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Langmuir 1999, 15, 3118-3122
Phase Behavior and Formation of Reverse Vesicles in Long-Polyoxyethylene-Chain Nonionic Surfactant Systems Hironobu Kunieda,*,† Kazuki Shigeta,† and Masao Suzuki‡ Division of Artificial Environment Systems, Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan, and Oleochemical Research Laboratory, NOF Corporation, Ohama-cho 1-56, Amagasaki 660-0083, Japan Received October 22, 1998. In Final Form: February 8, 1999
The phase diagram of a water/polyoxyethylene(50.8) oleyl ether (C18:1EO50.8)/m-xylene system was constructed at 25 °C. In the binary water/C18:1EO50.8 system, an aqueous micellar (Wm) phase and a discontinuous cubic (I1) phase form. In the relatively dilute region, an excess oil phase separates from these phases upon addition of m-xylene. In the concentrated region, however, the I1 phase changes to a lamellar (LR) phase via hexagonal (H1) and bicontinuous cubic (V1) phases with increasing oil content. According to the effective cross-sectional area of one surfactant molecule as in the liquid crystals calculated from the small-angle X-ray scattering (SAXS) data, the curvature of the surfactant molecular layer remains positive in the dilute region although m-xylene molecules penetrate into the surfactant palisade layer. The penetration effect of m-xylene is large and the I1-H1-V1-LR phase transition takes place in the concentrated region, because the hydration of the EO chain decreases. However, the negative surfactant curvature is not attained due to the steric hindrance of the very long polyoxyethylene chain. Consequently, the LR phase solubilizes a large amount of oil and very stable reverse vesicles form in the oil-rich region. The LR phase does not appear and the reverse vesicles are not formed in C18:1EO50.8 system upon addition of decane due to the weak tendency of the oil penetration.
Introduction Polyoxyethylene-type nonionic surfactants have been widely used in many fields of application. Different from the case of other surfactants, the hydrophile-lipophile balance (HLB) of the surfactant can be successively changed by changing the hydrophilic oxyethylene (EO) chain length. The curvatures of surfactant molecular layers in the self-organizing structures are also altered from negative to positive with increasing EO chain length.1 The types of self-organized structures are related to the functions or physicochemical properties of surfactant solutions, such as micelle formation, solubilization, emulsions, and interfacial tensions, etc. Recently, we constructed the phase diagram of water/ polyoxyethylene oleyl ether systems as a function of the EO chain length.1 Different from ordinary commercial oleyl surfactants, the surfactants contain the extremely pure oleyl group and the self-organized structures can be accurately determined. Almost all the self-organized structures from reverse micelles to normal micelles appear in the phase diagram. The surfactant molecular curvature is mainly dependent only on the EO chain length. When the EO chain length of the surfactant is more than 35 EO units, only aqueous micellar solution and discontinuoustype cubic phases are formed. Although the phase behavior of water/polyoxyethylenetype nonionic surfactant/oil systems has been extensively studied, nonionic surfactants with moderate EO chain lengths (0-10 EO units) were mainly used.2-5 The HLB * To whom correspondence should be addressed. † Yokohama National University. ‡ NOF Corporation. (1) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. 1997, 101, 7952. (2) Shinoda, K.; Kunieda, H. J. Colloid Interface Sci. 1973, 42, 381. (3) Kunieda, H.; Friberg, S. E. Bull. Chem. Soc. Jpn. 1981, 54, 1010.
