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Phase Transition between Microemulsion and Lamellar Liquid Crystal Naoyo Nakamura,† Toru Tagawa,‡ Kaoru Kihara,‡ Ichiro Tobita,§ and Hironobu Kunieda*,† Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240, Japan, Yokohama Research Center R&D Division, Mitsubishi Chemical Corporation, Kamoshida-cho 1000, Aoba-ku, Yokohama 227, Japan, and Department of Application Laboratories, Center for Applied Technology, Rigaku Corporation, Matsubara-cho 3-9-12, Akishima-shi, Tokyo 196, Japan Received June 19, 1996. In Final Form: December 10, 1996X A phase transition from a lamellar liquid crystal to a middle-phase microemulsion (or surfactant phase) takes place with decreasing surfactant content in the water/sucrose monododecanoate/hexanol/decane system. Considering the monomeric solubility of hexanol in oil, the phase transition from the lamellar liquid crystal to a microemulsion was investigated at equal weight ratio of water and decane by a phase behavior study, small-angle X-ray scattering, and electron spin resonance spin-probe methods. In this system, the interlayer spacing of the lamellar liquid crystal, d, gradually increases upon addition of water and oil. The hexanol molecules are distributed between the palisade layers and the oil core in the lamellar liquid crystal. The bilayer becomes more flexible upon dilution and, eventually, the lamellar liquid crystal changes to a bicontinuous microemulsion. The surfactant weight fraction at the microinterface inside the bicontinuous microemulsion is also discussed.
Introduction Microemulsions are isotropic surfactant solutions which can solubilize both water and oil. There are three types of microemulsions, oil-swollen micelles, water-swollen reverse micelles, and bicontinuous structures. The bicontinuous microemulsion coexists with excess water and oil phase and is often called a middle-phase microemulsion or surfactant phase.1,2 This kind of microemulsion appears when the HLB (hydrophile-lipophile balance) of the surfactant is just balanced in a given water-oil system.3 Due to the critical solution phenomenon, the solubilization capability of surfactant reaches its maximum and ultralow interfacial tensions are attained in the three-phase region.4-6 These properties of microemulsions have been extensively applied in basic study and practical applications.2,7,8 Bicontinuous microemulsions are produced in poly(oxyethylene)-type nonionic surfactant systems at a certain temperature called the HLB temperature.3,9 The HLB of the surfactant can be controlled with temperature since temperature causes a conformational change in the poly(oxyethylene) chain.10 On the other hand, cosurfactants such as medium-chain-length alcohols are usually used for the formation of middle-phase microemulsions in ionic surfactant systems, except those containing less hydrophilic ionic surfactants like Aerosol OT.4,11 When
a surfactant mixture or a surfactant-cosurfactant mixture is used to form microemulsions, one has to take into account the distribution of surfactants or cosurfactants between micro-water and oil domains and also at the interface inside the microemulsion.12,13 Although there is an argument on the detailed structure of the middle-phase microemulsions,14 it is considered to be in a melted state of the lamellar liquid crystal. Generally, a lamellar liquid crystalline phase is present in the vicinity of the three-phase region in many systems. Sucrose alkanoates are unique and biocompatible nonionic surfactants which have a strongly hydrophilic sucrose ring. With changes of the molar number and length of hydrocarbon chains attached to the sucrose moiety, the HLB can be controlled. The middle-phase microemulsion is also formed in a water/sucrose monoalkanoate/cosurfactant/oil system.15,16 In this paper, the phase equilibrium of the middle-phase microemulsion and phase transition from the lamellar liquid crystal to the microemulsion were investigated by means of phase study, small-angle X-ray scattering (SAXS), and ESR (electron spin resonance) spin probe method. Experimental Section
* To whom correspondence should be addressed. † Yokohama National University. ‡ Mitsubishi Chemical Corporation. § Rigaku Corporation. X Abstract published in Advance ACS Abstracts, March 15, 1997.
Materials. A reagent grade sucrose monododecanoate (SM1200 abbreviated as SMD) was kindly supplied by Mitsubishi Chemical Corp., and its monoester content is much greater than 95%. Dodecanoic acid (99%) was used to synthesize SMD. SMD was dehydrated using P2O5 under vacuum. Extra-pure-grade decane and hexanol were obtained from Tokyo Kasei Kogyo Co. The spin probes, 5- and 12-doxylstearic acids, were obtained from Aldrich Chemical Co. All chemicals were used without further purification.
