Urea Reversed

Carlos R. Brasil,, Laurence S. Romsted, and, Mario J. Politi. Effect of Urea on Biomimetic Systems: Neither Water 3-D Structure Rupture nor Direct...
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Langmuir 1996, 12, 4638-4643

Structure Determination of AOT/n-Hexane/Water/Urea Reversed Micelles by Light and Small Angle X-ray Scattering Carmem Lu´cia Costa Amaral,† Rosangela Itri,‡ and Ma´rio Jose´ Politi*,† Departamento de Bioquı´mica, Laborato´ rio Interdepartamental de Cine´ tica Ra´ pida, Instituto de Quı´mica da Universidade de Sa˜ o Paulo, Caixa Postal 26077, Sa˜ o Paulo, Sa˜ o Paulo 05599-97, Brasil, and Departamento de Fı´sica Aplicada, Laborato´ rio de Cristalografia, Instituto de Fı´sica da Universidade de Sa˜ o Paulo, Caixa Postal 66318, Sa˜ o Paulo, Sa˜ o 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 reversed micelles (RM) at molar concentration ratio [water]/[AOT] ) 10 is investigated by light and small angle X-ray scattering techniques. Scattering intensities are analyzed within the framework of repulsion and attractive 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 scattering intensities can be fitted only by including in the model an attractive term. X-ray data with 5 M urea show that RM preserves its discrete nature even for φ’s where ionic percolation occurs. Results are interpreted by preferential solubilization of urea at the interfacial region decreasing its stiffness (appearance of attractive interaction) without the formation of a bicontinuous phase. Accordingly the increase in the solution conductance previously observed with these mixtures is a result of clustering of RM droplets without their fusion.

Introduction Reversed micelles (RM) are thermodynamically stable isotropic dispersions consisting of microdomains of water in oil stabilized by an interfacial film of surface active molecules (surfactants). RM composed of sodium bis(2ethylhexyl) sulfosuccinate (AOT)/water/hydrocarbon are the most versatile ones due to their easy way of preparation and to their high capacity of water solubilization.1-3 The structure of these RM has been described as consisting of spherical droplets of water in oil up to high RM volume fraction (φ = 0.7) before the percolation threshold and appearance of bicontinuous phases.4 Droplet sizes of AOT’s RM is, for the same organic solvent, linearly dependent on molar concentration ratio W ) [water]/ [AOT].5-7 It has been shown that the percolation threshold can be altered by the presence of additives.8 Percolation is hindered by additives stiffening the micellar interface such as cholesterol and favored by additives which make the interface more flexible such as gramicidin8 or acrylamide.9 In particular, it has been recently observed that, by a sudden increase in the ionic conductance of the solution, the addition of urea10 as well as some of its * To whom correspondence may be addressed: e-mail, [email protected]; fax, (55)(011)8155579. † Instituto de Quı´mica da Universidade de Sa ˜ o Paulo. ‡ Instituto de Fı´sica da Universidade de Sa ˜ o Paulo. X Abstract published in Advance ACS Abstracts, August 15, 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.

