Structure of Polyglycerol Oleic Acid Ester Nonionic Surfactant Reverse

Feb 24, 2010 - The C18−1G1 and C18−1G4 were received from the Mitsubishi Co. Ltd., Tokyo, Japan, and C18−1G2 and C18−1G6 were received from Ri...
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Structure of Polyglycerol Oleic Acid Ester Nonionic Surfactant Reverse Micelles in Decane: Growth Control by Headgroup Size Lok Kumar Shrestha,† Martin Dulle,‡ Otto Glatter,‡ and Kenji Aramaki*,† †

Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, and ‡Department of Chemistry, University of Graz, Heinrichstrasse 28, 8010 Graz, Austria Received November 8, 2009. Revised Manuscript Received January 20, 2010

The structure of polyglycerol oleic acid ester nonionic surfactant micelles in n-decane has been investigated at room temperature by small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and densiometry techniques. The scattering data were evaluated by indirect Fourier transformation (IFT) or generalized indirect Fourier transformation (GIFT) methods depending on the volume fractions of the surfactants and also by model fit. A simple route to the growth control of reverse micelles by headgroup size of the surfactant was investigated. Additionally, the dependence of reverse micellar structure (shape and size) on temperature, composition, and added water was also investigated. The indirect Fourier transformation gives the real space pair-distance distribution function, p(r): a facile way for the quantitative estimation of structure parameters of the aggregates. It was found that the size of the reverse micelles increases with increasing the headgroup size of the surfactant. Globular type of micelles with maximum diameter ca. 6 nm observed in the monoglycerol oleic acid ester/decane system at 25 C transferred into elongated prolate type micelles with maximum diameter ca. 19.5 nm in the hexaglycerol oleic acid ester/decane system. In a particular surfactant and oil system, increasing temperature decreased the micellar size. The size of the micelle was decreased by ∼25% upon increasing temperature from 25 to 75 C in the 5 wt % diglycerol oleic acid ester/decane system. Concentration could not modulate the structure of micelles despite a wide variation in the surfactant concentration (5-25 wt %). Nevertheless, increasing surfactant concentration reduces the intermicellar distance, and a strong repulsive interaction peak was observed in the scattering curves at higher surfactant concentrations. Besides, the results obtained from the dynamic light scattering have shown the signature of diffusion hindrance relative to hard sphere with the surfactant concentration. Interestingly, the reverse micelles of the 10 wt % diglycerol oleic acid ester/ decane system could incorporate ∼1.2% water in the micellar core and cause a dramatic growth to the micelles size. The size of the water swollen micelles was ∼40% bigger than the empty micelle.

1. Introduction Studies on the formulation of reverse micelles (RMs) and their structure (shape and size) control have attracted a significant interest over the years in surfactant or polymer sciences due to its wide range of applications. RMs has been used as templates for the synthesis of nanomaterials and also as a size controlling microreactor for various aqueous chemical reactions.1-7 It has been found that the size of the nanoparticles largely depends on the size of the template micelles and also the shape.8 Reverse micellar solutions have also been used in the studies of enzyme kinetics reactions.9,10 Therefore, studies on the structural characterization of RMs have been increased over the past few years. RMs have an opposite structure compared to the normal micelles in aqueous systems and often known as inverse or inverted *To whom correspondence should be addressed: e-mail [email protected]. jp; Fax þ81-45-339-4300.

(1) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (2) Lopez-Quintela, M. A.; Tojo, C.; Blanco, M. C.; Garcı´ a Rio, L.; Leis, J. R. Curr. Opin. Colloid Interface Sci. 2004, 9, 264. (3) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137. (4) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893. (5) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (6) Pileni, M. P. Langmuir 1997, 13, 3266. (7) Pileni, M. P. In Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989; Vol. 65. (8) Pileni, M. P. Adv. Colloid Interface Sci. 1993, 46, 139. (9) Lissi, E. A.; Abuin, E. B. Langmuir 2000, 16, 10084. (10) Falcone, R. D.; Biasutti, M. A.; Correa, N. M.; Silber, J. J.; Lissi, E.; Abuin, E. Langmuir 2004, 20, 5732.

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micelles. The RMs core is made up of the hydrophilic headgroup and the shell of the lipophilic nonpolar part of the surfactant molecules. RMs are very common in the ternary mixtures of surfactant/water/oil systems mainly in oil-rich regions11-18 and are also equally been observed in aqueous systems of lipophilic surfactant in surfactant-rich regions.19,20 Although water has been regarded as an essential component in the formulation of RMs, there are some reports on the formation of reverse micelles in organic solvents without water addition.21-25 (11) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (12) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233. (13) Friberg, S. E.; Blute, I.; Kunieda, H.; Stenius, P. Langmuir 1986, 2, 659. (14) Kunieda, H.; Solans, C.; Parra, J. L. Colloids Surf. 1987, 24, 225. (15) Solans, C.; Pons, R.; Davis, H. T.; Evans, D. F.; Nakamura, K.; Kunieda, H. Langmuir 1993, 9, 1479. (16) Uddin, H. Md.; Rodriguez, C.; Watanabe, K.; Lopez-Quintela, M. A.; Kato, T.; Furukawa, H.; Harashima, A.; Kunieda, H. Langmuir 2001, 17, 5169. (17) Kunieda, H.; Tanimoto, M.; Shigeta, K.; Rodriguez, C. J. Oleo Sci. 2001, 50, 633. (18) Kaneko, M.; Matsuzawa, K.; Uddin, H. Md.; Lopez-Quintela, M. A.; Kunieda, H. J. Phys. Chem. B 2004, 108, 12736. (19) Kunieda, H.; Uddin, H. Md.; Horii, M.; Furukawa, H.; Harashima, A. J. Phys. Chem. B 2001, 105, 5419. (20) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (21) Desjardins, A.; van de Ven, T. G. M.; Eisenberg, A. Macromolecules 1992, 25, 2412. (22) Zhong, X. F.; Varsheny, S. K.; Eisenberg, A. Macromolecules 1992, 25, 7160. (23) Forster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956. (24) Rodriguez, C.; Uddin, Md. H.; Watanabe, K.; Furukawa, H.; Harashima, A.; Kunieda, H. J. Phys. Chem. B 2002, 106, 22.

