Langmuir 2006, 22, 1449-1454
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Phase Behavior of Diglycerol Fatty Acid Esters-Nonpolar Oil Systems Lok Kumar Shrestha,*,† Masaya Kaneko,† Takaaki Sato,‡ Durga P. Acharya,† Tetsuro Iwanaga,§ and Hironobu Kunieda† Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, DiVision of Physics and Applied Physics, Faculty of Science & Engineering, Waseda UniVersity, Okubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan, and Interface Solution DiVision, Taiyo Kagaku Co. Ltd.,Takara machi 1-3, Yokkaichi, Mie 510-0844, Japan ReceiVed September 27, 2005. In Final Form: December 13, 2005 Phase behavior of diglycerol fatty acid esters (Qn-D, where n represents the carbon number in the alkyl chain length of amphiphile, n ) 10-16) were investigated in different nonpolar oils, liquid paraffin (LP70), squalane, and squalene. There is surfactant solid at lower temperature, and the surfactant solid does not swell in oil, and the melting temperature is almost constant in a wide range of compositions. In all of the systems, a lamellar liquid crystal (LR) is formed in a concentrated region at a temperature between the solid melting temperature and the isotropic two- or single-phase regions. In the dilute regions, reverse vesicles are formed in LR + O regions. There are two liquid-phase regions above the LR present region. This two-phase boundary corresponds to the cloud-point curve of nonionic surfactant aqueous solutions. However, instead of being less soluble in water at high temperature for the cloud point, the surfactant becomes more soluble in the organic solvents at high temperature. Namely, the effect of temperature on the solubility is opposite to the clouding phenomenon. When the hydrocarbon chain of the diglycerol surfactant decreases, the two-phase region becomes wider. In the case of a fixed surfactant, the surfactant is most miscible with squalene (narrowest two-phase regions) and the order of dissolutions tendency is squalene > LP70 > squalane. These results show that the hydrophilic moiety (diglycerol group) is more insoluble in oil compared with that of a conventional poly(oxyethylene)-type nonionic surfactant. Formation of reversed rodlike micelles was confirmed by SAXS scattering curve. When the hydrocarbon chain of surfactant is short, the micellar size becomes larger. In a fixed surfactant system, the reverse micellar size increases by changing oil from squalene to LP70. A small amount of water induces a dramatic elongation of reverse micelles.
Introduction Poly(oxyethylene) alkyl ethers or alkanoic-acid esters are the most popular nonionic surfactants in industrial and household products. This type of surfactant forms various self-organized structure in water and their phase behavior has extensively been studied.1-5 However, since the hydrophilic moiety is soluble in many organic solvents, the surfactants have a tendency to dissolve monomericly in nonaqueous solvents, and they do not form a variety of self-organized structures in the absence of water.1 Hence, it is not efficient to use the conventional surfactants in the systems containing a large amount of oil because of high CMC or no CMC. On the other hand, most of the ionic surfactants are insoluble in many organic solvents, and they tend to be precipitated as a solid except some special surfactants such as AOT. Different from the conventional poly(oxyethylene) chains, other hydrophilic moieties such as sucrose, polyglycerol, and so on are more solvophobic;6,7 that is, they are not soluble in many organic solvents. Hence, these hydrophilic nonionic groups tend * To whom correspondence should be addressed. E-mail: lokkumar@ hotmail.com. Phone & Fax: +81-45-339-4300. † Yokohama National University. ‡ Waseda University. § Taiyo Kagaku Co. Ltd.. (1) Sjo¨blom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactnat; Shick, M. J., Ed.; Marcel Dekker: New York, 1987; Vol. 23, p 370. (2) Shinoda, K. J. Colloid Interface Sci. 1970, 34, 278. (3) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (4) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1983, 79, 975. (5) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (6) Herrington, T. M.; Shali, S. S. J. Am. Oil. Chem. Soc. 1988, 65, 1677. (7) Rodriguez, C.; Acharya, D. P.; Hinata, S.; Ishitobi, M.; Kunieda, H. J. Colloid Interface Sci. 2003, 262, 500.