temperature or the phase inversion temperature (PIT) is well-known as a characteristic temperature to evaluate the phase behavior and the formation of bicontinuous microemulsions.6,7 The HLB temperature is highly dependent on the types of oils because some oils tend to be solubilized in the surfactant palisade layers and the other oils are solubilized in the interior of the hydrocarbon core of aggregates.8 Among hydrocarbons, an aromatic hydrocarbon-like m-xylene has a strong tendency to penetrate into the surfactant palisade layer and to change the surfactant molecular curvature to negative. However, the effect of added oil on the phase behavior of the nonionic surfactant having an extremely long EO chain has not been extensively studied. Especially, the effect of added aromatic hydrocarbon on the phase transition in the very hydrophilic surfactant system has not been known. In this context, the phase behavior of a water/polyoxyethylene(50.8) oleyl ether/m-xylene system was investigated. The effect of added m-xylene on the self-organization was studied by small-angle X-ray scattering (SAXS). The result will be compared with that for the decane system. Finally, the formation of reverse vesicles is reported. Experimental Section Material. Four polyoxyethylene oleyl ethers (C18:1EOn) were obtained from NOF Corp. The average number of oxyethylene units is 10.7, 19.2, 30.1, and 50.8, respectively, and each EO chain has a distribution. On the other hand, the oleyl group is very pure, because the surfactants were synthesized from ultrapure oleic acid (99.999%). Extrapure-grade decane and (4) Kunieda, H. J. Colloid Interface Sci. 1989, 133, 237. (5) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technology 1982, 3, 233. (6) Arai, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 25, 396. (7) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1969, 30, 258. (8) Kunieda, H.; Ozawa, K.; Huang, K.-L. J. Phys. Chem. B 1998, 102, 831.
10.1021/la9814844 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/03/1999
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m-xylene were obtained from Tokyo Kasei Kogyo Co. and Wako Pure Chemical Industries, Ltd., respectively. Determination of Phase Diagram. Various amounts of water, surfactant, and oil were weighed and the mixtures were sealed in ampules. After the samples were mixed with a vortex mixer and centrifuged, they were kept in a thermostat at 25 °C. The phase state was determined by direct visual inspection and by crossed polarizers. The types of liquid crystals were determined by optical microscopy and small-angle X-ray scattering. Calculation of Volume Fraction. The volume fractions of surfactant (φs) and its lipophilic chain (φL) in the system are represented by
φs )
1 Fs Ww Wo 1+ + Ws Fw Fo
(
φL )
)
VL φ Vs s
(1)
(2)
where Ws, Ww, and Wo are the weight fractions of surfactant, water, and oil, respectively. Fs, Fw, and Fo denote the densities of surfactant, water, and oil, respectively. The density of C18:1EO50.8 is 1.09 g/cm3. Vs and VL are the molar volumes of surfactant and its lipophilic chain, respectively. Vs ) 2289 cm3/mol and VL ) 309 cm3/mol for C18:1EO50.8 at 25 °C1. Small-Angle X-ray Scattering (SAXS) Measurement. The interlayer spacing of the liquid crystal was measured by smallangle X-ray scattering, performed on a small-angle scattering goniometer with an 18 kV Rigaku Denki rotating anode generator (Rint 2500) at 25 °C. The samples were placed in a metal slit with plastic film (Mylar seal method). The types of liquid crystals were distinguished by the interlayer spacing ratio of the first, second, and third peaks. The peak ratios were 1:1/2:1/3 for the lamellar phase and 1:1/x3:1/2 for the hexagonal phase. Three peaks were observed in cubic phases (I1 or V1) and their ratios were 1:1/x2:1/x3 for the discontinuous type (I1) and 1:(3/4)1/2: (3/10)1/2 for the bicontinuous type (V1). The number of peaks for the cubic phase was not enough to identify the particular structure because there are many types of cubic phases. However, in the previous paper(s),1 we identified the bicontinuous cubic (V1) phase as an Ia3d group in a water/C18:1EO10.7 system. Moreover, it is known that most of the bicontinuous cubic phases belong to the Ia3d group.9 For this reason, the present V1 phase formed between hexagonal and lamellar phases is considered to belong to the Ia3d group. By using the geometrical relation, the effective cross-sectional area per surfactant molecule at the hydrophobic interface of aggregates was calculated from the interlayer spacing. The thickness of the polar domain consisting of water and the hydrophilic chain in the lamellar phase and the distance between cylinders in the hexagonal phase were also calculated (Figure 1). Hexagonal (H1) Phase. The radius of the cylinder r is expressed by
r)
{
2 (φL + φo) x3π
}
1/2
d
(3)
where d is the measured interlayer spacing. The effective crosssectional area as is given by
(
)
2VL φL + φo as ) rNA φL
2 d - 2r x3
(9) Rancon, Y.; Charvolin, J. J. Phys. 1987, 48, 1067.
