Phase Behavior and Microstructure of Water/Trisiloxane E 6 and E 10 Polyoxyethylene Surfactant/Silicone Oil Systems

Phase Behavior and Microstructure of Water/TrisiloxaneE6 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 MaterialsScience, 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 withwater 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 oil, MD2M. Microstructures wereinvestigated using small-angle X-ray scattering, polarized light microscopy, and cryogenic transmissionelectron microscopy. Our results illustrate the differences and similarities between the trisiloxane surfactantsand conventional hydrocarbon surfactants. Both water/surfactant binary systems form the isotropicspongelike phase, L3, in a narrow temperature band above the lamellar phase region. The L3 phase is foundat significantly higher temperatures for the E10 homolog. In the water/M(D′E6)M system, no two-phaseregion was detected between the region labeled L3 and the isotropic surfactant-rich region, L2. The differentbirefringent 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 featureof both the binary water/M(D′E6)M system and the ternary water/M(D′E6)M/silicone oil systems. In additionto L3 and LR, the E10 trisiloxane surfactant, with its larger hydrophilic group, also forms the normalhexagonal liquid crystal phase, H1, in the binary water/M(D′E10)M system and H1 and the cubic liquidcrystal phase, I1, in the ternary systems. The d spacings of the lamellar phase in both the binary andternary systems are inversely proportional to surfactant concentration and vary only weakly with oilcontent. Low molecular weight silicone oils shift the phase regions to higher temperature and lead to theformation of highly structured phases such as I1. Higher molecular weight oils shift microemulsion regionsto higher temperatures and surfactant concentrations.

Introduction

Siloxane surfactants consist of a permethylated siloxanehydrophobe coupled to one or more polar groups.1 Thisclass of surfactants finds a variety of uses in areas whereother types of surfactants are relatively ineffective, forexample, 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 manu-facture of polyurethane foam. The trisiloxane surfactants,shown in the diagram below, are particularly interestingbecause of their unique wetting behavior, and the simi-larity of their aqueous phase behavior to that of hydro-carbon polyoxyethylene nonionic surfactants.

A shorthand notation is used for the trisiloxane sur-factants which is derived from the organosilicon litera-ture,2,3 in which these surfactants are denoted M(D′En)Mwhere M stands for the trimethylsiloxy group, (CH3)3-SiO1/2-, D′ stands for -O1/2Si(CH3)(R)O1/2-, where R is apolyoxyethylene group attached to the silicon by way ofa propyl spacer, and En stands for polyoxyethylene, -(CH2-CH2O)nH-.

Substantial advances in our understanding of this classof surfactants have taken place in recent years includingtheir aqueous phase behavior4-11 and their unique abilityto rapidly wet hydrophobic substrates.12-14 Siloxanesurfactants are different from conventional hydrocarbon

* 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.

(4) . Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Langmuir1993, 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 SurfactantSolutions; 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.; Gruning, 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.

2278 Langmuir 1999, 15, 2278-2289

10.1021/la9804076 CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 03/05/1999

surfactants in being surface active in nonaqueous mediaand lowering surface tension to about 20 dyn/cm.15,16 Awide variety of structures can readily be prepared,17

ranging from high molecular weight graft copolymers tothe trisiloxane surfactants which are the subject of thispaper. Most early papers on siloxane surfactants dealtwithapplicationsandtheir surfacepropertiesssuchthingsas surface tension, adhesion, lubrication, and skin feel. Itwas once argued18 that because of the fluidity of thesiloxane species, this class of surfactants should not form“crystalline monolayers” or liquid crystal phases. Onlyrecently have the aggregation properties of this class ofsurfactants in aqueous solution received much attention.

The aqueous phase behavior of the homologous seriesof trisiloxane surfactants, M(D′En)M, has been reportedfor n ) 5, 7.5/8, 12, 16, and 18.5-7 The hydrophobicity ofthe MD′M19 group has been shown to be comparable tothat of a linear C12H25 group.8 However, its shape is quitedifferentsit is shorter and widersits length is about 9.7Å compared with 15 Å for C12H25 while its volume is larger,530 Å3 compared with 350 Å3 for C12H25.20 Siloxanesurfactants are similar to hydrocarbon surfactants inmany common features of surfactancy.

