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Langmuir 1999, 15, 2278-2289
Phase Behavior and Microstructure of Water/Trisiloxane E6 and E10 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 Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455
Randal M. Hill* Central R&D, Dow Corning Corp., 2200 West Salzburg, Midland, Michigan 48686-0994 Received April 9, 1998. In Final Form: December 17, 1998 The binary and ternary phase behavior of the trisiloxane E6 and E10 polyoxyethylene surfactants with water and three low molecular weight silicone oils has been determined. The silicone oils were the tetraand pentacyclosiloxanes, D4 and D5, and the short linear tetrasiloxane oil, MD2M. Microstructures were investigated using small-angle X-ray scattering, polarized light microscopy, and cryogenic transmission electron microscopy. Our results illustrate the differences and similarities between the trisiloxane surfactants and conventional hydrocarbon surfactants. Both water/surfactant binary systems form the isotropic spongelike phase, L3, in a narrow temperature band above the lamellar phase region. The L3 phase is found at significantly higher temperatures for the E10 homolog. In the water/M(D′E6)M system, no two-phase region was detected between the region labeled L3 and the isotropic surfactant-rich region, L2. The different birefringent appearance as one progresses between the L3, the L2, and the lamellar liquid crystal phase, LR, regions is photographically documented. A large region of lamellar phase is the dominating feature of both the binary water/M(D′E6)M system and the ternary water/M(D′E6)M/silicone oil systems. In addition to L3 and LR, the E10 trisiloxane surfactant, with its larger hydrophilic group, also forms the normal hexagonal liquid crystal phase, H1, in the binary water/M(D′E10)M system and H1 and the cubic liquid crystal phase, I1, in the ternary systems. The d spacings of the lamellar phase in both the binary and ternary systems are inversely proportional to surfactant concentration and vary only weakly with oil content. Low molecular weight silicone oils shift the phase regions to higher temperature and lead to the formation of highly structured phases such as I1. Higher molecular weight oils shift microemulsion regions to higher temperatures and surfactant concentrations.
Introduction Siloxane surfactants consist of a permethylated siloxane hydrophobe coupled to one or more polar groups.1 This class of surfactants finds a variety of uses in areas where other types of surfactants are relatively ineffective, for example, they are used as foam stabilizers in plastic foams, wetting agents, emulsifiers, lubricants, and release agents. They were introduced to the marketplace in the 1950s. Their first (and still the largest) use was in the manufacture of polyurethane foam. 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 literature,2,3 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-. * To whom correspondence should be addressed. † 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) . Noll, W. The Chemistry and Technology of Silicones; Academic Press: New York, 1968. (3) . Bailey, D. L. US 3299112, 1967.
Substantial advances in our understanding of this class of surfactants have taken place in recent years including their aqueous phase behavior4-11 and their unique ability to rapidly wet hydrophobic substrates.12-14 Siloxane surfactants are different from conventional hydrocarbon (4) . Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Langmuir 1993, 9, 2789. (5) . Hill, R. M.; He, M.; Davis, H. T.; Scriven, L. E. Langmuir 1994, 10, 1724. (6) . He, M.; Hill, R. M.; Lin, Z.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97, 8820. (7) . He, M.; Hill, R. M.; Doumaux, H. A.; Bates, F. S.; Davis, H. T.; Evans, D. F.; Scriven, L. E. In Structure and Flow in Surfactant Solutions; Herb, C., Prudhomme, R. K., Eds.; ACS Symp. Series 578; American Chemical Society: Washington, DC, 1994; p 192. (8) . Gradzielski, M.; Hoffmann, H.; Robisch, P.; Ulbricht, W. Tenside, Surfactant, Deterg. 1990, 27, 366. (9) . Stuermer, A.; Thunig, C.; Hoffmann, H.; Gru¨ning, B. Tenside, Surfactant, Deterg. 1994, 31, 90. (10) . Snow, S. A.; Fenton, W. N.; Owen, M. J. Langmuir 1991, 7, 868. (11) . Lin, Z.; He, M.; Scriven, L. E.; Davis, H. T.; Snow, S. A. J. Phys. Chem. 1993, 97, 3571. (12) . Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3, 247.
