nductivity,rmation off the sizel micelle

Journal of Colloid and Interface Science 294 (2006) 458–465www.elsevier.com/locate/jcis

The critical micelle concentration of tetraethylammoniumperfluorooctylsulfonate in water

José L. López-Fontána, Alfredo González-Péreza, Julian Costab, Juan M. Rusoa, Gerardo Prietoa,Pablo C. Schulzc,∗, Félix Sarmientoa

a Grupo de Biofísica e Interfases, Departamento de Física Aplicada, Facultade de Física, Universidade de Santiago de Compostela,15782, Santiago de Compostela, Spain

b Departamento de Matemáticas, Facultade de Informática, Universidade de A Coruña, 15071, A Coruña, Spainc Departamento de Química, Universidad Nacional del Sur, Bahía Blanca 8000, Argentina

Received 27 May 2005; accepted 19 July 2005

Available online 31 August 2005

Abstract

The aggregation characteristics of tetraethylammonium perfluorooctylsulfonate in water were studied by several techniques: copH, ion-selective electrodes, and surface tension. It was concluded that the aggregation process is gradual and starts with the fooligomers such as ion pairs that grow to give spherical micelles, which become wormlike with increasing concentration. Because oand hydrophobicity of the counterion, micelles quickly increase in ionization degree up to about 0.5. Differences among different criticaconcentration values in the literature are explained on the basis of the gradual formation of micelles. 2005 Elsevier Inc. All rights reserved.

Keywords: Tetraethylammonium perfluorooctylsulfonate; Critical micelle concentration; Premicelles; Perfluorinated surfactants; Air/solution interface

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1. Introduction

The physical properties of surfactant solutions, such as etrical conductivity, surface tension, light scattering, andtrasound velocity, change at the critical micelle concentra(CMC) [1]. These changes can be abrupt or slight. In thecase, the CMC is easily determined by the intersection betwtwo straight lines above and below the CMC. In the second cit is difficult to obtain a precise CMC value, as several strailines can be obtained by fitting the experimental points. Moover, in some cases the employed property does not follolinear relationship with concentration. To solve this problerecently a statistical method for determining the CMC with pcision, the local polynomial regression method (LPRM), wproposed[2]. This method is based on nonparametric estimtion of the regression function, which has the advantagbeing extremely flexible because it does not impose any p

* Corresponding author.E-mail address: pschulz@criba.edu.ar(P.C. Schulz).

0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2005.07.029

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metric model on the subjacent structure of the data. Hencemethod is applicable in practically all circumstances, givgood results.

Perfluorated amphiphiles are more surface-active thanmal hydrocarbon detergents. Solutions of perfluorated sutants usually have lower CMC and attain lower surface tenvalues than the solutions of their hydrogenated homologhaving the same chain length. They are also more temperaresistant. These properties are the reason for using perfluosurfactants to improve the performance of a large varietchemical industrial products (paints, varnish, surface-finisproducts, etc.). Ionic perfluorosurfactants form sphericathreadlike micelles in water, depending on the nature ofcounterion and the concentration of added electrolyte[3]. Inparticular, alkylammonium counterions are known to inducetransition from spherical to threadlike micelles in a seriesperfluorosulfonic and -carboxylic acids, producing a high sotion viscosity[4].

Perfluorooctanesulfonate (PFOS) salts or compoundsmay degrade to PFOS have been employed since a longago in common uses and industrial formulations, such as fa

J.L. López-Fontán et al. / Journal of Colloid and Interface Science 294 (2006) 458–465 459

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treatments, antistatic agents, paper coatings approved forcontact, shampoos, corrosion inhibitors, insecticides, andfighting foams (AFFs)[5,6]. However, PFOS salts are knowfor toxic effects on animals, including lethality[7–10]and otherbiological effects[11–16].

