Langmuir 1999, 15, 2307-2315
2307
Characterization of a Sucrose Ester Microemulsion by Freeze Fracture Electron Micrograph and Small Angle Neutron Scattering Experiments M. A. Bolzinger-Thevenin,† J. L. Grossiord,‡ and M. C. Poelman*,† De´ partement de Dermopharmacie et Biophysique Cutane´ e, Faculte´ des Sciences Pharmaceutiques et Biologiques, 4, av. de l’Observatoire, 75270 Paris Cedex 06, France, and Laboratoire de Physique Pharmaceutique, Faculte´ de Pharmacie, 5, rue Jean Baptiste Cle´ ment, 92 296 Chatenay Malabry Cedex, France Received April 15, 1998 A microemulsion system, containing biodegradable sugar surfactants and a nontoxic cosurfactant for pharmaceutical purposes, is characterized in a sucrose ester/ethyl or cetyl 2-(hexylethyl)-2-hexanoate (EHEC8 or EHEC16)/diethyleneglycol monoethyl ether (DME)/water system. The phase behavior and structure of these systems are intimately linked by the nature of the amphiphiles and the chain length of the oily ester. By adjusting the nature of the oil and the composition of the surfactant mixture, Winsor IV systems containing low amounts of a nonaggressive surfactant composition and equal amounts of water and oil could be formulated. For a given surfactant + cosurfactant-to-oil ratio, a dilution line is investigated in the sucrose monolaurate + sucrose dilaurate/DME/EHEC8/water system, in which numerous systems are prepared by adding from 20 up to 40% water (Φw). Along this dilution path, the microstructure of the system is characterized by rheological, small angle neutron scattering (SANS), and freeze fracture electron micrograph experiments (FFEM). The microemulsion nature of the samples is assessed by SANS. The SANS results are analyzed by means of the Teubner-Strey model, and the microemulsion nature of the three selected samples (Φw ) 25, 30, and 32%) is confirmed. The investigated samples exhibit a shear thinning process related to a reorganization mechanism, which occurs under shear. However, in the samples containing more than 30% water, a second rheological transition occurs, which is related to the steep decrease of the apparent viscosity around γ˘ ) 1000 s-1. This second shear thinning is explained by the presence of two structures in these samples. The microstructures of the two samples have not yet been completely determined with the FFEM technique. The specific bicontinuous structure of one sample has been confirmed and characterized by Φw ) 25%.
Introduction Sucrose fatty esters are biocompatible surfactants, synthesized from renewable resources such as fatty acids and sugars, which are easily biodegradable. These new surfactants are expected to replace or supplement traditional ones because they are nonethoxylated nonionic surfactants. These properties could enable the development of safe products.1-3 Single-phased microemulsions (MEs) are fine dispersions of oil and water stabilized by surfactant molecules. Unfortunately, the need for large quantities of surfactant in their formulation may present some drawbacks for pharmaceutical purposes. The phase behavior of sucrose alkanoates in water or oil has been previously studied.4-6 The hydrophilelipophile balance (HLB) of sucrose esters is largely unchanged with temperature due to the strong hydrophilicity of the sucrose part. Therefore the clouding phenomena are not observed. However, few examples of sucrose ester microemulsions have been reported in the literature. The investigation of † ‡
De´partement de Dermopharmacie et Biophysique Cutane´e. Laboratoire de Physique Pharmaceutique.
(1) Desai, N. B.; Lowicki, N. Cosmet. Toiletries 1985, 100, 55. (2) Brooks, G. J. Cosmet. Toiletries 1980, 95, 73. (3) Akoh, C. C. J. Am. Oil Chem. Soc. 1992, 69, 9. (4) Kunieda, H.; Ushio, N.; Nakano, A.; Miura, M. J. Colloid Interface Sci. 1993, 159, 37. (5) Arakami, K.; Kunieda, H.; Ishitobi, M.; Tagawa, T. Langmuir 1997, 13, 2266. (6) Pes, M. A.; Arakami, K.; Nakamura, N.; Kunieda, H. J. Colloid Interface Sci. 1996, 178, 666.
