Effect of Didodecyldimethylammonium Bromide on thePhase Behavior of Nonionic Surfactant-Silicone Oil

Microemulsions

James A. Silas and Eric W. Kaler*

Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering,University of Delaware, Newark, Delaware 19716

Randal M. Hill

Central R&D, Dow Corning Corporation, 2200 West Salzburg, Midland, Michigan 48686-0994

Received March 8, 2001. In Final Form: May 9, 2001

The effects of adding didodecyldimethylammonium bromide (DDAB) to mixtures of n-alkyl polyglycolether (CiEj), water, and silicone oil are systematically studied. In mixtures of C8E3 and C12E6 with water,small amounts of DDAB cause the upper miscibility gap to vanish and be replaced by a high-temperaturelamellar phase. In mixtures of C12E5 with octamethylcyclotetrasiloxane (D4), the addition of DDAB makesthe surfactant mixture more hydrophilic and expands the lower miscibility gap by increasing the (pseudo)-critical temperature. In ternary mixtures of CiEj, D4, and water, adding DDAB increases the surfactantefficiency by up to a factor of three, expanding the single-phase microemulsion region to higher tempera-tures and lower surfactant concentrations. The temperature limits of the single-phase microemulsioncorrelate with changes in the pseudo-binary phase diagrams upon addition of DDAB. Additionally, thesurfactant mixture stabilizes liquid crystalline regions, but at temperatures that do not obscure the gainsin surfactant efficiency offered by the ionic-nonionic surfactant mixture. Similar results are reported fortwo linear silicone oils, hexamethyldisiloxane and decamethyltetrasiloxane.

Introduction

Microemulsions are thermodynamically stable, isotro-pic, microstructured solutions of water, oil, and surfactant.There is a large and growing body of research about thethermodynamic, structural, and dynamic properties ofthese solutions,1,2 yet many unanswered questions remain.There are also many practical applications that make useof the different forms of microstructure available withinmicroemulsions,3 but economic factors nearly alwaysrequire the product or process to use the least surfactantpossible. Obtaining efficient microemulsions with a par-ticular oil usually involves the addition of cosurfactant,such as a copolymer4 or an ionic surfactant,5 or matchingthe surfactant structure to the oil in question.6,7 However,the addition of ionic cosurfactant usually introducesunwanted liquid crystalline regions that dominate thephase behavior and can obscure any enhanced efficiency.

Silicone oils are an industrially important class ofmaterials that show excellent chemical stability and areused in cosmetics and other personal care products, intextile manufacture, and as lubricants, foam controlagents, and mold release agents.8,9 Incorporating the

physical properties of silicone oils into efficient micro-emulsions is a first step to new products, and the uniquefeatures of silicone oils open new areas for scientific study.While there are empirical rules to guide the formation ofefficient microemulsions with hydrocarbon alkane oils,10,11

there has been relatively little examination of the ternaryphase behavior of silicone oils in solutions of either siloxaneor carbon-based surfactants. Ternary siloxane surfactant-silicone oil-water systems show phase behavior resultsthat are similar to those of mixtures of hydrocarbons andcarbon-based nonionic surfactants.6,7 However, siloxanesurfactants are not available with the range of moleculararchitectures found inhydrocarbonsurfactants.Generally,the studies of hydrocarbon surfactants with silicone oilsmostly report the inability to solubilize much silicone oilin water. Mixtures of hydrocarbon surfactants withmodified silicone oil (where the modification is an amine12

or a phenyl derivative13) and water show somewhatimproved solubilization over that achieved with unmodi-fied silicone oils,14-16 but the level of solubilization is stillnot comparable to that found in hydrocarbon systems.Similarly, a co-oil can increase solubilization of the siliconeoil and lead to the formation of microemulsions andinteresting liquid crystalline phases.17(1) Strey, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 402-410.

(2) Schubert, K.-V.; Kaler, E. W. Ber. Bunsen-Ges. Phys. Chem. 1996,100, 190-205.

(3) Solans, C.; Kuneida, H. Industrial Applications of Microemulsions;Surfactant Science Series Vol. 66; Marcel Dekker: New York, 1997.