Nonionic surfactants such as the linear alkyl ethoxy-lates, CiHj, have been extensively studied,21-25 partlybecause of the ease of systematically varying the size ofthe hydrophilic and hydrophobic groups. The generalpatterns of phase behavior of the trisiloxane surfactantsare similar to the CiEj series, but because of the differentsize and shape of the MD′M hydrophobe, the phasebehavior is shifted to longer EO groups. For instance, thephase behavior of M(D′E8)M is more similar to that ofC12H5 than to C12H8 .5 Since the water solubility of thetrisiloxane polyoxyethylene surfactants derives from thepolyoxyethylene group, they also become less soluble inwater with increasing temperature.6,26 However, thehydrophilicity of the siloxane surfactants cannot be relatedto molecular structure using the hydrophile/lipophilebalance (HLB) systemsno relationship analogous to the

HLB system has yet been developed to predict theemulsifying and cloud point behavior of siloxane surfac-tants.26

Gee and Keil have shown how to prepare transparentmixtures of silicone oils and water using siloxane poly-oxyalkylene copolymers,27,28 as well as methods forproducing very small particle size emulsions of siliconepolymers.29 Gum,30 Starch,31 and Guthauser32 have alsoclaimed transparent mixtures of water and silicone oils,which are sometimes described as microemulsions. Zottoet al.33 have also described the use of silicone surfactantsfor preparing transparent mixtures of silicone oils andwater.

Ternary water/trisiloxane surfactant/silicone oil sys-tems have not been previously studied despite theirpotential importance in several applications.34-36 We haverecently investigated the phase behavior of ternary water/E8

37 and E1238 trisiloxane polyoxyethylene surfactant/

silicone oil systems and demonstrated that these systemsform microemulsions and lyotropic lamellar, hexagonal,and cubic liquid crystal phases. The three silicone oilsstudied were found to promote the formation of lyotropicliquid crystals. In this paper, we report the phase behaviorof the E6 and E10 trisiloxane homologues in water andtheir corresponding ternary phase behavior with threelow molecular weight silicone oils.

Experimental Methods

Phase Identification and Analysis. Water/surfactant andwater/surfactant/oil mixtures were prepared in sealed glass testtubes using deionized water. The ternary systems were studiedat constant temperature and with varying temperature usingboth fish cuts and channel cuts through the phase prism. In thediscussion of the ternary results below, we will use the variablesR and γ to describe the composition of the system:

R ) weight oil/(weight oil + weight water)γ ) weight surfactant/(weight surfactant + weight oil + weightwater)

R is the weight ratio of oil to water and γ is the weight fractionof surfactant in the system. These variables are used becausethe fish cut varies γ while holding R constant, while the channelcut varies R while holding γ constant. Details of samplepreparation and the optical, X-ray, and electron microscopymethods we used are given elsewhere in this issue.38

Materials

The trisiloxane surfactants were prepared by hydrosi-lylation of 1,1,1,3,5,5,5-heptamethyltrisiloxane with theappropriate allyl E6 and E10 polyoxyethylene derivativesand chloropatinic acid catalyst. The 1,1,1,3,5,5,5-hep-tamethyltrisiloxane was distilled to >95 purity prior tohydrosilylation; thus, the trisiloxane hydrophobe wasessentially monodisperse, while the polyoxyethylenegroups were polydisperse (Mw/Mn ≈ 1.1 by gel permeationchromatography (GPC)).

(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.; WeissmullerJ.; 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) hydrophobeincludes three -CH2- groups linking the siloxane with the polyoxy-ethylene. These groups are part of the hydrophobe.