their phase behavior in the presence of oil and water is quiet new.6 We have previously studied the ability of formulating a microemulsion with a range of HLB sucrose esters.7 We have concluded that a cosurfactant such as middle-chain alcohol is needed to obtain large Winsor IV areas. The aim of the present study is to investigate the microstructure of large microemulsion areas containing large amounts of oil and water with a minimal amount of surfactants. Kunieda et al.8 and Ajith et al.9 have recently pointed out the effect of mixtures of surfactants on solubilization in a microemulsion system. In the present work, mixtures of sucrose esters are investigated to provide a large single microemulsion area in which large amounts of oil and water could be solubilized. Bicontinuous structures are expected in such systems, and their benefits in many industrial applications have been widely demonstrated. Their structures are determined by small angle neutron scattering (SANS) and freeze fracture electron micrograph (FFEM) experiments. The properties of surfactant systems depend mainly upon the molecular structure of the surfactant. The hydrophilic-lipophilic properties of sucrose esters are highly dependent on the number of alkanoic acids attaching sucrose.4 By controlling the amount of di- and triester in the formulation, it is possible to prepare a class of sucrose esters with a range of HLB values from 3 to 16 (7) Thevenin, M. A.; Grossiord, J. L.; Poelman, M. C. Int. J. Pharm. 1996, 137, 177. (8) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci. 1995, 170, 78. (9) Ajith, S.; Rackshit, A. K. J. Phys. Chem. 1995, 99, 14778.
10.1021/la9804278 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/25/1999
2308 Langmuir, Vol. 15, No. 7, 1999
Figure 1. Representative formula for sucrose esters. R is the alkyl group for a fatty acid. The esterification occurs in the methylhydroxy side chain groups, and the sucrose esters contain a mixture of mono-, di-, and triesters.
based on the extreme hydrophilicity of the sucrose monoester. Their chemical structure is illustrated in Figure 1. Although a sucrose ester is a mixture of isomers, Kunieda et al. have demonstrated that it can be treated as a pseudosingle component because monodisperse solubilities in water and oil are very small. A number of carbohydrate fatty acid polyesters are screened for their ability to produce ME systems. Their surfactant properties are examined in terms of their ability to produce large microemulsion monophasic areas with water and esters as the oily phase at room temperature.7 When used alone, these surfactants are mostly too hydrophilic or lipophilic (Figure 2a and f). According to Arakami5 and Kunieda,8 surfactant mixing increases the apparent solubilizing power of a surfactant and, when the hydrophilic-lipophilic properties of the surfactants are sufficiently different, the solubilization tends to increase. The more the HLB values of the surfactants are different, the more the real solubilizing power is increased. Since sucrose dilaurate (SDL) is a lipophilic surfactant compared with sucrose monolaurate (SML), the phase behavior of a mixture of both is investigated. The results confirmed that, in DME, the 82:18 ratio favors large microemulsion areas, as previously demonstrated with ethanol.7 Kunieda et al. have demonstrated that if SML is used alone, the three-phase behavior would not be observed even at high temperatures.4 As far as phase behavior is concerned, sucrose ester resembles an ionic surfactant. The three-phase body, including middle-phase microemulsions, appears in a water/poly(oxyethylene)-type surfactant/oil system at a defined temperature, called the HLB temperature or PIT (phase inversion temperature). The solubilization capacity reaches its maximum, and ultralow interfacial tensions are attained at the HLB temperature. According to Arakami et al., the HLB of sucrose alkanoate is largely unchanged with temperature. Judging from the structure of the hydrophilic part (a sucrose ring, which is compact) a large conformational change related to the hydration phenomenon is not expected.5 Cosurfactants, such as middle-chain alcohols, are needed to obtain the three-phase body in a water/oil system.10 There are two predominant effects of the alcohol: first a change in the effective hydrophilicity of the amphiphilic mixture11 and second an increase in the efficiency or solubilization capacity of the amphiphilic mixture. The addition of an alcohol acts similarly to an increase of temperature. It tunes the surfactant mixture to a more balanced state. Moreover the addition of another component to the mixture provides an additional degree of freedom with which to adjust the phase behavior. Middle-chain alcohols have been mainly used as cosurfactant. The removal of such cosurfactant from these microemulsions is required for pharmaceutical purposes.12 (10) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 10313. (11) Strey, R.; Jonstro¨mer, M. J. Phys. Chem. 1992, 96, 4537. (12) Malcomson, C.; Lawrence, M. J. Colloids Surf., B 1995, 4, 97.
Bolzinger-Thevenin et al.
