Phase Behavior and Formation of Reverse Cubic Phase Based

Effect of Lipophilic Tail Architecture and Solvent Engineering on the Structure of .... Alencar , S. A. Soares , V. M. M. Melo , L. R. B. Gonçalves ,...
0 downloads 0 Views 218KB Size
Download PDF
Langmuir 2001, 17, 5169-5175

5169

Phase Behavior and Formation of Reverse Cubic Phase Based Emulsion in Water/Poly(oxyethylene) Poly(dimethylsiloxane) Surfactants/Silicone Oil Systems Md. Hemayet Uddin,† Carlos Rodrı´guez,† Kenichi Watanabe,† Arturo Lo´pez-Quintela,‡ Tadashi Kato,§ Haruhiko Furukawa,| Asao Harashima,| and Hironobu Kunieda*,† Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan, Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago E-15706, Santiago de Compostela, Spain, Department of Chemistry, Tokyo Metropolitan University, 1-1 Minamiohsawa, Hachioji, Tokyo 192-0397, Japan, and Dow Corning Toray Silicone Co. Ltd., Chigusa-Kaigan 2-2, Ichihara 299-01, Japan Received February 23, 2001. In Final Form: June 5, 2001 The phase behavior of long hydrophobic A-B type silicone surfactants, Me3SiO-(Me2SiO)m-2-Me2SiCH2CH2CH2-O-(CH2CH2O)nH (SimC3EOn), in water and water + octamethylcyclotetrasiloxane (D4) was investigated by studying phase behavior and small-angle X-ray scattering. Si25C3EO15.8 forms a reverse micellar cubic phase (I2) in water and water + D4 systems. This cubic phase is highly thermally stable in a surfactant-water binary system. The thermal stability decreases monotonically with addition of silicone oil. Although the solubilization of water in the reverse cubic phase is low, a very large amount of excess water can be incorporated in a so-called reverse cubic phase based concentrated emulsion. The emulsion stability is enhanced upon addition of silicone oil. D4 molecules penetrate into the surfactant palisade layer in the reverse micelles forming the I2 phase and expand the effective cross-sectional area per surfactant, aS (penetration). The continuous penetration of oil destabilizes the I2 phase structure, and therefore the melting temperature of the phase decreases. The incorporation of D4 into the I2 phase in the aqueous mixtures of Si14C3EO7.8, Si25C3EO7.8, Si25C3EO12.2, and Si25C3EO15.8 varies with both the hydrophilic and lipophilic chain lengths of silicone surfactants.

Introduction There is symmetry on shapes of surfactant selforganized structures formed in water. Depending on the hydrophile-lipophile balance (HLB) of surfactant, a discontinuous micellar cubic phase, normal hexagonal phase, bicontinuous cubic phase with positive layer curvature, lamellar phase, bicontinuous cubic phase with negative layer curvature, reverse hexagonal phase, and discontinuous reverse cubic phase may form. In our previous papers,1-3 it is reported that the formation of a discontinuous cubic phase with normal micelles is promoted by adding long-chain alkanes to hydrophilic surfactant-water systems, and highly concentrated oil-inwater (O/W) type emulsions can be formed in the normal discontinuous cubic phase.3,4 Hence, it can be expected that water-in-oil (W/O) type highly concentrated emulsions can also be formed based on the use of the discontinuous reverse micellar cubic phase. However, the discontinuous reverse cubic phase has been found only in a few systems, such as lecithin,5 poly(oxyethylene)-poly(propylene) copolymer,6 sugar-related surfactant7 systems, and so forth. * Corresponding author. Phone & Fax: +81-45-339-4190. Email: [email protected]. † Yokohama National University. ‡ Universidad de Santiago. § Tokyo Metropolitan University. | Dow Corning Toray Silicone Co. Ltd. (1) Kunieda, H.; Ozawa, K.; Huang, K.-L. J. Phys. Chem. B 1998, 102, 831. (2) Kunieda, H.; Shigeta, K.; Suzuki, M. Langmuir 1999, 15, 3118. (3) Rodriguez, C.; Shigeta, K.; Kunieda, H. J. Colloid Interface Sci. 2000, 223, 197. (4) Uddin, Md. H.; Kanei, N.; Kunieda, H. Langmuir 2000, 16, 6891. (5) Seddon, J. M. Biochemistry 1990, 29, 7997.