Lamellar (LR) Phase. The thickness of the apolar domain consisting of oil and a lipophilic chain r is expressed by
r ) (φL + φo)d
(6)
as is given by
as )
(
)
2VL φL + φo rNA φL
(7)
The thickness of the polar domain consisting of water and a hydrophilic chain dH is expressed by the following equation.
dH ) d -r
(8)
Cubic (V1) Phase. As mentioned before, the bicontinuous cubic (V1) phase is considered to belong to the Ia3d group. In this case, as can be calculated.10,11 The midplane of the layer consisting of water and a hydrophilic chain in the bicontinuous cubic phase (oil-in-water) forms a periodic minimal surface.12 A water/oil interface is assumed as two parallel surfaces at distance L from the midplane.13 The total interfacial area per unit volume (A/V) is expressed by10
(
( ))
A φ sN A 2c¸ 2πχu L ) a ) 1+ V VL s a c¸ a
2
(9)
where χu is the Euler characteristic per unit cell of the minimal surface and c¸ is a dimensionless area constant.10,13 a is the lattice parameter and is calculated from the first SAXS peak (d). The values of a, χu, and c¸ for the space group Ia3d are x6d, -8, and 3.091, respectively.13 Furthermore, the volume fraction of the polar domain consisting of water and a hydrophilic chain (φH + φW) is given by the follow equation.13
φH + φW ) 1 - (φL + φo) )
(
( ))
2c¸ L 2πχu L 1+ a 3c¸ a
2
(10)
(4) By solving the simultaneous equations eqs 9 and 10, as can be obtained.
where NA is Avogadro’s constant. The distance between cylinders dH is expressed by the following equation.
dH )
Figure 1. Schematic figures of hexagonal and lamellar phases.
(5)
(10) Alexandridis, P.; Olsson, U.; Lindman, B. J. Phys. Chem. 1996, 100, 280 (11) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23. (12) Luzzati, V.; Vargas, R.; Mariani, P.; Gulik, A.; Delacroix, H. J. Mol. Biol. 1993, 229, 540. (13) Anderson, D.; Wennerstro¨m, H.; Olsson, U. J. Phys. Chem. 1989, 93, 4243.
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Figure 2. Phase diagram of the water/C18:1EO50.8/m-xylene system at 25 °C. Reverse vesicles are formed in the shaded region. Notation is as follows: Wm, micellar phase; I1, discontinuous cubic phase; H1, hexagonal phase; V1, bicontinuous cubic phase; LR, lamellar phase; S, solid present phase; Om, surfactant liquid or reverse micellar solution phase; O, excess oil phase. The spacing group of the V1 phase is considered as Ia3d from SAXS measurement.
