10490
J. Phys. Chem. B 2000, 104, 10490-10497
Mixed Concentrated Water/Oil Emulsions (Fluorinated/Hydrogenated): Formulation, Properties, and Structural Studies S. Rocca and M. J. Ste´ be´ * Equipe Physico-Chimie des Colloı¨des, UMR 7565 CNRS/UniVersite´ H. Poincare´ -Nancy 1, Faculte´ des sciences, BP 239, 54506 VandoeuVre-le` s-Nancy, Cedex, France ReceiVed: March 30, 2000; In Final Form: July 25, 2000
Highly concentrated water-in-oil emulsions consisting of mixtures of five compounds (a fluorinated surfactant, a hydrogenated surfactant, a fluorocarbon, a hydrocarbon, and water) have been prepared. The formulation and properties of these peculiar emulsions obtained from compounds with antagonistic behaviors have been examined. These mixed emulsions are stable and present macroscopic properties comparable to those of concentrated emulsions prepared with purely hydrogenated compounds. The study of the phase behavior shows that the mixed emulsions are triphasic systems constituted by two kinds of mixed microemulsions coexisting with an aqueous phase. The composition of these microemulsions has been determined by quantitative NMR studies. The five compounds are present in both microemulsions, but one of them is rich in fluorinated compounds and the other in hydrogenated ones. Small-angle neutron scattering experiments allowed us to propose a structure for these mixed emulsions. The bulk of the continuous medium is made of a mixed microemulsion rich in hydrogenated compounds, whereas the mixed microemulsion rich in fluorinated compounds is dispersed inside the continuous medium.
I. Introduction Highly concentrated water-in-oil (W/O) emulsions may be prepared with either a fluorinated surfactant and a fluorocarbon or a hydrogenated surfactant and a hydrocarbon.1,2 Despite the very low amount of surfactant needed to prepare the fluorinated concentrated emulsions (oil/surfactant ratio (rF) g 2 w/w; see Appendix), the fluorinated systems are stable for several months, whereas the hydrogenated emulsions stabilized by larger amounts of surfactant (3 < rH < 1.3) are stable for several weeks. The surfactants used display a very pronounced hydrophobic behavior, and the phase inversion temperature (temperature at which the type of the emulsion changes from “O/W” to “W/O”)3 of the ternary system is lower than 0 °C. Despite the large amount of water added (up to 98% w/w), the phase inversion is never reached.4 The optical properties of the concentrated emulsions prepared with fluorinated compounds are particularly interesting. Indeed, such emulsions may be transparent due to a refractive index effect. The very low refractive indexes of fluorocarbons allow the refractive index of the continuous phase (fluorocarbon + fluorinated surfactant + water) to match that of the dispersed water phase. Furthermore, by slightly modifying the fluorocarbon/ fluorinated surfactant ratio, one can obtain bluish or rosy gleams, which are especially appealing for possible cosmetic applications. The emulsions obtained from hydrogenated compounds are white or translucent. These emulsions are called “gel-emulsions” because of their viscoelastic behavior5,6 and are characterized by yield stress values. The elastic modulus is equal to 300 Pa for fluorinated emulsions as compared to 100 Pa for hydrogenated ones. The observation of hydrogenated or fluorinated emulsions with an optical microscope shows a compartment-like structure. The water droplets are polydisperse with a typical size of a few micrometers. The structure of these emulsions has been
Figure 1. Schematic representation of a concentrated emulsion.