The shape of the self-organized structure and the surfactant layer curvature are dependent on the interactions between each group of surfactant. According to the surfactant parameter or critical packing parameter theory,8-10 the effective cross-sectional area per surfactant in the aggregates in water, aS, is determined by the balance between the repulsion of hydrophilic headgroups and the attraction driven by the water-hydrophobic moiety interfacial tension. The surfactant layer curvature is determined by the critical packing parameter that involves aS, the volume of the hydrophobic part, and the effective hydrophobic chain length. In this theory, the effect of the hydrophobic chain on the surfactant layer curvature is essentially neglected. Recently, it has been found that the hydrophobic chain is also crucial to the surfactant layer curvature.11,12 When the hydrophobic chain of the surfactant becomes long, aS also increases but the surfactant layer curvature is changed toward negative values.12 It has been also reported that the types or shapes of self-organized structures are highly correlated with the volume ratio of the ethylene oxide (EO) chain to the total surfactant, nVEO/VS, or to the classical Griffin’s HLB (6) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1996, 12, 1419. (7) Minamikawa, H.; Hato, M. Langmuir 1998, 14, 4503. (8) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (9) Israelachvili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press: London, 1992; Chapter 17. (10) Hyde, S.; Andersson, S.; Larsson, K.; Blum, Z.; Landh, T.; Lidin, S.; Ninham, B. W. The Language of Shape; Elsevier: Amsterdam, 1997; Chapter 4. (11) Seddon, J. M. Biochim. Biophys. Acta 1990, 1031, 1. (12) Kunieda, H.; Uddin, Md. H.; Horii, M.; Furukawa, H.; Harashima, A. J. Phys. Chem. B 2001, 105, 5419.

10.1021/la010289d CCC: $20.00 © 2001 American Chemical Society Published on Web 07/31/2001

5170

Langmuir, Vol. 17, No. 17, 2001

number.13 However, in the conventional hydrocarbon-type surfactant systems, in general, the hydrophobic chain length is limited in a range of C8-C18. Therefore, in poly(oxyethylene)-type nonionic surfactant systems, very short EO chain surfactant has to be used to form the reverse micellar cubic phase, but such a surfactant hardly forms aggregates due to the very weak amphiphilicity. In other words, the segregation tendency of the short EO chain from oil is not enough to form reverse micelles.14 Similarly, a very short alkyl chain does not form normal micelles in water. On the other hand, silicone surfactants with a long hydrophobic chain (poly(dimethylsiloxane)) can be used because they are often in the liquid state up to very high molecular weights at room temperature.15,16 It is possible to form the discontinuous reverse micellar cubic phase (I2) in the poly(oxyethylene)-type silicone surfactant systems because the surfactant is lipophilic enough to produce an I2 phase even if the EO chain is long. In fact, we have found the I2 phase in several water-linear A-B type silicone surfactant systems.12 The solubilization of oil in different surfactant aggregates in the hydrocarbon surfactant systems has been studied earlier.17-21 Recently, there have been a few studies about the effect of low molecular weight silicone oil on the phase behavior of water-poly(oxyethylene) trisiloxane surfactant systems, mainly concerned with the normal type of liquid crystals.22,23 In the normal type of liquid crystals, oil is solubilized in the inner core of aggregates formed with the lipophilic part of the surfactant. On the other hand, in the reverse type, oil must be incorporated in the outer core of aggregates. It is very important to know the location of oil solubilization in the reverse-type liquid crystals, whether it is solubilized into a poly(dimethylsiloxane) palisade layer of surfactant or into an intermicellar space, and how much the solubilization is affected by changing the hydrophilic or lipophilic chain length of surfactant. In this context, we investigated the formation of I2 phase in a water-poly(oxyethylene) poly(dimethylsiloxane) (Si25C3EO15.8) system. We studied the effect of added low molecular weight silicone oil, octamethylcyclotetrasiloxane (D4), on the stability of the I2 phase in water-SimC3EOn systems and, eventually, the formation of highly concentrated W/O emulsions. Experimental Section Materials. The series of surfactants with the general formula Me3SiO-(Me2SiO)m-2-Me2SiCH2CH2CH2-O-(CH2CH2O)nH, abbreviated as SimC3EOn, were obtained from Dow Corning Toray Silicone Co. Ltd., Japan. Me is a methyl group attached to Si, (13) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (14) Kitahara, A. J. Phys. Chem. B 1965, 69, 2788. (15) Schmidt, G. L. F. In Industrial Applications of Surfactants; Karsa, D. R., Ed.; Royal Society for Chemistry: London, 1987; p 24. (16) Silicone Surfactants; Hill, R. M., Ed.; Surfactant Science Series 86; Marcel Dekker: New York, 1999. (17) Hoffman, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (18) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys. Chem. B 1986, 90, 842. (19) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. B 1988, 92, 774. (20) Eastoe, J.; Hetherington, K. J.; Sharpe, D.; Dong, J.; Heenan, R. K.; Steytler, D. Langmuir 1996, 12, 3876. (21) Kunieda, H.; Umizu, G.; Aramaki, K. J. Phys. Chem. B 2000, 104, 2005. (22) Kunieda, H.; Taoka, H.; Iwanaga, T.; Harashima, A. Langmuir 1998, 14, 5113. (23) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R. M. Langmuir 1999, 15, 2278.

