Phase Behavior and Microstructure of Water/Trisiloxane E 12 Polyoxyethylene Surfactant/Silicone Oil Systems

Phase Behavior and Microstructure of Water/TrisiloxaneE12 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 MaterialsScicence, 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 lowmolecular 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-angleX-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 nowfamiliar 2Φ (Winsor I) to three-phase (Winsor III) sequence with increasing temperature. Because of thelarge immiscibility gap between this very polar trisiloxane surfactant and the silicone oils, 2Φ (WinsorII) type behavior was not observed, even at elevated temperatures (up to 140 °C). The cubic I1 and I3 phaseswere found in the ternary systems along with hexagonal H1 and lamellar LR liquid crystal phases, whichpersisted to substantially higher temperatures than their counterparts in the binary surfactant in watersystem.

IntroductionSiloxane surfactants are used in commercial applica-

tions ranging from the manufacture of polyurethane foamto cosmetics and textile manufacture, to wetting agents,agricultural adjuvants, and coatings additives.1-6 Despitethis widespread usage, study and understanding of thesiloxane surfactants have until recently been mostlylimited to their surface tension lowering and wettingbehavior. The trisiloxane surfactants, shown in thediagram below, are particularly interesting because oftheir unique wetting behavior and the similarity of theiraqueous phase behavior to that of hydrocarbon polyoxy-ethylene nonionic surfactants.

A shorthand notation is used for the trisiloxane sur-factants, which is derived from the organosilicon litera-

ture,7,8 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-.

Recently the aggregation behavior of a number oftrisiloxane and polymeric siloxane surfactants in waterhas been reported.9-20 The phase behavior of the tri-siloxane surfactants depends on the size of the polyoxy-ethylene headgroup with microstructures of higher posi-tive curvature21 being favored by larger En groups, similarto the behavior of the linear alkyl polyoxyethylenesurfactants CiEj.22

* 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.

(7) Noll, W. The Chemistry and Technology of Silicones; AcademicPress: 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; AmericanChemical 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.; Gruning, B. Tenside,

Surfactants, Deterg. 1994, 31, 90.(21) In this context, positive curvature means concave toward the

hydrophobic side.

2267Langmuir 1999, 15, 2267-2277

10.1021/la980406d CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 03/04/1999

Figure 1 shows the binary phase diagram for the water/M(D′E12)M system (we have replotted the diagram fromHe et al.9,13). This surfactant forms one-phase isotropicmixtures with water at all concentrations in a widetemperature range from about 12 °C to the lower limit ofthe lower consolute temperature boundary at 43 °C. Heet al. demonstrated that this surfactant forms micro-structures in this one-phase region which evolve continu-ously from small spherical micelles to cylindrical micellesto a random interconnected bilayer to inverted micro-structures at high surfactant concentration. Below 12 °Ca normal hexagonal H1 and a lamellar LR region are alsofound. In this paper we extend our study of the trisiloxanesurfactant M(D′E12)M to consider the phase behavior andmicrostructures of ternary water/siloxane surfactant/silicone oil systems.

There have been many systematic studies of the phasebehavior and microstructures of ternary systems of water,alkyl polyoxyethylene surfactant CiEj, and hydrocarbonoils.23-26 The general features of such ternary systemsare determined by the interactions between three factors:27 (1) the upper miscibility gap of the water/surfactant

system, (2) the lower miscibility gap of the surfactant/oilsystem, and (3) the central miscibility gap of the water/oilsystem.

The ternary phase behavior of systems containingsiloxane surfactants and silicone oils has not been studiedbeforesit would be interesting to determine how suchsystems behave differently. There are a few studies thatdiscuss emulsification of hydrocarbon oils by siloxanesurfactants28 and emulsification of silicone oil by hydro-carbon surfactants.29-32 We present results here formixtures of the trisiloxane E12 polyoxyethylene surfactantand three low molecular weight volatile silicone oilscomparable to the volatile normal alkanes.

