Langmuir 2004, 20, 2607-2613
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Cubic Phase Prepared in an Anionic/Amphoteric Surfactant/Oleic Acid/Decane/Water System and the Relationship with the Neighboring Phase Kei Watanabe,*,†,‡ Yasunari Nakama,† Toshio Yanaki,† Christine Thunig,‡ Klaus Horbachek,‡ and Heinz Hoffmann‡ Shiseido Co. Ltd., Research Center (Shin-yokohama), 2-2-1 Hayabuchi, Tsuzuki-ku, Yokohama, Japan, and University of Bayreuth, Physical Chemistry I, D-95440 Bayreuth, Germany Received August 29, 2003. In Final Form: January 12, 2004 The formation, properties, and structure of discontinuous cubic phase in the pseudo-ternary system consisting of N′-carboxyethyl N′-hydroxyethyl N-aminoethyl dodecylamide (imidazoriniumbetain), sodium and triethanol amine salt of polyoxyethylene (1.5 mol) myristyl ether sulfate, oleic acid, decane, and water at a constant surfactant/water ratio of 4/6 were studied by means of small-angle X-ray scattering, freezefracture transmission electron microscopy, static light scattering, and dynamic rheology to gain an insight in its origin and interrelation with neighboring phases. It was found that the cubic phase occupied a rather wide region in a constructed ternary phase diagram, including from 25 to 45% of decane. Its properties and structural parameters varied with changing the oil content. The decane addition caused the swelling of spherical micellar aggregates. This resulted in an increase of their diameter up to 35 nm, which was ca. nine times larger than that of the initial micelles, and micellar volume fraction (packing fraction) up to 72 vol. %, which was close to the theoretically possible value of 74 vol. % for the close-packed spherical particles. The cubic phase was surrounded by a micellar L1 phase from the water-rich side (separated by a short two-phase region), two-phase region (cubic + oil) from the oil-rich side, and a lamellar phase from the surfactant-rich side. A transition from the L1 phase to the cubic state at the packing fraction of 60 vol. % was caused by an increase in the packing density of micellar aggregates, occurring with the decane addition. When it reached 72 vol. %, the oil started forming a separated phase owing to the inability of micelles to dissolve it. The important observation is that the adjacent phase from the surfactant-rich side was a lamellar one made up of flat bilayers. The preliminary data showed that the lamellar phase coexisted with cylindrical micelles in the intermediate two-phase region separating the cubic and lamellar phases.
* To whom correspondence may be addressed at the Basic Research Center, Shiseido Co. † Shiseido Co. Ltd. ‡ University of Bayreuth.
interfacial activity of mixed surfactants exceeded that of single surfactant systems. The synergetic effects are often obvious in surface or interfacial tension and viscosity of the solutions at certain mixing ratios.25 Their explanation is usually based on the idea that the headgroups of mixed surfactants can be packed more densely than ones of the same surfactant systems. Our previous study13 on an anionic-amphoteric system containing sodium and triethanol amine salt of polyoxyethylene (1.5 mol) myristyl ether sulfate (PMST)/N′carboxyethyl N′-hydroxyethyl N-aminoethyl dodecylamide [imidazoriniumbetain (IB)] showed that these surfactants in aqueous solutions have the strongest interactions with
(1) Clint J. H. In The structure, Dynamics and Equilibrium Properties of Colloid Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; Vol. 324, p 71. (2) Bucci, S.; Fagotti, C.; Degeorgio, V.; Piazza, R. Langmuir 1991, 7, 824. (3) Guering, P.; Nilsson, PG.; Lindman, B. J. Colloid Interface Sci. 1985, 105, 41. (4) Ogino, K.; Kato, K.; Abe, M. J. Am. Oil Chem. Soc. 1988, 65, 272. (5) Tsujii, K.; Okahashi, K.; Takeuchi, T. J. Phys. Chem. 1982, 86, 1437. (6) Hofmann, S.; Rauscher, A.; Hoffmann, H. Ber Bunsen-Ges. Phys. Chem. 1991, 95, 135. (7) Hoffmann, H.; Hofmann, S.; Illner, J. C. Prog. Colloid Polym. Sci. 1994, 97, 103. (8) Weer, J. G.; Rathman, J. F.; Scheuing, D. R. Colloid Polym. Sci. 1990, 268, 832. (9) Hoffmann, H.; Oetter, G.; Schwandner, B. Prog. Colloid Polym. Sci. 1987, 73, 95. (10) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972. (11) Kekicheff, P.; Mandelmont, G. G. J. Colloid Interface Sci. 1989, 131 , 1. (12) Watanabe, K.; Nakama, Y.; Yanaki, T.; Hoffmann, H. Langmuir 2001, 17, 7219.
