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Viscoelastic Wormlike Micelles of Long Polyoxyethylene Chain Phytosterol with Lipophilic Nonionic Surfactant in Aqueous Solution Suraj Chandra Sharma,*,† Lok Kumar Shrestha,‡ Koji Tsuchiya,† 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, Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, and Institute of Colloid and Interface Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: January 14, 2009
We have studied the formation, structure, and rheological behavior of viscoelastic wormlike micelles in the mixed system of long polyoxyethylene chain phytosterol (PhyEO30) and polyoxyethylene dodecyl ether (C12EOn, n ) 3 and 4) surfactants in water. Partial ternary phase diagrams of water/PhyEO30/C12EOn are constructed at 25 °C. Addition of C12EOn to the aqueous solution of PhyEO30 in a dilute region increases the viscosity by several orders and forms viscoelastic micellar solution of entangled wormlike micelles showing the Maxwellian behavior at low shear frequencies. With successive addition of C12EOn, ultimately a phase separation occurs with the formation of turbid solution of vesicular dispersion coexisting with the micellar phase. A rapid micellar growth has been observed when C12EO3 replaces the C12EO4. However, no viscosity maximum is seen with the C12EO3 system; the viscosity increases continuously until phase separation. The effect of temperature on water/PhyEO30/C12EO3 system has also been studied. Small-angle X-ray scattering measurements have shown the one-dimensional micellar growth induced by C12EOn and well support the conclusions derived from rheometry. Introduction Under appropriate conditions, micelles undergo enormous elongation and form very long and flexible aggregates, referred to as “wormlike” or “threadlike” micelles, for which the spontaneous curvature of the end-caps is higher than the curvature along the cylindrical body. The growth is therefore a consequence of the system to minimize the excess free energy by reducing the number of end-caps in spite of the counteracting entropy factor. At a certain concentration, wormlike micelles start to form a transient network, similar to a solution of flexible polymers, which display remarkable viscoelastic properties. Unlike polymers, wormlike micelles break and reform dynamically. The viscoelastic properties of wormlike micelles have been largely exploited for many applications such as fracturing fluids in oil fields, drag reducing agents, and also in formulation of home and personal care products.1 Formation and properties of viscoelastic wormlike micelles have been studied extensively, mostly in long hydrophobic chain cationic surfactant2-11 in the presence of high concentration of salt, which screens the electrostatic repulsions between the charged surfactant head groups. Viscoelastic solutions of wormlike micelles have also been reported in mixed cationic/ anionic, ionic/nonionic, and in some cases, in zwitterionic surfactant aqueous systems.12-19 Although the majority of wormlike micelles reported are charged systems, nonionic surfactant can also form wormlike micelles, usually in the presence of a cosurfactant. Acharya et al.20,21 found that highly * To whom correspondence should be addressed. E-mail: surajcsharma@ yahoo.com/ Phone/Fax: +81-4-7122-1442. † Department of Pure and Applied Chemistry, Tokyo University of Science. ‡ Yokohama National University. § Institute of Colloid and Interface Science, Tokyo University of Science.