Uddin et al. m is the total number of silicone, and n is the average number of ethylene oxide (EO) units. Their purities are 99.9% for Si14C3EO7.8, 96.2% for Si25C3EO7.8, 93.7% for Si25C3EO12.2, and 93.1% for Si25C3EO15.8. The main impurity is unreacted polyether, CH2dCHCH2-O-(CH2CH2O)nH, which is soluble in water. Octamethylcyclotetrasiloxane (C8H24O4Si4 or D4) was obtained from Dow Corning Toray Silicone Co. Ltd. All chemicals were used as received. Fresh deionized water was used to prepare mixtures of water, surfactant, and oil by weight. Calculation of the Volume Fraction of the Lipophilic Part of Surfactants. The volume fraction of the lipophilic part of the surfactant was calculated by using the following equation:

φL )

VL MS 1 - W′S VS + FW W′S

(1)

where VS and VL are the molar volumes of surfactant and its lipophilic moiety. FW is the density of water, MS is the molecular weight of surfactant, and W′S is the weight fraction of pure surfactant (excluding impurity) in the water-surfactant system. As the water-soluble polyether impurities are completely insoluble in poly(dimethylsiloxane) oil, they are considered to be solubilzed in the hydrophilic inner cores of reverse micelles (1 - φL). The values of VL are 1137 cm3/mol or 1.89 nm3/molecule for Si14C3- and 1985 cm3/mol or 3.29 nm3/molecule for Si25C3.12 In the presence of oil, eq 1 is rewritten to the following form:

φL )

VL MS 1 - (WO + W′S) MS WO VS + + FW W′S FO W′S

(2)

where FO is the density and WO is the weight fraction of D4 in the system. Determination of Phase Diagrams. Various amounts of constituents were weighed and sealed in ampules. Samples were mixed using a vortex mixer, and homogeneity was attained by repeated centrifugation through a narrow constriction in the sample tubes. The phase equilibria were determined by visual observation. The optical isotropic nature of the samples was checked with crossed polarizers. The structural characterization of liquid crystal was determined by means of small-angle X-ray scattering (SAXS) measurements. Small-Angle X-ray Scattering. SAXS measurements on surfactant samples with no added solvent were performed by using the synchrotron radiation SAXS spectrometer of the BL10C instrument installed at the Photon Factory (PF), High Energy Physics Research Institute (KEK), Tsukuba, Japan. The scattered X-ray was detected using a position sensitive proportional counter. A copper-made sample cell with Kapton windows was used, and the temperature of the sample was controlled within a precision of 0.01 K. The interlayer spacing of liquid crystals in the presence of solvent was also measured by SAXS, performed on a small-angle scattering goniometer with an 18 kW Rigaku Denki rotating anode generator (Rint-2500) at about 25 °C. The samples were covered with plastic films (Mylar seal method) for the measurement. It is assumed that spherical reverse micelles are packed in a cubic array in the I2 phase. According to the geometry of the I2 phase24 (Fd3m space group) shown in Figure 1, the following equations can be derived for the calculation of the radius of the hydrophilic core of micelle, rI2, and the effective cross-sectional area per surfactant molecule, a S:

