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Journal of Colloid and Interface Science 275 (2004) 560–569www.elsevier.com/locate/jcis

Interfacial aggregate growth process of Fe(II) and Fe(III) complexespyridylazophenol in solvent extraction system

Yoki Yulizar,1 Hideaki Monjushiro, and Hitoshi Watarai∗

Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

Received 8 October 2003; accepted 23 February 2004

Available online 19 March 2004

Abstract

The complexation mechanism and aggregate formation of bis[2-(5-bromo-2-pyridylazo)-5-diethylaminophenolate] iron(II) and iron(IIIcomplexes at the heptane–water interface were studied spectrophotometrically by the high-speed stirring method and the centrimembrane method. Furthermore, the reduction process of the Fe(III) complex with ascorbic acid at the interface was spectrophotometricalobserved. The chemical compositions of the interfacial aggregate of complexes have been proved by the X-ray photoelectron spThe aggregation of the complex at the interface was observed as a red-shifted, very strong and narrower absorption band withthe absorption band of the monomer complex. The aggregate of Fe(III)complex showed more shifted spectrum than that of Fe(II) complexwhich proposed the larger aggregation number of Fe(III) aggregate (n = 8) than that of Fe(II) aggregate (n = 3). The obtained rate constanof interfacial aggregation were smaller than rate constants of interfacial monomer complexation, because the formation of aggregathe assembly of the monomers. 2004 Elsevier Inc. All rights reserved.

Keywords: Liquid–liquid interface; Interfacial kinetics; Interfacial complexation;Aggregation; Solvent extraction; Metal pyridylazo complex; 5-Br-PADAP;High-speed stirring method; Centrifugal liquid membrane method; XPS

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1. Introduction

Interfacial reactions in two immiscible liquids providesignificant challenge to investigators, because liquid–liquiinterfaces are similar to some heterogeneous structurequently encountered in life. The elucidation of the intfacial reaction in liquid–liquid system and the mechanof charge separation is a fundamental problem of modchemistry and chemical technology[1]. The solvent extraction process of metal ions is extremely dependent onmass transfer across the interface. Therefore, the kinetic roleof the interface has to be studied to understand the extramechanism and to control the extraction rate[2]. Recently,the kinetics and the mechanism of metal complexation hbeen investigated at the liquid–liquid interface[3–6]. More-over, the interfacial aggregate formation of metal comphas been studied as an interesting subject in solvent ex

* Corresponding author.E-mail address: [email protected] (H. Watarai).

1 Present address: Department of Chemistry, Faculty of Mathemand Science, University of Indonesia, Depok 16424, Indonesia.

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.02.045

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-

tion kinetics of metal ions and molecular recognition cheistry [7–9]. Actually, the interfacial aggregate formationthe complex is poorly understood, because this subjevery new. For this reason, we have interested to explorefundamental concept of the aggregate formation of mcomplex.

The determination of the iron ion in various matces with 2-(5-bromo-2-pyridylazo)-5-diethylaminophe(5-Br-PADAP or HL) was reported several times in sotion [10–14]. However, there was no report that describedcomplex formation of Fe(II) and Fe(III) with 5-Br-PADAat the liquid–liquid interface.Moreover, the aggregation otheir complex might have characteristic absorption spectwith respect to the monomer complex. In this study,measurement of the interfacialcomplex formation of bis[2(5-bromo-2-pyridylazo)-5-diethylaminophenolate] iron(or iron(III), called the Fe(II) or Fe(III) complex, and thaggregate formation of their monomer complexes wereried out. The demonstration of the reduction reactionthe Fe(III) complex formed at the interface with ascoracid reductant in the heptane–water system were perfospectrophotometrically using a centrifugal liquid membr

http://www.elsevier.com/locate/jcis

Y. Yulizar et al. / Journal of Colloid and Interface Science 275 (2004) 560–569 561

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(CLM) method. It was supported by high-speed stirr(HSS) measurements as well.

5-Br-PADAP, known as a tridentate ligand, has been uas a highly sensitive colorimetric reagent for various mions, because the ligand can form stable and intensely coored metal complexes in bulk organic phase[15–18]. Thisligand has a tendency to adsorb at the interface, especin the heptane–water system[19]. The poor solubilities ofthe metal complex of 5-Br-PADAP in both phases providchance to observe the adsorption of the complex at theterface. Furthermore, when the two-dimensional saturatiostate of the complex was attained, it is very often possto observe the formation of the aggregate of the metal cplex [9].

