Charge-Free Reverse Wormlike Micelles in Nonaqueous Media

Feb 21, 2011 - (3-8) Reverse micelles can grow into cylinder and wormlike forms ..... type of behavior, and the flow activation energy calculated is v...
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Charge-Free Reverse Wormlike Micelles in Nonaqueous Media Lok Kumar Shrestha,*,† Mie Yamamoto,‡ Satoshi Arima,§ and Kenji Aramaki‡ †

International Center for Young Scientists (ICYS), WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba Ibaraki, 305-0044, Japan ‡ Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan § Technical Development Center, Mitsubishi-Kagaku Foods Co., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-0033, Japan

bS Supporting Information ABSTRACT: We report a facile method for the formation of charge-free reverse wormlike micelles in a nonionic surfactant/ oil system without addition of water under ambient conditions. This route involves the addition of sucrose dioleate (SDO) to semidilute solutions of sucrose trioleate (STO) in hexadecane. A reverse wormlike micelle was possible to achieve only with ionic surfactants in which water and/or salts are fundamentally required to induce micellar growth so far. In this contribution, we have shown that less lipophilic nonionic surfactant SDO promotes one-dimensional growth to STO reverse micelles and leads to the formation of transient networks of viscoelastic reverse wormlike micelles. The zero-shear viscosity increases by ∼4 orders of magnitude, and it is the mixing fraction of SDO to STO that determines the viscosity growth. The structure and dynamics of the reverse micelles are confirmed by small-angle X-ray scattering (SAXS) and rheometry measurements.

1. INTRODUCTION Reverse micelles comprise a hydrophilic polar core and a lipophilic nonpolar shell, have the reverse structure of conventional micelles in aqueous media, and are often called inverse or inverted micelles. Reverse micelles have attracted significant interest over the years due to their wide range of practical applications. They have been used as a size-controlling microreactor for several aqueous chemical reactions1,2 and also as templates for materials synthesis.3-8 Reverse micelles can grow into cylinder and wormlike forms under certain conditions, and exhibit viscoelastic behavior. An example of such a system is found in ternary mixtures of lecithin/water/oil.9 Lecithin is a zwitterionic phospholipid with two alkyl chains and usually forms globular or ellipsoidal micelles in a variety of organic solvents. When a trace amount of water is added to globular lechithin reverse micellar solutions, the micelles grow axially into flexible cylinders, which become entangled and form a transient network of reverse wormlike micelles having viscosity several orders of magnitude greater than those of pure solvent oils or water. Additionally, the solutions exhibit viscoelastic behavior due to the entanglement of long cylindrical micelles into a transient network.10-13 Thus, water is an essential component for the formation of reverse wormlike micelles in lecithin-based aggregates, and it is the molar ratio of water to lecithin, which controls the reverse micellar growth. Although there is a growing interest in the self-assembly of amphiphilic molecules both in aqueous and nonaqueous systems, it is surprising that self-assembly in nonaqueous media (formation of r 2011 American Chemical Society

reverse micelles or wormlike micelles) has been so far reported in only a few systems.14-22 The ternary mixture lecithin/water/oil continues to be the best system for studies of reverse wormlike micelles.23,24 Lecithin and water mixtures form reverse wormlike micelles in several nonpolar solvents. Lecithin itself forms small globular micelles in organic solvents, so it is the presence of water that promotes growth of micelles to long and flexible wormlike micelles.9 Recent studies have shown that replacing water with another polar additive, such as formamide or glycerol, can have a similar effect.25 Recently, Raghavan et al.26,27 proposed a new method for the formulation of reverse wormlike micelles without water addition involving the addition of biosurfactants such as bile salts to lecithin/oil mixtures. They have also illustrated the formation of light responsive reverse wormlike micelles in a combination of lecithin/para-coumaric acid/oils.28 Similarly, Hashizaki et al. prepared lecithin-based reverse wormlike micelles in decane by substituting water for urea and sucrose fatty acid esters.29,30 The uniting aspect of these additives is their ability to form hydrogen bonds with the lecithin headgroup. In this contribution, we report a new strategy for the formulation of reverse wormlike micelles in a nonaqueous system without charge and water. This method consists of use of a sucrosebased lipophilic nonionic surfactant, sucrose trioleate (STO) as the main surfactant, which spontaneously forms small globular Received: December 8, 2010 Revised: February 1, 2011 Published: February 21, 2011 2340

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Scheme 1. Schematic Molecular Structures of the Surfactants Used in the Present Study: (a) STO and (b) SDO.

reverse micelles in n-hexadecane in the absence of water under ambient conditions. Addition of sucrose dioleate (SDO) to the semidilute solution of STO promotes micellar growth of long cylinders, which entangle forming a transient network of viscoealstic wormlike micelles. Note that STO and SDO are environmentally friendly and biocompatible nonionic surfactants. Therefore formulation of reverse wormlike micelles from mixtures of such surfactants should offer better potential in practical applications.

