Structural Investigation of Diglycerol Polyisostearate Reverse Micelles

J. Phys. Chem. B 2009, 113, 12669–12679

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Structural Investigation of Diglycerol Polyisostearate Reverse Micelles in Organic Solvents Lok Kumar Shrestha,† Rekha Goswami Shrestha,† Keiichi Oyama,‡ Makoto Matsuzawa,‡ and Kenji Aramaki*,† Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, and The Nisshin OilliO Group, Ltd., 1 Shinmori-cho, Isogo-ku, Yokohama 235-8558, Japan ReceiVed: April 13, 2009; ReVised Manuscript ReceiVed: August 1, 2009

The structure of glycerol-based reverse micelles in the surfactant/oil binary system without external water addition has been investigated using a small-angle X-ray scattering technique, and different tunable parameters for the structure control of reverse micelles are determined. The scattering data were evaluated by the generalized indirect Fourier transformation (GIFT) method and complemented by model fitting. It was found that diglycerol polyisostearates (abbreviated as (iso-C18)nG2, n ) 2-4, where n represents the number of isosterate chains per surfactant molecule) form reverse micelles in a variety of organic solvents such as cyclohexane, n-decane, and n-hexadecane without the addition of water from outside, and their structure (shape and size) depends on solvent properties (alkyl chain length), tail architecture of the surfactant, temperature, and added water. Small globular types of micelles were observed in the (iso-C18)2G2/cyclohexane system at 25 °C. The micellar size and the aggregation number were increased with increasing the alkyl chain length of the oils resulting in elongated ellipsoidal prolate or rodlike type micelles in the (iso-C18)2G2/ hexadecane system. This structural evolution is caused by the different penetration tendency depending on the chain length of oils to the lipophilic chain of the surfactant. At fixed oil, composition, and temperature, the tail architecture of the surfactant played a crucial role in the micellar structure. The micellar size and, hence, the aggregation number decreased monotonically with increasing number of isostearate chain per surfactant molecule due to the voluminous lipophilic part of the surfactant. Composition could not modulate the structure of micelles but led to strong repulsive interactions among the micelles due to reduced osmotic compressibility of the system at higher concentrations. Increasing temperature decreased the micellar size, while the cross-section structure remains essentially the same. The structure was modified significantly in terms of micellar size and cross-section diameter upon solubilization of traces water in the surfactant/oil/ water system. Both the maximum size and the cross-section diameter of the micelles increase with water; i.e., reverse micelles swell with water forming a water pool in the micellar core. Furthermore, from the results of model fittings, it was found that the aggregation number increases with water concentration. 1. Introduction Reverse micelles have an inverse structure in comparison to the conventional normal micelles in aqueous systems. Therefore, they are often known as inverse or inverted micelles. In reverse micelles, the micellar cores consist of a hydrophilic polar component, and the shells consist of a lipophilic nonpolar part of the surfactant molecules. Reverse micelles are mostly observed in the ternary mixtures of surfactant/water/oil mostly in oil-rich regions.1-8 Furthermore, reverse micelles have also been observed in aqueous systems of lipophilic surfactant in surfactant-rich regions.9,10 In most of the studies carried out in the past, water was regarded as an essential component in the formulation of reverse micelles. Only few reports exist in the literature of surfactant science that describes the formation of reverse micelles in organic solvents without water addition.11-15 Studies on reverse micelles have attracted significant interest over the years because of their wide range of applications. Reverse micelles stabilize species that are insoluble in nonpolar solvents and are also used as a size-controlling microreactor * Corresponding author. E-mail: [email protected]. Phone & Fax: +81-45-339-4300. † Yokohama National University. ‡ The Nisshin OilliO Group, Ltd.

for different aqueous chemical reactions.16,17 Reverse micelles have been used as a template for the synthesis of nanomaterials for a long time.18-24 It has been found that the structure of the nanomaterials largely depends on the structure of the template micelles.25 In the studies of reverse micelles, ternary mixtures of surfactant/water/oil systems such as ternary mixtures of water/ Aerosol OT (AOT)/oils or water/lecithin/oils were mostly considered.26-31 Some authors have also formulated reverse micelles in mixed solvent systems.32,33 Formation of reverse micelles in surfactant/oil binary systems without the addition of water is still a matter of discussion as much fewer numbers of amphiphiles are reported to form reverse micelles in surfactant/oil binary systems. The hydrophilic moiety of polyglycerol fatty acid esters has been found to be more solvophobic in comparison to the conventional ethylene oxide (EO) based nonionic surfactants and, hence, tend to form micellar aggregates in nonpolar solvents.34,35 Recently, we have studied the phase behavior of short chain mono- and diglycerol fatty acid esters in a variety of organic solvents such as liquid paraffin, squalane, and squalene. Liquid crystalline phases were not observed in the temperature-composition diagrams of the monoglyceride/ oil systems but an isotropic solution of reverse micelles at

10.1021/jp903382y CCC: $40.75  2009 American Chemical Society Published on Web 09/01/2009

