Effect of Polyol on the Structure of Nonionic Surfactant Reverse

The real space structural functions of the reverse micelles were obtained by ..... change in the shape of the p(r) functions, indicating prolate-to-cy...
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Effect of Polyol on the Structure of Nonionic Surfactant Reverse Micelles in Glycerol Monoisostearate/Decane Systems Lok Kumar Shrestha,† Takaaki Sato,‡ Dharmesh Varade,§ and Kenji Aramaki*,† †

Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, ‡International Young Researchers Empowerment Center, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan, and §School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland Received August 17, 2009. Revised Manuscript Received September 30, 2009

Using small-angle X-ray scattering (SAXS), effects of different polyols used as polar additives, glycerol (GC), ethylene glycol (EG), and 1,2-butanediol (1,2-BD), on the structure of nonionic surfactant glycerol monoisostearate (iso-C18G1) reverse micelles in decane have been investigated as a function of polyol concentration and temperature. The real space structural functions of the reverse micelles were obtained by generalized indirect Fourier transformation (GIFT) evaluation of the SAXS data, letting the form factor virtually model-free, and the results were complemented by conventional theoretical model fittings. The iso-C18G1 forms spheroid type or slightly elongated prolate type micelles in n-decane. We have found that addition of these polyols causes two-dimensional (2-D) growth of the reverse micelles in a similar manner to that induced by added water, both the maximum length and micellar cross-section diameter continuously increasing with polyol concentration. EG is most effective to increase the micellar size. 1,2-BD, which is apparently more hydrophobic than EG, has a weakest effect on the micellar growth. Unexpectedly, GC exhibits a less pronounced effect than EG despite its stronger hydrophilicity, which may be related to the similarity of the molecular structure to the surfactant glycerol moiety. The data demonstrate that different polarities of additives can be an additional tunable parameter for controlling the structure of the reverse micelles. The results well complement our recent findings that slightly different properties of solvent oils can provide a significant effect on the reverse micellar structures in glycerol-based surfactant systems.

1. Introduction Lipophilic surfactants are assumed to form reverse micelles (RMs) in organic solvents above the critical micelle concentration (CMC).1 RMs usually exist in spherical shape with a small aggregation number compared to normal micelles.2 Studies on RMs have attracted significant interest over the years because of their applications in solubilization of hydrophilic agent,3 drug delivery,4 and material synthesis.5 RMs can act as stabilizers for reactive species that are insoluble in nonpolar solvents and are also used as size controlling microreactors for different aqueous chemical reactions.6,7 Many studies on enzyme kinetics in reverse micellar solutions have been published.8-10 Furthermore, RMs have also been used as a template for the synthesis of nano*Corresponding author. E-mail: [email protected]. Tel and Fax: þ8145-339-4300. (1) Rodriguez, C.; Uddin, Md. H.; Watanabe, K.; Furukawa, H.; Harashima, A.; Kunieda, H. J. Phys. Chem. B 2002, 106, 22. (2) Penfold, P.; Staples, E.; Tuckeer, I.; Cummins, P. J. Colloid Interface Sci. 1997, 185, 424. (3) Seoud, O.A. El.; Correa, N. M.; Novaki, L. P. Langmuir 2001, 17, 1847. (4) Kr€amer, M.; Stumbe, J.-F.; Tr€uk, H.; Krause, S.; Komp, A.; Delineau, L.; Prokhorova, S.; Kautz, H.; Haag, R. Angew. Chem., Int. Ed. 2002, 41, 4252. (5) Pileni, M. P. Langmuir 1997, 13, 3266. (6) Luisi, P. L., Strab, B. E., Eds.; Reverse Micelles: Biological and Technological relevance of Amphiphilc Structures in Apolar Media; Plenum Press: New York, 1987. (7) Pileni, M. P. Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elasevier: Amsterdam, 1989; Vol 65. (8) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78, 99. (9) Falcone, R. D.; Biasutti, M. A.; Correau, N. M.; Silber, J. J.; Lissi, E.; Abuin, E. Langmuir 2004, 20, 5732. (10) Menger, F. M.; Yamada, K. J. Am. Chem. Soc. 1979, 101, 6731. (11) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (12) Lopez-Quintela, M. A.; Tojo, C.; Blanco, M. C.; Garcı´ a Rio, L.; Leis, J. R. Curr. Opin. Colloid Interface Sci. 2004, 9, 264. (13) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137.