Tetraethylammonium perfluorooctylsulfonate (TEA.PFOsolutions have been studied in the literature using severalniques such as conductivity, kinetic data, NMR, and etric birefringence. However, there are some discrepancies.TEA.PFOS literature CMC values range from 8.8 × 10−4 to1.7×10−3 moldm−3 at similar temperatures[17,18]. This 93%difference (on the basis of CMC= 0.88 mM) is higher thanthat reasonably expected from experimental error. Bosseal. reported a value of 1.1 × 10−3 moldm−3 at 30◦C usingconductimetry measurements[18]. Micelles were reported athreadlike[17,18], but in some cases discoid micelles were sgested, the surfactant molecules being perpendicular to thesurfaces of micelles[17].

In this article we studied the tetraethylammonium perflrooctylsulfonate critical micelle concentration to elucidatereason for these discrepancies.

2. Experimental

The tetraethylammonium perfluorooctylsulfonate (TEPFOS) was from Aldrich and was recrystallized. Water wdouble-distilled, deionized, and degassed before use.

The individual samples were prepared by weight. Since Bsev et al.[18] and Hoffmann and Ulbricht[19] found that thepreparation of the TEA.PFOS samples in water was diffiand a long time is necessary to reach equilibrium, we shthe mixture vigorously; then it was sonicated to ensure hogeneity. Furthermore, samples were left to rest at least 4to reach equilibrium (one day more than Schorr and Hoffm[17] indicated).

Several difficulties were found in ensuring the attainmenequilibrium. It was necessary to perform some controls anconfirm that the measurements were reproducible. As an eple, we prepared each concentration separately and then isonicated and left to rest for 4 days. Next we checked tharesults of measurements were constant with the time: aftefirst measurement, another one was made several hoursand then another after 1 day.

When error intervals were computed, the Studentt functionwas employed with a confidence level of 0.90. To obtainaverage of several measurements having different errorslinear unbiased minimum variance estimation method[20] wasemployed. This method uses the inverse of the square of thror of each measurement as its statistical weight in the ave

The specific conductivity (κ) was measured with a KyotElectronic C-117 conductivity meter calibrated with KCl slutions and literature data[21,22]. Solutions were vigorouslyshaken during conductivity measurement in a thermostaticat 293.15 ± 0.01 K. The κ data were analyzed by the locpolynomial regression method[2] to determinate the inflectiopoint.

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We also used the�κ = κmeasured− κextrapolatedvs total sur-factant concentration (C) representation, whereκmeasuredandκextrapolatedare the measured value of the specific conducity at eachC and theκ value extrapolated at the same cocentration from the data below the CMC. This represetion magnifies the difference in slopes below and aboveCMC. Conductivity data were also studied by the differenconductivity Λd = 1000∂κ/∂C vs Caverage, where∂κ/∂C =1000(κ2 − κ1)/(C2 − C1) andCaverage= (C2 − C1)/2; C1 andC2 being two successive total surfactant concentrations wrespective specific conductivities areκ1 andκ2. This representation allows obtaining additional information from conductividata[23,24].

The pH determinations were made at 293.15 K with a CSON GLP 22 pH meter and a CRISON glass electrode.minutes after the immersion of the electrode in each sampseries of 10 measurements were made. Measurements werformed on the pH and the potential (mV) scale.

Since tetraethylammonium perfluorooctylsulfonate is aof a strong surface-active acid and a weak base (TEAOH)hydrolysis give acid solutions:

TEA+ + H2O ↔ TEAOH + H+. (1)

So there is some tetraethylammonium hydroxide (TEAOin solution in equilibrium with TEA+ and H+ ions.