( )

rI2 ) C

3 4πnm

aS )

1/3

(1 - φL)1/3d

3vL 1 - φL rI2 φL

(3)

(4)

where d is the interlayer spacing measured by SAXS, nm is the

Reverse Cubic Phase Based Emulsion

Langmuir, Vol. 17, No. 17, 2001 5171

Figure 1. Schematic representation of the I2 phase (Fd3m space group). The unit cell contains 8 reverse micelles of symmetry 4h 3m (white) and 16 reverse micelles of symmetry 3m (shaded). The lines are a guide to the eye. Adapted from ref 24.

Figure 2. Phase diagram of the water/Si25C3EO15.8 system: I2, discontinuous reverse cubic phase; Om, reverse micellar solution or surfactant liquid phase; W, an excess-water phase; II, two-phase region; S, solid present phase. number of micelles per unit cell, and C is a constant (C ) (h2 + k2 + l2)1/2, where h, k, and l are the Miller indices). vL is the volume of the lipophilic part of one surfactant molecule. In oilcontaining systems, eqs 3 and 4 can be modified considering that oil molecules are incorporated into the lipophilic moiety of surfactant in the reverse micelles forming the I2 phase as shown in Figure 1, which leads to the following equations:

( )

rI2 ) C

3 4πnm

aS )

1/3

(1 - (φL + φO))1/3d

3vL 1 - (φL + φO) rI2 φL

(5)

(6)

where φO is the volume fraction of silicone oil (D4) in the system.

Results and Discussion Phase Diagram of the Water-Si25C3EO15.8 Binary System. The phase diagram of the water-Si25C3EO15.8 system was constructed as a function of temperature and is shown in Figure 2. A considerably large isotropic cubic phase region appears, and no birefringent phase was found through the entire range of composition. The samples in this region are very stiff with a clear glassy appearance. A very narrow (24) Luzzati, V.; Vargas, R.; Gulik, A.; Mariani, P.; Seddon, J. M.; Rivas, E. Biochemistry 1992, 31, 279.