(1) Shinoda, K.; Kunieda, H. J. Colloid Interface Sci. 1973, 42, 381. (2) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Surfactant Science Series; Marcel Dekker: New York, 1988; Vol. 30. (3) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (4) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (5) Kunieda, H.; Friberg, S. E. Bull. Chem. Soc. Jpn. 1981, 54, 1010. (6) Kunieda, H.; Shinoda, K. Bull. Chem. Soc. Jpn. 1982, 55, 1777. (7) Prince, L. M., Ed. Microemulsions; Academic Press: New York, 1977. (8) Solans, C., Kunieda, H., Eds. Industrial Applications of Microemulsions; Marcel Dekker: New York, 1996. (9) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233. (10) Karlstroem, G. J. Phys. Chem. 1985, 89, 4962.
(11) Kunieda, H.; Shinoda, K. J. Jpn. Oil Chem. Soc. (Yukagaku) 1980, 29, 676. (12) Kunieda, H.; Sato, Y. Organized Solutions; Friberg, S. E., Lindman, B., Eds.; Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 44. (13) Kunieda, H.; Yamagata, M. Langmuir 1993, 9, 3345. (14) Zemb, T. N.; Barnes, J. S.; Derian, P. J.; Ninham, B. W. Prog. Colloid Polym. Sci. 1990, 81, 20. (15) Kunieda, H.; Ushio, N.; Nakano, A.; Miura, M. J. Colloid Interface Sci. 1993, 159, 37. (16) Pes, M. A.; Aramaki, K.; Nakamura, N.; Kunieda, H. J. Colloid Interface Sci. 1996, 178, 666.
S0743-7463(96)00606-3 CCC: $14.00
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2002 Langmuir, Vol. 13, No. 7, 1997 Procedures To Determine Phase Boundaries. Phase behavior of the water/SMD/hexanol/decane system was studied at constant temperature, 25 °C and at constant water/oil weight ratio, 50/50. Various amounts of water, SMD, hexanol, and decane were sealed in ampules. To attain homogeneity of samples, a vortex mixer was used for rather diluted samples and concentrated samples were mixed by repeated centrifugation through a narrow constriction at around 60 °C (heated samples in the water bath). These samples were kept at 25 °C in a thermostat for several hours to few days to equilibrate. Phase boundaries were determined by visual observation. Liquid crystals were detected by using crossed polarizers. The type of liquid crystal was distinguished by SAXS peaks.17 The ratios of interlayer spacing from the first and second peaks are 1:1/2 for the lamellar type and 1:(1/x3) for the hexagonal type, respectively. The type of liquid crystal was also identified by a polarizing microscope. Inaccuracy of dotted line in the phase diagram is (1.25 wt % (maximum). X-ray Scattering. The interlayer spacing of the lamellar liquid crystal was measured using small-angle X-ray scattering (SAXS), performed on a small-angle scattering goniometer with an 15 kW Rigaku Denki rotating anode generator (RINT-2500) at ∼30 °C. The samples were covered by plastic films for the SAXS experiment (Mylar seal method). Wide-angle X-ray scattering measurements showed a wide diffuse reflection with a position corresponding to a spacing of 0.45 nm for lamellar liquid crystals in the present system. These results strongly support that the hydrocarbon chain of SMD is in a liquid state. ESR Measurements. ESR spectra were recorded using a JEOL-ME-3X spectrometer with 100 kHz field modulation. Since the present system contains a large amount of water, a sealed narrow capillary tube (its internal diameter is 1.54 mm) was used to avoid the heating of samples18 and to prevent the evaporation of water by microwave radiation. The scan width was (50 G and the microwave power was 5 mW. Resonance absorption occurred at an external field strength about 3000 G. The ESR spin probe method is used to detect the changes within the hydrocarbon phase of the lipid model membranes, with the spin probes incorporated in the molecular aggregate, the lipid bilayer.19-23 In this study, the above method was applied in the surfactant bilayer system. Two kinds of stearic acid spin probes, 5- and 12-doxylstearic acids (abbreviated as 5- and 12NS), were used. The concentrations of the probes were always in the order of 10-4 M. The nitroxide radicals show triplet ESR spectra, which are interpreted as being due to the coupling of the nuclear spin of the nitrogen atom. The line