10.1021/la9804076 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999
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surfactants in being surface active in nonaqueous media and lowering surface tension to about 20 dyn/cm.15,16 A wide variety of structures can readily be prepared,17 ranging from high molecular weight graft copolymers to the trisiloxane surfactants which are the subject of this paper. Most early papers on siloxane surfactants dealt with applications and their surface propertiesssuch things as surface tension, adhesion, lubrication, and skin feel. It was once argued18 that because of the fluidity of the siloxane species, this class of surfactants should not form “crystalline monolayers” or liquid crystal phases. Only recently have the aggregation properties of this class of surfactants in aqueous solution received much attention. The aqueous phase behavior of the homologous series of trisiloxane surfactants, M(D′En)M, has been reported for n ) 5, 7.5/8, 12, 16, and 18.5-7 The hydrophobicity of the MD′M19 group has been shown to be comparable to that of a linear C12H25 group.8 However, its shape is quite differentsit is shorter and widersits length is about 9.7 Å compared with 15 Å for C12H25 while its volume is larger, 530 Å3 compared with 350 Å3 for C12H25.20 Siloxane surfactants are similar to hydrocarbon surfactants in many common features of surfactancy. Nonionic surfactants such as the linear alkyl ethoxylates, CiHj, have been extensively studied,21-25 partly because of the ease of systematically varying the size of the hydrophilic and hydrophobic groups. The general patterns of phase behavior of the trisiloxane surfactants are similar to the CiEj series, but because of the different size and shape of the MD′M hydrophobe, the phase behavior is shifted to longer EO groups. For instance, the phase behavior of M(D′E8)M is more similar to that of C12H5 than to C12H8 .5 Since the water solubility of the trisiloxane polyoxyethylene surfactants derives from the polyoxyethylene group, they also become less soluble in water with increasing temperature.6,26 However, the hydrophilicity of the siloxane surfactants cannot be related to molecular structure using the hydrophile/lipophile balance (HLB) systemsno relationship analogous to the (13) . Svitova, T.; Hill, R. M.; Smirnova, Y.; Stuermer, A.; Yakubov, G. Langmuir 1998, 14, 5023. (14) . Wagner R.; Wu Y. L.; Richter L.; Pfohl T.; Siegel S.; Weissmuller J.; Reiners J.; Stelzle M.; Frohlich R. Chem. Eng. Technol. 1998, 21, 172. (15) . Owen, M. J. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 97. (16) . Hill, R. M. In Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symp. Series 501; American Chemical Society: Washington, DC, 1992; p 278. (17) . Gruning, B.; Koerner, G. Tenside, Surfactant, Deterg. 1989, 26, 312. (18) . Kanner, B.; Reid, W. G.; Petersen, I. H. Ind. Eng. Chem. Prod. Res. Dev. 1967, 6, 88. (19) . Note that the MD′M ()heptamethyltrisiloxane) hydrophobe includes three -CH2- groups linking the siloxane with the polyoxyethylene. These groups are part of the hydrophobe. (20) . Estimates of molecular lengths of trisiloxane surfactants may be calculated from bond lengths and bond angles as described by Hill et al.4 Molecular volumes can readily be calculated from the molecular weight and density. The cross-sectional area can then be estimated by dividing the volume by the length. Molecular areas may also be derived from the slope of the surface tension vs log(concentration) curve below the critical aggregation concentration (CAC). The “best” values of crosssectional areas are derived from small-angle X-ray scattering (SAXS) measurements of lamellar phase liquid crystal. In the case of the trisiloxane surfactants, the calculations and the two experimental measurements give consistent results.6 (21) . Nonionic Surfactants; Schick, M. J., Ed.; Surface Science Series 1; Marcel Dekker: New York, 1967. (22) . Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; MacDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (23) . Nonionic Surfactants; Schick, M. J., Ed.; Surface Science Series 23; Marcel Dekker: New York, 1987. (24) . Strey, R.; Jahn, W.; Porte, G.; Bassereau, P. Langmuir 1990, 6, 1635. (25) . Strey, R. Colloids Polym. Sci. 1994, 272, 1005. (26) . Vick, S. C. Soap, Cosmet., Chem. Spec. 1984, May, 36.