Published on Web 02/24/2010

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Shrestha et al. Scheme 1. Schematic Molecular Structure of Polyglycerol Oleic Acid Esters (C18-1Gn, n = 1, 2, 4, and 6)

The ternary mixtures of water/Aerosol OT (AOT)/oils or water/lecithin/oils have mostly been considered for the studies of reverse micelles or W/O microemulsions so far.26-33 Some authors have also reported the formulation of ionic liquid-based reverse micelles, in which ionic liquids replace water.34 On the other hand, glycerol-based nonionic surfactants have attracted considerable attention these days because of its various applications in food and cosmetic. These surfactants are derived from natural sources and are environmentally friendly and also biocompatible. Because of lyophobic nature of polyglycerol, the glycerol-based surfactants tend to form different self-assembled structures in nonaqueous systems.35-37 In studies of surfactant self-assembly of polyglycrol fatty acid esters in a variety of organic solvents, we have found that in addition to the water/Aerosol OT (AOT)/oils or water/lecithin/oils systems glycerol-based nonionic surfactants could also be the good systems for the study of RMs. These surfactants spontaneously form micelles, vesicles, and liquid crystals in nonpolar solvents depending on temperature, composition, and other outer conditions.38,39 It has been found that the shape and size of the RMs depend on the solvent nature, temperature, composition, alkyl chain length of the surfactant, and also on water addition. Mono- and diglycerol fatty acid esters have been shown to form small spheroid type RMs in short chain oil cyclohexane or in aromatic oils ethylene benzene or phenyloctane.39 The structural evolution (in terms of both shape and size) has been found with increasing hydrocarbon chain length of the linear chain oils from octane to hexadecane.38,39 Similarly, increasing surfactant concentration favored growth of the monoand diglyceride RMs, whereas increasing the lipophilic chain length of surfactant or temperature favored micellar shrinkage.40 Effect of headgroup size on the reverse micellar structure has been (25) Shrestha, L. K.; Masaya, K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Kunieda, H. Langmuir 2006, 22, 1449. (26) De, T.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (27) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B 1997, 101, 8292. (28) Cason, J. P.; Roberts, C. B. J. Phys. Chem. B 2000, 104, 1217. (29) Li, Q.; Li, T.; Wu, J. J. Phys. Chem. B 2000, 104, 9011. (30) Novaki, L. P.; Correa, N. M.; Silber, J. J.; El Seoud, O. A. Langmuir 2000, 16, 5573. (31) Kanamaru, M.; Einaga, Y. Polymer 2002, 43, 3925. (32) Tung, S.-H.; Huang, Y.-E.; Raghavan, S. R. J. Am. Chem. Soc. 2006, 128, 5751. (33) Novaira, M.; Moyano, F.; Biasutti, M. A.; Silber, J. J.; Correa, N. M. Langmuir 2008, 24, 4637. (34) Falcone, R. D.; Correa, N. M.; Silber, J. J. Langmuir 2009, 25, 10426. (35) Herrington, T. M.; Shali, S. S. J. Am. Oil Chem. Soc. 1988, 65, 1677. (36) Rodriguez-Aberu, C.; Acharya, D. P.; Hinata, S.; Ishitobi, M.; Kunieda, H. J. Colloid Interface Sci. 2003, 262, 500. (37) Shrestha, L. K.; Aramaki, K. J. Dispersion Sci. Technol. 2007, 28, 1236. (38) Shrestha, L. K.; Sato, T.; Aramaki, K. Langmuir 2007, 23, 6606. (39) Shrestha, L. K.; Sato, T.; Aramaki, K. J. Phys. Chem. B 2007, 111, 1664. (40) Shrestha, L. K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Aramaki, K.; Kunieda, H. J. Phys. Chem. B 2006, 110, 12266.

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studied in short chain glycerol-based surfactants, which form RMs only at elevated temperatures. An ellipsoidal prolate-tocylinder type transition has been observed upon increasing headgroup size from mono- to diglycerol.41 In the present paper, we report the structural characterization of RMs in polyglycerol oleic acid esters (C18-1Gn, n = 1, 2, 4, and 6) in decane at room temperature. We have performed systematic studies on the effect of headgroup size, temperature, concentration, and water content on the reverse micellar structures. For the structural characterization of RMs, small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) techniques have been used. The SAXS data were analyzed by the indirect Fourier transformation (IFT), its updated version including particle interaction generalized indirect Fourier transformation (GIFT) methods, and also model fitting.

2. Experimental Section 2.1. Materials. Commercial grade nonionic surfactants polyglycerol oleic acid esters (designated as C18-1Gn, n = 1, 2, 4, and 6 are the number of glycerol molecule per surfactant molecule as hydrophilic moieties) were taken. The C18-1G1 and C18-1G4 were received from the Mitsubishi Co. Ltd., Tokyo, Japan, and C18-1G2 and C18-1G6 were received from Riken Vitamin Co. Ltd. and Nikko Chemicals Co. Ltd., Tokyo, Japan, respectively. The molecular structures of C18-1G1, C18-1G2, C18-1G4, and C18-1G6 are given in Scheme 1. The organic solvent n-decane (99.5% pure) was purchased from Tokyo Chemical Industry, Tokyo, Japan. Deionized and Milli-Q filtered water was used. 2.2. Methods. 2.2.1. Equilibrium Phases at 25 C. The equilibrium phases of the polyglycerol oleic acid ester (C18-1Gn, where n = 1, 2, 4, and 6) in n-decane in the dilute regions was identified by visual inspection through a crossed-polarizer. For this purpose, 5-25 wt % C18-1Gn (n = 1, 2, 4, and 6)/decane binary mixtures were prepared in clean and dry glass ampules (5 mL) with the screw cap. The samples were mixed using dry thermobath, vortex mixer, and repeated centrifugation to achieve homogeneity. After mixing, the samples were kept in a temperature-controlled water bath at 25 C for 2 h prior to observing the equilibrium phase. The accuracy of the temperature reading in thermometer is (0.5 C. Equilibrium phases of the surfactant/oil/water ternary system was also determined by the same procedure. Water was then added into the reverse micellar solution of C18-1G2/decane system (10 wt %) until phase separation. 2.2.2. Small-Angle X-ray Scattering (SAXS). A series of SAXS measurements were performed on the dilute solutions of C18-1Gn in decane as a function of headgroup size of the surfactant, temperature, and concentration and also on the (41) Shrestha, L. K.; Glatter, O.; Aramaki, K. J. Phys. Chem. B 2009, 113, 6290.

