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Colloids and Surfaces A: Physicochem. Eng. Aspects 315 (2008) 226–231
Effect of cationic micelles of cetytrimethylammoniumbromide on the oxidation of thiourea by permanganate
Naushad Ahmad, Parveen Kumar 1, Athar A. Hashmi, Zaheer Khan ∗Department of Chemistry, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi 110025, India
Received 4 February 2007; received in revised form 19 July 2007; accepted 3 August 2007Available online 7 August 2007
bstract
Kinetics of MnO4−-thiourea redox reaction have been investigated spectrophotometrically in presence of cationic micelles of cetyltrimethylam-
onium bromide (CTAB). Upon mixing aqueous solutions of permanganate and thiourea, a readily distinguishable brown colour appears and thenisappears slowly. Various experiments have been performed to confirm the nature of brown colored solution. The effects of [MnO4
−], [thiourea]nd [H+] on the reaction rate were determined in presence of CTAB (=8.0 × 10−4 mol dm−3). Absorbance of the reaction mixture increases with
CTAB] which suggest the incorporation/association of permanganate with the head group of CTAB micelles. Menger–Portnoy model was used toxplain the effect of CTAB micelles. The reaction proceeds through the fast formation and slow decomposition of water-soluble colloidal MnO2s an intermediate. A suitable mechanism is proposed for the oxidation of thiourea by permanganate. 2007 Elsevier B.V. All rights reserved.
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eywords: Micelles; Colloidal MnO2; Oxidation; Permanganate; Thiourea; CT
. Introduction
Research on surfactants is a rapidly developing field dueo their successful applications in many important appliednd fundamental sciences like petroleum, oil recovery, waternd water pollutions, understanding the mysterious role ofiological membranes, biotechnology and other systems. Inves-igations of reaction mechanism in organized assemblies is beingncreasingly carried out in view of the interest derived fromealization that many biological processes proceed in a micro-eterogeneous system which contains an aqueous and lipophilicoiety [1]. Among the biochemical functions, the redox pro-
esses represent reactions of primary importance [2].Polarity and water content in different regions of the micelle
lay an important role in the rate of reaction in these regions. Theurface layer of a micelle resembles a concentrated electrolyteolution with a dielectric constant lower than that of the bulk
∗ Corresponding author. Present address: Department of Chemistry, Facultyf Science, King Abdulaziz University, P.O. Box 80203, Jeddah, Saudi Arabia.el.: +91 11 26981717x3250/3252 (India).
E-mail address: [email protected] (Z. Khan).1 Present address: Solid State and Structural Chemistry Unit, Indian Institutef Science, Bangalore 560012, India.
dacRbrlort
927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.08.001
ater. The micellar phase is less polar than water and the ionicicelles have a polarity near to that of pure ethanol even at
he Stern layer [3–7]. An increase in the aggregation numberauses a decrease in the surface polarity [8]. Micelle catalyzedeactions had become an area of rapidly increasing interest andnumber of extremely important thermodynamic and kinetic
tudies of organic reactions have been performed in micellarolutions. There is extensive evidence on the ability of aqueousicelles and other associated colloids to influence reaction rates
nd equilibria, and concentration, or depletion, of reactants inhe interfacial region have major effects on the rates of reactions4,9–15].
Thiourea (NH2CSNH2), one of the simplest of the thioompound, has many industrial applications. Thiourea and itserivatives are known corrosion inhibitors. Thiourea is toxic [16]nd a cancer support agent [17]. These and other environmentaloncerns have promoted studies on the destruction of thiourea.edox reactions of sulfur compounds are generally known toe quite complex, which may be due to the, autoxidations, free-adical mechanisms and the formation of sulfur–sulfur bonds
eading to various polymeric sulfur species. Thiourea can bexidized by a wide variety of oxidizing agents [18–20]. Theeaction pathways and the final products depend on the pH andhe condition of the reaction mixtures. Kinetic studies of oxida-
hysic
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bmaM(t
(=1.0 × 10 mol dm ) and [CTAB] (=8.0 × 10 mol dm )at 40 ◦C. It was observed that as the initial [MnO4
−] increased,the values of kobs decreased (Table 1). The abnormal behaviorprobably was due to flocculation of colloidal particles.