(13) Mehreteab, A.; Loprest, F. J. J. Colloid Interface Sci. 1988, 125, 602. (14) Nakama, Y.; Harusawa, F.; Murotani, I. J. Am. Oil Chem. Soc. 1990, 67, 717. (15) Kalar, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698. (16) Herrington, K. L.; Kalar, E. W.; Miller, D. D.; Zasadzinski, J. A. N.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (17) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kalar, E. W.; Chiruvolu, S.; Zasadzinski, J. A. N. J. Phys. Chem. 1996, 100, 5874. (18) Soderman, O.; Herrington, K. L.; Kalar, E. W.; Miller, D. D. Langmuir 1997, 13, 5531. (19) Villeneuve, M.; Kaneshina, S.; Imae, T.; Aratono, M. Langmuir 1999, 15, 2029. (20) Safran, S. A.; MacKintosh, F. C.; Pincus, P. A.; Andelman, D. A. Prog. Colloid Polym. Sci. 1991, 84, 3. (21) Kaler, E. W.; Murthy, A. K.; Rodrigues, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (22) Safran, S. A.; Pincus, P. A.; Andelman, D. A.; MacKintosh, F. C. Phys. Rev. A 1991, 43, 1071. (23) Bergstroem, M.; Eriksson, J. C. Langmuir 1996, 12, 624. (24) Bergstroem, M. Langmuir 1996, 12, 2454. (25) Rosen, M. J. Langmuir 1991, 7, 885.
1. Introduction It is common practice to prepare formulations for various applications by mixing surfactants. It allows revealing synergetic effects, such as increased oil solubilization, foaming, emulsification, washing abilities, and desired structural organizations in mixed systems.1 Recent studies on nonionic-anionic,2,3 anionic-amphoteric,4-12 and anionic-cationic13-24 mixtures have shown that the
10.1021/la030346y CCC: $27.50 © 2004 American Chemical Society Published on Web 03/06/2004
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each other at the IB/PMST ratio of 7/3. By using systems at this ratio, the effect of addition of oleic acid (OA) on the properties was investigated. Upon the addition of OA to the spherical aqueous micellar solution of IB/PMST, a phase sequence of vesicle phase (LRl) to lamellar liquid crystalline phase (LRh) with flat bilayers to sponge phase (L3) took place. We also reported the vesicle one-phase region extending to very low mixed surfactant concentrations (75%) stabilized by the cubic phase was recently reported.40-42 The cubic phase is also known for a unique characteristic feature, a ringing sound, which is a reflection (26) Winsor, P. A. Chem. Rev. 1968, 68, 1. (27) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Surfactant Science Series Vol. 30; Marcel Dekker: New York, 1988. (28) Tardieu, A.; Luzzati, V. Biochim. Biophys. Acta 1970, 219, 11. (29) Larsson, K.; Fontell, K.; Krog, N. Chem. Phys. Lipids 1980, 27, 321. (30) Larsson, K. J. Phys. Chem. 1989, 93, 7301. (31) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (32) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831. (33) Oetter, G.; Hoffmann, H Colloids Surf. 1989, 38, 225. (34) Fontell, K. Colloid Polym. Sci. 1990, 268, 264. (35) Gradzielski, M.; Hoffmann, H.; Panitz, J. C.; Wokaun, A. J. Colloid Interface Sci. 1995, 103, 169. (36) Ekwall, P.; Mandell, L.; Fontell, K.. J. Colloid Interface Sci. 1970, 33, 215. (37) Nuernberg, E.; Pohler, W. Prog. Colloid Polym. Sci. 1984, 69, 48. (38) Jousma, H.; Joosten, J. G. H.; Gooris, G. S.; Junginger, H. E. Colloid Polym. Sci. 1989, 267, 353. (39) Li, X.; Ueda, K.; Kunieda, H. Langmuir 2000, 16, 10092. (40) Uddin, M. H.; Kanei, N.; Kunieda, H. Langmuir 2000, 16, 6891. (41) Uddin, M. H.; Rodriguez, C.; Watanabe, K.; Lopez-Quintela, A.; Kato, T.; Furukawa, H.; Harashima, A.; Kunieda, H. Langmuir 2001, 17, 5169.