to aggregate even in nonpolar media. Besides, the phase behavior of microemulsions in polyglycerol type nonionic surfactant systems is not largely influenced by temperature.8-12 To our knowledge, there are not many studies on the phase behavior of these surfactants in oils. Diglycerol fatty acid esters (designated as Qn-D, where n is the carbon number in the alkyl chain length of amphiphile) are one of the edible surfactants and hence mostly are used for food products, cosmetics, and pharmacy. Compared with conventional monoglycerides, the hydrophilic size is almost double, and it is possible to form a self-organized structure in oil. Recently, it was found that these diglycerol surfactants dramatically stabilize nonaqueous foams.13 This is very useful for practical applications such as a shaving cream, whip cream, and so on. The foaming properties are highly related to the phase behavior of surfactant in oils. In this contest, we studied the nonaqueous phase behavior of Qn-D in different oils: liquid petroleum (LP70), squalane, and squalene. We constructed phase diagrams of Qn-D/oils in a wide range of compositions by means of visual observation and X-ray scattering techniques. We have also studied the influence of alkyl chain length or added water on the surfactant self-assembled structure. (8) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160. (9) Takagi, K.; Hirai, M.; Fujinuma, Y.; Fujimatsu, H.; Usami, H.; Ogasawara, S.; Kasahara, Y.; Yuki, A. J. Oleo Sci. 1995, 44, 207. (10) Kunieda, H.; Uddin, Md. H.; Yamashita, Y:; Furukawa, H.; Harashima, A. J. Oleo Sci. 2002, 51, 113. (11) Kunieda, H.; Kaneko, M.; Fujiyama, R.; Ishitobi, M. J. Oleo Sci. 2002, 51, 761. (12) Kunieda, H.; Uddin, Md. H.; Furukawa, H.; Harashima, A. Macromolecules 2001, 34, 9093. (13) Kunieda, H.; Shrestha, L. K.; Acharya, D. P.; Kato, H.; Takase, Y.; Gutie´rrez, J. M. J. Dispers. Sci. Technol., accepted.
10.1021/la052622+ CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006
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Figure 1. Schematic molecular structure of diglycerol monomyristate (Q14-D), squalane and squalene.
Vesicles are microscopic spherical bilayer structures containing an aqueous compartment enclosed by a surfactant bilayer or lipid.14-16 The reverse vesicle has the opposite structure to conventional vesicles or liposomes in water. Reverse vesicles and their stability have extensively been studied in many nonpolar media.17-23 The diglycerol fatty acid ester was able to form reverse vesicles in the above-mentioned nonpolar oils. Experimental Section Materials. The surfactants diglycerol caprate (Q10-D), diglycerol monolaurate (Q12-D), diglycerol monomyristate (Q14-D), and diglycerol palmitate (Q16-D), and the solvents liquid paraffin (LP70), squalane, and squalene are kindly obtained from Taiyo Kagaku Company, Japan. The purities of diglycerol surfactants are 91.1% for Q12-D, 92.9% for Q14-D, and 90.2% for Q16-D. The main impurities are unreacted diglycerol, diglycerol difatty acid esters. They were used without further purifications. LP70 is paraffin oil, which contains hydrocarbons with different chain lengths and the average chain length is C24. The schematic molecular structures of Q14-D, squalane, and squalene are shown in Figure 1. Sample Preparations. To construct the phase diagram by visual observation, different samples composition ranges from 2 to 100 wt % of surfactants (Qn-D, n ) 10-16) were prepared in LP70, squalane, and squalene in a clean and dry glass ampules. The ampules were immediately flame-sealed, and the samples were mixed properly by using dry thermo bath, vortex mixer, and repeated centrifugation to get the homogeneity. The phase behavior was then studied over wide temperature range (0-130 °C) keeping the samples inside the temperature control water bath and glycerol bath for higher temperature (above 100 °C). The samples were left for at least 4045 min at each temperature. The optical properties of the samples (birefringent and nonbirefringent) were identified with crossed (14) Bangham, A. D.; Horne, R. W. J. Mol. Biol. 1964, 8, 660. (15) Machy, P., Leserman, L. Liposomes in Cell Biology and Pharmacology; John Libbey & Co.: London, 1987. (16) Ostro, J. J. Liposomes, Marcel Dekkeer, New York, 1987. (17) Kunieda, H.; Nakamura, K.; Davis, H. T.; Evans, D. F. Langmuir 1991, 7, 1915. (18) Kunieda, H.; Nakamura, K.; Evans, D. F. J. Am. Chem. Soc. 1991, 113, 3, 1051. (19) Nakamura, K.; Machiyama, Y.; Kunieda, H. J. Jpn. Oil Chem. Soc. (Yukagaku) 1992, 41, 480. (20) Kunieda H.; Yamagata, M. J. Colloid Interface Sci. 1992, 150, 277. (21) Kunieda H.; Makino, S., Ushio, N. J. Colloid Interface Sci. 1991, 147, 286. (22) Kunieda, H.; Akimaru, M.; Ushio, N.; Nakamura, K. J. Colloid Interface Sci. 1993, 156, 446. (23) Kunieda, H.; Nakamura, K.; Olsson, U.; Lindman, B. J. Phys. Chem. 1993, 97, 9925.