(20) . Estimates of molecular lengths of trisiloxane surfactants maybe calculated from bond lengths and bond angles as described by Hillet al.4 Molecular volumes can readily be calculated from the molecularweight and density. The cross-sectional area can then be estimated bydividing the volume by the length. Molecular areas may also be derivedfrom the slope of the surface tension vs log(concentration) curve belowthe critical aggregation concentration (CAC). The “best” values of cross-sectional areas are derived from small-angle X-ray scattering (SAXS)measurements of lamellar phase liquid crystal. In the case of thetrisiloxane surfactants, the calculations and the two experimentalmeasurements give consistent results.6

(21) . Nonionic Surfactants; Schick, M. J., Ed.; Surface Science Series1; 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 Series23; 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.

(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.

Water/Surfactant/Oil Systems Langmuir, Vol. 15, No. 7, 1999 2279

Themolecularstructureofoctamethylcyclotetrasiloxane(C8H24O4Si4 or D4) is

The molecular structure of decamethylcyclopentasiloxane(C10H30O5Si5 or D5) is

D4 and D5 were purchased from Fluka. The molecularstructure of decamethyltetrasiloxane (MD2M) is

MD2Mwaspurchased fromUnitedChemicalTechnologies,Inc. (Bristol, PA). All chemicals were used as received.

ResultsThe Binary Water/M(D′E6)M System. The critical

aggregation concentration (CAC) of M(D′E6)M in water is0.0055 wt %; the surface tension at the CAC is 20.3 dyn/cm and the area per molecule is 55.7 Å. The binary phasediagram for mixtures of M(D′E6)M and water is shown inFigure 1. A water-rich isotropic phase is found at verydilute M(D′E6)M concentrations (less than about 0.1 wt%). This isotropic phase with low surfactant concentrationis denoted with the letter W.39 We did not preciselydetermine the concentration at which turbidity firstappears (the transition from W to W + LR) or whethermicelles or other microstructures are present in the Wregion.Previouswork40 withM(D′E8OMe)Mindicates that,for that surfactant, the onset of turbidity is very close tothe CAC determined from the Gibbs plot. We suspect thatthis is also the case for M(D′E6)M.

An extensive region of lamellar phase liquid crystaloccupies the central portion of the phase diagram. Samplesfrom this region have no diffraction peaks in wide-angleX-ray scattering indicating that the trisiloxane group isabove its melting transition and establishing that this isLR and not Lâ. An example SAXS spectrum and a polarizedlight micrograph of a 50 wt % sample from the LR regionare shown in Figure 2. The small-angle X-ray scattering(SAXS) spectrum shows maxima at wave vectors, q, inthe ratio of 1:2 which is characteristic of lamellar

phases.41 The micrograph shows the typical extinction-cross texture of the LR phase.42 We mark a vertical dottedline on the phase diagram in Figure 1 at 15 wt % surfactantbecause the precise location of the phase boundary in thisregion is uncertain. Other workers have also noteddifficulties with other surfactants in this region due inpart to sensitivity to shear when a highly swollen lamellarphase is present.43

The d spacing in a one-phase lamellar liquid crystalregion obeys the formula

where ms is the mass fraction of surfactant, Fs and Fw arethe densities of surfactant and water, and ds is thethickness of the surfactant bilayer. Our d spacing resultsdetermined by SAXS are plotted vs 1/ms, in Figure 3. Thethickness of surfactant bilayer, ds, was estimated from alinear fit of the data to be about 34.4 Å.

The region of W + LR in Figure 1 was cloudy and weaklybirefringent. A cryogenic transmission electron microscopy(cryo-TEM) image of a 1 wt % sample from this two-phaseregion at room temperature is shown in Figure 4. Thisimage shows a dispersion of mostly globular unilamellarvesicles of 50-500 nm, along with some small vesiclestrapped in larger ones. The image demonstrates that theturbidity present in the two-phase region is due to bilayerstructures and is consistent with the notion that a dilutedispersion of the lamellar phase forms closed bilayerstructures, i.e., vesicles.

