Viscoelastic Micellar Solutions in a Mixed Nonionic Fluorinated

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Langmuir 2007, 23, 5324-5330

Viscoelastic Micellar Solutions in a Mixed Nonionic Fluorinated Surfactants System and the Effect of Oils Suraj Chandra Sharma,† Durga P. Acharya,†,‡ and Kenji Aramaki*,† Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan, and Food Science Australia, 671 Sneydes Road, Werribee, VIC 3030, Australia ReceiVed January 8, 2007. In Final Form: February 27, 2007 Formation and rheological behavior of viscoelastic wormlike micelles in aqueous solution of a mixed system of nonionic fluorinated surfactants, perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)nH (abbreviated as C8F17EOn) was studied. In the water-surfactant binary system C8F17EO20 forms an isotropic micellar solution over wide concentration range (>85 wt %) at 25 °C. With successive addition of C8F17EO1 to the aqueous C8F17EO20 solution, viscosity of the solution increases swiftly, and a viscoelastic solution is formed. The oscillatory rheological behavior of the viscoelastic solution can be described by Maxwell model at low-frequency region, which is typical of wormlike micelles. With further addition of C8F17EO1, the viscosity decreases after a maximum and phase separation occurs. Addition of a small amount of fluorinated oils to the wormlike micellar solution disrupts the network structure and decreases the viscosity sharply. It is found that polymeric oil, PFP (F-(C3F6O)nCF2CF2COOH), decreases the viscosity more effectively than the perfluorodecalin (PFD). The difference in the effect of oil on rheological properties is explained in terms of the solubilization site of the oils in the hydrophobic interior of the cylindrical aggregates, and their ability to induce rod-sphere transition.

Introduction Perfluorosurfactants, like their hydrocarbon analogues, form micelles, threadlike micelles, vesicles, lamellar aggregates,and other various liquid crystalline structures in solution, and the aggregate structures in fluorinated surfactants can be explained, like hydrocarbon surfactants, in terms of the value of critical packing parameter,1 CPP ) V/asl, where V is the volume of the hydrophobic part, l its length, and as the average area of head group at the interface. Similar phase sequence is observed in fluorinated surfactant and hydrocarbon surfactant systems when surfactant concentration is changed. In some respects, however, perfluorosurfactants are different from hydrocarbon surfactants. They are more hydrophobic, show unusually low surface tension in aqueous solution and exhibit much lower critical micelle concentration than the hydrocarbon chain surfactants of same length do.2,3 Perfluorosurfactants have stiff hydrophobic chains, with their skeleton covered by a dense electron-rich environment. It is easier to pack fluorocarbon chain closely because chains are in all-trans state and less entropy is lost. Moreover, the fluorocarbon chains are bulkier than the hydrocarbon chain, with the volume of -CF2 and terminal -CF3 being 22.9 and 55.4 cm3/mol, respectively, compared with 16.5 and 33.0 cm3/mol, respectively, for hydrocarbon analogues.4,5 To form a spherical aggregate (CPPe1/3), a fluorinated surfactant having a very large head group is needed to balance the effect of bulky fluorocarbon chain.6 Therefore, cylindrical micelles are observed in fluorinated surfactant systems at solution conditions where spherical micelles are expected in hydrocarbon surfactant * Corresponding author. E-mail: [email protected]. Phone and fax: +81-45-339 4300. † Yokohama National University. ‡ Food Science Australia. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2, 1976, 72, 1525. (2) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909. (3) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 2468. (4) Riess, J. G. Tetrahedron 2002, 58, 4113-4131. (5) Aramaki, K.; Kunieda, H. Colloid Polym. Sci. 1999, 277, 34.

