Structure and Dynamics of Poly(oxyethylene) Cholesteryl Ether

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Structure and Dynamics of Poly(oxyethylene) Cholesteryl Ether Wormlike Micelles: Rheometry, SAXS, and Cryo-TEM Studies Rekha Goswami Shrestha,*,† Ludmila Abezgauz,‡ Dganit Danino,‡ Kenichi Sakai,† Hideki Sakai,†,§ and Masahiko Abe†,§ †

Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ‡ Department of Biotechnology and Food Engineering, Technion, Haifa, Israel § Institute of Colloid and Interface Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan ABSTRACT: In this article, we provide direct evidence for 1-D micellar growth and the formation of a network structure in an aqueous system of poly(oxyethylene) cholesteryl ether (ChEO20) and lauryl diethanolamide (L-02) by rheometry, small-angle X-ray scattering (SAXS), and cryo-transmission electron microscopy (cryo-TEM). The ChEO20 self-assembles into spheroid micelles above the critical micelle concentration and undergoes a 1-D microstructural transition upon the incorporation of L-02, which because of its lipophilic nature tends to be solubilized into the micellar palisade layer and reduces the micellar curvature. The elongated micelles entangle with each other, forming network structures of wormlike micelles, and the system shows viscoelastic properties, which could be described by the Maxwell model. A peak observed in the zero-shear viscosity (η0) versus L-02 concentration curve shifted toward higher L-02 concentrations and the value of maximum viscosity (η0 max) increased with the increasing ChEO20 mixing fraction with water. We observed that η0 max increased by 2 to 4 orders of magnitude as a function of the ChEO20 concentration. The Maxwell relaxation time (τR) shows a maximum value at a concentration corresponding to η0 max (i.e., τR increases with L-02 concentration and then decreases after attaining a maximum value, whereas the plateau modulus (G0) shows monotonous growth). These observations demonstrate microstructural transitions in two different modes: L-02 first induces 1-D micellar growth and as a result the viscosity increases, and finally after the system attains its maximum viscosity, L-02 causes branching in the network structures. The microstructure transitions are confirmed by SAXS and cryo-TEM techniques.

1. INTRODUCTION Long, flexible linear micelles exhibiting viscous elastic properties are commonly referred to as wormlike or threadlike micelles. Under some conditions, they behave similarly to a solution of flexible polymers, thus also referred to as living polymers. However, they differ from classical polymers in that they constantly break and recombine; therefore, they do not exhibit a quenched distribution of lengths, and this unique rheological property profoundly affects their dynamic behavior. Research into living polymers has continued to attract experimentalists and theorists owing to numerous potential applications such as fracturing fluids in oil fields, drag-reducing agents, and also the formulation of home and personal care products.1 From an environmental point of view, wormlike micelles obtained from a mixture of nonionic surfactants are preferred over ionic systems in practical applications, where a viscoelastic property is often required. There are numerous reports on the formation and properties of viscoelastic wormlike micelles in charged systems. Most work centers on cationic surfactants2
11 in the presence of high salt concentrations, though ample reports on mixed r 2011 American Chemical Society

aqueous systems of cationic/anionic,12
15 ionic/nonionic,16
19 and gemini surfactant20
23 are also available. Importantly, nonionic surfactants can also form wormlike micelles, typically in the presence of a cosurfactant.24
26 Such viscoelastic solutions with a mixture of poly(oxyethylene) cholesteryl ether and alkanolamides can be promising candidates in many pharmaceutical and cosmetic formulations because both the cholesteryl as the hydrophobic part and the poly(oxyethylene) chain as the hydrophilic part are environmentally friendly and biocompatible27 and the alkanolamides are well known as foam boosting, thickening, antistatic, and anticorrosion agents in detergents. Acharya et al.28,29 and Toufiq et al.30 have shown that the incorporation of a lipophilic surfactant with a small headgroup, C12EOm (m = 1
4) or dodecanoyl-N-methylethanolamide, reduces the effective area per molecule, as, of ChEOn, (n = 10, 15, and 30) and favors micellar growth, eventually forming a network structure. Depending upon the molecular configuration of the Received: July 25, 2011 Revised: September 14, 2011 Published: September 22, 2011 12877

