Structure Determination of AOT/n-Hexane/Water/UreaReversed Micelles by Light and Small Angle X-ray

Scattering

Carmem Lucia Costa Amaral,† Rosangela Itri,‡ and Mario Jose Politi*,†

Departamento de Bioquımica, Laboratorio Interdepartamental de Cinetica Rapida,Instituto de Quımica da Universidade de Sao Paulo, Caixa Postal 26077,

Sao Paulo, Sao Paulo 05599-97, Brasil, and Departamento de Fısica Aplicada,Laboratorio de Cristalografia, Instituto de Fısica da Universidade de Sao Paulo,

Caixa Postal 66318, Sao Paulo, Sao Paulo 053389-910, Brasil

Received November 17, 1995. In Final Form: June 23, 1996X

The structural effect of urea in sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-hexane/water reversedmicelles (RM) at molar concentration ratio [water]/[AOT] ) 10 is investigated by light and small angleX-ray scattering techniques. Scattering intensities are analyzed within the framework of repulsion andattractive interaction potentials due to RM excluded volume (hard sphere term) and contact adhesion(attractive term). In the absence of urea the simple hard sphere droplet model up to a RM volume fraction(φ) of 0.32 applies for the entire scattering curves. In the presence of 3 and 5 M urea, light scatteringintensities can be fitted only by including in the model an attractive term. X-ray data with 5M urea showthat RM preserves its discrete nature even for φ’s where ionic percolation occurs. Results are interpretedbypreferential solubilizationofureaat the interfacial regiondecreasing its stiffness (appearanceofattractiveinteraction) without the formation of a bicontinuous phase. Accordingly the increase in the solutionconductance previously observed with these mixtures is a result of clustering of RM droplets without theirfusion.

Introduction

Reversed micelles (RM) are thermodynamically stableisotropic dispersions consisting ofmicrodomains of waterin oil stabilized by an interfacial film of surface activemolecules (surfactants). RM composed of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water/hydrocarbon arethemostversatile onesdue to their easywayofpreparationand to their high capacity of water solubilization.1-3 Thestructure of these RM has been described as consisting ofspherical droplets of water in oil up to high RM volumefraction (φ = 0.7) before the percolation threshold andappearance of bicontinuous phases.4 Droplet sizes ofAOT’s RM is, for the same organic solvent, linearlydependent on molar concentration ratio W ) [water]/[AOT].5-7 Ithasbeenshownthat thepercolation thresholdcan be altered by the presence of additives.8 Percolationis hindered by additives stiffening the micellar interfacesuch as cholesterol and favored by additives which makethe interface more flexible such as gramicidin8 or acryl-amide.9 In particular, it has been recently observed that,by a sudden increase in the ionic conductance of thesolution, the addition of urea10 as well as some of its

derivatives11 to AOT RM induces the system to percolateat relatively very low φ. The percolative transitionthreshold is dependent on both urea concentration andwater/urea volume fraction. Preferential solubilizationof urea at the micelle interface, which increases theinterfacial flexibilityandenhances theattractivepotentialamong micelles, was used to rationalize the transitionfrom discrete droplets to bicontinuous structures.10 How-ever, the formation of permanent bicontinuous structuresor rather the presence of discrete droplets above percola-tion threshold is still not clear.8,10,12,13In order to extend the previous work10 and to obtain

informationonRMstructural changesuponureaaddition,the presentworkwasundertaken. AOT/n-hexane/water/urea systems at a fixed W (W ) 10) are investigated bystatic lightscattering (LS)andsmallangleX-rayscattering(SAXS) as a function of φ for 3 and 5 M urea. SAXS isused to determine particle structure whereas LS is usedto obtain information on the interaction forces betweendroplets.