2. EXPERIMENTAL SECTION 2.1. Materials. Sucrose-based nonionic surfactants (SDO and STO) with >99% purity were obtained from Mistubishi-Kagaku Foods Co., Ltd., Tokyo, Japan. The nonpolar organic solvent n-hexadecane with >99.5% purity was purchased from Tokyo Chemical Industry, Tokyo, Japan. The surfactants and oil were used as supplied. The molecular structures of SDO and STO are given in Scheme 1. 2.2. Methods. 2.2.1. Identification of Equilibrium Phases at 25 C. Equilibrium phases in the dilute systems of SDO/hexadecane and STO/ hexadecane were identified by visual inspection and through a crossedpolarizer at 25 C. Similarly, the equilibrium phases in the ternary mixtures of STO/hexadecane/SDO system at different mixing fractions of SDO to STO (W1 = SDO/(SDO þ STO)] were determined. In ternary mixtures, the total surfactant concentration was fixed to 10 wt %. Binary mixtures of SDO and STO in hexadecane with surfactant concentrations between 5% and 25% were prepared in clean and dry 5 mL glass ampules with a screw cap. The samples were mixed using a dry thermobath and a vortex mixer to achieve homogeneity. 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 control was better than (0.5 C. 2.2.2. Rheological Measurements. Ternary mixtures of STO/hexadecane/SDO system at different mixing fractions of SDO to STO (W1 = SDO/(SDO þ STO)] were prepared keeping the total surfactant concentration at 10 wt %. These samples were mixed at higher temperature (50 C) with a constant stirring by a magnetic stirrer for several days. The samples were then kept in a thermostatted water bath at 25 C for at least one week before rheological measurements. The rheological measurements were performed on a stress-controlled AR-G2 rheometer (TA Instruments) using cone-plate geometries (cone diameter, 40 mm; cone angle, 1) equipped with Peltier-based temperature control. A sample cover provided with the instrument was used to minimize the change in sample composition by evaporation during the measurement. Frequency sweep measurements were performed in the linear viscoelastic regime of the samples, which was determined by dynamic strain sweep measurements. Temperature effect was studied by changing temperature from 15 to 45 C by peltier unit, which uses the peltier effect for rapidly and accurately controlled heating and cooling.

2.2.3. Small-angle X-ray scattering (SAXS). SAXS measurements were carried out on a series of samples: the total surfactant concentration was fixed to 10 wt % and the mixing fraction of SDO to STO [W1 = SDO/(SDO þ STO)] was varied from 0 to 0.90, to study the SDOinduced micellar growth. Additionally, in a particular sample with composition W1 = 0.90, SAXS measurements were carried out at different temperatures (20, 25, 40, 50, and 85 C) to monitor the temperature-induced microstructure change of the reverse wormlike micelles. A SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealed-tube anode X-ray generator (PANalytical, Netherlands) was operated at 40 kV and 50 mA. A G€obel mirror and a block collimator provided a focused monochromatic X-ray beam of Cu-KR radiation (λ = 0.1542 nm) with a well-defined line shape. A thermostat sample holder unit (TCS 120, Anton Paar) was used to control the sample temperature with an accuracy of (0.1 C. The two-dimensional (2-D) scattering pattern was recorded on an image plate (IP) detector (Cyclone, Perkin-Elmer, USA) and integrated into to one-dimensional scattering 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 the measured intensities were semiautomatically calibrated for transmission by normalizing a zero-q attenuated primary intensity to unity. 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.31 We have used the generalized indirect Fourier transformation (GIFT) method32-35 to obtain a pair-distance distribution function, p(r), i.e., structure information in real space. 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 averaged static structure factor, and n is the number of particles in unit volume. Since, theoretically, P(q) is the Fourier transformation of p(r) as Z ¥ sin qr dr pðrÞ PðqÞ ¼ 4π ð1Þ qr 0 one needs to calculate inverse Fourier transformation of an experimental P(q) to deduce p(r) from the scattering experiments. To suppress the influence of interparticle interference scattering on the evaluation of p(r) that generally leads to oscillations and to a highly underestimated maximum size of the scattering object, an interaction potential model for S(q) was to be involved in the analysis, where we chose the averaged structure factor model36,37 of the hard-sphere and Percus-Yevick closure relation to solve the Ornstein-Zernike equation. The detailed theoretical description on the method has been given elsewhere.38-40 When the axial length of a cylindrical scattering particle is long, say at least 3 times longer than the cross-sectional diameter, we can apply a model-free cross-section analysis. Theoretically, the radial electron density profile, ΔFc(r), is related to the cross-section pair-distance 2341