The apparent constant of acidity of the TEA+ ion Ka =[H+]2/C is very sensitive to changes of the polarity in the enronments of the hydrolyzable species. The measuredKa valueinvolves contributions of the value for the nonaggregated TE+ions in monomeric solution in water and of the counterionsthe Stern layer at the aggregates’ surface. The importancthese contributions depends in turn on the proportion of bspecies in solution. The contribution of TEA+ ions included inthe aggregates’ Stern layer is affected by the polarity ofregion[25,26], which in turn is strongly affected by its struture [27,28]. This structure (nature of head groups and coterions, degree of micelle ionization, distance between phead groups) is in turn affected by the general micelle stture (spherical, wormlike, or disklike shape). The Stern layeanionic micelles attracts H+ and repels OH− [29], and the dif-ference between the local pH in this layer and that in the bsolution may be as large as 2 pH units[30,31]. As a conse-quence, values of pKa may vary up to about 1.3 units whenweak acid is anchored at the micelle Stern layer[32].

The ion-selective measurements were made with a GLPpH meter with a saturated calomel reference electrode (CSON) and a perfluorooctanesulfonate ion-selective electroddetermine the concentration of free (nonaggregated) PF−ions at each total surfactant concentrationC, and a dode-cyltrimethylammonium (C12TEA+) ion-selective electrode tdetermine the free TEA+ ion concentration. The constructioof both surfactant ion-selective electrodes may be found inliterature ((PFOS−) [33]; (C12TEA+) [34]). Although the pri-mary ions for both ion-selective electrodes were not PFOS− andTEA+, their response to these ions was good, as frequentlycurs with ion-selective electrodes[35]. To ensure equilibrium30 min were waited before the first determination, and the

460 J.L. López-Fontán et al. / Journal of Colloid and Interface Science 294 (2006) 458–465

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series of 10 measurements were made with intervals of 30tween two successive potential readings. Two independentwere performed.

Surface tensions were measured with a Krüss ring tensioter, which acquired the data automatically. A series of 10 msurements were made on each sample, once surface teequilibrium was reached. Reproducibility was±10−2 mNm−1.Temperature was kept constant to 293.15±0.01 K by water cir-culation.

3. Results and discussion

Fig. 1 shows one of the�κ vs C plot. The CMC is clearlyseen at 9.8×10−4 moldm−3, and a second inflection point wita slight reduction in conductivity at about 2.5×10−3 moldm−3,indicated as “second CMC.”Fig. 2 shows the differential conductivity (Λd) plotted as a function ofCaverage. The CMC cor-responds to the inflection point. The nonzero slope belowCMC indicates that there is some kind of association be

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ee

micelles form[23,36–38]. The molar conductivity of micellesΛd

M was obtained from the region above the CMC[23,38],giving Λd

M = 20.1± 0.2 S cmmol−1. Another independent determination gaveΛd

M = 24±3 S cmmol−1. A weighted averageusing the linear unbiased minimum variance estimation megives Λd

M = 20.12 ± 0.04 S cmmol−1. This value is higherthan that for dodecylammonium trifluoroctanoate micelles[37],and sodium dodecyl sulfate[38] (∼10 S cm mol−1). The highervalue for TEA.PFOS may be due to a combination of smamicelle and higher charge because of the high micelle iontion degree (see the ion-selective results below).

The surface tension vs logC plot did not show any particular characteristic and is not shown. The CMC calculateding the LPRM method was 8.55 × 10−3 moldm−3 at 25◦C.The surface tension at the CMC was 21.7 mN mol−1, be-ing similar to that of monoethanolammonium perfluoroctysfonate [39]. The surface excess at the CMC, computed wthe Gibbs equation, was 5.67× 10−6 molm−2 in this run and5.51× 10−6 molm−2 in a second independent run. These v

Fig. 1. Difference between measured and extrapolated specific conductivity�κ vs total TEA.PFOS concentration plot. Lines are eye guides.

Fig. 2. Differential conductivity vs average concentration of TEA.PFOS. Lines are eye guides.

J.L. López-Fontán et al. / Journal of Colloid and Interface Science 294 (2006) 458–465 461

.

.