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particles.11-16 Earlier reports indicated that the structure of the nanoparticles depends on the size and shape of the template micelles.17 The great advantage of RMs or water-in-oil microemulsion is the solubilization of water and other polar solvents in the micellar core.7,18 In our previous studies, we found that the polyglycerol fatty acid ester-based RMs can solubilize a certain amount of water, resulting in a modulated structure of the micelles.19-22 Solubilization of water by polymeric RMs has been reported elsewhere.23,24 It has been found that water solubilization capacity of the polymeric RMs increases with temperature.23 Similarly, a notable increase (more than doubling) in the water solubilization has been observed when the PEO content of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymers is decreased from 40 to 20%.24 Polyols have been found to modify the self-assembled structures of surfactants or polymers either in aqueous or nonaqueous systems. The effects of polyols, such as glycerol, propylene glycol, and 1,3,-butanediol, on the phase behavior and microstructure of poly(oxyethylene) type nonionic surfactants have been (14) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893. (15) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (16) Sharma, S. C.; Kunieda, H.; Esquena, J.; Rodriguez-Aberu, C. J. Colloid Interface Sci. 2006, 299, 297. (17) Pileni, M. P. Adv. Colloid Interface Sci. 1993, 46, 139. (18) De, T. K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (19) Shrestha, L. K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Aramaki, K; Kunieda, H. J. Phys. Chem. B 2006, 110, 12266. (20) Shrestha, L. K.; Masaya, K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Kunieda, H. Langmuir 2006, 22, 1449. (21) Shrestha, L. K.; Sato, T.; Aramaki, K. Langmuir 2007, 23, 6606. (22) Shrestha, L. K.; Sato, T.; Aramaki, K. J. Phys. Chem. B 2007, 111, 1664. (23) Alexandridis, P.; Andersson, K. J. Colloid Interface Sci. 1997, 194, 166. (24) Alexandridis, P.; Andersson, K. J. Phys. Chem. B 1997, 101, 8103.

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studied.25,26 An increase or decrease of the lattice spacing in the hexagonal liquid crystalline phase upon the addition of polyols has been explained in terms of dehydration/hydration of the poly(oxyethylene) chain of the surfactants and a consecutive decrease/increase of the surfactant hydrophilicity. In the studies of effects of glycols on the self-assembly of amphiphilic block copolymers in water, Ivanova et al.27 and Alexandridis et al.28 have shown the pronounced effects on the concentration range of stability of the different lyotropic liquid crystalline phases, when polar additives are varied from ethanol (the least polar one in their study) to glucose (the most polar one). The glycerol and glucose swell the hexagonal phase to lower copolymer content, whereas propylene glycol and ethanol do not show this effect. The aim of the present study is to clarify the influence of polyols on the structure of glycerol monoisostearate (iso-C18G1) based RMs. Polyols like glycerol are widely used in humectants in cosmetics and pharmaceuticals, which also influence the physicochemical properties of surfactant solutions, such as cloud point.29 Therefore, the knowledge of the polyols effect on the reverse micellar structure is desired. In the present study, 10 wt % iso-C18G1/decane, in which ellipsoidal prolate type reverse micelles present, was used as the standard system, and different polyols, such as glycerol (GC), ethylene glycol (EG), and 1, 2-butanediol (1,2-BD), were added until phase separation occurs, and using a small-angle X-ray scattering (SAXS) technique, the structural modulations were monitored. The SAXS data were evaluated by generalized indirect Fourier transformation (GIFT) method. The GIFT results were complemented by conventional theoretical model fittings.