viscoelastic wormlike micelles can be formed in the salt-free condition when a lipophilic nonionic surfactant such as short hydrophilic chain polyoxyethylene alkyl ether or dodecanoylN-methylethanolamide is added to the dilute micellar solution of polyoxyethylene cholesteryl ether surfactant. Incorporation of a lipophilic surfactant with a small headgroup reduces the effective area per molecule as, which results in a decrease in the interfacial curvature of the aggregate. Polyoxyethylene phytosterol surfactants are composed of sterol as the hydrophobic part and polyoxyethylene chain as the hydrophilic part. They are claimed to have low toxicity and the natural origin of the hydrophobe makes them attractive from an environmental point of view.22 Being good emulsifiers, polyoxyethylene phytosterols are used in many cosmetic formulations. Recently Oka et al.23 prepared the ultrafine emulsions using the long polyoxyethylene chain phyotosterol surfactant. Like polyoxyethylene cholesteryl ether surfactants, nonionic phytosterol surfactants also possess a very strong segregation tendency between the hydrophilic and the hydrophobic group compared to conventional alkyl ethoxylated surfactants and hence show unique phase behavior in the presence of water and water/cosurfactant.24 Naito et al.25 reported the formation of viscoelastic wormlike micellar solution in polyoxyethylene phytosterol (PhyEO10 and PhyEO20) surfactant systems and studied effect of changing the headgroup size of cosurfactant on the viscosity growth. However, there is still a need for studies on the micellar growth in the surfactant with a very big headgroup. On the other hand, the direct evidence of the microstructural change of the aggregates is still lacking for such system. In the present contribution, we report the formation of viscoelastic wormlike micelles in mixed system of long polyoxyethylene chain phytosterol and short short-chain polyoxy-
10.1021/jp8102244 CCC: $40.75 2009 American Chemical Society Published on Web 02/19/2009
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SCHEME 1: Molecular Structure of PhyEO30
the solvent. The SAXS data of the micellar solutions are analyzed by generalized indirect fourier transformation’s (GIFT) method,26-29 which gives pair-distance distribution functions (PDDFs), p(r), and structures of particles in real space. The details on the theory of SAXS and data treatment methods are described elsewhere.30-34
ethylene alkyl ether type nonionic surfactants. Additionally, small-angle X-ray scattering (SAXS) technique has been used to study the micellar growth triggered by a cosurfactant and temperature. 2. Experimental Section 2.1. Materials. Polyoxyethylene phytosterol (PhyEO30) was kindly supplied from Cosmos Technical Centre, Co., Ltd., Japan. Homogeneous polyoxyethylene dodecyl ethers (C12EOn, n ) 3 and 4) were purchased from Nikko Chemicals Co., Ltd., Japan. All the chemicals were used as received. Millipore water was used throughout the experiment. The schematic molecular structure of PhyEO30 is shown in Scheme 1. 2.2. Phase Diagram. Samples for phase diagram construction were prepared by weighing the required amounts of reagent into test tubes fitted with screw caps and mixed using a vortex mixer. 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. Samples were homogenized and kept in a water bath at 25 °C for at least 24 h to ensure equilibration before performing measurements. The rheological measurements were performed in a stress-controlled rheometer, AR-G2 (TA Instruments) using cone-plate geometries (diameter of 60 mm with cone angle of 2° 1′ 9′′ for lowviscosity sample and diameter 40 mm with cone angle of 2° 0′ 4′′ for high-viscosity sample). Frequency sweep measurements were performed in the linear viscoelastic regime of the samples, as determined previously by dynamic strain sweep measurements. 2.4. SAXS. For the structural investigation of the micellar aggregates, SAXS measurements were carried out on water/ PhyEO30/C12EOn 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 (KR wavelength of 0.1542 nm) (PANalytical). The apparatus was operated at 40 kV voltages and 50 mA current respectively. The SAXSess camera is equipped with a focusing multilayer optics and a block collimator for an intense and monochromatic primary beam with low background and a translucent beam stop for the measurement of an attenuated primary beam at q ) 0. Samples were enclosed into a vacuum tight thin quartz capillary with an outer diameter of 1 mm and thickness of 10 µm, and the same capillary was used for all measurements to attain the exactly the same scattering volume and background contribution. Sample temperature was controlled with a thermostatted sample holder unit (TCS 120, Anton Paar). The scattered intensity was first measured on an image plate (IP) detection system Cyclone (Perkin-Elmer, USA), and the two-dimensional intensity data was finally transformed into onedimensional 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 the transmission calibration and were corrected for the background scattering from the capillary and