(4) Jakobs, B.; Sottman, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter,D. Langmuir 1999, 15, 6707.

(5) Kahlweit, M.; Faulhaber, B.; Busse, G. Langmuir 1994, 10, 2528-2532.

(6) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R.M. Langmuir 1999, 15, 2267-2277.

(7) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R.M. Langmuir 1999, 15, 2278-2289.

(8) O’Lenick, A. J., Jr. J. Surfactants Deterg. 2000, 3, 229-236.(9) Auner, N.; Weis, J. Organosilicon Chemistry IV; Auner, N., Weis,

J., Eds.; Wiley-VCH: New York, 2000.

(10) Kahlweit, M. J. Phys. Chem. 1995, 99, 1281-1284.(11) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci.

1995, 170, 78-84.(12) Katayama, H.; Tagawa, T.; Kuneida, H. J. Colloid Interface Sci.

1992, 153, 429-436.(13) Steytler, D. C.; Dowding, P. J.; Robinson, B. H.; Hague, J. D.;

Rennie, J. H. S.; Leng, C. A.; Eastoe, J.; Heenan, R. K. Langmuir 1998,14, 3517-3523.

(14) Binks, B. P.; Dong, J. Colloids Surf., A 1998, 132, 289-301.(15) Dowding, P. Ph.D. Thesis, University of East Anglia, Norwich,

U.K., 1995.(16) Messier, A.; Schorsch, G.; Rouviere, J.; Tenebre, L. Prog. Colloid

Polym. Sci. 1989, 79, 249-256.

4534 Langmuir 2001, 17, 4534-4539

10.1021/la010359g CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 06/27/2001

The most effective way to examine ternary systemsexperimentally is by constructing an upright ternary Gibbsphase prism and making planar cuts through the prismalong different lines of constant composition. The resultingvertical sections have temperature as the ordinate and acomposition variable as abscissa. All or part of the bodyof heterogeneous phases is contained in this Gibbs prism.Mapping the changes in phase behavior as a function oftemperature and composition provides essential thermo-dynamic information. Studies of mixtures of the n-alkylpolyglycol ethers (CiEj’s), alkanes, and water show howthe critical points on the surfactant-water and surfac-tant-oil binaries influence the behavior of ternarymixtures.18-21 A similar approach can show how theaddition of a cosurfactant can affect the critical pointsand formation of heterogeneous phases.5,22-24

As explained by Kahlweit and Strey,19 the lower criticalpoint on the surfactant-water binary, Tâ, is connected bya critical line to the lower critical temperature of thethree-phase body, Tl. Similarly, the upper critical pointon the surfactant-oil binary is connected by a critical lineto the upper critical temperature of the three-phase body,Tu. Therefore, movement of the critical points TR and Tâdirectly affect the extent of the three-phase body intemperature by moving Tu and Tl, respectively.

To make efficient microemulsions, surfactant mixtureshave been employed to tailor the properties of thesurfactant film to the particular application. For example,the addition of ionic surfactant to nonionic surfactantchanges the phase progression and temperature depen-dence of surfactant-water mixtures.25-29 Ionic-nonionicsurfactant mixtures are known to increase the surfactantefficiency in systems of CiEj-alkane oil-water. Phenom-enologically, these observations can be explained in termsof the relative location of the tricritical point5 or by thecombined hydrophilic-lipophilic balance (HLB) of thesurfactant mixture.30-32 Structurally, the addition ofcharge to nonionic membranes causes an increase in thebending moduli of the nonionic membrane33 and canchange themechanismofstabilization in lyotropic lamellarphases.34

In this paper, we present the results of phase behaviorstudies on the system CiEj-D4-water. In addition, wereport how addition of a cationic cosurfactant changes

the interactions of the surfactant-oil and surfactant-water binary, while greatly increasing the efficiency ofhydrocarbon surfactants with respect to silicone oils. Thisstudy parallels and supports other work with alkane andether oils.5,31,35,36