Theory

(1) Static Light Scattering. The Rayleigh ratio, Rθ,for vertically polarized incident light is proportional tothe micellar concentration (c), to the solution refractiveindex (n), to the particle form factor P(q), and to theinterparticle interference S(q), with q ) (4πn/λ) sen(θ/2),where θ is the scattering angle. The micellar size in theinvestigated range is usually much smaller than thewavelength of incident light (λ); therefore, the scatteredintensity is independent of the scattering angle, and P(q)∼ 1 for reversed micelles under study since their radiusis less than100Å, that is, scatters behaveas single points.

* To whom correspondence may be addressed: e-mail,mjpoliti@usp.br; fax, (55)(011)8155579.

† Instituto de Quımica da Universidade de Sao Paulo.‡ Instituto de Fısica da Universidade de Sao Paulo.X Abstractpublished inAdvanceACSAbstracts,August15, 1996.(1) Luisi, P. L.; Giomini, M.; Pilene, M. P.; Robinson, B. H. Biochim.

Biophys. Acta 1988, 947, 209.(2) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946.(3) Day, R. A.; Robinson, B. H.; Clarke, J. H.; Doherty, J. V. J.Chem.

Soc., Faraday Trans. 1 1979, 75, 132.(4) Kotlarchyk, M.; Chen, S. H.; Huang, J. S.; Kim, M.W. Phys.Rev.

A 1984, 29, 2054.(5) Fletcher, P. D. I. J.Chem.Soc., Faraday Trans. 1 1986, 82, 2651.(6) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480.(7) Robinson, B. H.; Toprakcroglu, C.; Dore, J. C. J. Chem. Soc.,

Faraday Trans. 2 1984, 80, 13.(8) Mathew, C.; Patanjali, P. K.; Nabi, A.; Maitra, A. Colloids Surf.

1988, 94, 3069.(9) Candau, F.; Leong, Y. S.; Pouyet, G.; Candau, S. J. J. Colloid

Interface Sci. 1984, 101, 167.

(10) Amaral, C. L. C.; Brino, O.; Chaimovich, H.; Politi, M. J.Langmuir 1992, 8, 2417.

(11) Garcia-Rıo, L.; Leis, J. R.; Mejuto, J. C.; Pena, M. E. Langmuir1994, 10, 1676.

(12) Gimmona, G.; Goffredi, F.; Turco, L. V.; Vassalo, G. J. ColloidInterface Sci. 1991, 154, 411.

(13) Borkoveck, M.; Eicke, H. F.; Hammerich, H.; Das Gupta, B. J.Phys. Chem. 1988, 92, 206.

4638 Langmuir 1996, 12, 4638-4643

S0743-7463(95)01051-1 CCC: $12.00 © 1996 American Chemical Society

In the limit of small concentration,Rθ is expressedunderthe well-known form14

withK)4π2n2(dn/dc)2/(NAλ4),wheredn/dc is the refractiveindex increment with respect to c; NA is Avogadro’snumber,Mh w is the micelle weight averaged molar mass,andA2 represents the second virial coefficient. Thus,Mh wcanbe obtained from the intercept of a plot ofKc/Rθ versusc. FromMh w values, it is straightforward to calculate theaggregation number Nh (eq 9).Rθ can be also expressed as a function of the micellar

volume fraction (φ) by9

where V is the particle volume, B/2 is the second virialcoefficient, and K′ ) 2π2n2(dn/dφ)2λ-4, where dn/dφ is therefractive index incrementwith respect toφ. The relationbetween A2 and B is given by B ) 2A2Mh w