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Figure 1. (a) Steady-shear viscosity at different mixing fraction of SDO, W1 [SDO/(STO þ SDO)] and (b) the corresponding zero-shear viscosity and the phase behavior. distribution function, pc(r), as41 pc ðrÞ ¼ rΔ~F c 2 ðrÞ

ð2Þ

Using a counterpart technique of indirect Fourier transformation (IFT), pc(r) can directly be calculated from the experimental scattering intensity I(q) based on Z ¥ pc ðrÞJ0 ðqrÞ dr ð3Þ IðqÞq ¼ πLIc ðqÞ ¼ 2π2 L 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 procedure.42,43

3. RESULTS AND DISCUSSION 3.1. Isothermal Phase Behavior of STO and SDO and Their Mixtures in Hexadecane. Equilibrium phases of STO and SDO

and their mixtures (at different mixing fractions) in n-hexadecane were identified by visual inspection through a cross-polarizer at 25 C. Binary mixtures of STO and SDO in hexadecane were prepared varying surfactant concentrations from 5 to 25 wt % and in ternary mixtures, the total surfactant concentration was fixed to 10 wt % and the mixing fractions of SDO to STO, W1, was varied from 0 to 0.90. It was found that STO surfactant forms an isotropic solution consisting of reverse micellar aggregates in hexadecane at ambient conditions; whereas, a solid dispersion is observed in the SDO/hexadecane system. The STO surfactant micelles could solubilize SDO surfactant up to W1 = 0.9, and with further increasing W1, phase separation to a turbid solution occurred. 3.2. Rheology of STO/Hexadecane/SDO System: Effect of SDO Mixing Fraction (W1). Figure 1 show the results obtained from the steady-shear rheological measurements of STO/hexadecane/SDO systems at different W1 at 25 C. The total surfactant concentration is fixed at 10 wt %. The STO surfactant forms globular micelles in hexadecane with a viscosity equivalent to that of pure hexadecane. The solution shows a Newtonian fluid-like behavior, i.e., the viscosity is independent of the applied shear. The addition of SDO up to W1 = 0.5 increases viscosity only slightly, indicating micellar growth; micelles are only slightly elongated, and the network structure is yet to form. With further increase in W1 (W1 e 0.8) the viscosity increases significantly but the solution still exhibits a Newtonian fluid-like behavior. A shear-thinning behavior is observed only at W1 g 0.83 (see Figure 1a), indicating the formation of an entangled network of wormlike micelles. The samples with compositions of W1 > 0.83 show a shear birefringent behavior (birefringent upon shaking or when a small force is

Figure 2. Dynamic rheological behavior of STO-SDO reverse wormlike micelles: (a) Variations of complex viscosity, |η*|, elastic (G0 ), and viscous (G00 ) modulus as a function of oscillatory frequency (ω) for a W1 = 0.90 system at 25 C as a typical example. (b) The rheological parameters, η0, G0, and τR versus W1. The G0 and τR are obtained from the Maxwell model fit to the data. In panel a, symbols (for G0 and G00 ) represent the experimental data points, and solid lines are the Maxwell model fit. Solid lines in panel b are the visual guide. Total surfactant concentration is fixed at 10%.

applied but isotropic at rest). Increasing W1 above this value not only increases the viscosity but also shifts the critical shear rate (shear rate at which shear-thinning occurs) toward the low shear rate, showing that the system is getting more structured. The effect of mixing fractions of SDO on the rheological behavior can better be seen in the zero-shear viscosity (η0) plot (see Figure 1b). Values of η0 were determined by extrapolation of the plateau viscosity to zero shear rates. Note that η0 increases only slightly until W1 reaches W1 = 0.70 and then increases 2342