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HLB system has yet been developed to predict the emulsifying and cloud point behavior of siloxane surfactants.26 Gee and Keil have shown how to prepare transparent mixtures of silicone oils and water using siloxane polyoxyalkylene copolymers,27,28 as well as methods for producing very small particle size emulsions of silicone polymers.29 Gum,30 Starch,31 and Guthauser32 have also claimed transparent mixtures of water and silicone oils, which are sometimes described as microemulsions. Zotto et al.33 have also described the use of silicone surfactants for preparing transparent mixtures of silicone oils and water. Ternary water/trisiloxane surfactant/silicone oil systems have not been previously studied despite their potential importance in several applications.34-36 We have recently investigated the phase behavior of ternary water/ E837 and E1238 trisiloxane polyoxyethylene surfactant/ silicone oil systems and demonstrated that these systems form microemulsions and lyotropic lamellar, hexagonal, and cubic liquid crystal phases. The three silicone oils studied were found to promote the formation of lyotropic liquid crystals. In this paper, we report the phase behavior of the E6 and E10 trisiloxane homologues in water and their corresponding ternary phase behavior with three low molecular weight silicone oils. Experimental Methods Phase Identification and Analysis. Water/surfactant and water/surfactant/oil mixtures were prepared in sealed glass test tubes using deionized water. 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. Details of sample preparation and the optical, X-ray, and electron microscopy methods we used are given elsewhere in this issue.38
Materials The trisiloxane surfactants were prepared by hydrosilylation of 1,1,1,3,5,5,5-heptamethyltrisiloxane with the appropriate allyl E6 and E10 polyoxyethylene derivatives and chloropatinic acid catalyst. The 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)). (27) . Gee, R. P.; Keil, J. W. US 4122029, 1978. (28) . Keil, J. W. US 4265878, 1981. (29) . Gee, R. P. US 4620878, 1986. (30) . Gum, M. L. US 4782095, 1988. (31) . Starch, M. S. US 4311695, 1982. (32) . Guthauser, B. US 5162378, 1992. (33) . Zotta, A. A.; Thimineur, R. J.; Raleigh, W. J. US 4988504, 1991. (34) . Smid-Korbar, J.; Krist, J.; Stare, M. Int. J. Cosmet. Sci. 1990, 12, 135. (35) . Hoffmann, H.; Sturmer, A. Tenside, Surfactant Deterg. 1993, 30, 5. (36) . Mayer, H. Tenside, Surfactant Deterg. 1993, 30, 90. (37) . Hill, R. M. Manuscript in preparation. (38) . Li, Xiangbing, Hill, M. R.; Washenberger, R.; Scriven, L. E.; Davis, H. T. Langmuir 1999, 15, 2267.
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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
MD2M was purchased from United Chemical Technologies, Inc. (Bristol, PA). All chemicals were used as received. Results
Figure 1. Binary phase diagram of M(D′E6)M/water.
phases.41 The micrograph shows the typical extinctioncross texture of the LR phase.42 We mark a vertical dotted line on the phase diagram in Figure 1 at 15 wt % surfactant because the precise location of the phase boundary in this region is uncertain. Other workers have also noted difficulties with other surfactants in this region due in part to sensitivity to shear when a highly swollen lamellar phase is present.43 The d spacing in a one-phase lamellar liquid crystal region obeys the formula
The Binary Water/M(D′E6)M System. The critical aggregation concentration (CAC) of M(D′E6)M in water is 0.0055 wt %; the surface tension at the CAC is 20.3 dyn/ cm and the area per molecule is 55.7 Å. The binary phase diagram for mixtures of M(D′E6)M and water is shown in Figure 1. A water-rich isotropic phase is found at very dilute M(D′E6)M concentrations (less than about 0.1 wt %). This isotropic phase with low surfactant concentration is denoted with the letter W.39 We did not precisely determine the concentration at which turbidity first appears (the transition from W to W + LR) or whether micelles or other microstructures are present in the W region. Previous work40 with M(D′E8OMe)M indicates that, for that surfactant, the onset of turbidity is very close to the CAC determined from the Gibbs plot. We suspect that this is also the case for M(D′E6)M. An extensive region of lamellar phase liquid crystal occupies the central portion of the phase diagram. Samples from this region have no diffraction peaks in wide-angle X-ray scattering indicating that the trisiloxane group is above its melting transition and establishing that this is LR and not Lβ. An example SAXS spectrum and a polarized light micrograph of a 50 wt % sample from the LR region are shown in Figure 2. The small-angle X-ray scattering (SAXS) spectrum shows maxima at wave vectors, q, in the ratio of 1:2 which is characteristic of lamellar
where ms is the mass fraction of surfactant, Fs and Fw are the densities of surfactant and water, and ds is the thickness of the surfactant bilayer. Our d spacing results determined by SAXS are plotted vs 1/ms, in Figure 3. The thickness of surfactant bilayer, ds, was estimated from a linear fit of the data to be about 34.4 Å. The region of W + LR in Figure 1 was cloudy and weakly birefringent. A cryogenic transmission electron microscopy (cryo-TEM) image of a 1 wt % sample from this two-phase region at room temperature is shown in Figure 4. This image shows a dispersion of mostly globular unilamellar vesicles of 50-500 nm, along with some small vesicles trapped in larger ones. The image demonstrates that the turbidity present in the two-phase region is due to bilayer structures and is consistent with the notion that a dilute dispersion of the lamellar phase forms closed bilayer structures, i.e., vesicles. Figure 1 also shows a narrow region above and extending to the left of the LR region which we have labeled L3 and which we show as contiguous with a region we have labeled L2. This and several other features of this region require
(39) . Sjoblom, J.; Stenius, P.; Danielesson, I. in Nonionic Surfactants; Schick, M. J., Ed.; Surface Science Series 23; Marcel Dekker: New York, 1987; p 369. (40) . Ananthapadmanabhan, K. P.; Goddard, E. D.; Chandar, P. Colloids Surf. 1990, 44, 281.