Experimental SectionMaterials. Water was filtered through a 0.2 µm filter, distilled,

and deionized until the specific resistance was 18.3 MΩ cm.N-Dodecyl pentaoxyethylene glycol ether (C12E5) (>99%),n-dodecyl hexaoxyethylene glycol ether (C12E6) (>99%), n-octyltrioxyethylene glycol ether (C8E3) (>99%), n-dodecyl trioxyeth-ylene glycol ether (C12E3) (>99%), and n-decyl tetraoxyethyleneglycol ether (C10E4) (>99%) were obtained from Nikko. Decane(>98%), octamethylcyclotetrasiloxane (D4) (>99%), hexameth-yldisiloxane (MM) (>99%), decamethyltetrasiloxane (MD2M)(>98%), n-hexyl diethylene glycol ether (C6E2) (>98%), andn-butyl ethlyene glycol ether (C4E1) (>98%) were purchased fromFluka. Didodecyl dimethylammonium bromide (>99%), DDAB,was obtained from TCI America. All materials were used withoutfurther purification.

Phase Behavior Determination. The procedure used forternary and quaternary phase diagram determination followsthe procedure introduced by Kahlweit and co-workers.19,35 Withfour-component mixtures, the phase space is defined by tem-perature, pressure, and three composition variables. To specifythe amount of (A) water, (B) oil, (C) nonionic surfactant, and (D)ionic surfactant, we use the following composition variables: themass fraction of oil neglecting surfactant, R, defined as

the mass fraction of surfactant, γ, defined as

andthemass fractionof ionic surfactant in thesurfactantmixture,δ, defined as

For a ternary mixture, only the first two mass variables areneeded, the third being δ ) 0.

If instead of an ionic surfactant the fourth component is a (E)co-oil, R is defined as

γ is defined as

and the mass fraction of co-oil in the oil is

To represent the phase space in two dimensions, only two of thedefining variables are allowed to vary. In this work, the pressurewill always be the ambient air pressure, and the phase behaviorwill be read as a function of temperature against one compositionvariable, all others being held constant.

In particular, several sections through the phase prism haveproven useful for studying microemulsions. Sections at R ) 0and 100 correspond to the surfactant-water or surfactant-oilbinary, respectively. These give valuable information on thespecific interactions between two of the species involved and

(17) John, A. C.; Uchiyama, H.; Nakamura, K.; Kuneida, H. J. ColloidInterface Sci. 1997, 186, 294-299.

(18) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1983, 87,5032-5040.

(19) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24,654-668.

(20) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88,1937-1944.

(21) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.;Schomacker, R. Langmuir 1988, 4, 499-511.

(22) Ryan, L. D.; Schubert, K.-V.; Kaler, E. W. Langmuir 1997, 13,1510-1518.

(23) Ryan, L. D.; Kaler, E. W. Langmuir 1997, 13, 5222-5228.(24) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 10313-

10318.(25) Douglas, C. B.; Kaler, E. W. J. Chem. Soc., Faraday Trans. 1994,

90, 471-477.(26) Douglas, C. B.; Kaler, E. W. Langmuir 1991, 7, 1097-1102.(27) Marszall, L. Langmuir 1990, 6, 347-350.(28) Marszall, L. Langmuir 1988, 4, 90-93.(29) Firman, P.; Haase, D.; Jen, J.; Kahlweit, M.; Strey, R. Langmuir

1985, 1, 718-724.(30) Aramaki, K.; Ozawa, K.; Kunieda, H. J. Colloid Interface Sci.

1997, 196, 74-78.(31) Kunieda, H.; Hanno, K.; Yamaguchi, S.; Shinoda, K. J. Colloid

Interface Sci. 1985, 107, 129-137.(32) Shinoda, K.; Kunieda, H.; Arai, T.; Saijo, H. J. Phys. Chem.

1984, 88 (21), 5126-5129.(33) Schomacker, R.; Strey, R. J. Phys. Chem. 1994, 98, 3908-3912.(34) Roux, D.; Safinya, C. R. J. Phys. France 1988, 49, 307-318.

(35) Kahlweit, M.; Strey, R. J. Phys. Chem. 1988, 92, 1557-1563.(36) Ryan, L. D.; Kaler, E. W. J. Phys. Chem. 1998, 102, 7549-7556.

