Langmuir monolayers of the zwitterionic surfactant hexadecyl 1-N-l-tryptophan glycerol ether

ARTICLE IN PRESSS0021-9797(04)00512-0/FLA AID:10267 Vol.•••(•••)ELSGMLTM(YJCIS):m5 2004/05/28 Prn:2/07/2004; 11:09 yjcis10267 P.1 (1-7)

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Journal of Colloid and Interface Science••• (••••) •••–•••www.elsevier.com/locate/jcis

Effect of the addition of water-soluble polymers on the interfacialproperties of aerosol OT vesicles

Margarita Valeroa, M. Mercedes Velázquezb,∗

a Departamento de Química Física, Facultad de Farmacia, Universidad de Salamanca, Campus Unamuno, Apartado 449, 37080 Salamanca, Spainb Departamento de Química Física, Facultad de Ciencias Químicas, Universidad deSalamanca, 37008 Salamanca, Spain

Received 17 September 2003; accepted 1 June 2004

Abstract

The properties of the interface of vesicles of pure sodium bis-(2-ethyl-hexyl) sulfosuccinate (AOT) and binary mixtures composewith poly(ethylene) glycol (PEG), poly(sodium 4-styrensulfonate) (PSS) and sodium chloride were investigated using absorption anstate fluorescence of nabumetone and electrophoretic mobility measurements. Resultsconfirm those obtained in aprevious work indicatingthat the addition of PEG, PSS, and NaCl stabilizes the AOT vesicles. The stabilization mechanism is the screening of the surfacethe case of binary mixtures of AOT/PSS and AOT/NaCl and the polymer adsorption on the interface for vesicles of AOT/PEG. 2004 Elsevier Inc. All rights reserved.

Keywords:Aerosol OT; Vesicles; Poly(ethylene) glycol; Poly(sodium 4-styrensulfonate); Nabumetone; Fluorescence probing; Electrophoretic mobility

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1. Introduction

Vesicles consist of a surfactant bilayer that separan inner region of water from a continuous phase ofsame fluid. Biological and industrial applications suchmicroencapsulation for drug delivery, catalysis, or cleing depend on a controlled method for the formationwell-defined size vesicles. Nonequilibrium methods suchsonication, dialysis, extrusion of lamellar suspensions,reverse-phase evaporation[1] are usually necessary to obtavesicles that in most cases are unstable and highly polyperse. Spontaneous formation of vesicles has been descin the literature. Most previous work on spontaneous vcle formation involved surfactant mixtures, catanionic[2],cationic/cationic[3,4], nonionic/ionic[5], and hydrophilicpolymers with surfactant[6,7]. These spontaneous vesiclare thermodynamically more stable than those formednonequilibrium methods. A theoretical model based onthermodynamics of a dispersed solution of aggregates aveloped by Tanford[8] and Israelachvili et al.[9] has beenused to explain vesicle formation. These models are basemolecular packing[9] and on concepts of curvature elas

* Corresponding author. Fax: 00-34-923-294574.E-mail address:[email protected] (M.M. Velázquez).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.06.003

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theory[10]. In vesicles formed by mixtures, novel thermdynamic approaches to calculating the vesicle curvatureenergy introduce contributions due to geometrical pack[11,12], electrostatic[13,14], hydrocarbon chain conformational contribution[15], or the influence of mixing surfactants[16,17].

In previous work, we studied the spontaneous vesformation of AOT and the effect of poly(ethylene) glyc(PEG) and poly(sodium 4-styrensulfonate) (PSS) on vesformation[18]. We found two critical vesicle concentratiocorresponding to different kinds of aggregates, smalllarge vesicles. The morphology of these aggregateseasily observed by video-enhanced optical microscopy.addition of PSS, PEG, and NaCl stabilizes the vesicdecreasing the critical concentrations[18].

We are interested in the mechanism of the stabilizationof AOT vesicles by the addition of polymers; thus, takiinto account that the structure of the interface playsimportant role in vesicle stability[19], the properties othe interfaces of these vesicles have been obtaineduse fluorescence probing to estimate structural propeof the vesicle interfaces. In this case it is necessarychoose a probe localized at the interface and sensto changes on the microenvironment properties[20]. Thefluorescence probe chosen was nabumetone (Scheme 1).