elucidated some years ago by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) experiments.1,7,8 The continuous phase is made of water-swollen reverse micelles (100-200 Å), and the water droplets are separated from this micellar phase by a surfactant monolayer. A gel-emulsion is therefore a water emulsion in a water-in-oil microemulsion (Figure 1). At equilibrium, these systems are biphasic; a reverse microemulsion coexists with an aqueous phase. Moreover, the study of the phase behavior of the ternary system (oil/surfactant/water) allows us to link the structure of the continuous phase of the emulsion to the existence of water-swollen micelles in the oilrich domain.7 The compartment-like structure of these emulsions suggests that they can be used for drug release. Model molecules have been incorporated into the water phase, and the influence of some parameters such as the amount of water, surfactant, drug, and the temperature has been investigated.9,10 It appears that the release is different according to the nature of the emulsion (fluorinated or hydrogenated).11 Consequently, mixtures of both
10.1021/jp001202k CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
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kinds of emulsions have been used to control the release rate.12 In this paper, we report the formulation, properties, and structure of these mixed concentrated emulsions. II. Materials and Methods All the compositions are given in w/w. II.1. Chemical Compounds. The fluorinated oil is perfluorodecalin (C10F18, denoted PFD) purchased from Interchim and used as received. The fluorinated surfactant is CmF2m+1C2H4SC2H4(OC2H4)2OH, denoted RfE2, and it has been synthesized according to a method described by Cambon et al.13 from a mixture of C6-C8-C10 perfluorinated alcohols (Elf Atochem) by condensation of a chemically pure diethyleneglycol chlorhydrin. The measurement by vapor-phase osmometry of the molecular weight of this surfactant gave an average value of 597 g mol-1, which corresponds to a C8 fluorinated tail. The hydrocarbon oil is decane (C10H22) purchased from Sigma. The hydrogenated surfactant (tetraethylene glycol mono n-hexadecyl ether, C16E4) was supplied by Nikko Chemicals Ltd. D2O was purchased from Euriso-top (>99.9%). II.2. Phase Behavior. The samples containing oil(s), surfactant(s), and water were sealed in vessels, thoroughly mixed, and kept at the experimental temperatures for several hours. For each observation, the number of phases and their volumes were recorded. The possible existence of anisotropic phases was checked by using crossed polarizers. II.3. Quantitative Analysis by NMR. The simultaneous presence of fluorinated and hydrogenated compounds has been detected using 19F and 1H NMR. The 19F spectra have been obtained using a BRUKER AC250 spectrometer at 235 MHz, and 1H spectra have been recorded on a BRUKER AN400 apparatus at 400 MHz. The analytical method is based on the measurement of the integrals of each peak standing for groups of protons or fluorine nuclei within each molecule. For the 1H experiments, a 10% w/w solution of hexamethyldisiloxane in CDCl3 is used as a calibration solution, whereas for 19F experiments the calibration solution is 10% w/w C6F6 in C6D6. The amounts of surfactants, oils, and water in the samples are deduced by comparing the integral of the calibration solution to those of the sample, taking into account molecular weights and densities. The 1H spectra allow us to determine the amount of C16E4, RfE2, and water, and the 19F spectra reveal the amount of RfE2 and PFD; the content of decane is evaluated from these values. II.4. Rheological Measurements. The rheological measurements were performed on a stress-controlled Carri-Med CSL 500 rheometer. Cone/plate geometry has been used with a solvent trap and a lid to avoid oil evaporation. The emulsions have been prepared using a Vortex and then transferred into the rheometer. The dynamic viscoelastic measurements, providing the elastic modulus G′, have been carried out at a frequency of 0.1 Hz, the stress (σ) varying from 2 to 100 Pa. II.5. SANS. The SANS experiments were performed at the Leon Brillouin Laboratory (CEA/CNRS Saclay, France) using the PAXY spectrometer. Two configurations have been used. For the first one, the incident neutron wavelength λ was set to 12 Å and the sample detector distance D was 3.5 m; the wave vectors q are then in the range 8 × 10-3 < q < 5 × 10-2 Å-1. For the second one, λ ) 5 Å and D ) 1.5 m, whence 3 × 10-2 < q < 1.5 × 10-1 Å-1. The scattering intensities on an absolute scale are obtained according to a calibration method based on the transmission of the sample and of H2O.1 Samples are placed in HELLMA quartz cells of 1 mm thickness.