3. Results and Discussion 3.1. Phase Behavior. The partial ternary phase diagrams of water/PhyEO30/C12EOn (n ) 3 and 4) systems in the dilute region at 25 °C are shown in Figure 1. In water/PhyEO30 binary system, PhyEO30 forms the micellar solution (Wm), micellar cubic (I1) phase, hexagonal (H1) phase, and solid phase with increasing surfactant concentration at normal room temperature. The Wm region appears up to nearly 25 wt % of the surfactant. Because of the very big headgroup, the present surfactant forms a spherical type of micelle above the critical micelle concentration (CMC), which has been found to 3 × 10-6 M at 25 °C despite its bulky and nonflexible sterol group. The detailed aqueous binary phase behavior of PhyEO30 is described elsewhere.22 It is known that C12EO3 and C12EO4 cannot form a micellar phase in water.35 In case of ternary system, lipophilic amphiphiles C12EOn tend to be solubilized in a surfactant palisade layer and reduce the surfactant layer curvature. Upon successive addition of C12EOn to the dilute micellar solution of PhyEO30, viscosity increases gradually at first then promptly, and a viscous solution is observed. The interesting point to be noted here is that despite the big headgroup possessed by PhyEO30, which tends to keep the curvature more positive, the one-dimensional micellar growth occurs. It is possible due to the rigid structure of the lipophilic part and the branching of the alkyl group, which create a hindrance in the packing of the surfactant in the spherical aggregate. At a higher mixing fraction of C12EOn, phase separation takes place, and vesicular dispersion is seen in the multiphase region. Hence, the surfactant layer curvature finally becomes zero at a high C12EOn content. It can be seen from Figure 1 that the width of the Wm phase, that is, the ability of the Wm phase to solubilize the C12EOn at the waterrich region, increases with the increasing number of EO units in C12EOn, and hence with C12EOn the Wm phase protrudes toward the C12EOn apex. The shaded area in the phase diagram indicates the high viscosity region inside the Wm phase. The samples inside the high viscosity zone show flow-birefringence, which is seen when the samples are viewed under crossed polarizers while being shaken. 3.2. Rheological Behavior. 3.2.1. Effect of Surfactant Concentrations. Figure 2 shows the variation of viscosity (η) as a function of shear-rate (γ˙ ) for 5 wt % PhyEO30 + C12EOn systems at different mixing fraction of C12EOn, expressed in weight fraction of C12EOn in total surfactant, X, at 25 °C. As can be seen from Figure 2a in the absence of C12EO4, η of surfactant solution is very small, close to that of solvent and constant regardless of γ˙ , that is, Newtonian flow behavior is observed up to γ˙ ) 1000 s-1. At lower value of X ≈ 0.40, η is still very low and independent of γ˙ , although a slight increase in viscosity is noticed at low γ˙ . At X ≈ 0.44, the behavior is still Newtonian over wide range of shear-rate but shear thinning occurs at large deformation (γ˙ > 10 s-1), which can be taken as evidence of the formation of wormlike micelles that undergo structural change, such as alignment of the long micelles at a high shear.9 With further increasing X up to X ≈ 0.51, the wormlike micelles become entangled into a transient network, thereby enhancing the viscoselasticity of the fluid and the critical γ˙ (the shear-rate at which shear thinning starts) shifts to lower value. However, with further increase in C12EO4 concentration
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Figure 1. Partial phase diagrams of (a) water/PhyEO30/C12EO4 and (b) water/PhyEO30/C12EO3 ternary systems at 25 °C. Wm stands for the isotropic micellar solution.
Figure 2. Steady shear-rate (γ˙ ) versus viscosity (η) curves for (a) 5 wt % PhyEO30 + C12EO4 and (b) 5 wt % PhyEO30 + C12EO3 systems at various mixing fraction of C12EOn in total surfactant, X.
(at X ≈ 0.57 and 0.63), the viscosity decreases and the Newtonian region appears up to higher γ˙ , which corresponds to the structural modification in the system. The explanation for this change in the rheological behavior is that with increasing X, the interfacial curvature of aggregate gradually decreases and, as a result, energy required for the formation of hemispherical end-caps of the cylindrical micelles increases. The end-cap energy can be minimized, only if the free ends fuse with cylindrical part of its own or other micelles, thus forming micellar joints, or branching in the network structure. Such joints can slip along the cylindrical body, thereby allowing a faster and easier way of stress relaxation.36,37 Branching points also restricts the alignment of micelles under shear38 causing an increase in critical γ˙ . In 5 wt % PhyEO30 + C12EO3 system, viscosity increases monotonically until phase separation and therefore rheological data above (X ≈ 0.39) are not available (see Figure 2b). At a comparable concentration (X ≈ 0.40 for C12EO4 and X ≈ 0.39 for C12EO3), Newtonian behavior is observed in C12EO4 system, whereas C12EO3 system shows a strong shear thinning effect. This clearly indicates that C12EO3 induces more pronounced micellar growth than C12EO4 under similar conditions. Steady-shear measurements on 10 wt % PhyEO30 + C12EOn systems show a similar trend with increasing concentration of C12EOn. The Newtonian behavior is noticed at low C12EOn concentration, while shear thinning is observed at high C12EOn concentration. Figure 3 shows the variation of zero-shear viscosity (η0) as a function of the mixing fraction of C12EOn in total surfactant, X, at different PhyEO30 concentration, viz., 5 and 10 wt %. The η0 values have been estimated by extrapolating the viscosity
Figure 3. Variation of zero-shear viscosity (η0) as a function of X for PhyEO30 + C12EOn systems at different PhyEO30 concentration. Open symbols, C12EO4; filled symbols, C12EO3.