R ) BA + B

× 100

γ ) C + DA + B + C + D

× 100

δ ) DC + D

× 100

R ) B + EA + B + E

× 100

γ ) CA + B + C + E

× 100

â ) EB + E

× 100

Effect of DDAB on Surfactant-Oil Microemulsions Langmuir, Vol. 17, No. 15, 2001 4535

have been shown to control the formation of the three-phasebody within the phase prism. Sections at R ) 50 are used indetermining the least amount of surfactant needed to solu-bilize equal weights of oil and water. This occurs at the x-point,where the one- and three-phase microemulsions meet at a pointat R ) 50, and has the coordinates of γ ) γ and T ) T. Theamount of surfactant at the x-point, γ, is the efficiency of thesurfactant being measured and can be compared to efficienciesof other surfactant systems. Varying the oil or surfactantcomposition allows the tracking of surfactant efficiency as afunction of â and δ.

Results

CiEj/DDAB/Water. Adding small amounts of DDABhas a profound effect on the phase behavior of solutionsof CiEj and water (Figure 1). The changes observed forC12E6 and C8E3 systems are similar to changes inducedby anionic cosurfactant in previous studies.25,26,37 Atconcentrations above δ ) 1, the lower consolute temper-ature is no longer evident, while a lamellar phase beginsto form roughly 15 °C above the former critical point. Whatwas a transition from a single-phase micellar solution totwo isotropic phases upon heating becomes a transitionfrom a single-phase micellar solution to a micellar solutionplus a liquid crystalline lamellar phase. The phaseboundaries do not change upon further addition of DDABfor δ values beyond that needed to form the lamellar phase.

C12E5/DDAB/D4. The binary of C12E5 with the siliconeoil D4 shows a lower miscibility gap that narrows with anincrease in temperature until the critical point TR isreached (Figure 2, with TR ) 21 °C). The addition of DDAB

raises the temperature required to obtain a one-phasemixture. At δ ) 7, the pseudo-TR ) 27 °C, while at δ )18, the pseudo-TR ) 35 °C. C12E3, on the other hand, iscompletely miscible with D4 for δ from 0 to 10.

C12E5/Decane/D4/Water. D4 was blended with decane,with which it is miscible at all concentrations, and theone-phase microemulsion of this oil mixture with C12E5was followed as a function of oil composition, â, at R ) 50.The results are shown in Figure 3. The shaded area forâ ) 0 indicates both single- and multiphase lamellarregions. As â increases, the one-phase region moves tohigher temperatures and surfactant concentrations. At â) 100, the efficiency γ ) 38, so the surfactant is actuallythe plurality component in the mixture (by weight). Thethree-phase region of D4 with C12E5 (â ) 100) extends toaround γ ) 5 in composition and over a temperature rangeat least 20 °C wide. The lamellar phase formed with decaneretreats to higher γ as â increases, until it is no longerevident in the oil mixture.

CiEj/D4/Water. Figure 4 shows the single-phase mi-croemulsion regions at R ) 50 for a selection of CiEj’s.Here, the change in temperature of the three-phase bodyupon varying i and j can be seen by comparison. Increasingi by 4, from C8E3 to C12E3, lowers T by 15 °C, whileincreasing j by 2, from C12E3 to C12E5, increases T by 40°C. Notice, however, that none of the CiEj’s have γ < 30,meaning it takes about as much nonionic surfactant as oilto form a one-phase microemulsion.

CiEj/DDAB/D4/Water. The addition of cosurfactantDDAB to C8E3 substantially increases the surfactantefficiency, as shown for DDAB and C8E3-D4-water in

(37) Rajagopalan, V.; Bagger-Jorgensen, H.; Fukuda, K.; Olsson, U.;Jonsson, B. Langmuir 1996, 12, 2939-2946.

Figure 1. Temperature-composition pseudo-binary phasediagram of CiEj-DDAB-water mixtures at constant δ. Areaslabeled LR contain both single- and multiphase lamellar regions.C12E6-DDAB-water mixtures (top) are shown at δ ) 0, 1, andC8E3-DDAB-water mixtures (bottom) are shown at δ ) 0, 2.

Figure 2. Temperature-composition pseudo-binary phasediagram of C12E5-DDAB-D4 mixtures at constant δ.