Figure 2. SANS spectra of a fluorinated emulsion prepared with H2O (φ ) 0.90, rF ) 2.3, T ) 25 °C).
As already explained in the Introduction, the gel-emulsions could be considered as water emulsions in a W/O microemulsion.7,8 Therefore, the scattered intensities may be approximated as the sum of two contributions. The first is due to the large particles embedded in a homogeneous medium whose scattering density is equal to the mean scattering length density of the mixture of oil and small particles. The second contribution stems from the small particles. Consequently, the scattered intensities of the gel-emulsion may be represented by the following equation:
I)
2πSVIS(q) q4
+ (1 - φ)Im(q)
The first term represents the diffusion due to the large particles and is approximated by Porod-Auvray’s law14 where SV is the specific surface and Is is a function of the scattering length density (Q) profile at the oil/water interface:
IS(q) ) (Qext - Qint)2 + q2f1 + ... where Qext and Qint are the scattering length densities of the external and internal phases of the emulsion, respectively, and f1 depends on the profile of the surfactant membrane at the interface. The second term, Im, is the contribution of the continuous phase alone.15,16 A typical spectrum has been plotted as shown in Figure 2. To elucidate the structure of the mixed emulsions, we took advantage of the large differences of the scattering lengths of the fluorine and hydrogen atoms. In that way, the contrast between the continuous medium and the dispersed phase depends on the composition of the emulsion (hydrogenated/ fluorinated compound ratio). The contrast variation method has been applied by using various H2O/D2O mixtures as the aqueous dispersed phase, considering only the smallest q values of the scattered intensities. Even for concentrated samples, the variation of the zero angle scattered intensity I(0) with the known isotopic composition of the water phase indicates whether the big droplets are definitely water-in-fluorinated-oil or water-inhydrogenated-oil particles. In our case, if Qint is made identical to Qext the slope of the scattered intensity will vary as q-2 and no longer as q-4. The plot of the square root of the scattered intensity for q values near zero should display a minimum for Qext ) Qint. The calculation of the scattered length density Qext
10492 J. Phys. Chem. B, Vol. 104, No. 45, 2000
Figure 3. Optical microphotograph of a mixed concentrated emulsion H50 (95% water, rF ) 2.3, rH ) 1.5), objective 40. The scale is given by a bar corresponding to 8.9 µm.
corresponding to the different continuous media (fluorinated, hydrogenated, or mixed) enables us to know the composition of the most probable continuous medium, by comparison with the experimental results. III. Results III.1. Formulation. The emulsions studied are composed of hydrogenated and fluorinated compounds. Prepared from decane, C16E4, PFD, RfE2, and water, they form mixed systems. They can be obtained through two processes: (1) by mixing, according to the chosen ratio, the fluorinated and hydrogenated gel-emulsions prepared independently and (2) by mixing the fluorinated and hydrogenated surfactants, the fluorocarbon, and the hydrocarbon. Because these compounds are not completely miscible, a vigorous stirring is needed to obtain a homogeneous sample. Water is then slowly added under stirring. Whatever the process followed, mixed emulsions can be obtained with any hydrogenated/fluorinated ratio, and the systems are stable for several days. The notations used in the following indicate the proportion of hydrogenated compounds (oil + surfactant) in the mixture before adding water (see Appendix). For example, H100 means the emulsion has been prepared only with hydrogenated compounds, H0 means the emulsion has been prepared only with fluorinated compounds, and H25 means the emulsion has been prepared with 25% of hydrogenated compounds (decane + C16E4) and 75% of fluorinated compounds (PFD + RfE2). The mixed emulsions are obtained for water contents higher than 65% for H25, 75% for H50, and 85% for H75. Whatever the emulsions (fluorinated, hydrogenated, or mixed), the oil/surfactant (O/S) ratios are always rH ) 1.5 and rF ) 2.3 (w/w). We display in Figure 3 the picture of a mixed emulsion obtained using an optical microscope (Olympus BX50), which shows classic features for concentrated emulsions. No macroscopic heterogeneity is observable, and the droplet size is in the range typical for the hydrogenated emulsions (i.e., 2-3 µm). The mixed emulsions are stable; nevertheless, a film of water appears in the top of the emulsion. To study the stability with respect to coalescence, we measured the amount of water spontaneously released. Two sets of mixed emulsions (95% water) containing various proportions of fluorinated/hydrogenated compounds have been prepared. Three grams of each emulsion is placed in a 1-cm-diameter tube. The tubes are closed hermetically and kept in a thermostated bath at 25 °C. The samples prepared according to the two methods described above are observed for a few days. After each observation, the amount
Rocca and Ste´be´
Figure 4. Stability with respect to coalescence for different concentrated mixed emulsions prepared according to the two methods as explained in the text. (1) By mixing H0 (0) and H100 (9), one obtains H25m (1), H50m (2), and H75m (b). (2) By mixing fluorinated and hydrogenanted surfactants and oils, one obtains H25 (3), H50 (O), and H75 (4).