Experimental Methods

Water/surfactant/oil mixtures were prepared with deionizedwater in 7 mL, 1 cm diameter sealed glass tubes with 0.1 mLvolumetric tick-marks. Each sample contained a small Teflon-covered magnetic stir bar and was submerged in a glass waterbath with precise temperature control ((0.1 °C). Samples wereobserved while in the water bath between crossed polarizerswith a strong light placed behind the water tank for theexamination of birefringence and turbidity. A laser beam wasoccasionally used to detect small changes in turbidity. Attemperatures 2 °C apart, and then 0.2 °C apart near cloud points,samples were well mixed with stirring or gentle hand shakingand allowed to reach equilibrium. Upon equilibration, sampleswere inspected for turbidity and birefringence. Turbidity, viscos-ity, and birefringence were used to identify the phases and thephase boundaries.31 Phase boundaries were determined byaveraging the upper and lower temperatures which contain thephase-transition region. The difference between the phase-transition temperatures reached from below and above wasalways smaller than 0.4 °C.

The ternary systems were studied at constant temperatureand with varying temperature using both fish cuts and channelcuts through the phase prism. In the discussion of the ternaryresults below, we will use the variables R and γ to describe thecomposition of the system: R ) weight oil/(weight oil + weightwater); γ ) weight surfactant/(weight surfactant + weight oil +weight water).

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.

The hydrolytic stability of the siloxane surfactants, especiallythe trisiloxanes, is an issue when working with them and isviewed by some as a serious problem see refs 33 and 34 andreferences therein). The Si-O-Si linkage is susceptible tohydrolysis in the presence of moisture:

This equilibrium is catalyzed by acid or base but is slow nearneutral pH.3,8,19,33 Cleaning glassware with strong mineral acidsor bases should be avoided, and glassware should be treatedwith a hydrophobizing agent such as octyltrichlorosilane, or workshould be done in plasticware.35 We have found that near ambienttemperatures, even dilute solutions of the trisiloxane surfactantswhich are carefully buffered to pH 7.0 using, for example,standard phosphate buffer, may safely be used for at least 2-4(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: NewYork, 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: NewYork, 1987.

(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.

Figure 1. Binary phase diagram of M(D′E12)M/water.9,13 L1denotes a water-rich isotropic phase. H1 denotes the normalhexagonal liquid crystal phase. LR denotes lamellar liquidcrystal phase. Hatching is used consistently throughout thispaper to denote a two-phase region.

tSi-O-Sit h tSi-OH + HO-Sit (1)

2268 Langmuir, Vol. 15, No. 7, 1999 Li et al.

weeks. At temperatures above 70 °C, we sometimes saw smallchanges in temperature boundaries after about 1 day of continu-ous exposure to high temperature. To avoid this problem, wedetermined temperature boundaries in this region using freshsamples and kept equilibration times as short as possible.

Small-angle X-ray scattering (SAXS) experiments were per-formed using a modified Kratky camera from Anton Paar KG,Graz, Austria, equipped with an extended flight tube and amovable beam stop.36,37 The X-ray generator was a rotating anode(“ROTAFLEX” model RU-200B, Rigaku Corp., Japan) operatingat 10 kW, with a copper target. The KR wavelength of 1.54 Å wasselected by means of Nichol filters. The energy window on amodel MBRAUN OED-100-M 10-cm linear position sensitivedetector (Innovative Technology, Inc., Newburyport, MA) wasset to accept only the scattering photons with energy close to1.54 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 distancewas 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 scatteringangle. The scattering data were accumulated over 30-120 minand were corrected for background scattering by subtracting thescattering intensity of water and the empty capillary. The slit-smeared 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 characteristicsequence of peaks in the SAXS spectrum corresponding to Braggreflections from each of the hkl planes36,39,40 which can be usedto identify the phase and determine unit cell dimensions.

Materials

M(D′E12OH)M was prepared by hydrosilylation of1,1,1,3,5,5,5-heptamethyltrisiloxane with the appropriateallyl E12 polyoxyethylene derivative and chloropatinic acidcatalyst. 1,1,1,3,5,5,5-Heptamethyltrisiloxane was dis-tilled to >95% purity prior to hydrosilylation; thus, thetrisiloxane hydrophobe was essentially monodisperse,while the polyoxyethylene groups were polydisperse (Mw/Mn ≈ 1.1 by gel permeation chromatography (GPC).

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.

Results

The Binary M(D′E12)M/D4 System. The binary phasediagram for M(D′E12)M/D4 is shown in Figure 2. Becauseof its long hydrophilic polyoxyethylene headgroup,M(D′E12)M has little solubility in D4 and a very large lowermiscibility gap exists between M(D′E12)M and D4. Thisgap extends well beyond the upper miscibility gap of theM(D′E12)M/water system shown in Figure 1. D4 has about30% solubility in M(D′E12)M at ambient temperature.Since M(D′E12)M also has large solubility in water, wecan expect a one-phase region in the ternary phasediagram which is surfactant rich instead of oil rich.