Figure1alsoshowsanarrowregionaboveand extendingto the left of the LR region which we have labeled L3 andwhich we show as contiguous with a region we have labeledL2. 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: NewYork, 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. ActaCrystallogr. 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.

Figure 1. Binary phase diagram of M(D′E6)M/water.

d ) (1 -Fs

Fw)ds +

Fs

Fw

ds

ms

2280 Langmuir, Vol. 15, No. 7, 1999 Li et al.

discussion. At the lower left terminus of the region, wehave marked a eutectic line to signify that the appearanceof the L3 phase splits the biphasic W + LR mixture intoW + 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 locationof the right-hand endpoint. Between 15 and 75 wt %,samples are transparent and isotropic above room tem-perature.With increasing temperature theybecomecloudybut remain birefringent (the solid circles) and then becometransparent and isotropic (the open circles) and finallycloudy and isotropic (the upper open circles). Between 50and 70 wt %, it was difficult to detect the cloudy biphasic

region above the LR phase. We assume that an azeotropicpoint occurs at the apex of the LR region.

We found a 3-5 °C wide, continuous band of transparentand isotropic phase above the LR phase at all concentra-tions between 1 and 80 wt % surfactant. We have labeledthe water-rich end of this band L3 and the surfactant-richend L2. We emphasize that we are using the notation L3

and L2 to signify the microstructures commonly associatedwith these regions22 rather than two distinct phases.Laughlin44 has recently shown that several previouslypublished phase diagrams with a similar appearance to

a

b

Figure 2. (a) SAXS spectrum of 50 wt % M(D′E6)M in waterat 20 °C shows diffraction peaks in the ratio 1:2. (b) Polarizedlight micrograph of 50 wt % M(D′E6)M in water at 20 °C showingthe typical LR extinction-cross texture.

Figure 3. Measured d spacings and linear fit for M(D′E6)Min water.

Figure 4. Cryo-TEM micrograph of a 1 wt % solution ofM(D′E6)M in water showing small globular vesicles.


Figure 1 are actually incorrect. Examining our samplesin the water bath with slow-temperature variation, wesaw no evidence of a biphasic region in this band. Sincethe L3 phase is shear sensitive, we also warmed the 55 wt% sample into the W + L2 region just above the clear-to-cloudy transition and allowed the sample to cool withoutstirring. As the sample cooled, it became transparent inthe same temperature band indicating that we have notmissed the biphasic region because of stirring the sample.If a transition is occurring in this region forming a biphasicmixture of L2 and L3, then the lack of visible turbidityrequires some explanation.

We have previously pointed out5 that there is nofundamental reason the microstructures associated withL3 and L2 must be separated by a first-order phasetransition. He7 demonstrated that the E12 trisiloxanesurfactant, M(D′E12)M, evolves continuously (in quite abroad temperature band) from a microstructure analogousto L1 to a spongelike microstructure analogous to L3 to amicrostructure analogous to L2 without undergoing anyphase transitions. Our phase diagram for M(D′E10OH)M,presented below, shows a similar behavior. We speculatethat a similar progression may be occurring within anarrower temperature band for M(D′E6)M. Far from beingforbidden, or even atypical, continuous evolution with nophase boundaries from bicontinuous microstructures

analogous to L3 to microstructures analogous to L2 is, infact, the rule in microemulsions as is illustrated by theschematic of the microstructures in a fish-cut phasediagram shown by Strey.24 Other examples include Figure8 below.

The L3 phase is often stated to be flow birefringent whilethe L2 phase is not. The L3 phase for M(D′E6)M begins atabout 1 wt % surfactant (which, incidentally, implies acharacteristic length of about 3400 Å) in a temperaturerange from 41 to 46 °C. Solutions in this region exhibitstrong flow birefringence but no static birefringence. Inthe L2 region above 70 wt %, samples are isotropic andshow no detectable flow or static birefringence. Todetermine if this difference could be used to distinguishthe L3 and L2 regions, we observed the flow birefringenceof samples in the transparent isotropic band with in-creasing concentration. Figure 5 shows seven samplesbetween 1 and 6 wt % in sealed glass tubes and submergedin a water bath at 43 °C. With stirring, the four sampleswith concentrations between 1 and 3 wt % showed flowbirefringence 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 solutionshad become isotropic. One hour after stirring was stoppedall of the solutions had become isotropic. None of thesolutions showed static birefringence at equilibrium. The2 and 2.5 wt % solutions required about an hour to reachequilibrium. Above 4 wt % we did not observe flowbirefringence; 4 wt % surfactant is well within the region