The difference in the reactivity of Fe(II) and Fe(III) witthe 5-Br-PADAP ligand to form the stable complexesthe interface, and the rate of the interfacial aggregatiothe complexes were investigated. Although the absorpspectra of Ni(II), Zn(II)[20], and Fe(III) complexes at thinterface are very similar, except for the Fe(II) compltheir interfacial aggregateshave a remarkably intense anred-shifted peak from the corresponding monomer compwhich showed the interaction of the ligand in the aggregof the complexes.

2. Materials and methods

2.1. Reagents

Stock solution of 0.1 M iron(II) and 0.1 M iron(III) werprepared by dissolving FeCl2 anhydrous and FeCl3 anhy-drous (Wako Pure Chemicals) in a small amount of percric acid (G.R., Nacalai Tesque, Inc.). The ionic strength othe aqueous phase was maintained at 0.1 M using soperchlorate (G.R., Aldrich) and perchloric acid. L-ascoracid (G.R., Katayama Chemicals) was used as a reduagent. The pH of the solution was adjusted with sodiacetate and acetic acid, which were obtained from NacTesque.

2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (5-BPADAP) was purchased from Tokyo Kasei and 1× 10−3 Mstock solution was preparedin heptane. Heptane (G.RNacalai Tesque) was used after purification by fractionaltillation.

All other chemicals used were of analytical grade and utrapure water from a Millipore Milli-Q system (18.2 M�cm)was used for all dilutions.

2.2. Liquid–liquid extraction

The batch method was used to investigate the extracbehavior of the complex. Samples of 5 ml of aqueous stions of 1.0× 10−3 M metal ion and 0.1 M (H+, Na+)ClO−

4in a glass tube were used in the pH range of 0.9–2.5Fe(III) system and 2.5–5.1 for Fe(II) containing the ascor

acid. A sample of 1.0× 10−5 M HL in heptane (5 ml) wascontacted with the aqueous phase and the two phasesshaken for 4 h in a thermostated room at 298± 1 K. The so-lutions were allowed to separate and both of the absorpspectra of the aqueous and organic phases were measuUV/vis/NIR spectrophotometry (V-570, Jasco).

2.3. Measurement of interfacial reaction ofextraction system

The high-speed stirring (HSS) apparatus for interfareaction and extraction measurement has been reported prviously [21–23]. A 49.5-ml aqueous acetate buffer soluti(5 × 10−3 M) without metal ions containing 0.1 M (H+,Na+)ClO−

4 , and 49.5 ml of heptane without HL were intrduced into a glass stirring cell thermostated at 298± 0.5 Kfor a blank measurement. The organic phase was conously separated with a PTFE phase separator, passed tha flow cell with optical path length 1 cm in a photodiodarray spectrophotometric detector, and returned to the gstirring cell. Then 0.5 ml of HL in heptane was addedthe mixture. The absorption spectrum of the organic phwas recorded continuously at stirring rates of 5000 r(high speed) and 200 rpm (low speed). The interfacial adsorption of the ligand was observed from the differenceabsorbances at both stirring speeds. Under the high-scondition a 0.5-ml Fe(III) solution or a 0.5-ml Fe(II) soltion containing 0.05 M ascorbic acid was injected intomixture and complexation was initiated. After complexatequilibrium was attained, the stirring rate was decrease200 rpm and the desorption of the adsorbed species frominterface to the organic phase was observed. For the cairon(III) complexation, after theattainment of equilibriumthe ascorbic acid was added to the stirred mixture andreduction of the Fe(III) complex at the interface was sptrophotometrically observed.

2.4. Interfacial kinetics measurement

The centrifugal liquid membrane (CLM) method usedthe same as previously described[9] with a slight modifica-tion in measurement procedure. A blank measurementconducted by introducing heptane (100 µl) and an aqusolution (150 µl) containing 5× 10−3 M acetate buffer and0.1 M (H+, Na+)ClO−

4 into a cylindrical rotating cell. Thecell was rotated at the speed of 10,000 rpm. A 50-µl soluof HL in heptane was added to the mixture and a compformation was initiated by the injection of a 100-µl Fe(Isolution or 100-µl Fe (II) solution containing 0.05 M ascbic acid. The reduction reaction of the Fe(III) complex wobserved by the addition of ascorbic acid (10 µl) intomixture of the Fe(III) system after the complexation retion reached the equilibrium. The sum absorption spectrthe interface and the bulk phases in the rotating cell wmeasured with an HP8453 diode-array spectrophotome298± 1 K.