Results and Discussion Phase Behavior of C18:1EO50.8 in Water/Oil. The phase diagram of a water/C18:1EO50.8/m-xylene system was constructed at 25 °C and is shown in Figure 2. In the binary water/surfactant system, an aqueous micellar solution (Wm) phase and a discontinuous cubic (I1) phase form. Upon addition of m-xylene, an excess oil phase separates from the phase boundary of the single Wm and I1 regions in a relatively dilute region. On the other hand, the I1 phase is changed to lamellar (LR) via hexagonal (H1) and bicontinuous cubic (V1) phases at high surfactant concentrations above Rsw ) 0.6, where Rsw is the weight fraction of surfactant in the water/surfactant. Although the LR phase swells a large amount of m-xylene, the phase transition from the LR phase to another type of self-organized structure with a negative surfactant curvature such as a reverse hexagonal (H2) phase does not take place, as is shown in Figure 2. When m-xylene is replaced with decane, the phase transition from the Wm or I1 phase to another phase does not occur in the whole range of the surfactant concentration. An excess oil phase always separates from the phase boundary of each single-phase region. To clarify the difference in the phase behavior of both systems, we measured the change in interlayer spacing (d) of the I1 phase as a function of the oil content at Rsw ) 0.5. The result is shown in Figure 3. The interlayer spacing largely increases upon addition of decane whereas it increases less in the m-xylene system. When an excess oil separates, the interlayer spacing becomes constant. The solubilization of m-xylene in the I1 phase is much larger than that of decane. Although it is considered that the I1 phase consists of discrete normal micelles, the exact shape of the micelle was not identified because only three SAXS peaks were observed and they are not enough to analyze the structure of the cubic phase. In the previous study, the effect of added m-xylene or decane on the structure of the H1 phase in the water/ polyoxyethylene dodecyl ether system was investigated by means of SAXS.8 In the decane system, the oil is mainly solubilized in the inner core of the aggregate. The effective
Kunieda et al.
Figure 3. Change of the interlayer spacing d of I1 in water/ C18:1EO50.8/m-xylene (b) and water/C18:1EO50.8/decane (O) systems.
cross-sectional area of one surfactant molecule (as) is almost unchanged and the radius of the rod micelle increases. As a result, the interlayer spacing largely increases with increasing decane content. In the m-xylene system, however, m-xylene molecules mainly penetrate in the surfactant palisade layer. In other words, m-xylene is solubilized in the vicinity of the interface of water/ hydrocarbon chains of surfactant. Hence, the as increases upon addition of m-xylene whereas the increase in the radius of micelles is rather small, and the interlayer spacing does not largely change. In the present system, the same tendency is observed, as is shown in Figure 3. Although the precise structure of the I1 phase was not identified, it is considered that m-xylene mainly penetrates into the surfactant palisade layer whereas decane is solubilized in the deep core of the hydrocarbon chains. This difference in the solubilization mechanism causes the difference in the phase behavior. Effect of Added m-Xylene on the Self-Organized Structures. Even in the m-xylene system, the effect of added m-xylene on the phase behavior is different depending on Rsw, as is shown in Figure 2. To understand the difference, we measured the change in interlayer spacing of the H1 and LR phases at each Rsw as a function of the m-xylene content. The as is calculated from the obtained d value and the result is shown in Figure 4. The as increases in the H1 phase upon addition of the oil. Especially, the as becomes larger at high Rsw as is shown in Figure 4. This means that the penetration of the m-xylene molecule into the palisade layer is large at higher Rsw. According to Israelachvili, the as is determined by the balance between the repulsion of the hydrophilic group and the attraction of the lipophilic moiety of the surfactant.14 It is also known that the as in polyoxyethylenetype nonionic surfactant systems is mainly dependent on the EO chain.15 Since the repulsion between the nonionic hydrophilic groups of surfactant molecules is attributed to the hydration force, it would decrease with decreasing water/EO chain ratio. Hence, the repulsion is large enough (14) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (15) Kunieda, H.; Umizu, G.; Yamaguchi, Y.; Suzuki, M. J. Jpn. Oil Chem. Soc. (Yukagaku) 1998, 47, 879.
Long-Polyoxyethylene-Chain Surfactant Systems
Figure 4. Change of the effective cross-sectional area as(0) at various Rsw values as a function of weight fraction of oil in a system, Wo, in a water/C18:1EO50.8/m-xylene system at 25 °C.