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of the elastic property. The sound occurs when a sample of the phase is tapped with a soft object.43,44 Additionally, quite a new type of cubic phase that consists of cubical packed vesicles was found in an octanol/sodium oleate/ water system recently.45 In a ternary phase diagram, the cubic phase with densely packed aqueous micelles generally exists in a water-rich region. The first cubic phase that was reported as “viscous isotropic”46 is located in the phase diagram bordered by an aqueous micellar solution phase on the water-rich side and by a hexagonal liquid crystalline phase on the water-poor side. A considerable number of cubic phases are found in this position.34 Cubic phases with various locations in phase diagrams have also been of interest, especially with respect to the correlation between their structure and the structure of neighboring phases.35,48,49 Such a study was reported on phase diagrams of a sodium sulfosuccinic acid bis-2-ethylhexyl ester (AOT)/ 1-octanol/water system35 and di(alkyl)dimethylammonium bromide/hydrocarbon/water system48 in which cubic phases are located adjacent to the reversed micellar solution region. This article considers a pseudoternary system instead of the previously investigated binary one, IB/PMST/OA/ water. It is shown that the addition of nonpolar solvent provided the formation of a discontinuous cubic phase. The aim was to elucidate conditions for its formation, relation with the neighboring phases, structure, and properties. 2. Materials and Methods 2.1. Materials. The anionic surfactant solution of sodium and triethanol amine salt of polyoxyethylene myristyl ether sulfate (PMST) aqueous solution (30%) was purchased from Toho Chemical Industry (Tokyo, Japan). The average moiety of ethylene oxide group in PMST was 1.5 mol. The weight ratio of the sodium salt and the triethanolamine salt is 7:3. An aqueous solution (30%) of amphoteric surfactant, N′-hydroxyethyl N′-carboxyethyl N-aminoethyl dodecylamide [imidazoriniumbetain (IB)] was also purchased from Toho Chemical Industry. It contained 1.1% NaCl. Oleic acid (OA) (ca. 99% purity) was delivered from NOF (Tokyo). Decane was purchased from Tokyo Kasei (Tokyo). These materials were used without further purification. To prepare samples, the surfactant solutions were concentrated by freeze-drying. The water was purified by an ion-exchange water purification system. 2.2. Methods. 2.2.1. Phase Diagram Determination. Various amounts of constituents were placed into test tubes and mixed using a vortex mixer. The samples were stored in a water bath at 25 °C. Their phase equilibria were checked by visual observation. The type of liquid crystalline phase was determined by a polarity microscope. The following abbreviations are used for the phases: L1, aqueous micellar solution; LR, lamellar liquid crystalline (42) Kunieda, H.; Tanimoto, M.; Shigeta, K.; Rodriguez, C. J. Oleo Sci. 2001, 50, 633. (43) Hoffmann, H.; Paulus, E. F. Fette, Seifen, Anstrichm. 1969, 71, 399. (44) Gradzielski, M.; Hoffmann, H.; Oetter, G. Colloid Polym. Sci. 1990, 268, 167. (45) Gradzielski, M.; Bergmeier, M.; Mueller, M.; Hoffmann, H. J. Phys. Chem. B 1997, 101, 1719. (46) Luzzati, V.; Mustacchi, H.; Skoulios, A. Discuss. Faraday Soc. 1958, 25, 43. (47) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (48) Fontell, K.; Jansson, M. Prog. Colloid Polym. Sci. 1988, 76, 169. (49) Tansho, M.; Imae, T.; Tanaka, S.; Ohki, H.; Ikeda, R.; Suzuki, S. Colloids Surf., B 1996, 7, 281.