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ternary mixtures of 10 wt % C18-1G2/decane þ water as a function of water content. In the studies of temperature effect on the micellar structure, a 5 wt % C18-1G2/decane system was considered, and temperature was varied from 25 to 75 C. For concentration effect, 5-25 wt % C18-1G2/decane samples were prepared. In the SAXS measurements, a SAXSess camera (Anton Paar, Austria) attached to a PW3830 X-ray generator (PANalytical, Netherlands) with a sealed-tube anode (Cu-KR wavelength of 0.1542 nm) was used. The generator was operated at 40 kV and 50 mA. The SAXSess camera is equipped with a focusing multilayer optics and a block collimator for an intense and monochromatic primary beam with low background. A semitransparent beam stop enables a measurement of an attenuated primary beam for the exact definition of the zero scattering vector and transmission correction. Samples were enclosed into vacuum-tight, reusable thin quartz capillary to attain exactly the same scattering volume and background contribution. The sample temperature was controlled with a thermostated sample holder unit (TCS 120, Anton Paar). The 2-D scattered intensities recorded on an imaging plate (IP) detector were read out by a Cyclone system (Perkin-Elmer) and were converted via SAXSQuant software (Anton Paar) to one-dimensional scattering curves as a function of the magnitude of the scattering vector q = (4π/λ) sin(θ/2), where θ is the total scattering angle. All the intensities were transmission-calibrated by normalizing the attenuated primary intensity at q = 0 to unity and were corrected for the background scattering from the capillary and the solvent. The absolute scale calibration was made using water as a secondary standard.42 For monodisperse globular particle systems having n particles in unit volume, the total (absolute) scattered intensity I(q) can generally be described by IðqÞ ¼ nPðqÞSðqÞ

ð1Þ

where P(q) is the averaged form factor and S(q) is the static structure factor. The P(q) is given by the Fourier transformation of the so-called pair-distance distribution function (PDDF), p(r), as Z

¥

PðqÞ ¼ 4π 0

sin qr dr pðrÞ qr

ð2Þ

S(q) describes the spatial distribution of the particles and is given by the Fourier transformation of the total correlation function, h(r) = g(r) - 1, as Z SðqÞ ¼ 1 þ 4πn

¥

½gðrÞ -1r2

0

sin qr dr qr

ð3Þ

where g(r) is the pair-correlation function. The SAXS data for the reverse micellar solutions were analyzed by the generalized indirect Fourier transformation (GIFT) technique.43-45 Its basic concept is the simultaneous determination of P(q) and S(q) with minimal assumptions. The P(q) is calculated via the model-free routine as the well-established IFT does, while an interaction potential model for S(q) is to be assumed. In the present study we have used the averaged structure factor S(q) model46,47 of hard-sphere interaction potential, which is described by the weighted average of Percus-Yevick analytical solutions of the Ornstein-Zernike equation for Gaussian-distributed hard-sphere

(42) Orthaber, D.; Bergmann, A.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 218. (43) Brunner, P., J.; Glatter, O. J. Appl. Crystallogr. 1997, 30, 431. (44) Glatter, O.; Fritz, G.; Lindner, H.; Brunner, P. J.; Mittelbach, R.; Strey, R.; Egelhaaf, S. U. Langmuir 2000, 16, 8692. (45) Bergmann, A.; Fritz, G.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 1212. (46) Pusey, P. N.; Fijnaut, H. M.; Vrijm, A. J. Chem. Phys. 1982, 77, 4270. (47) Salgi, P.; Rajagopolan, R. Adv. Colloid Interface Sci. 1993, 43, 169.

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radii. The detailed theoretical description on the method has been reported elsewhere.48-50 For cylindrical scattering objects with circular symmetry and an axial length at least 3 times longer than the cross-sectional diameter, a model-free cross-section analysis of the micellar structure is available under the assumption that the scattering length density profile of the cross section is simply a function of the radial position. The radial profile, ΔFc(r), is related to the cross-sectional PDDF pc(r) as51 pc ðrÞ ¼ rΔ~ F c 2 ðrÞ

ð4Þ

The cross-sectional PDDF can be calculated from the scattered intensity via Z

¥

IðqÞq ¼ πLIc ðqÞ ¼ 2π L 2

pc ðrÞJ0 ðqrÞ dr

ð5Þ

0

where J0(qr) is the zeroth-order Bessel function. The indirect Fourier transformation of eq 5 yields pc(r), which is then used to calculate ΔFc(r) by the deconvolution technique.52,53 2.2.3. Dynamic Light Scattering (DLS). Dynamic light scattering (DLS) measurements were performed on a laboratory built goniometer equipped with a diode pumped solid-state laser (Coherent Verdi V5, λ = 532 nm, Pmax = 5 W) used with a power of 300-500 mW with a single-mode fiber detection optics (OZ from GMP, Z€ urich, Switzerland), ALV/SO-SIPD/DUAL photomultiplier with pseudo-cross-correlation, and ALV 5000/E correlator with fast expansion (ALV, Langen, Germany). A number of 30 s DLS measurements were carried out at the scattering angle of 90 for each sample. Ten of the resulting intensity autocorrelation functions G2(τ) were averaged and transformed into the massweighted distribution of the hydrodynamic radii D(RH) by an inverse Laplace transformation utilizing the optimized regularization technique (ORT).54 2.2.4. Densimetry. A high-precision DSA5000 densimeter (Anton Paar, Austria) was used to measure densities of surfactants, oil, and their mixtures at the same temperature of SAXS experiments. The DSA5000 instrument is based on the conventional mechanical oscillator method, which measures the natural resonant frequency of a U-shaped glass tube, filled with 1 mL sample. The highly tuned temperature control of the apparatus enables an accuracy of 10 mK in an absolute value.