Shrestha et al. polarizer. Samples were prepared in a narrow neck glass ampules for the small-angle X-ray scattering (SAXS) measurements and kept in the water bath at 25 °C for several days before going to the measurement. Small- and Wide-Angle X-ray Scattering. SAXS measurements were carried out to examine the interlayer spacing of the liquid crystalline systems by using a NanoViewer, (Rigaku Corporation, Japan) equipped with a CCD camera as a detector, for which a rotating anode generator was operated at 40 kV and 20 mA. The samples were inserted into a slot of a metal sample holder, covered with two thin Mylar films. The SAXS patterns were obtained at different temperatures in an arbitrary unit, and the peak positions were measured for the determination of space groups and the interlayer spacing of the liquid crystalline phases. To characterize the structures of reverse micelles, in particular focusing on the shape and size depending on the types of diglycerol fatty acid ester surfactants and oils, and additionally the effects of added water, SAXS measurements on 5% Qn-D/oil systems were performed by the use of a SAXSess camera (Anton Paar, PANalytical), equipped with the PW3830 laboratory X-ray generator with a long fine focus sealed glass X-ray tube (KR wavelength of λ ) 0.1542 nm) (PANalytical), a focusing multiplayer optics, a block collimator, a translucent beam stop, an image plate (IP) detector, and temperature controlled sample holder unit (TCS 120). The X-ray tube generator was operated at 40 kV and 50 mA. The samples were enclosed into a well-engineered reusable vacuum tight capillary for all measurements to attain exactly the same scattering volume and background contribution. The scattered intensity recorded on IP was read off by means of a Cyclone storage phosphor system (PerkinElmer, USA) and transformed into a function of the magnitude of the scattering vector q)
4π sin (θ/2) λ
(1)
where θ is the total scattering angle. The raw date include an attenuated primary intensity at q ) 0. All of the data were normalized into the same primary intensity for the transmission calibration and were corrected for the background scattering from the capillary and the solvents. The absolute scale calibration was made by using water as a secondary standard. (Additionally, we also employed a SAXSess camera for the detection of R-gel structures, taking advantage of its extremely wide q-range coverage that achieves qmax ∼ 27 nm-1 without any further arrangement of optical setting.) The total scattered intensity I(q) from one-component globular particle systems having n particle density can generally be written as I(q) ) nP(q)S(q)
(2)
where P(q) is the form factor and S(q) is the structure factor. P(q) is given by the Fourier transformation of the pair-distance distribution function (PDDF), p(r), that is directly connected to the convolution square of the spatial electron density fluctuations, ∆F‚2(r), and thus contains information on the geometry of the particles in the real space P(q) ) 4π
∫
∞
0
sin qr dr qr
p(r)
(3)
where r is the distance between two scattering centers inside of the particle. On one hand, S(q) involves the information about spatial distribution of the particles. Even in case of uncharged systems, when the particle density is relatively high (>1 wt %), its contribution, that is, the effects of the interparticle interference scattering, is already not negligible. S(q) is given by the Fourier transformation of the total correlation function, h(r) ) g(r) - 1, as S(q) ) 1 + 4πn
∫
∞
0
sin qr dr qr
[g(r) - 1]r2
(4)
The SAXS data for the reverse micellar solutions were analyzed by
Diglycerol Fatty Acid Esters-Nonpolar Oil Systems
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Figure 2. Phase diagram of Q14-D with different nonaqueous solvent systems (S ) solid, O ) excess oil, LR) Lamellar liquid crystal, and I ) isotropic solution). Note that the dotted line up to 40% by wt of surfactant in Q14-D/squalane system represents the theoretical phase boundary. LC for Q10-D/oil systems may be LR unidentified. the generalized indirect Fourier transformation (GIFT) technique24,25 with the Boltzmann simplex simulated annealing (BSSA) algorithm. In the limited cases of well-defined colloidal systems, ex., purified proteins or well-prepared monodisperse latex particles, P(q), can be fixed from a long-time measurement at very low concentration or in some cases it is possible to calculate it theoretically, so that the experimental (interaction potential model-free) structure factor, S(q)exp, can be deduced from careful SAXS or SANS measurements in an absolute scale by dividing the concentration-normalized scattering function, I(q)/c, by P(q).26 However, for aggregating systems such as micellar solutions and microemulsion droplets, P(q) has to be determined from experiments because it is unknown and generally a function of concentration and temperature. The basic concept of GIFT is simultaneous determination of P(q) and S(q) with minimal assumptions, letting P(q) model-free together (24) Brunner, P. J.; Glatter, O. J. Appl. Crystallogr. 1997, 30, 431. (25) Weyerich, B.; Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1999, 32, 197. (26) Stradner, A.; Sedgwick, H.; Cardinaux, F.; Poon, C. K. W.; Egelhaaf, S. U.; Schurtenberger, P. Nature. 2004, 432, 492.