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Scheme 1. Molecular Structures of (a) ChEO20 and (b) L-02

surfactant and cosurfactants, the viscosity increases by several orders of magnitude. Moitzi et al.31 indicated the formation of viscoelastic wormlike micelles in mixed C12EOm (m = 1
4) and ChEOn (n = 10 and 15) systems on the basis of rheometry and small-angle neutron scattering (SANS). Although the formation and properties of wormlike micelle of ChEOn (n = 10, 15, and 30) surfactant systems have been reported in detail, ChEO20 has not been studied to that extent. In this context, we report here on the formation of viscoelastic wormlike micelles in mixed systems of the long poly(oxyethylene) cholesteryl ether surfactant, ChEO20, and alkanolamide-type nonionic surfactants. Rheology and small-angle X-ray scattering (SAXS) have been used to elucidate the micellar changes triggered by a cosurfactant. Cryo-TEM studies of the corresponding systems provided evidence for the formation of linear and branched micelles in the different regimes and supported the conclusions derived from the above techniques.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(oxyethylene) cholesteryl ether (ChEO20) and lauryl diethanolamide (L-02) were kindly received from Nihon Emulsion Co., Japan and Kao Chemicals, Japan, respectively. All of the chemicals were used as received. Millipore water was used throughout the experiment. The schematic molecular structures of ChEO20 and L-02 are shown in Scheme 1. 2.2. Phase Diagram. First, several aqueous solutions of ChEO20 with increasing concentrations were prepared in order to determine the dilute micellar
liquid-crystal (Wm
LC) phase boundary. Samples prepared by weighing the required amounts of reagents into test tubes fitted with screw caps were mixed using a vortex mixer. After the determination of the binary phase boundary, cosurfactant L-02 was added to several micellar solutions of ChEO20. Incremental concentration steps of ∼0.5 wt % were taken to locate the points accurately until phase separation occurred. Very viscous samples within the Wm regions were noted. The samples were kept in a water bath at 25 °C for equilibration. Phases were identified by visual observation through crossed polarizers. 2.3. Rheological Measurements. For the rheological measurements, 2
10 wt % ChEO20 micellar solutions were prepared with varying concentrations of L-02. Steady, dynamic rheological experiments were performed on a stress-controlled AR-G2 rheometer (TA Instruments). Samples were run on cone
plate geometries (diameter of 60 mm with a cone angle of 2° 10 900 for low-viscosity samples and a diameter of 40 mm with a cone angle of 2° 00 400 for high-viscosity samples). The zero-shear viscosity of the sample was determined from controlled-stress measurements by extrapolating the viscosity
shear-stress curve to zero-shear rate. Dynamic frequency spectra were obtained in the linear viscoelastic regime of each sample as determined by dynamic strain
sweep experiments in the frequency region from 0.01 to 100 rad s
1. 2.4. Small-Angle X-ray Scattering (SAXS). SAXS measurements were carried out on ChEO20/H2O/L-02 systems. The measurements were performed using a SAXSess camera (Anton Paar, PANalytical) attached to a PW3830 laboratory X-ray generator with a long, fine focus, sealed

glass X-ray tube (Kα wavelength of 0.1542 nm) (PANalytical). The apparatus was operated at 40 kV and 50 mA. The SAXSess camera is equipped with focusing multilayer optics and a block collimator for an intense monochromatic primary beam with low background and a translucent beam stop for the measurement of an attenuated primary beam at a scattering vector, q = 0, related to the scattering angle, θ, and wavelength of X-rays, λ, by q = (4π/λ) sin(θ/2). Samples were enclosed in a vacuum-tight thin quartz capillary with an outer diameter of 1 mm and a thickness of 10 μm, and the same capillary was used for all measurements to attain exactly the same scattering volume and background contribution. The sample temperature was controlled with a thermostatted sample holder unit (TCS 120, Anton Paar). The scattering intensity was first measured on a Cyclone image plate (IP) detection system (Perkin-Elmer, U.S.), and the 2-D intensity data were finally transformed into 1-D scattering curves as a function of the magnitude of the scattering vector by using SAXSQuant software (Anton Paar). All data were normalized to the same incident primary beam intensity for transmission calibration and were corrected for background scattering from the capillary and the solvent. The SAXS data of the micellar solutions were analyzed by the generalized indirect Fourier transformation (GIFT) method,32
36 which gives pair
distance distribution functions (PDDFs), p(r), and structures of particles in real space. Details on the SAXS theory and data treatment methods are described elsewhere.37
42 2.5. Cryo-TEM. Samples for cryo-TEM were prepared in a controlled environment vitrification system (CEVS). In this apparatus, samples at the desired temperature are quenched rapidly in liquid ethane at its freezing point of
183 °C, forming a vitrified specimen, and then transferred to liquid nitrogen for storage. The vitrified samples were examined on a Philips CM120 microscope operated at 120 kV using an Oxford CT-3500 cryo-holder. Additional samples were examined on a Tecnai T12 TEM at 120 kV using a Gatan 626 cryo-holder. All samples were kept below
170 °C during transfer and imaging. Digital images were recorded in low-dose mode on Gatan-cooled CCD cameras, as described before.43