3. Results and Discussion 3.1. Isothermal Phases in the Dilute Regions. The equilibrium phases in the dilute region of the polyglycerol oleic acid esters (C18-1Gn, n = 1, 2, 4, and 6) in n-decane were identified by visual inspection through a crossed polarizer at 25 C. Polyglycerol oleic acid esters in decane form isotropic solutions in the dilute regions (5-25 wt %) at 25 C. However, its counterpart stearic acid esters are immiscible in decane at this temperature, which could possibly be due to stiffer saturated hydrocarbon chain in the lipophilic skeleton. It was found that the dilute solutions of the C18-1G4/decane or C18-1G6/decane systems appear slightly translucent; however, phase separation did not take place even after a month and also not upon centrifugation, which confirms the presence of single-phase solutions. We did not determine a detailed phase behavior study (temperaturecomposition diagrams) since it is beyond the scope of the present (48) (49) (50) (51) (52) (53) (54)

Fritz, G.; Bergmann, A. J. Appl. Crystallogr. 2004, 37, 815. Glatter, O. Prog. Colloid Polym. Sci. 1991, 84, 46. Glatter, O. J. Appl. Crystallogr. 1980, 13, 577. Glatter, O. J. Appl. Crystallogr. 1980, 13, 7. Glatter, O. J. Appl. Crystallogr. 1981, 14, 101. Glatter, O.; Hainisch, B. J. Appl. Crystallogr. 1984, 17, 435. Schnablegger, H.; Glatter, O. Appl. Opt. 1991, 30, 4889.

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Figure 1. (a) Normalized X-ray scattering intensities, I(q), of the 5 wt % C18-1Gn (n = 1, 2, 4, and 6)/decane systems in absolute units at

25 C, (b) the corresponding p(r) curves, (c) the normalized p(r) curves, p(r)/p(rmax), and (d) the normalized p(r) curves vs r* (r/rmax). The solid and broken lines in (a) represent GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. Arrows in (b) and (c) indicate the maximum dimension of the RMs, Dmax.

study. In the following sections, we discuss about the growth control of RMs by variation of headgroup size of the surfactant and the effect of temperature and concentration on the reverse micellar structure. Finally, we will discuss the water induced microstructure transition of the host RMs. 3.2. Growth Control by Headgroup Size. Polyglycerol oleic acid esters spontaneously form RMs in n-decane at ambient condition and by tuning headgroup size of the surfactant the micellar size could flexibly be controlled. A monotonous growth to the micellar size was observed with increasing headgroup size of the surfactant. Figure 1 shows the scattering data obtained from the SAXS measurements with calculated form factors and the real space pair-distance distribution functions (PDDF), p(r), derived from the GIFT method. In Figure 1a, symbols represent the scattering data in absolute unit and solid and broken lines represent the GIFT fit and the calculated total form factor for n particles existing in unit volume, nP(q), respectively. A strong q dependence of I(q) in the I(q) vs q curve indicates the presence of self-aggregate structure (RMs in the present case) in the systems studied. As can be seen in Figure 1a, the scattering curves reach toward zero q horizontally, indicating the presence of globular type scattering particles in the C18-1G1/decane system. Besides, the absence of local maximum and minimum in the low-q regions highlights the present system as noninteracting. With increasing hydrophilic headgroup size of the surfactant, the scattering behavior is modified significantly both in the low-q and high-q regions. The low-q scattering intensity and also the low-q slope increase with the headgroup size of the surfactant. These features can be taken as the strong evidence for micellar growth. Furthermore, the scattering intensity in the high-q region (or cross-section region) shifts toward the forward direction showing the growth of the cross-section diameter of the micelles. Thus, the data show that headgroup size of the surfactant modulates both the maximum size and internal cross-sectional structure of the RMs in surfactant/oil binary systems. The effect headgroup size on the 7018 DOI: 10.1021/la904231t

structure of micelles can be seen best in real space in the PDDF, p(r), shown in Figure 1b. All the p(r) curves show pronounced peaks in the low-r side with slightly asymmetric tails in the higher-r side, indicating the presence of ellipsoidal prolate type of RMs in the systems. Note that the p(r) functions go to zero at different r values, indicating the size variation depending on the headgroup size of the surfactant. With increasing headgroup size, the maximum dimension of the particles, Dmax, as indicated by arrows, increases. From the viewpoint of spontaneous curvature, it is quite reasonable to anticipate bigger nonaqueous RMs (reverse micelles only in surfactant/oil system) with the large headgroup surfactant system. Increasing headgroup size of the surfactant keeping lipophilic tail identical decreases the critical packing parameter, which means aggregate with less negative curvature in nonaqueous systems, i.e., larger or elongated particles. In the studies of headgroup size effect on the reverse micellar structure of glycerol fatty acid ester, we have found that slightly elongated ellipsoidal prolate particle found in monoglycerol monomyristate (C14G1)/ tetradecane system transformed into a long cylinder type particle in the diglycerol monomyristate (C14G2)/tetradecane system at 60 C.41 Similar results have been found in the mono- and diglycerol monolaurate/decane systems at 60 C.55 It was also found that in both the monolaurate and monomyristate/oil systems increasing headgroup size increases the cross-section diameter of the particles. The mono- and diglycerol fatty acid esters do not form RMs at room temperature and thus limit their practical applications for which the present system of polyglycerol oleic acid ester would be more appropriate. Moreover, the size of the RMs increases greatly with increasing headgroup size of the surfactant (see Figure 1b,c). The micellar dimensions increases from 6.0 to 19.5 nm upon increasing headgroup size from mono- to hexaglycerol. (55) Shrestha, L. K.; Sato, T.; Aramaki, K. Phys. Chem. Chem. Phys. 2009, 11, 4251.