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2 M. Valero, M.M. Velázquez / Journal of Colloid and Interface Science••• (••••) •••–•••

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Scheme 1. Chemical structure of the fluorescence probe nabumeto

This molecule is a naphthalene derivative with spectroscproperties sensitive to the nature of the environmenwhich it is located[21] and has been successfully usedobtain information on the properties of the cationic miceinterface[22].

On the other hand, we are also interested in obtaithe vesicle surface potential; however, it is usually difficto obtain it. An alternative strategy is to use electrokinmeasurements, which can be interpreted in terms ofzeta potential,ζ . Experience shows that it is possiblecorrelate colloidal stability with this quantity[23]. Thereforethe electrophoretic mobility of vesicles formed by pure AOand binary mixtures of AOT with PSS or PEG were obtainFor comparative purposes, because polymer moleculePSS have Na+ as counterion, the influence of the additiof NaCl on the properties of AOT vesicles was also stud

2. Experimental

2.1. Materials and vesicle preparation

The surfactant sodium bis-(2-ethylhexyl) sulfosuccinwas purchased from Fluka and was purified accordinthe published method[24]. The purity was evaluated busing gas chromatography[25]. The results indicate that thsamples contain<0.5% (w) of 2-ethyl hexanol. This alcohois formed by hydrolysis of either one or both of the twhydrocarbon tails.

Poly(sodium 4-styrensulfonate) (Mr 75,000), was fromAldrich and poly(ethylene glycol) (Mr 17,000) was fromFluka. Nabumetone, 4-[6-methoxy-2-napthyl]-2-butanowas from Sigma Chemical Co. The polymers and probe wused as received without further purification.

The pure vesicles were prepared by adding the calcuamount of surfactant to the solvent water. The most disolutions were prepared by dilution from the stock conctrated solution. In polymer–surfactant mixtures the surtant was dissolved in the polymer aqueous solution wigiven concentration. In all cases care was taken not toexternal energy input except for gentle stirring.

The solutions were prepared with water purified withcombination of RiOs and Milli-Q systems from MilliporeAll solutions were prepared the day before to obtainfluorescence spectra and the electrophoretic mobility valueand were maintained at 30◦C.

f

2.2. Steady-state fluorescence measurements

The emission spectra of nabumetone incorporatedAOT vesicles were recorded with the LS-50B spectroflrimeter. The concentration of nabumetone was kept conat a value of 4× 10−5 M. The excitation wavelength wa317 nm and the excitation and emission slits were keptstant at values of 2.5/2.5 or 3/3 nm as a function of the flrescence intensity. The instrumental response at each wlength was corrected by means of the curve providedthe spectrofluorimeter.

The fluorescence quantum yield,Φf , was calculated usinthe equation[26]

(1)Φf = Φr

(n2

u

n2r

)(Ar

Au

)(Fu

Fr

),

where the subscripts r and u stand for reference and nabtone, respectively;F is the area under the corrected sptrum, n is the solvent refractive index,A is the absorbancat the exciting wavelength, andΦr is the reference quantuyield, 0.543 for quinine sulfate in 0.1 N sulfuric acid.

2.3. Zeta potential measurements

The Zetasizer 3000 device (Malvern, UK) was usedmeasure theζ -potential of vesicles. This apparatus uslaser Doppler velocimetry to measure the electrophormobility, µe. All experiments were made in a 5× 2-mmrectangular quartz capillary. Each experimental value is thaverage of three measurements and the standard deviatthese measurements was considered to be the experimerror.

The calculation ofζ -potential can be realized by thSmoluchowski equation[27],

(2)ζ = ηµe/ε,

whereη and ε are the absolute viscosity and permittivof the medium, respectively. The viscosity of aqueous pmer solutions was determined elsewhere and no significhanges with respect to water were observed[28]. SinceEq. (2) is not adequate for converting electrophoretic mbility into ζ -potential in several colloidal particles suchlatex polymers[29], we represent the electrophoretic mobity, µe, as a function of the polymers and of the electrolconcentrations.