2/Vh , where Vh isthe partialmolar volume ofAOTRM,which simply arisesfrom unit conversion from cm3‚mol/g2 to φ.When φ increases, intermicellar interactions introduce

deviations in the linear behavior relationship predictedby eq 1 (or eq 2). In this situation the deviation can beexpressed by the sum of two contributions: (i) particleexcludedvolumeand (ii) attractive term interaction.Thus,considering that reversedmicelleshavea spherical shape,the variation of the Rθ with φ is given by15,16

where Vm is the micellar volume and a ) φHS/φ, φHS is thevolume fraction occupied by hard spheres of radius RHS.The first termof the right side of eq3derivedbyCarnahanand Starling17 arises from the osmotic pressure due tohard sphere repulsion. The second termderives from theosmotic pressure associated with the attractive forceswhich are treated as perturbation. In this treatment,interactions between three and more droplets are notconsidered. The interaction coefficientA canbeexpressedby

where UA(r) is the interaction potential, kB is the Boltz-mann constant, and T is the temperature.Fitting of experimental Rθ to eq 3 allows the determi-

nation of Vm, a, and A from which its possible to deducethe micellar radius (Rm) and the second virial coefficientB′. The interaction coefficient A is related to B by15,16

It should be noticed that B′ is calculated for the entirerange ofφ’swhereasB is that calculated only for the linearportion, that is, low φ’s. The second virial coefficient

reflects the interdroplets interaction where as morenegative is its value, the stronger is the attractivepotential.(2) SAXS. The scattered intensity I(q) for an isotropic

system of monodisperse spheres in a small angle X-rayscattering (SAXS) experiment can be expressed as

wherenp is theparticle numberdensity andη is a constantthat depends on instrumental characteristics.For very diluted system or when the interference is not

pronounced over the scattering curve, S(q) ∼ 1 and I(q)) ηnpP(q). In this case one canuseGuinier’s law for smallvalues of qRm (qRm < 1) which gives18

with I(0)) ηnp(Fm- Fs)2Vm2,whereFmandFs are themicelle

andsolventelectrondensities, respectively;Rg is theradiusof gyration of the scattering particles. Rg is calculatedfrom plotting eq 7 (logarithmic form) in the limit of q f0.For large q values Porod19 has derived an expression

for I(q),which is valid for a systemcomposedby twophasesof different scattering densities separated by a sharpinterface, so that I(q) ∝ Σ/q4, where Σ ) np4πRm

2 ) totalinterface between the two phases. It should be remarkedthat Porod’s law remains valid for interacting systems,since S(q f 1) for large q. The experimental value of Σcan be deduced from the plateau in a Porod plot (q4I(q)versus q), if absolute intensity measurements are per-formed which is not the present case. However, changesin the particle shape lead to changes in the exponent ofq. Therefore, the analysis of the plot of ln I(q) versusln(q), focusing the slope for large q range, can give an ideaof possible structural changes in the system.

Experimental PartSamplePreparation. AOT (sodiumbis(2-ethylhexyl) sulfo-

succinate) of 99% purity obtained from Aldrich Chemical Co.was used as received. Urea (Merck) was triply recrystallizedfrom hot ETOH. n-Hexane was distilled prior to use and keptover molecular sieves (4 Å). Water was distilled and deionized.Toluene (Merck, spectroscopy grade) was used as supplied.Stock solutions of AOT were prepared in n-hexane. Appropri-

ate amounts of water or urea aqueous solution (3 and 5M) wereadded to the AOT solutions. Transparent reversed micellesolutions were obtained with gentle manual agitation.The molar ratio of water to surfactant was kept constant (W

) 10), while droplets concentrations were changed by addingn-hexane to the mixture. Droplet volume fractions (φ) werecalculated using the following relation:

where Vs is the volume of surfactant calculated by dividing themass of AOT by its density (FAOT ) 1.14 g/cm3),20 Vw is the watervolume in the droplets (Fw ) 1.00 g/cm3),21 Vu is the urea volume(Fu ) 1.3230 g/cm3),21 and Vtotal is the total solution volume. Theaverageaggregationnumber (Nh ) for the systemscanbecalculatedby

(14) Kratochvil, P.ClassicalLightScattering fromPolymerSolution;Elsevier: New York, 1987.

(15) Brunetti, S.; Roux, D.; Bellocq, A. M.; Fourche, G.; Bothorel, P.J. Phys. Chem. 1983, 87, 1028.