We have therefore chosen a short poly(ethylene glycol) alkyl ether, which is more hydrophilic than the longer chain investigated by Kunieda in such systems. The longer chains are more amphiphilic and could play the role of a surfactant.4 Kalhweit et al.13 have shown that the efficiency depends not only on the nature of the amphiphiles but also on the nature of the oil. In our study the temperature is fixed at 25 °C. By varying the nature of the oil, the efficiency of the amphiphile can be enhanced at a given temperature. Of the two assessed esters, ethyl 2-(hexylethyl)-2-hexanoate (EHEC8) has the shortest chain length and would be expected to penetrate the surfactant interface to the larger extent.14 It is known that the HLB temperature is highly dependent on the oil phase. In the case of saturated hydrocarbons, the HLB temperature decreases with decreasing molecular weight of the oil. The results in Figure 3 (illustrating the temperature as a function of the percentage of amphiphiles γ ) surfactant + cosurfactant/ (oil + water + surfactant + cosurfactant)) show that the three-phase body appears at a lower temperature in the EHEC8 system. Increasing the chain length of the oil decreases its miscibility with the surfactant mixture. According to Kalhweit et al.,15,16 our results suggest that the more polar or hydrophilic oils are required at room temperature. In agreement with Friberg,17 short-chain analogues result in a consequent increase of the microemulsion area. Schulman viewed microemulsions as optically isotropic fluid transparent oil and water dispersions consisting of uniform droplets of either oil or water in an appropriate continuous phase. Today one knows that the homogeneous mixtures of water, oils, and amphiphiles may show various microstructures with water and oil domains apparently separated by saturated monolayers of the amphiphile. Thus drawing a borderline between weakly structured mixtures and a microemulsion seems difficult.13,18 Moreover, the efficient use of microemulsions is directly related to the understanding of their microstructure. Scattering experiments may be used to explain the water and oil mutual arrangement. It can be deduced via direct imaging, which seems especially informative, as demonstrated by Jahn and Strey,19 who use the FFEM technique to visualize bicontinuous structures in a three-component microemulsion system. In the present work, the one-phase region (Winsor IV) of the phase diagram SML + SDL/DME/EHEC8/water has been mapped for the fixed surfactant + cosurfactant/ oil weight ratio 40/60, and its microstructure has been probed along a single water dilution line using FFEM and SANS experiments. This line is chosen because the water content Φw along it may be increased continuously from 20% to more than 40% while avoiding a two-phase or mesophase region. At low Φw, Winsor I and II systems are observed between 5 and 20%. However the most interesting area is located between 20 and 40% water, in which Winsor IV mixtures containing low amounts of surfactant are observed. (13) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. (14) Eastoe, J.; Hetherington, K. J.; Sharpe, D.; Dong, J. Langmuir 1996, 12, 3876. (15) Kahlweit, M.; Strey, R.; Haase, D.; Firman, P. Langmuir 1988, 4, 785. (16) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Scho¨macker, R. Langmuir 1988, 4, 499. (17) Friberg, S. E.; Gan Zuo, L. J. Soc. Cosmet. Chem. 1983, 34, 73. (18) Hoar, T. P.; Schulman, J. H. Nature 1943, 152, 102. (19) Jahn, W.; Strey, R. J. Phys. Chem. 1988, 92, 2294.
Sucrose Ester Microemulsion
Langmuir, Vol. 15, No. 7, 1999 2309
Figure 2. Isothermal phase diagram of the quaternary H2O/SML + SDL/DME/EHEC16 mixtures at different SDL/SML ratios: a, 100:0; b, 82:18; c, 64:36; d, 46:54; e, 28-72; f, 0:100. T ) 25 °C. The shaded area represents the Winsor IV region. The systems are fluid isotropic and transparent. Cosurfactant. Diethyleneglycol monoethyl ether (DME) However, an anisotropy appears along the titration line (C6H14O3 or C2H5-O-(CH2)2-O-(CH2)2-OH) is purchased from around 30% water, when the mixture is stirred, and we Gattefosse´ (France). tried to apply a rheological method to discriminate the Aqueous Phase. Distilled water (pH ) 6) is used as the truly isotropic mixture from the others. aqueous phase. Neutron Scattering Experiment. For the neutron scatMaterials tering study, deuterated water is used (EURISO-TOP, degree of Surfactants. Sucrose monoalkanoates are supplied by Mitdeuteration 99.9%). subishi Kagaku Foods Corporation. Their HLB values are All chemicals are used without further purifications. estimated to be around 16 for sucrose monolaurate (SML) and 5 for sucrose dilaurate (SDL). The monoester and diester contents Methods of SML are 83 and 16%, respectively. The monoester, diester, Phase Diagram. Pseudoternary Phase Diagram. The mixand triester contents in SDL are 29, 41, and 23%, respectively. tures of sucrose dilaurate and monolaurate have the following Oily Phase: Alkyl Esters. Ethyl and cetyl 2-(hexylethyl)ratios (% w/w): 100:0, 82:18, 64:36, 46:54, 28:72, 0:100. These 2-hexanoate (EHEC8 and EHEC16) are purchased from Dragoco ratios correspond respectively to HLB values of 5, 7, 9, 11, 13, (France).