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significantly. This is a clear indication of SDO-induced one-dimensional micellar growth. Above W1 > 0.80, the micelles are long enough to form a transient network of wormlike micelles. The η0 increases by ∼4 orders of magnitude as W1 increases from 0 to 0.90. Above W1 = 0.90, there is a phase separation to a turbid solution. Dynamic oscillatory-shear rheological measurements were carried out to characterize the viscoelastic behavior of STOSDO reverse wormlike micelles. Measurements were performed as at different W1 and at different temperatures at a fixed W1. Figure 2 shows the results obtained from the dynamic rheological measurements at different W1. Variation of complex viscosity |η*|, elastic modulus G0 , and viscous modulus G00 as a function of frequency ω for one sample at W1 = 0.90 is shown as a representative plot. In Figure 2a, the symbols (for G0 and G00 ) represent the experimental data points, and solid lines are the Maxwell model fits considering a single stress relaxation process. Plots of G0 and G00 as a function of ω for different W1 are supplied in the Supporting Information. The rheogram in Figure 2a clearly shows that these samples exhibit viscoelastic behavior: at higher ω or short time scales, the samples show elastic behavior with G0 greater than G00 ; on the other hand, at low ω or long time scales, the samples show viscous behavior with G00 higher than G0 , i.e., there is a crossover between G0 and G00 .44-48 The ω at which G0 and G00 intersect is called the crossover frequency, ωc, whose reciprocal (1/ωc) gives the relaxation time, τR, of these viscoelastic samples.44-47 It was found that with increase in W1, the rheogram shifts toward lowerω regime and values of G0 and G00 are higher throughout the entire ω range (see Figure S1a in the Supporting Information). This shows that the system is getting more structured. The rheological (viscoelastic) properties of present systems could be described by the Maxwell model based on the following equations:44-47 G0 ¼

ω2 τR 2 G0 1 þ ω2 τR 2

ð4Þ

G00 ¼

ωτR G0 1 þ ω2 τR 2

ð5Þ

where, G0 is the plateau modulus, which is equivalent to the value of G0 in the higher ω region. Maxwell model fits the data only at low frequencies, i.e., below the reciprocal of relaxation time, which is given by the square root of the product of reptation and micellar breaking time; τR = (τBreak  τRep)1/2 and deviates at higher frequencies.49 The agreement with Maxwell model is a result of motional averaging of the micellar dynamics, which occurs when the reptation time is at least 2 orders of magnitude longer than the breaking time. Very significant deviation from the Maxwell model at high frequencies may indicate that the reptation time is short, or the breaking time is long.49 The Cole-Cole plot (variation of G00 vs G0 ) supplied in the Supporting Information (Figure S1b) further verifies the viscoelastic behavior of wormlike micelles. It has found that the shape of the curve is semicircular up to crossover regime and then deviates from semicircular behavior as predicted by Maxwell model. We note that the experimental variations of G0 and G00 and those calculated from the Maxwell model show a strong agreement until the crossover frequency region, although the shape of the curves G0 and G00 at higher frequencies is indicative of the occurrence of fast modes superimposing on a slow relaxation

Figure 3. Variation of plateau modulus, G0, relaxation time, τR, and the longest relaxation time, TR, as a function of W1 at 25 C. G0 is determined by Maxwell model fit (circle) and also using eq 6 (triangle); TR is also determined using eq 6. The value of τR is included for the sake of comparison.

process. This shows the possibility of a multiple relaxation process. Figure 2b shows the variation of rheological parameters: plateau modulus, G0, and relaxation time, τR, determined from the Maxwell model fit to the experimental data, as a function of W1. Value of zero-shear viscosity, η0, is also included in the Figure 2b. As shown in Figure 2b, G0, τR, and η0 increase with W1. The increase of τR with the SDO mixing fraction can be attributed to the growth of reverse wormlike micelles. The G0 increase with W1 indicates an enhanced network and can be explained by the formation and increase in the number of entanglements accompanying the growth of the micelles. Similar results have been observed for lecithin reverse wormlike micelles upon addition of bile salt.26,27 In addition to the Maxwell model, from the evaluation of lowfrequency data, we have attempted to determine the plateau modulus, G0, and the longest relaxation time, TR, using the following relationships described elsewhere:50   1 G0 TR ¼ lim and ω f 0 ω G00  00  1 G lim ð6Þ G0 ¼ ω f 0 TR ω Figure 3 shows that the G0 curve determined from eq 6 lies below the curve predicted by the Maxwell model, and the G0 is less sensitive to the variation of W1. As expected from the viscosity data, the TR determined from low-frequency behavior increases with increasing W1, i.e., the relaxation time becomes longer at higher W1. Here we note that the relaxation time, τR, determined by the Maxwell model is smaller than the longest relaxation time, which indicates that the behavior of the complex shear modulus is a nonMaxwellian behavior at higher frequencies. Nevertheless, TR increases with the increase in W1 in a manner similar to that of τR. 3.3. Rheology of STO/Hexadecane/SDO System: Effect of Temperature. The thermoresponsive behavior of STO-SDO reverse wormlike micelles was tested by performing dynamic oscillatory-shear measurements at different temperatures. For this purpose, a sample from the maximum viscosity region was taken. Plots of complex viscosity |η*|, elastic modulus G0 , and viscous modulus G00 as a function of oscillation frequency ω at 20 C for W1 = 0.90 are shown in Figure 4 as a typical example. 2343

