Phase Behavior of Microemulsions Made with n-Alkyl Monoglucosides

The addition of CmG1 to water−octane−CiEj mixtures promotes formation of a three-phase region .... Biotechnology Progress 2011 27 (10.1002/btpr.v2...
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Langmuir 1997, 13, 1510-1518

Phase Behavior of Microemulsions Made with n-Alkyl Monoglucosides and n-Alkyl Polyglycol Ethers Larry D. Ryan, Kai-V. Schubert,† and Eric W. Kaler* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received September 6, 1996. In Final Form: December 27, 1996X The phase behavior of mixtures of water, octane, n-alkyl polyglycol ethers (CiEj), and n-alkyl β-Dglucopyranosides (CmG1) is systematically studied as a function of temperature and composition. The addition of CmG1 to water-octane-CiEj mixtures promotes formation of a three-phase region that is nearly independent of temperature in all mixtures studied. The size and extent of this three-phase region are shown as a function of CmG1 concentration and hydrophobicity and CiEj amphiphilicity. This unique behavior arises from the extremely low oil solubility of the alkyl monoglucosides.

Introduction Microemulsions are thermodynamically stable, isotropic mixtures of water, oil, and surfactant and may contain additives such as salt or alcohol. Preparing microemulsions for practical purposes requires the systematic study of phase behavior and microstructure, and a generalized strategy for formulating microemulsions has been recently discussed by Kahlweit.1 When an application calls for a nonionic surfactant, n-alkyl polyglycol ethers (CiEj) or related materials are typically used. These surfactants contain i number of carbon atoms in the hydrophobic alkyl chain and j number of ethoxy groups in the hydrophilic head group. The phase behavior of mixtures containing water, alkanes, and CiEj has been studied extensively2-5 and recently reviewed.6 Typically, temperature-composition phase diagrams made at a 1:1 ratio of oil-to-water show a one-phase microemulsion region at relatively high surfactant concentrations (so-called “fish tail”) in contact with a three-phase body at lower surfactant concentrations (“fish body”). The three-phase body, consisting of a middle-phase microemulsion in equilibrium with an excess oil and water phase, is surrounded by two-phase regions. At low temperatures CiEj’s are dissolved mainly in water and the two-phase region consists of a surfactant-rich water phase in equilibrium with an excess oil phase (2). At high temperatures, CiEj’s are more soluble in oil and form a surfactant-rich oil phase in equilibrium with an excess water phase (2). The bar denotes the phase in which the surfactant is mainly dissolved. This complex phase behavior is understood to arise through the interplay of features of the three corresponding binary systems: (1) the location of the upper miscibility * To whom correspondence should be addressed. † Current address: DuPont Central Research and Development, Experimental Station, P.O. Box 80356, Wilmington, DE 198800356. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Kahlweit, M. J. Phys. Chem. 1995, 99, 1281-1284. (2) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654-668. (3) Kahlweit, M.; Strey, R.; Haase, D.; Kunieda, H.; T., S.; B., F.; Borkovec, M.; Eicke, H.-F.; Busse, G.; Eggers, F.; Funck, T.; Richmann, H.; Magid, L.; So¨derman, O.; Stilbs, P.; Winkler, J.; Dittrich, A.; Jahn, W. J. Colloid Interface Sci. 1987, 118 (2), 436-453. (4) Kahlweit, M.; Strey, R.; Busse, G. Phys. Rev. E 1993, 47, 41974209. (5) Schick, M. Nonionic Surfactants; Surfactant Science Series 23; Marcel Dekker: New York, 1987. (6) Schubert, K.-V.; Kaler, E. W. Ber. Bunsenges. Phys. Chem. 1996, 100, 190-205.