(41) . Luzzati, V.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13, 660. (42) . Hartshorne, N. H. in Liquid Crystals and Plastic Crystals; Gary, G. W., Winsor, P. A., Ed.; Halsted Press: New York, Vol. 2, 1974. (43) . Olsson, U. Personal communication.
(
d) 1-
)
Fs Fs ds ds + Fw Fw m s
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a
b
Figure 2. (a) SAXS spectrum of 50 wt % M(D′E6)M in water at 20 °C shows diffraction peaks in the ratio 1:2. (b) Polarized light micrograph of 50 wt % M(D′E6)M in water at 20 °C showing the typical LR extinction-cross texture.
discussion. At the lower left terminus of the region, we have marked a eutectic line to signify that the appearance of the L3 phase splits the biphasic W + LR mixture into W + L3 to the left of the L3 region and L3 + LR to the right. This is a dotted line because we do not know the location of the right-hand endpoint. Between 15 and 75 wt %, samples are transparent and isotropic above room temperature. With increasing temperature they become cloudy but remain birefringent (the solid circles) and then become transparent and isotropic (the open circles) and finally cloudy and isotropic (the upper open circles). Between 50 and 70 wt %, it was difficult to detect the cloudy biphasic
Figure 3. Measured d spacings and linear fit for M(D′E6)M in water.
Figure 4. Cryo-TEM micrograph of a 1 wt % solution of M(D′E6)M in water showing small globular vesicles.
region above the LR phase. We assume that an azeotropic point occurs at the apex of the LR region. We found a 3-5 °C wide, continuous band of transparent and isotropic phase above the LR phase at all concentrations between 1 and 80 wt % surfactant. We have labeled the water-rich end of this band L3 and the surfactant-rich end L2. We emphasize that we are using the notation L3 and L2 to signify the microstructures commonly associated with these regions22 rather than two distinct phases. Laughlin44 has recently shown that several previously published phase diagrams with a similar appearance to
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Figure 5. Photographs of M(D′E6)M solutions at 43 °C between crossed polarizes. See text for commentary.
Figure 1 are actually incorrect. Examining our samples in the water bath with slow-temperature variation, we saw no evidence of a biphasic region in this band. Since the L3 phase is shear sensitive, we also warmed the 55 wt % sample into the W + L2 region just above the clearto-cloudy transition and allowed the sample to cool without stirring. As the sample cooled, it became transparent in the same temperature band indicating that we have not missed the biphasic region because of stirring the sample. If a transition is occurring in this region forming a biphasic mixture of L2 and L3, then the lack of visible turbidity requires some explanation. We have previously pointed out5 that there is no fundamental reason the microstructures associated with L3 and L2 must be separated by a first-order phase transition. He7 demonstrated that the E12 trisiloxane surfactant, M(D′E12)M, evolves continuously (in quite a broad temperature band) from a microstructure analogous to L1 to a spongelike microstructure analogous to L3 to a microstructure analogous to L2 without undergoing any phase transitions. Our phase diagram for M(D′E10OH)M, presented below, shows a similar behavior. We speculate that a similar progression may be occurring within a narrower temperature band for M(D′E6)M. Far from being forbidden, or even atypical, continuous evolution with no phase boundaries from bicontinuous microstructures (44) . Laughlin, R. G. In Micelles, Microemulsions and Monolayers; Shah, D. O., Ed.; Marcel Dekker: New York, 1998; p 73.