of water released by the emulsion is removed from the tube and weighed. We report the evolution of the cumulative amount of water removed with time (Figure 4). One can see that the mixed emulsions [H25m, H50m, H75m] obtained from H0 and H100 emulsions are a little less stable than those prepared following the second process. This result has convinced us that procedure 2 described above is more favorable. Therefore, the mixed emulsions are prepared by mixing the fluorinated and hydrogenated oils and surfactants; water is then incorporated under stirring. The stability of the mixed emulsions is not very different for samples of different compositions, although the more hydrogenated mixed emulsions (H75) appear to be the most fragile. After more than 100 hours, water domains appear inside the emulsions and phase separation occurs progressively. Fluorinated emulsions are the most stable (less than 3% water released after 60 h). III.2. Rheological Behavior. The rheological properties have already been examined for fluorinated and hydrogenated concentrated emulsions.5 In both cases, these emulsions display viscoelastic behavior with a high elastic modulus (G′H ) 110 Pa and G′F ) 350 Pa) and a large linear domain (until ∼15% strain for a fluorinated emulsion (H0) and ∼12% for a hydrogenated one (H100) (cf. Figure 5a)). In Figure 5a, the evolution of G′ for three mixed emulsions, H25, H50, and H75, is displayed with rH ) 1.5 and rF ) 2.3 which contain 95% water. It can be seen that the mixed emulsions behave in a similar way as the fluorinated and hydrogenated ones. The linear domain remains large (10% strain), and G′ values range from 115 to 180 Pa. The preparation of a large range of mixed emulsions allowed us to examine carefully the variation of the elastic modulus as a function of the composition in hydrogenated and fluorinated compounds (cf. Figure 5b). Surprisingly, from H100 to H20 the elastic modulus does not vary (∼150 Pa), but when the proportion of fluorinated compounds exceeds 80%, G′ rises up to 400 Pa. III.3. Phase Behavior. The study of the phase behavior of the above complex systems was performed step by step. First, the mixture of the two surfactants was examined. The binary diagram (Figure 6) displays the miscibility of C16E4 and RfE2. An isotropic phase exists above 35 °C. For temperatures lower than 25 °C, gel phases (Lβ) are obtained as indicated by the typical texture observed under an optical microscope. The isotropic phase appears between 25 and 35 °C depending on
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Figure 7. Phase diagram of C16E4/RfE2/H2O (a) at 35 °C and (b) at 50 °C. Figure 5. (a) Determination of the linear domain; variation of the elastic modulus (G′) as a function of strain for different mixed concentrated emulsions, φ ) 0.95. (b) Evolution of the elastic modulus G′ of mixed emulsions as a function of the mass fraction of hydrogenated constituents (surfactant + oil), φ ) 0.95.