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Figure 2. Direct cross-section (core) structure analysis. (a) The cross-sectional PDDF, pc(r), for the C18-1Gn (n = 2, 4, and 6)/decane systems and (b) the corresponding electron density profile, ΔFc(r), calculated via deconvolution from pc(r). Solid lines in (a) represent the DECON fit.

Figure 3. (a) Model fits (full lines) and experimental X-ray scattered intensities of 5 wt % C18-1G1/decane (square), 5 wt % C18-1G2/decane (circle), and 5 wt % C18-1G4/decane (triangle) as typical examples and (b) the p(r) functions derived from GIFT and model fittings. The data were fitted using the model of a homogeneous prolate.

Increasing headgroup size of the surfactant also caused growth to the cross-section diameter, which is clearly seen in the normalized p(r) function, p(r)/p(rmax) (see Figure 1c). The position of the maximum, i.e., rmax, also increases with the headgroup size. Finally, we have tested if the headgroup size causes any significant change in the RMs shape. Figure 1d shows the plot of p(r)/p(rmax) vs r* (r/rmax). One can see that all the curves lie mostly on top of each other, indicating no significant change in the shape of the RMs. Only the micellar size and so the scattering intensity vary with increasing headgoup size of the surfactant. Thus, the present SAXS data have clearly shown the importance of headgroup size to the growth control of the nonaqueous RMs. For the scattering particles with elongated geometries, a direct cross-section analysis is available under assumption that the scattering length density profile of the cross section is simply a function of the radial position. However, for such analysis the axial length of the elongated particles should be at least 3 times longer than the cross-sectional diameter. Symmetrical particles allow calculation of the convolution square root, resulting in the radial scattering length density profile. Figure 2 shows the crosssectional real space pair-distance distribution function, pc(r), and the corresponding electron density distribution profile, ΔFc(r). The pc(r) functions are typical for homogeneous particles, and the maximum diameter, Dc,max, increases with increasing headgroup size. The maximum cross-sectional diameter of the C18-1G2 system was ca. 3.7 nm and was increased to 7.6 nm in the C18-1G6 system. The maximum cross-sectional radius, Rc,max, estimated from the scattering length density profiles is exactly half of the maximum cross-section diameter and follows the similar increasing trend with headgroup size. Note that the cross-sectional radius of 1.9 nm for the C18-1G2 system is higher than twice Langmuir 2010, 26(10), 7015–7024

the extended chain length of the glycerol molecule as one glycerol would best account for 0.4-0.5 nm. This implies that the core of the RMs not only consists of diglycerol moiety, but there could be some water molecules, as the glycerol surfactants are highly hygroscopic and there is the possibility presence of some water as an impurity. It is always good to obtain structural information on scattering particles by more than one method. Therefore, we have performed model fitting for the selected systems, and the results are presented in Figure 3. The model fitting was performed testing different plausible models based on the method reported in detail elsewhere.56 The 5 wt % C18-1G1/decane, C18-1G2/decane and 5 wt % C18-1G4/decane systems were considered for the model fitting, and the form factor of these systems could be well fitted with elongated prolate structures with homogeneous electron density distribution. Note that the experimental scattering functions, I(q), lack the theoretically predicted minimum mainly due to polydispersity effect. The GIFT evaluation of the scattering data has shown an ellipsoid prolate RMs with maximum dimension Dmax ∼ 9 nm in the 5 wt % C18-1G2/decane system and the more elongated ellipsoid prolate with Dmax ∼ 15 nm in the 5 wt % C18-1G4/decane system. The short and long axes of the ellipsoidal prolate, a and b, obtained from the model fittings are in good agreement with the results obtained from the GIFT method. Furthermore, the p(r) functions derived from the model fittings are very similar to the p(r) functions deduced from the GIFT method (see Figure 3b). The micellar aggregation number calculated from the results of model fitting was found to increase with increasing headgroup size of the surfactant; the details of aggregation number and other structure (56) Glatter, O. Acta Phys. Austriaca 1980, 52, 243.

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Figure 4. (a) Normalized X-ray scattering intensities, I(q), of the 5 wt % C18-1G2/decane system in absolute unit at different temperatures (25, 50, and 75 C) and (b) the corresponding p(r) curves. The solid and broken lines in (a) represent GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. Arrows and broken line in (b) highlight the maximum dimension, Dmax, and the crosssection diameter.

parameters are supplied in the Supporting Information. Thus, the model fittings well supports the general results obtained directly from the model-free GIFT data analysis procedure. 3.3. Effect of Temperature. Temperature usually increases the penetration of oils to the lipophilic chain of the surfactant in nonaqueous systems forming aggregates with more negative spontaneous curvature. Figure 4 shows the I(q) and the resulting p(r) curves for the 5 wt % C18-1G2/decane system at different temperatures (25, 50, and 75 C). With increasing temperature the scattering intensity in the low-q region decreases keeping the scattering behavior practically the same at higher-q regions at q > 1.5 nm-1 (see Figure 4a and the inset). Such behavior in the I(q) vs q curves indicates the decrease in the micellar size virtually keeping cross-section diameter unchanged. As can be seen in real space p(r) functions, at all temperatures (25-75 C) the shape of the p(r) curves are apparently the same; the only change is in the Dmax value (see Figure 4b). Increasing temperature from 25 to 75 C leads to a decrease of the micellar size from ca. ∼9.5 to 7.2 nm, i.e., ∼25% micellar shrinkage. Judging from the trend in the microstructure transitions, one can anticipate the globular type of RMs at sufficient higher temperatures. Very similar results of temperature effect on the reverse micelles structure were observed in other glycerol-based nonionic surfactant/oil systems.38-41 Furthermore, it was also found that the miscibility of glycerol-based surfactants and oils increases with increasing temperature.25 The hydrophobic volume fraction of the surfactant increases with the rise of the temperature, and the van der Waals interaction between the hydrocarbon chain of the surfactant and the oils increases, leading to an increase in the oil solubilization. Increasing temperature increases the penetration of oil to the lipophilic chain of the surfactant. As a result, the packing parameter increases favoring the formation of aggregates with more negative curvature. However, in the studies of structure and dynamics of AOT-based RMs systems, Lang et al.57 reported that the aggregation of AOT-based RMs increases with the rise of temperature, which contradicts our present results, in which we have found the decreasing trend of aggregation number with temperature. This discrepancy can be attributed to the anomalous behavior of AOT in solution. From the studies of the solution behavior of AOT/oil/water containing some salt, Kunieda et al.58 found the existence of hydrophilic-lipophilic balance (HLB) temperature. This is the temperature at which the hydrophilic-lipophilic balance of adsorbed surfactant monolayer just balances toward a given oil/water system in a solution of a (57) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946. (58) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601.