2310 Langmuir, Vol. 15, No. 7, 1999
Bolzinger-Thevenin et al.
Figure 4. Phase behavior for the microemulsion H2O/SDL + SML (82:18)/DME/EHEC8 at 25 °C. Points 1, 2, and 3 represent the samples investigated by SANS. Compositions of the sample: sample 1, 25% water, 45% EHEC8, 3.24% SML, 14.76% SDL, 12% DME; sample 2, 30% water, 42% EHEC8, 3.03% SML, 13.77% SDL, 11.2% DME; sample 3, 32% water, 40.8% EHEC8, 2.94% SML, 13.38% SDL, 10.88% DME. The shaded region represents the Winsor IV systems (truly isotropic), while the dashed one represents the systems, which exhibit an anisotropy under stirring. Table 1. Composition in Percentage (w/w) of the Three Microemulsion Systems Investigated by SANS and FFEM sample water (%) EHEC8 (%) SML (%) SDL (%) DME (%) 1 2 3
Figure 3. Vertical section through a tetrahedron in the H2O/ SDL + SML/DME system with both esters (a, EHEC16; b, EHEC8) at R ) 50% and SML + SDL/DME ) 60/40 with increasing γ as a function of temperature. WIIIA indicates a three-phase body in which an anisotropic phase coexists with two microemulsions. WIIIO indicates a three-phase body in which the middle phase is an emulsion. and 16. The mixtures are dissolved in DME. The mixing ratio between the surfactants and the cosurfactant was 60/40. The mixture is mixed until a clear solution is obtained. Oily phase/surfactant + cosurfactant ratios varying from 10 to 90 are studied. The phase behavior of the samples is determined at 25 °C by titration with water, and the phase boundaries of the monophasic area (Winsor IV or I) are determined by visual observation. Crystalline phases are checked for anisotropy under polarized light. Fish Representation. The fish representation has been proposed by Kahlweit et al.13 This representation allows the determination of the three-phase body and of the minimal amount of amphiphile γmin required to obtain a homogeneous solution of the three components at a given ratio R (ratio oil/water + oil ) R ) 0.5) between oil and water related to the temperature. We choose R ) 50%, because the strength of the amphiphile to solubilize equal amounts of oil and water is assessed. The fish picture has been investigated between 20 and 60 °C for both esters. Moreover the microstructure of the systems could be estimated from this representation. The common phase sequence observed with rising temperature is the following: for high γ values, one finds generally a homogeneous Winsor IV solution between the melting and boiling points. At some γ, the sequence Winsor I-Winsor IIIWinsor II is observed. At low γ between the melting and boiling points, the sequence Winsor I-Winsor II is observed. The temperature T h , at which the three-phase body appears is precisely determined. FFEM Experiments. 1. The samples are homogenized by stirring with a Teflon-coated magnet at 25 °C.
25 30 32
45 42 40.8
3.24 3.03 2.94
14.76 13.77 13.38
12 11.2 10.88
2. Then, the samples are rapidly transferred into liquid propane at -196 °C. The cooling rate is of the order of 2000 °C/s. 3. For fracturing, the samples are clamped under liquid nitrogen inside the vacuum chamber of the freeze-etching apparatus (Balzers BAF 400 freeze-etching apparatus). Fracturing is achieved by displacing a microtome arm cooled by liquid nitrogen. The now-exposed fracture face is immediately shadowed unidirectionally by Pt-C. 4. The specimens are washed in THF acid and the replicas observed with a transmission electron microscope with a tilt device for stereoimaging (JEOL 100SX). The final pictures are observed at ×60000 magnification. SANS Experiments. The scattering experiments have been carried out with the PACE spectrometer at the Laboratoire Le´on Brillouin (CEA-CNRS, CE-Saclay, France). For each sample, two runs are performed, each run involving the recording of the spectrum simultaneously at 30 different scattering angles θ using neutrons of wavelength λ ) 6.43 Å and λ ) 9.48 Å, respectively. Samples are held at 25.0 ( 0.1 °C in a 2 mm fused silica cell. SANS spectra I(q) were measured as a function of the scattering vector q ) (4π/λ) sin(θ/2), which is in the range 0.00654 Å-1 e q e 0.0691 Å-1 for the higher wavelength and in the range 0.0277 Å-1 e q e 0.286 Å-1 for the lower one. The raw data are normalized to scattering from a 1.00 mm thick water (H2O) sample whose absolute cross section is determined independently. Corrections for background and multiple scattering, and further subtraction of the incoherent scattering are performed. The compositions of the investigated systems called 1, 2, and 3 are listed in Table 1. They are visualized in Figure 4. They respectively correspond to 25, 30, and 32% water. Rheological Measurement. Rheological measurements are performed on the SML + SDL/DME/EHEC8/water systems with the water amount varying from 25 to 40%. Steady-shear viscosity measurements are performed at 25 °C on a Rheostress RS 100 Haake working with a cone-and-plate geometry (angle ) 0.5°, diameter 35 mm). Shear stress measurements are carried out in experiments where the shear rate was increased stepwise (γ˘ ) 0.05-3000 s-1). The shear rate is increased by 50 s-1/min.