(44) . Laughlin, R. G. In Micelles, Microemulsions and Monolayers;Shah, D. O., Ed.; Marcel Dekker: New York, 1998; p 73.

Figure 5. Photographs of M(D′E6)M solutions at 43 °C between crossed polarizes. See text for commentary.


usually denoted as L3. We have also observed that othersurfactants (such as M(D′E8)M5) which show a clear gapbetween the L3 and L2 regions do not show visible flowbirefringence in the L3 region above about 5 wt %surfactant. Therefore, flow birefringence does not appearto 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 °Care shown in Figure 6. The two diagrams are quite similarand both have a large region of lamellar phase, LR. Viewedbetween crossed polarizers, samples in this region showbright birefringence similar to the samples in the lamellarphase domain of the binary water/M(D′E6)M system.Obviously, the LR phase in the ternary and binary systemsshare the same originsthe added oil is solubilized intothe hydrophobic part of surfactant bilayer causing it toswell. Polarized light microscopy of the lamellar phase inwater/M(D′E6)M/D4 tends to show the mosaic texture whilesamples of water/M(D′E6)M/D5 tend to show the oilystreaks texture.45,46 Both textures are fingerprints of thelamellar phase.

Plots of lamellar d spacing measured by SAXS as afunction of surfactant concentration, at fixed oil contentof R ) 20 wt % are shown in Figure 7a. The lamellarspacing, d, of the ternary mixtures is also inverselyproportional to surfactant mass fraction, ms, as we foundfor the binary mixtures. The three silicone oils almost fitthe same line. The greater the amount of surfactant atfixed oil-to-water ratio, the more surfactant bilayers dividethe same space and the smaller the lamellar d spacing.Figure 7b shows how the lamellar d spacing varies withoil content at fixed surfactant concentration. At constantsurfactant concentration, the number of bilayers does notchange; therefore, higher oil content results in a decreasein the thickness of the water layer and an increase in thethickness of the bilayer, ds. The overall changes in thebilayer spacing also depend on the relative densities ofthe oil and water. Because the density of MD2M is slightlylower than that of water, the d spacing change is largerthan those for D4 and D5.

There are two narrow regions of microemulsion in eachternary phase diagram. One is near the M(D′E6)M/siliconeoil axis. This is a water-lean microemulsion and existsalmost continuously from the surfactant corner to the oilcorner. The other microemulsion region is roughly parallelto the water-oil axis and contains about 10 wt %surfactant.

Fish-cut phase diagrams for the water/M(D′E6)M/D4system are shown in Figure 8. The diagrams at R ) 20,30, 40, and 50 wt % are similar in appearance. In all fourdiagrams, 2Φ, a water-continuous, two-phase mixturewith the surfactant predominantly in the aqueous phase,is formed in the low temperature, low surfactant con-centration corner. At higher temperatures 2Φ, an oil-continuous, two-phase mixture with the surfactant pre-dominantly in the oil phase is formed. Between is founda one-phase microemulsion region. The one-phase regionhas a wide temperature span on the surfactant side andnarrows on the water-rich side. A region of lamellar phase,LR, is found below the microemulsion region. These fourdiagrams are similar in appearance to Figure 1, the phasediagram for the binary water/M(D′E6)M system. No three-phase body is observedsthe oil is completely incorporatedinto the sponge phase at the lowest surfactant concentra-tions we investigated even at relatively high weightfractions of oil. In effect, the phase diagrams in Figure 8show only the upper half of the fish “tail”. Compared toFigure 1, the one-phase microemulsion region (analogousto the region labeled L3 in Figure 1) has shifted downwardabout 20° and becomes progressively broader with in-creasing weight fraction of oil. The similarity of phasebehavior between the binary and ternary systems andthe pattern of evolution with increasing oil level providea further indication that we have not missed a biphasicregion separating the L3 and L2 regions of Figure 1.