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derivatives11 to AOT RM induces the system to percolate at relatively very low φ. The percolative transition threshold is dependent on both urea concentration and water/urea volume fraction. Preferential solubilization of urea at the micelle interface, which increases the interfacial flexibility and enhances the attractive potential among micelles, was used to rationalize the transition from discrete droplets to bicontinuous structures.10 However, the formation of permanent bicontinuous structures or rather the presence of discrete droplets above percolation threshold is still not clear.8,10,12,13 In order to extend the previous work10 and to obtain information on RM structural changes upon urea addition, the present work was undertaken. AOT/n-hexane/water/ urea systems at a fixed W (W ) 10) are investigated by static light scattering (LS) and small angle X-ray scattering (SAXS) as a function of φ for 3 and 5 M urea. SAXS is used to determine particle structure whereas LS is used to obtain information on the interaction forces between droplets. Theory (1) Static Light Scattering. The Rayleigh ratio, Rθ, for vertically polarized incident light is proportional to the micellar concentration (c), to the solution refractive index (n), to the particle form factor P(q), and to the interparticle interference S(q), with q ) (4πn/λ) sen(θ/2), where θ is the scattering angle. The micellar size in the investigated range is usually much smaller than the wavelength of incident light (λ); therefore, the scattered intensity is independent of the scattering angle, and P(q) ∼ 1 for reversed micelles under study since their radius is less than 100 Å, that is, scatters behave as single points. (10) Amaral, C. L. C.; Brino, O.; Chaimovich, H.; Politi, M. J. Langmuir 1992, 8, 2417. (11) Garcia-Rı´o, L.; Leis, J. R.; Mejuto, J. C.; Pen˜a, M. E. Langmuir 1994, 10, 1676. (12) Gimmona, G.; Goffredi, F.; Turco, L. V.; Vassalo, G. J. Colloid Interface Sci. 1991, 154, 411. (13) Borkoveck, M.; Eicke, H. F.; Hammerich, H.; Das Gupta, B. J. Phys. Chem. 1988, 92, 206.

© 1996 American Chemical Society

Structure Determination of Reversed Micelles

Langmuir, Vol. 12, No. 20, 1996 4639

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

1 Kc ) + 2A2c Rθ M hw

(1)

with K ) 4π2n2(dn/dc)2/(NAλ4), where dn/dc is the refractive index increment with respect to c; NA is Avogadro’s number, M h w is the micelle weight averaged molar mass, hw and A2 represents the second virial coefficient. Thus, M can be obtained from the intercept of a plot of Kc/Rθ versus c. From M h w values, it is straightforward to calculate the aggregation number N h (eq 9). Rθ can be also expressed as a function of the micellar volume fraction (φ) by9

K′φ 1 ) (1 + Bφ) Rθ V

(2)

where V is the particle volume, B/2 is the second virial coefficient, and K′ ) 2π2n2(dn/dφ)2λ-4, where dn/dφ is the refractive index increment with respect to φ. The relation h w2/V h , where V h is between A2 and B is given by B ) 2A2M the partial molar volume of AOT RM, which simply arises from unit conversion from cm3‚mol/g2 to φ. When φ increases, intermicellar interactions introduce deviations in the linear behavior relationship predicted by eq 1 (or eq 2). In this situation the deviation can be expressed by the sum of two contributions: (i) particle excluded volume and (ii) attractive term interaction. Thus, considering that reversed micelles have a spherical shape, the variation of the Rθ with φ is given by15,16

Rθ )

K′Vmφ(1 - aφ)4 1 + 4aφ + 4a2φ2 - 4a3φ3 + a4φ4

+ Aφa(1 - aφ)4 (3)

where Vm is the micellar volume and a ) φHS/φ, φHS is the volume fraction occupied by hard spheres of radius RHS. The first term of the right side of eq 3 derived by Carnahan and Starling17 arises from the osmotic pressure due to hard sphere repulsion. The second term derives from the osmotic pressure associated with the attractive forces which are treated as perturbation. In this treatment, interactions between three and more droplets are not considered. The interaction coefficient A can be expressed by

A)

∫2R∞

4π 1 kBT Vm

HS

UA(r)r2 dr

(4)

where UA(r) is the interaction potential, kB is the Boltzmann constant, and T is the temperature. Fitting of experimental Rθ to eq 3 allows the determination of Vm, a, and A from which its possible to deduce the micellar radius (Rm) and the second virial coefficient B′. The interaction coefficient A is related to B by15,16

reflects the interdroplets interaction where as more negative is its value, the stronger is the attractive potential. (2) SAXS. The scattered intensity I(q) for an isotropic system of monodisperse spheres in a small angle X-ray scattering (SAXS) experiment can be expressed as

I(q) ) ηnpP(q)S(q)

(6)

where np is the particle number density and η is a constant that 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 can use Guinier’s law for small values of qRm (qRm < 1) which gives18

[

I(q) ≈ I(0) exp -

]