Figure 3. Temperature-composition phase diagram of C12E5-decane-D4-water at R ) 50 and various â. The shaded areafor â ) 0 (decane) contains both single- and multiphase lamellarregions. The three-phase body is not shown for clarity.

4536 Langmuir, Vol. 17, No. 15, 2001 Silas et al.

Figure 5a. Only the envelopes of the single-phase micro-emulsion regions are shown for clarity, but note that byδ ) 10, the three-phase body has disappeared. As δincreases, γ moves to higher temperatures and lower γ.At δ ) 0, γ ) 44 and T ) 45 °C, while at δ )12, γ ) 14and T ) 72 °C. The extent of the one-phase channel forδ ) 10 encompasses the one-phase channels that existedat lower δ. Thus, the addition of 12% ionic surfactant inthe surfactant mixture has reduced the amount of sur-factant needed to form a one-phase microemulsion by afactor of three from the original nonionic system. Further

addition of ionic surfactant begins to move the one-phasemicroemulsions above the experimental window in tem-perature at low surfactant concentrations. Note that asδ increases, the lower phase boundary of the one-phaseregion does not move. For example, at γ ) 45, the lowerphase boundaries overlap at T ) 45 °C for δ ) 0, 2, 10,12. The effect of addition of DDAB to C12E3 microemulsionsis shown in Figure 5b. Again, only the one-phase regionsare shown for clarity and the three-phase body disappearsas larger amounts of ionic surfactant are added. As forC8E3, DDAB raises T and lowers γ with an increase in δ.At δ ) 0, γ ) 31, while as δ increases to δ ) 18, γ ) 12.Again, notice how the lower phase boundaries of the one-phase region coincide for all δ.

C8E3/MM/Water. Adding DDAB to C8E3 with the two-unit linear silicone oil hexamethyldisiloxane, MM, alsolowers γ (Figure 6), with γ moving from 37 to 15 as δchanges from 0 to 11. The change in temperature withincreasing δ is not as large as it is for D4 but still showsthe same characteristic of a lower phase boundary thatcoincides with the original boundary, while the upperboundary increases in temperature.

C12E3/MD2M/Water. Similar effects are seen withC12E3, water, and decamethyltetrasiloxane, MD2M (Figure7). As δ increases, γ decreases while the temperaturerange of the microemulsions increases. At δ ) 11, γ islowered to 15, down from 40 when δ ) 0. At the sametime, the temperature of the one-phase region increasesfrom 40 to 65 °C. The lower phase boundaries coincide forall δ values, while the upper phase boundary increaseswith increasing δ.

Figure 4. Temperature-composition phase diagram of CiEj-D4-water mixtures at R ) 50. Only one-phase regions are shownfor clarity. The shaded area for C12E3 indicates a multiphaselamellar region.

Figure 5. Temperature-composition phase diagram of CiEj-DDAB-D4-water at R ) 50 and various δ. Only one-phaseregions are shown for clarity. C8E3-DDAB-D4-water mixtures(top) are shown for δ ) 0, 2, 10, and 12. C12E3-DDAB-D4-water mixtures (bottom) are shown for δ ) 0, 7, 12, 15, and 18.The shaded area for C12E3 indicates a multiphase lamellarregion.

Figure 6. Temperature-composition phase diagram of C8E3-MM-water at R ) 50 and various δ. Only one-phase regionsare shown for clarity.

Figure 7. Temperature-composition phase diagram of C8E3-MD2M-water at R ) 50 and various δ. Only one-phase regionsare shown for clarity.