Figure 5. Change of interlayer spacing d(O) and radius of a cylinder in the H1 phase or thickness of an apolar domain consisting of oil and a lipophilic chain in the LR phase, r(4), the thickness of a polar domain consisting of water and a hydrophilic chain in the LR phase, and the distance between cylinders in the H1 phase, dH()), and as(0) as a function of Wo in a water/ C18:1EO50.8/m-xylene system at 25 °C. Rsw is fixed at 0.75.
to prevent oil penetration into the surfactant palisade layer at low surfactant concentrations. As a result, even if m-xylene is solubilized in micelles, the surfactant molecular curvature is not largely changed and the phase transition does not take place at a low Rsw. On the other hand, m-xylene penetrates into the surfactant layer and make the surfactant curvature less positive at a high Rsw because of the decrease of the repulsion between hydrophilic groups. Eventually, the phase transition from the I1 phase to the LR phase via the H1 and V1 phases occurs. Solubilization of m-Xylene in the Lr Phase. Changes in d, dH, r, and as at fixed Rsw ) 0.75 are shown in Figure 5. With increasing oil content, the interlayer spacing is continuously increased. The as in the H1 phase also increases upon addition of m-xylene, but it becomes almost constant in the LR phase. The as for the V1 phase was also calculated by eqs 9 and 10 in the Experimental Section. As is shown in Figure 5, as increases in the V1 phase. Hence, it is considered that oil is mainly solubilized in the
Langmuir, Vol. 15, No. 9, 1999 3121
vicinity of the interface and expands the as in the H1 phase. In the LR phase, however, m-xylene is mainly solubilized in the deep core of the hydrophobic part and the thickness is dramatically increased, as is shown in Figure 5. In a double-chain cationic surfactant system, a similar result is obtained.16 For the phase transition from the H1 phase to the LR phase, the hydrophilic EO chains must approach each other because they should orient in a parallel way in the LR phase. This causes the increase in the repulsion, and as becomes larger. In this system, the self-organized structure with a negative surfactant curvature such as a reverse hexagonal phase is not formed, and the curvature remains positive or zero even in the m-xylene-rich region. However, Alexandridis reported that almost all the types of selforganized structures are formed in the phase diagram of the water/(polyoxyethylene)19(polyoxypropylene)43(polyoxyethylene)19/p-xylene system.17 The amphiphilic molecule consists of hydrophilic EO and less hydrophilic propylene oxide chains, whose chain lengths are comparable and are not very different. In this case, the surfactant curvature is mainly dependent on the water/oil ratio. However, the hydrophilic and lipophilic moieties of the surfactant are very asymmetric in the present very hydrophilic oleyl surfactant system. Consequently, aromatic hydrocarbon-like m-xylene tends to penetrate the surfactant palisade layer and expand as. This phenomenon induces the change in surfactant curvature from positive to zero. However, to form a negative surfactant curvature, the oil penetration has to overcome the large increase in the repulsion force due to the overlapping of hydrophilic moieties. Hence, it is difficult to form a self-organized structure with a negative curvature in long-EO-chain nonionic surfactant systems. Formation of Reverse Vesicles. As is shown in Figure 5, the LR phase solubilizes a large amount of m-xylene at high surfactant concentration. Finally, the bilayers form a closed structure called reverse vesicles18-22 in an oilrich region (shaded area in Figure 2), at which the reverse vesicle solution looks bluish against scattered light. The formation of reverse vesicles is highly dependent on Rsw. The stability is also related to this ratio. The reverse vesicles at Rsw ) 0.70 precipitate after a day. However, the reverse vesicles are more stable for at least a few weeks above Rsw ) 0.75. In particular, the most stable reverse vesicles are formed at Rsw ) 0.75. They are very stable, and there is no precipitation at least for one month after preparation. The correlation between the EO-chain length of the oleyl surfactant and Rsw for the formation of reverse vesicles is shown in Figure 6. The weight fraction of m-xylene in the system is kept at 0.9. When the EO chain length is in the range 10-50, the reverse vesicles are formed almost at constant Rsw. This Rsw corresponds to an equal molar ratio of water and (16) Aramaki, K.; Kunieda, H. Colloid Polym. Sci., accepted. (17) Alxandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, 2627. (18) Kunieda, H.; Nakamura, K.; Evans, D. F. J. Am. Chem. Soc. 1991, 113, 1051. (19) Kunieda, H.; Nakamura, K.; Davis, H. T.; Evans, D. F. Langmuir 1991, 7, 1915. (20) Kunieda, H.; Nakamura, K.; Olsson, U.; Lindman, B. J. Phys. Chem. 1993, 97, 9525. (21) Olsson, U.; Nakamura, K.; Kunieda, H.; Strey, R. Langmuir 1996, 12, 3045. (22) Nakamura, K.; Uemoto, A.; Imae, T.; Solans, C.; Kunieda, H. J. Colloid Interface Sci. 1995, 170, 363.