(16) Hou, M. J.; Kim, M.; Shah, D. O. J. Colloid Interface Sci. 1988,123, 398.

(17) Carnahan, N. F.; Starling, K. E. J. Chem. Phys. 1969, 51, 635.

(18) Guinier, A.; Fournet, G.Small Angle Scattering of XRay;Wiley:New York, 1955.

(19) Porod, G. Kolloid Z. 1951, 124, 83.(20) Hilfiker, R.; Eicke, H. F.; Sager, W.; Steeb, W.; Hofmeier, U.;

Gehrke, R. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 677.(21) Handbook ofChemistry andPhysics, 59th ed.; Chemical Rubber

Company Press: Cleveland, OH, 1978.

KcRθ

) 1Mh w

+ 2A2c (1)

K′φRθ

) 1V(1 + Bφ) (2)

Rθ )K′Vmφ(1 - aφ)4

1 + 4aφ + 4a2φ2 - 4a3φ3 + a4φ4+ Aφa(1 - aφ)4

(3)

A ) 4πkBT

1Vm∫2RHS

∞UA(r)r

2 dr (4)

B′ ) 8a + A (5)

I(q) ) ηnpP(q)S(q) (6)

I(q) ≈ I(0) exp[-(qRg)

2

3 ] (7)

φ )Vs + Vw + Vu

Vtotal(8)

Structure Determination of Reversed Micelles Langmuir, Vol. 12, No. 20, 1996 4639

whereMAOT,MH2O, andMurea are the molecular weights of AOT,water, and urea, respectively. XH2O and Xurea are the molefractions of water and urea, respectively.Methods. Static light scattering measurements were per-

formedusinga10mWvertically polarizedHe/Ne laser (Hughes),a home-made goniometer, photomultiplier (Thorn & EMI), anda BIC autocorrelator (Brookhaven). Scattered intensities weremeasured for a fixed time basis (1 s) and read directly fromaccumulated counts in the autocorrelator channel A. Lightscattering intensities were obtained at 90° observation angle at30 °C. Samples were filtered using a 0.2 µm nylon filter. Therelative intensity (solution scattering intensity minus puresolvent scattering) was converted into Rθ using as reference theintensity scattered by pure toluene It.Refractive index increments were determined with an ABBE

refractometer (λ ) 632 nm). Values of dn/dc (dn/dφ) are 0.04(0.042), 0.046 (0.05), and 0.054 (0.059) for 0, 3, and 5 M urea,respectively.For small angle X-ray scattering (SAXS) experiments the

samples were conditioned in sealed capillaries of 1-mm insidediameter and measured at room temperature 22 ( 1 °C.Scattering intensity curves were obtained with a small-angleRigaku-Denki goniometer, using a line beam-transmissiongeometry and Cu KR radiation (graphite monochromator) (λ )1.5418 Å). The scattered intensities (Jobs) were corrected bysubtracting a background (parasitic scattering plus electronicnoise). The subtracted parasitic scattering consisted of themeasured intensity without sample multiplied by the sampleattenuation factor. The radius of gyration Rg value evaluatedfrom the Guinier region is not affected by the beam heightsmearing once the geometry used is considered to be linearinfinite. A behavior of I(q)∝ q-3 is expected for thePorod region.

Results

(1) Light Scattering. Figure 1 shows experimentalpoints and fitted curves (eq 3) forRθ versus φ for AOTRM(W ) 10) with and without urea. For the investigatedsolutions it is observed initially (low φ) a linear increasein Rθ followed by a maximum and a decrease thereafter.For 0, 3, and 5 M urea the maximum occurs around φ ∼0.1. In the absence of urea the decrease in the scatteredintensity after themaximum isnot so pronounced (Figure1a) as that observed with urea (parts b and c of Figure 1).The existence of a maximum and the decrease of the Rθaredue to repulsive interactionsbetween theaggregates.22For AOT RM without urea, experimental results can befitted using only excluded volume interaction (A ) 0 inthe eq 3). That is, considering only second-order interac-tions as used in deriving eq 1 and 3. On the other hand,for 3 and 5 M of urea a good fitting can be obtained onlyif an attractive potential is included in the model.Table 1 presents the best values for Rm and B′. Note