solid present region exists at very high surfactant concentration, and judging from X-ray scattering data at wide angles, it seems to be in an amorphous state. This solid phase changes into the cubic phase even at 100% surfactant composition at room temperature. The cubic phase is considered a discontinuous reverse micellar cubic phase, represented as I2, previously observed in aqueous binary mixtures of the homologous series of these surfactants (Si14C3EO7.8, Si25C3EO7.8, and Si25C3EO12.2).12 In water-Si14C3EOn systems, surfactant liquid (Om) phase, reverse hexagonal (H2), and lamellar (LR) liquid crystals are obtained for the EO chain lengths of 3.2, 12, and 15.8, respectively. In longer hydrophobic chain Si25C3EOn systems, the Om phase for EO chain of 3.2 and LR for 51.6 are observed. The I2 phase appears between the reverse micellar solution (Om) and reverse hexagonal (H2) phases in the range of volume ratio of EO to the total surfactant (nVEO/VS) of 0.13-0.21 for Si14C3EOn and 0.09-0.24 for Si25C3EOn. On the other hand, polyoxyethylene dodecyl or oleyl surfactant does not form the I2 phase in water. In these systems, the Om phase extends up to nVEO/VS ) 0-0.24 for dodecyl and 0-0.2 for oleyl surfactant; however, the H2 phase is observed in the range of 0.2-0.26 in the oleyl surfactant/water systems.12 The features of the phase diagram of the I2 phase in the water-Si25C3EO15.8 system are similar to those of the I2 phase in water-Si14C3EO7.8, water-Si25C3EO7.8, and water-Si25C3EO12.2 phase diagrams. In the concentrated surfactant region, the I2 phase melts to a reverse micellar solution or a surfactant liquid (Om) phase via a two-phase region (I2 + Om). The maximum melting point of the I2 phase (>160 °C) is about 60 °C higher than that of the normal-type cubic (I1) phase formed in the similar molecular weight hydrophilic silicone surfactant (Si5.8C3EO51.6)-water system,25 indicating a high thermal stability of the reverse-type cubic phase. In comparison with other systems (Si14C3EO7.8, Si25C3EO7.8, and Si25C3EO12.2),12 the maximum melting point of the I2 phase increases with increasing both hydrophilic and lipophilic chain lengths of the surfactant, indicating that the lipophilic moiety also has an effect on the stability of mesophases. In other words, the maximum melting temperature of the I2 phase or its thermal stability increases with the increase of the molecular weight of surfactants. SAXS measurements were performed at different temperatures on a bulk (as received) Si25C3EO15.8 sample. Figure 3a shows a representative SAXS pattern at 80 °C. A total of eight Bragg peaks were identified (indicated by arrows) which can be indexed as the 111, 220, 311, 400, 331, 333, 440, and 533 reflections of the face-centered space group Fd3m (Q227). This group has been found in some binary26 and ternary systems.6,24 Since the SAXS pattern we obtained is similar to those previously observed for Fd3m cubic phases, it is quite possible that the Si25C3EO15.8 I2 phase has the same structure proposed by Charvolin and Sadoc27 and confirmed by Luzzati et al.,24 namely, a unit cell containing 24 quasi-spherical reverse micelles, 8 larger ones and 16 smaller ones. The plot of the reciprocal d spacings (1/dhkl) versus (h2 + l2 + k2)1/2 (Figure 3b) fits a straight line passing through the origin, in agreement with the Fd3m space group. The lattice parameter, from the reciprocal slope of the plot, is 32.4 nm. This value showed almost no variation in a temperature scan from 30 to 80 °C. However, it was observed (25) Rodriguez, C.; Uddin, Md. H.; Furukawa, H.; Harashima, A.; Kunieda, H. Prog. Colloid Polym. Sci., accepted. (26) Seddon, J. M.; Zeb, N.; Templer, R. H.; McElhaney, R. N.; Mannock, D. A. Langmuir 1996, 12, 5250. (27) Charvolin, J.; Sadoc, J. F. J. Phys. 1988, 49, 521.

5172

Langmuir, Vol. 17, No. 17, 2001

Uddin et al.

Figure 4. Variation of d, rI2, and aS in the I2 phase in the water/Si25C3EO15.8 system as a function of water content at 25 °C: d (0), interlayer spacing; rI2 (4), radius of the hydrophilic core of reverse micelle; aS (O), effective cross-sectional area per surfactant molecule.

Figure 3. SAXS data obtained from a bulk (as received) Si25C3EO15.8 sample at 80 °C. (a) Diffraction pattern. The arrows indicate reflections corresponding to the Fd3m space group. q is the scattering vector. (b) Plot of the reciprocal d spacings (1/dhkl) of the reflections marked in (a) versus (h2 + k2 + l2)1/2.

that the intensities and sharpness of the reflections increase significantly with temperature, which might indicate that the sample becomes more homogeneous and consequently there is a growth of crystalline domains. The solubilization of water into the reverse micelle forming cubic phase increases with increasing surfactant EO chain length. The excess-water phase (I2 + W) region appears beyond the solubilization limit of the I2 phase, and the phase boundary was determined by SAXS at 25 °C. The interlayer spacing, d, corresponding to the 311 Bragg reflection (the most intense one in the Figure 3a) of the reverse cubic phase was measured by SAXS as a function of surfactant concentration. Studies on other Fd3m cubic structures24 have shown that micelles display a high degree of spherical symmetry and that the difference in the size of the two types of micelles (Figure 1) is not large. We therefore assumed monodisperse spherical micelles ordered in a Fd3m structure and used eqs 3 and 4 (nm ) 24) to calculate the radius of the hydrophilic core of micelles, rI2, and the effective cross-sectional area per surfactant molecule, aS. Variation of d, rI2, and aS with surfactant concentration is shown in Figure 4.