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The rheograms show that the wormlike micelles observed in the present system are highly sensitive to temperature; viscoelastic behavior is observed in a limited temperature range 20-30 C. The rheogram and the crossover frequency, ωc, shift toward higher frequency region upon heating (see Figure S2a in Supporting Information), and the Maxwell model fits well the data in the low and intermediate frequency regions and deviates in the higher frequency regime (above ωc). We note that the crossover between G0 and G00 is well beyond 100 rad 3 s-1 above 30 C. The shift of ωc to higher values indicates that Maxwellian relaxation time (= 1/ωc) decreases with the rise of temperature, i.e., the sample regains its equilibrium faster when deformed by applying some mechanical stress, and can be explained by decreasing micellar length. Temperature effect on rheology is well-supported by SAXS data, which shows a monotonous decrease of micellar length with increase in temperature see Figure 8b. The variation of G0 and τR with temperature is shown in Figure 4b in a semilog plot. It can seen that the value of τR decreases almost linearly with increasing temperature, indicating the exponential decay that can be explained by an Arrhenius-type equation.46   Ea τR ¼ A exp ð7Þ RT where Ea is the flow activation energy, R is the gas constant, T is the absolute temperature, and A is a constant. We have also calculated the values of η0 from the dynamic oscillatory-shear rheology data using the Maxwell model relation η0 = G0τR. It is found that the values of η0 are nearly the same as those obtained from steady-shear rheology and decrease with the rise of temperature and fall on a straight line with approximately the same slope as that obtained for the τR line (see Figure 4b). This shows that η0 also follows an Arrhenius-type relation defined as51   Ea η0 ¼ G0 A exp ð8Þ RT This equation shows that when G0 is constant, η0 will decrease exponentially with temperature. Although the value of G0 is not practically constant in the present system, η0 decays exponentially, and the value of flow activation energy determined from eq 7 (ca. 178.4 kJ/mol) is close to the value obtained from the τR plot (136.4 kJ/mol). The values of flow activation energy determined from eq 7, and eq 8 for STO-SDO reverse wormlike micelles in hexadecane are comparable to the reported values for normal and reverse wormlike micelles.27,47,51-53 Here again we have determined the values of G0 and TR from eq 6 and compared with the values obtained from Maxwell model fits. It has found that though the curve of G0 determined from eq 6 lies below the values obtained from Maxwell model fit, follows the similar behavior with increase in temperature. The longest relaxation time, TR, determined from eq 6 is higher than the values of τR obtained from the Maxwell model but lie on a straight line with nearly the same slope in a semilog plot (see Figure S2b in Supporting Information). The higher values of longest relaxation time compared to the relaxation time determined by the Maxwell model indicates again that the behavior of the complex shear modulus is a non-Maxwellian behavior at higher frequencies. 3.4. SAXS Study of STO/Hexadecane/SDO System. Herein we discuss on the microstructure transition induced by the addition of SDO and temperature. SAXS measurements were performed as a function of mixing fraction of SDO, W1, and then

Figure 4. Effect of temperature on the dynamic rheological behavior of STO-SDO reverse wormlike micelles for W1 = 0.90: (a) variation of elastic (G0 ) and viscous (G00 ) modulus as a function of oscillatory frequency (ω) at 20 C as a representative plot, and (b) zero-shear viscosity, η0, and the rheological parameters (G0 and τR) obtained from Maxwell model fittings at different temperatures. Solid lines in panel a represent the Maxwell model fit. In panel b solid lines are for η0, and τR curves represent the fit based on eqs 7 and 8.