562 Y. Yulizar et al. / Journal of Colloid and Interface Science 275 (2004) 560–569

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To investigate the dependence of interfacial reacon HL concentration, the initial concentration of HL wchanged from 6.6× 10−7 to 1.0× 10−4 M with a constantconcentration of Fe(III)= 4.0 × 10−4 M at pH 1.5. As forthe Fe(II) system, the concentration of Fe(II) was fixed1.0 × 10−3 M, containing 5.0× 10−2 M ascorbic acid apH 5.0 (buffer acetate); then the initial HL concentratwas changed from 9.9× 10−7 to 5.0× 10−5 M. The ionicstrength was maintained at 0.1 M (H+, Na+)ClO−

4 .

2.5. X-ray photoelectron spectrometry (XPS)

A 400-µl Fe(III) solution containing 0.1 M (H+, Na+)ClO−

4 with pH 2.0 or Fe(II) solution containing 0.05 Mascorbic acid, 0.1 M (H+, Na+)ClO−

4 with pH 5.0, and thesame volume of 2.0× 10−5 M HL in heptane were introduced in a cylindrical glass cell (inner diameter and inheight were 11 mm and 25 mm, respectively). Thephases were allowed to form the interfacial aggregate ocomplex at 298± 1 K. The aggregate of the Fe(II) or Fe(IIcomplex formed at the interface was collected on a gplate (diameter 10 mm) that was placed on the bottom ocylindrical glass cell by flowing of the two-phase solution

XPS spectra of the aggregates on the glass platemeasured by a Shimadzu HIPS-70 photoelectron spectrter with unmonochromated MgKα radiation as an excitatiosource. The X-ray source was operated at 8 kV and 30and the takeoff angle of the spectrometer was set at◦from the sample surface. An energy calibration of electanalyzer was carried out using photoelectron lines at 7932.7, and 335.0 eV for Cu3p, Cu2p3/2, and CuL3MM, re-spectively, for bare Cu metal[24]. The spectra for bindinenergy region of Fe2p, N1s, O1s, Br3p3/2, and C1s weremeasured. The binding energies were referred to thesbinding energy of adventitious carbon at 285.0 eV.

3. Results and discussion

3.1. Interfacial reaction in the extraction system

The Fe(III)–5-Br-PADAP complex was not extractedthe organic phase. A very small amount of the comp(λmax = 511 nm) was soluble in aqueous phase at pHbut the water-soluble complex became smaller and negble at pH 2.5. Therefore, the Fe(III) complex was suggeto completely adsorb at the interface at pH 2.5. In the casthe Fe(II) system, a significant amount of the Fe(II) comp(24.9% of the total complex) was observed in the orgaphase that showed the two maxima,λmax = 533 and 756 nmin the absorption spectrum.

The HSS measurement was performed to investigate thinterfacial adsorption of the ligand and its complex.Figs. 1a and 1b(solid line), before the addition of metal ionthe stirring effect on the absorbance of the ligand in hep

-

Fig. 1. Typical absorbance changes in heptane as a function of timesured by HSS: (a) formation of Fe(III) complex, [HL]= 4.9 × 10−6 M,[Fe(III)] = 1.0 × 10−4 M, pH 1.5; (b) formation of Fe(II) complex[HL] = 5.4 × 10−6 M, [Fe(II)] = 1.0 × 10−3 M, pH 5.7, [Ascorbicacid] = 5.0 × 10−2 M; (c) interfacial reduction of aggregate of Fe(IIcomplex, [HL] = 1.0 × 10−5 M, [Fe(III)] = 5.0 × 10−6 M, [Ascorbicacid]= 1.0× 10−2 M, pH 4.4.

phase at the absorption maximum,λmax = 452 nm, was observed. From the change in absorbance (�A), the adsorptionconstants of HL at the heptane–water interface,K ′ were es-timated by using the equation