3. Results and discussion

3.1. Steady-state fluorescence measurements

The absorption and emission spectra of nabumesolubilized in vesicles of aerosol OT and in vesicles of mipolymer–AOT in aqueous solutions were obtained at 30◦C.Some of these spectra are presented inFigs. 1 and 2. Fig. 1shows the absorption spectra of nabumetone solubilize


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M. Valero, M.M. Velázquez / Journal of Colloid and Interface Science••• (••••) •••–••• 3

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AOT/PEG mixtures. The PEG was kept constant at a valu1.2 × 10−4 M while the AOT concentration varied betwee0 and 3× 10−2 M. The spectrum is typical of a naphthale2-substituted compound[30] and is a three-band systemAs expected, the longer wavelength band correspondin

Fig. 1. Absorption spectra of nabumetone dissolved in 1.2 × 10−4 M PEGaqueous solutions without AOT and withdifferent AOT concentrations(1) [AOT] = 0; (2) [AOT] = 0.005; (3)[AOT] = 0.031 M.

1Ag → 1Lb is the most sensitive to changes in the solvpolarity [31] and a linear correlation between the relativepermittivity of the drug environment and the absorptmaximum was found[21].

In vesicles of AOT/PSS the polymer concentration wkept constant at a value of 1.3×10−5 M, while in AOT/NaClvesicles the NaCl concentration remained constant at 4.9 ×10−3 M. This NaCl concentration is the free Na+ concentra-tion left by 1.3× 10−5 M of PSS polymer.

As can be seen inFig. 1 the absorption spectrumshifted to longer wavelengths when the AOT concentraincreases. This trend has been observed in all systemsied, indicating that nabumetone molecules are transfefrom water to a less polar environment inside the aggregas AOT concentration increases. This behavior was prevously observed in micelles ofN -cetyl-N ,N ,N -trimethyl-ammonium bromide (CTAB)[22].

In addition, no changes in the maximum positionthe nabumetone absorption spectrum were observeaqueous solutions of polymers and NaCl, indicatinginteractions between the probe and these additives.relative permittivity of the probe microenvironment w

(A) (B)

(C) (D)

Fig. 2. Fluorescence spectra of nabumetone solubilized in AOT vesicles containing different surfactant concentrations: (A) pure AOT vesicles; (B)AOT/PEGvesicles,[PEG] = 1.2× 10−4 M; (C) AOT/NaCl vesicles,[NaCl] = 4.9× 10−3 M; and (D) AOT/PSS vesicles,[PSS] = 1.3× 10−5 M.


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Table 1Spectroscopic parameters of the probe nabumetone dissolved in difmedia

System Nabumetone maximum ofabsorption (nm)

εa Φf

Water 330.0 79 0.0301.2× 10−4 M PEG 330.0 79 0.0301.3× 10−5 M PSS 330.0 79 0.0304.9× 10−3 M NaCl 330.0 79 0.030AOT 331.6 34 0.056AOT/1.2× 10−4 M PEG 332.0 24 0.098AOT/1.3× 10−5 M PSS 332.0 24 0.053AOT/4.9× 10−3 M NaCl 332.0 24 0.060

Note. In pure and mixed AOT vesicles the AOT concentration was alw0.03 M.

a Values obtained in this work; see text.

calculated from the position of the absorption maximumnabumetone incorporated intothe AOT vesicles. Results acollected inTable 1. For the sake of clarity,Table 1presentsonly results corresponding tonabumetone solubilized ilarge AOT vesicles,[AOT] = 0.03 M. For comparisonvalues obtained in water and in polymer aqueous solutwithout surfactant are also included. As can be seenrelative permittivities of the environment of the drugaggregates formed in the absence and presence oadditives are clearly different. Nabumetone is solubilizea more hydrophobic microenvironment in vesicles formby binary mixtures of surfactant–polymers or surfactaNaCl than in pure AOT vesicles.

