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Phase Behavior and Microstructure of Water/Trisiloxane E12 Polyoxyethylene Surfactant/Silicone Oil Systems X. Li,† R. M. Washenberger,‡ L. E. Scriven, and H. T. Davis Center for Interfacial Engineering and Department of Chemical Engineering and Materials Scicence, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455
Randal M. Hill* Central R&D, Dow Corning Corporation, 2200 West Salzburg, Midland, Michigan 48686-0994 Received April 9, 1998. In Final Form: December 17, 1998 The ternary phase behavior of the trisiloxane E12 polyoxyethylene surfactant with water and three low molecular weight silicone oils has been determined. The silicone oils were the tetra- and pentacyclosiloxanes, D4 and D5, and the short linear tetrasiloxane, MD2M. Microstructures were investigated using small-angle X-ray scattering and polarized light microscopy. All three ternary systems exhibit similar phase behavior, forming surfactant-rich and water-rich microemulsions, and liquid crystal phases. They follow the now familiar 2Φ (Winsor I) to three-phase (Winsor III) sequence with increasing temperature. Because of the large immiscibility gap between this very polar trisiloxane surfactant and the silicone oils, 2Φ (Winsor II) type behavior was not observed, even at elevated temperatures (up to 140 °C). The cubic I1 and I3 phases were found in the ternary systems along with hexagonal H1 and lamellar LR liquid crystal phases, which persisted to substantially higher temperatures than their counterparts in the binary surfactant in water system.
Introduction Siloxane surfactants are used in commercial applications ranging from the manufacture of polyurethane foam to cosmetics and textile manufacture, to wetting agents, agricultural adjuvants, and coatings additives.1-6 Despite this widespread usage, study and understanding of the siloxane surfactants have until recently been mostly limited to their surface tension lowering and wetting behavior. The trisiloxane surfactants, shown in the diagram below, are particularly interesting because of their unique wetting behavior and the similarity of their aqueous phase behavior to that of hydrocarbon polyoxyethylene nonionic surfactants.
A shorthand notation is used for the trisiloxane surfactants, which is derived from the organosilicon litera* To whom correspondence should be directed. † Present address: Applied Materials Inc., 2151 Mission College Blvd., M/S 2554, Santa Clara, CA 95054. ‡ Present address: Exxon Company USA, 16945 North Chase Dr., P.O. Box 4697, Houston, TX 77210. (1) Hill, R. M. In Specialist Surfactants; Robb, I. D., Ed.; Chapman & Hall: London, 1996. (2) Schaefer, D. Tenside, Surfactants, Deterg. 1990, 27, 154. (3) Gruning, B.; Koerner, G. Tenside, Surfactants, Deterg. 1989, 26, 312. (4) Schmidt, G. Tenside, Surfactants, Deterg. 1990, 27, 234. (5) Stevens, P. J. G. Pestic. Sci. 1993, 38, 103. (6) Gould, C. Spec. Chem. 1991, 354.
ture,7,8 in which these surfactants are denoted M(D′En)M where M stands for the trimethylsiloxy group, (CH3)3SiO1/2-, D′ stands for -O1/2Si(CH3)(R)O1/2-, where R is a polyoxyethylene group attached to the silicon by way of a propyl spacer, and En stands for polyoxyethylene, -(CH2CH2O)nH-. Recently the aggregation behavior of a number of trisiloxane and polymeric siloxane surfactants in water has been reported.9-20 The phase behavior of the trisiloxane surfactants depends on the size of the polyoxyethylene headgroup with microstructures of higher positive curvature21 being favored by larger En groups, similar to the behavior of the linear alkyl polyoxyethylene surfactants CiEj.22 (7) Noll, W. The Chemistry and Technology of Silicones; Academic Press: New York, 1968. (8) Bailey, D. L. US 3299112, 1967. (9) He, M.; Hill, M.; Lin, Z.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97, 8820. (10) Hill, R. M.; He, M.; Davis, H. T.; Scriven, L. E. Langmuir 1994, 10, 1724. (11) Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Langmuir 1993, 9, 2789. (12) He, M. Ph.D. Thesis, University of Minnesota, 1993. (13) He, M.; Hill, R. M.; Doumaux, H. A.; Bates, F. S.; Davis, H. T.; Scriven, L. E. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; ACS Symposium Series 578; American Chemical Society: Washington, DC, 1994; p 192. (14) Lin, Z. Ph.D. Thesis, University of Minnesota, 1993. (15) Lin, Z.; Hill, R. M.; Davis, H. T.; Scriven, L. E.; Talmon, Y. Langmuir 1994, 10, 1008. (16) Doumaux, H. Ph.D. Thesis, University of Minnesota, 1995. (17) Lin, Z.; He, M.; Scriven, L. E.; Davis, H. T.; Snow, S. A. J. Phys. Chem. 1993, 97, 3571. (18) Snow, S. A. Langmuir 1993, 9, 424. (19) Gradzielski, M.; Hoffmann, H.; Robisch, P.; Ulbricht, W. Tenside, Surfactants, Deterg. 1990, 27, 366. (20) Stuermer, A.; Thunig, C.; Hoffmann, H.; Gru¨ning, B. Tenside, Surfactants, Deterg. 1994, 31, 90. (21) In this context, positive curvature means concave toward the hydrophobic side.