Sucrose Ester Microemulsion
Results and Discussion Mixing of SML and SDL. The phase behavior of mixtures of sucrose alkanoates in the presence of diethyleneglycol monoethyl ether is investigated. The isothermal phase diagrams at 25 °C of the pseudoternary SML + SDL/water/EHE C16/DME/water system at various SML/SDL ratios are presented (Figure 2). If the two pseudo-three-component phase diagrams of each surfactant are surimposed, then a smaller area is observed where both the systems show one phase microemulsion (Figure 2a and f). This result was previously reported by Ajith et al.9 in a mixed surfactant system. With increasing the HLB value of the surfactant mixture, the monophasic area is shifted from the water-rich side (along the surfactant axis) of the pseudoternary diagram toward the center of the diagram (ratio 82:18) and finally toward the oil-rich part of the diagram. (A microemulsion system is interesting when the phase diagram presents a continuous path from pure oil to pure water.) When the hydrophile-lipophile property of a mixed surfactant system is just balanced, a large single-phase microemulsion is formed.20 The ratio 82:18 provides the more balanced mixture, and the phase diagram is almost symmetrical in oil and water, but the minimal amount of surfactant and cosurfactant needed to obtain a single-phased system is still high (40% surfactant and cosurfactant, 42% water, and 18% oil). Influence of the Nature of the Oil Phase. The dependence of the three-phase body on the chemical nature of the oil and the amphiphile represented by plotting T versus γ as they change with the oil chain length is shown in Figure 3. The results (Figure 3) show the phase behavior of the SML + SDL/DME/water system as the alkyl chain length of the ester is changed from C16 to C8. In the EHEC16 system, a truly Winsor I area is located around γ ) 40%. This large area seems insensitive to changes in temperature. The WI region is shifted toward higher temperature (50 °C) at lower γ. This Winsor I system appears twice at a fixed temperature. This type of distortion of the microemulsion region is quite common in a mixed surfactant system, as surfactant molecules are distributed between water and oil domains and the interface inside the microemulsion.21 Moreover, the addition of an alcohol to a ternary microemulsion induces a shift of the three-phase region to lower temperatures and its distortion. The distortion of the three-phase region can be explained by the fact that the solubility of DME in EHEC8 or EHEC16 is larger than that of the surfactant mixture.10 With this long-chain ester, the three-phase bodies are largely skewed to a high temperature (around 60 °C). Before this temperature, three-phase bodies with an anisotropic middle phase are found in a narrow range of γ (30-40%). In the EHEC8 system, a large truly Winsor III system appears, whereas a WI system was present in the former system with EHEC16. Decreasing the alkyl chain length of the ester shifts the three-phase body to lower temperatures. Around 60 °C, similar three-phase bodies are found, such as in the former system. Around γ ) 45%, a large Winsor II area is found, which is shifted to lower γ, when the temperature is increased. The theoretical fish representation is quite informative on the system behavior, but the complete determination (20) Kunieda, H.; Ozawa, K.; Arakami, K.; Nakano, A.; Solans, C. Langmuir 1998, 14, 260. (21) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107.
Langmuir, Vol. 15, No. 7, 1999 2311
of the ratio R providing the lower γmin is quite long. Under our conditions, any single-phase system is found. The fish representation for R ) 50% is too restrictive. Moreover, in the present case the surfactant-to-cosurfactant ratio is fixed and the exact determination of the tricritical point is missed, as demonstrated in Figure 3 and predicted by Kalhweit et al.22 For these reasons, the pseudoternary phase diagram for the SDL + SML/DME/EHEC8/water system has been mapped at 25 °C (Figure 4). On this diagram, a large monophasic area is observed. Up to 30% water, the samples appear truly isotropic. Above this point, the samples exhibit a slight anisotropy when they are stirred. Compared to the case of Figure 2b, the presence of the shorter alkyl ester enhances the microemulsion area. The HLB temperature is highly dependent on the oil nature.17,21 If polar oils are used, the HLB temperature decreases. SANS Spectral Analysis. The SANS spectra exhibit a single peak in the small q range, followed by a characteristic q-4 dependence at large q. In microemulsion systems, microscopically, the waterand oil-rich domains are separated by a monolayer across which a sharp change of scattering length density is often observed.23 In SANS, this structure is reflected in the characteristic limit of the Porod law: I(q) ∼ q-4.