Fish-cut phase diagrams for the water/M(D′E6)M/D5system at R ) 20, 48.93, and 70 wt % are shown in Figure9. The fish has been shifted upward and to the rightcompared with D4 (which is the usual trend with molecularweight), and now the three-phase body and some of thelower branch of the tail have become visible. We did notprecisely determine the boundaries of the three-phasebody. With oil weight fraction increase, the three-phasebody moves to higher temperatures, the lamellar phaseshrinks, and the high-concentration side of the one-phasemicroemulsion 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 thisconcentration the surfactant forms a dispersion of lamellar

(45) . Laughlin, R. G. The Aqueous Phase Behavior of Surfactants;Academic Press: New York, 1994.

(46) .Hartshorne,N.H. The MicroscopyofLiquidCrystals;MicroscopePublications Ltd.: New York, 1974.

Figure 6. (a) Isothermal ternary phase diagram of the water/M(D′E6)M/D4 system at 20 °C. (b) Isothermal ternary phasediagram of the water/M(D′E6)M/D5 system at 20 °C.


phase at low temperatures and L3 near 45 °C. Additionof oil shifts the L3 region sharply downward to form achannel of bicontinuous microemulsion and initiates theformation of a lower branch of droplet microemulsion. Athigher oil levels, the system shows the usual progressionfrom 2Φ to one-phase microemulsion to 2Φ.

The Binary Water/M(D′E10)M System. The binaryphase diagram of mixtures of M(D′E10)M and water isshown in Figure 10. Between 30 and 44 °C mixtures ofM(D′E10)M and water form a single isotropic liquid phase,L, at all concentrations. A similar broad band of singlephase was observed for M(D′E12OH)M, and the micro-structural evolution within that region analyzed.7 Below30 °C a region was of lamellar phase, LR, is found near 72wt % 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 dilutesurfactant region was investigated using cryo-TEM. InFigure 11, a cryo-TEM image of 5 wt % M(D′E10)M inwater at 20 °C shows spherical and elongated micelles.Polarized light micrographs of samples from the hexagonaland lamellar phases are shown in Figure 12. The textureof Figure 12a identifies that sample as hexagonal phase.The extinction-cross pattern and characteristic pinwheel-ing of the extinction crosses upon rotation of the microscopestage identify the sample in Figure 12b as lamellar phasebatonettes.42 The SAXS spectrum of a sample from thelamellar phase, with wave vectors in the ratio 1:2 is shownin Figure 12c. The interlayer spacing the lamellar phaseat 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 fromthe lower region are statically birefringent identifyingthem as LR, while samples in the upper region exhibitflow birefringence, identifying that region as L3. Bire-fringence in the two regions is documented in Figure 13.Figure 13a shows the birefringence of 0.5, 1.25, 2.5, and5 wt % solutions while being stirred at 74 °C. All foursamples are birefringent when stirred, but Figure 13bshows that the first two quickly lose their birefringence

when stirring is stopped. Thus the first two are L3 whilethe latter two are LR. Parts c and d of Figure 13 show thatat 76 °C, the 2.5 wt % solution has transformed from theLR phase into the L3 phasesit has become flow birefringentrather than static birefringent.