The emission spectra of nabumetone dissolved inAOT and AOT/PEG, AOT/PSS, and AOT/NaCl mixturshow significant differences when the surfactant concentrtion increases (Fig. 2). In AOT aqueous solutions the spetrum consists of a band centered around 350 nm (Fig. 2A).The intensity of the emission spectrum increases as thefactant concentration increases, indicating that the pnabumetone passes to a more hydrophobic environmentil it reaches a particular surfactant concentration at whicthe intensity practically remains constant. In vesiclesconcentration corresponds to the cvc. No changes inmaximum position of the emission spectrum are obsewhen the surfactant concentration increases. It is interestito note that the vibrational structure appears at the higAOT concentrations. This fact indicates that the probe isubilized in a rigid microenvironment of the aggregates.

A similar trend in the nabumetone emission spectraubilized in vesicles of AOT/PEG and AOT/NaCl was fou(Figs. 2B and 2C); however, significant differences in vescles of AOT/PSS were observed.Fig. 2Dshows the nabumetone emission spectra in AOT/PSS mixtures. The sptrum consists of a wide and asymmetric band centere375 nm with a shoulder centered around 365 nm. The esion intensity increases as the surfactant concentration increases. This band could be the sum of two contributionsnabumetone and polymer emissions. To confirm this pthe emission spectrum of 1.3 × 10−5 M PSS aqueous solu

t

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Fig. 3. Effect of the AOT concentration on the fluorescence quantuyield of nabumetone solubilized on AOT vesicles: open circles (AOsquares (AOT/PSS,[PSS] = 1.3× 10−5 M); circles (AOT/NaCl,[NaCl] =4.9× 10−3 M); and triangles (AOT/PEG,[PEG] = 1.2× 10−4 M).

tion was recorded (seeFig. 2D). The spectrum shows a maimum emission at 378 nm, and no significant changes wdetected when different amounts of AOT were added.PSS emission spectrum was subtracted from that of nabtone in vesicles of AOT/PSS of variable surfactant conctrations. The resultant spectrum was corrected, and thtensity of emission presents a dependence on surfactancentration similar to that in vesicles of AOT and AOT/PEand AOT/NaCl.

To quantify differences in intensity of emission, tquantum yield is preferred to the fluorescence intenstherefore the fluorescence quantum yield was calculateaccording toEq. (1).

Fig. 3 presents the nabumetone fluorescence quanyield as a function of surfactant concentration for the stems studied in this work. As can be seen, the quantum yof nabumetone increases with AOT concentration untreaches a plateau at a particularsurfactant concentrationcvc. To interpret the dependence of the nabumetone quatum yield on AOT concentration, it is necessary to consthat nabumetone can exist in two preferred conformatione ascribed to a folded conformation responsible forfluorescence quenching of the naphthalene ring by thetanone side chain, lowΦf , and the other correspondingcompletely extended a side chain and highΦf . The forma-tion of each conformation is affected mainly by the prence of water. Thus the folded conformation predominateaqueous solutions (∼91%)[21]. Therefore at low surfactanconcentration nabumetone was dissolved in water at lowΦf .When the surfactant concentration increased, the probetransferred to a more hydrophobic microenvironment ptecting it from water quenching, and the quantum yieldcreased. When the AOT concentration reached the cvcprobe nabumetone was incorporated into the aggregatethe quantum yield remained constant. Accordingly, thecan be evaluated from the nabumetone quantum yield vsfactant concentration curves. The values are collected inTa-ble 2.


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Table 2Critical vesicle concentration values found for aerosol OT vesicles

Additive Fluorescence

cvc (mM)

Electrical conductivitya

cvc1 (mM) cvc2 (mM)

0 7.3 7.8 19.71.2× 10−4 M PEG 5.4 3.5 181.3× 10−5 M PSS 8.1 4.5 154.9× 10−3 M NaCl 5 5.9 10.4

a Data from Ref.[18].