2. Experimental Section 2.1. Materials. The surfactant iso-C18G1 with purity >97% was purchased from Wako Chemical Industry, Tokyo, Japan. The surfactant was used without further purification. n-Decane (purity >99.5%) was purchased from Tokyo Chemical Industry, Tokyo, Japan. The reagents of polyols, glycerol, ethylene glycol, and 1,2-butanediol are the product of Wako Chemical Industry, Tokyo, Japan. The schematic molecular structure of iso-C18G1 is given in Scheme 1. 2.2. Methods. 2.2.1. Sample Preparations. A 10 wt % of iso-C18G1 solution in decane was prepared in clean and dry glass ampules with a screw cap. The sample was thoroughly mixed by using a dry thermo bath and a vortex mixer to achieve homogeneity. After mixing, different concentrations of polyols, e.g., EG, GC, and 1,2-BD, were added and mixed using a similar method. After mixing, all of the samples were placed in a temperature-controlled water bath at 25 C for several hours before SAXS measurements. 2.2.2. Small-Angle X-ray Scattering (SAXS). To investigate the effects of polyols on the geometry of iso-C18G1-based reverse micelles, SAXS measurements were carried out in the ternary mixtures of iso-C18G1/decane/polyols as a function of polyol concentration. A SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealed-tube anode X-ray generator (PANalytical, Netherlands) was used. The X-ray generator was operated at 40 kV and 50 mA. An equipped G€ obel mirror and a block collimator provide a focused monochromatic X-ray beam of Cu KR radiation (λ=0.1542 nm) with a well-defined line-shape. A thermostatted sample holder unit (TCS 120, Anton Paar) was used to control the sample temperature. (25) Iwanaga, T.; Suzuki, M.; Kunieda, H. Langmuir 1998, 14, 5775. (26) Aramaki, K.; Olsson, U.; Yamaguchi, Y.; Kunieda, H. Langmuir 1999, 15, 6226. (27) Ivanova, R.; Lindman, B.; Alexandridis, P. Langmuir 2000, 16, 3660. (28) Alexandridis, P.; Ivanova, R.; Lindman, B. Langmuir 2000, 16, 3676. (29) Sagitani, H.; Hirai, Y.; Nabeta, K.; Nagai, M. J. Jpn. Oil Chem. Soc. 1986, 35, 102.

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Scheme 1. Schematic Molecular Structure of iso-C18G1

The 2-D scattering pattern was recorded by an imaging-plate (IP) detector (a Cyclone, Perkin-Elmer, USA) and integrated into one-dimensional scattered intensities I(q) as a function of the magnitude of the scattering vector q = (4π/λ) sin(θ/2) using SAXSQuant software (Anton Paar), where θ is the total scattering angle. All measured intensities were calibrated for transmission by normalizing an attenuated primary intensity to unity, by taking advantage of a semitransparent beam stop. All I(q) data were corrected for the background scattering from the capillary and the solvents, and the absolute scale calibration was made using water as a secondary standard. To obtain the real-space structure function, the so-called pairdistance distribution function, p(r), of the reverse micelles via a virtually model-free routine, we used the GIFT technique.30-33 This procedure relies on a basic equation of one-component globular particle systems, I(q) = nP(q) S(q), and its extension to polydisperse systems, where P(q) is the averaged form factor, S(q) is the static structure factor, and n is the number of particles in unit volume. p(r) is mathematically connected to P(q) as Z ¥ sin qr PðqÞ ¼ 4π pðrÞ dr ð1Þ qr 0 An experimental p(r) can be calculated as inverse Fourier transformation of P(q). To suppress the influence of interparticle interference scattering on the evaluation of p(r), which generally leads to underestimated maximum size of the scattering object, an interaction potential model for S(q) was involved. We chose the averaged structure factor model34,35 of hard-sphere and Percus-Yevick closure relation to solve the Ornstein-Zernike equation. A detailed theoretical description on the method has been given elsewhere.36-38 For an elongated rod-like or cylindrical scattering particle with axial length at least three times longer than the crosssectional diameter, a model-free cross-section analysis of the micellar core can be carried out. Theoretically, the radial electron density profile of the cross section, ΔFc(r), is connected to the cross-sectional pair-distance distribution function, pc(r), as39 pc ðrÞ ¼ rΔ~ F c 2 ðrÞ