The Ternary Water/M(D′E12)M/D4 System. Theisothermalternaryphasediagramforthewater/M(D′E12)M/D4 system at 25 °C is shown in Figure 3. There is a one-phase isotropic microemulsion region extending along thewater/M(D′E12)M axis and around the surfactant corner.This ternary phase diagram has a rich liquid crystal phasebehavior, containing regions of hexagonal H1, lamellarLR, and cubic I1 and I3 phases. There is also a small regionof low-viscosity isotropic liquid phase on the low surfactantconcentration side of the I2 cubic phase which will bediscussed later with regard to the fish-cut phase diagrams.

(35) We particularly caution those working with trisiloxane surfac-tants to carefully analyze samples which have hydroxyl end-cap groupson the polyoxyethylene chain. Such materials can be contaminated withsignificant amounts of a reaction byproduct containing Si-O-Clinkages. In water, this byproduct may hydrolyze to form a low molecularweight 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.

Figure 2. Binary phase diagram of M(D′E12)M/D4.

Water/Surfactant/Oil Systems Langmuir, Vol. 15, No. 7, 1999 2269

Polarized light micrographs of samples from the hex-agonal H1 and lamellar LR phase regions are shown inFigure 4, illustrating the characteristic textures whichidentify them. Samples in the regions labeled I1 and I3 inthe ternary diagram are transparent, optically isotropic,

and extremely viscous. There are three types of lyotropiccubic liquid crystal phases: I1, I2, and I3. The first, I1, isusually identified as close-packed spherical micelles oroil-swollen micelles with water as the continuous phase.I3 is closely packed inverse micelles or water-swollenmicelles with either surfactant or oil as the continuousphase. I2 is a bicontinuous intermediate between the twoextremes. We identify the cubic-phase region closest tothe water corner as I1 because of its location (especiallywith respect to the H1 region) and because pulsed fieldgradient NMR self-diffusion measurements indicated thatit is water continuous. Similarly, NMR self-diffusionmeasurements indicated that samples from the regionwe have labeled I3 are oil continuous.

Figure 5 shows examples of SAXS results for samplesfrom each of the four liquid crystal phases. In each case,the phase is identified by the sequence of peak ratiosobserved. Figure 5a shows a SAXS spectrum of an H1sample. Figure 5b shows results for an LR sample. Theinterlayer spacing calculated from the first peak positionis 57.4 Å. Figure 5c shows a SAXS spectrum of an I1 sample.Five peaks were detected with wave vectors in the ratiosx3:x4:x8:x11:x12. We interpret this to mean that thefive peaks represent diffraction from the (111), (200), (220),(311), and (222) planes, respectively.41 The indices of theplanes indicate that this particular cubic phase is probablya 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. Thesimilarity in the SAXS spectra of the two cubic phasesindicates that the two have a similar cubic structure ofclose packed spherical micelles.

Figure 6 shows fish-cut phase diagrams for the ternarywater/M(D′E12)M/D4 system at R ) 10%, 42%, 48.85%(equal volume of oil and water), and 60%. In each phasediagram, the upper boundary of the one-phase fish “tail”and the lower boundary of the three-phase region originatefrom the upper miscibility gap of water and M(D′E12)M,whereas the lower boundary of the fish-tail and upperboundary of the three-phase region come from the lowermiscibility gap between M(D′E12)M and D4. The overlapof the two boundaries creates the three-phase region inwhich 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 thesurfactant resides primarily in the oil-rich upper phase.In the 2Φ region, M(D′E12)M is soluble in water but notin D4, so the surfactant resides primarily in the water-rich lower phase. Other two-phase regions in other partsof the phase diagram are simply marked as “2-phases”.As the surfactant concentration increases, the middlephase in three-phase region incorporates more and morewater and oil. Beginning at the concentration at whichthere is just enough surfactant to incorporate all of theoil and water into the middle phase, only one phase isfound. This concentration is called Cmin. Embedded in thefish tail are liquid crystals and their mixtures. The thermalstability of these liquid crystals is higher than theircounterparts in the binary water M(D′E12)M phasediagram shown in Figure 1swhere liquid crystal phasesare found only well below ambient temperature. Forexample, the cubic phase, I1, in Figure 6b is stable up toabout 80 °C.