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system, (2) the lower miscibility gap of the surfactant/oil system, and (3) the central miscibility gap of the water/oil system. The ternary phase behavior of systems containing siloxane surfactants and silicone oils has not been studied beforesit would be interesting to determine how such systems behave differently. There are a few studies that discuss emulsification of hydrocarbon oils by siloxane surfactants28 and emulsification of silicone oil by hydrocarbon surfactants.29-32 We present results here for mixtures of the trisiloxane E12 polyoxyethylene surfactant and three low molecular weight volatile silicone oils comparable to the volatile normal alkanes. Experimental Methods
Figure 1. Binary phase diagram of M(D′E12)M/water.9,13 L1 denotes a water-rich isotropic phase. H1 denotes the normal hexagonal liquid crystal phase. LR denotes lamellar liquid crystal phase. Hatching is used consistently throughout this paper to denote a two-phase region.
Figure 1 shows the binary phase diagram for the water/ M(D′E12)M system (we have replotted the diagram from He et al.9,13). This surfactant forms one-phase isotropic mixtures with water at all concentrations in a wide temperature range from about 12 °C to the lower limit of the lower consolute temperature boundary at 43 °C. He et al. demonstrated that this surfactant forms microstructures in this one-phase region which evolve continuously from small spherical micelles to cylindrical micelles to a random interconnected bilayer to inverted microstructures at high surfactant concentration. Below 12 °C a normal hexagonal H1 and a lamellar LR region are also found. In this paper we extend our study of the trisiloxane surfactant M(D′E12)M to consider the phase behavior and microstructures of ternary water/siloxane surfactant/ silicone oil systems. There have been many systematic studies of the phase behavior and microstructures of ternary systems of water, alkyl polyoxyethylene surfactant CiEj, and hydrocarbon oils.23-26 The general features of such ternary systems are determined by the interactions between three factors: 27 (1) the upper miscibility gap of the water/surfactant (22) Mitchell, J. D.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (23) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233. (24) Kahlwiet, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. (25) Xie, M.; Zhu, X.; Miller, W. G.; Bohlen, D. S.; Vinson, P. K.; Davis, H. T.; Scriven L. E. In Organized Solutions; Friberg, S. E., Lindman, B., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 44, pp 145-158. (26) Schubert, K.-V.; Kaler, E. W. Nonionic Microemulsions. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 190. (27) Kahlweit, M.; Strey, R. In Microemulsion Systems; Rosano, H. L., Clausse M., Eds.; Surfactant Science Series 24; Marcel Dekker: New York, 1987.