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gap and its critical point (Cpβ at Tβ) in the binary waterCiEj mixture; (2) the location of the critical point (CpR at TR) in the binary alkane-CiEj mixture; (3) the location of the critical point in the water-alkane mixture. For CiEj’s, Tβ has a relatively weak dependence on i and j,7,8 which enables the miscibility gap to be tuned by appropriately adjusting i and j. This tuning of the miscibility gap controls the location of the three-phase body for a given alkane as a function of temperature. Likewise, TR can be moved up or down on the temperature scale by adjusting i and j ,7 again moving the three-phase body up or down on the temperature scale. Finally, as the carbon number of the alkane is lowered, TR drops for a given CiEj2 due to an increase in solubility of CiEj, and the three-phase body moves to lower temperatures. Concern for the natural environment has added to formulation constraints by increasing the desire for nontoxic, biodegradable surfactants and microemulsions. As a consequence, another class of nonionic surfactants, alkyl polyglucosides (CmGn), have become increasingly important9 because of their excellent biodegradability and ease of manufacture from renewable resources. These surfactants contain m number of carbon atoms in the hydrophobic alkyl chain and n number of glucose groups in the hydrophilic head group, with commercial blends typically containing noninteger values of both m and n. Although they have been known for more than 35 years,10,11 relatively little work has been done to characterize their phase behavior. Experimental studies have been done using commercial blends of CmGn,12-16 with few systematic studies using pure CmGn.17-23 (7) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 38813894. (8) Schubert, K.-V.; Strey, R.; Kahlweit, M. J. Colloid Interface Sci. 1991, 141, 21-29. (9) Nilsson, F. INFORM 1996, 7, 490-497. (10) Shinoda, K.; Yamanaka, T.; Kinoshita, K. J. Phys. Chem. 1959, 63, 638-650. (11) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. Soc. Jpn. 1961, 34, 237-241. (12) Balzer, D. Tenside, Surfactants, Deterg. 1991, 28, 419-426. (13) Balzer, D. Langmuir 1993, 9, 3375-3384. (14) Aleksejczyk, R. A. In Pesticide Formulations and Application Systems: 12th Volume; Devisetty, B. N., Chasin, D. G., Berger, P. D., Eds.; ASTM: Philadelphia, PA, 1993; Vol. 12, pp 22-32. (15) Parker, W. O.; Genova, C.; Carignano, G. Colloids Surf., A 1993, 72, 275-284. (16) Platz, G.; Polike, J.; Thunig, C. Langmuir 1995, 11, 4250-4255. (17) Hill, R. M.; Dieker, S. O. Presented at 78th AOCS Meeting, New Orleans, 1987. (18) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1990, 67, 996-1001. (19) Saito, K.; Saijo, H.; Kato, S.; Deguchi, K. AOCS 82nd Annual Meeting; Chicago, IL,1991

© 1997 American Chemical Society

Phase Behavior of Microemulsions

In contrast to n-alkyl polyglycol ethers, alkyl monoglucosides have very different phase behavior in the binary water-CmG1 and alkane-CmG1 mixtures. In binary water-CmG1 mixtures, for m g 10, a miscibility gap exists throughout the experimental temperature range (0-80 °C) at low surfactant concentrations, so Tβ is located below 0 °C. For m < 8, CmG1 are totally miscible throughout the experimental window, indicating that Tβ > 80 °C. Therefore, the value of Tβ has a large dependence on temperature, changing by more than 100 °C when m changes by 2.19,24 These large changes in Tβ for small changes in CmG1 structure make tuning the miscibility gap with changes in surfactant structure less practical than is the case for the relatively small temperature steps occurring with CiEj surfactants. The phase behavior of binary alkane-CmG1 mixtures is also very different than alkane-CiEj behavior. CmG1’s are nearly insoluble in octane (99%), water (filtered through a 0.2 µm filter, distilled and deionized so the electrical conductivity was 1.8 × 107 Ω cm), and pentaethylene glycol monododecyl ether, C12E5 (>98%), were purchased from Fluka Chemical. 2-Butoxyethanol, C4E1 (>99%), and diethylene glycol monohexyl ether, C6E2 (>99%), were purchased from Aldrich Chemical. Tetraethylene glycol monodecyl ether, C10E4 (>99%), was purchased from Nikko Chemical. n-Octyl β-D-glucopyranoside, C8G1 (>98%), n-decyl β-D-glucopyranoside, C10G1 (98%), and n-dodecyl β-Dglucopyranoside, C12G1 (98%), were purchased from Sigma Chemical. All materials were used without further purification. Phase Diagram Determination. Kahlweit and co-workers2,27 introduced a way of studying the phase behavior of ternary or quaternary mixtures. Using a quaternary mixture of water (A)-oil (B)-surfactant 1 (C)-surfactant 2 (D), the phase space is unambiguously defined by five independent variables: pressure, temperature, and three mass fractions. The most convenient variables for our use are the pressure P, the temperature (20) Kano, K.; Ishimura, T. J. Chem. Soc., Perkin Trans. 2 1995, 8, 1655-1660. (21) Kutschmann, E.-M.; Findenegg, G. H.; Nickel, D.; von Rybinski, W. Colloid Polym. Sci. 1995, 273, 565-571. (22) Nilsson, F.; So¨derman, O. Langmuir 1996, 12, 902-908. (23) Stubenrauch, C.; Kutschmann, E.-M.; Paeplow, B.; Findenegg, G. I. Tenside, Surfactants Deterg. 1996, 33, 237-241. (24) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 33823387. (25) Fukuda, K.; So¨derman, O.; Lindman, B.; Shinoda, K. Langmuir 1993, 9, 2921-2925. (26) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1996, 12, 861862. (27) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 1937-1944.