I(q) )
2π(∆F)2S -4 q +B V
(1)
where ∆(F) is the difference in scattering power between oil and water and B is the value of the background scattering. Porod-like behavior was verified by fitting a linear function to plots of I(q)q4 versus q4. The large-q data for the three samples investigated are plotted in Figure 5. The characteristic q-4 dependence of the intensity distribution at large wave vector q is attributed to the existence of a well-defined internal interface of the three samples at large q and confirms the presence of a monolayer, which separates the oil- and water-rich domains. The scattering patterns in bulk contrast, for small values of q, were fitted to the expression proposed by Teubner and Strey.24
I(q) )
1 +b a2 + c1q2 + c2q4
(2)
a2, c1, c2, and b are obtained from the fitting of the experimental data. This expression for the scattered intensity corresponds to a real space density-density correlation function of the form
γ(r) )
sin kr -r/ξ e kr
(3)
This correlation function describes a structure of periodicity d ) (2π/k) damped as a function of the correlation length ξ. d and ξ are related to the constants in the intensity function by (22) Kalhweit, M.; Strey, R.; Aratono, M.; Busse, G.; Jen, J.; Schubert, K. V. J. Chem. Phys. 1991, 95, 2842. (23) Strey, R.; Winkler, J.; Magid, L. J. Phys. Chem. 1991, 95, 7502. (24) Teubner, M.; Strey, R. J. Phys. Chem. 1987, 87, 3195.
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Figure 6. Fit of SANS data by the Teubner-Strey model at small q for samples 1 (a), 2 (b), and 3 (c). Table 3. Relationship between the Conduit Diameter d and the Surfactant Volume Fraction Φs
Figure 5. Scattering data presented as Iq4 versus q4 for the following: a, sample 1; b, sample 2; c, sample 3. Table 2. Characteristic Length Scales ξ and d Obtained from Fit of Eqs 2, 4, and 5 to Scattering Data for Samples 1, 2, and 3 sample
a2
c1
c2
d (Å)
ξ (Å)
1 2 3
3.27 E-11 1.88 E-11 1.35 E-11
-5.39 E-8 -4.00 E-8 -2.83 E-8
8.44 E-5 6.91 E-5 5.49 E-5
290 311 323
81 92 91
[( ) [( )
k)
1 a2 2 c2
ξ)
1 a2 2 c2
1/2
1/2
-
+
] ]
c1 4c2
c1 4c2
1/2
(4)
-1/2
(5)
d is characteristic for the domain size, and ξ is the correlation length. d is a measure of the quasi-periodic polar-nonpolar repeat distance. Less obvious is the meaning of ξ, but it has been conceived as a measure for the dispersion of d.25 The parameters are given in Table 2. SANS studies confirm that the surfactant is exclusively present at the interface. (25) Schubert, K. V.; Strey, R. J. Chem. Phys. 1991, 11 (1), 8532.
sample
d (Å)
Φs (%)
1 2 3
290 311 323
30 28 27.2
In our monophasic systems 1, 2, and 3, at small q values, the SANS data are well suited by the Teubner Strey model (Figure 6). The sample microstructures are microemulsions which are characterized by two length scales, the domain size d of the domains and the correlation length ξ. With increasing water content, the position of the peak moves to higher angles. The increase in correlation length, with increasing water content, can be explained by the fact that increasing Φw dilutes the scattering units, as Regev et al. described.26 Dilution acts on the microemulsion structure as a simple dilation. However, the conservation of the total area of the membrane then implies that
d ∼ Φs-1
(6)
where Φs is the volume fraction of the membrane. Equation 6 is found to fit the data reasonably well (Table 3 and Figure 7).27 (26) Regev, O.; Ezrahi, S.; Aserin, A.; Garti, N.; Watchel, E.; Kaler, E. W.; Khan, A.; Talmon, Y. Langmuir 1996, 12, 668. (27) Snabre, P.; Porte, G. Europhys. Lett. 1990, 13 (7), 641.