analogous to L3 to microstructures analogous to L2 is, in fact, the rule in microemulsions as is illustrated by the schematic of the microstructures in a fish-cut phase diagram shown by Strey.24 Other examples include Figure 8 below. The L3 phase is often stated to be flow birefringent while the L2 phase is not. The L3 phase for M(D′E6)M begins at about 1 wt % surfactant (which, incidentally, implies a characteristic length of about 3400 Å) in a temperature range from 41 to 46 °C. Solutions in this region exhibit strong flow birefringence but no static birefringence. In the L2 region above 70 wt %, samples are isotropic and show no detectable flow or static birefringence. To determine if this difference could be used to distinguish the L3 and L2 regions, we observed the flow birefringence of samples in the transparent isotropic band with increasing concentration. Figure 5 shows seven samples between 1 and 6 wt % in sealed glass tubes and submerged in a water bath at 43 °C. With stirring, the four samples with concentrations between 1 and 3 wt % showed flow birefringence whereas the three samples >4 wt % did not. Seconds after stirring was stopped, the 2 and 2.5 wt % solutions were still visibly birefringent. The other solutions had become isotropic. One hour after stirring was stopped all of the solutions had become isotropic. None of the solutions showed static birefringence at equilibrium. The 2 and 2.5 wt % solutions required about an hour to reach equilibrium. Above 4 wt % we did not observe flow birefringence; 4 wt % surfactant is well within the region
Water/Surfactant/Oil Systems
Figure 6. (a) Isothermal ternary phase diagram of the water/ M(D′E6)M/D4 system at 20 °C. (b) Isothermal ternary phase diagram of the water/M(D′E6)M/D5 system at 20 °C.
usually denoted as L3. We have also observed that other surfactants (such as M(D′E8)M5) which show a clear gap between the L3 and L2 regions do not show visible flow birefringence in the L3 region above about 5 wt % surfactant. Therefore, flow birefringence does not appear to be a useful tool to distinguish L3 and L2. The Ternary Water/M(D′E6)M/D4 and D5 Systems. Isothermal phase diagrams for the ternary water/ M(D′E6)M/D4 and water/M(D′E6)M/D5 systems at 20 °C are shown in Figure 6. The two diagrams are quite similar and both have a large region of lamellar phase, LR. Viewed between crossed polarizers, samples in this region show bright birefringence similar to the samples in the lamellar phase domain of the binary water/M(D′E6)M system. Obviously, the LR phase in the ternary and binary systems share the same originsthe added oil is solubilized into the hydrophobic part of surfactant bilayer causing it to swell. Polarized light microscopy of the lamellar phase in water/M(D′E6)M/D4 tends to show the mosaic texture while samples of water/M(D′E6)M/D5 tend to show the oily streaks texture.45,46 Both textures are fingerprints of the lamellar phase. (45) . Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: New York, 1994. (46) . Hartshorne, N. H. The Microscopy of Liquid Crystals; Microscope Publications Ltd.: New York, 1974.
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Plots of lamellar d spacing measured by SAXS as a function of surfactant concentration, at fixed oil content of R ) 20 wt % are shown in Figure 7a. The lamellar spacing, d, of the ternary mixtures is also inversely proportional to surfactant mass fraction, ms, as we found for the binary mixtures. The three silicone oils almost fit the same line. The greater the amount of surfactant at fixed oil-to-water ratio, the more surfactant bilayers divide the same space and the smaller the lamellar d spacing. Figure 7b shows how the lamellar d spacing varies with oil content at fixed surfactant concentration. At constant surfactant concentration, the number of bilayers does not change; therefore, higher oil content results in a decrease in the thickness of the water layer and an increase in the thickness of the bilayer, ds. The overall changes in the bilayer spacing also depend on the relative densities of the oil and water. Because the density of MD2M is slightly lower than that of water, the d spacing change is larger than those for D4 and D5. There are two narrow regions of microemulsion in each ternary phase diagram. One is near the M(D′E6)M/silicone oil axis. This is a water-lean microemulsion and exists almost continuously from the surfactant corner to the oil corner. The other microemulsion region is roughly parallel to the water-oil axis and contains about 10 wt % surfactant. Fish-cut phase diagrams for the water/M(D′E6)M/D4 system are shown in Figure 8. The diagrams at R ) 20, 30, 40, and 50 wt % are similar in appearance. In all four diagrams, 2Φ, a water-continuous, two-phase mixture with the surfactant predominantly in the aqueous phase, is formed in the low temperature, low surfactant concentration corner. At higher temperatures 2Φ, an oilcontinuous, two-phase mixture with the surfactant predominantly in the oil phase is formed. Between is found a one-phase microemulsion region. The one-phase region has a wide temperature span on the surfactant side and narrows on the water-rich side. A region of lamellar phase, LR, is found below the microemulsion region. These four diagrams are similar in appearance to Figure 1, the phase diagram for the binary water/M(D′E6)M system. No threephase body is observedsthe oil is completely incorporated into the sponge phase at the lowest surfactant concentrations we investigated even at relatively high weight fractions of oil. In effect, the phase diagrams in Figure 8 show only the upper half of the fish “tail”. Compared to Figure 1, the one-phase microemulsion region (analogous to the region labeled L3 in Figure 1) has shifted downward about 20° and becomes progressively broader with increasing weight fraction of oil. The similarity of phase behavior between the binary and ternary systems and the pattern of evolution with increasing oil level provide a further indication that we have not missed a biphasic region separating the L3 and L2 regions of Figure 1. Fish-cut phase diagrams for the water/M(D′E6)M/D5 system at R ) 20, 48.93, and 70 wt % are shown in Figure 9. The fish has been shifted upward and to the right compared with D4 (which is the usual trend with molecular weight), and now the three-phase body and some of the lower branch of the tail have become visible. We did not precisely determine the boundaries of the three-phase body. With oil weight fraction increase, the three-phase body moves to higher temperatures, the lamellar phase shrinks, and the high-concentration side of the one-phase microemulsion region expands. A channel-cut phase diagram for the water/M(D′E6)M/ D5 system at γ ) 10 wt % is shown in Figure 9d. At this concentration the surfactant forms a dispersion of lamellar
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Figure 7. (a) Lamellar d spacings for water/M(D′E6)M/silicone oil systems at fixed oil content at R ) 20 wt % and T ) 20 °C. (b) Lamellar d spacings for water/M(D′E6)M/silicone oil systems at fixed M(D′E6)M concentration at T ) 20 °C.