Langmuir, Vol. 13, No. 6, 1997 1511 T, and the following composition variables: the mass ratio of oil to water in the mixture, R, defined as

R)

B × 100 A+B

in wt %

(1)

the mass fraction of surfactant in the mixture, γ, defined as

γ)

C+D × 100 A+B+C+D

in wt %

(2)

and when two surfactants are used the mass ratio of surfactants in the mixture, δ, defined as

δ)

D × 100 C+D

in wt %

(3)

To make a two-dimensional representation of the phase space, three of these variables must be held constant. Pressure in these experiments is always kept constant (ambient), leaving two of the remaining four variables to be held constant to make a suitable two-dimensional representation of the phase behavior. Several sections through the phase prism have been shown to be especially useful when studying microemulsion phase behavior. First, sections at equal masses of oil and water (R ) 50) as a function of temperature and surfactant concentration are useful for determining the least amount of surfactant necessary to completely solubilize equal masses of oil and water [or the efficiency, denoted by (γ˜ )], and the extent and average temperature (T ˜ ) of the three-phase body for a given oil.4 Another section made at constant surfactant concentration as a function of temperature and oil concentration shows the ability of the surfactant to solubilize oil in water (low R) and water in oil (high R).3 Yet another section at constant R (R ) 50) and temperature, as a function of surfactant ratio (δ), shows the evolution of the one-phase microemulsion region and three-phase body as the surfactant ratios are changed.28-30 These sections are a sampling of possible two-dimensional representations of sections through the phase space for the quaternary mixture and are used to describe the phase behavior of the mixtures studied. The procedure used to determine the phase boundaries for sections through the phase prism for the water-octane-CiEj-CmG1 mixtures closely follows that of Schubert and Strey.31 For the sections through the phase prism as a function of temperature the phase boundaries were determined to within 0.05 °C, while the phase boundaries of sections at constant temperature were determined to within 1 wt %.

Results I. Phase Behavior for Water (A)-Octane (B)-C6E2 (C)-C10G1 (D). 1. As a Function of δ, γ, and Temperature at r ) 50. Figure 1 shows the temperaturecomposition phase diagram for the water-octane-C6E2C10G1 mixture for δ ) 10 (top), 25 (center), and 50 (bottom) at R ) 50. The water-octane-C6E2 “fish” (δ ) 0) is shown for reference in each plot. With the addition of C10G1, the homogeneous microemulsion region (“fish tail”) becomes wider and the efficiency of the surfactant mixture increases slightly (i.e., γ˜ ) 33 for δ ) 0 and drops to γ˜ ) 31 for δ ) 10). The temperature interval for the three-phase body increases with increasing δ, and the whole “fish body” moves upward on the temperature scale. The dominating feature of these phase diagrams is the emergence of a region in the three-phase body that is almost independent of temperature (a so-called “chimney”) which begins at γ* (shown by the dashed line in Figure 1 (top)) as γ is decreased in the presence of C10G1. The concentration (28) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1991, 95, 53445352. (29) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 15761583. (30) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 60916095. (31) Schubert, K.-V.; Strey, R. J. Chem. Phys. 1991, 95, 8532-8545.

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Ryan et al.

Figure 1. Vertical sections through the pseudoternary phase prism for water-octane-C6E2-C10G1 with varying C10G1 concentration (δ) at equal mass fractions of water and octane (R ) 50) for δ ) 10 (top), δ ) 25 (center), and δ ) 50 (bottom). Dashed line shows the wt % of surfactant (γ*) where the temperature-independent region of the three-phase body begins.