Sucrose Ester Microemulsion
Figure 7. Φs versus the quasi-periodic size of the polarnonpolar domain d (Å).
Figure 8. Variation of the steady-state viscosity as a function of the shear rate in the Winsor IV area, along the 60:40 path in the H2O/SDL + SML (82:18)/DME/EHEC8 system. The percentages represent the water content in the investigated samples.
It is therefore interesting to follow the evolution of the physical properties of the investigated systems upon dilution. Along the dilution line, between 25 and 35% water, the progression of the structural transformations needs to be elucidated. The rheological behavior along the dilution line has been checked as Φw increases from 20 to 40%. Rheological Behavior. The investigated samples exhibit a low viscosity, which is related to the fluid character of the overall structure. This result, previously described,27,28 is quite surprising if we are dealing with bicontinuous structures characterized by their multiconnected network in the three directions of space. The investigated samples exhibit two newtonian regions: the graphs of η(γ˘ ) include two plateaus, a lowshear-rate region and a high-shear-rate region, which are linked by a diminishing portion (Figure 8). The viscosity of each Newtonian plateau increases with Φw. Such behavior could be analyzed as one of shear thinning, with high-shear and low-shear viscosities. Surprisingly, all microemulsions exhibit shear thinning at the same shear rate, between 20 and 30 s-1. This shear thinning of these systems is sometimes attributed to a reorganization mechanism previously proposed for bicontinuous microemulsions and sponge phases, which involves the shrinking of conduits and then their pinching off.29,30 (28) De Gennes, P. G.; Taupin, C. J. Phys. Chem. 1982, 86, 2294. (29) Chen, C. M.; Warr, G. J. Phys. Chem. 1992, 96, 9492. (30) Warr, G. Colloids Surf., A 1995, 103, 273.
Langmuir, Vol. 15, No. 7, 1999 2313
Figure 9. SDL + SML (82:18) in the DME fracture face at 60000×. Bar ) 100 Å.
Figure 10. SDL + SML (82:18)/DME/EHEC8 freeze-fractured face at 60000×. Bar ) 100 Å. Table 4. Comparison between the Calculated γ3 c(theo) Values Obtained from Eq 7 and the γ3 c(exp) Obtained from the Shear Data (s-1)
γ˘ c(theo) γ˘ c(exp) (s-1)
sample 2
sample 3
2.8 × 2.1 × 103
1.5 × 103 1.8 × 103
103
The shrinking of the passages for a critical shear rate γ˘ c has been estimated as
γ˘ c )
kbT 6πηsd3
(7)
where ηs is the solvent viscosity, d is the conduit diameter, and kb is the Boltzmann constant. Using ηs ) 3.6 mPa‚s, d ) 290 Å, and T ) 298 K, eq 7 gives a γ˘ c of 2500 s-1 for sample 1 containing 25% water instead of 20-30 s-1. If this mechanism was the main process of reorganization of the membranes in this sample, the scaling law would be respected. The shrinking of the passages does not appear to be the main mechanism involved. It is not able to explain the first shear-thinning process in the investigated samples. Above 30% water, the dynamic response at high shear is quite different from that for sample 1, which exhibits a quasi-Newtonian behavior up to 2 × 104 s-1 (plate/plate geometry with a gap of 30 µm). For shear rates approximately higher than 103 s-1, the viscosity profiles have revealed that samples undergo a large decrease in viscosity: the microemulsions (Φw g 30%) undergo two reorganization processes of their structure. It is interesting to emphasize that eq 7 provides theoretical values of γ˘ c which are in good agreement with
2314 Langmuir, Vol. 15, No. 7, 1999
Bolzinger-Thevenin et al.