(qRg)2 3

(7)

with I(0) ) ηnp(Fm - Fs)2Vm2, where Fm and Fs are the micelle and solvent electron densities, respectively; Rg is the radius of gyration of the scattering particles. Rg is calculated from plotting eq 7 (logarithmic form) in the limit of q f 0. For large q values Porod19 has derived an expression for I(q), which is valid for a system composed by two phases of different scattering densities separated by a sharp interface, so that I(q) ∝ Σ/q4, where Σ ) np4πRm2 ) total interface between the two phases. It should be remarked that Porod’s law remains valid for interacting systems, since S(q f 1) for large q. The experimental value of Σ can be deduced from the plateau in a Porod plot (q4I(q) versus q), if absolute intensity measurements are performed which is not the present case. However, changes in the particle shape lead to changes in the exponent of q. Therefore, the analysis of the plot of ln I(q) versus ln(q), focusing the slope for large q range, can give an idea of possible structural changes in the system. Experimental Part Sample Preparation. AOT (sodium bis(2-ethylhexyl) sulfosuccinate) of 99% purity obtained from Aldrich Chemical Co. was used as received. Urea (Merck) was triply recrystallized from hot ETOH. n-Hexane was distilled prior to use and kept over 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. Appropriate amounts of water or urea aqueous solution (3 and 5 M) were added to the AOT solutions. Transparent reversed micelle solutions were obtained with gentle manual agitation. The molar ratio of water to surfactant was kept constant (W ) 10), while droplets concentrations were changed by adding n-hexane to the mixture. Droplet volume fractions (φ) were calculated using the following relation:

φ)

Vs + Vw + Vu Vtotal

(8)

It should be noticed that B′ is calculated for the entire range of φ’s whereas B is that calculated only for the linear portion, that is, low φ’s. The second virial coefficient

where Vs is the volume of surfactant calculated by dividing the mass of AOT by its density (FAOT ) 1.14 g/cm3),20 Vw is the water volume 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. The average aggregation number (N h ) for the systems can be calculated by

(14) Kratochvil, P. Classical Light Scattering from Polymer Solution; 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 X Ray; 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 of Chemistry and Physics, 59th ed.; Chemical Rubber Company Press: Cleveland, OH, 1978.

B′ ) 8a + A

(5)

4640 Langmuir, Vol. 12, No. 20, 1996 N h )

M hw MAOT + W(XH2OMH2O + XureaMurea)

Costa Amaral et al. (9)

where MAOT, MH2O, and Murea are the molecular weights of AOT, water, and urea, respectively. XH2O and Xurea are the mole fractions of water and urea, respectively. Methods. Static light scattering measurements were performed using a 10 mW vertically polarized He/Ne laser (Hughes), a home-made goniometer, photomultiplier (Thorn & EMI), and a BIC autocorrelator (Brookhaven). Scattered intensities were measured for a fixed time basis (1 s) and read directly from accumulated counts in the autocorrelator channel A. Light scattering intensities were obtained at 90° observation angle at 30 °C. Samples were filtered using a 0.2 µm nylon filter. The relative intensity (solution scattering intensity minus pure solvent scattering) was converted into Rθ using as reference the intensity 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 inside diameter and measured at room temperature 22 ( 1 °C. Scattering intensity curves were obtained with a small-angle Rigaku-Denki goniometer, using a line beam-transmission geometry and Cu KR radiation (graphite monochromator) (λ ) 1.5418 Å). The scattered intensities (Jobs) were corrected by subtracting a background (parasitic scattering plus electronic noise). The subtracted parasitic scattering consisted of the measured intensity without sample multiplied by the sample attenuation factor. The radius of gyration Rg value evaluated from the Guinier region is not affected by the beam height smearing once the geometry used is considered to be linear infinite. A behavior of I(q) ∝ q-3 is expected for the Porod region.