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SCHEME 1: Schematic Molecular Structure of Diglycerol Polyisostearate (iso-C18)nG2 (n ) 1-4)

elevated temperatures.36 Contrary to the monoglycerides, the diglyceride surfactants efficiently formed a variety of selfassembled structures.14 It has been found that the diglyceride surfactants form a lamellar liquid crystal (LR) phase in the surfactant axis and swell with oils and a dispersion of reverse vesicles in the dilute regions at 25 °C. The LR phase transforms into an isotropic solution phase consisting of reverse micelles upon heating. Similarly, the surfactants (diglycerol monolaurate and diglycerol monomyristate) formed reverse micelles in n-alkanes at higher temperatures after melting of the LR phase.37,38 In short chain oil such as cyclohexane or in aromatic oils such as ethylene benzene or phenyl octane, isotropic solutions of reverse micelles have been observed with these surfactants at room temperature.39,40 However, the formulation of reverse micelles with long chain lipophilic glycerol-based nonionic surfactants at room temperature has not carried out, so far. Glycerol-based nonionic surfactants are drawing considerable interest from food or cosmetic scientists due to its biocompatibility and biodegradability. Therefore, studies on the selfassemblies of such surfactants would offer various practical applications. In this context, we have carried out structural investigation of diglycerol polyisostearate (abbreviated as (isoC18)nG2, n ) 2-4) reverse micelles in different nonpolar oils such as cyclohexane, n-decane, and n-hexadecane in the surfactant/oil binary systems at 25 °C and determined the tunable parameters to the structural control of the reverse micelles. We have mainly focused on the studies of the effect of solvent properties, nature of surfactant (tail architecture of surfactant), composition, and temperature. Finally, we have investigated the effect of a small amount of solubilized water on the structure of the reverse micelles, and the results are compared with the monoglycerol isostearate/oil systems. For the structural characterization of reverse micelles, a small-angle X-ray scattering

(SAXS) technique has been used, and the data were evaluated by the generalized indirect Fourier transformation (GIFT) method and complemented by model fitting. 2. Experimental Section 2.1. Materials. The glycerol-based nonionic surfactants diglycerol polyisostearates (abbreviated as (iso-C18)nG2, n ) 1-4) with purity >99% were a generous gift from the Nisshin OilliO Group, Ltd., Japan. The surfactants were used without further purification. The nonpolar organic solvents, cyclohexane, n-decane, and n-hexadecane, were purchased from Tokyo Chemical Industry, Tokyo, Japan. All the oils were 99.5% pure. Millipore-filtered water was used to investigate water-induced reverse micellar structure. The schematic molecular structures of (iso-C18)nG2 (n ) 1-4) are given in Scheme 1. 2.2. Methods. 2.2.1. Phase Equilibrium at Room Temperature. The equilibrium phases in the dilute region of the surfactant/oil binary mixtures were identified by visual inspection through a crossed-polarizer. For this purpose, 5-25 wt % of (iso-C18)nG2 (n ) 1-4)/oil mixtures (∼2 g) were prepared in cyclohexane, n-decane, and n-hexadecane in a 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 to observe the equilibrium phases. The accuracy of the temperature reading in a thermometer is ( 0.5°. An isotropic solution phase was observed in all the systems except the diglcyerol monoisostearate (iso-C18)1G2/oil system, in which a turbid solution was observed. The turbid solution was stable with temperatures up to 75 °C. A further temperature scan was not carried out. Equilibrium phase of the surfactant/oil/water system was also determined following the same procedure. Water was added into

Diglycerol Polyisostearate Reverse Micelles

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the reverse micellar solution of the (iso-C18)2G2/decane system (10 wt %) until phase separation. 2.2.2. Small-Angle X-ray Scattering (SAXS). SAXS measurements were carried out on a series of samples to investigate the effect of solvent properties, nature of surfactant, composition, and water addition on the reverse micellar structure. The samples (after mixing) were placed in the water bath at 25 °C for 2 h before SAXS measurements. In the SAXS measurements, a SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealed-tube anode X-ray generator (PANalytical, Netherlands), which was operated at 40 kV and 50 mA, was used. An equipped Go¨bel mirror and a block collimator enabled us to obtain 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. 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 semiautomatically calibrated for transmission by normalizing a zero-q 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 generalized indirect Fourier transformation (GIFT) method.41-44 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. As p(r) is mathematically connected to P(q) as

P(q) ) 4π

∫0∞ p(r) sinqrqr dr

(1)

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) that generally leads to highly underestimated maximum size of the scattering object, an interaction potential model for S(q) was to be involved, where we chose the averaged structure factor model45,46 of hard-sphere and the Percus-Yevick closure relation to solve the Ornstein-Zernike equation. The detailed theoretical description on the method has been given elsewhere.47-49 When the axial length of an elongated rodlike cylindrical scattering particle is at least three times longer than the crosssectional diameter, direct cross-section analysis of the micellar core can be carried out. Theoretically, the radial electron density profile, ∆Fc(r), is connected to the cross-sectional pair-distance distribution function, pc(r), as50

pc(r) ) r∆F ˜2c (r)

(2)

Using an extended technique of indirect Fourier transformation (IFT), pc(r) can directly be calculated from the experimental scattered intensity I(q) via