phase at low temperatures and L3 near 45 °C. Addition of oil shifts the L3 region sharply downward to form a channel of bicontinuous microemulsion and initiates the formation of a lower branch of droplet microemulsion. At higher oil levels, the system shows the usual progression from 2Φ to one-phase microemulsion to 2Φ. The Binary Water/M(D′E10)M System. The binary phase diagram of mixtures of M(D′E10)M and water is shown in Figure 10. Between 30 and 44 °C mixtures of M(D′E10)M and water form a single isotropic liquid phase, L, at all concentrations. A similar broad band of single phase was observed for M(D′E12OH)M, and the microstructural evolution within that region analyzed.7 Below 30 °C a region was of lamellar phase, LR, is found near 72 wt % surfactant. A region of normal hexagonal phase, H1, is found below 20 °C near 50 wt % surfactant. Above 44 °C a lower consolute temperature boundary is found. The microstructure of the isotropic phase in the dilute surfactant region was investigated using cryo-TEM. In Figure 11, a cryo-TEM image of 5 wt % M(D′E10)M in water at 20 °C shows spherical and elongated micelles. Polarized light micrographs of samples from the hexagonal and lamellar phases are shown in Figure 12. The texture of Figure 12a identifies that sample as hexagonal phase. The extinction-cross pattern and characteristic pinwheeling of the extinction crosses upon rotation of the microscope stage identify the sample in Figure 12b as lamellar phase batonettes.42 The SAXS spectrum of a sample from the lamellar phase, with wave vectors in the ratio 1:2 is shown in Figure 12c. The interlayer spacing the lamellar phase at 75 wt % at 20 °C was determined to be 50.7 Å. Inside the lower consolute temperature boundary, additional LR and L3 phases were identified. Samples from the lower region are statically birefringent identifying them as LR, while samples in the upper region exhibit flow birefringence, identifying that region as L3. Birefringence in the two regions is documented in Figure 13. Figure 13a shows the birefringence of 0.5, 1.25, 2.5, and 5 wt % solutions while being stirred at 74 °C. All four samples are birefringent when stirred, but Figure 13b shows that the first two quickly lose their birefringence
when stirring is stopped. Thus the first two are L3 while the latter two are LR. Parts c and d of Figure 13 show that at 76 °C, the 2.5 wt % solution has transformed from the LR phase into the L3 phasesit has become flow birefringent rather than static birefringent. The Ternary Water/M(D′E10)M/D4 System. Figure 14 shows the isothermal phase diagram of the ternary water/M(D′E10)M/D4 system at 20 °C. Two microemulsion regions are present, one in the surfactant-rich corner and the other along the water/surfactant axis. In addition to the lamellar phase, there is a hexagonal phase which originates from the H1 phase in the binary diagram shown in Figure 10. In the binary system, the H1 phase is only seen below 20 °C. Addition of 7 wt % D4 raises the melting point of the phase causing it to appear at room temperature. The LR phase shows the mosaic texture between crossed polarizers and SAXS peaks with wave vectors in the ratio of 1:2. The H1 phase shows a fanlike angular texture and SAXS peaks with wave vectors in the ratio of 1:x3:2, representing hexagonally packed semi-infinite rodlike micelles. At lower surfactant concentrations we found a cubic liquid crystal phase. This phase is isotropic and highly viscous. The SAXS spectrum of a sample from this region is shown in Figure 15. Three peaks are observed in the ratio of x3:x4:x14, which is consistent with identification of the phase as Ia3d, body center cubic phase,47 but positive identification cannot be made from these three peaks alone. The presence of this highly ordered phase only in the ternary system and the higher melting point of the H1 phase indicate that addition of oil promotes increased ordering of the surfactant microstructures. Discussion Microstructure and Wetting Ability of Aqueous M(D′En)M Solutions. The patterns of phase behavior in binary water/M(D′En)M systems parallel those previously (47) . The spacing of peaks in powder spectra of the cubic phases obeys dhkl ) a/(h2 + k2 + l2)1/2 according to which the x3:x4:x14 peaks are reflections from the 111, 200, and 123 planes, respectively.