with appropriate choices for the interparticle interaction potential model and the closure relation for S(q). Hence, GIFT can be a crucial technique that provides access to dense interacting systems without changing structures of systems under investigation by dilution. The detailed theoretical description on the method has been reported elsewhere.27,28 Microscopic Analysis. Micrograph of reverse vesicle structure was taken by the use of Nomarski-type phase contrast interference microscope.
Results and Discussion Phase Behavior of Qn-D in Oils. The phase diagrams of surfactant/oil binary systems for Qn-D (n ) 10-16) surfactants in different oils in the whole concentration range at atmospheric pressure are shown in Figure 2. (27) Sato, T.; Hossain, Md. K.; Acharya, D. P.; Glatter, O.; Chiba, A.; Kunieda, H. J. Phys. Chem. B 2004, 108, 12927-12939. (28) Acharya, D. P.; Sato, T.; Kaneko, M.; Singh, Y.; Kunieda, H. J Phys. Chem. B, in press.
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Figure 3. (a) SAXS patterns of Q14-D/LP70 systems at 35 °C at various surfactant weight fraction, Ws, (b) the corresponding variation of the interlayer spacing, d, as a function of 1/Ws for the same series shown in panel a.
With decreasing hydrocarbon chain length of surfactant, the melting temperature is monotonically reduced. In each surfactant system, the temperature is practically unchanged upon addition of oil as shown in Figure 2. After the solid surfactant is melted, a liquid crystal is formed, whose structure is identified as a lamellar liquid crystal confirmed by SAXS. The lamellar liquid crystal (LR) phase can solubilize some amount of oil and get swollen, but with successive addition of oil, a two-phase region consisting of excess oil and LR phase, in which the interlayer spacing is unchanged with the compositions, is obtained as shown in Figures 3 and 4. On the other hand, the d spacing of the surfactant solid is unchanged in a whole range of compositions and slightly larger than that for liquid crystal, ∼5 nm for both systems. We observed the melting of the surfactant in WAXS patterns. As shown in Figure 5, there appears one sharp peak in a wideangle regime of the SAXS data at lower temperatures whose position (q value) corresponds to the characteristic distance ∼0.42 nm in real space. This high-q peak is the signature of solid state of surfactant that arises from the crystalline structure of the hydrophobic tail. It diminishes at a temperature between 25 and 35 °C. The solid shows a lamellar structure in the SAXS region, whereas strong single peak is observed in the WAXS region. Hence this crystal is considered to be R-crystal or Lβ phase.29-32 (29) Krog, N. J. In Food Emulsion, 2nd ed.; Larsson, K., Friberg, S. E., Eds.; Marcel Dekker: New York, 1990; p 127. (30) Larsson, K., Dejmek, P. In Food Emulsion, 2nd ed.; Larsson, K., Friberg, S. E., Eds.; Marcel Dekker: New York, 1990; p 97. (31) Hernqvist, L. In Crystallization and Polymorphism of Fats and Fatty Acids; Garti, N., Sato, K., Eds.; Marcel Dekker: New York, 1988; p 97. (32) Larsson, K. Nature. 1967, 213, 383.