The Ternary Water/M(D′E10)M/D4 System. Figure14 shows the isothermal phase diagram of the ternarywater/M(D′E10)M/D4 system at 20 °C. Two microemulsionregions are present, one in the surfactant-rich corner andthe other along the water/surfactant axis. In addition tothe lamellar phase, there is a hexagonal phase whichoriginates from the H1 phase in the binary diagram shownin Figure 10. In the binary system, the H1 phase is onlyseen below 20 °C. Addition of 7 wt % D4 raises the meltingpoint of the phase causing it to appear at room temper-ature. The LR phase shows the mosaic texture betweencrossed polarizers and SAXS peaks with wave vectors inthe ratio of 1:2. The H1 phase shows a fanlike angulartexture and SAXS peaks with wave vectors in the ratioof 1:x3:2, representing hexagonally packed semi-infiniterodlike micelles. At lower surfactant concentrations wefound a cubic liquid crystal phase. This phase is isotropicand highly viscous. The SAXS spectrum of a sample fromthis region is shown in Figure 15. Three peaks are observedin the ratio of x3:x4:x14, which is consistent withidentification of the phase as Ia3d, body center cubicphase,47 but positive identification cannot be made fromthese three peaks alone. The presence of this highlyordered phase only in the ternary system and the highermelting point of the H1 phase indicate that addition of oilpromotes increased ordering of the surfactant micro-structures.

Discussion

Microstructure and Wetting Ability of AqueousM(D′En)M Solutions. The patterns of phase behavior inbinary water/M(D′En)M systems parallel those previously

(47) . The spacing of peaks in powder spectra of the cubic phasesobeys dhkl ) a/(h2 + k2 + l2)1/2 according to which the x3:x4:x14 peaksare reflections from the 111, 200, and 123 planes, respectively.

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.


reported for water/CiEj systems.48-50 The phases whichwe observed in our binary water/M(D′En)M systems arelisted in Table 1. For n ) 5-8, the phase behavior ofM(D′En)M is dominated by the LR and L3 phases, whichboth consist of surfactant bilayers with zero net curvature.This indicates that for these polyoxyethylene chain

lengths, the cross-sectional area of the polar headgroupand that of the trisiloxane hydrophobe are very similar-in other words, the molecule occupies a cylindrical volumeelement. 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 sur-factants 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.

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

surfactantlow cncnregion

middle concnregion

high concnregion

M(D′E5)M W + L3 L3 LR L2

M(D′E6)M W + LR L3 LR L2

M(D′E8)M W + L1 L3 LR L2

M(D′E10)M L1 L3 LR H1 L2

M(D′E12)M L1 LR H1 L2

M(D′E18)M L1 H1 L2


linear alkyl ethoxylates, form surfactant microstructureswhose curvature shifts toward more positive with in-creasing polyoxyethylene chain length. Dilute aqueoussolutions of M(D′En)M surfactants divide themselvesin a similar way with respect to their wetting abilityon hydrophobic surfacessfor n ) 5-8, dilute aqueoussolutions of M(D′En)M are cloudy suspensions andspread rapidly on low-energy surfaces such as polyeth-ylene, whereas for n ) 10-18, the mixtures are clearisotropic solutions which form sessile drops on suchsurfaces.

Trends in Ternary Water/M(D′En)M/Silicone OilSystems. Figure 16 illustrates the relationship betweenmicrostructure and the hydrophilic En chain length forthe ternary water/M(D′En)M/silicone oil systems. It reveals

a similar pattern as in the binary water/M(D′En)Msystemsslonger polyoxyethylene chain lengths tendto give more positive curvature microstructures. For-example, the water/M(D′E6)M/D4 system forms thezero curvature lamellar liquid crystal phase LR, whichdominates 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 sys-tem,38 additional cubic phases appear which consist ofclose-packed highly curved spherical micelles. Therefore,ternary water/M(D′En)M/silicone oil systems form morepositive curvature microstructures for larger polyoxyeth-ylene chain lengths, and the oils tend to promote theformation of more ordered microstructures at highertemperatures.

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).


We discuss the microstructures of the ternary systemsonly in terms of the liquid crystal phases they form becausewe have not directly determined the microstructures ofthe microemulsion regions. We presume that the micro-structures of the microemulsions formed in our systemsevolve from droplet to bicontinuous to inverse droplet inthe same way as do previously studied organic systems,but determination of these microstructures is the subjectof ongoing work in our laboratories.