(1)K ′ = aεl�AVo

(aεlSi − �AVo)A′ ,

derived from Ref.[19], where a refers to the maximuminterfacial concentration at the saturation state, for whloga = −10.29 mol cm−2 [25], ε is the molar absorptivityof HL in heptane (5.16× 107 mol−1 cm2) [19], l is the opti-cal path length of the flow cell (1 cm),A′ is the absorbance othe organic phase under high-speed stirring,Si is the total in-terfacial area under the stirring (1.7×104 cm2), andVo is thevolume of the organic phase (50 cm3). Using the above values in theEq. (1), we could estimate the value of interfacadsorption constant of HL,K ′ at pH 1.5 (Fig. 1a) and at pH5.7 (Fig. 1b), to be 5.48×10−3 and 2.59×10−3 cm, respec-tively. TheK ′ value at pH 1.5 was larger than that at pH 5because of the protonation ofligand, considering the vaues of pKa2 and pKa3 for 5-Br-PADAP in the ethanol:wate


Fig. 2. Absorption spectral change of complexation and aggregation of Fe(III) complex at the heptane–water interface by the CLM method: [HL]=1.7× 10−5 M, [Fe(III)] = 1.0× 10−3 M, pH 1.5.

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(1:1) are 2.02 and 11.30, respectively[26]. The protonatedstate of HL at pH 1.5 was turned out more adsorbed atinterface than that at pH 5.7.

Under the high-speed stirring state (seeFig. 1a), the ad-dition of Fe(III) solution to the mixture showed a decreaof HL absorbance atλmax = 452 nm (solid line). Howeverno extraction of the Fe(III) complex into the heptane phwas observed at 554 nm (dashed line), as well as undelow-speed stirring state. This result suggested that the cplexation took place only at the interface.

In the case of Fe(II) system (seeFig. 1b), the addition ofFe(II) solution to the mixture containing HL at 5000 rpcaused an abrupt decrease of HL absorbance at 452(solid line). However, there was no increase of Fe(II) coplex adsorbance observed at 533 nm (dashed line). Notraction of the complex in the organic phase was obseunder high-speed stirring. Since the stirring rate wascreased to 200 rpm a significant increase of Fe(II) compabsorbance was observed in the time range of 4300–49(dashed line). A part of the complex was extracted intoorganic phase. From this result, we observed that the Fcomplex was formed and adsorbed completely at the inface under high-speed stirring due to the high interfaarea, while a significant amount of complex would betracted in the organic phase, when the interfacial areashrunk. The observation of HL absorbance increase intime range of 4300–4900 s (Fig. 1b, solid line) as well asin the range of 4300–5000 s (Fig. 1a, solid line) under low-speed stirring described the desorption of ligand (HL) frthe interface into the heptane.

-

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3.2. Interfacial formation of the complex and its aggregate

To confirm the adsorption of the complex at the interfawe used the CLM spectrophotometry.Fig. 2shows the CLMabsorption spectra for the process of Fe(III) complexmation and the aggregation of the complex at pH 1.5 atheptane–water interface. The result of the HSS experimeshowed no extraction of the Fe(III) complex in the heptaphase and a negligible amount of aqueous soluble comHowever, in the CLM measurement (Fig. 2), immediatelyafter the initiation, the absorption spectrum of the monomecomplex band atλmax = 554 nm was observed at first. Thband position was different from that of the aqueous spe(λmax= 511 nm), therefore the complex was confirmed toformed at the interface. Further, the lapse of time caused thappearance of a new absorption band atλmax= 630 nm witha remarkably red shift. This ischaracteristic for the J-banwith a very intense and narrow absorption band compawith that of the monomer complex, due to strong electrocoupling of several ligands in the monomer[27]. Therefore,this result indicated clearly the J-type aggregate formaof Fe(III) complex at the interface.

Fig. 3 shows the absorbance change observed by Cmethod for the ligand, Fe(III) complex formation, and agregation of the complex at the interface. After the inittion of the process at 100 s, the absorbance of the ligat 452 nm (dotted line) decreased, while the absorbancthe monomer complex at 554 nm (solid line) increased vfast. At the same time, the absorbance at 630 nm (daline) was gradually increased. The peak at 554 nm andshifted peak at 630 nm observed by CLM measurement w


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Fig. 3. Profile of the interfacial complexation and aggregate formation knetics observed by the CLM method: [HL]= 1.7 × 10−5 M, [Fe(III)] =4.0× 10−4 M, pH 1.5.

directly assigned to the formation of the Fe(III) complex aits aggregation at the interface, respectively.