I(q)q ) πLIc(q) ) 2π2L

∫0∞ pc(r)J0(qr)dr

(3)

where J0(qr) is the zero-order Bessel function. The yielded pc(r) can then be used to calculate ∆Fc(r) by the deconvolution procedure.51,52 2.2.3. Rheometry. The steady shear rheological measurements were performed in a stress-controlled rheometer, ARG2 (TA Instruments), using a cone-plate geometry (diameter 60 mm with a cone angle of 1°) with the plate temperature controlled by a peltier unit, which uses the peltier effect to rapidly and accurately control heating and cooling. 2.2.4. Densimetry. Using a high-precision DSA5000 densimeter (Anton Paar, Austria),53 density measurements were carried out on diglycerol polyisostearate surfactants, oils (cyclohexane, decane, and hexadecane), and the reverse micellar solutions 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 a 1 mL sample. The highly tuned temperature control of the apparatus enables an accuracy of 10 mK in an absolute value. 3. Results and Discussions 3.1. Isothermal Phases in the Dilute Regions. The equilibrium phases in the dilute region of the diglycerol polyisostearate (iso-C18)nG2, (n ) 1-4) surfactants in different organic solvents were identified by visual inspection through a crossed polarizer at 25 °C. It was found that these surfactants (isoC18)nG2, (n ) 2-4) form isotropic solutions in cyclohexane, n-decane, and n-hexadecane in the dilute regions at 25 °C. Contrary to the glycerol monoisostearate, (iso-C18)1G1, the diglycerol monoisostearate, (iso-C18)1G2, could not form micellar aggregates in the above-mentioned oils at 25 °C. A turbid solution appeared in the dilute regions of the (iso-C18)1G2/oil systems over a wider temperature range (up to 75 °C). This may possibly be caused due to increased hydrophilic counterpart in the surfactant molecule. We did not carry out the detail phase behavior (temperature-composition diagrams) study since it is beyond the scope of the present study. In the following sections, we discuss the solvent dependence of reverse micellar structure and the effect of nature of surfactants (mainly tail architecture), composition, and temperature, on the structure of reverse micelles. Furthermore, we will discuss how a small amount of solubilized water modulates the structure of host reverse micelles. 3.2. Solvent Dependence of Reverse Micellar Structure. Here, we discuss about solvent dependence of reverse micelle structure and their rheological behavior. Figure 1 shows the scattering functions, I(q), and the pair-distance distribution functions (PDDFs), p(r), derived from the GIFT method for the (iso-C18)2G2/oil binary systems in different oils. The composition and temperature are fixed to 5 wt % (iso-C18)2G2 and 25 °C, respectively. The oil is varied from cyclohexane to hexadecane via decane. Some interesting features can be seen in the Figure 1a, first being the q dependence of I(q) in all the systems studied. This clearly indicates the presence of aggregate structure in the 5 wt % (iso-C18)2G2/oil binary systems. Second is the dependence of low-q scattering intensity on the nature of the solvents. The continuous increase of low-q scattering intensity (I (q ) 0)) upon changing oil from cyclohexane to hexadecane via decane can be taken as a solvent-induced micellar growth. Very similar results were observed in other glycerol-based surfactant/

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Figure 1. Solvent dependence of reverse micellar structures as obtained by SAXS: (a) the scattered intensities, I(q), of 5 wt % (iso-C18)2G2/oil binary systems at 25 °C in absolute units, (b) the corresponding pair-distance distribution functions, p(r), and (c) the model fitting (full lines). 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 arrows in panel b indicate the maximum length of the micellar core, Dmax. Symbols and dashed lines in panel c indicate the scattering data and GIFT fit.

oil systems.54 Thus, one can say that the longer the alkyl chain length of hydrocarbon oil, the bigger the micellar size. It is very important to note that in the surfactant/oil nonaqueous systems the contrast (the electron length density difference) between the oils and the lipophilic part of the surfactant is low. As a result, the SAXS selectively detects the hydrophilic core of the reverse micelles. Therefore, p(r)-function must be recognized as a measure of the micellar core structure. The solvent dependence of the reverse micellar structure in the present systems can be clearly seen in the p(r)-functions presented in Figure 1b. A slightly asymmetric, bell shaped p(r)function of the 5 wt % (iso-C18)2G2/cyclohexane system confirms a slightly elongated globular type reverse micelle with a maximum core diameter, Dmax, ca. ∼3.9 nm at 25 °C. The shape of the p(r)-function in the (iso-C18)2G2/decane system shows more asymmetry with considerably greater Dmax (ca. ∼5.9 nm), demonstrating the formation of a prolate-type elongated aggregate structure. Further micellar growth is achieved upon gradual increase of the alkyl chain length of oil from decance to hexadecane. As shown in Figure 1b, the p(r)-function of the (iso-C18)2G2/hexadecane system shows the typical behavior of an elongated particle with a Dmax of ∼6.8 nm. If we compare the micellar size of the (iso-C18)1G1 and (iso-C18)2G2/oil systems, we will see that the micellar size is higher in the latter systems. For example, the maximum dimensions of the (iso-C18)1G1-based reverse micelles were found to be ca. 2.5, 4.0, and 6.2 nm in cyclohexane, decane, and hexadecane, respectively, at 25 °C.54 These values are much smaller than the present values (see Figure 1b). Thus, one can say that increasing headgroup size of the surfactant increases the micellar size in reverse systems. The inflection point seen after the maximum of p(r)-function as indicated by a dotted line at r ∼ 2 nm in Figure 1b qualitatively accounts for the cross-sectional diameter of the reverse micellar core. Minute observations of the p(r)-functions reveal that the position of the maximum in the p(r)-function (rmax) firmly shifts toward higher r side upon changing oil from