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balanced ionic surfactant as well as nonionic surfactant. AOT is dissolved in the oil phase below this temperature and forms revere micellar solution, which is in equilibrium with excess water phase. On the other hand, AOT is dissolved in water phase above this temperature forming normal micellar solution, which is in equilibrium with oil. That is to say, in the AOT/oil/water system, with increasing temperature W/O phase changes to O/W phase. However, present glycerol-based nonionic surfactant/oil systems behave in the opposite way, and hence, micellar aggregation number decreases with temperature. It is interesting to note that the net decrease in the size of the RMs with increasing temperature was also found to depend on the nature of surfactant or oils. In some system such as C12G1/ tetradecane increasing temperature by 10 C decrease the micellar size by ∼40%.55 Similarly, in the case of glycerol monoisostearate/hexadecane system, it was found that increasing temperature from 25 to 75 C reduces the micellar size by ∼50%.59 Note that with increasing temperature the contrast of such systems becomes worse, and interpretation of scattering data may suffer from artifacts, which may lead to underestimation of Dmax. However, the RMs systems possess a positive electron density fluctuation in the hydrophilic core, whose hydrophobic shell has almost negligible contrast with the oils. This offers rather a better situation for the interpretation of scattering data despite worse contrast at higher temperatures. As shown in Figure 4b, the decreasing micellar size by ∼25% upon increasing temperature from 25 to 75 C cannot be truly caused by artifact. Thus, the SAXS data suggest that the length of the aggregates decreases with the rise of temperature. Figure 5 shows the results of 5 wt % C18-1G2/ decane system at two different temperatures 25 and 50 C obtained from model fitting as typical examples. As can be seen from this figure, a model of a homogeneous prolate ellipsoid fits the data well. The micellar size and the aggregation number are decreasing with temperature. 3.4. Effect of Surfactant Concentration. Figure 6 shows the I(q) and the corresponding p(r) curves for the C18-1G2/decane system at different concentrations (5-25 wt %) at 25 C. It was found that the RMs size does not increase despite a large variation in the surfactant concentrations. As one can see in the scattering curves (Figure 6a), with increasing surfactant concentration from 5 to 15 wt %, the scattering intensities increase throughout the q-range due to increase in the number density of the RMs in unit volume. However, with further increasing concentration, say above 15 wt %, the scattering intensity in the low-q region (59) Shrestha, L. K.; Shrestha, R. G.; Varade, D.; Aramaki, K. Langmuir 2009, 25, 4435.

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Figure 5. (a) Experimental X-ray scattered intensities, model and GIFT fits of 5 wt % C18-1G2/decane systems at two different temperatures as typical examples and (b) the p(r) functions derived from model fits (lines) and GIFT method (symbol). The data were fitted using a model of a homogeneous prolate ellipsoid.

Figure 6. (a) Scattering curves I(q) of the C18-1G2/decane systems at different surfactant concentrations obtained on absolute scale at 25 C, (b) the corresponding real space p(r) functions, (c) the p(r) functions normalized by volume fraction of the surfactant, and (d) the structure factor curves obtained from the GIFT evaluation of the SAXS data. Solid and broken lines in (a) represent GIFT fit and the calculated total form factor for n particles existing in unit volume, nP(q), respectively. The arrows and broken lines in (b) and (c) indicate the maximum dimension and cross-sectional diameter of the micelles, respectively.

decreases and a weak but growing interaction peak appears at intermediate q-range (q ∼ 0.9 nm-1). This peak comes from the repulsive intermicellar interactions.60,61 The structure of the reverse micelles has been found to depend on the nature of the surfactants or oils and depending on the systems the RMs may or may not grow with concentration. Usually, elongated micelles such as cylindrical micelles show growth with concentration. On the other hand, spherical micelles, which are normally formed by the lipophilic surfactants above critical micelle concentration (cmc) in nonaqueous systems, do not grow with composition. With increasing surfactant concentration, the number density of micelles increases and a repulsive (60) Stradner, A.; Sedgwick, H.; Cardinaux, F.; Poon, W. C. K.; Egelhaaf, S. U.; Schurtenberger, P. Nature 2004, 432, 492. (61) Stradner, A.; Glatter, O.; Schurtenberger, P. Langmuir 2000, 16, 5354.

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intermicellar force, which works perpendicular to the interface, comes into play. The repulsive intermicellar interactions induce micelle ordering with a symmetry depending on the micellar shape. In the present study, we have found a clear signature of slightly elongated prolate type RMs in the C18-1G2/decane system at 25 C; however, increasing surfactant concentration from 5 to 25 wt % could not increase the micellar size. The only change occurred with concentration is the increased number density of scattering particles at higher surfactant concentrations. As can be seen in Figure 6b, the maximum dimension of the micelles, Dmax, remains practically the same for 5-25 wt % surfactant systems. Furthermore, the inflection point seen on the higher-r side of the maximum of p(r), as highlighted by a broken line at r ∼ 3.5 nm in Figure 6b, which is a semiquantitative measure of the cross-section diameter of the micellar core, remains the same by a wide variation in the surfactant concentration. DOI: 10.1021/la904231t

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Figure 7. Results obtained from DLS measurements. (a) Diffusion coefficient and (b) apparent hydrodynamic radius as a function of volume fraction of the surfactant (C18-1G2) at 25 C.