Water/surfactant/oil mixtures were prepared with deionized water in 7 mL, 1 cm diameter sealed glass tubes with 0.1 mL volumetric tick-marks. Each sample contained a small Tefloncovered magnetic stir bar and was submerged in a glass water bath with precise temperature control ((0.1 °C). Samples were observed while in the water bath between crossed polarizers with a strong light placed behind the water tank for the examination of birefringence and turbidity. A laser beam was occasionally used to detect small changes in turbidity. At temperatures 2 °C apart, and then 0.2 °C apart near cloud points, samples were well mixed with stirring or gentle hand shaking and allowed to reach equilibrium. Upon equilibration, samples were inspected for turbidity and birefringence. Turbidity, viscosity, and birefringence were used to identify the phases and the phase boundaries.31 Phase boundaries were determined by averaging the upper and lower temperatures which contain the phase-transition region. The difference between the phasetransition temperatures reached from below and above was always smaller than 0.4 °C. The ternary systems were studied at constant temperature and with varying temperature using both fish cuts and channel cuts through the phase prism. In the discussion of the ternary results below, we will use the variables R and γ to describe the composition of the system: R ) weight oil/(weight oil + weight water); γ ) weight surfactant/(weight surfactant + weight oil + weight water). R is the weight ratio of oil to water, and γ is the weight fraction of surfactant in the system. These variables are used because the fish cut varies γ while holding R constant, while the channel cut varies R while holding γ constant. The hydrolytic stability of the siloxane surfactants, especially the trisiloxanes, is an issue when working with them and is viewed by some as a serious problem see refs 33 and 34 and references therein). The Si-O-Si linkage is susceptible to hydrolysis in the presence of moisture:
tSi-O-Sit h tSi-OH + HO-Sit
(1)
This equilibrium is catalyzed by acid or base but is slow near neutral pH.3,8,19,33 Cleaning glassware with strong mineral acids or bases should be avoided, and glassware should be treated with a hydrophobizing agent such as octyltrichlorosilane, or work should be done in plasticware.35 We have found that near ambient temperatures, even dilute solutions of the trisiloxane surfactants which are carefully buffered to pH 7.0 using, for example, standard phosphate buffer, may safely be used for at least 2-4 (28) Smid-Korbar, J.; Krist, J.; Stare, M. Int. J. Cosmet. Sci. 1990, 12, 135. (29) Hoffmann, H.; Sturmer, A. Tenside, Surfactants, Deterg. 1993, 30, 5. (30) Mayer, H. Tenside, Surfactants, Deterg. 1993, 30, 90. (31) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (32) Laughlin, R. G. Aqueous Phase Science of Cationic Surfactant Salts. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series 37; Marcel Dekker: New York, 1991. (33) Knoche, M.; Tamura, H.; Bukovac, M. J. J. Agric. Food Chem. 1991, 39, 202. (34) Knoche, M. Weed Res. 1994, 34, 221.
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weeks. At temperatures above 70 °C, we sometimes saw small changes in temperature boundaries after about 1 day of continuous exposure to high temperature. To avoid this problem, we determined temperature boundaries in this region using fresh samples and kept equilibration times as short as possible. Small-angle X-ray scattering (SAXS) experiments were performed using a modified Kratky camera from Anton Paar KG, Graz, Austria, equipped with an extended flight tube and a movable beam stop.36,37 The X-ray generator was a rotating anode (“ROTAFLEX” model RU-200B, Rigaku Corp., Japan) operating at 10 kW, with a copper target. The KR wavelength of 1.54 Å was selected by means of Nichol filters. The energy window on a model MBRAUN OED-100-M 10-cm linear position sensitive detector (Innovative Technology, Inc., Newburyport, MA) was set to accept only the scattering photons with energy close to 1.54 Å. The Kratky linear collimation produced a 15 × 0.13 mm2 X-ray area on the sample sealed in a 1.5-mm-i.d. glass capillary (CharlesSuper Co., Natick, MA). The sample-to-detector distance was 68.2 cm. The detectable wave vector range was 0.01 Å-1 < q < 0.3 Å-1, where q ) (4π/λ) sin(θ/2) and θ is the scattering angle. The scattering data were accumulated over 30-120 min and were corrected for background scattering by subtracting the scattering intensity of water and the empty capillary. The slitsmeared SAXS intensities were converted numerically to pinhole, or unsmeared intensities by means of Vonk’s method.38 Each of the liquid crystal phases exhibits a characteristic sequence of peaks in the SAXS spectrum corresponding to Bragg reflections from each of the hkl planes36,39,40 which can be used to identify the phase and determine unit cell dimensions.
Materials M(D′E12OH)M was prepared by hydrosilylation of 1,1,1,3,5,5,5-heptamethyltrisiloxane with the appropriate allyl E12 polyoxyethylene derivative and chloropatinic acid catalyst. 1,1,1,3,5,5,5-Heptamethyltrisiloxane was distilled to >95% purity prior to hydrosilylation; thus, the trisiloxane hydrophobe was essentially monodisperse, while the polyoxyethylene groups were polydisperse (Mw/ Mn ≈ 1.1 by gel permeation chromatography (GPC). The molecular structure of octamethylcyclotetrasiloxane (C8H24O4Si4 or D4) is
The molecular structure of decamethylcyclopentasiloxane (C10H30O5Si5 or D5) is
D4 and D5 were purchased from Fluka. The molecular structure of decamethyltetrasiloxane (MD2M) is
Figure 2. Binary phase diagram of M(D′E12)M/D4.