The critical concentration of vesicle formation was otained in previous work by conductivity and pyrene fluorcence[18]. Two critical vesicle concentrations corresponing to small and large vesicles were determined. The mphology of the aggregates was observed by video-enhaoptical microscopy[18]. For comparative purposes thevalues are also presented inTable 2. For the sake of clarity, the table only presents values obtained from condtivity measurements because they agree acceptably withose from pyrene fluorescence. Comparison of theseues shows an acceptable agreement between the cvcues from nabumetone fluorescence probing and the1values obtained from conductivity measurements. Howenabumetone fluorescence only detects one critical concentration value. Consequently, electrical conductivity measments are more sensitive than nabumetone probing fodetermination of the critical concentrations of AOT vesicl

The cvc values found in this work are consistent wthose obtained previously by electrical conductivity[18]or surface tension measurements[32]. These values showthat the addition of PSS or PEG polymers or NaClcreases the cvc of pure AOT vesicles. This classical beior in polymer–surfactant mixtures indicates the existencepolymer–surfactant interactions responsible for the stabiliztion of the aggregates[33].

Table 1 also presents the quantum yield of the drsolubilized in vesicles of AOT, AOT/PSS, AOT/PEG, aAOT/NaCl. The values correspond to vesicles at surfacconcentrations greater than cvc2, or large vesicles. The results show that the quantum yield is practically the saexcept in vesicles containing PEG polymer. In this stem the quantum yield is higher than in the other AOvesicles studied in this work. The fluorescence quanyields of nabumetone incorporated into AOT, AOT/PSS,AOT/NaCl vesicles have values of around 0.06. This vais higher than the quantum yield of nabumetone aqueoulutions (0.03), while it is lower than the value in organsolvents[21]. This indicates that the probe is partially prtected from the water quenching. However, neither the ption of the maximum of the nabumetone absorption spectnor theΦf of nabumetone indicates the complete protecof water [21]. We compare theΦf values with those obtained for nabumetone partially protected from water, sas nabumetome–cyclodextrin complexes. In these systhe drug forms a complex through the inclusion of the nathalene ring inside the cyclodextrin[22], partially protect-

d

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s

Fig. 4. Effect of the PEG concentration on the fluorescence quantum yieof nabumetone incorporated into vesicles composed of 0.03 M of AOT

ing nabumetone from water quenching. The quantum yof nabumetone complexed by different cyclodextrins vabetween 0.045 and 0.065[22]. These values are very closto the values found in this work. This fact seems to incate that the probe is located at the vesicle interface in AAOT/PSS, and AOT/NaCl vesicles; therefore it is in parcontact with water.

The quantum yield of nabumetone in 0.03 M solutioof AOT and different PSS concentrations ranging from 1×10−6 to 1.3× 10−5 M were obtained. Results show thatΦfremains constant at a value around 0.06. This value agvery well with the quantum yield of nabumetone incorprated into empty vesicles formed by 0.03 M AOT. Conquently, the interactions between AOT and PSS or Ndo not affect the fluorescence properties of the fluorescprobe nabumetone.

From the results inTable 1we can see that theΦf of thenabumetone incorporated into AOT/PEG vesicles is alwhigher than that in vesicles formed by AOT, AOT/PSS, aAOT/NaCl. In addition, the relative permittivity values esmated from the position of the maximum of the nabumetabsorption spectrum are similar in all the filled vesicles stied in this work (Table 1). This behavior can be explaineif one considers that PEG is adsorbed at the vesicle inface, protecting the fluorescence probe from water queing. To confirm this fact, the quantum yield of nabumetosolubilized in vesicles formed by 0.03 M surfactant and vable PEG concentration was obtained and results are reprsented inFig. 4. One can see thatΦf remains constant atvalue close to that of pure AOT until the PEG concentratreaches a value of 7× 10−5 M; up to this polymer concentration the quantum yield increases with PEG and reachplateau at a polymer concentration around 1× 10−4 M. Thequantum yield at the plateau is 0.090± 0.02.

This behavior confirms the polymer adsorption atinterface. Thus, at low polymer concentration,[PEG] <

5 × 10−5 M, there is a weak interaction between PEG aAOT molecules not detected byfluorescence but observeby electrophoretic mobility measurements described below


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When the polymer concentration increases, the interactiobecomes important and the quantum yield increases badsorption of PEG at the interface, protecting nabumetonfrom water quenching until polymer saturation. Under tcondition, the quantum yield remains constant at 0.090.