Figure 8. Direct cross-section (core) structure analysis. (a) The cross-sectional pair-distance distribution functions, pc(r), for the C18-1G2/ decane systems at different surfactant concentrations (5-25 wt %) and (b) the corresponding electron density profile, ΔFc(r), calculated via deconvolution from pc(r). Solid lines in (a) represent the DECON fit.

The concentration independence of the micellar structure can be seen best in Figure 6c, where the p(r) curves are divided by the surfactant volume fraction Φs. All the curves lie on top of each other. If there would a micellar growth, the curves height would increase with the volume fraction of the surfactant.41 Here we discuss the structure factor, S(q), curves (Figure 6d) obtained from the GIFT method. The S(q) curves reveal the presence of considerable intermicellar interactions mainly at higher concentrations, say above 15 wt %. Minute observations of the S(q) curves reveal that the S(q) peak position, which measures the mean distance between the micelles, shifts slightly toward higher-q regions with increasing concentration, indicating the decrease in the intermicellar distance. Next, the S(q = 0) value decreases monotonically with surfactant concentration due to the reduced osmotic compressibility of micelles. The actual structure factor peaks might differ from what is predicted for monodisperse hard spheres.62 Nevertheless, the polydispersity derived from the GIFT method was found to be very low (∼0.07). As was mentioned earlier, SAXS detects only the micellar core of the nonaqueous RMs due to negligible contrast between the solvent and lipophilic part of the surfactant. From the virtually concentration independent structure factor peak at q ∼ 0.9 nm-1 in the S(q) curves, the mean distance between the micelles is estimated to ∼7.0 nm, which is slightly smaller than the average maximum diameter of the micelles. This highlights the fact that the micelles are not approaching laterally but from other possible orientations. Next, it is possible to anticipate the interpenetration of RMs in the studied systems, mainly at higher volume fraction of the surfactant. The detailed GIFT parameters (interaction radius, effective volume fractions, polydispersity, (62) Svensson, B.; Olsson, U.; Alexandridis, P.; Mortensen, K. Macromolecules 1999, 32, 6725.

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etc.) as a function of surfactant concentration are supplied in the Supporting Information. We have also performed DLS measurements as a function of concentration in order to confirm if any interpenetration of RMs taking place at higher concentrations. This technique is sensitive to the collective diffusion. The collective diffusion shows an increase of the effective diffusion coefficient with volume fraction for hard spheres, which was clearly shown for polymeric micelles in aqueous systems.63 However, RMs can also show a strong decrease of the collective diffusion with concentration due to interpenetration.64 For the system studied here the effective diffusion coefficient tends to increase with the surfactant concentration following D0(1 þ 1.14Φ) behavior, which is a sign of diffusion hindrance relative to hard sphere. For hard sphere the diffusion coefficient follows D0(1 þ 1.56Φ) behavior. The inflection point of the p(r) curves after the maximum (dashed line in Figure 6b,c) of the C18-1G2/decane systems semiquantitatively estimates the cross-section diameter to ∼3.5 nm. For the quantitative estimation, we have used the direct crosssection analysis by using IFT and deconvolution procedures. The real space cross-sectional pair-distance distribution function, pc(r), can directly be calculated from the experimental scattered intensity I(q) via eq 3, and the deconvolution of the pc(r) gives the difference electron density profile ΔFc(r). Figure 8 shows the cross-sectional pc(r) functions and the corresponding contrast profile for the C18-1G2/decane systems at different surfactant concentrations at 25 C. Note that the positive electron density profiles in Figure 8b came from the electron-rich hydrophilic reverse micellar core. The (63) Lindner, H.; Scherf, G.; Glatter, O. Phys. Rev. E 2003, 67, 061402-1–9. (64) Glatter, O.; Orthaber, D.; Stradner, A.; Scherf, G.; Fanun, M.; Garti, N.; Clement, V.; Leser, M. E. J. Colloid Interface Sci. 2001, 241, 215.

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Figure 9. (a) X-ray scattering intensities I(q) of 10 wt % C18-1G2/decane at different concentration of added water obtained on absolute scale at 25 C, (b) the corresponding p(r) curves, and (c) a “master curve” plot of normalized p(r) function, p(r)/p(rmax) vs r* = r/rmax. Solid and broken lines in (a) represent GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The arrows in (b) indicate the maximum dimension of the RMs.

resulting pc(r) functions represent typical homogeneous aggregates with maximum diameter of ∼3.8 nm. This value is close to the value estimated from the inflection point in total p(r) functions and is independent of the composition. The maximum core radius (Rcore) estimated from the contrast profile, Rcore ∼ 1.9 nm, is exactly half of the cross-section diameter but higher than the extended chain of the twice the glycerol moiety, again indicating presence of some water in the micellar core. 3.5. Effect of Water. One of the most interesting features of RMs is their potential to solubilize water or other polar additives. It has been shown that added water or other polar additives tend to solubilize in the interior of the RMs and so cause an increase in the aggregation number.65 Moreover, RMs are mostly formulated in water-incorporated systems.66-71 Note that traces of water can always be present as an impurity in hygroscopic surfactants such as glycerol-based nonionic surfactants and facilitate RMs formation. Previous reports have shown that water or other additives induce a change the geometry of the host RMs and the size is much bigger than the empty micelles.38,39,41,72 Therefore, the study of water effect on the structure of RMs could be interesting. First we have determined the water solubilization at fixed surfactant/oil system and then performed SAXS measurements in the mixed surfactant/oil/water systems as a function of water concentration. It is interesting to note that 10 wt % C18-1G2/ (65) Mathews, M. B.; Hirschhorn, E. J. Colloid Sci. 1953, 8, 86. (66) Angelico, R.; Palazzo, G.; Colafemmina, G.; Crikel, P. A.; Giustini, M.; Ceglie, A. J. Phys. Chem. B 1998, 102, 2883. (67) Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356. (68) Schurtenberger, P.; Magid, L. J.; King, S. M.; Lindner, P. J. Phys. Chem. 1991, 95, 4173. (69) Wu, G.; Zhou, Z.; Chu, B. Macromolecules 1993, 26, 2117. (70) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (71) Alexandridis, P.; Andersson, K. J. Colloid Interface Sci. 1997, 194, 166. (72) Li, J.; Zhang, J.; Han, B.; Gao, Y.; Shen, D.; Wu, Z. Colloids Surf., A 2006, 279, 208.