MD2M was purchased from United Chemical Technologies, Inc. (Bristol, PA). All chemicals were used as received. Results The Binary M(D′E12)M/D4 System. The binary phase diagram for M(D′E12)M/D4 is shown in Figure 2. Because of its long hydrophilic polyoxyethylene headgroup, M(D′E12)M has little solubility in D4 and a very large lower miscibility gap exists between M(D′E12)M and D4. This gap extends well beyond the upper miscibility gap of the M(D′E12)M/water system shown in Figure 1. D4 has about 30% solubility in M(D′E12)M at ambient temperature. Since M(D′E12)M also has large solubility in water, we can expect a one-phase region in the ternary phase diagram which is surfactant rich instead of oil rich. The Ternary Water/M(D′E12)M/D4 System. The isothermal ternary phase diagram for the water/M(D′E12)M/ D4 system at 25 °C is shown in Figure 3. There is a onephase isotropic microemulsion region extending along the water/M(D′E12)M axis and around the surfactant corner. This ternary phase diagram has a rich liquid crystal phase behavior, containing regions of hexagonal H1, lamellar LR, and cubic I1 and I3 phases. There is also a small region of low-viscosity isotropic liquid phase on the low surfactant concentration side of the I2 cubic phase which will be discussed later with regard to the fish-cut phase diagrams. (35) We particularly caution those working with trisiloxane surfactants to carefully analyze samples which have hydroxyl end-cap groups on the polyoxyethylene chain. Such materials can be contaminated with significant amounts of a reaction byproduct containing Si-O-C linkages. In water, this byproduct may hydrolyze to form a low molecular weight silicone oil which will seriously perturb phase studies. (36) Kaler, E. W. Ph.D. Thesis, University of Minnesota, 1982. (37) Foster, M. D. Ph.D. Thesis, University of Minnesota, 1986. (38) Vonk, C. G. J. Appl. Crystallogr. 1971, 4, 340. (39) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; John Wiley & Sons: New York, 1974. (40) Luzatti, V.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13, 660.
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Figure 3. Isothermal ternary phase diagram of water/ M(D′E12)M/D4 at 25 °C. L2 denotes a surfactant-rich isotropic phase. I1 denotes a water-continuous cubic liquid crystal phase. I3 denotes an oil (or surfactant) continuous cubic liquid crystal phase.
Figure 4. Polarized light micrographs of samples of water/ M(D′E12)M/D4 mixtures at 25 °C: (a) hexagonal H1 phase at R ) 10% and γ ) 50%; (b) lamellar LR phase at R ) 20% and γ ) 70%. R and γ are defined in the text.
Polarized light micrographs of samples from the hexagonal H1 and lamellar LR phase regions are shown in Figure 4, illustrating the characteristic textures which identify them. Samples in the regions labeled I1 and I3 in the ternary diagram are transparent, optically isotropic,
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and extremely viscous. There are three types of lyotropic cubic liquid crystal phases: I1, I2, and I3. The first, I1, is usually identified as close-packed spherical micelles or oil-swollen micelles with water as the continuous phase. I3 is closely packed inverse micelles or water-swollen micelles with either surfactant or oil as the continuous phase. I2 is a bicontinuous intermediate between the two extremes. We identify the cubic-phase region closest to the water corner as I1 because of its location (especially with respect to the H1 region) and because pulsed field gradient NMR self-diffusion measurements indicated that it is water continuous. Similarly, NMR self-diffusion measurements indicated that samples from the region we have labeled I3 are oil continuous. Figure 5 shows examples of SAXS results for samples from each of the four liquid crystal phases. In each case, the phase is identified by the sequence of peak ratios observed. Figure 5a shows a SAXS spectrum of an H1 sample. Figure 5b shows results for an LR sample. The interlayer spacing calculated from the first peak position is 57.4 Å. Figure 5c shows a SAXS spectrum of an I1 sample. Five peaks were detected with wave vectors in the ratios x3:x4:x8:x11:x12. We interpret this to mean that the five peaks represent diffraction from the (111), (200), (220), (311), and (222) planes, respectively.41 The indices of the planes indicate that this particular cubic phase is probably a face-centered cubic (fcc) belonging to the F23 space group. Three peaks are detected in the spectrum of the I3 sample, shown in Figure 5d, in the ratios of x3:x4:x8. The similarity in the SAXS spectra of the two cubic phases indicates that the two have a similar cubic structure of close