3. RESULTS AND DISCUSSION 3.1. Effect of L-02 on the Aqueous Phase of ChEO20. ChEO20 forms spherical micelles above the critical micelle concentration (cmc) because of the balance between the bulky, nonflexible cholesterol headgroup and the long, flexible ethylene oxide chain. Before the addition of L-02, the aqueous ChEO20 solution is transparent, with a viscosity that is almost the same as that of the solvent. The micellar region, Wm, appears to up to ∼28 wt % surfactant, and then the micellar phase evolves to an optically isotropic, highly viscous cubic liquid crystal (I1) phase with space group Pm3n as determined by SAXS.44 Upon successive addition of L-02 to the dilute aqueous ChEO20 micellar solution, the viscosity increases gradually at first and then promptly, and a viscous solution is observed. This is because L-02 tends to reside in the palisade layer of the micelle, thereby reducing the surface curvature. This is possible because of the rigid structure of the lipophilic part and the branching of the alkyl 12878

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Langmuir group, which create a hindrance in the packing of the surfactant into spherical aggregates. The samples corresponding to the shaded area within the Wm region in the phase diagram are transparent and have a high viscosity. Although optically isotropic at rest, they exhibit shear birefringence and can support their weight for tens of seconds when placed upside down. At higher mixing fractions of L-02, phase separation takes place and a vesicular dispersion (diameter ∼10 μm in 2 wt % ChEO20/H2O + 3 wt % L-02) is seen in the multiphase region. The surfactant layer curvature finally becomes zero at higher L-02 content. 3.2. Steady and Dynamic Rheological Properties of Max_ plot well Fluids. Figure 2 shows the viscosity (η)
shear-rate (γ) for 2 wt % ChEO20 as a function of L-02 content at 25 °C. The η of the ChEO20 micellar solution in the absence of L-02 is close to that of the solvent and is constant regardless of γ_ (Newtonian flow behavior). At around 1.4
2 wt % L-02, the flow curves show two distinct regions; at low shear rates, the shear viscosity remained unvaried with the plateau value equal to the zero-shear viscosity, η0, and with increasing shear rate, a shear-thinning behavior can be observed (non-Newtonian behavior), which is an indication of the presence of a transient network of wormlike micelles. At 2 wt %, the system attains its maximum viscosity, η0max, and the critical shear rate, γ_ c (the shear rate at which shear thinning starts), shifts to lower values (i.e., shear thinning begins at lower shear rates, suggesting that the transient network density increases, thereby enhancing the

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fluid viscoelasticity). When the deformation is faster than the time required to regain equilibrium, the network structure of wormlike micelles is deformed and hence shear thinning occurs because of the alignment of aggregates under flow that are unable to retain their structure in the particular range of the shear rate. However, with further increases in L-02 concentration, the viscosity drops and the Newtonian region shifts toward _ corresponding to the structural modification of the higher γ, system. This change in the rheological behavior accounts for the fact that with increasing concentration of L-02 the interfacial curvature of aggregates gradually decreases and, as a result, the energy required for the formation of hemispherical end-caps of the cylindrical micelles increases. The end-cap energy of a system can be compromised if the free ends fuse with the cylindrical part of within any given micelles or with other micelles, thus forming micellar joints or branching in the network structure. The overall entropy gain associated with the branch points is greater than that of end-caps. Such joints can slip along the cylindrical body, thereby allowing a faster and easier route to stress relaxation.45,46 Branching points also restrict the alignment of micelles under shear, causing an increase in γ_ c.47,48 The zero-shear viscosity (η0) plot of micellar ChEO20 solutions with L-02 (Figure 2b) remarkably shows a maximum for all of the solutions. The η0
L-02 curves shift toward the right side with the concentration of ChEO20 (from 2 to 10 wt %); higher concentrations of L-02 are needed to induce effective micellar growth for elevated ChEO20 concentrations. The viscoelastic properties of the wormlike micellar solutions were investigated by oscillatory-shear measurements. Oscillatory-shear (frequency-sweep) measurements were performed on the viscous and transparent (gel-like) solutions formed around the viscosity maximum (Figure 2). In this region, the data points of the elastic modulus, G0 , viscous modulus, G00 , and complex viscosity, |η*| (obtained especially in low-ω (0.001
100 s
1) regions) well fit the Maxwell model governed by the equations49

Figure 1. Partial ternary phase diagram of ChEO20/H2O/L-02 at 25 °C. Wm and I1 represent the isotropic micellar solution and discrete cubic phase, respectively. The shaded area within the Wm region stands for the highly viscous solutions, and the dotted lines represent the compositions along which rheological measurements were performed.