systems. These cylindrical micelles often undergo enormous one-dimensional growth and form flexible threadlike aggregates called “wormlike micelles”. When the packing properties of amphiphile, or in other words, when the spontaneous curvature of the micelle favors the formation of cylindrical aggregates, the molecules packed at the hemispherical ends have excess free energy in comparison to the molecules at the cylindrical part. This excess free energy, called “end cap energy”, is the thermodynamical driving force for the linear growth of cylindrical micelles. The system can reduce the free energy by reducing the number of free ends, that is, by end-to-end fusion of several short cylindrical aggregates to form a long aggregate. When the number density of the wormlike aggregates exceeds a certain threshold value, they entangle with each other to form a transient network and exhibit viscoelastic properties. Formation and properties of viscoelastic wormlike micelles have been studied extensively for hydrocarbon surfactants, mostly in long hydrophobic chain (C16 or longer) cationic surfactant in presence of excess counterions,7-13 and in some cases, even in absence of excess counterions if counterions are strongly bound at hydrophilic-hydrophobic interface.14-17 Viscoelastic solutions (6) El Moujahid, C.; Ravey, J. C.; Schmitt, V.; Ste´be´, M. J. Colloid Surf. A.1998, 136, 289. (7) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (8) Kern, F.; Lemarechal, P.; Candau, S. J.; Cates, M. E. Langmuir 1992, 8, 437. (9) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (10) Khatory, A.; Kern, F.; Lequeux, F.; Appell, J.; Porte, G.; Morie, N.; Ott, A.; Urbach, W. Langmuir 1993, 9, 933. (11) Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456. (12) Berret, J.-F.; Appell, J.; Porte, G. Langmuir 1993, 9, 2851. (13) Lin, Z.; Cai, J. J.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1994, 98, 5984. (14) Soltero, J. F. A.; Puig, J. E.; Manero, O.; Schulz, P. C. Langmuir 1995, 11, 3337. (15) Soltero, J. F. A.; Puig, J. E.; Manero, O. Langmuir 1996, 12, 2654. (16) Narayanan, J.; Manohar, C.; Kern, F.; Lequeux, F.; Candau, S. J. Langmuir 1997, 13, 5235.

10.1021/la0700523 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/17/2007

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of wormlike micelles have also been reported in cationic or anionic fluorinated surfactant aqueous systems.18-20 The effect of the concentrations of counterions and surfactant on the rheological behavior and micellar growth is more or less similar in both types of surfactants.20 There are reports of theromoresponsive viscoelasticity in some hybrid anionic surfactants containing both fluorocarbon and hydrocarbon chain in their molecules.21-24 Another way to induce one-dimensional micellar growth is to add lipophilic nonionic surfactants such as polyoxyethylene alkylether (CmEOn) or long-chain monoglyceride to the aqueous solution of hydrophilic surfactants. We found that this approach works not only for ionic surfactants but also for nonionic surfactants and highly viscoelastic wormlike micellar solution have been reported recently in ionic-nonionic surfactant systems25-29 as well as mixed nonionic surfactant systems.30-33 Knowledge about formation and rheological properties of nonoionic systems is important for the better understanding of the underlying basic principle of the phenomenon. However, not much is known about the formation and rheological behavior of the wormlike micelles in nonionic systems. Evolution of viscoelastic properties of wormlike micelles as a function of surfactant and cosurfactant concentration in mixed surfactant systems has been studied.29-32 It is known that addition of solubilizates, such as oils, affect the self-aggregation behavior of surfactants. There are some reports on the effect of oil on the rheological behavior of hydrocarbon surfactant systems and it was found that the effect depends on the nature of the oil.34-36 Little is known about the effect of oil in the wormlike micellar solution in fluorinated surfactant system.37 Recently, it was found that an aqueous solution of perfluoroalkyl sulfonamide ethoxylate type of surfactant (structure shown in Scheme 1) having a medium chain length (C8) fluoroalkyl chain and relatively large headgroup (EO10) formed highly (17) Hassan, P. A.; Candau, S. J.; Kern, F.; Manohar, C. Langmuir 1998, 14, 6025. (18) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237. (19) Knoblich, A.; Matsumoto, M.; Murata, K.; Fujiyoshi, Y. Langmuir 1995, 11, 2361. (20) Hoffmann, H.; Wu¨rtz, J. J. Mol. Liq. 1997, 72, 191. (21) Abe, M.; Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Kasahara, Y.; Matsuzawa, H.; Iwahashi, M.; Momozawa, N.; Nishiyama, K. Langmuir 1997, 13, 2932. (22) Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Iwahashi, M.; Momozawa, N.; Abe, M. Langmuir 1997, 13, 5054. (23) Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Kamogawa, K.; Momozawa, N.; Abe, M. Langmuir 1998, 14, 4753. (24) Danino, D.; Weihs, D.; Zana, R.; Ora¨dd, G.; Lindblom, G.; Abe, M.; Talmon, Y. J. Colloid Interface Sci. 2003, 259, 382. (25) Herb, C. A.; Chen, L. B.; Sun, W. M. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; ACS Symposium Series 578; American Chemical Society: Washington, DC, 1994; p 153. (26) Rodriguez, C.; Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Langmuir 2003, 19, 8692. (27) Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Langmuir 2003, 19, 9173. (28) Acharya, D. P.; Sato, T.; Singh, Y.; Kunieda, H. J. Phys. Chem. B 2006, 110, 754. (29) Acharya, D. P.; Shiba, Y.; Aratani, K.-i. Kunieda, H. J. Phys. Chem. B 2004, 108, 1790. (30) Acharya, D. P.; Kunieda, H. J. Phys. Chem. B. 2003, 107, 10168. (31) Acharya, D. P.; Hossain, Md. K.; Jin-Feng, Sakai, T.; Kunieda, H. Phys. Chem. Chem. Phys. 2004, 6, 1627. (32) Naito, N.; Acharya, D. P.; Tanimura, K.; Kunieda, H. J. Oleo Sci. 2004, 53, 599. (33) Maestro, A.; Acharya, D. P.; Furukawa, H.; Gutie´rrez, J. M.; Lo´pezQuintela, M. A.; Ishitobi, M.; Kunieda, H. J. Phys. Chem. B 2004, 108, 14009. (34) Sato, T.; Acharya, D. P.; Kaneko, M.; Aramaki, K.; Singh, Y.; Ishitobi, M.; Kunieda, H. J. Dispersion Sci. Technol. 2006, 27, 611. (35) Rodriguez-Aberu, C.; Aramaki, K.; Tanaka, Y.; Lopez-Quintela, M. A.; Ishitobi, M.; Kunieda, H. J. Colloid Interface Sci. 2005, 291, 560. (36) Pokhriyal, N. K.; Joshi, J. V.; Goyal, P. S. Colloids Surf. A 2003, 218, 201. (37) Sharma, S. C.; Kunieda, H.; Esquena, J.; Rodriguez-Abreu, C. J. Colloid Interface Sci. 2006, 299, 297.