width of three spectra is termed A| in the case where the principal chain of spin probe is parallel to the magnetic field whereas it is termed A⊥ in the case where the principal chain is vertical to the magnetic field. The angle between the chain in the surfactant bilayer and the magnetic field has a distribution. The observed spectrum is a combination of individual spectra originating in all radicals present in the system. It is known that a measure of the maximum hyperfine splitting (Amax) is equal to 2A| and that of the minimum hyperfine splitting (Amin) is equal to 2A⊥. The apparent order parameter, S, associated with the orientation of the membrane can be calculated by the following equation (17) Fontell, K. Liquid Crystals & Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; John Wiley & Sons: New York, 1974; Vol. 2, Chapter 4. (18) Alger, R. S. Electron Paramagnetic Resonance; John Wiley & Sons: New York, 1968. (19) McConnell, H. M.; McFarland, B. G. Q. Rev. Biophys. 1970, 3, 91. (20) Smith, I. C. P. Biological Applications of Electron Spin Resonance; Swartz, H. M., Bolton, J. R., Borg, D. C., Ed.; Wiley-Interscience: New York, 1972. (21) Berliner, L. J., Ed. Spin Labeling; Academic Press: New York, 1976 (Vol. I) and 1978 (Vol II). (22) Nakagawa, T.; Jizonoto, H. Kolloid. Z. Z. Polym. 1972, 250, 594. (23) Ohnishi, S.; Cyr, T. J. R.; Fukushima, H. Bull. Chem. Soc Jpn. 1970, 43, 673. (24) Hubbell, W. L.; McConnell, H. M. J. Am. Chem. Soc. 1971, 93, 314. (25) Graffney, B. J.; McConnell, H. M. J. Magn. Reson. 1974, 16, 1. (26) Kunieda, H.; Kanei, N.; Tobita, I.; Kihara, K.; Yuki, A. Colloid Polym. Sci. 1995, 273, 584.
Nakamura et al. S)
a0 aN
A| - A⊥ 1 Azz - (Axx + Ayy) 2
(1)
where the principal values of the hyperfine splitting tensor were taken as Axx ) 6.3 G, Ayy ) 5.8 G, and Azz ) 33.6 G for the nitroxide radicals. a0/aN is the polarity correction factor, where a0 and aN are
a0 ) 1/3(Axx + Ayy + Azz) and
aN ) 1/3(A| + 2A⊥) respectively.21,24,25
Results Phase Diagram of the Water/Sucrose Monododecanoate/Hexanol/Decane System. Since SMD is a very hydrophilic surfactant, mixing with a cosurfactant is required to form a middle-phase microemulsion, similar to an ionic surfactant system.16 The phase diagram of a water/SMD/hexanol/decane system at 25 °C is shown in Figure 1, where the water/decane ratio is unity. The phase diagram is a section of the whole phase equilibrium represented by the composition tetrahedron.3 An isotropic single-phase region extends from the SMDhexanol axis. This phase is considered to be a reverse micellar solution phase (Om) or W/O microemulsion.2, 5 A solid phase is present in the vicinity of the SMD apex, but the region of existence was not determined. On the axis, SMD and hexanol form a mixed layer and the average interfacial area per amphiphilic molecule, As ()ASMDXSMD + AhexanolXhexanol, where X and A mean the mole fraction and the effective interfacial area of each components), decreases with increasing hexanol content. Eventually the layer structure is not maintained and turns to reverse micelles.26 It is unclear whether the layer structure in the absence of water is in a liquid crystal or layered gel, because the system is extremely viscous and the appearance is like a translucent solid. Further investigation is needed on this axis.
Figure 1. Phase diagram of the water/SMD/hexanol/decane system at 25 °C. The weight ratio of water/decane is kept constant (50/50). I, II, and III indicate single-, two-, and threephase regions, respectively. The concentrations are given in weight fractions.
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Figure 3. Change in interlayer spacing, d, along line A in Figure 1. The weight fraction of hexanol in the system is 0.15 on line A. d is plotted against the reciprocal of the volume fraction of the sum of SMD and hexanol, 1/φs. The solid line is the theoretical line calculated by eq 2.
Figure 2. Phase diagram of the water/SMD/hexanol/decane system at 25 °C. (a) The total surfactant concentration (SMD + hexanol) is 20 wt % in the system. (b) The total surfactant concentration (SMD + hexanol) is 35 wt % in the system.