that the Rm and B′ values were calculated by consideringthe whole φ range. Below the percolation limit, that is,in the dilute regime of small concentration, eq 1 can beapplied to determineMh w, of AOT RM, and B values fromeq 2. Mh w, Nh , and B of the micelles are summarized inTable 1. Systems containing urea are characterized bylarger particle sizes, since Rm increases with amount ofurea. These Rm values include the AOT’s hydrocarbontail, once φ is calculated considering all AOT monomervolume (eq8). FromB′values it is clear that theattractiveintermicellar interaction becomes larger as urea concen-tration increases. This effect, as will be shown below,arises from RM clustering and not from particle fusion.

It is important to note that Mh w and Nh values remainconstant within the experimental error and the B valuesbecome attractive in the presence of urea. Experimentalaccuracy for the determination ofMh w and B for RM withurea is lower than that with water due to its larger linearregion; i.e., the number of the data points used for thelinear regression in the fitting procedure is smaller forthe system. From B and B values it is clear that

(22) Wanka, G.; Hoffman, H.; Ulbricht, W.Colloid Polym. Sci. 1990,268, 101.

Figure 1. Light scattering intensity as a function of φ for (a)AOT/n-hexane/water and (b and c) AOT/n-hexane/water/urea(3 and 5 M), respectively: (+) experimental points; the thickline corresponds to the fitting of eq 3. W ) 10 (see text).

Table 1. Micellar Radius (Rm), Second VirialCoefficients (B′) (eq 5) and B (eq 2), Weight AverageMolar Mass (Mh w), and AOT Aggregation Number (Nh ),

Deduced from Light Scattering Data

[urea](M) Rm (Å) B′ B

Mh w × 104(g/mol) Nh

0 35 ( 0.9 6.0 ( 0.1 5.1 ( 0.4 12.6 ( 0.9 202 ( 153 43 ( 1.5 -2.4 ( 1.1 -3.6 ( 0.6 13.0 ( 2.5 200 ( 385 55 ( 2.9 -6.5 ( 0.8 -4.3 ( 0.8 13.3 ( 3.2 197 ( 48

Nh )Mh w

MAOT + W(XH2OMH2O

+ XureaMurea)(9)

4640 Langmuir, Vol. 12, No. 20, 1996 Costa Amaral et al.

intermicellarattractive interactionbecomes largerasureaconcentration increases.SAXS. Figure 2 shows the small angleX-ray scattering

intensitiesJobs(q) normalizedbyφ forAOT/n-hexane/water(Figure 2a) and AOT/n-hexane/water/5 M urea (Figure2b) as a function of φ. In Figure 2a for theAOT/n-hexane/water system, a usual behavior of single scatteringparticles is observed for the lowest studiedvolume fraction(φ ) 0.06). The coincidence of the data, for φ ) 0.13, athigh q values implies that the form factor P(q) remainsconstant, since the interference factor S(q) ∼ 1 in this qrange. The observed decrease in Jobs(q) for q < 0.05 Å-1

is therefore due to repulsive interparticle interaction thatdepresses S(q) for q f 0.23,24 This effect becomes morepronounced over the scattering curve for higher concen-trationswhich showsdecrease in the slope ofJobs for smallq values.A big increase in the intensity is therefore observed by

adding 5M urea (Figure 2b) from φ) 0.06 up to φ) 0.19.This increase may be related to the appearance ofattractive forces betweenmicelles,25-27 in agreementwithLSresultswhichshowthat incorporationofureaenhancesattractive interactions between RM. On the other hand,depressing of Jobs for q < 0.05 Å-1 is again observed,although less accentuated, forφg0.2. X-ray results seemto indicate that repulsive interactions can also dominatefor φ g 0.2 for AOT/n-hexane/water/urea.