Langmuir, Vol. 23, No. 10, 2007 5325 Scheme 1: Molecular Structure of C8F17EOn (Amphiphiles with n ) 20 and 1 Were Used in This Study)

viscoelastic solution of wormlike micelles at low temperatures.38 Although nonionic surfactants like C12EO5 are known to form wormlike in water-surfactant binary system39,40 they do not form viscoelastic solution. Formation of cylindrical aggregates and highly favored linear growth on the aggregates in the perfluorosurfactant system in spite of the large headgroup and short fluroalkyl chain of amphiphile reflects the effect of stiff and voluminous fluoroalkyl chain on the packing constraint in the hydrophobic core. These surfactants are promising candidates to be used as a model system to understand the rheological behavior of viscoelastic solution in purely nonionic surfactant system. By careful tuning of the interfacial curvature, the extent of micellar growth and, consequently, the flow properties can be controlled, which is important in industrial applications. In this context, we studied the formation and rheological behavior of viscoleastic solutions in mixed perfluoroalkyl sulfonamide ethoxylates having similar tail group but differing in the size of polyoxyethylene head group. We also studied the effect of two different kinds of fluorinated oils on the rheological properties of the solutions. In this paper, at first, a partial phase diagram and the rheological properties of the solution as a function of composition at fixed temperature is presented. Then the effect of oils on the rheological properties is presented and the microstructural changes in the aggregate as indicated by the rheological study is discussed. Experimental Materials. Perfluorosurfactants, N-polyoxyethylene-N-propyl perfluorooctane sulfonamide, designated as C8F17EO20, and N-propylN-(2-hydroxyethyl) perfluorooctane sulfonamide, designated as C8F17EO1, were obtained from Mitsubishi Materials, Japan. Schematic molecular structure of the surfactant is shown in Scheme 1. Perfluoropolyether oil having structure F-(CF2CF2CF2O)n-CF2CF2COOH with n ≈ 21 (average molecular weight 3600, polydispersity 1.14), abbreviated as PFP was kindly provided by Daikin Industries Ltd., Japan. Octadecafluorodecahydonaphthalene (perfluorodecalin, abbreviated as PFD) of purity 95% (mixture of cis and trans) was the product of Aldrich. The chemicals were used as received. Millipore water was used. Phase Diagram. For the study of phase behavior, sealed ampoules containing required amount of reagents were homogenized and kept in a water bath at 25 °C for equilibration. Phases were identified by visual observation (through crossed polarizers). Rheological Measurements. Samples for rheological measurements were homogenized and kept in water bath at specified temperature for at least 24 h to ensure equilibration before performing measurements. The rheological measurements were performed in a stress controlled rheometer, AR-G2 (TA instrument) using coneplates geometries (diameters 60 mm for low-viscosity sample and 40 mm for high-viscosity sample, each with a cone angle of 1°) with the plate temperature controlled by peltier unit. 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, as determined previously by dynamic strain sweep measurements. (38) Acharya, D. P.; Sharma, S. C.; Rodriguez-Abreu, C.; Aramaki, K. J. Phys. Chem. B 2006, 110, 20224. (39) Kato, T.; Nozu, D. J. Mol. Liq. 2001, 90, 167. (40) Bernheim, A.; Wachtel, E.; Talmon, Y. Langmuir 2000, 16, 4131.