packed spherical micelles. Figure 6 shows fish-cut phase diagrams for the ternary water/M(D′E12)M/D4 system at R ) 10%, 42%, 48.85% (equal volume of oil and water), and 60%. In each phase diagram, the upper boundary of the one-phase fish “tail” and the lower boundary of the three-phase region originate from the upper miscibility gap of water and M(D′E12)M, whereas the lower boundary of the fish-tail and upper boundary of the three-phase region come from the lower miscibility gap between M(D′E12)M and D4. The overlap of the two boundaries creates the three-phase region in which the surfactant is immiscible in both water and D4, causing formation of a third phase. In the 2Φ region, M(D′E12)M is soluble in D4 but not in water, so the surfactant resides primarily in the oil-rich upper phase. In the 2Φ region, M(D′E12)M is soluble in water but not in D4, so the surfactant resides primarily in the waterrich lower phase. Other two-phase regions in other parts of the phase diagram are simply marked as “2-phases”. As the surfactant concentration increases, the middle phase in three-phase region incorporates more and more water and oil. Beginning at the concentration at which there is just enough surfactant to incorporate all of the oil and water into the middle phase, only one phase is found. This concentration is called Cmin. Embedded in the fish tail are liquid crystals and their mixtures. The thermal stability of these liquid crystals is higher than their counterparts in the binary water M(D′E12)M phase diagram shown in Figure 1swhere liquid crystal phases are found only well below ambient temperature. For example, the cubic phase, I1, in Figure 6b is stable up to about 80 °C. With the proportion of oil increased, the three-phase region expands and moves to higher temperatures and Cmin shifts to higher surfactant concentrations. The liquid (41) The spacing of peaks in powder spectra of the cubic phases obeys dhkl ) a/(h2 + k2 + l2)1/2.
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Figure 5. SAXS spectra of samples in liquid crystal phases from the water/M(D′E12)M/D4 system: (a) hexagonal phase H1 at R ) 10% and γ ) 50%; (b) lamellar phase LR at R ) 20% and γ ) 70%; (c) cubic phase I1 at R ) 20% and γ ) 40%; (d) cubic phase I3 at R ) 60% and γ ) 57%.
crystal phases progressively transform from water continuous to oil continuous. For example, Figure 6b shows that I1 is formed at R ) 42%, while Figure 6d shows a region of I3 at R ) 60%. There is also a two-liquid-phase region (2φ) at the beginning of the fish tail in the phase diagrams at R ) 42%, 48.85%, and 60%. In the R ) 48.85% phase diagram (Figure 6c), there is a small one-phase isotropic region
above the I1 + D4 two-phase region. Examination of parts c and d of Figure 6 shows a narrow region of microemulsion (one-phase isotropic liquid) between the I1 and I3 regions. This is the same phase that shows up in Figure 3 as an isolated pocket of liquid isotropic phase between the I1 and I3 regions. At 25 °C, this region is separated from the surfactant-rich microemulsion while the fish cuts show that the two regions are connected at higher temperature.
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Figure 6. Fish-cut phase diagrams for the water/M(D′E12)M/D4 system: (a) R ) 10%; (b) R ) 42%; (c) R ) 48.85%; (d) R ) 60%.
Channel-cut phase diagrams at fixed surfactant concentration γ for the water/M(D′E12)M/D4 system are shown in Figure 7. The horizontal axes in these diagrams start with water plus surfactant and end with oil plus surfactant. At a surfactant concentration of 10%, shown in Figure 7a, there is a narrow one-phase isotropic region near the water side in the lower temperature region of the diagram. This one-phase region narrows with higher temperatures and oil content. A three-phase region starts at the end of the one-phase region, which becomes wider with increasing temperature and oil content. Below the one- and three-
phase regions is a wide domain of 2Φ, while above these regions 2Φ is found. The phase behavior at γ ) 20%, shown in Figure 7b, is quite similar. Figure 7c shows the phase behavior at γ ) 37.5%. Now the one-phase region has expanded to become a continuous one-phase (isotropic) channel from the water side of the phase diagram to the oil side and the three-phase region has disappeared. A cubic I1 phase appears, along with regions consisting of mixtures of phases in the 2Φ region and a small region of H1 + L1 in the water corner. With further increase in
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Figure 7. Channel-cut phase diagrams for the water/M(D′E12)M/D4 system: (a) γ ) 10%; (b) γ ) 20%; (c) γ ) 37.5%; (d) γ ) 50%.