Effect of DDAB on Surfactant-Oil Microemulsions Langmuir, Vol. 17, No. 15, 2001 4537

Discussion

Surfactant-WaterBinary.ThephasebehaviorofCiEjsurfactants with water is well understood,1 and thechanges resulting from the addition of cationic surfactantare similar to those observed in anionic-nonionicsystems.26,38-40 The suppression of the upper miscibilitygap shown in Figure 1 increases the temperature rangeof the micellar region of the pseudo-binary mixture,effectively raising Tâ until it is obscured by a liquidcrystalline region. It has been suggested that the changein Tâ is due to the adsorption or depletion of salts at thesurfactant monolayer, depending on the nature of the saltin question.41 Phenomenologically, increasing Tâ shouldraise the temperature of the body of heterogeneousphases.29,42

Surfactant-Oil Binary. The typical phase behaviorofnonionic surfactantwithoil consistsofa lowermiscibilitygap, which, depending upon the differences in hydropho-bicity between the compounds, may or may not be visiblewithin the observable temperature range. As surfactantsbecome more hydrophobic, the critical point TR falls below0 °C. Such is the case for C12E3 with D4. However, TR forC12E5 is around 20 °C with D4. The addition of the ionicsurfactant increases the hydrophilicity of the surfactantmixture, raising the pseudo-TR incrementally with eachaddition of DDAB, as shown in Figure 2. In a ternarymixture, such a change in TR increases the temperatureat which the three-phase body forms. Since the three-phase body and one-phase microemulsion generally occurin the same temperature range, the temperature of theone-phase microemulsion should also rise.

C12E5/Decane/D4/Water. The technique of blending anew “unknown” oil with one whose phase behavior isalready known is helpful in locating the three-phase regionof the new oil.43 Figure 3 shows that γ and T for D4 withC12E5 are higher in surfactant concentration and tem-perature relative to that of decane. The movement up intemperature is expected since TR for C12E5 and decane isbelow the experimental window, while TR ) 20 °C for C12E5and D4. In addition, the lamellar phase that forms in theone-phase channel at higher surfactant concentrationswith decane diminishes and retracts with the addition ofD4. This implies a decrease of long-range order within themicrostructure of this system in parallel with the loss ofefficiency. Since only the composition of the oil phase ischanging in this experiment, the simultaneous loss ofmicrostructural order and surfactant efficiency in themicroemulsion clearly reflects changes in oil-surfactantinteractions.

CiEj/D4/Water. The efficiency of C12E5 with D4 isrepresentative of the results for all the CiEj’s. Indeed, allof the nonionic surfactants shown in Figure 4 require γabove 30 in order to form a single-phase microemulsion.The trends in T of the CiEj’s shown in Figure 4 are similarto the trends in T of CiEj’s with alkane oils and n-alkylmethacrylates. In particular, as i increases, T decreases,and as j increases, T increases. When the phase behaviorof CiEj-D4-water is compared to the results of CiEj-n-

hexyl methacrylate-water44 or CiEj-octane-water,45 D4is generally more hydrophobic than either of these twooils. Higher temperatures are required to partition aparticular surfactant into D4 than the oil phase of eitheroctane or n-hexyl methacrylate systems. In fact, extrapo-lating the observed temperature dependence of the alkaneswith CiEj’s indicates that D4 behaves as approximately aC20 alkane,46 as one would expect from comparisons ofmolecular weights or volumes. However, the uniquecharacter of the silicone moiety makes linear comparisonswith alkanes difficult.47

The body of heterogeneous phases moves in temperaturedepending on the relative hydrophobicity of the surfactanttested. More hydrophobic surfactants have lower TR’s andTâ’s and partition into the silicone oil at lower temper-atures. This lowers the temperature of the three-phasebody, so three liquid phases are formed with C12E3 at lowertemperatures than with C8E3 or C12E5. The results oftracking T and γ from a series of CiEj’s with i decrementedby 2 and j by 1 (i.e., C12E5, C10E4, etc.) are similar to theresults reported for octane and n-hexyl methacrylate. Inthose cases, the nonlinear changes in T and γ as thesurfactant becomes less amphiphilic (decreasing i and j)signal the approach to a tricritical point as the three-phase bodies shrink, and that is likely what is happeninghere also. However, C4E1 still forms a three-phase bodywith D4 and water, so the tricritical point must be “past”C4E1.