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Figure 8. Fish-cut phase diagrams of the water/M(D′E6)M/D4 system at (a) R ) 20 wt %, (b) R ) 30 wt %, (c) R ) 40 wt %, and (d) R ) 50 wt %. R ) weight of oil/(weight of oil + weight of water). Table 1. Phases in Binary Water/M(D′En)M Systems surfactant
low cncn region
M(D′E5)M M(D′E6)M M(D′E8)M M(D′E10)M M(D′E12)M M(D′E18)M
W + L3 W + LR W + L1 L1 L1 L1
L3 L3 L3 L3
middle concn region
high concn region
LR LR LR LR LR
L2 L2 L2 L2 L2 L2
H1 H1 H1
reported for water/CiEj systems.48-50 The phases which we observed in our binary water/M(D′En)M systems are listed in Table 1. For n ) 5-8, the phase behavior of M(D′En)M is dominated by the LR and L3 phases, which both consist of surfactant bilayers with zero net curvature. This indicates that for these polyoxyethylene chain
lengths, the cross-sectional area of the polar headgroup and that of the trisiloxane hydrophobe are very similarin other words, the molecule occupies a cylindrical volume element. In contrast, the M(D′En)M surfactants with n ) 10-18 in water form isotropic micellar solutions and H1, both of which contain positive curvature microstructures.51 This indicates that the polar headgroups of these surfactants are larger than the trisiloxane hydrophobe. Therefore, the trisiloxane surfactants in water, like the (48) . Li, X.; Lin, Z.; Cai, J.; Scriven, L. E. and Davis, H. T. J. Phys. Chem. 1995, 99, 10865. (49) . Conroy, J. P.; Hall, D.; Leng, C. A.; Rendall, K.; Tiddy, G. J. T.; Walsh, J.; Lindblom, G. Prog. Colloid Polym. Sci. 1990, 82, 253. (50) . Doumaux, H. Ph.D. Thesis, University of Minnesota, 1995. (51) . In this context, positive curvature means concave toward the hydrophobic side.
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Figure 9. Fish-cut phase diagrams of the water/M(D′E6)M/D5 system at (a) R ) 20 wt %, (b) R ) 48.93 wt %, and (c) R ) 70 wt %. (d) Channel-cut phase diagram of the water/M(D′E6)M/D4 system at γ ) 10 wt %. γ ) weight of surfactant/(weight of surfactant + weight of oil + weight of water).
linear alkyl ethoxylates, form surfactant microstructures whose curvature shifts toward more positive with increasing polyoxyethylene chain length. Dilute aqueous solutions of M(D′En)M surfactants divide themselves in a similar way with respect to their wetting ability on hydrophobic surfacessfor n ) 5-8, dilute aqueous solutions of M(D′En)M are cloudy suspensions and spread rapidly on low-energy surfaces such as polyethylene, whereas for n ) 10-18, the mixtures are clear isotropic solutions which form sessile drops on such surfaces. Trends in Ternary Water/M(D′En)M/Silicone Oil Systems. Figure 16 illustrates the relationship between microstructure and the hydrophilic En chain length for the ternary water/M(D′En)M/silicone oil systems. It reveals
a similar pattern as in the binary water/M(D′En)M systemsslonger polyoxyethylene chain lengths tend to give more positive curvature microstructures. Forexample, the water/M(D′E6)M/D4 system forms the zero curvature lamellar liquid crystal phase LR, which dominates the ternary diagram, whereas the water/ M(D′E8)M/D4 system forms the H1 phase. In the water/ M(D′E10)M/D4 system and the water/M(D′E12)M/D4 system,38 additional cubic phases appear which consist of close-packed highly curved spherical micelles. Therefore, ternary water/M(D′En)M/silicone oil systems form more positive curvature microstructures for larger polyoxyethylene chain lengths, and the oils tend to promote the formation of more ordered microstructures at higher temperatures.