) 7 and extends to γ ) 2, while at δ ) 50 the “chimney” region begins at γ* ) 25 and still extends to γ ) 3, making the entire “fish body” almost independent of temperature. To identify the minimum amount of C10G1 required to observe this behavior, δ was lowered to δ ) 1, but the region of interest for the “chimney” was located at very low surfactant concentrations, and it is therefore difficult to determine if a “chimney” exists. 2. As a Function of CiEj Amphiphilicity at r ) 50, δ ) 25. Determining if this observed “chimney” present in the preceding phase diagrams correlates with the efficiency (i.e., amphiphilicity) of the CiEj requires the study of the effect of C10G1 on mixtures containing different CiEj. Figure 2 shows the temperature-composition phase behavior for mixtures of water-octane-CiEj-C10G1 with δ ) 25 and R ) 50. The series of CiEj’s with C2j+2Ej (from ˜ C4E1 to C12E5) were chosen because for these homologs T remains almost constant while the amount of surfactant needed to completely solubilize equal masses of oil and water, γ˜ , decreases.32 The three-phase “chimney” exists in all mixtures studied, regardless of the CiEj amphiphilicity. The weaker the amphiphile, the higher the surfactant concentration (γ*) where the “chimney” region begins. For example, in the C4E1 mixture, the “chimney” region begins at γ* ) 30, while for the C10E4 mixture the “chimney” begins at γ* ) 8. The efficiency of the surfactant mixtures increases, compared to the pure CiEj case, with C4E1/C10G1 and C6E2/C10G1. The efficiency remains approximately constant in the C8E3/C10G1 mixture and decreases in the C10E4/C10G1 and C12E5/C10G1 mixtures (not shown). This implies that C10G1 is a more efficient amphiphile than either C4E1 or C6E2, is of comparable efficiency to C8E3, and is less efficient than both C10E4 and C12E5 for the mixtures studied. Also, the temperature interval of the “fish body” becomes larger as C10G1 is added, with the C4E1/C10G1 “fish body” showing the greatest effect. 3. As a function of CmG1 at r ) 50, δ ) 25. With the effect of CiEj amphiphilicity on the phase behavior known, we next examine the effect of CmG1 hydrophobicity in the mixture. Figure 3 shows the temperature-composition phase behavior for the water-octane-C6E2-CmG1 mixtures, with δ ) 25 and R ) 50. The water-octane-C6E2 “fish” (δ ) 0) is shown for reference. The series begins with C8G1 (top), and the surfactant is made more hydrophobic by increasing m in two-carbon increments to C10G1 (middle) (redrawn from Figure 2 (middle)) and C12G1 (bottom). Increasing the chain length of the hydrophobic portion of CmG1 has a qualitatively similar effect on the threephase body as observed with the previous mixtures. A “chimney” region exists in all mixtures and starts at lower γ* and narrows as m is increased. For example, in the C8G1/C6E2 mixture the “chimney” region begins at γ* ) 18, while for the C12G1/C6E2 mixture it begins at γ* ) 8. Also, the efficiency of the surfactant mixtures increases as m is increased (compared to the pure C6E2 case), showing that C12G1 is the most efficient of the CmG1’s studied. The C8G1/C6E2 mixture shows almost no change in efficiency, indicating that C8G1 and C6E2 have approximately the same efficiency with octane. Thirdly, with increasing m the phase behavior is moved to lower temperatures and the three-phase temperature interval decreases, a trend typically observed when a more efficient nonionic surfactant is used.4 For example, the temper-

range of the “chimney” region is a function of δ. For example, at δ ) 10, the “chimney” region emerges at γ*

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

Phase Behavior of Microemulsions

Figure 2. Vertical sections through the pseudoternary phase prism for water-octane-CiEj-C10G1 mixtures with δ ) 25 at equal mass fractions of water and octane (R ) 50) for C4E1 (top), C6E2 (center), and C10E4 (bottom). The liquid crystalline phases (LR) present at high surfactant concentrations in the C10E4 mixture have been omitted.

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Figure 3. Vertical sections through pseudoternary phase prism for water-octane-C6E2-CmG1 mixtures with δ ) 25 at equal mass fractions of water and octane (R ) 50) for C8G1 (top), C10G1 (center), and C12G1 (bottom).