data points to zero shear-rate for low viscosity samples. For viscoelastic samples, η0 can be calculated from oscillatory-shear measurements (see eq 3). Decreasing the EO chain length from EO4 to EO3 increases the extent of the micellar growth with a higher viscosity value occurring at a lower value of X. As can be seen from Figure 3, after a certain concentration further addition of C12EO4 increases η0 sharply till a maximum is reached and afterward decreases. However, no such maximum in the η0 curve is seen in the C12EO3 system. Addition of C12EO3 to the micellar solution of PhyEO30 increases η0 more steeply until phase separation. The η0-X curves shift toward left side when PhyEO30 concentration is increased from 5 to 10 wt %
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Figure 4. Variation of elastic modulus, G′ (filled symbols) and viscous modulus, G′′ (open symbols) as a function of oscillatory shear frequency (ω) as obtained by frequency sweep measurement at 25 °C in (a) 10 wt % PhyEO30 + C12EO4 and (b) 10 wt % PhyEO30 + C12EO3 systems at different mixing fraction of C12EOn in total surfactant, X. Solid lines represent the best fit to the Maxwell model.
indicates that rapid micellar growth occurs at lower concentration of C12EOn due to the decrease in the effective area as, of the surfactant headgroup at the interface with increasing PhyEO30 concentration. The maximum zero-shear viscosity obtained in the present system is comparable to that obtained by Naito et al.25 for similar PhyEO20/water/C12EOn systems with the exception that a relatively higher concentration of C12EOn is needed to form viscoelastic wormlike micellar solution. This is attributed to the higher interfacial curvature of the aggregates because of a longer EO chain length. Viscoelastic properties of the wormlike micellar solutions were investigated by oscillatory-shear measurements. Oscillatory-shear (frequency sweep) measurements were performed on the viscous samples formed around the viscosity maximum. Figure 4 shows plots of elastic or storage modulus (G′) and viscous or loss modulus (G′′) as a function of oscillatory shear frequency (ω) for 10 wt % PhyEO30 + C12EOn systems. At highω, the sample exhibits elastic bahavior (G′ > G′′); whereas at low-ω, the sample shows viscous behavior (G′′ > G′). The arrows highlight the intersection point of G′ and G′′. The viscoelasticity of the solution is attributed to the entanglement of very long and flexible wormlike micelles to form a transient network. Cates et al.39 describes the viscoelastic behavior of wormlike micelles by considering two processes of stress relaxation, namely, reptation or reptilelike motion of the micelle along a tube and reversible scission of micelles, taking place at two time scales, that is, reptation time τrep and scission time τb. The viscoelastic behavior of such a system at low shear frequency often follows the Maxwell model described by the following equations, which considers a single stress relaxation time τR given by (τbτrep)1/2
G′(ω) )
G′′(ω) )
ω2τR2 1 + ω2τR2
Go
(1)
Go
(2)
ωτR 1 + ω2τR2
where Go is called the shear (plateau) modulus. At high-ω, G′ tends to attain a constant value equal to Go. The relaxation time, τR may be estimated from the G′-G′′ crossover frequency, that is, τR ) 1/ω, when G′ ) G′′. Once Go and τR are available, η0 can be calculated using following relation
η0 ) GoτR
(3)
Solid lines in Figure 4 show the Maxwellian fitting to the data points. The rheological behavior in low-ω region can be described by the Maxwell model, but at high-ω region experimental data show significant deviation from the Maxwell model showing an upturn in the viscous modulus G′′. This deviation arises from the transition of the relaxation mode from “slower” reptation to “faster” Rouse modes.39,40 As it can be seen from Figure 4a, with increasing X, the G′-G′′ crossover frequency shifts to the lower value and attains the smallest value at X ) 0.45 which corresponds to a longer relaxation time. With a further increase in X, the G′-G′′ crossover frequency moves to the higher value with a shorter relaxation time and exhibits a poorer viscoelasticity. Unlike 10 wt % PhyEO30 + C12EO4 system, with increasing X the G′-G′′ crossover frequency shifts to lower values continuously with longer relaxation times (τR ) 2.6 s at X ) 0.33, τR ) 27.6 s at X ) 0.35, and τR ) 59.6 s at X ) 0.38) in 10 wt % PhyEO30 + C12EO3 system till the phase separation occurs as shown in Figure 4b. It can be seen in