Some important results can be directly extracted fromthescatteringcurves, apartof the interdroplet interferenceeffects, through the Guinier and Porod analysis, as willbe shown below. A detailed analysis of the scatteringcurves bymodeling of P(q) and S(q) (eq 6) is in progress.28(1) Guinier and Porod Analysis. (a) AOT/n-Hex-

ane/Water System. Guinier plots are shown in Figure3 for φ ) 0.06, 0.13, and 0.19, where S(q) is not sopronounced over the interesting q range. A set of straightparallel lines is observed for these concentrations, al-though the q interval for the fitting is reduced for φ) 0.13and0.19. Theseeffects suggest that repulsive interactionsappear already for φ) 0.13. Table 2 shows the respectivebest fittedvaluesofRg andthecalculatedRm (Rg

2)3/5Rm2).

A plot of ln(Jobs) versus ln(q) (Porod’s plot) (Figure 4)presents a linear region for 0.1 < q < 0.2 Å-1 withinclination of -4.3, for φ up to 0.19. A slope of -3 wasexpected due to the X-ray line beam geometry used. Thedeviation of Porod’s law could be attributed, in a firstglance, to the assumption that RM is composed of twodifferent phases with a sharp interface between them.

(23) Percus, J. K.; Yevick, G. J. Phys. Rev. 1958, 110, 1.(24) Hayter, J. B.; Pendolf, J. Mol. Phys. 1981, 42, 109.(25) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023.(26) Bendedouch, D.; Chen, S. H. J. Phys. Chem. 1984, 88, 648.(27) Huang, J. S.; Safran, S.; Kim, M. W.; Grest, G.; Kotlarchyk, M.;

Quirke, N. Phys. Rev. Lett. 1984, 53, 592. (28) Itri, R.; Amaral, C. L.C.; Politi,M. J.Manuscript in preparation.

Figure 2. (a) SAXS intensity of AOT/n-hexane/water and (b)AOT/n-hexane/water/5Murea systems, normalized by dropletvolume fraction φ, forW ) 10 and different φ. (The error barshave the same size as the data symbol.)

Figure 3. Guinier plot for AOT/n-hexane/water (W ) 10) forφ up to 0.19.

Figure 4. Porod plot for AOT/n-hexane/water (W ) 10) fordifferent φ.

Table 2. Droplet Size of AOT/n-Hexane/Water andAOT/n-Hexane/Water/5 M Urea Reversed Micelles

Deduced from SAXS Data: Rg ) Radius of Gyration andRm ) Spherical Radius

0 M urea 5 M urea

φ Rg (Å) Rm (Å) Rg (Å) Rm (Å)

0.06 22.8 ( 0.1 29.4 ( 0.2 23.3 ( 0.3 30.1 ( 0.30.13 22.1 ( 2.0 28.5 ( 2.4 21.9 ( 2.0 28.3 ( 2.40.19 22.0 ( 2.0 28.4 ( 2.4 25.2 ( 2.0 32.5 ( 2.4

Structure Determination of Reversed Micelles Langmuir, Vol. 12, No. 20, 1996 4641

However, modeling results28 show that this slope can bestrongly affectedby including somepolydispersity degree.The deviation of the expected value of Porod’s exponentis thus probably due to size polydispersity. An effect ofthe polydispersity was also considered for a similarsystem.29 It should be remarked that we are neglectingpolydispersity effect to calculateRg. TheobtainedRg valueis then an overestimate of the mean Rg.28,30A striking result of the Porod plot, therefore, refers to