N. Ahmad et al. / Colloids and Surfaces A: P
ion of ascorbic acid by permanganate have, until recently, beenacking in presence of surfactant.
We are, therefore, interested in examining micellar effectspon rates of oxidation of thiourea by permanganate. These stud-es are useful when discussing the effects of micelles on electronransfer reactions. In this paper, the results corresponding to theitle reaction in aqueous micelle-forming surfactant of CTABre presented for the first time.
. Experimental
.1. Materials
Potassium permanganate (E. Merck, India, 99%), thiourea (E.erck, India, 99%), cetyltrimethylammonium bromide (sigma,
9%), sodium fluoride (E. Merck, India, 99%), MnCl2 (E.erck, India, 99%) were used as received. Double-distilled (first
ime from alkaline KMnO4), deionized, and CO2-free water wassed throughout the studies. To maintain hydrogen ion concen-ration constant, H2SO4 (E. Merck, 99%) was used.
.2. Kinetic measurements
The required solution of all the reactants (except oxidant)as taken in a three-necked reaction vessel equipped with aouble-surface condenser to prevent evaporation. The reactionessel was kept immersed in the oil bath thermostated at desiredemperature and the solution was left to stand for 30 min tottain equilibrium.The reaction was initiated with the additionf required volume of thermally equilibrated oxidant solution.he zero time was taken when half of the thiourea solutionas been added. The progress of the reaction was followedpectrophotometrically by pipetting out aliquots at differentime intervals and measuring the decay in the absorbance ofermanganate at 525 nm using Bausch & Lomb Spectronic-0 D spectrophotometer. Pseudo-first-order conditions wereaintained by keeping the thiourea in excess. Values of the
seudo-first-order rate constants (kobs, s−1) were calculated fromhe slopes of the plots of log (absorbance) versus time by least-quares regression analysis of the data. Multiple kinetic runshowed that the data were reproducible within ±3%.
.3. Product identification
The oxidation product of thiourea was confirmed as fol-ows: solution of thiourea (=1.0 × 10−3 mol dm−3) was addedlowly to a solution of permanganate (=1.0 × 10−4 mol dm−3).he reaction mixture rapidly turned dark brown to color-
ess and 10 cm3 ethanol was added followed by concentratedydrochloric acid. After sometime, white crystals of dithiobisformamidinium) were obtained. The compound was identifiedy the reported method [21].
.4. Free radical detection
Permanganate solution (=5.0 cm3, 1.0 × 10−3 mol dm−3)as added to a mixture of [thiourea] (=5.0 cm3, 1.0 × 10−3
ochem. Eng. Aspects 315 (2008) 226–231 227
ol dm−3) and saturated solution of HgCl2 (=5.0 cm3) in a vol-me of 50 cm3 and the reaction mixture was heated at 40 ◦Cor 30 min. A white precipitate of mercurous chloride appearedlowly. Controlled experiments (with thiourea or MnO4
− only)id not show any precipitate formation. These results indicatehat the reaction proceed through the formation of free radicals.
. Results and discussion
In order to see the role of cationic surfactant (CTAB), aeries of experiments were carried out under different exper-mental conditions. Preliminary observations showed that theolution of CTAB became turbid in presence of HClO4. Tur-idity increases with [HClO4] at constant [CTAB]. Therefore,2SO4 was used to maintain the acidic strength constant. It wasbserved that pink colour of MnO4
− solution (λmax = 525 nm)ecomes brown immediately after the addition of small amountf thiourea at room temperature. To confirm the nature of brownolour, spectrum of the MnO4
− (=1.0 × 10−4 mol dm−3) andhiourea (=1.0 × 10−3 mol dm−3) solution was recorded. Thelot of log (absorbance) versus log (wavelength) was linear withlope (=−5.7). Thus, we may conclude that brown colour is dueo the formation of water-soluble colloidal MnO2 as an interme-iate. These results are in good agreement with the observationsf other investigators [22,23].