cyclohexane to decane. The position of the rmax is unaffected by changing oil from decane to hexadecane; i.e., the crosssectional diameter of the reverse micellar structure is slightly increased upon changing oil from cyclohexane to decane and remains practically the same upon changing oil from decane to hexadecane. Thus, the present results show that the solvents not only change the size but also modulate the internal structure of the reverse micelles. It is known that oils penetrate into the palisade layer of surfactant molecules at a water/oil interface.55,56 This penetration effect modifies the control parameter and makes spontaneous curvature negative. The observed difference in the aggregate structure in the (iso-C18)2G2/oil systems depending on the solvent nature can be explained in terms of their penetration tendency to the hydrophilic/lipophilic interface of the surfactant molecule. In the studies of nonaqueous phase behavior of diglycerol monomyristate, it has been found that the solubility of surfactant decreases with increasing molecular weight of oils. The surfactant is soluble with cyclohexane, and the solubility decreases when cyclohexane is replaced with the straight chain hydrocarbons. Among hydrocarbon oils, the solubility decreases with increasing alkyl chain length of the hydrocarbon oils.39 Thus, one can anticipate that cyclohexane has a stronger penetration tendency to the hydrophilic/lipophilic interface of the surfactant compared to linear chain solvents. The observed globular type reverse micelles in the (iso-C18)2G2/cyclohexane system further confirm the fact that cyclohexane goes very close to the interface and makes spontaneous more negative. As the chain length of linear chain hydrocarbon oils increases, the penetration tendency goes on decreasing, and hence, aggregates with less negative curvature are formed. The present results well support the results of Kunieda et al.,57 in which they studied the effect of oil on the surfactant molecular curvature in liquid crystals. They found that that m-xylene penetrates into the surfactant palisade layer, and the H1-LR liquid crystal transition occurs in the hydrophilic C12EO7 system. This implies that the penetration leads to



Figure 2. Surfactant dependence of reverse micellar structures: (a) the scattered intensities, I(q), for the 5 wt % ((iso-C18)nG2, n ) 2-4))/decane binary systems at 25 °C in absolute units, (b) the pair-distance distribution functions, p(r), and (c) the normalized p(r)-function (p(r)/p(rmax)). 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 arrows in panel b and c indicate the maximum dimension of the micellar core, Dmax, and the broken lines in panel b indicate the cross-sectional diameter.

aggregates with negative or less positive curvature. Furthermore, they found that long chain oils such as squalane have a smaller degree of penetration compared to a short chain hydrocarbon oil decane. Decane-induced LR-H2 phase transitions in the lipophilic C12EO3 surfactant system are due to penetration effect. Detailed information about the effect of oil solubilization in surfactant self-assemblies can be found elsewhere.57-59 The results obtained from the GIFT method were complemented by model fittings as shown in Figure 1c. The data were fitted using a model of a homogeneous prolate. The fitting was carried out based on the method reported in detail elsewhere.60 To fix the scattering length density difference of the micelle, densities of the investigated mixtures, pure surfactants, and oils were measured at 25 °C (the data are supplied in the Supporting Information). Note that the dashed lines (GIFT fit) coincide with the full lines (model fit) throughout the q-range with a small deviation in the high-q range, i.e., the region of theoretical minima of the I(q) curves. This is due to polydispersity in the size of the micelles. Model fitting was performed assuming the systems having homogeneous and monodispersed particles. The short and long axes of the ellipsoidal prolate, a and b, obtained from the theoretical model fittings are consistent with the results obtained from the GIFT method. From the fitting results, we have calculated the micellar aggregation number (Nagg) in different oil systems. It was found that the Nagg increases with increasing alkyl chain length of oils. For example, the Nagg was ca. 26 for the 5 wt % (iso-C18)2G2/cyclohexane system, whereas Nagg was ca. 56 for the 5 wt % (iso-C18)2G2/hexadecane system. The detail structure parameters obtained from the results of model fittings are tabulated in the Supporting Information. 3.3. Effect of Tail Architecture on the Reverse Micellar Structure. Figure 2 shows the results of SAXS measurements (I(q)- and p(r)-functions) for the 5 wt % ((iso-C18)nG2, n ) 2-4)/ decane systems at 25 °C. The scattering behavior differs depending on the tail architecture of the surfactant. As can be seen in Figure 2a, with increasing number of isostearate chain,