Cubic Phases
Langmuir, Vol. 20, No. 7, 2004 2609 Table 1. Chemical Structure of Materials Used in This System material
chemical structure
N′-carboxyethyl N′-hydroxyethyl N-aminoethyl dodecylamide (imidazoriumbetain) sodium and triethanolamine salts of polyoxyethylene (1.5 mol) myristyl ether sulfate oleic acid
C11H23CONHCH2CH2N(CH2CH2OH)(CH2CH2COONa)
IB
C13H27(C2H4O)1.5SO3Na0.7[NH(CH2CH2OH)3]0.3
PMST
CH3(CH2)7CHdCH(CH2)7COOH
OA
phase; I1, aqueous discontinuous cubic liquid crystalline phase. An abbreviation LC is used for a multiphase region with an unidentified liquid crystalline phase. 2.2.2. Small-Angle X-ray Scattering (SAXS). Interlayer spacing measurements on a cubic liquid crystalline phase were performed by a Bruker AXS Nanostar (Bruker, Germany) equipped with two-dimensional detector. 2.2.3. Freeze-Fracture Transmission Electron Microscopy (FF-TEM). The observations50 were performed by using replicas of samples with a CEM 902 electron microscope (Zeiss, Oberkochen, Germany). For the preparation of replication, a small amount of sample was placed on a 0.1 mm thick copper disk covered by another copper disk. It was plunged into liquid propane that was precooled by liquid nitrogen. The frozen sample was fractured and replicated in a freeze-fracture apparatus, BAF 400 (BalTec, Balzer, Liechtenstein) at -140 °C. The Pt/C layer was deposited at an angle of 45°. 2.2.4. Static Light Scattering. The effective molecular weight of micelles in a L1 phase was determined with a low-angle light scattering photometer, KMX-6 (Chromatix Inc., Sunnyvale, CA). The radius of the spherical micelle and the length of the rodlike ones were also calculated. 2.2.5. Rheological Measurements. They were carried out on the cubic phase by using a Bohlin CS 10 rheometer (Pforzheim, Germany). The viscoelastic properties were determined from oscillatory measurements at a frequency that varied from 0.001 to 10 Hz. The stress or stress control regime was alternatively used. 3. Results and Discussion 3.1. Phase Diagram of Pseudoternary Decane/IB/ PMST/OA/Water System. This study was restricted to a discontinuous cubic phase produced from the micellar solution of mixed surfactants after the decane addition. As shown in our previous article,12 the OA introduced into the IB/PMST mixture with optimal ratio of 7/3 resulted in a shift of hydrophilic-lipophilic balance (HLB) into the lipophilic side. Consequently, the phase sequence of micellar solution (L1) to lamellar liquid crystalline phase (LR) to sponge phase (L3) was observed. Therefore, the ratio of IB/PMST/OA was fixed to be 7/3/1 because of the L1 to LR phase transition at the IB/PMST/OA ratio of 7/3/ 1.7. The phase diagram of pseudoternary decane/IB/PMST/ OA/water system is shown in Figure 1. As seen, the L1 phase exists around the water corner, extending to the surfactant corner. It is followed by a two-phase region coexisting with the vesicle phase, vesicle one-phase, and solid locating in the surfactant-rich region. The most notable point is a narrow channel extending toward the center of the phase diagram that presents a cubic phase produced in the course of a decane solubilization by the L1 phase. The cubic phase region has a large extension with respect to the content of decane, from 25% to 45% at an approximately constant surfactant/water ratio of 4/6. It was optically isotropic and completely transparent. It was as a stiff gel that demonstrated characteristic ringing property. The cubic phase obtained in this system (50) Imae, T. J. Jpn. Oil Chem. Soc. 1996, 45, 1115.