ð2Þ

Using the indirect Fourier transformation (IFT) technique, a cross-sectional pair-distance distribution function, pc(r), can be directly calculated from the scattered intensity, I(q), via Z ¥ IðqÞq ¼ πLIc ðqÞ ¼ 2π2 L pc ðrÞJ0 ðqrÞ dr ð3Þ 0

where J0(qr) is the zeroth-order Bessel function. The yielded pc(r) can then be used to calculate ΔFc(r) by the deconvolution technique.40,41 (30) Fritz, G.; Bergmann, A.; Glatter, O. J. Chem. Phys. 2000, 113, 9733. (31) Glatter, O.; Fritz, G.; Lindner, H.; Brunner, P. J.; Mittelbach, R.; Strey, R.; Egelhaaf, S. U. Langmuir 2000, 16, 8692. (32) Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Shubert, K. V.; Kaler, E. W.; Glatter, O. J. Chem. Phys. 1999, 110, 10623. (33) Weyerich, B.; Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1999, 32, 197. (34) Pusey, P. N.; Fijnaut, H. M.; Vrijm, A. J. Chem. Phys. 1982, 77, 4270. (35) Salgi, P.; Rajagopolan, R. Adv. Colloid Interface Sci. 1993, 43, 169. (36) Fritz, G.; Bergmann, A. J. Appl. Crystallogr. 2004, 37, 815. (37) Glatter, O. Prog. Colloid Polym. Sci. 1991, 84, 46. (38) Glatter, O. J. Appl. Crystallogr. 1980, 13, 577. (39) Glatter, O. J. Appl. Crystallogr. 1980, 13, 7. (40) Glatter, O. J. Appl. Crystallogr. 1981, 14, 101. (41) Glatter, O.; Hainisch, B. J. Appl. Crystallogr. 1984, 17, 435.

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Figure 1. Effect of EG on the reverse micellar structures of the 10 wt % iso-C18G1/decane systems as obtained by SAXS; (a) the scattered intensities, I(q), in absolute unit, (b) the pair-distance distribution functions, p(r), and (c) the normalized p(r) [p(r)/pmax(r)] as a function of EG concentration at 25 C. The solid and broken lines in panel a represent GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The inset in panel (a) shows the scattering behavior in the low-q regions. The arrows in panels (b) and (c) indicate the maximum length of the micellar core, Dmax. The broken lines in panel (b) represent the position of inflection points after the maximum, which measures cross-section diameter semiquantitatively. The inset in panel (c) shows p(r) curves in the low-r side, and the arrows highlight the position of the maximum in the p(r) curves.

3. Results and Discussion The water or other polar additives tend to be solubilized in the reverse micellar core and generally cause an increase in the aggregation number.42 Contrary to the conventional poly(oxyethylene) type nonionic surfactants, the iso-C18G1 nonionic surfactant forms RMs in a variety of nonpolar organic solvents at ambient temperature without adding water or polar additives.43 The hydrophilic nature of the surfactant’s headgroup is supposed to be responsible for the formation of such aggregate structures. In the present study, we have investigated the effects of different polyols on the geometry of iso-C18G1 reverse micelles in the iso-C18G1/decane system at 25 C. The different polyols such as EG, GC, and 1,2-BD are mainly used. 3.1. Effect of EG on the Reverse Micellar Structure. Figure 1 shows the scattering functions, I(q), and the resulting pair-distance distribution functions, p(r), of 10 wt % iso-C18G1/ decane/EG systems at different EG concentrations (0.5, 0.75, and 1.0 wt %) at 25 C. Phase separation occurs above 1 wt % EG. In nonaqueous systems, the electron density difference between solvent oil and lipophilic part of surfactant is certainly small, giving negligibly small contrast to the reverse micellar shell. As a result, SAXS detects only a hydrophilic core of the reverse micelles; that is to say, the p(r) must be recognized as a measure of the micellar core structure. The details of the structure of micelles depending on solvent nature, temperature, compositions, and other outer conditions of the iso-C18G1 have been described elsewhere.43 As can be seen in Figure 1a, the feature of I(q) indicates the formation of aggregate structures in the system. The scatteing intensities in the low-q region increase as the EG concentration is increased, and simultaneously those in the high-q regions (42) Mathews, M. B.; Hirschhorn, E. J. Colloid Sci. 1953, 8, 86. (43) Shrestha, L. K.; Shrestha, R. G.; Varade, D.; Aramaki, K. Langmuir 2009, 25, 4435.