Figure 6. Phase diagram of the C16E4/RfE2.
the composition of the mixture. The melting temperature of the hydrophobic tails increases with the amount of the fluorinated surfactant in the mixture. Second, water is added to the two surfactants. The ternary system C16E4/RfE2/water is studied at 25, 35, and 50 °C. At 25 °C, the mixture of surfactants is crystallized and is not modified by the water added. At 35 °C, two isotropic phases exist in the C16E4- and RfE2-rich area. For C16E4-rich mixtures, 5-15% water can be incorporated compared to 7% for RfE2-rich
mixtures. At this temperature, an extended mixed lamellar phase (LR) is obtained for larger water contents; a gel phase (Lβ) appears in the RfE2-rich mixtures (Figure 7a). At 50 °C (Figure 7b), the two surfactants are miscible in any proportion and the resulting isotropic phase (L2) can incorporate a small amount of water. For larger water contents, two lamellar phases are observed. In the following step, decane is added to the C16E4/RfE2 mixtures. At a temperature as low as 25 °C, C16E4 is solubilized in decane up to ∼95%. The oil/surfactant mixtures can incorporate up to 12% of RfE2 (Figure 8a). At 35 °C, C16E4 is soluble in decane in any proportion and the solubilization of the fluorinated surfactant is of the same magnitude (15%) as at 25 °C. At 50 °C, a large isotropic phase (L2) is obtained, but the presence of a large amount of fluorinated surfactant in decane leads to separation (Figure 8b). Perfluorodecalin may be added to the RfE2/C16E4 mixtures instead of decane (Figure 9). In this case, the incorporation of fluorinated oil is not possible at 25 °C whatever the C16E4/RfE2 ratio. At 35 °C, an isotropic phase exists in the domain rich in fluorinated surfactant and may contain ∼40% of C16E4. At 50 °C, this L2 phase is larger; the surfactants RfE2 and C16E4 are miscible and may solubilize up to 10% of PFD. At 35 °C, 5% of water could be solubilized in the PFD/C16E4/ RfE2 mixtures and 10% in the decane/C16E4/RfE2 mixtures. In conclusion, all the systems studied above lead to the formation of mixed reverse microemulsions. The last step is the determination of the phase behavior of the system with five compounds, which allows us to prepare mixed gel-emulsions. To realize this study, the decane/C16E4 (rH) and PFD/RfE2 (rF) ratios were fixed at 1.5 and 2.3,
10494 J. Phys. Chem. B, Vol. 104, No. 45, 2000
Figure 8. Phase diagram of C16E4/RfE2/decane (a) at 25 °C (solid line) and 35 °C (dashed line) and (b) at 50 °C. II means biphasic domain.
respectively; this corresponds to the composition of oils and surfactants of the concentrated emulsions studied. The phase behavior is reported on a ternary diagram where the corners represent the decane/C16E4 mixture (denoted mH), the PFD/RfE2 mixture (denoted mF), and water (Figure 10). A set of samples were prepared by mixing the mH and mF solutions in various proportions. When equilibrium is reached, the number and the volume of the different phases are noted. Then water is added progressively in 1% steps, and after each addition the same observations are performed. Three temperatures were studied (25, 35, and 50 °C), and no notable modification of the behavior has been observed. Therefore, only the diagram at 25 °C is reported (Figure 10). It may be noticed that the use of the triangular representation does not allow applying the phase rule as in a ternary phase diagram; as a matter of fact, the tie lines are not all in the same plane. Only the phase boundaries can be represented. Mixing the mH and mF microemulsions leads to the formation of a mixed microemulsion if the ratio mH/mF is >6 or 6), mHf constitutes the upper phase and water the lower phase (domain c). Concerning the other mH/mF ratios, mixing mH and mF leads to triphasic systems with two mixed microemulsions mHf and mFh and an aqueous phase (domain d). The separation into three phases is slow (from several hours to several days). For water contents larger than 80%, gel-emulsions are formed and their stability increases with the water content. III.4. Quantitative Analysis. To assess the composition of the microemulsions mHf and mFh, NMR analyses were performed according to the method described in Section II. Two sets of mixed emulsions with two different ratios of fluorinated and hydrogenated compounds, H25 and H50, were prepared. For each composition, three water contents (10%, 50%, and 70%) were considered. Then the emulsions were destabilized until total phase separation occurred, and the two microemulsions were removed and analyzed. The results of these analyses are summarized in Table 1. In fact, only mixed microemulsions are obtained because the five compounds are present in every case. The composition of the microemulsions obviously depends on the initial mH/mF ratio, but one may notice that the microemulsion mHf, rich in hydrogenated compounds, contains a noticeable amount of fluorinated compounds (∼15%). Within the limit of experimental error, the composition of the microemulsions does not vary with the water content, which means that the formation of the emulsions does not influence
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Figure 10. Representation of the phase boundaries of the system mH/mF/water (T ) 25 °C); a, b, and c are biphasic domains, and d is a triphasic domain (see text).