Fig. 3. Response of the PFOS− ion-selective electrode as a function of the surfactant concentration. Full line is the response to unaggregated ions

Fig. 4. Response of the TEA+ ion-selective electrode as a function of the surfactant concentration. Full line is the response to unaggregated ions

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ues are higher than those in the literature (between 1.45× 10−6

and 4.05× 10−6 molm−2) [39]. This gives a molecular areathe air/solution interface ofa = 0.293 and 0.302 nm2, respec-tively, which are smaller than those obtained for other perflrated surfactants (0.41–0.55 nm2 [39]). These differences mabe due to the higher hydrophobicity of the TEA+ counterion, al-lowing the formation of a more compact monolayer. Sincesection of the perfluorated chain, computed with the bondgles and lengths and the covalent radii of F and C atoms,0.175 nm2, the above results are plausible. The smalla valuemay be explained from the analysis of the TEA.PFOS micStern layer made by Bossev et al.[18,40]. These authors suggest that at a high surfactant concentration a compact layTEA+ ions may cover the surface of TEA.PFOS micelles, wclosely packed PFOS− ions at the micelle surface. Taking inaccount the hydrophobicity and volume (closely related topolarizability and thus to its adsorbability on a nonpolar sface) of TEA+ ions, the general structure of the Stern labelonging to the monolayer adsorbed onto the air/solutionterface probably is similar to that of TEA.PFOS micelles, ia very compact packing, which may explain the lowa value.

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Fig. 3 shows the response of the PFOS− ion-selective electrode, andFig. 4 that of the TEA+ one. Concentrations showing changes in the electrode response are marked on thures. The data were studied with the usual technique[41] todetermine the concentration of free (nonmicellized) ions,cause micellized ones do not affect the response of theselective electrodes[42]. These computations gave the daplotted in Fig. 5. It may be seen that aggregation startsabout 5× 10−4 moldm−3, giving aggregates having the samamount of TEA+ and PFOS−, which suggest that the aggrgates may be ion pairs TEA.PFOS. However, there is a chat 6.6× 10−4 moldm−3, which suggests that the ion pairs cature some PFOS− ions, giving charged structures, probably pmicelles. At about 8.2×10−4 moldm−3 there is a change in thaggregation, showing the formation of micelles.

Fig. 6 shows the ionization degree of aggregates, cputed asα = 1 − ([TEA+]T − [TEA+]free)/([PFOS−]T −[PFOS−]free), where [X]T and [X]free indicate the total molar concentration and that of nonaggregated ions of specierespectively. It may be seen that at 6.4 × 10−4 moldm−3

the premicellar aggregates change in nature from uncha

462 J.L. López-Fontán et al. / Journal of Colloid and Interface Science 294 (2006) 458–465

Fig. 5. Concentration of the different species as a function of the total surfactant concentration. Open circles: free TEA+ ions; squares: nonmicellized PFOS− ions;triangles: aggregated PFOS− (on a monomer basis). Full line: behavior of ions if no aggregation occurs.

Fig. 6. The aggregates’ ionisation degree determined from ion-selective electrodes results plotted as a function of the total surfactant concentration. The curve is aneye guide.

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ion pairs TEA.PFOS to the general structure TEA2PFOS−3at 7.4 × 10−4 moldm−3. Bossev et al.[18] suggested theexistence of oligomers even in equilibrium with micelleshigh concentration, but they did not suggest any comption for such submicellar aggregates. Thus, micelles form7.9 × 10−4 moldm−3 having α ≈ 0.18, and the increase oconcentration gives an increase inα up to 0.00103 mol dm−3,where the maximum value ofα is attained. Hoffmann and Ulbricht determinedα = 0.37 for TEA.PFOS at the CMC and a20◦C [19], and 0.17 at 25◦C [43] by conductivity. SinceFig. 6shows a sudden increase ofα from 0.18 at 7.9×10−4 moldm−3

to 0.46 at 8.86 × 10−4 moldm−3, small differences in theadopted CMC value or in the value of slopes in the condtivity plots may explain the difference of theα values in lit-erature. The value ofα ≈ 0.5 found between 9× 10−3 and1.1×10−2 moldm−3 may be explained from the analysis of tTEA.PFOS micelle Stern layer made by Bossev et al.[18,40].These authors suggested that at high surfactant concentra