Results (1) Light Scattering. Figure 1 shows experimental points and fitted curves (eq 3) for Rθ versus φ for AOT RM (W ) 10) with and without urea. For the investigated solutions it is observed initially (low φ) a linear increase in 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 scattered intensity after the maximum is not so pronounced (Figure 1a) as that observed with urea (parts b and c of Figure 1). The existence of a maximum and the decrease of the Rθ are due to repulsive interactions between the aggregates.22 For AOT RM without urea, experimental results can be fitted using only excluded volume interaction (A ) 0 in the eq 3). That is, considering only second-order interactions 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 only if 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 considering the whole φ range. Below the percolation limit, that is, in the dilute regime of small concentration, eq 1 can be applied to determine M h w, of AOT RM, and B values from h , and B of the micelles are summarized in eq 2. M h w, N Table 1. Systems containing urea are characterized by larger particle sizes, since Rm increases with amount of urea. These Rm values include the AOT’s hydrocarbon tail, once φ is calculated considering all AOT monomer volume (eq 8). From B′ values it is clear that the attractive intermicellar interaction becomes larger as urea concentration increases. This effect, as will be shown below, arises from RM clustering and not from particle fusion. (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 thick line corresponds to the fitting of eq 3. W ) 10 (see text). Table 1. Micellar Radius (Rm), Second Virial Coefficients (B′) (eq 5) and B (eq 2), Weight Average Molar Mass (M h w), and AOT Aggregation Number (N h ), Deduced from Light Scattering Data [urea] (M) 0 3 5

Rm (Å)

B′

B

M h w × 104 (g/mol)

N h

35 ( 0.9 6.0 ( 0.1 5.1 ( 0.4 12.6 ( 0.9 202 ( 15 43 ( 1.5 -2.4 ( 1.1 -3.6 ( 0.6 13.0 ( 2.5 200 ( 38 55 ( 2.9 -6.5 ( 0.8 -4.3 ( 0.8 13.3 ( 3.2 197 ( 48

h values remain It is important to note that M h w and N constant within the experimental error and the B values become attractive in the presence of urea. Experimental accuracy for the determination of M h w and B for RM with urea is lower than that with water due to its larger linear region; i.e., the number of the data points used for the linear regression in the fitting procedure is smaller for the system. From B and B values it is clear that

Structure Determination of Reversed Micelles

Langmuir, Vol. 12, No. 20, 1996 4641

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

Figure 2. (a) SAXS intensity of AOT/n-hexane/water and (b) AOT/n-hexane/water/5 M urea systems, normalized by droplet volume fraction φ, for W ) 10 and different φ. (The error bars have the same size as the data symbol.)

intermicellar attractive interaction becomes larger as urea concentration increases. SAXS. Figure 2 shows the small angle X-ray scattering intensities Jobs(q) normalized by φ for AOT/n-hexane/water (Figure 2a) and AOT/n-hexane/water/5 M urea (Figure 2b) as a function of φ. In Figure 2a for the AOT/n-hexane/ water system, a usual behavior of single scattering particles is observed for the lowest studied volume fraction (φ ) 0.06). The coincidence of the data, for φ ) 0.13, at high q values implies that the form factor P(q) remains constant, since the interference factor S(q) ∼ 1 in this q range. The observed decrease in Jobs(q) for q < 0.05 Å-1 is therefore due to repulsive interparticle interaction that depresses S(q) for q f 0.23,24 This effect becomes more pronounced over the scattering curve for higher concentrations which shows decrease in the slope of Jobs for small q values. A big increase in the intensity is therefore observed by adding 5 M urea (Figure 2b) from φ ) 0.06 up to φ ) 0.19. This increase may be related to the appearance of attractive forces between micelles,25-27 in agreement with LS results which show that incorporation of urea enhances attractive interactions between RM. On the other hand, depressing of Jobs for q < 0.05 Å-1 is again observed, although less accentuated, for φ g 0.2. X-ray results seem to indicate that repulsive interactions can also dominate for φ g 0.2 for AOT/n-hexane/water/urea. (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.