Figure 5. Phase diagram of the water-Si25C3EO15.8-D4 system at 25 °C. Phase notation is as in Figure 2. III denotes a threephase region.

Water is incorporated in the reverse micellar core, micelles grow, and d and rI2 increase until an excess-water phase separates. Hence, the solubilization of water in the I2 phase can be estimated by the inflection point in the d curve. aS is not largely changed with water content, suggesting a small variation in the hydration of the ethylene oxide groups, and water incorporates mainly by swelling of the micellar cores. Ternary Phase Behavior of the Water-Si25C3EO15.8-D4 System. The phase behavior of the waterSi25C3EO15.8-D4 ternary system at 25 °C is represented in Figure 5. The accuracy in the location of the phase boundaries is (1.5 wt %. There are only two single phases, namely, a large isotropic liquid crystalline reverse cubic phase (I2) and a reverse micellar solution phase (Om). About 59 wt % of Si25C3EO15.8 can be dissolved in D4. Although the monomeric solubility of CmEOn-type nonionics into alkanes is considerably large,28 Si25C3EO15.8 forms reverse micelles at a very low surfactant concentration (0.1 mmol/L) in D4.29 The Om phase can solubilize small amounts of water (28) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1971, 35, 359. (29) Rodrı´guez, C.; Uddin, Md. H.; Watanabe, K.; Furukawa, H.; Harashima, A.; Kunieda, H. J. Phys. Chem. B, submitted.

Reverse Cubic Phase Based Emulsion

Figure 6. Variation of d in the I2 phase in the water-Si25C3EO15.8-D4 system as a function of water content at 25 °C. Lines A and B indicate the weight ratios of Si25C3EO15.8/D4 of 80/20 and 70/30, respectively.

up to a maximum of about 15 wt %. Beyond the solubilization limit, Om phase coexists with excess water. A large discontinuous reverse cubic phase is found at the surfactant corner of the phase diagram, which is separated from the Om phase by two- or three-phase regions. The two- or three-phase regions are indicated tentatively in accordance with the Gibbs phase rule. A considerable amount of water up to a maximum of 28 wt % (on the water-surfactant binary axis) can be solubilized into the I2 phase. With further addition of water, an excess-water phase separates at the solubilization limit, introducing a little turbidity of the samples in these regions, making it difficult to determine the phase boundary between I2 and I2 + W phases only by visual observation. To ascertain the phase boundary, the interlayer spacing (d) of the samples at constant Si25C3EO15.8/ D4 mass ratios of 80/20 and 70/30 was measured by using SAXS, and the results are shown in Figure 6 as a function of increasing water content. Assuming that the aggregation number of micelles is constant at a fixed surfactant/oil mass ratio, d increases as the micellar size increases upon solubilization of water into the reverse micellar core. The d curves become flat after reaching the solubilization limit of the I2 phase. Hence, the phase boundary was determined from the inflection point of the curve. Although the solubilization of water in the I2 phase is not very much (28 wt %), a large amount of water (up to 90 wt %) can be incorporated as water droplets in the I2 + W region forming a turbid gellike emulsion having a cubic phase (I2) as the external continuous phase. Water incorporation in the stable emulsions can be increased (up to 96 wt % water) in oil present systems as described later. In the surfactant-D4 binary system, about 35 wt % of D4 can be incorporated into the I2 phase. Upon addition of water to the system, the incorporation of D4 in the I2 phase increases up to a maximum of 54 wt % along the surfactant-water mass ratio of 85/15, which is the middle of the binary water-Si25C3EO15.8 I2 phase. Oil mostly incorporates into the corona of reverse micelles, affecting the stability of the I2 phase. Effect of Octamethylcyclotetrasiloxane (D4) on the Phase Behavior of Water-SimC3EOn Systems. The effect of addition of low molecular weight silicone oil, D4, on the I2 phase of water-Si14C3EO7.8, -Si25C3EO7.8, -Si25C3EO12.2, and -Si25C3EO15.8 (water/SimC3EOn weight ratios were kept constant at 10/90) systems was investigated as a function of temperature. The melting point of the reverse cubic phase, that is, the phase transition