as a function of temperature at fixed W1. Figure 5 shows the SAXS results for the 10 wt % STO/hexadecane/SDO systems at different W1 at 25 C. Note that the scattering length density of hydrocarbon oil and the lipophilic part of the surfactant are almost identical. As a result, SAXS selectively detects the hydrophilic core of the reverse micelles in nonaqeuous media.54,55 Therefore p(r) must be recognized as a measure of the micellar core structure. Considering the scattering functions, it can be clearly seen that the low-q scattering intensity reaches horizontally to zero-q, i.e., the low-q scattering intensity follows q0 behavior in the forward direction, indicating the formation of globular micelles in the SDO free system. We note that forward scattering intensity, I(q = 0), as well as the low-q slope increase with increase in W1, and eventually at W1 = 0.90 the I(q) decays following q-1 behavior in the low-q region. Growth of cylinder or wormlike micelles leads to this kind of behavior in the scattering curve.56-60 Thus, SAXS data provide strong evidence for the SDO-induced one-dimensional micellar growth. The evidence of rapid micellar growth induced by SDO can be better seen in the real-space p(r) curves shown in Figure 5b. Nearly symmetric bell-shaped p(r) curves of 2344

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Figure 5. (a) X-ray scattered intensities, I(q), of the 10 wt % STO/hexadecane/SDO systems at different W1 at 25 C in absolute scales, and (b) the corresponding pair-distance distribution functions, p(r). 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 and dashed line after the maximum of p(r) curve in panel b indicate the maximum length, Dmax, and cross-sectional diameter of the micellar core.

Figure 6. (a) The cross-sectional pair-distance distribution function, pc(r), of the 10 wt % STO/hexadecane/SDO systems at different W1, and (b) the resulting cross-section radial electron density profile, ΔFc(r), calculated with the deconvolution procedure of pc(r) using the program DECON.

Figure 7. Schematic representation of nonionic reverse micelles formed by STO without and with SDO. STO alone forms spherical geometry in nonpolar oil hexadecane. The less lipophilic nonionic surfactant SDO, which does not form any structure in hexadecane, can be solublized at the palisade layer of STO micelles, and, as a result, the curvature decreases favoring micellar growth.

the systems from W1 = 0-0.30 indicates a typical feature of globular-type micelles with maximum micellar diameter below 7.0 nm. An asymmetry in the shape of the p(r) curves appears upon further increases in W1 showing a shape transition from globular to elongated ellipsoidal prolate or short rodlike micelles at W1 = 0.50 to 0.70 (Dmax increases to ∼10 nm for W1 = 0.50, and ∼16.5 nm for W1 = 0.70), which eventually transforms into a long cylinder or wormlike micelles as illustrated by a pronounced peak in the low-r regime and an extended nearly linear tail in the high-r side of the p(r) curves; Dmax, for W1 = 0.90 is calculated to ∼45 nm. Note that in Figure 5b Dmax represents the maximum dimension of micelles (diameter in case of globular micelles and length in the case of elongated rodlike or wormlike micelles). On

the other hand, the inflection point after the pronounced maximum in the lower-r regime, as indicated by the dashed line, indicates the maximum cross section diameter of the micellar core, Dcmax. It was found that increasing W1 increases the axial length of the cylindrical or wormlike micelle, Dmax, as indicated by the extended tail in the high-r side of the p(r) curves. Thus, judging from the SAXS and rheology data, one can conclude that the viscosity growth with increasing W1 is due to micellar growth and entanglement of cylindrical micelles. The appearance of a deviation in the intermediate r region, mainly at W1 = 0.85 and W1 = 0.90, more significant in the latter case, can be attributed the attractive interaction of the particles during the approach of the phase separation boundary (see Figure 1b). Glatter et al.33 have shown the contribution of a strong attractive interaction near the critical point in aqueous systems of nonionic surfactant. A similar effect of attractive interactions has been found in nonionic surfactant micelles in nonaqueous media.61 Note that the inflection point in the p(r) curves after the maximum at the low-r side, which measures the cross-sectional diameter of the aggregates semiquantitatively, remains apparently the same over a wide range of W1, indicating that increasing W1 does not modify the internal core structure of the micelles. Judging from the inflection point, the cross-sectional diameter of the cylinder or wormlike aggregates is estimated to be ∼3.8 nm. For a quantitative evaluation of the cross-section structure, we used IFT, by which the cross-sectional pair-distance distribution function, pc(r), can be directly calculated from the experimental scattered intensity I(q) based on eq 3. The further deconvolution procedure of the pc(r) yields the radial difference electron density 2345

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Figure 8. (a) X-ray scattered intensities, I(q), of the 10 wt % STO/hexadecane/SDO (W1 = 0.90) system at different temperatures (25, 40, 50, and 85 C) in absolute scales, and (b) the pair-distance distribution functions, p(r). The solid and dashed lines in panel (a) represent GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The arrows and dashed line after the maximum of p(r) curve in panel b indicate the maximum length, Dmax, and cross-sectional diameter of the micellar core, Dcmax.