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Figure 4. Vertical section through the pseudoternary phase prism for water-octane-C6E2-C10G1 at γ ) 5 and δ ) 25 for varying mass fractions of water and octane (R). The low oil concentrations (0 < R < 5) are shown in the inset. A dark, bold line on the R ) 100 boundary denotes the presence of a onephase region. Below the dark, bold line a gel-like, precipitate exists which extends to R ) 99 (shaded region).

ature interval for the C8G1/C6E2 “fish body” is very large (>30 °C), while that for the C12G1/C6E2 mixture is not much larger than that of the δ ) 0 case. The shrinking of the “chimney” as CmG1 hydrophobicity increases suggests that the CmG1 solubility in the oil might be an important factor in determining the origin and characteristics of this region. 4. As a Function of Temperature and r at Constant γ (the Channel Cut). Constructing a phase diagram at constant γ and δ as a function of R and temperature allows exploration of the solubilizing ability of the C6E2/C10G1 surfactant mixture. Figure 4 shows such a diagram for the water-octane-C6E2-C10G1 mixture at γ ) 5, δ ) 25. This surfactant concentration was chosen because it is located within the “chimney” region of the three-phase body. The phase diagram is dominated by a large three-phase region that extends from R ) 3 to R ) 95. The upper phase boundary of the three-phase body rises sharply in temperature between R ) 3 and R ) 10, passes through an inflection point between R ) 10 and R ) 30, then again increases sharply at R ) 30 to >80 °C at R ) 34. The upper phase boundary remains above 80 °C for all samples with R > 34. The lower phase boundary remains almost constant at 10 °C, with only a slight increase with temperature until R ) 85, when the boundary rises sharply to >80 °C at R ) 95. The lone point at R ) 100 shows the solubility of the surfactant mixture in pure octane with samples below 53 °C (and extending to R ) 99, shown by shaded region) containing a surfactant-rich, swollen precipitate and samples above 53 °C (dark line) containing a clear, isotropic mixture. This phase diagram provides a measure of the solubilizing power of the surfactant mixture. At R < 3 there is a small one-phase region at low temperatures (shown in the inset), indicating that only a small amount of octane can be solubilized by this surfactant mixture. The lack of a one-phase region in any temperature range at high R indicates that, similarly, essentially no water can be solubilized into octane with this surfactant mixture. The large, temperature-independent three-phase region of the phase diagram and the negligible solubilizing ability

Ryan et al.

Figure 5. Unfolded phase tetrahedron for the quaternary water-octane-C6E2-C10G1 mixture at 25 °C. The dashed line in the C10G1-C6E2-water ternary diagram is estimated from the data in Saito.19 The tie-lines shown are schematic.

of the surfactant at high R again indicate that the oil solubility of the surfactant mixture is an important component to the origin of the “chimney”, since the surfactant mixture never completely partitions into the oil (2). To further determine the origin of the “chimney”, the four component phase tetrahedron is constructed to show the effects of each component on the phase behavior. II. Phase Behavior at Constant Temperature (T ) 25 °C). 1. Phase Tetrahedron for Water-OctaneC6E2-C10G1. Understanding the phase behavior of the quarternary mixture and the origin of the “chimney” requires examination of the four constituent ternary mixtures. Figure 5 shows the unfolded phase tetrahedron at 25 °C for the water - octane - C6E2 - C10G1 mixture. (1) Water-Octane-C6E2 (Center). Figure 4 (center) shows that at 25 °C this mixture exhibits 2 (shown by the schematic tie-lines) behavior at R ) 50 because the threephase body is located at temperatures below 14 °C. Therefore, the Gibbs triangle is dominated by a large twophase region that originates at the water-C6E2 binary mixture (which has a Tβ of 11 °C) and extends toward the octane-rich corner (C6E2 and octane are miscible at 25 °C). (2) Water-C6E2-C10G1 (Left). This phase diagram shows a one-phase channel between two-phase regions. One two-phase region emerges from the miscibility gap on the water-C6E2 side and extends across the diagram to the miscibility gap on the water-C10G1 side. The two phases in this region are slow to separate. The second two-phase region is located toward the C10G1-rich corner and is comprised of turbid, birefringent, gel-like phases, because C10G1 forms a lamellar phase in water at high γ.19 The two surfactants show a mutual solubility of 22.5 wt % of C10G1 in C6E2. (3) Octane-C6E2-C10G1 (Right). The octane-C6E2C10G1 ternary phase diagram shows a one-phase region that begins at the C6E2-C10G1 side at 22.5 wt % C10G1 and extends to the octane corner. The rest of the diagram consists of a two-phase region containing turbid, gellike swollen precipitates, even at high octane concentrations. This behavior is not unexpected because C10G1 is nearly insoluble in octane.24 (4) Water-Octane-C10G1 (Bottom). The ternary phase diagram for the water-octane-C10G1 mixture shows a narrow one-phase region emerging from the water-C10G1