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Figure 10. Binary phase diagram of M(D′E10)M/water.
c
Figure 11. Cryo-TEM micrograph of 5 wt % M(D′E10)M in water.
We discuss the microstructures of the ternary systems only in terms of the liquid crystal phases they form because we have not directly determined the microstructures of the microemulsion regions. We presume that the microstructures of the microemulsions formed in our systems evolve from droplet to bicontinuous to inverse droplet in the same way as do previously studied organic systems, but determination of these microstructures is the subject of ongoing work in our laboratories. Near ambient temperature, mixtures of M(D′E12)M and M(D′E10)M with low molecular weight silicone oils exhibit Winsor type I phase behaviorsforming 2Φ type dispersions. Mixtures of M(D′E8)M with these oils form a Winsor type III systemsthere is a three-phase coexistence region
Figure 12. Polarized light micrographs of (a) 50 wt % M(D′E10)M in water at 10 °C showing the typical texture of H1 and (b) 75 wt % M(D′E10)M in water at 20 °C showing the typical texture of LR. (c) SAXS spectrum of the hexagonal phase of 75 wt % M(D′E10)M in water at 20 °C.
with the third phase being a midrange microemulsion. M(D′E6)M exhibits Winsor type II phase behaviors forming 2Φ type dispersions. Thus, with decreasing En in M(D′EnOH)M at room temperature, the Winsor sequence of I to III to II, or 2Φ to three-phase or one-phase to 2Φ
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Figure 13. Photographs of M(D′E10)M solutions between crossed polarizes. See text for commentary.
occurs, similar to what occurs in a single ternary system by raising the temperature. This implies that raising the temperature effectively reduces the hydrophilicity of En. Conclusions The phase behaviors of the binary water/M(D′E6)M and water/M(D′E10)M systems provide further illustration of how the principles of self-association and phase behavior which have been worked out for other classes of surfactants
can be applied to the trisiloxane polyoxyethylene nonionic surfactants. The trisiloxane surfactants form isotropic and liquid crystalline phases containing microstructures of increasing positive curvature with larger hydrophilic polyoxyethylene groups. A region of isotropic, but flowbirefringent, spongelike bicontinuous phase, L3, was found for both the E6 and the E10 trisiloxane surfactants. The temperature range of the L3 phase is significantly higher for the more hydrophilic E10 homologue. We observed a continuous band of transparent isotropic phase above the lamellar phase region from very low concentrations, where
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Figure 16. Microstructures found in water/M(D′En)M/silicone oil systems.
Figure 14. Isothermal phase diagram of the water/M(D′E10)M/ D4 system at 20 °C.
Figure 15. SAXS spectrum of a sample from the cubic phase liquid crystal, I1 at R ) 20 wt % and γ ) 40 wt % at 20 °C.
such a region is usually identified as L3, all the way to neat surfactant. Although this region is separated into
two distinct phases, L3 and L2, for many linear alkyl ethoxylate surfactants, we found no evidence of a biphasic region in this band. Since this feature of the binary phase diagram evolves very simply into the ternary diagrams where continuity from bicontinuous to droplet microstructures is common, we suspect that the microstructure of the water/M(D′E6)M binary system evolves continuously through this region without a first-order phase transition. Ternary mixtures of M(D′E6)M and M(D′E10)M with low molecular weight cyclic and linear silicone oils form waterrich and surfactant-rich microemulsions and LR, H1, and I1 liquid crystal phases. Fish-cut phase diagrams for M(D′E6)M with D4 show no three-phase region, and only the upper half of the fish “tail” indicating that this surfactant, which forms a highly swollen L3 phase at 1 wt % surfactant, is able to solubilize D4 into the bilayers and evolve continuously into bicontinuous microemulsion. The higher molecular weight oil, D5, shifts the fish to higher temperatures and surfactant concentrations, leading to the appearance of the three-phase body and the lower branch of the fish “tail”. Mixtures of M(D′E10)M with D4 form a cubic phase, which is not seen in the binary system, and increase the temperature range of the H1 phase. The oil seems to shift the microstructures toward more positive curvature aggregates and increases the temperature stability of the phases. At ambient temperature, the water/ M(D′E10)M/D4 system shows Winsor type I phase behavior while the water/M(D′E6)M/D4 system shows Winsor type II phase behavior. Acknowledgment. The project was supported by the National Science Foundation through the Center for Interfacial Engineering (CIE) at the University of Minnesota. X.L. acknowledges financial sponsorship from Dow Corning Corp. LA9804076