Near ambient temperature, mixtures of M(D′E12)M andM(D′E10)M with low molecular weight silicone oils exhibitWinsor type I phase behaviorsforming 2Φ type disper-sions. Mixtures of M(D′E8)M with these oils form a Winsortype III systemsthere is a three-phase coexistence region

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 inM(D′EnOH)M at room temperature, the Winsor sequenceof I to III to II, or 2Φ to three-phase or one-phase to 2Φ

Figure 10. Binary phase diagram of M(D′E10)M/water.

Figure 11. Cryo-TEM micrograph of 5 wt % M(D′E10)M inwater.

c

Figure 12. Polarized light micrographs of (a) 50 wt %M(D′E10)M in water at 10 °C showing the typical texture of H1and (b) 75 wt % M(D′E10)M in water at 20 °C showing the typicaltexture of LR. (c) SAXS spectrum of the hexagonal phase of 75wt % M(D′E10)M in water at 20 °C.


occurs, similar to what occurs in a single ternary systemby raising the temperature. This implies that raisingthe temperature effectively reduces the hydrophilicityof En.

Conclusions

The phase behaviors of the binary water/M(D′E6)M andwater/M(D′E10)M systems provide further illustration ofhow the principles of self-association and phase behaviorwhich have been worked out for other classes of surfactants

can be applied to the trisiloxane polyoxyethylene nonionicsurfactants. The trisiloxane surfactants form isotropic andliquid crystalline phases containing microstructures ofincreasing positive curvature with larger hydrophilicpolyoxyethylene groups. A region of isotropic, but flow-birefringent, spongelike bicontinuous phase, L3, was foundfor both the E6 and the E10 trisiloxane surfactants. Thetemperature range of the L3 phase is significantly higherfor the more hydrophilic E10 homologue. We observed acontinuous band of transparent isotropic phase above thelamellar phase region from very low concentrations, where

Figure 13. Photographs of M(D′E10)M solutions between crossed polarizes. See text for commentary.


such a region is usually identified as L3, all the way toneat surfactant. Although this region is separated into

two distinct phases, L3 and L2, for many linear alkylethoxylate surfactants, we found no evidence of a biphasicregion in this band. Since this feature of the binaryphase diagram evolves very simply into the ternarydiagrams where continuity from bicontinuous to dropletmicrostructures is common, we suspect that the micro-structure of the water/M(D′E6)M binary system evolvescontinuously through this region without a first-orderphase transition.

Ternary mixtures of M(D′E6)M and M(D′E10)M with lowmolecular weight cyclic and linear silicone oils form water-rich and surfactant-rich microemulsions and LR, H1, andI1 liquid crystal phases. Fish-cut phase diagrams forM(D′E6)M with D4 show no three-phase region, and onlythe upper half of the fish “tail” indicating that thissurfactant, which forms a highly swollen L3 phase at 1 wt% surfactant, is able to solubilize D4 into the bilayers andevolve continuously into bicontinuous microemulsion. Thehigher molecular weight oil, D5, shifts the fish to highertemperatures and surfactant concentrations, leading tothe appearance of the three-phase body and the lowerbranch of the fish “tail”. Mixtures of M(D′E10)M with D4form a cubic phase, which is not seen in the binary system,and increase the temperature range of the H1 phase. Theoil seems to shift the microstructures toward more positivecurvature aggregates and increases the temperaturestability of the phases. At ambient temperature, the water/M(D′E10)M/D4 system shows Winsor type I phase behaviorwhile the water/M(D′E6)M/D4 system shows Winsor typeII phase behavior.

Acknowledgment. The project was supported by theNational Science Foundation through the Center forInterfacial Engineering (CIE) at the University of Min-nesota. X.L. acknowledges financial sponsorship from DowCorning Corp.

LA9804076

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 phaseliquid crystal, I1 at R ) 20 wt % and γ ) 40 wt % at 20 °C.

Figure 16. Microstructures found in water/M(D′En)M/siliconeoil systems.


Documents

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