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Figure 1. Partial phase diagram of water/C8F17EO20/C8F17EO1 ternary system. Wm stands for the isotropic micellar solution and LR is the lamellar liquid crystalline phase.

Results and Discussion Phase Behavior. Partial phase diagram of water/C8F17EO20/ C8F17EO1 ternary system at 25 °C is shown in Figure 1. In the water/ C8F17EO20 binary system, isotropic solution is formed over wide range of surfactant concentration at given temperature. At high surfactant concentrations (>87 wt %), solid surfactant is observed. A large headgroup of the surfactant molecule might have favored the small and discrete aggregates of large interfacial curvature and prevented the formation of liquid crystalline structures of relatively low interfacial curvature, such as hexagonal (H1) and lamellar (LR) phases. It is interesting to note that even the isotropic discontinuous micellar cubic (I1) phase is not formed. It is possible that the aggregates are not spherical but somewhat elongated, at least at higher concentrations where I1 phase is expected, due to packing constrains caused by bulky hydrophobic part of fluorinated chain, and therefore, packing of the aggregates in a cubic structure is hindered. The packing constraints of the perfluoroalkyl chain is considered to be reason for the formation of cylindrical aggregates in aqueous C8F17EO10 system at a very low surfactant concentration (1 wt %), where spherical aggregates are normally expected for a surfactant of the given head and tail group. Detail study on the structure of the aggregate and the phase behavior as a function of temperature was not carried out as it is beyond the scope of this study and requires a separate treatment. The micellar solution of C8F17EO20 can solubilize significant amount of C8F17EO1, which is evident from the height of the Wm domain in the ternary phase diagram. Due to bulky and stiff hydrophobic tail and small hydrophilic group, C8F17EO1 itself cannot form discrete aggregates in water. Due to the same reason, incorporation of C8F17EO1 in the aggregates of C8F17EO20 reduces average headgroup area at the interface or, in other words, reduces the interfacial curvature, and beyond the solubilization limit of the Wm phase, the LR phase separates out from the isotropic solution. Upon successive addition of C8F17EO1 to the micellar solution of C8F17EO20, no significant change in viscosity occurred in the dilute solution of the hydrophilic surfactant, but at higher concentration (above 20 wt % of C8F17EO20), viscosity increased gradually at first, then swiftly and a viscous solution was observed. These solutions are isotropic at rest but are birefringent when applied a shear, such as sudden jerk. With further addition of C8F17EO1, viscosity decreased and ultimately a phase separation occurred. The tentative region of viscous solution inside the Wm domain is shown in the phase diagram. The viscous region extends toward higher surfactant concentration also, but its boundary was not determined at compositions above 70 wt % of C8F17EO20. Note that the range of surfactant concentrations in which viscoelastic micellar solutions are formed is noticeably higher than the range reported in the literature. Rheological Behavior. Effect of Surfactant Concentrations. Figure 2 shows the steady shear-rate (γ˘ )-viscosity (η) curves for 35% C8F17EO20 + C8F17EO1 system at different mixing fraction

Figure 2. Steady shear-rate (γ˘ )-viscosity (h) curves for 35% C8 + C8F17EO1 systems at various weight fraction of C8F17EO1 in total surfactant, W: (a) 0, (b) 0.016, (c) 0.029, (d) 0.042, (e) 0.055, (f) 0.087, (g) 0.107, and (h) 0.118.