Phase Behavior of Microemulsions

Figure 6. Planes through the water-octane-C6E2-C10G1 phase tetrahedron at T ) 25 °C and constant wt % C6E2 with schematic sections shaded (top) and experimental data for 4% C6E2 (center) and 10% C6E2 (bottom). The dark, shaded sections in the experimental data show the three-phase body.

binary axis at concentrations above 10 wt % C10G1. The one-phase region extends to octane concentrations of only 10 wt %, even at relatively high C10G1 concentrations (50 wt %). At concentrations greater than 10 wt % octane there is a large two-phase region consisting of creamy, stable emulsions. Formation of emulsions, compounded with the low solubility of C10G1 in octane, created difficulties in determining a large portion of this phase diagram. The phase behavior of the quaternary mixture originates from the behavior of the four ternary systems. As a result, at high C10G1 concentrations turbid, gellike phases will form regardless of the nature of the other two components. When no C6E2 is present, the water-octaneC10G1 mixture is dominated by emulsions that obscure any underlying microemulsion behavior. Therefore, it is necessary to determine the effect of each surfactant on the phase behavior. This is best accomplished by keeping the concentration of one surfactant constant while varying the other three components in the mixture, thereby exploring a plane in the phase tetrahedron that is parallel to one of the ternary diagram edges. 2. Sections through the Phase Tetrahedron at Constant C6E2 Concentration. Figure 6 (top) shows a schematic of the phase tetrahedron for the water-octaneC6E2-C10G1 mixture, with the shaded areas denoting sections of constant C6E2 concentration. Observations were made of mixtures containing 4 and 10 wt % C6E2 at 25 °C (Figure 6 (middle) and Figure 6 (bottom), respectively). Since the three-phase region (dark, shaded area) was the main point of interest, measurements were made only for C10G1 concentrations below 30%. The weight fractions of water, octane, and C10G1 in this section are calculated on a C6E2-free basis in the mixture (i.e., wt % C10G1 ) mass C10G1 / (mass C10G1 + mass water + mass octane) ×100). When the C6E2 concentration is held constant at 4 wt % (Figure 6 (middle)) and the mass fractions of the other

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three components are varied, a long, narrow, three-phase region emerges at low C10G1 concentration (99 wt %) with the data for 25 °C shown for comparison.

5. Octane-C6E2-C10G1 Ternary Diagram at T ) 80 °C. In addition to the previous planes through the phase tetrahedron, the ternary phase diagram for the octane-C6E2-C10G1 mixture at 80 °C is especially useful for the discussion, and Figure 9 shows this diagram for octane concentrations greater than 99 wt %. The ternary data for 25 °C are included for comparison from Figure 5. At 80 °C, the extension of the two-phase region (consisting of gellike, turbid phases) into the triangle from the C10G1-octane side has not substantially changed from the 25 °C case at these high oil concentrations. This diagram also shows that the solubility of C10G1 in octane at 80 °C (∼0.055 wt %) is not greatly enhanced by the presence of C6E2 when octane is the major component in the mixture.

Phase Behavior of Microemulsions

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Figure 10. Vertical section through the pseudoternary phase prism for the water-octane-C6E2-C12E8 mixture with δ ) 25 at equal mass fractions of water and octane (R ) 50).