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compact layer of TEA+ ions may cover the surfaces of PFOmicelles, in a proportion of roughly two PFOS− ions eachTEA+ one, because TEA+ ions are rather bulky and hydrophbic, which would explain the limited accommodation of thecounterions in the Stern layer[40]. This might give an ionization degree of about 0.5.

On the other hand, if aggregates are small, the polar hgroups are far each other and may accommodate a largeportion of the bulky TEA+ ions, thus giving a smaller value oα at low micelle concentration, just at the CMC.

Fig. 7 shows the variation of pH of solution with concetration. It may be seen that the concentration of hydrogenwas almost constant up to the CMC, which is consistent wthe strong reduction in the increase of the concentration ofTEA+ ions with the total concentration (seeFig. 5). There is achange atC = 2.2× 10−3 moldm−3. The apparent constant oacidity Ka = [H+]2/C is very sensitive to changes in the plarity in the environments of the hydrolyzable species.Fig. 8

J.L. López-Fontán et al. / Journal of Colloid and Interface Science 294 (2006) 458–465 463

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Fig. 7. pH vs the logarithm of the total concentration of tetraethylammonium perfluorooctylsulfonate.

Fig. 8. Representation of the logarithm of the apparent acidity constant of TEA+ as a function of the logarithm of the total surfactant concentration. Lines aeye guide.

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shows the dependence of logKa on logC, and the concentrations at which changes in the dependence occur are markethe figure. A change at 6.6×10−4 moldm−3 suggests the modfication of the structure of premicellar aggregates and coincwith the capture of PFOS− ions by the ion pairs as suggestby the ion-selective data (Fig. 5). It can be seen that betweeC = 9.1× 10−4 and 1.02× 10−3 moldm−3, there is a graduatransition. This means that the formation of micelles is grual in this concentration range, confirming that the CMCnot a unique, well-defined concentration but an interval. Tsituation explains the differences in CMC reported in the liature, since different methods have different sensitivity tochanges occurring in that concentration range. The chang1.6×10−3 and 2.6×10−3 moldm−3 probably indicate changein micelle structure.

Table 1shows the different critical concentrations founddifferent methods and the average values. ConcentrationC1 =(4.77± 0.27) × 10−4 mol dm−3 corresponds to the formatioof ion pairs (TEA.PFOS), which change in structure at ab

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C2 = (6.4± 1.0) × 10−4 moldm−3, giving charged aggregateof structure TEA2PFOS−3 . The average CMC isC3 = (9.00±0.44) × 10−4 moldm−3, but this is an estimate of the centrconcentration inside an interval ranging from about 8× 10−4

to 1.1 × 10−3 moldm−3, corresponding to gradual formatioof micelles. Values of the CMC of perfluorated surfactants hing the same chain length are 9×10−4 moldm−3 for potassiumperfluorononanoate[44], 8.5× 10−3 moldm−3 for sodium per-fluorooctylsulfonate (NaPFOS)[45], 6.3× 10−3 moldm−3 forLiPFOS [45], 8.0 × 10−3 moldm−3 for KPFOS[45], 5.5 ×10−3 moldm−3 for NH4PFOS, and 4.6 × 10−3 moldm−3 forNH3C2H4OH.PFOS[45]. Hoffmann and Ulbricht[19] deter-mined TEA.PFOS CMC= 9.45× 10−4 moldm−3 by conduc-tivity.