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Figure 4. (a) SAXS patterns of Q14-D/Squalane system at 35 °C at various surfactant weight fraction, Ws, and (b) the corresponding variation of the interlayer spacing, d, as a function of 1/Ws for the same series shown in panel a.
Figure 5. Small- and wide-angle X-ray scattering patterns of pure Q14-D at different temperatures.
After disappearance of the WAXS peak, we still observe equidistant sharp reflections in the small-angle region (q/nm-1 < 2), indicating that the solid surfactant is melted and the LR phase is formed. Above the LR present region, a turbid two-phase region or a single isotropic phase region are formed as is shown in Figure 2. The turbid solution, on standing for few minutes, separates into two transparent liquid phases. Judging from the solubility curve, one phase is an oil-rich phase with less surfactant and the other phase is the surfactant phase containing a considerable amount of oil. The boundary of single and two-phase regions corresponds to the cloud-point curve for a water-poly(oxyethylene) type of surfactant system,33-35 in which the surfactant becomes less hydrophilic with increasing temperature and the phase separation takes place. In the oil systems, however, the miscibility of oil and surfactant increases with an increase in the (33) Schick, M. J. J. Colloid Sci. 1962, 17, 801. (34) Hey, M. J.; Ilett, S. M.; Davidson, G. J. Chem. Soc., Faraday Trans. 1995, 91, 3897. (35) Tasaki, K. J. Am. Chem. Soc. 1996, 118, 8459.
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Figure 6. Micrograph of reverse vesicle structure observed in a 5% Q14-D/squalene.
temperature. With increasing temperature, the penetration of oil in the surfactant chain increases and the packing parameter decreases; that is, flat or nearly flat aggregates transform to cylindrical or spherical aggregates, depending on the lipophilic chain length of the amphihile. Thus, the present result confirms the penetrating theory of Ninham and co-workers.36 If the water is added, one can expect that three coexisting phases (water, surfactant, and oil phases) would appear. In fact, in the water/ Qn-D/oil systems, we observed the three-phase regions. Judging from the width of two-phase regions, Q14-D/squalene is most soluble compared with Q14-D/LP70 and Q14-D/squalane. This tendency is the same for other three-surfactant systems. Squalene has six double bonds in the molecule and is more polar than the other two. LP70 is a liquid-paraffin, but the molecular weight of squalane (saturated hydrocarbon) is larger. The difference in the oil nature may correspond to the dissolution tendency of the diglycerol surfactant. Reverse Vesicle Formation. Although the single LR region is narrow and exists only in surfactant rich regions, two-phase regions containing the LR phase, which is actually a dispersion of liquid crystals in oil, exists in a wide range of compositions as shown in Figure 2. In the dilute regions, the dispersion of the LR phase or reverse vesicles is formed. Figure 6 shows the images of vesicular aggregate as seen by an optical microscope. Reverse Micellar Structure. The upper critical solution point (top temperature for the liquid-liquid miscibility curve) is close to the solvents axis as shown in Figure 2. This is the same phenomenon of polymer solutions, namely, the apparent molecular weight of the surfactant in solution is as high as the polymers. In other words, surfactant molecules are aggregated and form reverse micelles in solution. We obtained SAXS curves for 5 wt % Q14-D/squalene and Q16-D/squalene solutions at 50 °C, at which a single isotropic phase is formed. In the GIFT calculation, we selected the averaged structure factor for the hard sphere interaction, S(q)av,37,38 with a Percus-Yevick (PY) closure relation, which considers simply the Gaussian distribution of the interaction radius, R, for individual monodisperse systems for polydispersity. Even in cases of elongated particles, the GIFT approach allows us to deduce realistic p(r), effectively suppressing the influence of S(q) on the calculation of p(r), whereas the output parameters from S(q) analysis become no longer physically correct, especially for highly elongated systems. (36) Israelachvilli, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568. (37) Pusey, P. N.; Fijnaut, H. M.; Vrijm, A. J. Chem. Phys. 1982, 77, 4270. (38) Salgi, P.; Rajagopolan, R. AdV. Colloid Interface Sci. 1993, 43, 169.