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(q > 2.5 nm-1) shift toward the forward direction. These observations can be taken as clear evidence for two-dimensional micellar growth. The micellar growth can be understood more intuitively from Figure 1b, which shows the p(r) curves at different EG concentrations. The maximum length of the core and the core cross-section diameter simultaneously increase with increasing EG concentration. The micellar core having the maximum length of ca. 5 nm in the EG free system transforms into that with ca. 8 nm upon addition of 1 wt % EG. Minute observation of Figure 1b appears to reveal that the inflection point (indicated by broken lines) found at the higher-r side of the maximum in p(r), which gives a semiquantitative measure of the cross-sectional diameter, gradually shifts to the higher-r side with increasing EG concentration. To confirm it, we plot the normalized p(r) defined as p(r)/pmax(r), see Figure 1c. The position of the normalized p(r) shifts towards higher-r side with EG concentration. These simultaneous changes in p(r) indicate that EG not only increases the micellar length, but also modifies the internal structure of the micelles, increasing the cross-section radius of the micellar core. Thus the SAXS data have shown the tendency of EG to promote the micellar core swelling the iso-C18G1 based reverse micelles. 3.2. Effect of GC on the Reverse Micellar Structure. Glycerol (GC) also induces a similar structural transition to the reverse micellar structure of the iso-C18G1/decane systems. Both the maximum length and the micellar cross-section diameter increase with increasing GC concentration up to 1 wt %. Further addition of GC causes phase separation. Figure 2 shows I(q) and the corresponding p(r) curves at different GC concentration at 25 C. For better visibility, the normalized p(r) is also presented in Figure 2. Addition of GC to the 10 wt % iso-C18G1/decane enhances the scattering intensity mainly in the forward direction. A clear realspace picture of the micellar growth is given in the p(r) functions as shown in Figure 2b; all of the p(r) functions show a pronounced maximum in the low-r region with extended and downward DOI: 10.1021/la9030602

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Figure 2. Effect of GC on the reverse micellar structures of 10 wt % iso-C18G1/decane systems as obtained by SAXS; (a) the scattered intensities, I(q), in absolute unit, (b) the p(r) curves, and (c) the normalized p(r) curves as a function of EG concentration at 25 C. The broken lines and arrows in panel (b) represent the cross-section diameter and maximum length of the micellar core, Dmax. The inset in panel (c) shows p(r) curves in the low-r side and the arrows highlight the position of maximum in the p(r) curves.

Figure 3. Effect of 1,2-BD concentration on the reverse micellar structures in the 10 wt % iso-C18G1/decane systems at 25 C as obtained by SAXS; (a) the scattered intensities, I(q), in absolute unit and (b) the p(r) functions. The solid and broken lines in panels (a) represent GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The arrows and a broken line in panel (b) indicate the maximum length of micellar core and the cross-sectional diameter, respectively. The inset in panel (b) shows p(r) curves in the low-r side, and the arrows in the inset highlight the position of the maximum in the p(r) functions.

convex tails in the higher-r region, implying the elongated ellipsoidal prolate type micelles. The maximum length of the micelles increases with increasing GC concentration. The normalized p(r) function reveals the gradual increase of the cross-section diameter of the micellar core with GC concentration. This indicates that similar to the case of EG, the induced micellar growth is two-dimensional, but GC is less effective for the growth of iso-C18G1 micelles than EG. 3.3. 1,2-BD-Induced 2-D Micellar Growth. In this section, we describe the effect of 1,2-BD on the micellar structure. Compared to EG and GC, a higher concentration of 1,2-BD can be solubilized in the micellar core, and it causes a significant effect on the aggregate structure of iso-C18G1-based reverse micelles. First, we describe the effect of 1,2-BD concentration at a fixed temperature of 25 C, and then we will discuss the effect of temperature in mixed system of iso-C18G1/decane/1,2-BD at a fixed concentration of 1,2-BD. 3.3.1. Effect of 1,2-BD Concentration. A notable change in the scattering function in terms of low-q slope and/or magnitude 3118 DOI: 10.1021/la9030602

of the forward scattering intensity is observed with increasing 1,2BD concentration. In the absence of 1,2-BD, the scattering intensity follows q0 behavior of the low-q slope in the forward direction indicating the presence of spheroid type or slightly elongated prolate type particles. The scattering intensities increase with increasing 1,2-BD concentration mainly in the low-q region, which eventually follow I(q) ∼ q-1 behavior, indicating the formation of cylindrical particles. As can be seen in Figure 3b, the characteristics of the shape of the p(r) functions, and thus the (ellipsoidal) shape of the micelles, are almost identical up to 2 wt % of 1,2-BD, whereas Dmax is fairly different depending on the 1,2-BD concentration, which is ∼5 nm in 1,2-BD free system, reaching ∼8 nm at 2 wt % of 1,2-BD. Further increase of the 1,2-BD concentration leads to a drastic change in the shape of the p(r) functions, indicating prolate-tocylinder transition. An extended linear tail in higher-r side of the p(r) functions is clear evidence for the formation of cylindrical particles. From these observations, we can infer that added 1,2-BD first goes to the deepest core of the micelles inducing Langmuir 2010, 26(5), 3115–3120