On the bottom axis in Figure 1, lamellar liquid crystal (L.LC), hexagonal liquid crystal (H.LC), viscous isotropic (cubic phase), or aqueous micellar solution phases (Wm) coexist with excess oil phase with decreasing SMD content. It is worth noting that no single phase exists in the absence of hexanol. The self-organizing structure is changed from a layer to a spherical structure upon addition of water.27,28 The mixed amphiphile (SMD + hexanol) is hydrophilic in the vicinity of the bottom axis of the phase diagram. A single lamellar liquid crystal phase (IL.LC) is extended from the SMD-hexanol axis to a dilute region along line A as is shown in Figure 1. The weight fraction of hexanol in the system is kept constant, 0.15, along this line. The lamellar liquid crystal is changed to the middle-phase microemulsion (D) which coexists with excess water (W) and oil phases (O). The solubilization capacity reaches its maximum at point a in Figure 1. The three-phase region (III) is not oriented toward the water/oil apex, as a considerable amount of hexanol is monomerically dissolved in the oil phase.16 In fact, the monomeric solubility of hexanol is about 20 wt % in the oil phase according to analysis of the phase behavior and HPLC (high-performance liquid chromatography) measurement at 25 °C.16 Figure 2 shows the phase diagram at a fixed total surfactant concentration (SMD + hexanol). The concentrations of SMD + hexanol are 20 wt % (Figure 2a) and 35 wt % (Figure 2b) in the system. (27) Herrington, T. M.; Sahi S. S. J. Am. Oil Chem. Soc. 1988, 65, 1677.
The weight fraction of hexanol in SMD + hexanol is plotted vertically, whereas the weight fraction of decane in water + decane is plotted horizontally. The three-phase and single-phase microemulsion region are shifted to the hexanol-rich side with increasing decane content as is shown in Figure 2a. This distortion of phase behavior also indicates a high monomeric solubility of hexanol in decane. When the concentration of total surfactant is 35 wt % in the system (shown in Figure 2b), a lamellar liquid crystalline phase is present in the area between two microemulsion phases. These phases also shift a little toward the hexanol-rich region with increasing decane content. This fact strongly suggests that hexanol molecules are monomerically dissolved in the oil part of the lamellar liquid crystal and total hexanol content in SMD + hexanol increases with increasing decane content. If all hexanol molecules are present only at the interface (the surfactant palisade layer), the lamellar liquid crystal region (LC present in the phase diagram) should be parallel to the horizontal axis because the mixing ratio of SMD + hexanol at the interface is unchanged in Figure 2b. Interlayer Spacing of Lamellar Liquid Crystal. Water- and oil-swollen lamellar liquid crystal is extended along line A in Figure 1. In order to investigate the structural change of lamellar liquid crystal with increasing water-oil content, the interlayer spacing, d, was measured along line A by means of SAXS. In the two-phase region (IIL.LC), the separated liquid crystal was used for the measurement. This result is plotted against the reciprocal of the volume fraction of the sum of SMD and hexanol and is shown in Figure 3. The interlayer spacing increases gradually with increasing water-oil content because the lamellar liquid crystal is swollen with water and oil. If it is assumed that all the surfactants (SMD + hexanol) form the bilayer and the As in the bilayer are unchanged upon addition of water and decane, the following equation holds in the single lamellar liquid crystal region
d ) dS/φS
(2)
where φs is the volume fraction of total surfactants and dS is the bilayer thickness. We can obtain the volume of (28) Kunieda et al. Unpublished data.
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Figure 5. Schematic representation of lamellar liquid crystal and microemulsion. Figure does not show the real configuration. Figure 4. Apparent order parameter, S, of 5-NS and 12-NS along line A (b), B (O), and C (×) in Figure 1. The weight fractions of hexanol in the system are 0.15, 0.125, and 0.10 on lines A, B, and C.