thebehavior fordifferent studiedφvalues: the samevalueof the slope obtained for φ up to 0.19 indicates that thereverse micelle does not change its shape as a function ofφ. These results togetherwith the observedRg values areevidence that the structure of the droplets is preserved,in good agreement with modeling results.28 The Porodanalysis forφ)0.25 and0.32 shows, therefore adeviationfrom the slope observed at low φ’s (Figure 4). It shouldbe remarked that the interference effects due to excludedvolume are pronounced for higher concentrations and theassumption of S(q) f 1 for large angles (where Porod’sregion is analyzed) is no longer valid. A detailed analysisof these curves through P(q) and S(q) modeling will bepresented in a continuing paper.28(b) AOT/n-Hexane/Water/5 M Urea. In parts a and

b of Figure 5 treatment of SAXS data by Guinier plot forφ)0.06,φ)0.13,andφ)0.19, respectively, arepresented.In Figure 5a two straight lines of different slopes areobserved: the inner small portion leads to Rm ) 40 Å,while a Rm value of 30 Å is obtained for the larger one.This result could be interpreted as two populations ofdifferent particle sizes are present in the system. How-

ever, the inner region is intrinsically influenced by thedivergenceofS(q) forqf0givenbyattractive interactions,while the second region has the same slope, and thereforethe same Rm value, as in the system without urea (Table2). Smaller slopes of Guinier plots are observed for φ upto 0.19 (Figure 5b). Data indicate that for concentrationsof φ ) 0.13 and φ ) 0.19, the attractive forces are not asstrong as for φ ) 0.06; that is, for larger φ’s, repulsiveinteraction overwhelms attractive ones. In such cases,an average value ofRm ) 30 Å is therefore obtained in thepresence or absence of urea.An interesting result comes from Porod’s plot: Figure

6a shows the same slope in the Porod region for φ ) 0.06for 0 and 5 M of urea. The same behavior is observed forconcentrations up to φ) 0.19. Itmeans that the reversedmicelles are not changing their shape by urea addition,since this would lead to deviation of the exponent of q(and so of the slope) in the Porod plot, in comparison withthe systemwithouturea. Further, it seems that the shapeof discrete droplets is also preserved at higher concentra-tions. The same tendency is observed for higher φ and 5M of urea as shown in Figure 6b by comparing φ ) 0.06and φ ) 0.32.

Discussion

In a previous study, the formation of discrete AOT RMcontaining urea in the water pool and the limits wherethese discrete aggregates begin to percolate and formhigher order structures were reported.10 Preferentialsolubilization of urea at the micellar interface and theresulting increase in the solution conductivity could arisefrom either clustering of AOT RM or formation ofbicontinuous structures. In the present study this pointis investigatedwith thehelpof techniqueswhichcanafford

(29) Robinson, B. H.; Toprakcioglu, C.; Dore, J. C. J. Chem. Soc.,Faraday Trans. 1 1984, 80, 13.

(30) Kotlarchyk, M.; Chen, S. H. J. Chem. Phys. 1983, 79, 2461.

Figure 5. Guinier plot for AOT/n-hexane/water/5 M urea (W) 10): (a) φ ) 0.06; (b) φ ) 0.13; φ ) 0.19.

Figure 6. Porod plot for: (a) φ ) 0.06 to 0 and 5 M urea, and(b) φ ) 0.06 and 0.32 for 5 M urea (W ) 10).

4642 Langmuir, Vol. 12, No. 20, 1996 Costa Amaral et al.

to obtain structural analytical parameters for answeringthe question.The addition of urea results in the appearance of

attractive forces in the system as evidenced by LS andX-ray results. It is clear, from LS results (Table 1) thatin the presence of urea the decrease in B and B valuesexpresses the enhancement of the attractive interactionbetween RM droplets.Preferential solubilization of urea at the interface