of C8F17EO1, expressed in weight fraction of C8F17EO1 in total surfactant (W) at 25 °C. At lower value of W ≈ 0.016, η is independent of γ˘ , i.e., Newtonian flow behavior is observed up to γ˘ ≈ 100 s-1 and shear thinning effect is outside the measurement regime (γ˘ > 1000 s-1). At W ≈ 0.029, behavior is still Newtonian over wide range of shear rate, but shear thinning occurs at large deformation (γ˘ g 100 s-1). With increasing W up to W ≈ 0.087, the critical γ˘ for shear thinning shifts gradually to lower value and also the viscosity in the plateau region (low γ˘ region) increases which shows that the system is getting more structured. This rheological behavior is typical of systems consisting of network structure formed by wormlike micelles. When network structure is deformed by applying a shear, shear thinning occurs due to alignment of aggregates under flow if the deformation is faster than the time required to regain equilibrium network structure, and with increasing network density the relaxation becomes slower, i.e., shear thinning begins at lower shear rate. However, with further increase in C8F17EO1 concentration (at W ) 0.107 and 0.118) the viscosity decreases and higher deformation rate is required to induce shear thinning. This indicates that some structural transformation occurs at W > 0.087. One of the possibilities is that the system becomes less structured, that is, micellar length decreases, and gradually network structure is lost. However, such a reversal in structural changes do not seem to be convincing because the interfacial curvature should be continuously decreasing with increasing W and aggregates with flat bilayer are formed ultimately at phase separation. A more convincing explanation for the change in the rheological behavior is that with increasing W, spontaneous interfacial curvature of aggregate gradually decreases and, with this, energy cost for the formation of hemispherical end caps of the cylindrical aggregates becomes higher. The endcap energy is minimized if the free ends fuse with cylindrical part of its own or another micelles, thus forming micellar joints, or branching.10,41 Since the surfactant molecules in the aggregate are not chemically connected, surfactants are expected to form this type of saddleshaped interconnection readily if they do not strongly resist a negative Gaussian curvature.42 Such joints reduce the viscosity because when a stress is applied micellar joint can slide along the cylindrical body (contour) thereby allowing a fast stress relaxation process. In some surfactant systems, micellar con(41) Candau, S. J.; Oda, R. Colloid Surf. A 2001, 183-185, 5. (42) Tlusty, T.; Safran, S. A. J. Phys.: Condens. Matter 2000, 12, A253.

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Figure 3. Variation of zero shear viscosity (ηo) as a function of the weight fraction of C8F17EO1 in total surfactant, W, for 25% C8F17EO20 + C8F17EO1 (a) and 35% C8F17EO20 + C8F17EO1 (b) systems at 25 °C.

nections or branching points have been detected by cryogenic transmission electron microscopy (cryo-TEM), especially in the region where the viscosity decreases after the maximum.43-46 Figure 3 shows the plot of zero-shear viscosity (ηo), obtained from γ˘ -η data such as shown in Figure 2 by extrapolating the viscosity to zero shear-rate, as a function of W at two different initial concentrations of C8F17EO20. When C8F17EO20 concentration is decreased from 35% to 25%, the general trend of the viscosity growth is essentially similar but the ηo-W curve shifts toward higher W values, or in other words, higher concentration of C8F17EO1 is required to attain comparable viscosity, which can be attributed to the decrease in packing constraints in the hydrophobic core with decreasing water content. The trend is essentially similar to that observed in the wormlike micellar solutions formed in mixed systems of hydrocarbon surfactants. Note that upon decreasing the concentration of C8F17EO20 the maximum viscosity has also decreased. At the compositions around the viscosity maxima, the systems exhibit viscoelasticity. Oscillatory-shear (frequency sweep) measurements were performed on the viscous samples. Figure 4 shows plots of elastic modulus (G′) and loss modulus (G′′) as a function of oscillatory shear frequency (ω) for a sample of 35% C8F17EO20 + C8F17EO1 system at W ) 0.069, 0.087 and 0.113, where W ) 0.087 is the composition corresponding to the viscosity maximum (see Figure 3). The result shows that these samples are viscoelastic in the time scale of measurement, with G′ < G′′ in low-ω region and G′ > G′′ in high-ω region. In the low-ω region the data points of G′ and G′′ fits well to the following equations of Maxwell’s mechanical model for a viscoelastic material of shear modulus Go and single stress relaxation time τR:

G′(ω) ) G′′ (ω) )

ω2τR2

Go

(1)

Go 1 + ω2τR2

(2)

1 + ω2τR2 ωτR

The equations indicate that at low-frequencies G′ ∼ ω2 and G′′ ∼ ω, a crossover occurs at a frequency ωc ) 1/τR, and in the high-w region (ω . ωc) G′ attains a plateau value, Go, whereas (43) Lin, Z. Langmuir 1996, 12, 1729. (44) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (45) In, M.; Aguerre-Chariol, O.; Zana, R. J. Phys. Chem. B 1999, 103, 7747. (46) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205.