Figure 9. (a) The cross-sectional pair-distance distribution function, pc(r), of the 10 wt % STO/hexadecane/SDO (W1 = 0.90) system at different temperatures, and (b) the resulting cross-section radial electron density profile, ΔFc(r), calculated with the deconvolution procedure of pc(r) using the program DECON.

distribution profile, ΔFc(r).42,43 Figure 6 shows the resulting pc(r) and ΔFc(r) for the same system with different W1 (W1 = 0.70 to 0.90). The positive values of ΔFc(r) in the entire r-range as shown in Figure 6b confirm the electron-rich hydrophilic reverse micellar core. The calculated cross-section diameters from the position of Dcmax in pc(r) are almost identical, giving diameters of ca. 3.6 nm, and close to those roughly estimated from the inflection point of the total p(r) curves. The core radius, Rcore ∼ 1.75 nm, is read out from ΔFc(r), is nearly half of the cross-section diameter, and remains unchanged at all W1. The geometries of self-assembled structures formed by amphiphilic molecules depend on their molecular structure and can be explained in terms of the critical packing parameter (cpp), which is given by a simple equation, cpp = v/aslc, where v is the volume of the lipophilic portion of the amphiphile, and as and lc are the area of hydrophilic headgroup and the extended chain length of the lipophilic portion, respectively.62 It is well-known in aqueous systems of ionic surfactants in the absence of salts or in nonionic surfactants with large headgroups that the value of cpp is ∼1/3, which implies the formation of spheroid micelles. In the case of ionic surfactants, when salt is added, the effective area of headgroup decreases due to a reduction in the electrostatic screening length, and the value of cpp increases to ∼1/2. Similarly, in the case of nonionic surfactants, the addition of lipophilic surfactants increases the cpp to ∼1/2, as the less hydrophilic surfactants tends to be soluble in the surfactant palisade layer, thereby increasing the overall headgroup area. In

both cases, spherical micelles transform into cylindrical micelles. However, in the case of surfactant self-assembly in nonaqueous media, for a spherical micelle to be formed, the cpp must have a value well above 1, i.e., the molecular geometry of surfactant should have an inverted cone shape, which is possible in lipophilic surfactants.26 Note that unlike in aqueous systems for a reverse sphere to grow into a cylinder, the cpp must decrease.26,62 Here we anticipate that the SDO, which itself is insoluble in hexadecane, is soluble at the surfactant palisade layer of STO micelles. Consequently, the spontaneous curvature of the system tends to be less negative. On the other hand, under this condition, the cpp tends to decrease, favoring transition from spheres to long cylinders or wormlike micelles. Although we do not have any strong evidence, it is possible that the headgroups of SDO and STO surfactants are fastened together by hydrogen bonding and, as a result, the overall headgroup size of the surfactant increases so that cpp decreases and favors micellar growth. The possible mechanism of micellar growth is shown in Figure 7. Next we discuss the effect of temperature on the microstructure transition of wormlike micelles based on SAXS measurements. The SAXS measurements were carried out on the sample corresponding to maximum viscosity in Figure 1b at different temperatures (25, 40, 50, and 85 C) and results are presented in Figure 8. There is an obvious decrease of low-q forward scattering intensity as well as the low-q slope with an increase in temperature. This is a clear indication of micelles' shortening. Minute 2346

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Langmuir observation reveals that I(q) in the cross-section regions tends to shift toward the higher-q side. This indicates that temperature not only changes the maximum length of cylinder or wormlike micelles but also tends to decrease the cross-section diameter. The effect of temperature on the micellar structure can better be seen in real-space p(r) function presented in Figure 8b. Here the maximum length of elongated micelles is decreasing with increasing temperature. Therefore viscosity decay with temperature can be attributed to micelle shortening or a wormlike-toprolate-type transition in the reverse micellar structure. In nonaqueous systems, increasing temperature enhances the penetration of oil to the surfactant chain, and thus favors the formation of aggregates with more negative curvature. We have also performed direct cross-section analysis as a function of temperature, and the results are presented in Figure 9. As can be seen from Figure 9a, the cross-section diameter of the micelle decreases from ∼3.8 to 3.0 nm with the rise of temperature from 25 to 85 C. Similarly, the cross-sectional radius calculated from the radial electron density profile decreases with temperature. The microstructure transition induced by temperature can be understood in terms of cpp, which tends to increase with temperature, as the penetration of oil to the lipophilic tail of the surfactant increases at higher temperature.