parameters along the lines A and B have almost the same value, but for line C they are higher. Discussion
each component, dividing the weight by density. d ) ds should hold at 1/φs ) 1 in the absence of water and oil. However, as described before, it is unclear whether the system is in a liquid crystal state or a layered gel. Instead of using this value, we obtain the straight line fitted to the experimental data in the concentrated region. In the concentrated region, the system contains relatively small amounts of water and oil, and the deviation from eq 2 would be small. The experimental values are not fitted to the calculated line in a dilute region. We will discuss about this deviation later. Order Parameter. In order to investigate how the bilayer is changed upon dilution, the order parameters, S, of 5- and 12-NS were measured along lines A, B, and C in Figure 1. The weight fraction of hexanol in the system is kept constant, 0.15, 0.125, and 0.10 in lines A, B, and C, respectively. Figure 4 shows the relationships between the ESR order parameter, S, and the inverse volume fraction of amphiphiles, which contain the spin probes with spin labels at 5- and 12-positions. In order to compare the SAXS result, the inverse volume fraction of amphiphiles is plotted. We know the mobility near the interfacial part of the bilayer from the spectra of the 5-NS labeled system. The mobility of the hydrophobic part of the bilayer can be estimated from the spectra of 12-NS. The large S value indicates high orientation or a low mobility. Consequently, the lower values observed for the 12-NS labeled system compared to those of the 5-NS labeled system are due to a large mobility of the hydrocarbon chains in the hydrophobic part of the bilayer. Initially, the S value decreases and then becomes constant with increasing water and oil content. In the concentrated region, the hydrocarbon parts of the bilayers are in close contact, but they are gradually separated upon addition of oil. In the case of 5-NS, the S values are very high in the concentrated region due to the interbilayer interaction, because the hydrophilic parts are also packed compactly. The order parameter also gradually decreases upon addition of water because the hydrophilic part is expanded due to hydration. All experimental values continuously decrease upon addition of water and oil as is shown in Figure 4. For 12-NS, measured values overlap on all lines, A, B, and C, and the hydrophobic part of the bilayer has almost the same mobility. On the other hand, for 5-NS, the order
Change of Bilayer Composition in Lamellar Liquid Crystal. We use the following model of the lamellar liquid crystal and the microemulsion as is shown in Figure 5. It is assumed that the monomeric solubility of hexanol in excess oil of the three-phase body is equal to that in the micro-oil domain inside the microemulsion. It is also assumed that the monomeric solubility of SMD in water or oil is negligible, because SMD has a low critical micelle concentration CMC.15,29 The monomeric solubility of hexanol in water is also neglected. In these conditions, SMD is present only at the interfaces inside the microemulsion or the lamellar liquid crystal whereas hexanol molecules are distributed between the micro-oil domains and interfaces. The center of the three-phase body included in line D (Figure 6) is represented by the following equation,12,13
W1 ) S1S +
S1S2S R (1/X - 1) 1 - S1 OW
(3)
where W1 is the weight fraction of hexanol in SMD + hexanol, and ROW is the weight fraction of oil in water + oil (0.5 in the present study). X is the weight fraction of total surfactant in the system, S1S and S2S are the weight fractions of hexanol (hydrophobic surfactant) and SMD (hydrophilic surfactant), respectively, in the surfactant monolayer inside the microemulsion. S1 is the monomeric solubility of hydrophobic surfactant (hexanol) in oil (microoil domain in the single microemulsion phase or the excess oil phase in the three-phase tie triangle). According to the above assumption, we consider the several dilution paths in the phase diagram and construct the most appropriate model of changing composition in the lamellar liquid crystal as is shown in Figure 6. Line A is the dilution path in the center of the lamellar liquid crystal as is shown in Figure 1. The conditions of each dilution paths, the lines D, E, and F are shown in Table 1. The solubilities of hexanol in the micro-oil domain and in the interface are regarded constant along each lines D, E, and F. Line F is very close to the line A but a deviation is still observed. Hence, the mixing fraction of hexanol in the palisade layer would be changed along line A. In this case, it is possible that the solubility of hexanol in the oil part would be also decreased. Since we (29) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1989; Chapter 3.
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Figure 7. Change in interlayer spacing, d, along line A in Figure 1. The expression φs - kφo is the volume fraction of the sum of SMD and hexanol located at the bilayer interface.