formed by ionic surfactants results in higher counteriondissociation degree and larger interfacial monomerarea.10,31,32 However,Mh w andNh values presented inTable1 show that the interfacial area does not change withurea. This indicates that the presence of urea in theAOTRM interface atW ) 10 provides only disorganization inthe interface, rendering it more flexible and fluid and asa result leading to an increase in the interdropletsattractive interaction. It is convenient tomention that atW ) 10 the free aqueous space for an increase in thecounterion dissociation degree and the effects associatedto it to manifest is obviously almost none.The Rg values (Table 2) observed for AOT/n-hexane/

water are in good agreement with that obtained from thelinear relationship Rg ) 8.2 + 1.4W (Å) found from SAXSstudies.20 TheRm values found byX-ray are smaller thanthose found by LS once these techniques access distinctparticle scattering volumes (Table 1). X-ray scattersconsist of water core plus the surfactant hydrated headgroups including the sulfosuccinate portion of the AOTmolecule,28 since the hydrocarbon chains of AOT and theorganic solvent (n-hexane) have quite similar electrondensities. On the other hand, LS includes the AOThydrocarbon tail since what counts is the region wherethe refractive index begins to change. In other words thedimension sized bySAXS is shorter than that sized byLS.The larger values of Rm obtained by LS as a function

of urea amount can be also related to interpenetration ofthe surfactant tails, due to attractive forces, which leadsto a greater effective radius for LS observations. X-rayis not sensitive to such interpenetration effect since thiseffect should not alter the contrast of electron density. Ofcourse, if droplet fusion (bicontinuousphase)hadoccurred,Rg obtained by SAXS would be larger.X-ray analysis shows that for the systemwith 5Murea,

the influence of attractive forces is more intense in the

lowest studied concentration φ ) 0.06. Such a concentra-tion corresponds to the value ofφwhere a sudden increasein theconductivityof thesolution,associated topercolationthreshold, was observed.10 X-ray results indicate, there-fore, that the structure of the discrete spherical dropletsis kept after the percolation phenomenon. An interplaybetween attractive and repulsive interactions decreasingthe attractive component at higher concentration28 ex-plains the observed plateau in the conductivity measure-ments. This is also reflected in LS where the scatteredintensity decreases as φ increases (Figure 1). This effectcan be simply understood by the fact that whereas theattractive termdue tourea remains fixed, thehard sphereexcluded volume increases steadily as φ increases.To further check the interaction potential between

droplets, we also included a repulsive term arising fromthe micelle counterion dissociation (R) in the model usedto fit SAXS data. R values were varied and the onlyreasonable fit was obtained when the net charge is zero(datanot shown). Thus, although counteriondissociationof the interior AOT RM certainly occurs, the repulsiveinteraction between droplets in the percolative regionwhere RM’s retain their discrete droplets structure canbemodulated simply by thehard sphere exclusion volumewith no electrostatic effects.

Conclusion

The present study was performed to provide a betterunderstanding of the micellar aggregates formed whenurea is added to the AOT RM. LS results indicate thatthe attractive intermicellar interaction becomes larger asurea concentration increases. This is shown by B and B′values.From Guinier’s and Porod’s analysis on X-ray data it

can be concluded thatAOT/n-hexane/water/urea systemskeeptheir structureuptoφ)0.32,aswell as, theirmicellardimension. Results indicate that thepercolative transitiondoes not occur from spherical droplets to a bicontinuousphase but from noninteracting droplets (φ ) 0.06) toclustering onesdue to interdroplet attractive interactions.

Acknowledgment. We thank to the expertise of Dr.Ourides Santin Filho of the Physics Institute of theUniversity of Sao Paulo for helping with SAXS measure-ments. We acknowledge financial support from theBrazilian Granting FAPESP, CNPq and FINEP.

LA951051Q

(31) Souza, S. M. B.; Chaimivoch, H.; Politi, M. J. Langmuir 1995,11, 1715.

(32) Almeida, F. C. L.; Chaimovich, H.; Schreier, S. Langmuir 1994,10, 1786.

Structure Determination of Reversed Micelles Langmuir, Vol. 12, No. 20, 1996 4643