The most satisfactory mechanism to fit the experimental datas represented by Scheme 1
In Scheme 1, Eq. (2) indicates the formation of a complexetween Mn(IV)-thiourea complex, which may break down uni-olecularly in the subsequent rate determining step gives radical
nd Mn(III) (Eq. (3)). In presence of large amount of thiourea,n(III) immediately gets converted into the product (Mn(II), Eq.
4)) and dimerization of radicals yields the oxidation product ofhiourea.
The effect of [MnO4−] was studied at constant [thiourea]
−3 −3 −4 −3
Scheme 1.
228 N. Ahmad et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 315 (2008) 226–231
Table 1Dependence of rate constants on the factors influencing the oxidation of thiourea by MnO4
−
Non-variables Variables (mol dm−3) 104 kobs (s−1)
[CTAB] = 8.0 × 10−4 mol dm−3, [thiourea] = 1.0 × 10−3 mol dm−3, temperature = 40 ◦C, λmax = 525 nm, and[H2SO4] = 0.0 mol dm−3
1.0 × 104 [MnO4−] 5.2
1.2 × 104 [MnO4−] 1.8
1.4 × 104 [MnO4−] 1.5
1.6 × 104 [MnO4−] 1.4
1.8 × 104 [MnO4−] 1.3
2.0 × 104 [MnO4−] 1.3
[CTAB] = 8.0 × 10−4 mol dm−3, [MnO4−] = 1.0 × 10−4 mol dm−3, temperature = 40 ◦C, λmax = 525 nm, and
[H2SO4] = 0.0 mol dm−30.2 × 103 [thiourea] 1.9
0.4 × 103 [thiourea] 2.70.6 × 103 [thiourea] 3.31.0 × 103 [thiourea] 3.51.2 × 103 [thiourea] 3.8
[CTAB] = 8.0 × 10−4 mol dm−3, [MnO4−] = 1.0 × 10−4 mol dm−3, [thiourea] = 1.0 × 10−3 mol dm−3,
temperature = 40 ◦C, and λmax = 525 nm0.5 [H2SO4] 5.2
0.9 [H2SO4] 9.21.8 [H2SO4] 17.02.6 [H2SO4] 22.42.7 [H2SO4] 27.03.7 [H2SO4] 38.0
[MnO4−] = 1.0 × 10−4 mol dm−3, [thiourea] = 1.0 × 10−3 mol dm−3, temperature = 40 ◦C, λmax = 525 nm, and
[H2SO4] = 0.0 mol dm−30.0 × 104 [CTAB] 5.4
4.0 × 104 [CTAB] 5.08.0 × 104 [CTAB] 3.5
10.0 × 104 [CTAB] 2.9
p[olooaMM
F1
pcivorder dependence on the [H+].
At constant [MnO4−] (=1.0 × 10−4 mol dm−3) and tem-
erature (=40 ◦C), the reaction rate increased (Table 1) withthiourea] (=0.2 × 10−3 to 1.2 × 10−3 mol dm−3) in presencef [CTAB] (=8.0 × 10−4 mol dm−3). The plot of log kobs versusog [thiourea] was linear with slope = 0.57, indicating fractional-rder dependence on [thiourea]. On the other hand, the plot
f k−1obs versus [thiourea]−1 was linear with a positive interceptnd positive slope (Fig. 1). Such plot is indicative of Michaelis-enten behavior (kinetic proof for complex formation betweennO2 and thiourea).
ig. 1. Plot of k−1obs vs. [thiourea]−1. Reaction conditions: [MnO4
−] = 1.0 ×0−3 mol dm−3, [CTAB] = 8.0 × 10−4 mol dm−3, and temperature = 40 ◦C.