abbreviation
consists of discontinuous micelles (water continuous) having the positive curvature of a surfactant layer; i.e., its convex is toward the water. There is a lamellar phase with flat bilayers in the neighborhood of the cubic phase. They should be separated by a two-phase region, but it was unable to determine the phase boundary for the lamellar phase region owing to its high viscosity. All the phase boundaries, which could be inadequately specified by us, were drawn by dashed lines. In the oil-rich region, there are two-phase regions in which the cubic or the lamellar phase coexists with oil phase. Though the most usually observed position for the discontinuous micellar cubic phase was between the L1 and hexagonal liquid crystalline phases,34 it was unable to find a hexagonal phase in the considered system. 3.2. Characterization through the Cubic Phase Region. The cubic phase extends from 25 to 45% of the decane in the system (Figure 1). To characterize its structure and properties in more detail, a series of samples with constant water/surfactant ratio of 6/4 was studied by means of SAXS, phase transition temperature measurement, FF-TEM, and dynamic rheology. The typical small-angle X-ray scattering chart is shown in Figure 2. There are two peaks at 2θ ) 0.38 (d ) 23 nm) and 2θ ) 0.63 (d ) 14 nm). The ratio of the d values is equal to 1/1.7, which is an indication of a structure formed from the body- or face-centered cubical packed array (first and third order). An effect of the concentration of decane on the d value, which characterizes the micellar dimension, is shown in Figure 3. The experimental data are presented by points. Micelles in the presence of the smallest amount of decane
Figure 1. Pseudo-ternary phase diagram for decane/IB/PMST/ OA/water system at 25 °C. The mixture of IB/PMST/OA was regarded as one component. The abbreviations of L1, LR, I1, and O represent a micellar solution phase, a lamellar liquid crystalline phase, a discontinuous micellar cubic phase, and a excess oil phase, respectively. The subscripts of lamellar liquid crystalline phases, l and h, represent vesicle and classic lamellar with flat bilayer.
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Watanabe et al.
Figure 2. Typical small-angle X-ray scattering pattern on the cubic one-phase region. The sample contains 28.5% of total surfactant, 30% decane, and 41.5% water. Two peaks of 2θ ) 0.38 (d ) 23 nm) and 2θ ) 0.63 (d ) 14 nm) could be observed corresponding to the structure of body- or face-centered cubical packed array (first and third order).
Figure 3. The effect of the concentration of decane on the diameter of the micelles in a series of cubic phase samples for constant surfactant/water ratio of 4/6. Measured values by SAXS are represented by open circles. The calculated diameter from the concentration and the density of each ingredient by the following equation is represented by a line. R = 3l(1 + Voil/ VSAA), where l is length of hydrophobic group of the surfactant mixture that is assumed to be 2 nm. Voil and VSAA are volume fraction of decane and the surfactant mixture, respectively. Density of the surfactant layer is assumed to be 0.9 g/cm. Dashed lines represent the phase boundaries between the two-phase regions.