3122 Langmuir, Vol. 15, No. 9, 1999
Figure 6. Phase diagram of water/C18:1EOn/m-xylene at 25 °C as a function of Rsw and number of oxyethylene units. The weight fraction of oil in the system is fixed at 0.9. Reverse vesicles are formed in the shaded region. The number of water molecules to an oxyethylene unit is indicated by the gray line.
one EO unit, which is half of full hydration of the EO unit.23 In a range of EO chain length between 10 and 50, it is difficult to make a surfactant curvature negative upon addition of m-xylene. This means that a long-EO-chain surfactant causes a very big energetic barrier to form a negative curved interface. On the other hand, if Rsw is low, even zero curvature is not attained and the H1 or I1 phases are formed, as is shown in Figure 6. According to the phase study24 and the geometrical packing model,25 reverse vesicles are formed when the HLB of surfactant is balanced or a little hydrophilic in a saturated hydrocarbon system. In the present m-xylene system, however, even a very-long-EO-chain surfactant forms reverse vesicles due to the tendency of strong oil penetration into the palisade layer. Hence, if a hydrocarbon such as decane is used, a reverse vesicles region would be shrunk and they would form only in a relatively shortEO-chain surfactant system. When the I1 phase is soaked in m-xylene in the C18:1EO50.8 system, myelin figures grow and reverse vesicles are spontaneously formed, as is shown in Figure 7. This phenomenon is similar to the case when lecithin touches water and normal vesicles are formed.26 This also (23) Rosh, M. In Nonionic Surfactants; Schick, M., ed.; Marcel Dekker: New York, 1967; Chapter 22. (24) Kunieda, H.; Ymagata, M. J. Colloid Interface Sci. 1992, 150, 277. (25) Kunieda, H.; Shigeta, K.; Nakamura, K.; Imae, T. Prog. Colloid Polym. Sci. 1996, 100, 1. (26) Hotani, H. J. Mol. Biol. 1984, 178, 113.
Kunieda et al.
Figure 7. Picture of reverse vesicles and myelin figures in the system.
suggests that the m-xylene molecule has a strong tendency to dissolve into the surfactant layer. Conclusion The phase behavior of C18:1EO50.8 in a water/oil system is discussed. The phase behavior is different with oil. In the m-xylene system, the I1 phase changes to the LR phase via the V1 and H1 phases with increasing oil content, depending on Rsw. On the other hand, in the decane system, the phase transition does not occur in the whole range of Rsw. This is because the solubilization mechanisms of the oil are different from each other. The decane is mainly solubilized in the inner core of the self-organized structure, whereas m-xylene is mostly solubilized in the palisade layer. The penetration of oil into the palisade layer induces the decrease of the curvature of the surfactant layer. However, if the EO chain length is more than 10, the surfactant molecular curvature remains positive in the dilute region because the EO chain length is fully hydrated and the repulsion between the hydrophilic groups is large. On the other hand, the I1-H1-V1-LR transition takes place in the concentrated region because the hydration decreases. The self-organized structure with negative curvature is not formed due to the steric hindrance of the long EO chain. As a result, the LR phase solubilizes a large amount of oil, and reverse vesicles are formed in the oil-rich region. The region and stability of reverse vesicles depend on Rsw. However, the region of reverse vesicles is independent of the hydrophilic EO chain length. LA9814844