G0 ðωÞ ¼

ω2 τR 2 G0 1 þ ω2 τ R 2

ð1Þ

G}ðωÞ ¼

ωτR G0 1 þ ω2 τ R 2

ð2Þ

G0 τR jηj ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ ω2 τR 2

ð3Þ

_ for 2 wt % ChEO20/H2O with varying concentrations of L-02 from 0 to 2.6 wt% and (b) zero shear Figure 2. (a) Viscosity (η) with steady shear-rate (γ) viscosity (η0) for 2, 5, and 10 wt% ChEO20/H2O at 25 °C. 12879

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Figure 3. (a) Dynamic frequency spectrum and (b) Cole
Cole plot for 2 wt % ChEO20/H2O + 2 wt % L-02 at 25 °C. The solid lines represent the best fit to the Maxwell model.

Figure 4. Variation of G0 and τR for (a) 2, (b) 5, and (c) 10 wt % ChEO20/H2O as a function of L-02 concentration. The dotted lines represent the compositions corresponding to η0 max.

The relaxation time, τR, may be estimated from the G0
G00 crossover frequency (i.e. τR = 1/ωc when G0 = G00 ). The zeroshear viscosity, η0, can also be calculated from G0 and τR by the following relation: η0 ¼ G0 τR

ð4Þ

Figure 3 shows the rheograms of the 2 wt % ChEO20/H2O + 2 wt % L-02 system. The plot shows G0 and G00 as a function of the angular frequency, ω. We note that the sample exhibits the viscoelastic response from wormlike micelles, with elastic behavior at high ω or short timescales (G0 dominating G00 ) and viscous behavior at low ω or long timescales (G00 exceeding G0 ). At frequencies below the crossover, G0 and G00 increase with ω2 and ω, respectively, whereas |η*| is constant. Clearly, the storage modulus (G0 ) and the loss modulus (G00 ) have one crossover. G0 progresses into a plateau, G0 at

high frequencies, and G00 reaches a maximum at ωc and then decreases continuously. This is typical viscoelastic behavior shown by wormlike micellar solutions. It is noteworthy that the experimental data (symbols) for G0 and G00 deviate from the Maxwell model at higher frequencies. Granek and Cates have shown that the high-frequency deviations can be accounted for by the Rouse modes and the primitive path fluctuations along the micelle chain.50 Here, we focus on the linear wormlike micelles. Similar results were also obtained for other viscoelastic samples in the higher-concentration range.51,52 A Cole
Cole plot (G00 as a function of G0 ) provides an alternative way to illustrate the Maxwellian behavior. The semicircle feature of a Maxwell fluid can be expressed as G 12880

002

   2 G0 2 G0 0 þ G
¼ 2 2

ð5Þ

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Figure 5. (a) Normalized X-ray scattering intensities, I(q), for the 2 wt % ChEO20/H2O system at different concentrations of L-02 obtained on absolute scales and (b) the corresponding pair
distance distribution function, p(r), at 25 °C. The solid and broken lines in panel a represent the GIFT fit and the calculated form factor for n particles existing in a unit volume, nP(q), respectively. Arrows on the high-r side of panel b represent the maximum dimension of the micelles.