Figure 4. Porod plot for AOT/n-hexane/water (W ) 10) for different φ. Table 2. Droplet Size of AOT/n-Hexane/Water and AOT/n-Hexane/Water/5 M Urea Reversed Micelles Deduced from SAXS Data: Rg ) Radius of Gyration and Rm ) Spherical Radius 0 M urea

5 M urea

φ

Rg (Å)

Rm (Å)

Rg (Å)

Rm (Å)

0.06 0.13 0.19

22.8 ( 0.1 22.1 ( 2.0 22.0 ( 2.0

29.4 ( 0.2 28.5 ( 2.4 28.4 ( 2.4

23.3 ( 0.3 21.9 ( 2.0 25.2 ( 2.0

30.1 ( 0.3 28.3 ( 2.4 32.5 ( 2.4

Some important results can be directly extracted from the scattering curves, apart of the interdroplet interference effects, through the Guinier and Porod analysis, as will be shown below. A detailed analysis of the scattering curves by modeling of P(q) and S(q) (eq 6) is in progress.28 (1) Guinier and Porod Analysis. (a) AOT/n-Hexane/Water System. Guinier plots are shown in Figure 3 for φ ) 0.06, 0.13, and 0.19, where S(q) is not so pronounced over the interesting q range. A set of straight parallel lines is observed for these concentrations, although the q interval for the fitting is reduced for φ ) 0.13 and 0.19. These effects suggest that repulsive interactions appear already for φ ) 0.13. Table 2 shows the respective best fitted values of Rg and the calculated Rm (Rg2 ) 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 with inclination of -4.3, for φ up to 0.19. A slope of -3 was expected due to the X-ray line beam geometry used. The deviation of Porod’s law could be attributed, in a first glance, to the assumption that RM is composed of two different phases with a sharp interface between them. (28) Itri, R.; Amaral, C. L. C.; Politi, M. J. Manuscript in preparation.

4642 Langmuir, Vol. 12, No. 20, 1996

Costa Amaral et al.

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).

However, modeling results28 show that this slope can be strongly affected by including some polydispersity degree. The deviation of the expected value of Porod’s exponent is thus probably due to size polydispersity. An effect of the polydispersity was also considered for a similar system.29 It should be remarked that we are neglecting polydispersity effect to calculate Rg. The obtained Rg value is then an overestimate of the mean Rg.28,30 A striking result of the Porod plot, therefore, refers to the behavior for different studied φ values: the same value of the slope obtained for φ up to 0.19 indicates that the reverse micelle does not change its shape as a function of φ. These results together with the observed Rg values are evidence that the structure of the droplets is preserved, in good agreement with modeling results.28 The Porod analysis for φ ) 0.25 and 0.32 shows, therefore a deviation from the slope observed at low φ’s (Figure 4). It should be remarked that the interference effects due to excluded volume are pronounced for higher concentrations and the assumption of S(q) f 1 for large angles (where Porod’s region is analyzed) is no longer valid. A detailed analysis of these curves through P(q) and S(q) modeling will be presented 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 for φ ) 0.06, φ ) 0.13, and φ ) 0.19, respectively, are presented. In Figure 5a two straight lines of different slopes are observed: 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 of different particle sizes are present in the system. How-

ever, the inner region is intrinsically influenced by the divergence of S(q) for q f 0 given by attractive interactions, while the second region has the same slope, and therefore the same Rm value, as in the system without urea (Table 2). Smaller slopes of Guinier plots are observed for φ up to 0.19 (Figure 5b). Data indicate that for concentrations of φ ) 0.13 and φ ) 0.19, the attractive forces are not as strong as for φ ) 0.06; that is, for larger φ’s, repulsive interaction overwhelms attractive ones. In such cases, an average value of Rm ) 30 Å is therefore obtained in the presence or absence of urea. An interesting result comes from Porod’s plot: Figure 6a shows the same slope in the Porod region for φ ) 0.06 for 0 and 5 M of urea. The same behavior is observed for concentrations up to φ ) 0.19. It means that the reversed micelles 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 with the system without urea. Further, it seems that the shape of discrete droplets is also preserved at higher concentrations. The same tendency is observed for higher φ and 5 M of urea as shown in Figure 6b by comparing φ ) 0.06 and φ ) 0.32.

(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.