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Figure 4. (a) The cross-section pair-distance distribution function, pc(r), for the iso-C18G1/decane þ 1,2-BD systems at different 1,2-BD concentration and (b) the corresponding cross-section radial electron density profile, ΔFc(r), deconvoluted from pc(r) shown in panel (a). The arrows in panels (a) and (b) represent the cross-sectional diameter and radius of the micellar core, respectively.

Figure 5. (a) The model calculation and fit to the experimental SAXS intensities of the 5 wt % iso-C18G1/decane (square) and the 10 wt % isoC18G2/decane þ 4.0 wt % 1,2-BD (circle) systems as typical examples. (b) The p(r)-functions calculated from the geometrical models of the spheroid and cylinder respectively for the 1,2-BD free and the 4.0 wt % 1,2-BD added systems. The broken lines and arrows in panel (b) represent the cross-section diameter and maximum length of the micellar core, Dmax.

three-dimensional (3-D) core swelling without significant change of the axial ratio. Further addition of 1,2-BD above 2% promotes reduction of the critical packing parameter favoring aggregates with less negative curvature. As already mentioned, the inflection point seen on the higher-r side of the maximum of p(r), as highlighted by a dotted line at r ∼ 2.5 nm in Figure 3b, gives a semiquantitative measure of the cross-section diameter of the micellar core. Minute observation of the p(r) functions reveals that the distance corresponding to the core cross-section diameter gradually increases with 1,2-BD concentration from ca. 1.8 to 2.5 nm. This can also be seen in the inset of Figure 3b, in which normalized p(r) curves are shown. The position of pmax(r) shifts toward higher-r side with 1,2-BD concentrations. For the quantitative estimation of the core diameter, we performed a model-free cross-section structure analysis, whose typical results are given in Figure 4. The core cross-section diameter from Dcmax in pc(r) and the radius rcmax in ΔFc(r) for the 10 wt % iso-C18G1/decane are almost identical with those estimated from the total p(r) functions. With successive addition of 1,2-BD, the cross-section diameter of the micelles increases from ca. 1.8 to 2.5 nm when 1,2-BD concentration is increased from 1.0 wt % to 4.0 wt %. All these results demonstrate a simultaneous increase in the maximum length and the cross-section, i.e., the 2-D growth, instead of purely 1-D growth. To provide convincing structural information, we attempt to complement the GIFT/IFT results presented in the previous sections; we performed model fitting for selected systems by testing different plausible models for the investigated reverse micelles. The results are presented in Figure 5. The calculation of the theoretical scattering functions was carried out based on the method described elsewhere.19,44 We found that for successful description (44) Glatter, O. Acta Phys. Austriaca 1980, 52, 243.