Figure 4b that with increasing X, G′ develops to a welldefined plateau value (Go ) 75.74 Pa, at X ) 0.38) and also the minima of G′′ decreases, which implies that the degree of entanglement of wormlike micelles forming a transient network increases and that rheology approaches Maxwellian behavior. The Cole-Cole plots of the 10 wt % PhyEO30 + C12EO4 systems at different X are presented in Figure 5a. The experimental data points closely follow Maxwell behavior with a semicircular behavior in the Cole-Cole plot. The estimated values for Go and τR are plotted in Figure 5b as a function of X in 5 wt % PhyEO30 + C12EO4, and 10 wt % PhyEO30 + C12EO4 systems. These parameters were calculated by fitting of the experimental data from frequency sweep measurements, especially the data in low-ω region, to the Maxwell equations. The value of Go is related to the number of entanglements between wormlike micelles or mesh size of the network9 and τR given by the Maxwell equation reflects the length of wormlike micelles. The increase in Go with X corresponds to the increase in the network density of the wormlike micelles. On the other hand, τR increases initially along with the increasing Go and X may be associated with the micellar growth leading to longer micelles and increased number of entanglements that make the system more viscoelastic. It can be expected that micelles with longer lengths would form an entangled network and therefore undergo stress relaxation slowly.41 With further increase in X, τR decreases indicating a structural change in the network that allows stress relaxation by additional faster mechanism. A plausible explanation for this observation is that the wormlike micelles get connected with each other, forming joints that can slip along their length, thereby
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Figure 5. (a) Cole-Cole plots for the 10 wt % PhyEO30 + C12EO4 system shown in Figure 4a and (b) variation of Go (squares) and τR (triangles) as a function of mixing fraction of C12EO4 in total surfactant, X for 5 wt % PhyEO30 + C12EO4 (open symbols) and 10 wt % PhyEO30 + C12EO4 (filled symbols) systems.
Figure 6. (a) Variation of zero-shear viscosity (η0) as a function of temperature (T) and (b) change of elastic modulus, G′ (filled symbols) and viscous modulus, G′′ (open symbols) as a function of ω as obtained by frequency sweep measurement at different temperatures for 5 wt % PhyEO30 + C12EO3 system at X ) 0.36. Solid lines represent the best fit to the Maxwell model in panel b.
allowing a faster and easier way of stress relaxation as mentioned earlier. The shift of τR curves toward the lower X values in τR-X plot (Figure 5b) upon increasing the PhyEO30 concentration in PhyEO30 + C12EO4 system corresponds to the higher extent of micellar growth. This fact is also supported by the increase of network density or Go with increasing PhyEO30 concentration. 3.2.2. Effect of Temperature. Temperature affects the micellar growth and rheological behavior of wormlike micellar solution, and the effect of temperature should be more pronounced in aqueous systems of nonionic surfactants, especially those having polyoxyethylene head groups because the hydration of oxyethylene unit is sensitive to the temperature. Since the addition of a short EO-chain lipophilic cosurfactant C12EOn reduces the average cross-sectional area of surfactant in the mixed aggregate and hence induces a unidimensional growth, the amphiphile with shorter EO chain length is capable of increasing viscosity to a greater extent. Thus, the effect on temperature on the rheology of a sample with the composition of 5 wt % PhyEO30 + C12EO3 system at X ) 0.36 is presented in Figure 6. Contrary to cationic surfactants, viscosity data of the mixed nonionic system does not follow an Arrhenius type behavior, i.e., the exponential decay of viscosity with temperature. As can be seen from Figure 6a, viscosity of the solution increases with increasing temperature to a maximum at 30 °C. With the further increase of temperature, viscosity starts decreasing continuously within the temperature range studied. The increase of viscosity with temperature in the nonionic surfactant can be understood in terms of the decrease in the interfacial curvature