3.2. Electrophoretic mobility results

The effect of the addition of PSS, PEG polymers, aNaCl on the electrophoretic mobility of AOT vesicles winvestigated and the results are presented inFig. 5. Thefigure shows the electrophoretic mobility values for AOTvesicles,[AOT] = 0.03 M, as a function of PSS, PEG, aNaCl concentration, respectively. The trend of these cuis similar to those found for vesicles containing 0.01 M AO

All the electrophoretic mobility curves show a minimumhowever, the shapes of these curves are significantlyferent.Fig. 5 shows the electrophoretic mobility values fmixed vesicles of AOT/PSS and AOT/NaCl. For compative purposes the polymer concentration is expressed asNa+ concentration left by the polymer PSS. The valuescoincident within experimental error showing that the intface potential of AOT/PSS vesicles is controlled by theadsorption.

The electrophoretic mobility vs Na+ curve shows aminimum reported elsewhere for positively and negativcharged latices[34]. Different models, including surfacconductance, surface roughness, ion adsorption, andlayer, have been proposed to account for this nonibehavior[34]. Even though the hairy layer model is tmost widely invoked for polymer colloids[35] it does notseem to be accurate for interpreting the results obtaineAOT/PSS and AOT/NaCl vesicles, because NaCl moleculdo not contain hairs protruding into the solution. In thecases the results can be interpreted with the ion adsormodel [36]. According to this model, for the caseanionic polystyrene, three competing processes are invoin determining the shape of the mobility vs ionic strencurve [34]: (1) neutralization of the negative chargethe surface by the adsorption of counterions, causingincrease in the electrophoreticmobility and less negativvalues—in our systems this behavior appears at[Na+] =5 × 10−4 M; (2) approach of counterions to the surfacausing a decrease in mobility and more negative va5 × 10−4 M = [Na+] = 3 × 10−3 M; (3) compression othe diffuse double layer due to the high electrolyte bconcentration, causing an increase in mobility observed[Na+] = 3× 10−3 M; seeFig. 5.

The electrokinetic results obtained for vesicles compoof binary mixtures of AOT/PEG present significant diffeences from those of AOT/PSS aggregates. To interpretfact, we consider that the fluorescence results of nabumeincorporated into these aggregates show evidence of PEpolymer adsorption at the vesicle interface. Consequethe electrokinetic results can be interpreted on the basthe diffuse double layer theory. According to it, the adso

e

y

,

e

Fig. 5. Effect of the mixture composition on the electrophoretic mobilityvesicles composed of 0.03 M of AOT and variable concentrations of(triangles), PSS (squares), and NaCl (circles). For comparative purposes tPSS concentrations are expressed as free Na+ concentration left by PSSmolecules.

tion on the interface of a nonionic polymer such as Pgives a relatively thick layer on the surface, which shiftsslip boundary toward the bulk solution, decreasing the etrophoretic mobility[36].

4. Conclusions

The effect of the addition of PSS, PEG polymers, aNaCl on the surface properties of AOT vesicles was invegated by fluorescence probing and electrophoretic mobmeasurements. Results obtained in this work confirm thsults of a previous work indicating that the addition of PEPSS, and NaCl favors the formation of spontaneous vesof AOT.

Fluorescence results and electrokinetic measuremenshow two different mechanisms of vesicle stabilizatdepending on the type of polymer added. When PSadded to the surfactant solutions, the stabilization is duthe screening of the surface charge by Na+ counterions. Incontrast, the addition of PEG stabilizes the AOT vesiclesthe adsorption of the polymer on the interface.

Acknowledgments

This work was financially supported by the Ministede Ciencia y Tecnología of Spain (Project BQU-201507). The authors acknowledge the Centro de Investiga


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y Desarrollo Tecnológico del Agua for making availabthe fluorescence andζ -potential facilities. They also thanProfessor De las Nieves for his remarks on electrophomobility results.

References

[1] D.F. Evans, H. Wennerström, The Colloidal Domain, secondWiley–VCH, New York, 1999, p. 308.