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decane system could solubilize 1.2% water at 25 C, showing that the present system can be a good reservoir for water-soluble drug molecules, which can be encapsulated inside the RMs. We have studied the effect of water on the geometry of RMs in C18-1G2/ decane systems at 25 C. Figure 9 shows the I(q) and the corresponding p(r) curves for the 10 wt % C18-1G2/decane systems at different concentrations of water. Interestingly, the forward scattering intensity I(q = 0) is strongly enhanced, and the scattering curve of the high-q region (∼1.9 nm-1) shifts toward the forward direction upon addition of water (see Figure 9a). Such behavior in the scattering functions indicates the waterinduced simultaneous changes in the maximum dimension and the cross-section structure of the RMs. As can be seen from Figure 9b, the position of the rmax and maximum dimensions of the micelles (Dmax) increase with increasing water concentration, indicating the micellar growth. Minute observation of the p(r) curves reveals that the shape of the curves up to 1.0% water are almost identical; however, the p(r) curve of the 1.2% water system is close to rodlike micelles as can be judged from pronounced peak in the low-r side and extended tail in the higher-r side. In a “master plot” plot of p(r)/p(rmax) vs r* (= r/rmax) all the p(r) curves fall on a “master curve” up to 1.0% water, as shown in Figure 9c. This shows that the shape does not change only the size is changing with increasing the amount of water up to 1.0%. A linear tail seen in the “master curve” of the 1.2% water content system is the indication of the shape transition as well. The present results are in good agreement with our previous findings in other glycerolbased nonionic surfactant systems.38,39 Such changes are due to the fact that the water molecules have the tendency to make a water pool at the micellar core. Besides, water may form hydrogen bonding with the glycerol molecule so that the overall hydrophilic size of the surfactant increases and favors micellar growth due to a lower critical packing parameter. Figure 10 shows the results obtained from the direct crosssection analysis for the water added systems at different concentrations DOI: 10.1021/la904231t

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Figure 10. Direct cross-section (core) structure analysis of water incorporated systems. (a) The cross-sectional pair-distance distribution function, pc(r), for the C18-1G2/decane þ water system at different concentration of water and (b) the corresponding electron density profile, ΔFc(r), calculated via deconvolution from pc(r). Solid lines in (a) represent the DECON fit.

Figure 11. (a) Model fittings (full lines) and experimental X-ray scattered intensities of 10 wt % C18-1G2/decane þ water systems at different water contents (symbol) as typical examples and (b) the p(r) functions derived from model fittings (lines) and GIFT method (symbol). In (a) square = 0% water, triangle = 0.8% water, and circle = 1.2% water. The data were fitted using a model of a homogeneous prolate.

of water. As can be seen from Figure 10a, the cross-section diameter of the RMs increases with increasing water concentration, again supporting the fact that the RMs incorporates water forming a water pool in the micellar core. This is also further confirmed by the contrast profiles, in which one can see the increasing cross-sectional radius of RMs with water. It should be noted that addition of 1% water increases the cross-sectional radius by ∼47%. Figure 11 compares the results obtained from GIFT evaluation of the scattering data and model fittings for the 10 wt % C18-1G2/decane and 10 wt % C18-1G2/decane þ water systems at two different water concentration as typical examples. A homogeneous prolate model fits the scattering data well, and the structure parameters obtained from the model fittings are close to the values obtained from the GIFT method. Besides the position of minimum obtained in the experimental form factor in the cross-section region is almost identical with the minimum of the theoretical form factor curve. Thus, the model fit confirms the general results obtained directly from the model-free data analysis procedure. The aggregation number calculated from the results of model fitting was found to increase with water content. This shows that added water not only swells the reverse micelles but also increases the number of surfactant molecules per micellar aggregate. The detail structure parameters as a function of water content is supplied in the Supporting Information.

4. Conclusion We have investigated the self-assembled structures of the polyglycerol oleic acid ester (C18-1Gn, n = 1, 2, 4, and 6) nonionic surfactants in decane at normal room temperature, 7024 DOI: 10.1021/la904231t

and the effects of headgroup size, temperature, composition, and added water on the structure of nonaqueous reverse micelles have been studied systematically. The structure of the RMs were determined by the GIFT evaluation of the SAXS data and supported by the model fits. It was found that the micellar growth could be controlled by headgroup size of the surfactant. Increasing headgroup size increases the maximum dimension and the cross-sectional diameter of the RMs; i.e., headgroup size favors reverse micellar growth. The size of the RMs decreases with the rise of temperature. Concentration variation could not modulate the structure of micelles but led to a repulsive interaction among the micelles due to excluded volume effects in the system at higher concentrations. The RMs formulated in the C18-1G2/decane system could incorporate a significant amount of water, enhancing the potential application of the present system in the solubilization and water-soluble drug encapsulation. Interestingly, water caused a drastic change in the structure of host RMs; the size and cross section increase with the amount of water. Acknowledgment. L.K.S. thanks the Japan Society for the Promotion of Science (JSPS) for postdoctoral fellowship for foreign researchers. Supporting Information Available: Densities of surfactants and oil, electron density difference of the hydrophilic part in different systems, the structure parameters obtained from the GIFT method and model fittings, and the micellar aggregation number obtained from the results of model fittings. This material is available free of charge via the Internet at http://pubs.acs.org. Langmuir 2010, 26(10), 7015–7024