surfactant concentration to 50%, shown in Figure 7d, the one-phase isotropic channel further widens in temperature. The cubic I1 phase disappears and the hexagonal H1 phase becomes larger. The Ternary Water/M(D′E12)M/D5 System. The silicone oil, D5, contains one more dimethylsiloxane unit than does D4. The isothermal ternary phase diagram of the water/M(D′E12)M/D5 system at 25 °C is shown in Figure 8. This diagram is similar to the D4 ternary shown in Figure 3. Both form a narrow band of microemulsion along the water/surfactant axis and a region of surfactant-rich
microemulsion. The D5 ternary also forms the cubic I1 phase and the hexagonal H1 phase, but the regions are smaller than those for D4. No lamellar LR or inverse cubic I3 phases were found in the D5 ternary. The SAXS spectrum of a sample from the cubic I1 region has peaks with wave vector ratios of x3:x4:x8:x11:x12 very similar to the I1 in the D4 ternary. Figure 9 shows a fish-cut phase diagram at R ) 48.93% which contains equal volumes of water and D5. Compared to the D4 system in Figure 6c, the three-phase region is larger and occurs at higher temperatures. The I3 phase
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Figure 8. Isothermal ternary phase diagram of the water/ M(D′E12)M/D5 system at 25 °C.
Figure 9. Fish-cut phase diagram for water/M(D′E12)M/D5 at R ) 48.93%.
inside the fish tail of the D4 system is absent for D5, consistent with the isothermal phase diagram in Figure 8. Cmin at the 1:1 water-to-oil (volume ratio) for the D5 system has been shifted to higher surfactant concentration (γ ) 48%) and temperature (T ) 126 °C) compared with the D4 system (γ ) 32% and T ) 108 °C). Thus, more surfactant and higher temperature are required to form microemulsion for D5 than for D4, consistent with the higher molecular weight of the D5. Note that 32-48% surfactant is a rather high level of surfactant indicating that M(D′E12)M is not very efficient in forming microemulsions with these oils.42-44 (42) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (43) Kahlweit, M.; Strey, R.; Aratono, M.; Busse, G.; Jen, J.; Schubert, K. V. J. Chem. Phys. 1991, 95, 2842. (44) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schomacker, R. Langmuir 1988, 4, 499.
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Figure 10. Isothermal ternary phase diagram of the water/ M(D′E12)M/MD2M system at 25 °C
The Ternary Water/M(D′E12)M/MD2M System. MD2M is a linear tetrasiloxane oil which contains two more methyl groups and one less oxygen atom than D4. Figure 10 shows the isothermal ternary phase diagram for the water/M(D′E12)M/MD2M system at 25 °C. This system also forms a narrow band of microemulsion along the water/surfactant axis, a region of surfactant-rich microemulsion, and cubic I1, and hexagonal H1 liquid crystal phases. The liquid crystal regions are substantially larger than either of the cyclic oils. MD2M is a very flexible short chain molecule45 whereas D4 is a relatively stiff ring with the approximate proportions of a hockey puck,46 which may explain why the MD2M ternary generally resembles the D5 system more than the D4 system. Figure 11 shows fish-cut phase diagrams of the water/ M(D′E12)M/MD2M system at R ) 20% and 46.06%, the latter contains equal volumes of water and MD2M silicone oil. Both diagrams show large fish-tail regions, almost all of which is isotropic phase except for small two-phase regions near Cmin. The oil-continuous cubic I3 phase is absent, as it is for D5. The three-phase region shifts between R ) 20% and 46.06% to higher temperatures and a wider concentration span. Cmin has also shiftedsat R ) 20%, Cmin occurs at γ ) 16% and T ) 91.5 °C, while at R ) 46.06%, it is at γ ) 47.5% and T ) 121.5 °C. Figure 12 plots the relationship of Cmin and T(Cmin) against R for each of the three oils. Both Cmin and T(Cmin) increase linearly with R for D4 in the R range studied. At R ) 50%, where the solutions contain approximately equal volumes of water and oil, Cmin value for D5 and MD2M are about twice the value for D4. T(Cmin) values for D5 and MD2M are also about 20 °C higher than that for D4. So at elevated temperature, D4 needs less surfactant to form a microemulsion with water than either D5 or MD2M, although the three oils have similar ternary phase behavior at ambient temperature. Discussion Evolution of Ternary Phase Behavior with Temperature. The systematic variation of ternary phase behavior is well described in the generic temperaturecomposition ternary phase prism.47-52 As temperature (45) Owen, M. J.; Kendrick, T. C. Macromolecules 1970, 3, 458. (46) Grigoras, S.; Lane, T. H. J. Comput. Chem. 1988, 9, 25. (47) Kahlweit, M.; Strey, R. J. Phys. Chem. 1987, 91, 1553.