The change in γ of these solutions correlates to the lengthof the hydrophobic portion of the surfactant (that is, i).Surfactants with longer hydrocarbon chains are moreefficient; they require less surfactant, regardless oftemperature, as seen from increasing i from 4 to 12. Thisis most likely because of the same factors that lead togreater ordering of the surfactant monolayer, such asrigidity and insolubility in the adjacent bulk phases. Theordering ability of the surfactant in solution is seenexperimentally by the extent of liquid crystalline phasesevident on the surfactant-water binary. For comparison,the phase behavior of C8E3 with water shows just an uppermiscibility gap, while C12E5 with water shows a lamellarphase at low concentration and C12E3 with water isdominated by lyotropic phases.48 As γ decreases from C8E3to C12E5 to C12E3, it follows that the addition of acosurfactant that can increase the order within thesurfactant layer should also affect the efficiency of thesurfactant mixture.

CiEj/DDAB/D4/Water. The addition of DDAB yieldslarge increases in the efficiency of CiEj-rich surfactantmixtures. Unfortunately, a rise in the temperature of theone-phase region also accompanies the increase in ef-ficiency, and this rise can move the one-phase micro-emulsion beyond accessible temperatures. Since thehomologous series of CiEj surfactants act similarly andpredictably, however, the temperature of the originalnonionic microemulsion can be controlled by judiciouschoice of i and j, and thus there is some control of thetemperature range over which a microemulsion forms.There are two main effects of adding ionic surfactant tothe nonionic microemulsions; T increases and γ decreases.

(38) Nishikido, N. J. Colloid Interface Sci. 1989, 136, 401.(39) Carvell, M.; Leng, C. A.; Leng, F. J.; Tiddy, G. J. T. Chem. Phys.

Lett. 1987, 137, 188.(40) De Salvo Souza, L.; Corti, M.; Cantu, L.; Degiorgio, V. Chem.

Phys. Lett. 1986, 131, 160.(41) Kabalnov, A.; Olsson, U.; Wennerstrom, H. J. Phys. Chem. 1995,

99, 6220.(42) Kahlweit,M.;Busse,G.;Faulhaber,B. Langmuir 2000, 16, 1020-

1024.(43) Kahlweit,M.;Busse,G.;Faulhaber,B. Langmuir 1997, 13, 5249-

5251.

(44) Lade, O.; Beizai, K.; Sottman, T.; Strey, R. Langmuir 2000, 16,4122.

(45) Burauer, S.; Sachert, T.; Sottman, T.; Strey, R. Phys. Chem.Chem. Phys. 1999, 1, 4299.

(46) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90,671-677.

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Properties of Selected Anionic, Cationic and Nonionic Surfactants;Elsevier: New York, 1993.

4538 Langmuir, Vol. 17, No. 15, 2001 Silas et al.

Understanding the temperature increase of the single-phase microemulsion caused by the addition of DDAB isaided by a closer inspection of the component phasediagrams. In a ternary mixture of surfactant, oil, andwater, theGibbsphaseprismencompassesall temperatureand composition combinations, allowing one to track therelationship between the binary and ternary phasediagrams. Critical lines directly connect the binarymiscibility gaps and the three-phase body, relating TR toTu, the upper temperature of the three-phase body, andTâ to Tl, the lower temperature of the three-phase body.19

In a quaternary mixture, however, a phase tetrahedronexists for each temperature, so precise relationshipsbetween the pseudo-TR and Tâ and the temperaturestability of any interior phase are difficult to ascertain.Inasmuch as the surfactant mixture can be regarded asa pseudo-component, the pseudo-binaries will control thetemperature dependence of the pseudo-ternary mixture,but the connection is not as rigorous as it is for a ternarymixture.

Keeping this in mind, the addition of DDAB to CiEj’sincreases both the pseudo-TR and Tâ (Figures 1 and 2). Anincrease in both Tâ and TR indicates that a highertemperature is needed to partition the surfactant out ofthe water phase and a still higher temperature is neededto partition the surfactant into the oil phase. Therefore,the three-phase body (when it exists) and the one-phasemicroemulsion move to higher temperatures, as seenexperimentally in Figures 5-7.