The concentrationsC4 = (1.49± 0.08) × 10−3 mol dm−3,C5 = (2.30 ± 0.15) × 10−3 moldm−3, and C6 = (5.17 ±0.19) × 10−3 moldm−3, probably reflect changes in the mcelle structure. Perfluorsurfactants of similar structure hvery small micelles with aggregation numbern = 15 [46]–

464 J.L. López-Fontán et al. / Journal of Colloid and Interface Science 294 (2006) 458–465

Table 1Critical concentrations found in the TEA.PFOS aqueous system at 20◦C

Concentration (mol dm−3)

Property C1 C2 C3 C4 C5 C6

pH (logC) 0.000525 0.000525 0.00081 0.00223logKa (logC) 0.00046 0.00097 0.00264 0.00525

0.00102a

TEA+ ion-electrode 0.000443 0.00066 0.00113 0.00148 0.00241PFOS− ion-electrode 0.00048 0.00078 0.0015 0.00201 0.0050

0.000801a

0.00115a

α (C) 0.00074 0.00079σ (logC) 0.000875 0.00171

0.000829a

0.000892a

κ (C) 0.000962 0.002280.000945a 0.0015a 0.00195a

0.0014a 0.00260a

�κ (C) 0.000892 0.0014 0.00226Λd (Caverage) 0.00090Λ (

√C) 0.00078 –

Average value 0.000477 0.00064 0.000900 0.001493 0.00230 0.00517±2.7× 10−5 ±0.00010 ±4.4× 10−5 ±7.8× 10−5 ±0.00015 ±0.00019

Note. The meanings of the different symbols are as follows:α (C): micelle ionization degree vs total surfactant concentration;σ (logC): surface tension vsthe logarithm ofC; logKa (logC): logarithm of the apparent acidity constant vs logC; κ (C): specific conductivity vsC; �κ (C): excess conductivity vsC;Λd (Caverage): differential conductivity vs average total surfactant concentration, andΛ (

√C): equivalent conductivity vs the square root ofC.

a Data from different runs.

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20 [47] for sodium perfluoroctanoate andn = 20 ± 2 forlithium perfluorononanoate[48]. These aggregation numbeare compatible with spherical micelles[49,50]. Schorr andHoffmann [17] studied the electric birefringence on micelsolutions of TEA.PFOS. It may be seen inFig. 1 of their ar-ticle that the Kerr constantB extrapolates to zero at abo1.5 × 10−3 moldm−3, which is ourC4 value. BeforeC4 thereis a gradual increase inB. This increase follows a curve between C5 and C6, and then increases almost linearly. ThSchorr and Hoffmann’s results also indicate changes instructure of aggregates. Extrapolation to zero micelle numdensity in Fig. 12 of the Schorr and Hoffmann article[17](which was computed on the supposition of the existencrodlike micelles) indicates that rodlike micelles disappea1.25 × 10−3 moldm−3. Since an increase in the Kerr costant can be caused by aggregates that become more andanisotropic[17], Schorr and Hoffmann results reinforce ointerpretation of the phenomena giving rise to the chanat C4 to C6. Moreover, Schorr and Hoffmann reported thTEA.PFOS micelles are rodlike with a radius of 0.22 nm[17].The same authors found that at about 6× 10−3 moldm−3, theaverage distance (D) between the wormlike micelles becomcomparable with their length (L) and then micelles can interaby hydrodynamic and electrostatic interactions. This situawas also verified by viscosity measurements. However, atconcentrationD = 63.5 nm andL = 30 nm[17], which meansthat overlap between neighboring micelles does not occur,that micelles can rotate freely, as verified by the same autusing the decay of the birefringence. In view of the values oα

we found by using ion-selective electrodes, this means thainteraction between micelles aboveC6 is mainly electrostatic.