mc
Fma
12.0 × 104 [CTAB] 2.616.0 × 104 [CTAB] 2.5
To study the effect of [H+], a series of kinetic runs wereerformed by increasing the [H+] (range: 0.5–3.7 mol dm−3) atonstant concentrations of other reactants. The rate constantsncreased with increase in [H+] (Fig. 2). The plot of log kobsersus log [H2SO4] was linear with slope = 1.0, indicating first
The effect of added manganese(II) was also explored becauseanganese(II) in permanganate redox reactions acquire a spe-
ial place due to their ability to react with intermediate(s)
ig. 2. Plot of kobs vs. [H2SO4]. Reaction conditions: [MnO4−] = 1.0 × 10−4
ol dm−3, [thiourea] = 1.0 × 10−3 mol dm−3, [CTAB] = 8.0 × 10−4 mol dm−3,nd temperature = 40 ◦C.
hysicochem. Eng. Aspects 315 (2008) 226–231 229
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N. Ahmad et al. / Colloids and Surfaces A: P
manganese(IV)) formed during the reduction of perman-anate by organic reductants. In addition to these, Mn(II)lso plays an important role in deciphering the auto catalyticature of the reaction. The effect of [Mn(II)] was studiedn presence of 8.0 × 10−4 mol dm−3 CTAB at 40 ◦C. Fig. 3absorbance-time profiles) indicate the sigmoid behavior ofermanganate-thiourea reaction in presence of externally addedn(II). Interestingly, the reaction time curves (Fig. 3b and c)
re not a true path of the oxidation of thiourea by MnO4−. It is
mixture of the oxidation rates of thiourea and Mn(II) by per-anganate and colloidal MnO2. Therefore, it can be concluded
hat the oxidation of thiourea is autocatalyzed by Mn(II) formeds reaction product. In presence of externally added Mn(II),here is competition between Mn(II) and thiourea to react withermanganate.
In order to visualize how cationic CTAB micelles maynhibit or catalyze the oxidation of thiourea by permanganate, aeries of kinetic runs were performed in presence of [CTAB]range: 0.0–20.0 × 10−4 mol dm−3) at constant [MnO4
−]=1.0 × 10−4 mol dm−3), [thiourea] (=1.0 × 10−3 mol dm−3)nd temperature (=40 ◦C). The observed pseudo-first-order rateonstants are summarized in Table 1. The observed data revealearly 3-fold decrease in kobs with increase in [CTAB]. Onhe other hand, the absorbance of the reaction mixture firstncreases until it reaches a maximum, and then decreases withCTAB]. These results are shown graphically in Fig. 4 asbsorbance—[CTAB] profile. The increase in absorbance withCTAB] may be attributed to the association/incorporation of
nO4− to the positive head group of CTAB aggregates through
lectrostatic interactions.From the present data, one can obtain a preliminary pic-
ure of the reaction sites. The key facts are: (i) the reactionroceeds more slowly in the micellar phase than in the bulkhase, (ii) only one reactant (MnO4
−) proceeds towards cationic
ig. 3. Plots of absorbance vs. time for three kinetic runs. Reaction con-itions: [MnO4
−] = 1.0 × 10−4 mol dm−3, [thiourea] = 1.0 × 10−3 mol dm−3,CTAB] = 8.0 × 10−4 mol dm−3, temperature = 40 ◦C, [Mn(II)] = 0.0 (�), 4.0�), and 12.0 × 10−4 mol dm−3 (�).
k
Et
As1Am
ig. 4. Absorbance vs. [CTAB] plot at 525 nm for the oxidation ofhiourea by MnO4
−. Reaction conditions: [MnO4−] = 1.0 × 10−3 mol dm−3,
thiourea] = 1.0 × 10−3 mol dm−3, and temperature = 40 ◦C.
icellar phase. The second reactant (thiourea) has a positiveharge, therefore, because of repulsion; it exists only in theater phase. The observed inhibitory effect of CTAB may be
xplained in terms of the Menger–Portnoy model [24] whichakes into consideration of incorporation/solubilization of onlyne reactant into the micellar phase. The reaction scheme in pres-nce of cationic micelles are consistent with reactions (6), (7),nd (8) (Scheme 2), where m and w denote MnO4
− in micellesnd the water, respectively, Ks = micellar binding constant andDn] = [CTAB]—cmc.