n, i.e., increasing lipophilicity of the surfactant, the forward scattering intensities (in the low-q regions) are suppressed greatly. This highlights the fact that the size of the reverse micelles decreases with increasing lipophilicity of the surfactant. The micellar shrinkage can be clearly seen in the p(r)-function presented in Figure 2b. The maximum size of the micelles, Dmax, as indicated by down arrows decreases upon changing surfactant from (iso-C18)2G2 to (iso-C18)4G2 via (iso-C18)3G2. It is interesting to note that the micellar size is reduced by ∼50% upon changing surfactant from (iso-C18)2G2 to (iso-C18)4G2. Furthermore, the inflection point seen after the maximum in the p(r)-functions (indicated by the broken line in Figure 2b) also changes with the change of nature of the surfactant. This indicates that the lipophilic tail architecture of the surfactant not only modifies the micellar size but also the internal structure of reverse micelles. The shift of inflection point toward low-r side with increasing n indicates that the cross-sectional diameter of the micellar core decreases. To obtain a clear picture on the internal structure depending on the nature of the surfactant, we present the plot of the normalized p(r)-functions (p(r)/p(rmax)) for all the surfactant systems (see Figure 2c). Here, one can clearly see the position shift of the p(r) maximum with the value of n. We have also performed model fitting of the data (data are not shown). From the results of the model fittings, we have found that the micellar aggregation number (Nagg) decreases with increasing number of isostearate chain per surfactant molecules, which support the results obtained from the GIFT method. The Nagg was ca. 52 in the 5 wt % (iso-C18)2G2/decane system and decreases to 11 in the 5 wt % (iso-C18)4G2/decane system. The detailed results of model fitting are supplied in the Supporting Information. The difference in micellar structure (mainly size) depending on the tail architecture of the surfactant can be explained in terms of control parameter. As n increases, the volume fraction of the lipophilic part per surfactant molecule increases, and due

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Figure 3. (a) Steady shear rheology data of 5 wt % (iso-C18)2G2/oil systems at 25 °C, and (b) steady shear rheology behavior of 5 wt % ((isoC18)nG2, n ) 2-4))/decane systems at 25 °C.

Figure 4. (a) Scattering curves, I(q), of the (iso-C18)2G2/decane system at different surfactant concentrations: 5, 10, 15, 20, and 25 wt % in absolute scales at 25 °C. (b) The corresponding real-space p(r)-function, (c) the normalized p(r) functions (p(r)/Φs), and (d) the structure factor curves. 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 arrow and broken line in panel b highlight the maximum dimension of the micellar core and the cross-section diameter of the micellar core, respectively.

to the voluminous lipophilic part, the critical parameter increases favoring smaller reverse micelles. In Figure 3, we present the steady shear rheological behavior of the reverse micellar solutions depending on solvent properties at fixed surfactant systems and also depending on lipophilic tail architecture in a fixed solvent systems. All the rheological measurements were carried out at 25 °C. As can be seen in Figure 3a, the viscosity (η) shows shear independent behavior (Newtonian fluid) irrespective to the solvent nature. With increasing molecular weight of the oil, the viscosity increases, which is in good agreement with the SAXS data. The 5 wt % (iso-C18)2G2/cyclohexane solution has a low viscosity due to the formation of small globular types of reverse micelles. On the other hand, the viscosity of the 5 wt % (iso-C18)2G2/ hexadecane system is much higher compared to the other two systems due to the formation of elongated prolate or rodlike micelles. Very similar structure dependence of steady shear rheology behavior was observed in 5 wt % ((iso-C18)nG2, n ) 2-4))/decane binary systems. As can be seen in Figure 3b, all the systems show a Newtonian fluid-like behavior, and the viscosity decreases with increasing value of n, which is in good agreement with the SAXS data. As mentioned earlier, increasing isostearate chain per surfactant molecule increases the lipophi-

licity of the surfactant resulting in smaller size particles, and hence, viscosity decreases. Thus, the present data have shown a good correlation between the flow properties and the structure of reverse micelles. 3.4. Effect of Surfactant Concentration on the Reverse Micellar Structure. SAXS measurements were carried out on the (iso-C18)2G2/decane system over a wider concentration range to study the composition effect on the micellar structure. In Figure 4, we present the X-ray scattered intensities, I(q), and the corresponding p(r)-functions at different compositions (5 to 25 wt % surfactant) at 25 °C. As can be seen in Figure 4a, with increasing surfactant concentration from 5 to 15 wt %, the scattering intensities increase throughout the q-range due to the increase in the number density of the scattering particles in the unit scattering volume. Minute observation of the scattering curves reveals that with further increasing concentration above 15 wt % the forward scattering intensity is suppressed and led to the formation of a weak, but growing, interaction peak at intermediate q values (q ∼ 1.8 nm-1). This growing peak indicates the strong repulsive intermicellar interactions owing to the reduced osmotic compressibility of the system at higher surfactant concentrations.61,62



Figure 5. (a) Cross-sectional pair-distance distribution function, pc(r), for the (iso-C18)2G2/decane systems at different surfactant concentrations (5-25 wt %) and (b) the corresponding cross-section radial electron density profile, ∆Fc(r), calculated from the deconvolution of the pc(r).