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a
b
Figure 11. Fish-cut phase diagrams of the water/M(D′E12)M/ MD2M system at (a) R ) 20% and (b) R ) 46.06%.
increases, the ternary phase diagrams systematically progress from Winsor I to III to II behavior.53,54 Winsor (48) Shinoda, K; Knieda, H. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1. (49) Davis, H. T.; Bodet J. F.; Scriven, L. E.; Miller, W. G. In Microemulsions and their precursors. In Physics of Amphiphilic Layers; Langevin, D., Meunier, J., Eds.; Springer-Verlag: 1987. (50) Kilpatrick, P. K.; Gorman, C. A.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J. Phys. Chem. 1986, 90, 5292. (51) Davis, H. T. Colloids Surf. 1994, 91, 9.
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I has a two-liquid-phase coexistence region with the surfactant predominantly in the water-rich lower phase, denoted 2Φ. Type II also has a two-liquid-phase coexistence region, but the surfactant is now predominantly in the oil-rich upper phase, denoted 2Φ. In the type III system, there is a three-phase coexistence region, denoted 3-phase. The third phase, or mesophase, is a midrange microemulsion. At every temperature, the surfactant hydrophile/lipophile balance (HLB)55 determines the type of ternary phase behavior. Water-in-oil (W/O) and oil-inwater (O/W) emulsions are favored for low and high HLB values, respectively, while at intermediate HLB values one finds a three-phase coexistence region in the ternary diagram. Polyoxyethylene groups dehydrate as the temperature rises causing EO-based surfactants to become more hydrophobic and shifting solubility from water to oil. The evolution of the ternary water/M(D′E12)M/oil diagrams with temperature is shown in Figure 13. M(D′E12)M, with its very polar E12 group, has a large HLB and is very soluble in water. Its ternary systems favor Winsor I phase behavior, forming 2Φ at ambient temperature, as shown in all of the phase diagrams in this paper. At sufficiently high temperature, the water/M(D′E12)M axis of the ternary triangle moves into the upper miscibility gap of the binary water/M(D′E12)M system. Then a 2Φ region appears near the water-surfactant axis and interacts with the 2Φ region to form a three-phase region, exhibiting type III behavior as shown in Figure 13b. The upper miscibility gap of water/M(D′E12)M starts at about 43 °C whereas the lower miscibility gap extends to very high temperature (>140 °C) for these silicone oils and M(D′E12)M. Because of this large overlap between the two miscibility gaps, the three-phase regions tend to extend over a wide temperature range. Winsor II phase behavior was observed in this work only at very high temperatures. Thus, the ternary water/trisiloxane surfactant/silicone oil systems follow the same generic patterns of phase behavior previously found in water/CiEj/hydrocarbon oil systems. Both classes of surfactant have polyoxyethylene hydrophilic groups and therefore share the property of having an upper miscibility gap in water. The most significant difference is the extent of the lower miscibility gap between the trisiloxane surfactant and these silicone oils. We attribute the insolubility of the trisiloxane polyoxyethylene surfactants in low molecular weight silicone oils to phobicity between the silicone oils and the polyoxyethylene groups of the surfactant. The insolubility of the surfactant/oil pair causes the three-phase region to occupy a broad temperature range, along with shifting Cmin to relatively large values. We present results for other M(D′En)M/silicone oil/water systems with smaller polyoxyethylene groups elsewhere.56,57 Evolution of Microstructure from Binary to Ternary Systems. Surfactant solutions above their critical aggregation concentration (CAC) contain aggregates of various morphologies including spherical, cylindrical, and (52) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1968, 26, 70. (53) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1967. (54) Kunieda, H.; Friberg, S. E. Prog. Bull. Chem. Soc. Jpn. 1981, 54, 1010. (55) Griffin, W. D. J. Soc. Cosmet. Chem. 1949, 1, 311. (56) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis H. T.; Hill, R. M. Langmuir 1999, 15, 2278. (57) Hill, R. M.; Li, X.; Davis, H. T. Manuscript in preparation.