With the proportion of oil increased, the three-phaseregion expands and moves to higher temperatures andCmin shifts to higher surfactant concentrations. The liquid

(41) The spacing of peaks in powder spectra of the cubic phases obeysdhkl ) a/(h2 + k2 + l2)1/2.

Figure 3. Isothermal ternary phase diagram of water/M(D′E12)M/D4 at 25 °C. L2 denotes a surfactant-rich isotropicphase. I1 denotes a water-continuous cubic liquid crystal phase.I3 denotes an oil (or surfactant) continuous cubic liquid crystalphase.

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.


crystal phases progressively transform from water con-tinuous to oil continuous. For example, Figure 6b showsthat I1 is formed at R ) 42%, while Figure 6d shows aregion of I3 at R ) 60%.

There is also a two-liquid-phase region (2φ) at thebeginning 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 partsc 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 anisolated pocket of liquid isotropic phase between the I1and I3 regions. At 25 °C, this region is separated from thesurfactant-rich microemulsion while the fish cuts showthat the two regions are connected at higher temperature.

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 phaseI3 at R ) 60% and γ ) 57%.


Channel-cut phase diagrams at fixed surfactant con-centration γ for the water/M(D′E12)M/D4 system are shownin Figure 7. The horizontal axes in these diagrams startwith water plus surfactant and end with oil plus surfac-tant. At a surfactant concentration of 10%, shown in Figure7a, there is a narrow one-phase isotropic region near thewater side in the lower temperature region of the diagram.This one-phase region narrows with higher temperaturesand oil content. A three-phase region starts at the end oftheone-phaseregion,whichbecomeswiderwith increasingtemperature and oil content. Below the one- and three-

phase regions is a wide domain of 2Φ, while above theseregions 2Φ is found. The phase behavior at γ ) 20%,shown in Figure 7b, is quite similar. Figure 7c shows thephase behavior at γ ) 37.5%. Now the one-phase regionhas expanded to become a continuous one-phase (isotropic)channel from the water side of the phase diagram to theoil side and the three-phase region has disappeared. Acubic I1 phase appears, along with regions consisting ofmixtures of phases in the 2Φ region and a small regionof H1 + L1 in the water corner. With further increase in

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%.


surfactant concentration to 50%, shown in Figure 7d, theone-phase isotropic channel further widens in tempera-ture. The cubic I1 phase disappears and the hexagonal H1phase becomes larger.

The Ternary Water/M(D′E12)M/D5 System. Thesilicone oil, D5, contains one more dimethylsiloxane unitthan does D4. The isothermal ternary phase diagram ofthe water/M(D′E12)M/D5 system at 25 °C is shown in Figure8. This diagram is similar to the D4 ternary shown inFigure 3. Both form a narrow band of microemulsion alongthe water/surfactant axis and a region of surfactant-rich

microemulsion. The D5 ternary also forms the cubic I1phase and the hexagonal H1 phase, but the regions aresmaller than those for D4. No lamellar LR or inverse cubicI3 phases were found in the D5 ternary. The SAXS spectrumof a sample from the cubic I1 region has peaks with wavevector ratios of x3:x4:x8:x11:x12 very similar to the I1in the D4 ternary.

Figure 9 shows a fish-cut phase diagram at R ) 48.93%which contains equal volumes of water and D5. Comparedto the D4 system in Figure 6c, the three-phase region islarger and occurs at higher temperatures. The I3 phase

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%.


inside the fish tail of the D4 system is absent for D5,consistent with the isothermal phase diagram in Figure8. Cmin at the 1:1 water-to-oil (volume ratio) for the D5system has been shifted to higher surfactant concentration(γ ) 48%) and temperature (T ) 126 °C) compared withthe D4 system (γ ) 32% and T ) 108 °C). Thus, moresurfactant and higher temperature are required to formmicroemulsion for D5 than for D4, consistent with thehigher molecular weight of the D5. Note that 32-48%surfactant is a rather high level of surfactant indicatingthat M(D′E12)M is not very efficient in forming micro-emulsions with these oils.42-44