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) 35%C8F17EO20 + C8F17EO1 surfactant systems at different mixing fraction of C8F17EO1 in total surfactant, W: 0.069 (rectangles), 0.087 (triangles), 0.113 (circles) and (b) in 35%C8F17EO20 + C8F17EO1, W ) 0.087 (triangles) and 25%C8F17EO20 + C8F17EO1, W ) 0.123 (rectangles), which correspond to the compositions at the viscosity-maxima for each system. The Maxwellian fittings to the experimental data are shown by solid lines.

G′′ shows a monotonic decrease. Considering reptation or diffusion of wormlike micelles along its own contour as the mechanism of stress relaxation in the entangled network, as proposed by Cates et al., the magnitude of τR is related to the average length of the wormlike micelles whereas Go is related to the number density of entanglement in the transient network.47-49 The parameters Go and τR are related to ηo by following relation:

ηo ) GoτR

(3)

In Figure 4 Maxwellian fit to the data points are shown by solid lines. The rheological behavior in low-ω region (below ωc) can be described by the mechanical model but in high-ω region, experimental data show significant deviation, which is considered to be related to other faster relaxation processes such as Rouse modes. As it can be seen from Figure 4(a), with increasing W, Go increases monotonically whereas ωc shifts to the lower value and attains the lowest value at a composition corresponding to viscosity maximum (W ) 0.087) but shifts to higher value again. This change in ωc, which corresponds to a faster relaxation process, may be due to the formation of micellar joints. Similar changes in rheological behavior with increasing W are observed in 25% C8F17EO20 + C8F17EO1 system also (data not shown). Figure 4(b) allows one to compare the dynamic rheological behavior of 25%C8F17EO20 + C8F17EO1 and 35% C8F17EO20 + C8F17EO1 systems at compositions corresponding to viscosity maxima. Upon decreasing the C8F17EO20 concentration, τR shows (47) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869. (48) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758. (49) Cates, M. E.; Candau, S. J. J. Phys. (Paris) 1988, 49, 1593.

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Figure 5. Variation of shear modulus (Go) and relaxation time (τR) as a function of the mixing fraction of C8F17EO1 in total surfactant (W) in 25%C8F17EO20 + C8F17EO1 systems (a) and 35%C8F17EO20 + C8F17EO1 systems (b) at 25 °C. The solid lines are given for visual guide only.

a small increase but Go decreases, which corresponds to an increase in the average micellar length but decrease in network density. Figure 5 shows the variation of Go and τR with W in 35%C8F17EO20 + C8F17EO1 and 25%C8F17EO20 + C8F17EO1. These parameters were obtained by fitting of the experimental data from frequency sweep measurements, especially the data in lowfrequency region, to the Maxwell equations. As in the case of the systems described in Figure 4, G′ at the high-ω region (say G′∞) is often higher than perfect plateau value (Go) as predicted by Maxwell equations. Therefore, the values of Go estimated from Maxwell equations should be considered as the lower limit for the shear modulus. The shift of the ηo and τR curves toward the lower W values in the ηo-W (Figure 3) and τR-W (Figure 5) plots upon increasing the C8F17EO20 concentration in the mixed system corresponds to the higher extent of linear micellar growth. This is also evident from the increase in Go or network density up on increasing surfactant concentration (Figure 5). The lower value of τR at the maximum in 35%C8F17EO20 + C8F17EO1 system in comparison to that in 25%C8F17EO20 + C8F17EO1 system should not be considered as a lower extent of micellar growth in the former system. Instead, it might have arisen from the fact that with increasing surfactant concentration the spontaneous curvature decreases and the micellar branching is favored even at lower value of W so as to minimize energy cost of the formation of end caps. Continuous growth of Go in the given composition range where ηo and τR decrease shows that after branching the network density grows until the phase separation occurs. There are indications that the local structure at branching points evolve toward bilayer structure38 and ultimately separates out. Effect of Added Oil. Effect of adding two different types of oils, namely PFP and PFD on the rheological behavior of the wormlike micelles was studied by adding oils to 35%C8F17EO20 + C8F17EO1 system at W ) 0.087 which corresponds to the composition with maximum viscosity. Figure 6 shows the changes in oscillatory-shear rheological behavior brought about by PFP and PFD at similar compositions. Both oils decrease the magnitude of Go and shift ωc to higher values or reduce τR, but the effect of polymeric oil PFP on the rheological behavior is more than that of PFD. Upon addition of a small concentration of PFP (Wo ) 0.0075, where Wo is the weight fraction of oil in total solution), Go and τR decrease from 177 to 94 Pa and from 1.28 to 0.20 s, respectively. At similar concentration of PFD, Go and τR values are 113 Pa and 0.75 s,