A similar result was obtained for the system of Fewith 5-Br-PADAP. We observed the absorption spectrumthe monomer complex with two maxima,λmax = 556 and740 nm, which are different from those of the extracted cplex in heptane phase (λmax = 533 and 756 nm); thereforthe Fe(II) complex was thought to be adsorbed at the inface. The lapse of time caused the J-band shift toλmax = 579and 759 nm assigned to the interfacial J-type of aggregThe magnitude of the shift of the band for the aggregfrom the monomer Fe(II) complex was smaller than thathe Fe(III) system. The Fe(III) complex was formed copletely at the interface; therefore it is possible that the sting interaction between ligands in the Fe(III) complex wlarger than that in the Fe(II) complex. The spectrum ofFe(II) complex formed at the interface was close to thathe bulk heptane phase, suggesting that both were thetral Fe(II)L2 complex. This characteristic of the aggregate othe Fe(II) complex was similar with that of the Ni(II) complex [9].

The stoichiometry of Fe(II) or Fe(III) ion against theBr-PADAP ligand was determined using CLM spectroptometry (Figs. 4a and 4b). The mole ratio of both complexewas obtained as 1:2 for metal:ligand.

3.3. XPS spectra of complex aggregate

XPS measurement was performed to characterizeaggregate of complex of Fe(II) or Fe(III)–5-Br-PADAobtained from the liquid–liquid interfacial reaction. TXPS spectra of Fe2p (Fig. 5a) were observed at 708.5 an710.0 eV for the aggregates of Fe(II) and Fe(III) complexrespectively. The Fe2p peak for Fe(II) in the aggregate wsharper than that of Fe(III).Fig. 5bshows that the N1s sig-nal was observed at 399.6 and 399.7 eV for aggregateFe(II) and Fe(III) complexes, respectively. The N1s peaksof both aggregates were shifted a little from that of theand (399.4 eV). The spectral shape of the N1s signal of theFe(III) complex was a little broader around 402 eV than thof the Fe(II) complex. It was probably caused by the diffence of the strength of coordination bonding between

.

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Fig. 4. Determination of the composition of the interfacial complexthe mole ratio method in CLM spectrophotometry: (a) Fe (III) coplex, (b) Fe(II) complex, [HL]= 1.0 × 10−6–3.5 × 10−5 M, [Fe(III)] =4.9× 10−6 M at pH 2.6, [Fe(II)]= 5.0× 10−6 M at pH 4.6.

Table 1Values of electron binding energies,Eb (eV), of complex aggregates

Species Fe2p N1s O1s Br3p3/2 C1s

HL – 399.4 532.3 184.1 284.Fe(II)L2 708.1 399.6 533.1 184.0 284Fe(III)L+

2 710.1 399.7 532.9 184.0 284

Note. HL = 5-Br-PADAP.

N, O) atoms in the ligand with Fe(II) or Fe(III) to form thneutral Fe(II)L2 complex or the charged Fe(III)L+2 complex.Fe(III)L+

2 was neutralized by the ion pair with ClO−4 . Thesame features were observed in the spectra of O1s (Fig. 5candTable 1). For Br3p3/2 and C1s signals, no shift was observed, as listed inTable 1. From these experimental resulwe confirmed that the aggregates of the complex weresorbed at the interface with the composition proposed strophotometrically from the CLM and HSS experiments.

3.4. Interfacial complexation and aggregation mechanisms

From the above results, we postulated the reaction manism of Fe(III) and Fe(II) ions with 5-Br-PADAP (HL) athe heptane–water interface that is shown inFig. 6a. First,


Fig. 5. Photoelectron spectra using MgKα X rays of the 5-Br-PADAP (HL) ligand, and the aggregates of Fe(III)L+2 and Fe(II)L2 complexes obtained from

the heptane–water interfacial reaction: XPS spectra of (a) Fe2p, (b) N1s, and (c) O1s.

Fig. 6. (a) Reaction mechanism of interfacial complexation and aggregate formation, (b) mechanism of interfacial reduction reaction: After the complexationand aggregate formation of Fe(III) complex completely at the interface, ascorbic acid was introduced in the system and immediately observed the interfacialreduction reaction of the complex to form Fe(II) complex that was demonstrated spectrophotometrically. At the high concentration state of Fe(II) complexcould form a new aggregate of complex, whichwas assigned by the shifted absorption band.