of the aggregates due to progressive dehydration of the EO chain. Formation of end-caps in the cylindrical aggregates becomes unfavorable with increasing temperature because of the high-energy cost of the formation of hemispherical ends and consequently, one-dimensional growth is favored. The trend observed up to 30 °C is consistent with that view. However, above 30 °C a turning point occurs and viscosity begins to decrease as if the micellar length changes in opposite direction possibly due to an increase in the extent of micellar scission kinetics. Figure 6b shows a plot of elastic modulus, G′ and viscous modulus, G′′ as function ω for the system described in Figure 6a at different temperatures (25, 30, and 40 °C). It can be seen from Figure 6b that the G′-G′′ crossover frequency shifts to the lower value with a longer relaxation time when temperature increases from 25 to 30 °C. As temperature is increased further from 30 to 40 °C, the G′-G′′ crossover frequency moves to a higher value, which suggests the poor viscoelastic character with a shorter relaxation time. Figure 7 shows the variation of Go and τR as a function of temperature (T) for 5 wt % PhyEO30 + C12EO3 system at X ) 0.36. With increasing temperature Go increases to a certain temperature (30 °C); after that it starts decreasing. Similarly, τR displays a maximum at a temperature corresponding to the viscosity maximum, which means that the relaxation first slows as micelles grow and the number of entanglements between the wormlike micelles increases. Above 30 °C, there is a faster relaxation mechanism acting in the system and shortening of micelles seems to be a possible reason for the viscosity drop as Go decreases.17 Further validation of the results can be obtained using SAXS.
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Figure 7. Variation of Go (squares) and τR (triangles) as a function of T for 5 wt % PhyEO30 + C12EO4 system at X ) 0.36.
3.3. SAXS Measurements. In order to provide a supportive structural evidence for the rheological data, SAXS measurements were carried out on the 5 wt % PhyEO30 + C12EOn systems at different mixing fraction of C12EOn in total surfactant, X. Figure 8 shows scattering functions, I(q), and the corresponding pairdistance distribution functions (PDDF), p(r), deduced with the GIFT analysis of the SAXS data for the 5 wt % PhyEO30 + C12EO4 system at different X at 25 °C. As can be seen in Figure 8a, the scattering function reaches to zero asymptotically with a local minimum at q ∼ 0.8 nm-1 in 5 wt % PhyEO30 solution. Such behavior in the scattering function can be taken as the evidence of spherical particle with a core-shell structure.42,43 Addition of low contrast cosurfactant C12EO4 to the dilute aqueous solution of PhyEO30 worsens the overall contrast of the system, and as a result the scattering intensity in the low-q region decreases. However, with the available contrast and sensitivity of the SAXS equipment, we are able to trace out some qualitative information regarding the modulation in the structure of PhyEO30 micelles induced by C12EOn. Note that the actual length of wormlike micelles may be in the order of hundreds of nanometers and sometimes up to micrometer range. Therefore, SAXS cannot estimate the real size of the wormlike micelles. However, the general trend in the structural transition caused by composition or temperature change can be observed, if SAXS measurements and data treatment are carried out carefully. At present, our basic strategy is to investigate the composition and temperature-induced microstructural changes and its impact on the rheological behavior. With increasing C12EO4 concentration, the position of minimum in the I(q) versus q curves shifts toward the low-q region and the low-q slope increases with increasing X from X ) 0.315 to 0.507. Both these features indicate micellar growth.44,45 Minute observation of the scattering function reveals that the scattering intensity follows I(q) ∼ q-1 behavior at higher mixing fraction of X. The decay of I(q) following such behavior in the low-q region is a signature of long cylindrical type particles.46 The growth of the PhyEO30 micelles induced by C12EO4 can be clearly seen in the PDDF curves shown in Figure 8b. A symmetric bell shaped PDDF, p(r), curve with a small bump in the low-r side (r ) ∼2 nm) indicates the core-shell type of spherical particles in the absence of C12EO4. In the p(r) curves, the value of r at which p(r) becomes zero estimates the maximum dimension of the aggregates (Dmax). When C12EO4 is added to the 5 wt % PhyEO30 solution, the PDDF curves exhibit a pronounced local maximum and minimum in the low-r side, and Dmax increases monotonically with the value of X. The local maximum and minimum in the low-r region indicates a