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4. Conclusions

The use of different techniques (pH, ion-selective electrosurface tension, and conductivity) enabled us to elucidateorigin of differences in the published CMC values, as cauby a gradual aggregation when concentration is raised. Othis situation is known, the CMC values obtained by differtechniques are mutually consistent. This study also enableto know the complex aggregation mechanism of this surfac

The aggregation of TEA.PFOS follows a gradual proces(C1 = (4.77± 0.27) × 10−4 mol dm−3) ion pairs (TEA.PFOS)are formed, which transform into charged oligomers of proble structure TEA2PFOS−3 atC2 ((6.4±1.0)×10−4 moldm−3).The average CMC isC3 = (9.00±0.44)×10−4 moldm−3, butthis is an estimation of the central concentration inside anterval ranging from about 8× 10−4 to 1.1 × 10−3 moldm−3,corresponding to a gradual formation and growth of sphermicelles havingα = 0.18 at the lower limit andα ≈ 0.5 at theupper limit of this interval.

The air/solution interface monolayer at the CMC is mcompact than that for other perfluorated surfactants.

At the concentrationC4 ((1.49± 0.08) × 10−4 moldm−3)spherical micelles became wormlike. There is a structchange atC5 = (2.30± 0.15) × 10−3 moldm−3, and the in-teractions among micelles become important atC6 = (5.17±0.19) × 10−3 moldm−3. These interactions are probably eletrostatic in nature.

Acknowledgments

This research was founded by the Spanish Ministry of Sence and Technology (Project MAT 2002-00608, Europ

J.L. López-Fontán et al. / Journal of Colloid and Interface Science 294 (2006) 458–465 465

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FEDER support included) and by the Xunta de Galicia (ProPGIDIT 03 PXIC20615PN). P.C.S. thanks the Consejocional de Investigaciones Científicas y Técnicas de la RepúArgentina (CONICET) for PIP 2739, which enabled himtravel to the University of Santiago de Compostela and won this research.

References

[1] M. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, 1[2] J.L. López-Fontán, J. Costa, J.L. Ruso, G. Prieto, F. Sarmiento, Eur. P

J. E 13 (2004) 133.[3] H. Hoffmann, J. Würtz, J. Mol. Liq. 72 (1997) 191.[4] A. Knoblich, M. Matsumoto, K. Murata, Y. Fujiyoshi, Langmuir 11 (199

2361.[5] U.S. Environmental Protection Agency, Fed. Regist. 65 (2000) 62319[6] U.S. Environmental Protection Agency, Fed. Regist. 67 (2002) 11008[7] A.M. Seacat, P.J. Thomford, K.J. Hansen, G.W. Olsen, M.T. Case,

Butenhoff, Toxicol. Sci. 68 (2002) 249.[8] M.T. Case, R.G. York, M.S. Chistian, Int. J. Toxicol. 20 (2001) 101.[9] A. Tullo, Chem. Eng. News (May 29, 2000) 12.

[10] H. Sanderson, T.M. Boudreau, S.A. Mabury, W.-J. Cheong, K.R. Smon, Environ. Toxicol. Chem. 21 (2002) 1490.

[11] R.L. Darwin, R.E. Ottman, E.C. Norman, J.E. Gott, C.P. Hanauska, NFire Protect. Assoc. (1995) 67.

[12] J. Berthiaume, K.B. Wallace, Toxicol. Lett. 129 (2002) 23.[13] M. Derbel, M. Hosokawa, T. Satoh, Biol. Pharm. Bull. 19 (1996) 765.[14] T. Ikeda, K. Fukuda, I. Mori, M. Enomoto, T. Komai, T. Suga, in: H.

Fahimi, H. Sies (Eds.), Peroxisomes in Biology and Medicine, SprinBerlin, 1987, pp. 304–308.