The Ternary Water/M(D′E12)M/MD2M System.MD2M is a linear tetrasiloxane oil which contains twomore methyl groups and one less oxygen atom than D4.Figure 10 shows the isothermal ternary phase diagramfor the water/M(D′E12)M/MD2M system at 25 °C. Thissystem also forms a narrow band of microemulsion alongthe water/surfactant axis, a region of surfactant-richmicroemulsion, and cubic I1, and hexagonal H1 liquidcrystal phases. The liquid crystal regions are substantiallylarger than either of the cyclic oils. MD2M is a very flexibleshort chain molecule45 whereas D4 is a relatively stiffring with the approximate proportions of a hockey puck,46

which may explain why the MD2M ternary generallyresembles 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%, thelatter contains equal volumes of water and MD2M siliconeoil. Both diagrams show large fish-tail regions, almost allof which is isotropic phase except for small two-phaseregions near Cmin. The oil-continuous cubic I3 phase isabsent, as it is for D5. The three-phase region shiftsbetween R ) 20% and 46.06% to higher temperatures anda 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. AtR ) 50%, where the solutions contain approximately equalvolumes of water and oil, Cmin value for D5 and MD2M areabout twice the value for D4. T(Cmin) values for D5 andMD2M are also about 20 °C higher than that for D4. Soat elevated temperature, D4 needs less surfactant to forma microemulsion with water than either D5 or MD2M,although the three oils have similar ternary phasebehavior at ambient temperature.

Discussion

Evolution of Ternary Phase Behavior with Tem-perature. The systematic variation of ternary phasebehavior is well described in the generic temperature-composition ternary phase prism.47-52 As temperature

(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.

(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.

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 atR ) 48.93%.

Figure 10. Isothermal ternary phase diagram of the water/M(D′E12)M/MD2M system at 25 °C


increases, the ternary phase diagrams systematicallyprogress from Winsor I to III to II behavior.53,54 Winsor

I has a two-liquid-phase coexistence region with thesurfactant predominantly in the water-rich lower phase,denoted 2Φ. Type II also has a two-liquid-phase coexist-ence region, but the surfactant is now predominantly inthe oil-rich upper phase, denoted 2Φ. In the type IIIsystem, there is a three-phase coexistence region, denoted3-phase. The third phase, or mesophase, is a midrangemicroemulsion. At every temperature, the surfactanthydrophile/lipophile balance (HLB)55 determines the typeof ternary phase behavior. Water-in-oil (W/O) and oil-in-water (O/W) emulsions are favored for low and high HLBvalues, respectively, while at intermediate HLB valuesone finds a three-phase coexistence region in the ternarydiagram. Polyoxyethylene groups dehydrate as the tem-perature rises causing EO-based surfactants to becomemore hydrophobic and shifting solubility from water tooil.

The evolution of the ternary water/M(D′E12)M/oil dia-grams with temperature is shown in Figure 13. M(D′E12)M,with its very polar E12 group, has a large HLB and is verysoluble in water. Its ternary systems favor Winsor I phasebehavior, forming 2Φ at ambient temperature, as shownin all of the phase diagrams in this paper. At sufficientlyhigh temperature, the water/M(D′E12)M axis of the ternarytriangle moves into the upper miscibility gap of the binarywater/M(D′E12)M system. Then a 2Φ region appears nearthe water-surfactant axis and interacts with the 2Φregion to form a three-phase region, exhibiting type IIIbehavior as shown in Figure 13b. The upper miscibilitygap of water/M(D′E12)M starts at about 43 °C whereas thelower miscibility gap extends to very high temperature(>140 °C) for these silicone oils and M(D′E12)M. Becauseof this large overlap between the two miscibility gaps, thethree-phase regions tend to extend over a wide temper-ature range. Winsor II phase behavior was observed inthis work only at very high temperatures.

Thus, the ternary water/trisiloxane surfactant/siliconeoil systems follow the same generic patterns of phasebehavior previously found in water/CiEj/hydrocarbon oilsystems. Both classes of surfactant have polyoxyethylenehydrophilic groups and therefore share the property ofhaving an upper miscibility gap in water. The mostsignificant difference is the extent of the lower miscibilitygap between the trisiloxane surfactant and these siliconeoils. We attribute the insolubility of the trisiloxanepolyoxyethylene surfactants in low molecular weightsilicone oils to phobicity between the silicone oils and thepolyoxyethylene groups of the surfactant. The insolubilityof the surfactant/oil pair causes the three-phase region tooccupy a broad temperature range, along with shiftingCmin to relatively large values. We present results for otherM(D′En)M/silicone oil/water systems with smaller poly-oxyethylene groups elsewhere.56,57

Evolution of Microstructure from Binary to Ter-nary Systems. Surfactant solutions above their criticalaggregation concentration (CAC) contain aggregates ofvarious morphologies including spherical, cylindrical, and

(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. InMicroemulsions 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.