Sharma et al. core-shell type aggregate structure. This comes from the productive contribution of the negative and positive electron densities from the hydrophobic core and hydrophilic shell, respectively, which provides the quantitative information about the internal structure of the micelles without any modeling for the micellar structure. The inflection point between the local maximum and minimum, and that after the local minimum, semiquantitatively gives the cross-sectional radius and diameter of the micellar core. The position of the first and second inflections in the p(r) functions for 5 wt % PhyEO30 + C12EO4 system are virtually unchanged despite the increasing C12EO4 content indicates nearly the same internal micellar structure, that is, core diameter. The linear decay of p(r) curve at high-r region with a continuous increases of Dmax with the value of X (see Figure 8b) gives direct evidence of the one-dimensional micellar growth. Note that the Dmax depends on the maximum resolution (qmin, the Dmax ) π/qmin) of the SAXS measurement and may not provide the actual length of the wormlike micelles. Nevertheless, trends in the variation of Dmax values as obtained from SAXS measurements can be taken as the evidence of the micellar growth. Figure 9 shows the scattering functions, I(q), and the corresponding PDDF, p(r), curves for the 5 wt % PhyEO30 + C12EO3 system at different X at 25 °C. Here also, the position of the minimum in the I(q) versus q curves shifts toward the low-q region and the low-q slope increases with increasing X indicating micellar growth. The comparison of Dmax values for the 5 wt % PhyEO30 + C12EO3 (X ) 0.363) and 5 wt % PhyEO30 + C12EO4 (X ) 0.433) systems reveals that the Dmax is higher in the former system, that is, the C12EO3 is more effective to induce micellar growth compared to the C12EO4. Next, we present the temperature induced microstructural change for the 5 wt % PhyEO30 + C12EO3 system at X ) 0.363. Figure 10 shows the scattering functions, I(q), and the corresponding pair-distance distribution functions (PDDF), p(r), for 5 wt % PhyEO30 + C12EO3 system at X ) 0.363 as a function of temperature. For better visibility the I(q) versus q curves are multiplied by factors 10 (25 °C), 100 (30 °C), 500 (35 °C), and 5000 (40 °C). Temperature generally induces micellar growth mainly in aqueous system of polyoxyethylene-based hydrocarbon or fluorocarbon nonionic surfactants.27,47 In the present system also, temperature favored micellar growth, but up to a certain temperature. Dmax increased with increasing temperature from 15 to 30 °C and after that decreases (see Figure 10b). As can be seen in Figure 10a, the position of the minimum in the I(q) versus q curves shifts toward forward direction, that is, low-q side with increasing temperature from 15 to 30 °C. Furthermore, the low-q slope increases. With further increasing temperature, the position of the minimum tends to shift toward higher-q side and slope of the scattering function in the low-q region tends to decreases. These features in the scattering functions indicate that the micelles size increases with increasing temperature from 15 to 30 °C and then decreases. As can be seen in Figure 10b, all the PDDF curves exhibit a local maximum and minimum in the low-r side indicating that the core-shell type of aggregate structure is present in the system. The inflection point in the low-r side is practically unchanged with increasing temperature. This indicates that the crosssectional diameter of the micellar core is almost constant, that is, temperature does not modulate the internal structure of the aggregates. As the temperature increases from 15 to 30 °C, the Dmax, as indicated by the downward arrows in Figure 10b, increases, and a long tailing in the higher-r side of the p(r)
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Figure 8. (a) X-ray scattered intensities of the 5 wt % PhyEO30 + C12EO4 system at different weight fraction of C12EO4 in total surfactant, X at 25 °C, obtained in absolute scale and (b) the corresponding PDDF, p(r), functions. The arrows in panel b represent the maximum dimension (Dmax) of the micelles.
Figure 9. (a) X-ray scattered intensities of 5 wt % PhyEO30 + C12EO3 system at different weight fraction of C12EO3 in total surfactant, X at 25 °C, obtained in absolute scale and (b) the corresponding PDDF, p(r), functions. The arrows in panel b represent the maximum dimension (Dmax) of the micelles.