Additionally, as ionic surfactant is added to the nonionicmicroemulsion, the lower phase boundary coincides withthat of the original microemulsion, while the upper phaseboundary increases in temperature with each DDABaddition. This phenomenon is also evident in previousaccounts.36 In terms of the pseudo-binaries, the surfac-tant-oil binary changes only slightly upon addition ofDDAB, as does the lower phase boundary of the one-phasemicroemulsion. In contrast, the surfactant-water binarychanges dramatically, as does the upper phase boundaryof the one-phase microemulsion. Since the lower phaseboundary corresponds to saturation of the single-phasemicroemulsion with oil (a Winsor IV to Winsor I transi-tion), it follows that the location of this boundary wouldbe most influenced by the changes in interactions betweenthe surfactant and oil. Similarly, as the upper phaseboundary corresponds to saturation of the single-phasemicroemulsion with water (Winsor IV to Winsor II), thelocation of this phase boundary is influenced most by thechanges in interactions between surfactant and water.Overall, the large movement of the upper phase boundarywith DDAB addition indicates that the microemulsionphase is more stable with regard to an excess water phaseor that the balanced or HLB temperature has dramaticallyincreased.

Recall that the final γ of the CiEj-DDAB surfactantmixture correlates with the formation of liquid crystallineregions with water (Figure 4). The addition of ionicsurfactant increases the ability of the surfactant mixtureto order in water (as evidenced by the low-concentrationlamellar phase), most likely as a consequence of increasingthe rigidity of the surfactant monolayers.33 As noted before,the difference in efficiency between alkane oils and silicone

oils can be traced simplistically to the differences in thesurfactant-oil interactions. To mitigate the changes ininteractions on the oil side of the surfactant monolayer,the interactions on the water side of the surfactantmonolayer can be modified by the addition of an ionicsurfactant. Therefore, a relatively “stronger” surfactantmixture is needed to solubilize silicone oils and yieldsurfactant efficiencies similar to those of CiEj’s with alkaneoils.

The same arguments and similar formulations havebeen proposed for alkane systems. However, increasingthe amount of lyotropic phases on one side of the phaseprism, whether this is done by adding a cosurfactant orby increasing the amphiphilicity, generally obscuresisotropic phases throughout the phase prism. For example,the lamellar phase formed with CiEj-DDAB-waterextends into the pseudo-phase prism with D4 and obscuresthe phase behavior of this system for R < 35. However,at R ) 50, observations indicate that the lamellar phaseexists only in the multiphase region above the one-phaseregion plotted in Figures 5-7. Increased surfactantefficiency can be realized only by evading the lyotropicphases in highly amphiphilic surfactant systems.

CiEj/DDAB/LinearOil/Water.Adding ionic surfactantalso increases the surfactant efficiency for mixtures withtwo different linear oils (Figures 6 and 7). As with D4, thetemperature of the one-phase region increases withincreasing δ, indicating a difference in the partitioning ofthe surfactant mixture with increasing ionic content.Similarly, the lower phase boundary of the one-phasemicroemulsions remains relatively constant while theupper phase boundary increases. As the T and γ increasewith the molecular weight of the oil, it becomes moreimportant to choose i and j to lower the temperature ofthe nonionic microemulsion. Therefore, MM is solubilizedwith C8E3 while MD2M requires the more hydrophobicC12E3 in order to lower the temperature of the one-phaseregions to accessible ranges. This example illustrates thecontrol over the formulation variable, T, that is obtainedby using a homologous series10 and suggests a strategy tostudy increasing molecular weight linear silicone oils.49

ConclusionSystematic study of water-D4-CiEj-DDAB mixtures

shows that adding a cationic cosurfactant greatly changesthe phase behavior of water-CiEj and D4-CiEj mixtures.The miscibility gaps in binary CiEj-water mixturesdisappear, and a lamellar phase appears at highertemperatures. For CiEj-D4 mixtures, the addition of ionicsurfactant increases the temperature at which the mis-cibility gap appears on the surfactant-oil side of the phaseprism. Coincidentally, the addition of DDAB to mixturescontaining equal weights of silicone oil and water greatlyincreases the efficiency of nonionic surfactants but alsoshifts the single-phase microemulsion to higher temper-atures.

Acknowledgment. This work was supported by DowCorning.

LA010359G

(49) Silas, J. In preparation.

Effect of DDAB on Surfactant-Oil Microemulsions Langmuir, Vol. 17, No. 15, 2001 4539