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of the data, homogeneous prolate ellipsoid and homogeneous cylinder are required respectively for the 5 wt % iso-C18G1/decane and the 5 wt % iso-C18G1/decane þ 4.0 wt % 1,2-BD. The short and long axes of the ellipsoid a and b, and core radius R and the length of the cylinder L obtained from the model fit are consistent with the results of the GIFT analysis (see Figure 5). The deviation of the theoretical scattering function from the experimental one seen in the high-q region (q >2 nm-1) are possibly due to the polydispersity effect and/or moderate electron density distribution in the particle (reverse micellar core), which normally hinder a theoretically predicted minimum in the scattering function. 3.3.2. Effect of Temperature. Figure 6 shows the results of SAXS experiments, I(q) and p(r), at different temperatures (T = 25, 50, and 75 C) for the 10 wt % iso-C18G1/decane þ 4 wt % 1,2-BD system. Increasing temperature rapidly decreases the forward scattering intensity and simultaneously causes a slight high-q shift of the I(q) curve in the high-q region, giving evidence for the micellar shrinkage at high T. The p(r) shown in Figure 6b demonstrates that increasing temperature from 25 to 75 C results in 2-D shrinkage of the micelles. These observations seem to indicate that increasing temperature dissociates polyols from the surfactant headgroup, and the resulting increase of cpp decreases the micellar size. Such behavior is rather similar to that recently found in the monomyristin/decane/glycerol system.45 Figure 7 compares the effects of different polyols on the structural modification of the iso-C18G1 micelles in decane at fixed surfactant and additive concentrations, in which I(q) and p(r)-functions for the 10 wt % iso-C18G1/decane þ 1.0 wt % polyol (EG, GC, and 1,2-BD) systems are displayed. The data show that if the concentration of polyols is fixed EG is most effective for the formation of larger micelles. Use of more (45) Shrestha, L. K.; Glatter, O; Aramaki, K. J. Phys. Chem. B 2009, 113, 6290.

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Figure 6. Effect of temperature on the reverse micellar structures for 10 wt % iso-C18G1/decane þ 4.0 wt % 1,2-BD system. (a) The normalized X-ray scattering intensities, I(q), in absolute unit at different temperatures of 25, 50, and 75 C and (b) the corresponding real-space p(r) functions. The solid and broken lines in panel (a) represent GIFT fit and the calculated form factor for n particles in unit volume nP(q), respectively. The broken lines and arrows in panel (b) represent the cross-section diameters and maximum length of the micellar core, Dmax.

Figure 7. Effect of different additive polyols (EG, GC, and 1,2-BD) on the iso-C18G1 reverse micellar structure in decane. (a) The scattering

curves I(q) of the 10 wt % iso-C18G1/decane þ 1.0 wt % polyol systems in absolute scale at 25 C, (b) the corresponding real-space p(r)-functions, and (c) “master curves” the normalized p(r) functions [p(r)/pmax(r)] vs r* (= r/rmax). The inset in panel (a) highlights the scattering behavior in the low-q side. The broken lines and arrows in panel (b) represent the cross-section diameters and maximum length of the micellar core, Dmax.

hydrophobic 1,2-BD represents a weaker effect on the micellar growth than EG. The influence of GC is not very straightforward; GC exhibits a less pronounced effect than EG despite its stronger hydrophilicity. The phenomenon may be related to the similarity of the molecular structure of GC to the surfactant glycerol moiety. All p(r)-functions collapse on a “master curve” when p(r)/pmax(r) is plotted against normalized axes r/rmax as shown in Figure 7c. The shape or aspect ratio of the micellar core does not significantly differ for different polyols, but only the size depends on the nature of polyols.

4. Conclusion In this paper, we have described the effects of polyols used as polar additives on the structure of iso-C18G1/decane reverse micelles at ambient condition. We have also discussed temperature effects for selected systems. The SAXS data were evaluated by GIFT technique, and the results were complemented by theoretical model fittings. We have found that polyols, depending on their nature and concentration, modulate the structure of iso-C18G1 reverse micelles. Contrary to the classical concept that reverse micelles generally exist 3120 DOI: 10.1021/la9030602

in spheroid shape, we are able to produce a variety of aggregate structures, such as sphere, ellipsoid prolate, short-rod to cylindrical type, depending on the nature of polar additives. Efficiency of EG for producing larger aggregates is apparently higher than 1,2-BD, which is less hydrophilic than EG due to additional two hydrocarbon group. However, GC exhibits less pronounced effect than EG despite its higher hydrophilicity. Our data demonstrate that the influence of polar additives is rather complicated, which is not simply determined by polarity or hydrophilicity of the additives. We postulate that similarity of the molecular structure of GC to the polar headgroup of iso-C18G1 counterbalances its strong polarity, and thus suppresses the micellar growth. Acknowledgment. L.K.S. is thankful for a JSPS Postdoctoral Fellowship for Foreign Researchers. Supporting Information Available: The structure factor parameters effective volume fraction (φ), effective interaction radius (R), and polydispersity obtained from the GIFT evaluation of the SAXS data as a function of polyols concentration. This material is available free of charge via the Internet at http://pubs.acs.org. Langmuir 2010, 26(5), 3115–3120