(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.

b

a

Figure 11. Fish-cut phase diagrams of the water/M(D′E12)M/MD2M system at (a) R ) 20% and (b) R ) 46.06%.


disklike micelles and dispersed bilayer microstruc-tures.15,58-60 At higher surfactant concentrations, attrac-tive interactions between aggregates lead to the formationof liquid crystal phases. Liquid crystal phases progressthrough a sequence from positive to neutral to negativecurvature21 with decreasing (effective) size of the polarheadgroup. Temperature, salt, surfactant concentration,and incorporation of oil into the aggregate all influencethe effective headgroup size.61

The structures of the liquid crystal phases in the ternarywater/M(D′E12)M/silicone oil system progress from cubic

I1 to hexagonal H1 to lamellar LR to inverse cubic I3 withincreasing surfactant + oil content. The liquid crystalphases observed in the ternary systems differ from thosein the binary water/M(D′E12)M system in two respects:(1) the cubic phases were not observed in the binarysystem; (2) the hexagonal H1 and lamellar LR phasespersist to higher temperatures in the presence of thesilicone oils.

We suppose that addition of the silicone oils causes thespherical micelles in dilute solutions of M(D′E12)M tobecome larger, decreasing the surface-to-surface distanceand increasing the attractive interaction. This leads tocloser packing of these spherical micelles and eventuallya phase transition to cubic phase liquid crystal. Similarly,the wormlike micelles in the intermediate concentrationregion are also swollen with the addition of silicone oil tohave larger cross sections, leading to the formation ofhexagonal phase. Apparently, increased intermicellarinteraction raises the liquid crystal melting temperaturesubstantially. At still higher surfactant concentrationsthe added silicone oil swells the spongelike bilayerstructure found in the binary water/M(D′E12)M system,causing a microstructural evolution toward a bicontinuousmicroemulsion and then to lamellar phase. The details ofthe energetics leading to microstructural transitions andformation of liquid crystal phases from disordered “melted”microstructures remains a challenging research problem,but the trends we observed are similar to trends found foranalogous hydrocarbon systems.

Conclusions

The ternary phase behavior of water/M(D′E12)M/siliconeoil systems has been determined for three low molecularweight silicone oils, including two cyclic silicone oils, D4-and D5, and one short linear silicone oil, MD2M. Becauseof the relatively close molecular weights, the three oilsbehave similarly in the ternary system. The water/M(D′E12)M binary system shows an upper miscibility gapas do the water/CiEj systems, while the M(D′E12)M/siliconeoil binary shows a much larger lower miscibility gapcompared to CiEj/alkane systems. The ternary systemsprogress through the now-familiar sequence of Winsor Ito III to II phase behavior with rising temperature.Because the lower miscibility gap of the surfactant/oil pairextends to such high temperature, the three-phase regionis quite broad in both concentration and temperature. Thehigher molecular weight oils shift the fish to highertemperatures 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.

Figure 12. (a) Plot of Cmin vs oil content R. (b) Plot of T(Cmin) vs oil content R.

Figure 13. Evolution of the ternary phase diagrams withtemperature for the water/M(D′E12)M/oil systems.


temperature, M(D′E12)M forms isotropic solutions withwater at all concentrations. The isothermal ternarydiagrams show that all of these solutions are able tosolubilize a small amount of oil to form microemulsion. Athigher oil levels, liquid crystal phases are formed nearambient temperature, including the cubic I1 and I3,hexagonal H1, and lamellar LR phases.

Acknowledgment. The project was supported by theNational Science Foundation through the Center forInterfacial Engineering (CIE) at University of Minnesota.Dow Corning Corporation’s financial sponsorship andtechnical supports are greatly appreciated.

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Phase Behavior and Microstructure of Water/Trisiloxane E 12 Polyoxyethylene Surfactant/Silicone Oil Systems