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the ligand mainly adsorbed at the interface, and it reawith Fe(III) or Fe(II) ion at the interface. The interfaciconcentration of the ligand was dependent on pH; thejority was the protonated ligand, H2L+ [3], in pH 0.9–2.5and the neutral form, HL, in pH 2.6–6.0 as confirmed byHSS measurement[20]. The protonation of the ligand occurred in the Nβ of the azo group, which did not coordinato metal ion directly. Therefore the reactivity of the ligandto form a complex was weakly affected by the protonatiWe also confirmed that the charged complex, Fe(III)L+ was
2
not extracted at all in the heptane phase, but adsorbethe interface. However, the neutral Fe(II)L2 complex is com-pletely adsorbed at the interface when the specific interfaarea became higher (the high-speed stirring state) and a paof the Fe(II)L2 complex is extracted in the heptane phawhen the specific interfacialarea is decreased (200 rpm).

3.5. Kinetics of interfacial complex formation

From the absorbance change versus time at the incomplex formation or aggregation stage (seeFig. 3), we


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obtained a second-order fitted equation by least-squaregression, and by differentiating the second-order equatiothe time of initiation we calculated the observed initial r(r0

obs).Taking into account that the concentration of complex

the aqueous phase in the CLM system is negligibly smthe initial rate was represented as

(2)r0obs= ki

([Fen+][HL]iSi/Vo

)2εl,

whereki andn are the interfacial complexation rate constand the valence of the metal ion, respectively. Factorrelated to the light beam passed twice through the liquliquid interface and both bulk phases in the rotating gcell. The ligand concentration at the interface,[HL]i , wascalculated from a Langmuir adsorption isotherm[19,21],

(3)[HL]i = aK ′[HL]oa + K ′[HL]o ,

where[HL]o is the bulk organic phase concentration of HAlthough the reaction for the Fe(III) system was conducat pH 1.5, for which the value of the adsorption constwasK ′ = 5.48× 10−3 cm, in the calculation of interfacialigand concentration from complex formation we usedvalueK ′ = 2.59× 10−3 cm determined at pH higher tha2.6, because the ligand was in the unprotonated statereactivity of the ligand to form a complex was made througcoordination binding the between metal ion, the N atomthe pyridyl group, the Nα atom of the azo group, and theatom of the OH group after the release of a proton. Thfore, the coordination bond was not formed at Nβ of theazo group of the ligand, where the protonation occurrepH 1.5. TheK ′ value of HL at pH 1.5 was assumed cotaining a mixture of HL in the protonated and unprotonastates, and only the latter form of the ligand will affect tcomplexation reaction.

Fig. 7 represents the dependence of the observed incomplexation rate,r0

obs/2εl, on the interfacial ligand concentration, [HL]iSi/Vo, which shows good linearity. From thslope of the plot, we calculated the complexation rate cstant at the interface,ki , as shown inTable 2. In the case oFe(III) system, the existence of the protonated ligand shohigher adsorptivity at the interface, so that it facilitatedinterfacial reaction of Fe(III) complex formation. This effebecame larger in the complexation rate of Fe(III) at theterface under the acidic condition of pH 2.5. Unfortunatit became difficult to investigate due to the formation of

-t

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Fig. 7. Interfacial ligand concentration,[HL]iSi/Vo dependence of the observed initial formation rate of the Fe(III) and Fe(II) complexes,r0

obs/2εl;

[Fe(III)] = 4.0× 10−4 M at pH 1.5, [Fe(II)]= 1.0× 10−3 M at pH 4.9.

precipitation of iron(III) hydroxide at pH� 3.0, when theconcentration of Fe(III) was 1.0× 10−3 M.

3.6. Kinetics of interfacial aggregate formation

Aggregate formation of Fe(II) or Fe(III) complex at thinterface started just after the critical aggregation concentration (cac),[FeL(n−2)+

2 ]ci , which is obtained as

(4)[FeL(n−2)+

2

]ci= Amc

2

1

103ε,

where Amc is the absorbance of the monomer compat the cac, factor 2 is related with the light beam pasthrough twice the interface and both bulk phases in thetating glass cell, andε is the molar absorptivity of monomecomplex, which is obtained from the slope of the plotsthe absorbance versus[FeL(n−2)+

2 ]iSi/Va in the range of5.0× 10−7–5.5 × 10−6 M. Using the values ofAmc andε

for FeL2 at 576 nm and for FeL+2 at 630 nm (seeTable 2),we calculated the values of cac at the interface as showTable 2.