TABLE 1: NMR Analysis of the Mixed Microemulsions mHf and mFh for Two Series of Mixed Emulsions (H25 and H50), Containing Various Water Amounts H25 mHf mFh
10% 50% 70% 10% 50% 70%
H50 mHf mFh
10% 50% 70% 10% 50% 70%
C16E4
R f E2
decane
PFD
water
22.0 23.0 24.5 1.5 1.0 1.5
17.0 15.5 14.5 25.5 26.0 27.5
41.5 52.0 44.5 2.5 2.0 1.5
9.0 3.0 3.5 66.0 60.0 63.5
10.5 6.5 12.5 4.5 10.0 6.0
C16E4
R f E2
decane
PFD
water
28.5 29.0 29.0 0.5 2.0 1.0
14.5 13.0 13.5 17.0 21.0 20.0
33.0 43.0 39.0 2.0 1.0 1.0
10.5 5.0 7.5 77.0 75.0 76.0
13.5 10.0 11.0 2.5 1.0 2.0
the phase equilibrium. In the following, we will examine the structure of these concentrated mixed emulsions. III.5. Structural Study (SANS). In a first step, using the contrast variation method (see Section II), the composition of the continuous medium of these emulsions is explored. Eight emulsions are prepared (H50) with various H2O/D2O contents, and their spectra are recorded in a range of small wave vectors q varying from 8 × 10-3 to 5 × 10-2 Å-1. The scattered intensity is a function of the isotopic composition of the aqueous phase; for small q, the scattered intensity varies as q-2 to q-4 according to the contrast. The scattered intensities are monitored at q ) 8 × 10-3 Å-1 and reported as a function of the isotopic composition of the aqueous phase (Figure 11a). The graph
presents a minimum for an aqueous phase with 20% D2O. To understand this result, three possibilities are proposed: (i) the continuous medium consists of the five compounds; (ii) the continuous medium contains only the hydrogenated surfactant, the hydrocarbon, and some water; (iii) the continuous medium is constituted of the two surfactants, the hydrocarbon, and some water. The calculation of the volumic scattering lengths indicates that whatever the composition of the continuous phase, there is no modification of the corresponding value for the dispersed phase, which is constituted, in its major part, of water. The extinction of contrast between the dispersed phase and the continuous medium occurs for case i with 22% D2O, for case ii with 3% D2O, and for the last case with 8% D2O (Figure 11b). The experimental results show that the extinction is obtained near 20% D2O. In conclusion, it seems that the continuous medium is composed of the fluorinated and the hydrogenated surfactants, the hydrocarbon, the fluorocarbon, and some water. To obtain further information, a set of emulsions (H0, H20, H60, H80, and H100) is prepared with D2O; the SANS spectra of these emulsions are reported in Figure 12. The pattern of the spectra of the mixed emulsions is analogous to that of purely hydrogenated or fluorinated emulsions. At small values of q, the scattered intensity follows a q-4 law, and at higher q values, a maximum appears which characterizes the continuous medium. The position and the intensity of this maximum depend on the composition (fluorinated/ hydrogenated) of the emulsions. The scattered intensity is highest with H100 and decreases with the content of fluorinated
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Figure 12. SANS spectra for five concentrated emulsions (H0, H20, H60, H80, and H100), 90% D2O, rF ) 2.3, rH ) 1.5, T ) 25 °C.