The structure factor profiles (see Figure 4d) evaluated from the GIFT method reveal the presence of considerable intermicellar interactions, which are responsible for the growing peaks in the low-q regions as shown in Figure 4a. Minute observation of the S(q) curves reveals that the position of the structure factor peak slightly moves toward the higher-q side with increasing surfactant concentration indicating the decreasing mean distance between the neighboring micelles. On the other hand, the increase in the amplitude of the peaks indicates an increase in the volume fraction of the scattering entities. The S(0) value decreases with the concentration of surfactant, which is due to the reduced osmotic compressibility of the system at higher concentrations. At higher q values, all the curves converge to unity since at length scales smaller than the dimensions of the scattering entities interparticle interactions are not relevant. The actual structure factor peaks might differ from what is predicted for monodisperse hard spheres.63 Apart from polydispersity effects, some differences could arise due to a pair interaction softer than that predicted by the used hard-sphere model.64 Presently, the polydispersity derived from the fittings to the present experimental data was ∼0.05. The shape and size of the micelles largely depend on the nature of the surfactants. Most of the surfactants with a bulky headgroup form small spherical micelles above critical micelle concentration (cmc) in the aqueous system. In general, such micelles grow with increasing volume fraction of the surfactant, and hence, the aggregation number increases. On the other hand, lipophilic surfactants (cpp > 1) usually form spherical type reverse micelles above the cmc in nonaqueous systems, and the micellar size increases with composition. Surfactants with cpp greater than 1 form globular or slightly elongated ellipsoidal prolate type reverse micelles in nonaqueous systems. With increasing surfactant concentration, the number density of micelles increases leading to repulsive intermicellar forces, which work perpendicular to the interface. 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 on the formation of slightly elongated ellipsoidal prolate-type micelles in the (iso-C18)2G2/ decane system at 25 °C. Micelles are generally expected to grow when they are elongated; however, in the present system, increasing surfactant concentration from 5 to 25 wt % could not modulate the structure of (iso-C18)2G2-based reverse micelles. The only change that occurred with composition is the increased number density of scattering particles at higher surfactant concentrations. As can be seen in Figure 4b, the maximum dimension of the micelles, Dmax, estimated from the point at which the p(r)-functions reach zero at the higher-r side, remains practically the same with a wide variation in the surfactant concentration from 5 to 25 wt %. Besides, the position of the

inflection point seen on the higher-r side of the maximum of p(r), as highlighted by a dotted line at r ∼ 2.0 nm in Figure 4b, is virtually unchanged despite the change in composition, indicating that the cross sectional structure of the reverse micelles is also unaffected by the compositional variation in the present system. The composition independence on the micellar structure can also be seen best in Figure 4c, where the curves are divided by the surfactant volume fraction Φs. All the curves lie on top of each other. If there would be a micellar growth, the curve height would increase with the volume fraction of the surfactant.65 The cross-sectional diameter of the (iso-C18)2G2/decane system was estimated ∼2.0 nm from the inflection point of the p(r)-function after the maximum (Figure 4b). To quantify this structure parameter, we have used the direct cross-section analysis by using the indirect Fourier transformation (IFT) and deconvolution procedure. Using IFT, the pc(r) can directly be calculated from the experimental scattered intensity I(q) via eq 3. The deconvolution of the cross-sectional pair-distance distribution function, pc(r), gives the radial difference electron density distribution profile ∆Fc(r).51,52Figure 5 shows the crosssectional pc(r)-functions and the corresponding contrast profile for the (iso-C18)2G2/decane systems at different surfactant concentrations at 25 °C. Note that the positive electron density profiles in Figure 5b came from the electron-rich hydrophilic reverse micellar core. Quantitative estimation of the crosssection diameter ∼1.98 nm judged from the Dc,max in the pc(r)functions of the (iso-C18)2G2/decane systems is close to the value estimated from the inflection point in total p(r)-function and is independent from the composition. The maximum core radius (Rcore) estimated from the contrast profile, Rcore ∼ 0.99 nm, is nearly equal to the extended chain of twice the glycerol moiety as the glycerol group alone would account for 0.4-0.5 nm. 3.5. Temperature-Induced Microstructural Transition. Figure 6 shows the results of SAXS measurements (I(q)- and p(r)-functions), at different temperatures (25, 50, and 75 °C), for the 10 wt % (iso-C18)2G2/decane system. One can clearly see the decreasing trend of the forward scattering intensity in the low-q region with increasing temperature from 25 to 75 °C without affecting the scattering behavior in the high-q regions. The decreasing trend of the low-q scattering intensity I(q ) 0) with increasing temperature can be taken as evidence of temperature-induced micellar shrinkage. As shown in the p(r)functions (Figure 6b), increasing temperature from 25 to 75 °C, the reverse micellar size gradually decreases from ∼5.9 to 4.9 nm without showing a notable modification of the micellar cross section structure; i.e., the micellar size decreased by ∼ 17%. It should be noted that the SAXS data treatments on surfactant systems at higher temperatures might suffer from an artifact caused by the different contrast at different temperatures. In

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Figure 6. Effect of temperature on the reverse micellar structures of the 10 wt % (iso-C18)2G2/decane system as obtained by SAXS. (a) The normalized X-ray scattering intensities, I(q), in absolute units at different temperatures 25, 50, and 75 °C, and (b) the corresponding real-space p(r)-functions, obtained by GIFT. 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. Arrows in panel b highlight the maximum diameter, Dmax, of the micellar core.