4. CONCLUSION In this paper, we have investigated the formation and properties of charge-free nonionic reverse wormlike micelles in nonaqeuous media. The proposed system is unique and far from common sense in this field. For surfactant self-assembly, the copresence of polar and nonpolar solvent is usual, and strong amphiphilicity of surfactant with bearing charge is common. From these viewpoints, the present system looks out of common sense in surfactant science. Nevertheless, such an unusual system could be highly interesting mainly from viewpoint of practical application, where one should avoid both charge and water. Sucrose molecules, being hydrophilic in nature, are practically insoluble in nonpolar organic solvent hexadecane. The sucrosebased lipophilic nonionic surfactant STO when added into hexadecane, the hydrophilic sucrose group tends to avoid direct contact with solvent molecules, and as a result of strong dipoledipole interactions among sucrose molecules, the STO molecules spontaneously self-assemble into globular micelles in the absence of water at 25 C. On the other hand, SDO, which is less lipophilic compared to STO, does not form any structure in hexadecane and separates out as a solid phase. Nevertheless, due to strong interaction between sucrose molecules, STO reverse micelles potentially solubilize SDO molecules into a micellar palisade layer and induce a microstructure transition from globular to long cylindrical micelles, which are entangled, forming networks of reverse wormlike micelles. Zero-shear viscosity rises by ∼4 orders of magnitude relative to pure solvent. The wormlike micelles exhibit a viscoelastic response with a single stress relaxation mode as confirmed by the Maxwell model. The zero-shear viscosity decays exponentially with temperature following the Arrhenius type of behavior, and the flow activation energy calculated is very close to that of typical wormlike micelles. The microstructure transition caused by the addition of SDO and variation of temperature were further confirmed by SAXS measurements. Micellar growth induced by SDO addition is attributed to the decrease of cpp; the SDO molecule goes to the surfactant palisade layer and decreases cpp by increasing the

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overall headgroup size of the surfactant. On the other hand, cpp tends to increase with the rise of temperature and, hence, micelles with more negative curvature are inevitable at higher temperature.

’ ASSOCIATED CONTENT

bS

Supporting Information. Results and discussion. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ81-29851-3354 ext. 8903. Fax: þ81-29-860-4706.

’ ACKNOWLEDGMENT L.K.S. thanks the International Center for Young Scientists (ICYS), the National Institute for Materials Science (NIMS), and the International Center for Materials Nanoarchitectonics (MANA) for partial financial support. Authors are thankful to Mistubishi-Kagaku Foods Co., Ltd., Tokyo, Japan, for the supply of surfactants. Fruitful discussion with Prof. Dr. Katsuhiko Ariga, WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS) is highly acknowledged. ’ REFERENCES (1) Luisi, P. L., Strab, B. E., Eds. Reverse Micelles: Biological and Technological Relevance of Amphiphilc Structures in Apolar Media; Plenum Press: New York, 1987. (2) Pileni, M. P. Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989; Vol 65. (3) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (4) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (5) Pileni, M. P. Langmuir 1997, 13, 3266. (6) L opez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137. (7) L opez-Quintela, M. A.; Tojo, C.; Blanco, M. C.; García Rio, L.; Leis, J. R. Curr. Opin. Colloid Interface Sci. 2004, 9, 264. (8) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893. (9) Scartazzini, R.; Luisi, P. L. J. Phys. Chem. 1988, 92, 829. (10) Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28, 372. (11) Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356. (12) Schurtenberger, P.; Scartazzini, R.; Magid, L. J.; Leser, M. E.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3694. (13) Schurtenberger, P.; Magid, L. J.; King, S. M.; Lindner, P. J. Phys. Chem. 1991, 95, 4173. (14) Shrestha, L. K.; Kaneko, M.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Kunieda, H. Langmuir 2006, 22, 1449. (15) Shrestha, L. K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Aramaki, K.; Kunieda, H. J. Phys. Chem. B 2006, 110, 12266. (16) Shrestha, L. K.; Aramaki, K. J. Dispersion Sci. Technol. 2007, 28, 1236. (17) Harrison, W. J.; McDonald, M. P.; Tiddy, G. J. T. J. Phys. Chem. 1991, 95, 4136. (18) Mukherjee, K.; Moulik, S. P.; Mukherjee, D. C. Langmuir 1993, 9, 1727. (19) Yu, Z. J.; Neuman, R. D. Langmuir 1994, 10, 2553. (20) Steytler, D. C.; Jenta, T. R.; Robinson, B. H.; Easto, J.; Heenan, R. K. Langmuir 1996, 12, 1483. 2347

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