Figure 11. (a) Experimental results, plot of xI(q) at q ) 8 × 10-3 Å-1 as a function of the water isotopic composition. (b) Calculation of the contrast (Qint - Qext) as a function of the isotopic composition of the aqueous phase for different types of continuous media. The explanation for i, ii, and iii is given in the text.
compounds, in agreement with the variation of the contrast between the aqueous and continuous phases. The spectra of two mixed emulsions, H25 and H50, prepared with H2O and D2O are analyzed. They are reported in parts a and b of Figure 13, respectively. When the dispersed phase is prepared with D2O, the evolution of the scattered intensity is analogous to that of a hydrogenated emulsion. The scattered intensity varies as q-4 at small q values, and at higher q values the maximal intensity is characteristic of a continuous medium rich in hydrogenated compounds. On the contrary, when the dispersed phase is prepared with H2O, the scattered intensity at small q values is low and no longer varies with q-4. Given that hydrogenated emulsions prepared with H2O cancel the contrast (i.e., Qext ≈ Qint), it can be deduced that the continuous medium of the mixed emulsion resembles a hydrogenated emulsion. Therefore, we conclude that the water droplets are surrounded by a medium rich in hydrogenated compounds (a mHf microemulsion). Nevertheless, the presence of a maximum at higher q values shows the existence of a microemulsion mFh in the continuous medium, because the contrast used in this case excludes the observation of a mHf microemulsion with H2O.
Figure 13. SANS spectra of the mixed concentrated emulsions prepared with H2O and D2O: (a) H25 and (b) H50 (90% water, rF ) 2.3, rH ) 1.5, T ) 25 °C).
IV. Conclusion The formulation of mixed W/O concentrated emulsions, containing a hydrogenated and a fluorinated surfactant, a hydrocarbon, a fluorocarbon, and water, is achieved in the same way as that used for hydrogenated or fluorinated emulsions. Surfactants and oils are mixed together, and then water is added under stirring. The water content and the stability of these emulsions are comparable to those of hydrogenated emulsions (H100). As to the rheological properties, the mixed emulsions display viscoelastic behavior with a large linear domain and high elastic modulus values. Nevertheless, the elastic modulus does not vary linearly with the composition (fluorinated/hydrogenated) of the emulsions. Up to very large amounts of fluorinated compounds, G′ stays at the value of the hydrogenated emulsions (G′ ≈ 150 Pa). The elastic modulus value increases only for large contents (>80%) of fluorinated compounds. According to Princen’s work,17 the elasticity of the emulsions depends on the nature of
Mixed Concentrated Water/Oil Emulsions the film of surfactant stabilizing the water droplets. Because the rheological behavior of mixed emulsions is identical to that of hydrogenated emulsions except for highly fluorinated compound contents (>80%), it may be suggested that the interface of the water droplets is constituted mainly by the hydrogenated surfactant C16E4. The study of the phase behavior of mixed systems containing two, three, four, and five compounds clearly shows the existence of reverse mixed microemulsions and liquid crystals. An investigation of the mixed emulsions, once phase equilibrium is reached, allows us to describe these systems as triphasic ones, consisting of two microemulsions, one