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Li et al.
Figure 12. (a) Plot of Cmin vs oil content R. (b) Plot of T(Cmin) vs oil content R.
I1 to hexagonal H1 to lamellar LR to inverse cubic I3 with increasing surfactant + oil content. The liquid crystal phases observed in the ternary systems differ from those in the binary water/M(D′E12)M system in two respects: (1) the cubic phases were not observed in the binary system; (2) the hexagonal H1 and lamellar LR phases persist to higher temperatures in the presence of the silicone oils. We suppose that addition of the silicone oils causes the spherical micelles in dilute solutions of M(D′E12)M to become larger, decreasing the surface-to-surface distance and increasing the attractive interaction. This leads to closer packing of these spherical micelles and eventually a phase transition to cubic phase liquid crystal. Similarly, the wormlike micelles in the intermediate concentration region are also swollen with the addition of silicone oil to have larger cross sections, leading to the formation of hexagonal phase. Apparently, increased intermicellar interaction raises the liquid crystal melting temperature substantially. At still higher surfactant concentrations the added silicone oil swells the spongelike bilayer structure found in the binary water/M(D′E12)M system, causing a microstructural evolution toward a bicontinuous microemulsion and then to lamellar phase. The details of the energetics leading to microstructural transitions and formation of liquid crystal phases from disordered “melted” microstructures remains a challenging research problem, but the trends we observed are similar to trends found for analogous hydrocarbon systems. Figure 13. Evolution of the ternary phase diagrams with temperature for the water/M(D′E12)M/oil systems.
Conclusions
disklike micelles and dispersed bilayer microstructures.15,58-60 At higher surfactant concentrations, attractive interactions between aggregates lead to the formation of liquid crystal phases. Liquid crystal phases progress through a sequence from positive to neutral to negative curvature21 with decreasing (effective) size of the polar headgroup. Temperature, salt, surfactant concentration, and incorporation of oil into the aggregate all influence the effective headgroup size.61 The structures of the liquid crystal phases in the ternary water/M(D′E12)M/silicone oil system progress from cubic
The ternary phase behavior of water/M(D′E12)M/silicone oil systems has been determined for three low molecular weight silicone oils, including two cyclic silicone oils, D4and D5, and one short linear silicone oil, MD2M. Because of the relatively close molecular weights, the three oils behave similarly in the ternary system. The water/ M(D′E12)M binary system shows an upper miscibility gap as do the water/CiEj systems, while the M(D′E12)M/silicone oil binary shows a much larger lower miscibility gap compared to CiEj/alkane systems. The ternary systems progress through the now-familiar sequence of Winsor I to III to II phase behavior with rising temperature. Because the lower miscibility gap of the surfactant/oil pair extends to such high temperature, the three-phase region is quite broad in both concentration and temperature. The higher molecular weight oils shift the fish to higher temperatures and surfactant concentrations. At ambient
(58) Li, X.; Lin, Z.; Cai, J.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1995, 99, 10865. (59) Vinson, P. K.; Talmon, Y.; Walter, A. Biophys. J. 1989, 56, 669. (60) Zana, R.; Kaplun, A.; Talmon, Y. Langmuir 1993, 9, 1948. (61) Balmbra, R. R.; Clunie, J. S.; Coldman, J. F. Nature 1969, 222, 1159.
Water/Surfactant/Oil Systems
temperature, M(D′E12)M forms isotropic solutions with water at all concentrations. The isothermal ternary diagrams show that all of these solutions are able to solubilize a small amount of oil to form microemulsion. At higher oil levels, liquid crystal phases are formed near ambient temperature, including the cubic I1 and I3, hexagonal H1, and lamellar LR phases.
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Acknowledgment. The project was supported by the National Science Foundation through the Center for Interfacial Engineering (CIE) at University of Minnesota. Dow Corning Corporation’s financial sponsorship and technical supports are greatly appreciated. LA980406D