The Cole
Cole plot in Figure 3b shows that the experimental data closely follow the Maxwell model except at high frequencies. Generally, the viscoelastic wormlike micelles behave like a Maxwell fluid; these results indicate that the 2 wt % ChEO20/ H2O + 2 wt % L-02 system consists of wormlike micelles. The estimated G0 and τR obtained after fitting the experimental data to the Maxwell model are plotted in Figure 4 as a function of L-02 for 2, 5, and 10 wt % ChEO20 aqueous solutions. Increases in G0 and τR have been observed up to the composition corresponding to η0 max (Figure 2b) at all concentrations of ChEO20; a maximum is observed in τR at the same concentration of L-02 as that for η0 max; however, a thoroughly increasing pattern for G0 throughout the measured concentration range is noticed. It is known that for entangled polymers the plateau modulus, G0, is related to the entanglement density, Fe, by the equation G0 = FekBT.13 Therefore, it can be considered that the entanglements of wormlike micelles increased with the concentration until the concentrations corresponding to η0 max, where Fe reached a maximum and therefore underwent stress relaxation slowly, corresponding to higher value of τR.53 τR decreases upon further increases in L-02, indicating another microstructure transition in the network that allows stress relaxation by an additional faster mechanism. A plausible explanation for this observation is that the wormlike micelles get connected with each other, forming junctions that can slip along their length, thereby allowing a faster and easier route to stress relaxation as mentioned earlier.48 Branching occurs if the energy required to create a 3-fold junction is less than that needed to form a hemispherical end-cap.54,55 Further validation of the results is obtained using SAXS techniques. 3.3. SAXS Measurements. To confirm the micellar growth induced by L-02, SAXS measurements were carried out on a 2 wt % ChEO20 solution as a function of L-02 concentration at 25 °C. Figure 5 shows the normalized X-ray scattering intensities, I(q), and the corresponding real-space pair
distance distribution functions p(r) deduced with the GIFT analysis of the SAXS data. The scattering behavior of the 2 wt % ChEO20/H2O system shows the spheroid core
shell type of ChEO20 micelles; the lowq scattering intensity and the calculated form factor P(q) reach zero q following q0 behavior. The scattering behavior is modified upon addition of L-02. As seen in Figure 5a, the position of the local minimum in the intermediate q range (q ≈ 0.75 nm
1

without L-02) shifts toward low q and achieves a minimum value of q ≈ 0.53 nm
1 at a composition corresponding to η0 max. Furthermore, the P(q) curve follows ∼q
1 behavior in the low-q region. These features in the scattering functions demonstrate 1-D micellar growth induced by L-02.56,57 The L-02-induced microstructural transition can best be seen in the real-space p(r) curves shown in Figure 5b. A symmetrical bell-shaped p(r) curve with a small bump on the low-r side (r ∼ 2 nm) indicates the core
shell type of spherical particles with a maximum dimension, Dmax, of ∼12 nm for aqueous micellar solution of ChEO20 in the absence of L-02. When L-02 is incorporated into the system, the p(r) curves exhibit a pronounced local maximum and minimum on the low-r side, followed by the asymmetric decay of p(r) on the higher-r side. Moreover, Dmax increases with L-02 concentration. The overall contrast of the system seems to decrease upon addition of L-02 as indicated by the pronounced local maximum and minimum in the low-r region. Nevertheless, the inflection point between the local maximum and minimum and that after the local minimum, which semiquantitatively estimates the cross-sectional radius and diameter of the micellar core, remain virtually unchanged despite the variation in the L-02 concentration, demonstrating that the incorporation of L-02 does not modify the internal structure of the micelles. The linear decay of the p(r) curve at high r and a continuous increase in Dmax with L-02 can be taken as direct evidence of 1-D micellar growth. We note that an increase in the micellar size Dmax, from approximately 12 to 18 nm, could not explain the huge increase in the viscosity (Figure 2b). Dmax depends on the maximum resolution (qmin, Dmax = π/qmin) of the SAXS measurement and on the overall contrast of the system. Here, we note that the maximum length of long cylindrical or wormlike micelles determined from the total p(r) curves (∼18 nm) is underestimated and may not be the actual micellar length. Nevertheless, with the available contrast and sensitivity of the SAXS equipment (qmin = 0.08 nm
1), we are able to outline the qualitative information regarding the L-02-induced modulation of the structure of ChEO20 micelles; the internal structure remains essentially the same, whereas the maximum length of the micelles tends to increase with the L-02 concentration. SANS usually has a high resolution with the low q ≈ 0.03 nm
1, and hence an estimation of the particle size of up to ∼105 nm is possible. Because the scattering-length density of 12881

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finally vesicles are also observed in the partial tertiary phase diagram (Figure 1). The micelles found at low L-02 concentrations are somewhat flattened on the border between threadlike micelles and very narrow ribbons. Additionally, a number of 3-fold connections are found in the cryo-TEM images even at low L-02 concentrations (not shown), likely because of local composition inhomogeneity.58,59 Overall, the results from the cryo-TEM study are in good agreement with the predictions from the rheology, and the increase in viscosity is attributed to the transformation from spherical micelles to long cylinders and entanglements of the latter. The decrease in the viscosity is shown to be correlated with a shift from linear wormlike micelles to branched wormlike micelles and eventually to vesicles.