rich in fluorinated compounds and the other rich in hydrogenated compounds, and an aqueous phase. On the contrary, the mixed emulsions with a large majority of either mH or mF are biphasic systems. Finally, as to the organization and composition of the continuous phase, the contrast variation method shows that all five compounds are present in the continuous phase of the mixed emulsions. It has clearly been demonstrated that the two microemulsions are coexisting inside the continuous medium. The SANS experiments have shown that a q-4 slope is obtained in the case of mixed emulsions prepared with D2O, whereas when H2O is used a q-2 slope is obtained, meaning that the profile of the scattering length density across the aqueous dispersed phase and the continuous phase is similar to that of a hydrogenated emulsion. In conclusion, it seems that the mHf microemulsion surrounds the water droplets. As to the mFh microemulsion, which is also present in the continuous phase, it could be encapsulated in the microemulsion rich in hydrogenated compounds. Acknowledgment. The authors thank L. Rodehu¨ser for his critical reading of the manuscript and E. Dubois and J. Oberdisse, local contacts at the LLB, for their assistance during SANS experiments. Appendix RfE2: fluorinated surfactant C16E4: hydrogenated surfactant PFD: perfluorodecalin
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10497 H0: fluorinated emulsion H100: hydrogenated emulsion H25, H50, and H75: mixed emulsions with 25, 50, and 75% of decane/C16E4, respectively, in the mixture of oils and surfactants H25m, H50m, H75m: mixed emulsions obtained by mixing H0 and H100 prepared independently rH: decane/C16E4 (w/w) rF: PFD/RfE2 (w/w) mH: mixture of decane and C16 E4 with rH ) 1.5 mF: mixture of PFD and RfE2 with rF ) 2.3 mHf: mixed microemulsion rich in hydrogenated compounds mFh: mixed microemulsion rich in fluorinated compounds φ: water fraction (w/w) References and Notes (1) Ravey, J. C.; Ste´be´, M. J.; Sauvage, S. Colloids Surf., A 1994, 91, 237. (2) Solans, C.; Azemar, N.; Parra, J. L. Prog. Colloid Polym. Sci. 1989, 76, 224. (3) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1985, 107, 271. (4) Ravey, J. C.; Ste´be´, M. J.; Sauvage, S. J. Chim. Phys. Phys.-Chim. Biol. 1994, 91, 259. (5) Pons, R.; Erra, P.; Solans, C.; Ravey, J. C.; Ste´be´, M. J. J. Phys. Chem. 1993, 97, 12320. (6) Langenfeld, A.; Schmitt, V.; Ste´be´, M. J. J. Colloid Interface Sci. 1999, 218, 522. (7) Ravey, J. C.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1990, 82, 218. (8) Ravey, J. C.; Ste´be´, M. J. Physica B 1989, 156-157, 394. (9) Caldero, G.; Garcia-Celma, M. J.; Solans, C.; Plaza, M.; Pons, R. Langmuir 1997, 13, 385. (10) Rocca, S.; Muller, S.; Ste´be´, M. J. J. Controlled Release 1999, 61, 251. (11) Caldero, G.; Garcia-Celma, M. J.; Solans, C.; Ste´be´, M. J.; Ravey, J. C.; Rocca, S.; Pons, R. Langmuir 1998, 14, 1580. (12) Rocca, S.; Garcia-Celma, M. J.; Caldero, G.; Pons, R.; Solans, C.; Ste´be´, M. J. Langmuir 1998, 14, 6840. (13) Cambon, A.; Delpuech, J. J.; Matos, L.; Serratrice, G.; Szonyi, F. Bull. Soc. Chim. 1986, 6, 965. (14) Auvray, L.; Cotton, J. P.; Ober, J.; Taupin, C. J. Phys. 1984, 45, 913. (15) Regnaut, C.; Ravey, J. C. J. Chem. Phys. 1989, 91, 1211. (16) Robertus, C.; Philipse, W. H.; Joosten, J. G. H.; Levine, H. K. J. Chem. Phys. 1989, 90, 4482. (17) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1989, 128, 176.