have a diameter of slightly over 20 nm. An increase of its concentration resulted in micellar swelling. Their diameter was increased up to 35 nm. It is worth mentioning that it is approximately 9 times larger than the diameter of micelles without the added oil (ca. 4 nm as shown below). The line in Figure 3 represents the theoretical d values calculated by the following equation
R = 3l(1 + Voil/VSAA)
(1)
where l is the length of hydrocarbon chains of surfactants in the mixture that was taken to be 2 nm, Voil and VSAA are the volume fractions of decane and the surfactant mixture, respectively. The density of the surfactant layer was taken to be 0.9 g/cm. As can be seen, the theoretical
Figure 4. Phase transition temperature from the cubic onephase to a two-phase region in which lamellar phases coexist. The temperature of a series of cubic phase samples for constant surfactant/water ratio of 4/6 was determined by checking the anisotropy with cross polarizers. The vertical dashed lines represent the phase boundaries between the two-phase regions at 25 °C.
d values are in good agreement with experimentally measured ones. An effect of the amount of solubilized decane on the temperature range of the cubic one-phase existence is shown in Figure 4. A phase separation accompanied by turning an optically isotropic solution into the anisotropic ones was observed with raising the temperature. This is caused by a formation of a lamellar LR phase that coexists with the cubic one in the two-phase region. The addition of decane resulted in an increase of the phase transition temperature. The reason for that is a change of HLB in the formulation. A similar temperature rise, but of the cloud point with the hydrocarbon oil solubilization, was normally observed for systems containing nonionic surfactants. The effect was related to the swelling of micelles. This changes the curvature of interfacial boundary in such a manner that hydrophilic groups of surfactants approach closely to each other.44 This could also explain the extension of the cubic phase region toward higher temperatures in the considered system. It is of interest to consider how the volume fraction occupied by the micellar aggregates in the cubic phase (packing fraction) varied with the amount of added decane in the system. It can be calculated on the basis of density of each ingredient. Results are presented in Figure 5. In the case of the smallest amount of decane, the micelles, when taken that they include all the oil and surfactants, occupy 60% of the total volume of the phase. With increase of the concentration of decane, the micellar packing fraction is increased. As calculated, it reaches 72 vol. % in the vicinity of the phase boundary separating the twophase (I1 + O) region. It is well-known that the maximum packing fraction achievable in the close-packed structures like body- or face-centered cubic can be equal to 74 vol. %. The fact that the calculated value is close to the maximum possible one is indicative of fairly well structural organization of considered cubic phase that solubilized decane as much as theoretically possible. Rheological properties of the cubic phase are presented in Figure 6 as frequency dependencies of the storage modulus G′, loss modulus G′′, and complex viscosity η*. The main peculiarity of the rheogram is that the storage modulus, whose value is nearly undependable on the
Cubic Phases
Figure 5. Calculated volume fraction of the micelles in the total volume of the cubic phase for constant surfactant/water ratio of 4/6. Density of the surfactant layer is assumed to be 0.9 g/cm. The vertical dashed lines represent the phase boundaries between the two-phase regions at 25 °C.
Langmuir, Vol. 20, No. 7, 2004 2611
Figure 7. Freeze-fracture electron micrograph of a sample containing 32.5% decane, 40.5% water, and 27% surfactant mixture. Volume fraction of the micelle in this sample was calculated to be 65%. Table 2. Values for the Effective Molecular Weight Mw (104 g mol-1), Length L (Å), and Radius R (Å) (A) for Various Concentrations of Total Surfactant in Aqueous Solution and (B) for Concentration of Added Decane to 8.8% IB/PMST/OA Aqueous Solution at 25 °C (A) IB/PMST/OA aqueous solution + decane
Figure 6. Rheological measurement of the typical cubic phase sample containing 32.5% decane, 40.5% water, and 27% surfactant mixture. Magnitude of complex viscosity η* (0), storage modulus G′ (O), and loss modulus G′′ (/) are expressed as a function of angular frequency ω.