We can define the rate law of the aggregate formatiothe interface as

(5)r0obs,agg= kagg

([Fen+]f [HL]i,f Si/Vo

)2εl,

wherekaggrefers to the interfacial aggregation rate constThe concentration of free metal ion in the aqueous ph[Fen+]f and the interfacial concentration of free ligand,[HL]i,f were calculated by assuming that the adsorptioligand at the interface was not limited in the presence of

Table 2Values of complexation rate constant,ki , aggregation rate constants,kagg, and critical aggregation concentration, cac, at the heptane–water interface

Complexes Amc ε ki kagg cac(M−1 cm−1) (M−1 s−1) (M−1 s−1) (mol cm−2)

Fe(II)L2 1.5× 10−2 1.04× 105 6.8× 104 2.8× 102 2.1× 102 1.1× 10−10

(556 nm) (576 nm)Fe(III)L+

2 2.1× 10−3 9.8× 104 1.6× 104 1.0× 103 4.4× 10 6.3× 10−11

(554 nm) (630 nm)


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theui-ed.7.wase(II)

ringthe-fternceatthe

Fig. 8. Dependence of free ligand concentration at the interface,[HL]i,f ×Si/Vo, on the observed initial aggregation rate of Fe(III) and Fe(II) coplexes, r0

obs,agg/2εl ; [Fe(III)] = 4.0 × 10−4 M at pH 1.5, [Fe(II)] =1.0× 10−3 M at pH 4.9.

complex,

(6)[Fen+]f = [Fen+]T − [FeL(n−2)+2 ]ci Si/Va,

(7)[HL]i,f = aK ′([HL]T − 2[FeL(n−2)+2 ]ci Si/Vo)

a + K ′([HL]T − 2[FeL(n−2)+2 ]ci Si/Vo)

,

Va and subscriptT are the volume of aqueous phase andtotal concentration.

The plot ofr0obs,agg/2εl against[HL]i,f Si/Vo (seeFig. 8)

gave a linear correlation. From the slope ofFig. 8, we cancalculate the values of the interfacial aggregation rate cstant for Fe(III) complex and Fe(II) complexes,kaggas listedin Table 2.

The values of the interfacial aggregation rate conswere smaller than those of the corresponding interfacomplexation rate constant. It was caused by the factthe rate of interfacial aggregation was governed by themation rate of the monomer complex from Fe(III) or Fe(with the ligand at the interface. The rate constant of infacial aggregation of the Fe(III) complex was smaller ththat of the Fe(II) complex, while the interfacial formatiorate constant of the Fe(III) complex was larger than thathe Fe(II) complex. This suggested that the aggregatemation of the Fe(III) complex required a larger numbercomplexes than that of the Fe(II) complex and took a lontime in the arrangement of the complex to form aggregAs for the Fe(II) complex, although it has a slower complation rate at the interface, the aggregation is faster becof a small aggregation number.

The value of aggregation number can be estimated byequation[28]

(8)Neff =[

FW2/3M(m)

FW2/3M(agg)

]2

,

where FW2/3M is the full width at two-thirds of the maimum absorption spectrum of monomer (m) and its agggate (agg). The value of FW2/3M(m)= 3181 cm−1 for theFe(III) system was obtained from the spectrum after

e

Fig. 9. Kinetics profile of the interfacial reduction reaction of Fe(III) coplex with the addition of ascorbic acid in the heptane–water systemfixed wavelength by CLM method: [HL]= 5.0 × 10−5 M, [Fe(III)] =1.0× 10−3 M, pH 1.7, [Ascorbic acid]= 5.0× 10−2 M.

from the initiation of the complexation (Fig. 2). This valueis larger than that in the equilibrium condition of aggregateformation at 340 s with FW2/3M(agg)= 1145 cm−1. As forthe Fe(II) system, from the experiments conducted undeconditions [HL]= 1.7× 10−5 M, [Fe(III)] = 1.0× 10−3 M,[Ascorbic acid]= 5.0 × 10−2 M, pH 5.0, the values oFW2/3M for monomer at 5 s and for the aggregate at 37were 1965 and 1159 cm−1, respectively. By usingEq. (8),the average values of the effective coherence length (Neff)for Fe(III) and Fe(II) systems were 8 and 3, respectivwhich means the numbers of effectively coupled monomin the aggregates.

3.7. Interfacial reduction reaction

To demonstrate the complexation and aggregation rtion of Fe(III) or Fe(II) complexes at the heptane–waterterface, a direct observation of the interfacial reductionattempted.