the experimental data reported in Table 4. This second shear-thinning process could be related to the topological reorganization of the multiconnected membranes. But this good correlation must be considered carefully: it is difficult indeed to understand why samples 2 and 3 are well described by the scaling law, while it is not the case with sample 1. Previous works on L3 and lamellar phases have indicated that the L3-LR phase transition under shearing may be shear thinning.31,32 Although bicontinuous microemulsions are different from such systems, their rheological behaviors display very similar features. As proposed by Majhoub et al. for L3 sponge phases,31 the following interpretation could be suggested: 1. The rheological behavior of the samples under shearing shows the existence of two shear rate critical values γ˘ c. Low γ˘ c values involve topological changes (relaxation of passages), while higher γ˘ c values are associated with a narrowing of passages by hydrodynamic diffusion. It should be noticed that when the γ˘ c values are of the same order of magnitude for the first shear-thinning process (2.5 s-1 for the L3 sponge phase versus 25 s-1 for bicontinuous microemulsions), they are rather different for the second shear-thinning process (3 105 s-1 for the L3 sponge phase versus 2.5 × 103 s-1 for the bicontinuous microemulsion). This result could be explained by the intrinsic structure of each dispersion. This rheological behavior can be correlated with the transient birefringence detected between the cross polarizers, when the samples are stirred, and observed with Φw g 30%. The freeze fracture technique yields direct imaging of the microemulsion microstructure, as described by Jahn and Strey.19,33 Figures 9 and 10 show the replicas of SDL + SML in DME and of the SDL + SML/DME/EHEC8 system. The surfactant mixture is solubilized in DME. When oil is added no structure is detected. When water is added (between 20 and 40%), sufficiently large structures appear, which are easily observed with this technique (Figure 11). We find structures which occur reproducibly on separate grids and which are expected on theoretical grounds, and we consider them as probably close to reality. 2. Sample 1 exhibits regular-shaped structures. The discrimination between water- and oil-rich domains is not possible with the shadow material selected for the experiment, as demonstrated by Jahn and Strey. The characteristic repeat distances observed appear to be close to the 100 Å measured by SANS. The mutual intertwined domains observed are consistent with the notion of bicontinuity concluded from SANS. The images agree with the observation of a bicontinuous structure. 3. Samples 2 and 3 are not easy to classify. The structures are rougher for the second sample. For sample 3, big drops and small drops coexist. The smaller drops seem to be embedded in the big drops. Apparently, the specific structures of the two last samples seem difficult to determine despite the SANS results. For the third sample, scattering data have provided precise data on the length scale of the structure, which corresponds to the small drops, but the larger structure could have been missed. In this last sample, two structures may coexist: a bicontinuous network (small
Figure 11. Images of the freeze-fractured microemulsions in the system H2O/SDL + SML/DME/EHEC8 at points 1, 2, and 3. Note the bicontinuous network of sample 1. Bar ) 100 Å.
(31) Mahjoub, H. F.; McGrath, K. M.; Kle´man, M. Langmuir 1996, 12, 3131. (32) Yamamoto, J.; Tanaka, H. Phys. Rev. Lett. 1996, 77 (21), 4390. (33) Strey, R.; Jahn, W.; Porte, G.; Bassereau, P. Langmuir 1990, 6, 1635.
Sucrose Ester Microemulsion
drops) in a larger structure, which is not evidenced by SANS. The anisotropy may be explained by the fact that bicontinuous structures may scatter light. We note here that apparently, for a variety of different bicontinuous microemulsions, very similar curves are obtained by SANS, as evoked by Strey et al.23 The scattering data are a probe, which confirms that correlated interfaces do exist in the three investigated samples and gave an estimation for the diameter d of the domain.34 However, local structures from the neutron scattering experiment are obtained as statistical averages, while microscopy may yield additional information on the details of the structure.33 The observation of the images of samples 2 and 3 suggests that two structures may coexist in the system, which are only visible on a large scale. Rheology is a useful tool, sensitive to the largest scale structures in a material and to their rate of rearrangement. It takes into account the overall structure. The striking observation is the two reorganization processes for samples 2 and 3 at low and high shear never described previously for bicontinuous structures. The rheological behavior of these samples allows confirmation of the existence of two structures detected by the FFEM technique. (34) Marignan, J.; Appell, J.; Bassereau, P.; Porte, G.; May, R. P. J. Phys. (Paris) 1989, 9, 447.
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Conclusion In this paper, we have discussed the microstructure of three pseudoternary systems. A system accepting large amounts of water has been found in a sucrose ester/oil/water system. Adjusting the nature of the oil and of the cosurfactant allows us to obtain a model of transparent systems containing from 20% up to 40% water. Along this path, mixtures containing equal amounts of oil and water do exist and are expected to be bicontinuous. Even though two samples could not yet be clearly defined, the various experiments conducted confirm the specific bicontinuous microstructure of one of these microemulsions. Acknowledgment. We are indebted to Pr. R. Strey for his support and useful discussions. The authors express their gratitude to T. Pouget and J. F. Tranchant (Parfums Christian DIOR, St Jean de Braye) for their technical assistance with electron microscopic observation and useful discussions. M.A.B. and M.C.P. acknowledge Prof. D Clausse and Prof. Kunz for the SANS experiment. M.A.B. thanks N. Jager-Lezer for her support during the course of the rheological experiments. We thank Mitsubishi Kagaku Foods Corporation (Tokyo) for their financial support. LA9804278