frequency, is higher than the loss one by about a factor of 10. The complex viscosity decreased monotonically with the frequency. These rheological features are characteristic of the system being in the gel state. They were found in the whole region of cubic phase. A micrograph of the sample containing 32.5% decane, 40.5% water, and 27% surfactant mixture is obtained by FF-TEM observation (Figure 7). One can see densely packed spherical micelles. They are the main structural element of cubic phase. The size of micelles determined from the TEM micrograph is approximately 25 nm. This is in good agreement with the d value of 24.5 nm determined from the SAXS measurements (Figures 2 and 3). 3.3. Characterization through the L1 Phase. This phase lies in the water-rich side (Figure 1). The micelles were characterized by means of a light scattering technique. The results thus obtained are summarized in Table 2. The initial micellar system without decane consisted of aggregates the shape of which depended on the concentration of surfactants. When their total amount did not
CSAA
Mw
1.0 2.0 4.0 6.0 9.0
2.84 2.44 3.83 4.86 4.44
La
69.7 84.9 78.6
(B) 8.8% IB/PMST/OA aqueous solution
Rb
Coil
Mw
L
23.2 22.1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.0 1.2 1.38
4.39 4.49 4.47 4.42 4.33 4.02 3.43 3.03 2.58 2.20 2.22 2.25
77.9 79.3 78.9 78.6 77.7 72.8 63.0 56.5
R
22.1 21.3 21.4 21.5
a Length (Å), calculated from M assuming cylindrical aggregates w with density of 0.9 g cm-3 and a short radius of 20 Å. b Radius (Å), also calculated from Mw assuming spherical aggregates with density of 0.9 g cm-3.
exceed 2%, the micelles were spherical. Their diameter was around 2 nm and molecular weight, 30 000. A molecular weight increase was observed at the surfactant concentration above 2%. This was indication of a growth of the aggregation number that was caused by a transformation of globular micelles into rodlike ones. Their length was varied from 7.0 to 8.5 nm. The effect of decane on the rodlike micelle was examined at total surfactant concentration equal to 9%. A steplike decrease in the molecular weight was found. It occurs, as seen from the results in Table 2, in the range of 0.5-0.8% of added decane. The molecular weight and correspondingly the aggregation number of the micelles containing decane were around a half of the initial ones before the oil addition. It was also found that the introduced decane resulted in a decrease of the viscosity of solutions (Figure 8). The observed effect correlates with the change in the aggregation number (Table 2). These data make it obvious that the solubilization of decane inside the micelles promotes
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Figure 8. The effect of solubilized decane on the viscosity of the L1. Decane was solubilized with micellar solution containing the total surfactant 9% that form short rod micelles in the absent of decane.
Figure 9. Freeze-fracture electron micrograph of a L1 phase sample containing 30% surfactant mixture, 17.5% decane, and 52.5% water. The sample is on the very narrow L1 phase channel leading to the cubic one-phase region. Their arrangement is already quite dense and leads to a viscous solution. The volume fraction that the micelles occupy (packing fraction) on this sample was calculated to be 54%.
a rod-to-globular transformation. This is in line with observations of work47 in which the effect of various oils on the micellar shape was studied in detail. The structure of the L1 phase was characterized by the FF-TEM in the narrow channel being in the vicinity of the cubic phase. Figure 9 shows a micrograph taken for a solution containing 30% of the total surfactants, 52.5% water, and 17.5% decane. One can see spherical micelles of which the diameter is in the range of 10-15 nm. They are rather closely packed, which accounts for an increased viscosity of the solution. The volume fraction of micelles (packing fraction) was found to be 54 vol. %. This provides an explanation for jellification of solution, phase transition to the two-phase region, and then to the cubic one-phase region with the further addition of decane. 3.4. Characterization through the Two-Phase (I1 + Oil) Region. This region occurs on the oil-rich side with relation to the cubic phase (Figure 1). Figure 10 demonstrates an effect of decane concentration on the weight fraction of cubic phase and calculated packing fraction of micelle in it. Obviously, with increasing the concentration of decane toward the oil corner, the former decreases monotonically from 100% to 0, while the latter remains constant. It is worthy of mentioning that the packing fraction value is close to the maximum possible
Watanabe et al.