Figure 6. Several dilution paths in the water/SMD/hexanol/ decane system. Table 1. Hypothesis of the Change in Hexanol Compositions upon the Dilution Paths in Figure 6 dilution solubility of hexanol path in the oil part line D line E line F line A
S1 0 S1 S1
mixing fraction of hexanol in the palisade layer composition of point c in Figure 6 composition of point b composition of point b changing from point b
cannot measure the distribution directly, it is assumed that the solubility of hexanol in the oil part is constant, S1, whereas the mixing fraction in the palisade layer would be changed as is shown in Table 1. The error would be rather small because the S1 is the maximum solubility in the oil part and the amount of hexanol in the oil part would be decreased in the concentrated region. Change in Interlayer Spacing. The change in interlayer spacing of the lamellar liquid crystal in the water/SMD/hexanol/decane system is not fitted to eq 2. By using the above consideration, we can revise the calculation of the interlayer spacing. If 20 wt % hexanol is dissolved in the oil core inside the lamellar liquid crystal and does not participates in the formation of the bilayer along line A, we have to subtract this amount of hexanol from the volume fraction of total surfactant (φs) in eq 2, to obtain
d ) ds/(φs - kφO)
(4)
where kφO is the volume fraction of hexanol in the oil core (φO is the oil volume fraction) and k ()(S1/Fh)/((1 - S1)/FO)) is a constant. Fh and FO are the densities of hexanol and decane, respectively. The measured interlayer spacing, d, is plotted versus 1/(φs - kφO) in Figure 7. The dS in Figure 7 is the same as that in Figure 3. The solid line in Figure 7 is calculated by eq 4. Both experimental and theoretical data are coincident with each other in the dilute region. In eq 4 we assume that the solubility of hexanol in the micro-oil domain is constant and is the same as S1. Although the mixing fraction of hexanol in the palisade layer is likely changed as discussed before, this assumption of the constant solubility is considered to be valid. However, it is not perfect because the experimental data are still slightly deviated from the line as is shown in Figure 7.
Figure 8. Weight fraction of hexanol in the surfactant layer, S1S along lines A, B, and C in Figure 1 as a function of total surfactant weight fraction, X.
Change in HLB of mixed Surfactant at the Interface. If it is assumed that oil always contains 20 wt % hexanol in micro-oil domains in the lamellar liquid crystal and the middle-phase microemulsion, we can calculate the weight fraction of hexanol in the surfactant layer, S1S on lines A, B, and C in Figure 1 by using eq 3. The change in S1S with X is shown in Figure 8. As is shown in Figure 8, the hexanol content in the bilayer interface, S1S, is constant in the concentrated region (at large X value) but increases dramatically in the dilute region along line A and B. Therefore, the surfactant layer becomes flexible upon dilution. On the other hand, S1S shows the opposite tendency and the long-chain SMD content increases on line C. Hence, it is considered that the surfactant layer along line C is more rigid than in the other lines. The ESR data in the previous section support this tendency. As is shown in Figure 1, the three-phase body is narrow and the microemulsion is changed from O/W to W/O via bicontinuous with a small change in composition. Figure 8 shows that the interfacial composition of the surfactant layer inside the microemulsion is very different around point a in Figure 1, although lines A, B, and C are very close. The hexanol weight fraction is increased abruptly along line A whereas it decreases along line C. Therefore, the HLB of the mixed surfactant film is largely changed to a large extent with a small change in total composition. It is known that the HLB of the surfactant at the wateroil interface inside a microemulsion is directly related to
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the formation of a three-phase microemulsion.30 For example, if the temperature is fixed at the three-phase temperature of a homogeneous pentaethyleneglycol dodecyl ether system, the average hydrocarbon and oxyethylene chain lengths at the interface are always 12 and 5, when three-phase microemulsions are formed by mixing polyoxyethylene-type nonionic surfactants of different hydrophilic or hydrophobic chains.30 If this observation is also true in the present system, the weight fraction of hexanol at the interface should be (30) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci. 1995, 170, 78. (31) Olsson, U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986, 90, 4083.
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constant along line A. However, the volume ratio of excess water/excess oil is much smaller than unity at point a, since a considerable amount of hexanol is dissolved in the oil and the density of the oil phase is much smaller than that of the water phase. Hence, it is considered that the spontaneous curvature of the surfactant layer is not completely flat but slightly convex toward water at point a. The previous work (Olsson et al.)31 suggests that the spontaneous curvature of the surfactant layer is changed with the change in the water-to-oil phase volume ratio. In the present system, it is also considered that the spontaneous curvature of SMD-hexanol is changed from flat to concave toward water along the line A. LA960606U