Discussion The most striking experimental observation is that adding CmG1 to water-alkane-CiEj mixtures promotes the formation of a nearly temperature-independent region (“chimney”) in the three-phase body at low surfactant concentrations. Increasing the amount of C10G1 in the mixture increases the width of the “chimney” (Figure 1). As the CmG1 hydrophobicity increases (by increasing m), the “chimney” narrows (Figure 3). As CiEj amphiphilicity increases (with increasing i and j) the “chimney” narrows, but with a parallel shrinkage of the whole three-phase body (Figure 2). Observations of samples with constant γ as a function of R shows that much of the phase space is also nearly independent of temperature (Figure 4). These phase diagrams show that the presence of CmG1 in the mixture prevents the 2-3-2 h phase sequence always seen as a function of increasing temperature in wateroctane-CiEj mixtures. This failure to reach 2 in the “chimney” is a unique feature of this phase behavior. The complexities uncovered by the addition of the nonionic alkyl monoglucoside surfactants (CmG1) to the water-octane-CiEj mixtures are usefully compared to the known phase behavior of quaternary mixtures of water, octane, and two nonionic alkyl polyglycol ether surfactants (CiEj and CiEj*).33,34 For example, for a mixture of water-octane-C6E2-C12E8 (Figure 10) at δ ) 25 and R ) 50, the three-phase body is tilted to high temperatures at low surfactant concentration. This behavior, while in some sense similar to the phase behavior seen with CmG1CiEj mixtures, does not show the sharp, temperatureindependent phase boundaries seen in the glucoside mixtures. Also, the 2 region is eventually reached at high temperatures (∼66 °C) at all surfactant concentrations, indicating that both surfactants have partitioned into the octane, which is also not seen in the mixtures containing CmG1. The low Tβ and TR effective in the water-octaneC6E2 mixture make C6E2 partition into the octane at a low temperature (∼14 °C per Figure 1). Once C6E2 partitions into the octane, it acts as a “co-oil” that increases the effective hydrophilicity of the octane and influences the partitioning of the C12E8 into the “oil”, moving the threephase region to lower temperatures. As the surfactant concentration decreases, less C6E2 is available to act as (33) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107-121. (34) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci. 1995, 170, 78-84.

Figure 11. Schematic of a section through the prism for the water-octane-CiEj mixture (top) at high temperatures showing the central miscibility gap (2). Replacing CiEj with CmG1 produces an immiscibility in the octane-CiEj “oil”-rich corner which leads to the three-phase body formation (bottom). The inset shows the possible scenarios for the two-phase region present in the “oil” rich corner for T > TR and T < TR.

a “co-oil” and the “oil” behaves more like octane. Therefore, the three-phase region moves upward on the temperature scale, closer to the location of the water-octane-C12E8 three-phase body (not shown). This continuous change in the “oil” hydrophilicity as surfactant concentration decreases produces the tilted three-phase body. As is the case for the water-octane-C6E2-C12E8 mixture, the phase behavior of the quaternary wateroctane-CiEj-CmG1 mixtures can be explained through the analysis of the constituent ternary mixtures and the corresponding binary water-surfactant and octanesurfactant pairs. In water-CmG1 mixtures for m g 10, Tβ is located below 0 °C, while for m ) 8, Tβ is above 80 °C.24 For octane-CmG1 mixtures, regardless of the value of m, TR is located above 80 °C and CmG1 are essentially insoluble in octane.24 Since the phase behavior of the wateroctane-C6E2-CmG1 mixtures remains qualitatively similar as m is varied (Figure 3), even though Tβ changes drastically over the same range of m, we conclude that the value of Tβ must play a minor role in setting the phase behavior of these mixtures. On the other hand, the universal insolubility of CmG1 in octane and the corresponding high values of TR, coupled with the similarities in the quarternary phase behavior for water-octaneCiEj-CmG1 mixtures for various m values, show that TR and the solubility of CmG1 in oil have a major influence on the phase behavior. The effect of CmG1 solubility in oil on the phase behavior is best explained through examination of the schematic diagrams shown in Figure 11. In Figure 11 (top) is the usual central (oil-water) miscibility gap (2) for a CiEj surfactant with water and oil at a temperature well above

1518 Langmuir, Vol. 13, No. 6, 1997

both Tβ and the temperature where the upper critical tieline exists. For CiEj’s, TR is usually low and the miscibility gap on the surfactant-oil rich side of the triangle disappears at low temperatures, closing the three-phase body at the upper critical tie-line.35 In contrast, TR is extremely high in CmG1-alkane mixtures, and as shown in Figure 9, the presence of C6E2 as a “co-oil” does not appreciably change the C10G1 solubility in the “oil” at high octane concentrations, even at elevated temperatures. Also, Figure 4 shows that at low surfactant concentrations, even when the surfactant mixture is soluble in octane (above 53 °C), only a minute amount of water (