Figure 6. Cryo-TEM micrographs of micelles formed by (a) 2 wt % ChEO20 and with L-02 concentrations equal to (b) 1, (c) 2.3, and (d) 2.6 wt %.

neutrons is much higher than that of hydrogen (electrons), the system possesses better contrast in D2O, thus offering better scattering data. Furthermore, with the contrast variation facilities in the SANS technique, one can estimate the core, shell, and corona structures separately. These are the reasons that people prefer SANS, in particular, in long cylindrical or wormlike micellar solutions. At present, we do not have access to SANS. To elucidate the structures and directly resolve the micelle shape and length, we used cryo-TEM. Several solutions were studied, as discussed in the following section. 3.4. Cryogenic Transmission Electron Microscopy (CryoTEM). Direct images of vitrified ChEO20/H2O/L-02 samples were obtained by cryo-TEM. The image of an aqueous micellar solution of 2 wt % ChEO20 (no L-02, see Figure 6a) shows the presence of abundant small, spheroidal, slightly elongated micelles. Similar predictions were obtained from rheology and SAXS. When L-02 is added to the system, long wormlike micelles start to form. The large size distribution in the populations of the two coexisting micelle types at the cost of increasing the viscosity is clearly visible (1 wt % L-02, Figure 6b). The wormlike micelles continue to elongate, start to overlap, and create a network of entangled linear micelles, thereby increasing the viscosity as well as the elasticity of the system. As a consequence, considerably fewer micelles ends, spherical micelles, short wormlike micelles, and micellar end-caps are observed, which implies that the micelles are at or are close to their longest form. Micellar elongation continues up to the concentration of L-02 corresponding to η0 max. With further increases in L-02 concentration, there is a decrease in η0 beyond η0 max, and the pattern seen is that of a saturated network of branched micelles (Figure 6c). Finally, at even higher L-02 concentrations vesicles start to form (Figure 6d); black dots in the background of image are overlapping structures, and micelles are positioned parallel to the electron beam. Together, the images in Figure 6 clearly show that with the successive addition of L-02 there is micellar elongation of up to η0 max, and then the creation of connected structures and

4. CONCLUSION The addition of a lipophilic nonionic surfactant, L-02, to the dilute aqueous solution of ChEO20 dramatically increases the viscosity by several orders of magnitude as a result of the formation of very long wormlike micelles that entangle into a network and show viscoelastic behavior described by the single-mode Maxwell model at low shear frequency. The micellar growth can be explained by the reduction in the effective area per headgroup of surfactant upon the addition of L-02 (a decrease in the spontaneous curvature of the aggregates) and hence a progressive increase in the energy cost of the formation of hemispherical end-caps. The drop in the viscosity after the maxima with further increasing L-02 concentration is associated with the branching of already existing entangled micelles. The cross section of the wormlike micelles is not affected by the addition of L-02 as confirmed by SAXS. Cryo-TEM provided direct evidence for the microstructural changes in the micellar aggregates and is consistent with the conclusions drawn from the rheological and scattering measurements. Viscoelastic wormlike micelles in mixed nonionic surfactant systems would be extremely useful for practical applications such as cosmetics formulations, where viscoelasticity without adding salt is often a required property. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +81-47-124-1501 ext. 3662. Fax: +81-47-121-2439.

’ ACKNOWLEDGMENT R.G.S. is grateful to the Japan Society for the Promotion of Science (JSPS) for financial support. Fruitful discussion with Dr. Lok Kumar Shrestha, National Institute for Materials Science (NIMS), Japan, and Dr. Takaaki Sato, Shinshu University, Japan, are gratefully acknowledged. The support of the Israel Science Foundation (grant no. 1137/08) and RBNI's support of D.D. are gratefully acknowledged. ’ REFERENCES (1) Yang, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 276. (2) Oelschlaeger, C.; Schopferer, M.; Scheffold, F.; Willenbacher, N. Langmuir 2009, 25, 716. (3) Abezgauz, L.; Kuperkar, K.; Hassan, P. A.; Bahadur, P.; Danino, D. J. Colloid Interface Sci. 2010, 342, 83. (4) Kern, F.; Lemarechal, P.; Candau, S. J.; Cates, M. E. Langmuir 1992, 8, 437. 12882

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