Langmuir, Vol. 17, No. 17, 2001 5173

temperature from the cubic phase to reverse micellar solution phase, is plotted in Figure 7a-d as a function of oil concentration. In all systems, with addition of D4 to the surfactantwater binary systems the melting point of the I2 phase decreases monotonically and sharply. In Si14C3EO7.8 and Si25C3EO7.8 systems, the I2 phase changes to a turbid I2 + W phase with increasing temperature and finally the I2 phase melts to an Om + W phase. Between the twophase regions, there should be a three-phase region according to the phase rule, but it is probably very narrow and it could not be identified. The incorporation of D4 into the intermicellar spaces in the I2 phase varies with both the hydrophilic and lipophilic chain lengths of the surfactant. The maximum incorporation of oil into the I2 phase at room temperature is 38 wt % for Si14C3EO7.8, 42 wt % for Si25C3EO7.8, 55 wt % for Si25C3EO12.2, and 53 wt % for Si25C3EO15.8 systems. For a constant ethylene oxide chain length, solubility of oil increases with increasing the lipophilic chain length of the surfactant. In the case of the normal type of liquid crystals, oil molecules are solubilized in the aggregates by two possible ways.1 One is the “penetration effect” in which oil molecules penetrate into the surfactant palisade layer and expand the effective cross-sectional area, aS. The other is the “swelling effect”, in which oil molecules are solubilized in the core of aggregates and increase the volume of aggregates so that aS remains almost constant. To understand the process of oil incorporation into the I2 phase, we measured the interlayer spacing d of waterSimC3EOn-D4 systems by SAXS. rI2 and aS were calculated by using eqs 5 and 6, and the results are shown in Figure 8a-d as a function of oil content. In all systems, both the interlayer spacing, d, and the radius of the hydrophilic core of reverse micelles in the I2 phase, rI2, decrease with increasing oil content except in the Si14C3EO7.8 system in which there is a slight increase in the d values. The decrease of d and rI2 by the addition of oil can be attributed to the decrease of aggregation number of the reverse micelles forming the I2 phase. In all cases, the effective cross-sectional area per surfactant, aS, continuously increases with the incorporation of D4. Even in the oil-lean regions, the absolute values of aS are much larger than those found in water-in-oil microemulsions with conventional CmEOn nonionic surfactants.30 The molecular sizes of the present silicone surfactants (SimC3EOn) are 3-6 times larger than those referenced CmEOn nonionics. We reported that aS increases with increasing surfactant molecular size, namely, with both the dimethylsiloxane (m) or EO chain (n) lengths of the surfactant as the long chain has a shrunk-bulky structure compared with a short chain.12 But the surfactant layer curvature becomes negative (concave toward water) while increasing the aS with the increase of m. As the inner cores of reverse micelles are formed with the hydrophilic part of the surfactant and water, oil molecules have to be incorporated in the outer part of micelles formed with the lipophilic moiety of the surfactant. Actually, oil molecules are mainly incorporated in the palisade layer of the lipophilic part of the surfactant, instead of the intermicellar spaces, and hence the aS increases. However, the extent of increase of the aS varies for different systems. The penetration of oil molecules into the surfactant palisade layer induces the change in curvature and flexibility of the surfactant layer that causes the I2 phase to change into Om phase at lower temperatures. Hence, (30) Strey, R.; Glatter, O.; Schubert, K.-V.; Kaler, E. W. J. Chem. Phys. 1996, 105, 1175.

5174

Langmuir, Vol. 17, No. 17, 2001

Figure 7. Phase diagram of the water-SimC3EOn-D4 systems as a function of temperature. Phase notation is as in Figure 2. The water/SimC3EOn weight ratios are 10/90. (a) Si14C3EO7.8; (b) Si25C3EO7.8; (c) Si25C3EO12.2; (d) Si25C3EO15.8.

Uddin et al.