general, temperature worsens the contrast of the systems and may lead to the underestimation of micellar size at higher temperatures. In aqueous systems (mostly highly elongated micellar system) in which SAXS mostly can detect core and shell structure due to negative and positive electron density fluctuations in the micellar core and shell, respectively, this problem becomes more critical. However, the reverse micelles possess a positive electron density fluctuation in the hydrophilic core, whose hydrophobic shell has almost negligible contrast in the oil solvents. This offers a better situation for the reverse systems despite worse contrast at higher temperature. As shown in Figure 6b, the decreased micellar size by ca. ∼17% with increasing temperature from 25 to 75 °C, cannot be solely attributed to the different contrast. Thus, the SAXS data show that the length of the aggregates decreases with the rise of temperature, which is essentially a short rod-to-sphere type transition in the reverse micellar structure. In nonaqueous systems, increasing temperature enhances the penetration of oil in the surfactant chain, and as a result, aggregates with more negative curvature are likely to form. Increasing temperature has a similar effect of the decreasing alkyl chain length of the hydrocarbon oils in solvent dependence of reverse micellar structure see section 3.1. The present results support our previous reports, where temperature effects on the reverse micellar structure were more pronounced.37,66 3.6. Effect of Solubilized Water on the Structure of Host Micelles. Reverse micelles have a tendency to solubilize water or other polar solvents and thus offer various practical applications. They can be used to encapsulate water-soluble drug molecules and also as a reactor for several chemical reactions. Water usually induces micellization in nonaqueous systems.67 Surfactants usually do not form reverse micelles in organic solvents in absolute dry conditions unless a small amount of water or other polar additives is added.68-73 However, traces of water can always be present as impurities mainly in glycerolbased surfactants, as glycerol is hygroscopic. It is practically difficult to remove water completely and to measure SAXS in an absolutely dry condition. Solubilized water or other polar species have been shown to modulate the structure of host micelles.36-38,74 In this context, we have determined the water solubilization capacity of the reverse micelles in the 10 wt % (iso-C18)2G2/ decane system at 25 °C and performed SAXS measurements as a function of water concentration to investigate the waterinduced microstructure transition. The 10 wt % (iso-C18)2G2/ decane system could solubilize 0.3% water. Above this concentration, a turbid solution appeared. Figure 7 shows the SAXS

results on the water-induced micellar growth for the 10 wt % (iso-C18)2G2/decane + water systems at different concentrations of water. As can be seen in Figure 7a, addition of traces of water (0.1%) strongly enhances the forward scattering intensity, which goes on increasing with further addition of water. Such behavior in the scattering function would best account for the micellar growth, which is more clear in the corresponding real space structural information based on p(r)-functions shown in Figure 7b. The Dmax (as indicated by arrows) shows a monotonic increase with water concentration. Upon careful observation, one can see that the position of the maximum in the p(r)-function shifts toward the higher-r side with increasing amount of water indicating that the solubilized water not only favors micellar growth but also increases the cross-sectional diameter of the micellar core. For better visibility, we present the normalized pair-distance distribution functions p(r)/p(rmax) in Figure 7c, which clearly shows that the position of the rmax shifts toward higher-r side as the amount of water increases. Thus, the present results confirmed the formation of swollen micelles with a water pool in the micellar core.36 To confirm whether the solubilized water brought any significant change in the shape of the particles, we have plotted the normalized p(r)-functions versus r* ) r/rmax (master curve) for the 10 wt % (iso-C18)2G2/decane systems at different concentrations of added water at 25 °C (see Figure 7d). The shape of all the curves is almost identical in the master curve. This indicates that water does not modulate the shape of the reverse micelles. Only the micellar size and the scattering power vary. The solubilized water induced internal structural transition was further confirmed by cross-section analysis, and the results are presented in Figure 8. The core cross-section diameter of ∼1.98 nm judged from Dc,max in pc(r) and maximum Rcore ∼ 0.99 nm in ∆Fc(r) for the 10 wt % (iso-C18)2G2/decane system without water addition are much smaller than those of all other water added systems. An addition of water significantly increases the cross-section diameter of the micelles, giving Dc,max ∼ 3.12 nm and Rcore ∼ 1.56 nm. Thus, the results clearly indicate the formation of water pool at the micellar core and emphasize an efficiency of the solubilized water for producing a thicker core of the reverse micelles. Figure 9 shows the results obtained from the model fittings for the systems with and without water added at 25 °C. For the model calculations, two systems, 10 wt % (iso-C18)2G2/decane and 10 wt % (iso-C18)2G2/decane + 0.30% water, were considered. GIFT analysis of the SAXS data has confirmed slightly elongated ellipsoid prolate type micelles with maximum



Figure 7. (a) X-ray scattered intensities I(q) of 10 wt % iso-C18G2/decane at different concentrations of added water in absolute unit at 25 °C, (b) the corresponding p(r)-functions, (c) the normalized pair-distance distribution p(r)/p(rmax) versus r, and (d) a master curve (plot of normalized p(r)-function, p(r)/p(rmax) vs r*(r/rmax)). Solid and broken lines in panel a represent GIFT fit and the calculated form factor for n particles in unit volume, nP(q), respectively.

Figure 8. (a) Cross-sectional pair-distance distribution function, pc(r), for the 10 wt % (iso-C18)2G2/decane + water systems at different concentrations of water, and (b) the corresponding cross-section radial electron density profile, ∆Fc(r), calculated from the deconvolution of the pc(r).