Figure 7. (a) Scattering functions of 5 wt % Q14/squalene (O) and Q16/squalene (0) systems in absolute unit. Solid and broken lines respectively represent GIFT fit and the form factor. (b) The pairdistance distribution functions (PDDFs) of 5% Qn-D/oils; Q14/ squalene (solid line), Q16/squalene (dashed line), and Q16/LP70 (dashed dotted line), and (c) those of 5% Q14-D/squalene with different water content; without added water (solid line), 0.04 wt % water (dashed line), and 0.14 wt % water (dashed dotted line). The arrows in panels b and c approximately indicate the cross section diameter of reverse micelles.
As shown in Figure 7b, all PDDF curves exhibit the typical feature of a rodlike particle judging from a pronounced peak in the low-r regime and an extended tail to the high-r side. The absence of local maximum and minimum on the lower-r side of the peak position indicates homogeneous electron density distribution (no core-shell structure) in observing particles. However, since the contrast for oils and that for the hydrophobic part of the surfactants is almost identical, one has to recognize that SAXS measurements can detect only the hydrophilic core of the reverse micelles. The position indicated by the arrow in Figure 7, panels b and c, r ∼ 2.5 nm, semiquantitatively indicates the cross section diameter of the hydrophilic core, which is well fitted to the length of two diglycerol moieties. This cross section diameter is almost unchanged in different Qn-D/oil systems. On the other hand, the maximum length of the hydrophilic part, Dmax, read out from the truncation point of the PDDFs to zero, differs for different surfactant/oil systems. When the hydrocarbon chain length of surfactant decreases, Dmax increases. This result is in agreement with the phase diagrams. Namely, the shorthydrocarbon chain surfactant tends to separate from oil as shown in Figure 2, resulting in the reduction of surface area per surfactant on the micellar hydrophobic/hydrophilic interface. As for Q16-
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D, the reverse micelle in squalene is much shorter than that in LP70. This tendency is also in good agreement with the anticipated tendency from the phase diagram. In the squalene system, the mutual dissolution of surfactant and oil is the highest among the three oils. Effects of Added Water on the Micellar Structure: In Q14D/squalene systems, we also investigated the effects of added water on the micellar size by means of SAXS with GIFT analysis. It is well-known that a small amount of water induces the micellization in nonaqueous media in poly(oxyethylene) type nonionic surfactant systems. Upon addition of 0.14 wt % of water to 5 wt % Q14-D/squalene system, the dramatic elongation of reverse micelles takes place as proven by Figure 7c. A slight increase of the cross-section of the hydrophilic core is observed in terms of the slight shift of the inflection point after the maximum in PDDF. Hence, water induces mainly the elongation of micelles. Note that surfactant samples themselves may contain a small amount of water. It is practically difficult to remove water completely and to measure SAXS data in an absolutely dry state. For example, the present Q14-D contains 0.66% of water, which means that in the 5 wt % Q14-D/squalene solution, at least 0.03 wt % water is already involved before adding water. Hence, upon addition of water, the water content in the system is roughly changed from 0.03 to 0.17 wt %. It is therefore difficult not only to characterize the micellar structure under a perfectly dry condition but to discuss the effect of added water on the micellar shape in the exact manner. Nevertheless, our analysis unambiguously provides the direct structural evidence that water induces drastic elongation of reverse micelles.
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Conclusion Phase behavior of diglycerol fatty acid esters in nonpolar oils has been investigated. Two liquid phase regions above the melting temperature of the either solid or liquid crystalline phase in QnD/oil systems corresponds to the cloud point curve of the nonionic surfactant-aqueous solution. Diglycerol fatty acid ester becomes more soluble in nonpolar oils at higher temperature. On decreasing the amphiphilic chain length of the surfactant, two liquid phase regions become wider and wider. In common with all investigated surfactants, they are most soluble in squalene, directly linked with the narrowest two-phase regions, and least soluble in squalane. All of the Qn-D, surfactant/oil systems form a lamellar (LR) liquid crystalline phase in the surfactant rich region between the solid melting temperature and isotropic two- or single-phase regions. Reverse vesicles are formed in LR+ O region at lower surfactant concentration region. Diglycerol fatty acid esters form reversed rodlike micelles at higher temperature (50 °C) and its size increases with decreasing the hydrophobic chain length of the surfactant. In a fixed surfactant system, reverse micellar size increases by changing the oil from squalene to LP70. Even though the surfactant contains a very small amount of water, addition of traces amount of water (0.14 wt %) dramatically elongates the reverse micelles. Acknowledgment. This work was supported by Core Research for Evolution Science and Technology (CREST) of JST Corporation. LA052622+