Figure 8. Variation of d, rI2, and aS in the I2 phase in the water-SimC3EOn-D4 systems as a function of volume fraction of D4 at 25 °C. Phase notation is as in Figure 4. The water/ SimC3EOn weight ratios are 10/90. (a) Si14C3EO7.8; (b) Si25C3EO7.8; (c) Si25C3EO12.2; (d) Si25C3EO15.8.

Reverse Cubic Phase Based Emulsion

Langmuir, Vol. 17, No. 17, 2001 5175

Figure 9. VEM photomicrograph of a reverse cubic phase based emulsion containing 96 wt % water. The Si25C3EO15.8/D4 weight ratio is 50/50. Bar ) 50 µm.

the melting point curves fall down monotonically with oil incorporation. Formation of Reverse Cubic Phase Based Emulsions. As mentioned in the previous section, W/O type highly concentrated emulsions can also be formed based on the use of the discontinuous reverse micellar cubic phase. In the water-Si25C3EO15.8 binary system, water was continuously added and the samples became turbid beyond the solubilization capacity of the I2 phase, and we were able to incorporate about 90 wt % of water in viscous emulsions. With further addition of water, excess water was separated out as an excess phase. In other binary (water-Si14C3EO7.8, -Si25C3EO7.8, and -Si25C3EO12.2) systems, the maximum incorporation of water was around 40-50 wt %. However, in all surfactant systems, in the presence of D4 more than 90 wt % water can be incorporated in gel-like emulsions. The presence of oil molecules in the surfactant palisade layer induces the change in curvature and flexibility of the surfactant layer that allows higher amounts of water to be incorporated in the gelemulsions. The emulsions are quite stable, and excess water cannot be separated out even by centrifuging the samples for at least 12 h over 4000 rounds per minute at room temperature, since coalescence and creaming could not take place because of the extremely high viscosity of the external I2 phase. The emulsions are also thermally stable and cannot be diluted with a lot of excess water because the I2 phase exists as an external phase. However, the gel-emulsions can be diluted with D4 to form normal emulsions with a small droplet size (1-10 µm). To prepare a cubic phase based emulsion, the I2 phase should be melted to enable proper mixing during the emulsification process. Water incorporates gradually into the surfactant phase, and finally a gel is obtained by cooling the sample. The gels look turbid over the entire range of compositions in which they can be produced, because of the difference in the values of refractive indices of the cubic phase and water. A photomicrograph of the

gel-emulsion, obtained by video-enhanced microscopy (VEM), is shown in Figure 9. The emulsion is polydisperse, and some of the droplets are polyhedral, since the volume fraction of the internal phase (water) exceeds the maximum value for a packing of spheres, 0.74. Conclusion The reverse micellar cubic (I2) phase appears in Si25C3EO15.8 concentrated regions in water and water + D4 systems, with a structure corresponding to the Fd3m space group. The I2 phase in the binary water system is highly thermally stable. The addition of low molecular weight silicone oil, D4, destabilizes the I2 phases formed in waterSi14C3EO7.8, -Si25C3EO7.8, -Si25C3EO12.2, and -Si25C3EO15.8 systems by reducing their melting temperature. However, there is a maximum oil incorporation into the I2 phase in the water-Si25C3EO15.8-D4 ternary systems. The silicone oil penetrates into the palisade layer of the hydrophobic moiety of the surfactant and increases the effective cross-sectional area per surfactant aS, changing the flexibility at the hydrophobic-hydrophilic interface that induces the transition I2-reverse micellar solution phase at lower temperatures. The solubilization of water in the SimC3EOn-D4 cubic phases is low; however, very high water content, stable gel-emulsions, so-called reverse cubic phase based emulsions (W/I2), can be obtained. The emulsions are more likely to form in oil-containing ternary systems rather than the water/SimC3EOn binary systems. Acknowledgment. We thank Dr. Hirohisa Yoshida and Mr. Koji Minewaki (Tokyo Metropolitan University) for performing synchrotron radiation SAXS measurements. M.A.L.Q. acknowledges a grant by the “Ministerio de Educacio´n, Cultura y Deporte”, Spain, to spend part of a sabbatical term at the University of Yokohama, Japan. LA010289D