Figure 10. The effect of decane concentration on the phase behavior in the two-phase region in which the cubic phase coexists with excess oil phase. The weight fraction of the cubic phase and the packing fraction of micelle in the cubic phase calculated with the measured values are represented by open circles (O) and closed circles (b), respectively. Surfactant/water ratio is 4/6. The dashed line represents the phase boundaries between the cubic one-phase regions.
Figure 11. Freeze-fracture electron micrograph of a sample containing 27.5% of surfactant mixture, 40% decane, and 32.5% water. The sample is in the two-phase region in which lamellar phase coexists with the cubic phase. Around the bottom center and middle center of the micrograph, stacked bilayers of large multilamellar vesicles followed by network-like arrangements of micelles were observed.
one (74%). This fact means that the micelles in the cubic phase retained their closest packing even in the two-phase region. It enables us to conclude that the phase separation is due to the reaching of the solubilization limit of micelles. 3.5. Characterization through the Two-Phase (I1 + LR) Region. This two-phase region is located in the surfactant-rich side, representing a mixture of coexisting cubic and lamellar phases (Figure 1). The presence of the latter was confirmed by means of the FF-TEM. One can see large multilamellar vesicles in a micrograph given in Figure 11. The intervening space between them is filled by densely packed micelles. The micellar diameter ranged from 20 to 30 nm. This value is close to that found from the SAXS measurements and FF-TEM micrographs for the cubic phase being in the immediate neighborhood to this region. It is worth mentioning that there are also cylindrical micelles of various length. This observation means that the variously arranged micelles coexist with bimolecular structures. There is a transition from one structural organization into another. This opportunity, as justified
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Langmuir, Vol. 20, No. 7, 2004 2613
in a review article,51 is quite possible. It allows us suggesting a model of a phase transition between a cubic phase and a lamellar phase in the considered system. 4. Conclusion The presented results on the formation, properties, and structure of discontinuous cubic phase existing in the pseudo-ternary decane/IB/PMST/OA/water system provided an insight in its origin and interrelation with neighboring phases. As shown by means of FF-TEM, SAXS, static light scattering, and rheology, the cubic phase was formed after the decane addition owing to the swelling of micelles. This caused an increase of micellar volume fraction (packing fraction), resulting in a transition into the cubic state at 60 vol. %. In the constructed phase diagram, the cubic phase was found in a rather wide area containing from 25 to 45% decane. The packing fraction of micellar aggregates was increased up to 72 vol. %, reaching the theoretically possible value of 74 vol. % for the close-packed structures such as body- or face-centered cubic phase, whereupon a (51) Bouligand, Y. Liq. Cryst. 1999, 26, 501.
decane separation was observed owing to the inability of micelles to dissolve further the oil. It was mentioned that the cubic phase is interrelated with neighboring phases. One of them is a L1 phase representing a narrow channel in the phase diagram that extended from the water corner to the cubic phase. As revealed, a close correlation between both the phases is in the tight packing of swollen micelles that was increased with growing the amount of decane. It was found that the intermediate two-phase region separated the cubic and lamellar phases contained micellar and bimolecular structures. The presented preliminary data demonstrated that the lamellar phase coexisted with cylindrical micelles. Acknowledgment. The authors are grateful to Dr. M. Yamaguchi, Basic Research Division, Shiseido Co., for encouraging this study and for providing useful suggestions and sincerely thank Dr. H. Nakajima and Dr. N. Ueno for fruitful discussion about the results and Mr. S. Stangler and Mr. K. Lauterbach for performing SAXS measurement and static light scattering measurement, respectively. LA030346Y