Figure 9. Model calculation (full lines) and experimental X-ray scattered intensities (symbols) with GIFT fit (dashed lines) of 10 wt % (iso-C18)2G2/decane (square) and 10 wt % (iso-C18)2G2/decane + 0.30% water (diamond) as typical examples. The data were fitted using a model of a homogeneous prolate.

size ∼5.9 nm in the former system and sufficiently elongated micelles with maximum length ∼8.5 nm in the latter system. The short and long axes of an ellipsoidal prolate, a and b, obtained from the theoretical model fittings are consistent with the results obtained from the GIFT method despite a visible difference of the depth of the minimum in the experimental and

theoretical I(q) functions. As mentioned in the previous section, the deviation of model fit mainly in the high-q region is caused by the polydispersity effect. So far, the SAXS data have given a clear picture of the micellar growth with increasing water concentration. However, this could be the result of merely incorporating more water molecules in the micelles. The interesting question here is whether aggregation number of the reverse micelles increases with water concentration. To answer this question, we have calculated the micellar aggregation number (Nagg) as a function of water concentration from model fittings. It was found that the Nagg increases with water concentration. The Nagg without the water added system, i.e., the 10 wt % (iso-C18)2G2/decane system, was found to be 52, which goes on increasing with water concentration, and reached 124 in the 10 wt % (iso-C18)2G2/ decane + 0.30% water system. The detailed structure parameters including Nagg as a function of water concentration obtained from the results of model fit are supplied in the Supporting Information. 4. Conclusion Structural characterization of the glycerol-based nonionic surfactant reverse micelles has been performed using a small-

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angle X-ray scattering technique. The structure of scattering particles (reverse micelles) in real space was obtained by the inverse Fourier transformation technique, and the results were complemented by model fitting. Solvent dependence of reverse micellar structure, effect of surfactant’s tail architecture, composition, and temperature on the reverse micellar structure were investigated. Furthermore, water solubilization in the reverse micellar core and its impact on the structure of the host micelles were studied. It was found that diglycerol polyisostearates, (isoC18)nG2, n ) 2-4, form reverse micelles in a variety of oils: cyclohexane, n-decane, and n-hexadecane at 25 °C without external water addition. The micellar size was increased in parallel to the alkyl chain of the oil as the long chain oil less penetrates to the hydrophilic/lipophilic interface of the micelles. With increasing number of the isostearate chain, the micellar size was decreased due to the voluminous lipophilic part. Composition could not modulate the structure of micelles, rather a signature of intermicellar repulsion was observed at higher concentrations. However, temperature favored rod-to-sphere type transition in the micellar structure. The reverse micelles observed in the present systems efficiently solubilized some amount of water in the micellar core enhancing the practical application of the studied systems. The solubilized water induced micellar growth and core swelling. The results derived from the GIFT method are well supported by theoretical model fitting and rheometry. Acknowledgment. LKS thanks JSPS for a Postdoctoral Fellowship for Foreign Researchers. The authors are thankful to Dr. Takaaki Sato Shinshu University, Japan, and Prof. Dr. Stig E. Friberg University of Virginia, USA, for fruitful discussion. Supporting Information Available: Densities of surfactants and oils, electron density difference of the hydrophilic part in different systems, and the structure parameters obtained from the results of the GIFT method and model fittings. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (2) Kunieda, H.; Shinoda, K. J. Dispers. Sci. Technol. 1982, 3, 233. (3) Friberg, S. E.; Blute, I.; Kunieda, H.; Stenius, P. Langmuir 1986, 2, 659. (4) Kunieda, H.; Solans, C.; Parra, J. L. Colloids Surf. 1987, 24, 225. (5) Solans, C.; Pons, R.; Davis, H. T.; Evans, D. F.; Nakamura, K.; Kunieda, H. Langmuir 1993, 9, 1479. (6) Uddin, H. Md.; Rodriguez, C.; Watanabe, K.; Lo´pez-Quintela, M. A.; Kato, T.; Furukawa, H.; Harashima, A.; Kunieda, H. Langmuir 2001, 17, 5169. (7) Kunieda, H.; Tanimoto, M.; Shigeta, K.; Rodriguez, C. J. Oleo Sci. 2001, 50, 633. (8) Kaneko, M.; Matsuzawa, K.; Uddin, H. Md.; Lo´pez-Quintela, M. A.; Kunieda, H. J. Phys. Chem. B 2004, 108, 12736. (9) Kunieda, H.; Uddin, H. Md.; Horii, M.; Furukawa, H.; Harashima, A. J. Phys. Chem B 2001, 105, 5419. (10) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (11) Forster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956. (12) Zhong, X. F.; Varsheny, S. K.; Eisenberg, A. Macromolucules 1992, 25, 7160. (13) Desjardins, A.; van de Ven, T. G. M.; Eisenberg, A. Macromolecules 1992, 25, 2412. (14) Shrestha, L. K.; Masaya, K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Kunieda, H. Langmuir 2006, 22, 1449. (15) Rodriguez, C.; Uddin, Md. H.; Watanabe, K.; Furukawa, H.; Harashima, A.; Kunieda, H. J. Phys. Chem. B 2002, 106, 22. (16) Luisi, P. L., Strab, B. E., Eds. ReVerse Micelles: Biological and Technological releVance of Amphiphilc Structures in Apolar Media; Plenum Press: New York, 1987.

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