Fig. 9 shows the kinetic profile of the interfacial redution of the Fe(III) complex and its aggregate measured afixed wavelengths. In the first stage, the concentration oHL (452 nm) gradually decreased due to a reaction withFe(III) ion, while the concentration of the Fe(III) compleFe(III)L+

2 increased very fast as observed at 554 nm. Simtaneously, the aggregate formation was started at 630 nwell.

Soon after the addition of ascorbic acid solution tomixture of Fe(III) complex and its aggregate in the eqlibrium state at 1340 s, the Fe(III) complex was reducquantitatively to the chargeless Fe(II) complex in pH 1The decrease of the absorbance of the Fe(III) complexcompensated by the increase of the absorbance of Fcomplex.

Fig. 10 shows the change in absorbance spectra ducomplex and aggregate formation and the reduction ofFe(III) system shown inFig. 9. The decreasing of the absorbance of the aggregate of Fe(III) complex at 630 nm athe addition of ascorbic acid was followed by the appearaof the two new peaks of Fe(II) complex with the maxima556 and 740 nm. In a high concentration of the ligand,


dsed.acid

reaseand

thatnm)uldon a

nt ofII)nder

s de

wasnt osult

okm.eac-III)sed

fore,m-theugalec-

ratere-,yce.

II)e

arast,theThed

tate.III)umnd,

idtifictry

a-

oc.

.

76

343

57.

47.998)

Fig. 10. Absorption spectral change ofinterfacial aggregate formation anreduction reaction of Fe(III) complex. First, aggregate at 630 nm increaWhen the aggregation attained the equilibrium reaction, the ascorbicwas added in the system and the absorption spectrum at 630 nm decand appeared a new spectrum with two absorption maxima at 579759 nm. All experimental conditions are the same as those inFig. 9.

reduction produced the aggregation of Fe(II) complexwas assigned from the spectral red shift (579 and 759from the monomer complex. Thus, the CLM method codemonstrate direct measurement of the reduction reactithe liquid–liquid interface.

In the HSS measurement also (Fig. 1c, dashed line), theaddition of ascorbic acid showed an indirect measuremethe interfacial reduction of the Fe(III) complex. The Fe(complex was completely adsorbed at the interface uhigh-speed stirring (5000 rpm)until the equilibrium reactionstate was reached (8400 s). When the stirring speed wacreased to 200 rpm in the range 8400–9000s (Fig. 1c, dashedline), the increase in absorbance at maximum 533 nmobserved and this fact demonstrated a significant amouFe(II) complex extracted in the heptane phase. This reagreed well with that obtained fromFig. 1b. Therefore it re-ally confirmed that the reduction reaction completely toplace at the interface under high-speed stirring, 5000 rp

From the above experimental results, the detailed rtion mechanisms for interfacial reduction reaction of Fe(complex with the presence of ascorbic acid was proposchematically as shown inFig. 6b.

4. Conclusion

We have studied the interfacial growth of complexesFe(II) and Fe(III) ions with pyridylazophenol derivativthe following interfacial aggregate formation of the coplexes, and the interfacial reduction of Fe(III) complex inheptane–water system using high-speed stirring, centrifliquid membrane spectrophotometry, and X-ray photoe

d

t

-

f

l

tron spectroscopy. It is concluded that the formationof the Fe(II) or Fe(III) complex was governed by theaction between the iron ion and the ligand at the interfacewhile the rate of interfacialaggregation was governed bthe formation rate of the monomer complex at the interfaThe mole ratio of interfacial complex formation of Fe(or Fe(III) with 5-Br-PADAP ligand was determined to b1:2 (metal:ligand). The neutral Fe(II)L2 complex completelyadsorbed at the interface if the state of the interfacial arewas large at 5000 rpm by HSS measurement. In contin the small interfacial area state (200 rpm), a part ofFe(II) complex would be extracted in the heptane phase.positively charged Fe(III)L+2 complex would be adsorbecompletely at the interface with ClO−4 , without extractionin the heptane phase even in the small interfacial area sThe interfacial aggregate formations of Fe(II) and Fe(complexes showed a red shift of the absorption maximwith the growth of very strong and sharper absorption bawhich are characteristic for J-aggregates.

Acknowledgments

This work was financially supported by a Grant-in-Afor Scientific Research (A) (No. 12304045) and a ScienResearch of Priority Area (No. 13129204) from the Minisof Education, Science, Sports and Culture, Japan.

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interfacial aggregate growth process of fe(ii) and fe(iii) complexes with pyridylazophenol in...

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