[2] C. Tondre, Caillet, Adv. Colloid Interface Sci. 93 (2001) 115.[3] M.I. Viseu, K. Edward, C.S. Campos, S.M.B. Costa, Langmuir

(2000) 2105.[4] M.I. Viseu, M.M. Velázquez, C.S. Campos, I. García-Mateos, S.M

Costa, Langmuir 16 (2000) 4882.[5] P. De Moor, T. Beclen, B. Komancheck, O. Díaz, R. van San

J. Phys. Chem. B 101 (1997) 1077.[6] O. Regev, E.F. Marques, A. Khan, Langmuir 15 (1999) 642.[7] W. Meier, Curr. Opin. Colloid Interface Sci. 4 (1999) 6, and referenc

therein.[8] C. Tanford, The Hydrophobic Effect, Wiley, New York, 1980.[9] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, J. Chem. Soc. Fara

Trans. 272 (1976) 1525.[10] W. Helfrich, Naturforsch. C 28 (1996) 2454.[11] M. Bergtröm, J.C. Eriksson, Langmuir 12 (1996) 624.[12] M. Bergtröm, Langmuir 12 (1996) 2454.[13] M. Winterhalfer, W. Helfrich, J. Phys. Chem. 96 (1992) 327.[14] G. Porte, C. Ligure, J. Chem. Phys. 102 (1995) 4290.[15] S. May, C. Ben-Shaul, J. Chem. Phys. 103 (1995) 3834.[16] S.A. Safran, P.A. Pincus, D. Andelman, F.C. Mackintosh, Phys. R

A 43 (1991) 1071.

[17] M. Bergtröm, J. Colloid Interface Sci. 240 (2001) 294.[18] J.I. Briz, M.M. Velázquez, J. Colloid Interface Sci. 247 (2002) 437[19] S.A. Walker, J.A. Zasadzinski, Langmuir 13 (1997) 5076.[20] K. Kalyanasundaran,Photochemistry in Microheterogeneous System

Academic Press, Orlando, FL, 1987.[21] M. Valero, S.M.B. Costa, M.A. Santos, J. Photochem. Photobiol.

Chemistry 132 (2000) 67.[22] M. Valero, A. Del Arco, L.J. Rodríguez, J. Inclus Phenom. Macrocy

Chem. 42 (2002) 121.[23] R.J. Hunter, Zeta Potential in Colloidal Science, Academic Pr

London, 1988.[24] F.M. Menger, K. Yamada, J. Am. Chem. Soc. 101 (1979) 6731.[25] S.S. Datwani, K.J. Stebe, Langmuir 17 (2001) 4287.[26] M.M. Velázquez, M. Valero, L.J. Rodríguez, S.M.B. Costa, M.

Santos, J. Photochem. Photobiol. B Biol. 29 (1995) 23.[27] M.V. Smoluchowski, Handbuch der Elektrizität und des Magnetism

vol. 11, Graetz, Leipzig, 1921, p. 385.[28] J.I. Briz, M.M. Velázquez, unpublished results.[29] K. Makino, K. Suzuki, Y. Sakurai, T. Okano, H. Ohshima, Colloid

Surf. A 103 (1995) 221.[30] J.N. Murrel, The Theory of Electronic Spectra of Organic Molecules

Wiley, New York, 1963, chap. 6.[31] K. Nishimoto, R. Fujishiro, Bull. Chem. Soc. Jpn. 37 (1964) 1660.[32] A.B. Pedrosa, J.I. Briz, M.M. Velázquez, J. Surf. Deterg. 5 (2002) 36[33] E.D. Goddard, Colloids Surf. 19 (1986) 255.[34] F.J. De las Nieves, A. Fernández-Barbero, R. Hidalgo-Alvarez, in

K. Esumi (Ed.), Polymer Interfaces and Emulsions, Dekker, NYork, 1999, chap. 5, p. 167, and references therein.

[35] B.M. Verdegan, M.A. Anderson, J. Colloid Interface Sci. 158 (19372.

[36] A.A. Baran, N.M. Soboleva,L.M. Dukina, Kolloid-Z. 46 (1983)840.

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Langmuir monolayers of the zwitterionic surfactant hexadecyl 1-N-l-tryptophan glycerol ether