Figure 10. (a) X-ray scattered intensities of 5 wt % PhyEO30 + C12EO3 system, X ) 0.363 at different temperatures and (b) the corresponding PDDF, p(r), functions. For better visibility the I(q) vs q curves are multiplied by factors 10 (25 °C), 100 (30 °C), 500 (35 °C), and 5000 (40 °C), and the arrows in panel b represent the maximum dimension (Dmax) of the micelles.
function appears. The long tail in the high-r side of p(r) curves indicates elongated long cylindrical type of particles. With further increasing temperature from 30 to 40 °C, the micellar size starts decreasing and the micellar disruption continues with temperature. Rheological measurements have shown that the viscosity increases with increasing temperature up to a certain point and then decreases, that is, there is a viscosity maximum in the η0 versus T plot (see Figure 6a). Thus, the present SAXS results support the rheological data. The viscosity increase from 15 to 30 °C is due to one-dimensional micellar growth. On the other hand, the viscosity decay above 30 °C is caused by the micelles shortening or breaking. Conclusions Addition of short polyoxyethylene chain nonionic surfactant (C12EOn, n ) 3 and 4) to the dilute aqueous solution of PhyEO30
dramatically increases the viscosity by several orders and forms very long wormlike micelles, which entangle in a network and show viscoelastic behavior described by the Maxwell model at low shear frequency. It is found that the replacement of C12EO4 by C12EO3 in the mixed surfactant system increases the extent of micellar growth. The micellar growth can be simply explained by reducing the effective cross-sectional area per amphiphile in the aggregate upon the addition of C12EOn. The mixing fraction of C12EOn in total surfactant for the maximum viscosity decreases on increasing the surfactant concentration. The increase in viscosity with increasing temperature for water/ PhyEO30/C12EO3 system is mainly attributed to the decrease in the spontaneous curvature of the aggregates and hence a progressive increase in the energy cost for the formation of the hemispherical end-caps of the aggregates and consequently favors one-dimensional growth. The viscosity drops with
3050 J. Phys. Chem. B, Vol. 113, No. 10, 2009 increasing temperature is associated with shortening of micelles. Consistent with the rheological measurements, SAXS provided a direct evidence of microstructural changes in micellar aggregates. We hope that the prepared viscoelastic wormlike micelles in the mixed nonionic surfactants system would be extremely useful for practical applications such as cosmetics, where increase in viscoelastic properties is often required without adding salt. Acknowledgment. SCS is thankful to the Japan Society for the Promotion of Science (JSPS) for financial support. Supporting Information Available: Figures showing form factor. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yang, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 276. (2) Kern, F.; Lemarechal, P.; Candau, S. J.; Cates, M. E. Langmuir 1992, 8, 437. (3) Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456. (4) Kim, W.-J.; Yang, S.-M.; Kim, M. J. Colloid Interface Sci. 1997, 194, 108. (5) Kim, W.-J.; Yang, S.-M. J. Colloid Interface Sci. 2000, 232, 225. (6) Imai, S.; Shikata, T. J. Colloid Interface Sci. 2001, 244, 399. (7) Vethamuthu, M. S.; Almgren, M.; Brown, W.; Mukhtar, E. J. Colloid Interface Sci. 1995, 174, 461. (8) Hartmann, V.; Cressely, R. Colloids Surf., A 1997, 121, 151. (9) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (10) Montalvo, G.; Rodenas, E.; Valiente, M. J. Colloid Interface Sci. 2000, 227, 171. (11) Ponton, A.; Schott, C.; Quemada, D. Colloids Surf., A 1998, 145, 37. (12) Schubert, B. A.; Kaler, E. W.; Wagner, N. J. Langmuir 2003, 19, 4079. (13) Kunieda, H.; Rodrı´guez, C.; Tanaka, Y.; Kabir, M. H.; Ishitobi, M. Colloids Surf., B 2004, 38, 127. (14) Rodrı´guez, C.; Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Langmuir 2003, 19, 8692. (15) Sharma, S. C.; Shrestha, R. G; Varade, D.; Aramaki, K. Colloids Surf., A 2007, 305, 83. (16) Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Langmuir 2003, 19, 9173. (17) Sharma, S. C.; Shrestha, R. G; Shrestha, L. K; Aramaki, K. Colloid Polym. Sci. 2008, 286, 1613. (18) Hoffmann, H.; Rauscher, A.; Gradzielski, M.; Schulz, S. F. Langmuir 1992, 8, 2140.
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