Scheme 2 yield Eq. (9):
obs = kw + kmKs[Dn]
1 + Ks[Dn](9)
q. (9) can be rearranged to give Eq. (10), which may be usedo calculate Ks and Km:
1
kw − kobs= 1
kw − km+ 1
(kw − km)Ks[Dn](10)
ccording to Eq. (10), the plot of 1/(kw − kobs) versus 1/[Dn]hould be linear. But, no linearity was observed between
/(kw − kobs) and 1/[Dn] implying that the model is inadequate.s the rate was decrease monotonically in presence of CTABicelles, km can be neglected in Scheme 2, which resulted inScheme 2.
2 : Physicochem. Eng. Aspects 315 (2008) 226–231
E
TEsssmape
nstrrt(s
D
Tac(sf
ppioept
F(
roptft
30 N. Ahmad et al. / Colloids and Surfaces A
q. (11):
1
kobs= 1
kw+ Ks[Dn]
kw(11)
he results obtained in presence of CTAB were analyzed usingq. (11). According to Eq. (11), the plot of 1/kobs versus [Dn]hould be linear (Fig. 5). The value of Ks calculated from thelope of the plot is 427 mol−1 dm3. Sufficiently large Ks valueuggests that the reactant (MnO4
−) is strongly bound to theicellar phase, as can be expected from electrostatic consider-
tions. It is the removal of MnO4− species from the aqueous
hase into the micellar pseudo phase that produces retardationffect of CTAB on the reaction rate with increasing [CTAB].
Table 1 data clearly demonstrate the CTAB inhibitory effectot only above but even below the cmc, i.e., micellar as well asubmicellar inhibition are observed. Micelles are not fixed enti-ies but have a transient character [25]. Surfactant monomersapidly join and leave micelles, and the aggregation numberepresents only an average over time. According to the mul-iple equilibrium model, therefore, the distribution of surfactantD1) between various states of aggregation is controlled by aeries of dynamic association–dissociation equilibria:
1 + D1 � D2, D2 + D1 � D3, . . . , Dn−1 + D1 � Dn
he inhibition below cmc (i.e., submicellar inhibition) is not newnd conforms to various available results [26]. The feasibilityan be sought in the fact that small aggregates of the surfactantsdimmers, trimers, tetramers, etc.) exist below cmc; these smallubmicellar aggregates can interact physically with the reactantsorming catalytically inactive entities.
Electrostatic, hydrophobic and hydrogen bonding seem tolay an important role in bringing the reactants together. Theermanganate (MnO4
−) has both hydrophilic and ionic sites tonteract with the cationic micelles of CTAB. The association
f MnO4− with cationic head group of CTAB micelle involveslectrostatic interactions. Therefore, MnO4
− is expected to beresent in stern layer. Although the exact locations of thiourea athe surface of micelles can not be ascertained with any degree of
ig. 5. Plot of 1/kobs vs. [Dn] for the oxidation of thiourea=1.0 × 10−3 mol dm−3) by MnO4
− (=1.0 × 10−4 mol dm−3) at 40 ◦C.
Cts
R
[
[[
Scheme 3.
eliability, the inhibitory effect of CTAB micelles on the rate ofxidation of thiourea by permanganate reveals that this reactionrobably occurs in the aqueous phase (the positive charge on thehiourea molecule is responsible for the repulsion of thiourearom the micellar phase). Thus, only one factor, namely elec-rostatic interaction/repulsion is left for the inhibitory role ofTAB. Therefore, the orientation and stabilization of the reac-
ants by cationic CTAB micelles can be visualised as representedchematically in Scheme 3.
eferences
[1] C. Tanford, The hydrophobic Effects: Formation of Micelles and BiologicalMembranes, Wiley-Interscience, New York, 1973.
[2] N. Sutin, G.I. Eichhorn (Eds.), Inorganic Biochemistry, vol. 2, Elsevier,Amsterdam, 1973.
[3] L.S. Romsted, E.H. Cordes, J. Am. Chem. Soc. 90 (1968) 4404.[4] J.H. Fendler, E.J. Fendler, Catalysis in Micellar and Macromolecular Sys-
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hysic
[[[
[
[
[
[
[[[
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[[[
[
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