Discussion In a previous study, the formation of discrete AOT RM containing urea in the water pool and the limits where these discrete aggregates begin to percolate and form higher order structures were reported.10 Preferential solubilization of urea at the micellar interface and the resulting increase in the solution conductivity could arise from either clustering of AOT RM or formation of bicontinuous structures. In the present study this point is investigated with the help of techniques which can afford

Structure Determination of Reversed Micelles

to obtain structural analytical parameters for answering the question. The addition of urea results in the appearance of attractive forces in the system as evidenced by LS and X-ray results. It is clear, from LS results (Table 1) that in the presence of urea the decrease in B and B values expresses the enhancement of the attractive interaction between RM droplets. Preferential solubilization of urea at the interface formed by ionic surfactants results in higher counterion dissociation degree and larger interfacial monomer h w and N h values presented in Table area.10,31,32 However, M 1 show that the interfacial area does not change with urea. This indicates that the presence of urea in the AOT RM interface at W ) 10 provides only disorganization in the interface, rendering it more flexible and fluid and as a result leading to an increase in the interdroplets attractive interaction. It is convenient to mention that at W ) 10 the free aqueous space for an increase in the counterion dissociation degree and the effects associated to 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 the linear relationship Rg ) 8.2 + 1.4W (Å) found from SAXS studies.20 The Rm values found by X-ray are smaller than those found by LS once these techniques access distinct particle scattering volumes (Table 1). X-ray scatters consist of water core plus the surfactant hydrated head groups including the sulfosuccinate portion of the AOT molecule,28 since the hydrocarbon chains of AOT and the organic solvent (n-hexane) have quite similar electron densities. On the other hand, LS includes the AOT hydrocarbon tail since what counts is the region where the refractive index begins to change. In other words the dimension sized by SAXS is shorter than that sized by LS. The larger values of Rm obtained by LS as a function of urea amount can be also related to interpenetration of the surfactant tails, due to attractive forces, which leads to a greater effective radius for LS observations. X-ray is not sensitive to such interpenetration effect since this effect should not alter the contrast of electron density. Of course, if droplet fusion (bicontinuous phase) had occurred, Rg obtained by SAXS would be larger. X-ray analysis shows that for the system with 5 M urea, the influence of attractive forces is more intense in the (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.

Langmuir, Vol. 12, No. 20, 1996 4643

lowest studied concentration φ ) 0.06. Such a concentration corresponds to the value of φ where a sudden increase in the conductivity of the solution, associated to percolation threshold, was observed.10 X-ray results indicate, therefore, that the structure of the discrete spherical droplets is kept after the percolation phenomenon. An interplay between attractive and repulsive interactions decreasing the attractive component at higher concentration28 explains the observed plateau in the conductivity measurements. This is also reflected in LS where the scattered intensity decreases as φ increases (Figure 1). This effect can be simply understood by the fact that whereas the attractive term due to urea remains fixed, the hard sphere excluded volume increases steadily as φ increases. To further check the interaction potential between droplets, we also included a repulsive term arising from the micelle counterion dissociation (R) in the model used to fit SAXS data. R values were varied and the only reasonable fit was obtained when the net charge is zero (data not shown). Thus, although counterion dissociation of the interior AOT RM certainly occurs, the repulsive interaction between droplets in the percolative region where RM’s retain their discrete droplets structure can be modulated simply by the hard sphere exclusion volume with no electrostatic effects. Conclusion The present study was performed to provide a better understanding of the micellar aggregates formed when urea is added to the AOT RM. LS results indicate that the attractive intermicellar interaction becomes larger as urea 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 that AOT/n-hexane/water/urea systems keep their structure up to φ ) 0.32, as well as, their micellar dimension. Results indicate that the percolative transition does not occur from spherical droplets to a bicontinuous phase but from noninteracting droplets (φ ) 0.06) to clustering ones due to interdroplet attractive interactions. Acknowledgment. We thank to the expertise of Dr. Ourides Santin Filho of the Physics Institute of the University of Sa˜o Paulo for helping with SAXS measurements. We acknowledge financial support from the Brazilian Granting FAPESP, CNPq and FINEP. LA951051Q