[15] A.A. Starkov, K.B. Wallace, Toxicol. Sci. 66 (2002) 244.[16] D.J. Leubker, K.J. Hansen, N.M. Bass, J.L. Butenhoff, A.M. Seacat, T

icology 176 (2002) 175.[17] W. Schorr, H. Hoffmann, J. Phys. Chem. 85 (1981) 3160.[18] D.P. Bossev, M. Matsumoto, M. Nakahara, J. Phys. Chem. B 103 (1

8251.[19] H. Hoffmann, W. Ulbricht, Z. Phys. Chem. (N.F.) 106 (1977) 167.[20] J. Mandel, Statistical Analysis of Experimental Data, Interscience, N

York, 1964.

t-a

.s.

.

-

.

,

-

)

[21] T. Shedlovky, J. Am. Chem. Soc. 54 (1932) 1411.[22] J.F. Chambers, J.M. Stokes, R.H. Stokes, J. Phys. Chem. 60 (1956)[23] T. Asakawa, T. Shiraishi, S. Sunasaki, S. Muyaguishi, Bull. Chem.

Jpn. 68 (1995) 2503.[24] T. Matsumoto, Colloid Polym. Sci. 270 (1992) 492.[25] D.C. Grahame, J. Chem. Phys. 23 (1955) 1725.[26] P. Mukerjee, K. Banerjee, J. Phys. Chem. 68 (1964) 3567.[27] F. Booth, J. Chem. Phys. 19 (1951) 391, 1327, 1615.[28] M.J. Sparnaay, Rec. Trav. Chim. 77 (1958) 872.[29] A.L. Loeb, J.Th.G. Overbeek, P.H. Wiersema, The Electrical Dou

Layer around a Spherical Colloid Particle, MIT Press, Cambridge, M1960.

[30] A.D. James, B.H. Robinson, J. Chem. Soc., Faraday Trans. I 74 (1978[31] M. Montal, G. Gitler, Bioenergetics 4 (1973) 363.[32] H.W. Hoyer, A. Marmo, J. Phys. Chem. 65 (1961) 1807.[33] J.L. López-Fontán, F. Sarmiento, P.C. Schulz, Colloid Polym. Sci. 283

(2005) 862.[34] M.A. Morini, P.C. Schulz, J.E. Puig, Colloid Polym. Sci. 274 (1996) 66[35] P.L. Bayley, Analysis with Ion-Selective Electrodes, second ed., Hey

London, 1980.[36] G.S. Hartley, Aqueous Solutions of Paraffin Chain Salts, Herman, P

1936.[37] G. Sugihara, Y. Era, M. Tunatsu, T. Kunitake, S. Lee, Y. Sasaki, J. Co

Interface Sci. 187 (1987) 435.[38] D. Stigter, Rec. Trav. Chim. Pays. Bas. 73 (1954) 611.[39] E. Kissa, Fluorinated Surfactants and Repellents, second ed., in: Surf

Science Series, vol. 97, Dekker, New York, 2001, p. 111.[40] D.P. Bossev, M. Matsumoto, T. Sato, Y. Watanabe, M. Nakahara, J. P

Chem. 103 (1999) 8259.[41] P.C. Schulz, Colloids Surf. 34 (1988/1989) 69.[42] M.A. Morini, P.C. Schulz, Colloid Polym. Sci. 275 (1997) 802.[43] H. Hoffmann, B. Tagesson, Z. Phys. Chem. (N.F.) 110 (1978) 113.[44] H.B. Klevens, M. Raison, J. Chim. Phys. 51 (1954) 1.[45] K. Shinoda, M. Hato, T. Hayashi, J. Phys. Chem. 76 (1972) 909.[46] G. Sugihara, P. Mukerjee, J. Phys. Chem. 85 (1981) 1612.[47] E. Fisicaro, E. Pelizzetti, R. Bongiovanni, E. Borgarello, Colloids Surf.

(1990) 529.[48] C. La Mesa, B. Sesta, J. Phys. Chem. 91 (1987) 1450.[49] P.C. Schulz, Colloid Polym. Sci. 269 (1991) 612.[50] P.C. Schulz, J. Colloid Interface Sci. 152 (1991) 333.