may

M.A. Malik, Z. Khan / Colloids and Surfaces B: Biointerfaces 64 (2008) 42–48 47

be expressed as Eq.(7).

kobs1 = k1Kes1[EDTA]

1+Kes1[EDTA] (7)

1

kobs1 = 1

k1Kes1[EDTA]+ 1 k1

(8) Thus, a plot of 1/kobs1versus 1/[EDTA] should give a straight line (Fig. 2). The values ofk1andKes1 were calculated from the intercept and slope of the Fig. 2, and are 5.0×10⁻⁴s⁻¹ and 403 mol⁻¹dm³, respectively. Therefore, it was conformed that the redox reaction of MnO4− and EDTA occurs in two kinetically distinguishable steps. The first step is a fast forma- tion of complex between MnO4− and EDTA (Eq. (2)). The second step is a slower electron transfer from EDTA to the manganese(VII) with in the complex (Eq. (3)). Using values of k1, [EDTA] andKes1, the kcal1, can be generated for vari- ous kinetic runs (Table 2). The close agreement betweenkobs1

and kcal1, provides the supportive evidence for the proposed mechanism (Scheme 1) and conforms the validity of the rate- law (Eq. (7)). The proposed mechanism is further supported by analysis of the products. Formaldehyde has been detected as 2,4-dinitrophenylhydrozone derivative. Ethylenediaminetri- acetic acid was detected by reported methods[33,34]. Carbon dioxide was also identified by Ba(OH)2[35]. Formation of rad- icals during the redox process was confirmed by the addition of saturated solution of HgCl2 in the reaction mixtures (white precipitate was observed).

It has been established that Mn(IV) species is responsible for the auto catalysis observed in many permagnate reactions [10–12,36]. Therefore, in order to confirm the formation of Mn(IV) as an intermediate, some kinetic runs were also per- formed at 420 nm, where the contribution from MnO4− is negligible. All attempts to observe Mn(IV) at 420 nm were unsuccessful. Therefore, EDTA did not form Mn(IV) complex stable enough to be detected under conditions used in this present kinetic measurements. In the light of above observations and dis- cussion, the proposed mechanism is given inScheme 2for the autoacceleration pathway.

Scheme 2. Mechanism for the autoacceleration path of EDTA oxidation.

In presence of large amount of EDTA, the Mn(IV) immedi- ately gets converted into stable product (Mn(II); Eqs. (9) and (10)). The reduction of Mn(IV) to Mn(III) by Mn(II) has also been observed[21](Eq. (11)).Scheme 2clearly indicates that the autocatalytic pathway is not a true path of MnO4−–EDTA reaction. It may be a mixture of a series of reactions (Eqs. (9) to (14)). Therefore, the exact dependence ofkobs2on [EDTA] can- not be estimated. On the other hand, permanganate also oxidizes –CHO and –NH2 to –COOH and –NO2, respectively. Hence, the final products (formaldehyde and ethylenediaminetriacetic acid, which are produced in reaction (6), could, in principle, react with MnO4−. Finally, we can state that when an excess of permanganate over EDTA is used, further oxidation of the intermediate(s) occurs to yield CO2and nitro compounds as the final products.

6. Conclusion

It is difficult to ascertain the exact reaction site of a permi- cellar and micellar mediated reaction[37,8]. From the present data, one can obtain the preliminary picture of the reaction sites. The key fact are: (1) the reaction proceeds more fast in presence of cationic, (CTAB) surfactant than in aqueous phase;

(2) the anionic (SDS) and non-ionic (TX-100) surfactants has no effect on the reaction rate; (3) both the reactants (MnO4−

and EDTA) proceeds towards the cationic head group of CTAB dimer, tetramer, etc.); (4) the presence [H⁺] is not essential for the oxidation of EDTA in presence of CTAB; and (5) the oxida- tive degradation of EDTA by MnO4−is first order in [EDTA], but in the presence of CTAB the reaction is fractional-order in [EDTA]. This gives an indication to the relationship between the charge effect of surfactants and the molecularities and or order of the reaction[38].

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