Figure 6. Variation of G′ (filled symbols) and G′′ (open symbols) as a function of ω for a 35%C8F17EO20 + C8F17EO1 system, W ) 0.087 in absence of oil (a) and in presence of oil (PFD in plot A and PFP in plot B) at a weight fraction of oil in the total system, Wo ) 0.0075 (b). The solid lines are the Maxwellian fits.

respectively. Note that addition of C8F17EO1 to the system at the viscosity maximum also decreases τR, but Go shows a steady increase. Hence the C8F17EO1 and oils affect the network structure in different way. Figure 7 shows how the rheological parameters, namely, ηo, Go, and τR decrease as a function of Wo for both oils when each of these oils are added to 35%C8F17EO20 + C8F17EO1 system at the viscosity-maximum (W ) 0.087). The plots show that the viscosity of the solution decreases sharply with increasing concentration of PFP up to Wo ∼ 0.036. With successive addition of the oil up to Wo ≈ 0.086, the viscosity remains nearly constant (increases slightly) over large interval of Wo, and ultimately an excess oil phase separates out when oil is added beyond the solubilization limit. The behavior of PFD is essentially similar to the behavior of PFP in the sense that it also reduces the viscosity sharply, but the solubilization limit is nearly half of PFP and the ability of reducing of the viscosity, especially at lower oil concentrations, is less than with PFP, which is evident from the ηo-Wo plot (Figure 7A). The difference in the behavior of these two oils is also observed in the τR- and Go-Wo plots (Figure 7B). It can be seen that addition of PFP decreases τR monotonically, whereas with PFD there is almost no significant decrease in τR at a low oil concentration (Wo < 0.005), but at higher concentrations of oil (Wo > 0.005), the decay of τR is comparable to that observed with PFP. Comparison of the Go-Wo curves for these two oils also show that PFP has a greater ability of reducing Go at a lower values of Wo, but at higher values of Wo, the decrease of Go is effectively same. These results show that both oils disrupt the network structure and reduce the average micellar length, which is evident from the decrease in the values of Go and τR with increasing Wo, but the impact of polymeric oil PFP on the rheological behavior is more than that of PFD, especially at low oil concentration. Disruption of micellar network and decrease in the viscosity

Viscoelastic Micellar Solutions

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upon solubilization of oil arises from the effect of solubilizates on the packing properties of the amphiphile in aggregates and its interfacial curvature. Oils which are solubilized in the hydrophobic core of the micellar aggregates increase the interfacial curvature and stabilize the endcaps and, hence, decrease the length of rod micelles or induce a rod-sphere transition. This effect is opposite to that caused by cosurfactant including alcohols which are incorporated in palisade layer. The ability of oils to perturb the interfacial curvature depends on the location of the oil molecules in the hydrophobic interior of the aggregate. It has been shown flat or elongated molecules with rigid skeleton, such as cyclohexane, xylene and decalene, are preferentially solubilized in the region between the interface and the core whereas molecules with high molecular volume and flexibility such as triglycerides and long chain n-alkanes are solubilized in the core.50-52 In the former type of solubilzation, which is often known as “penetration”, the area of amphiphilic molecule at the interface increases but the diameter of the hydrophobic core does not increase and does not exceed the extended chain length of the surfactant whereas in the later case called “swelling” the area of surfactant at the interface remains unchanged but the diameter of the hydrophobic core increases due to formation of oil pool in the core. Many oils, however, do not exhibit perfect penetration or swelling but a behavior in between these two extremes.50,51 The difference in ability of PFP and PFD to reduce the viscosity and disrupt the network structure of the wormlike micelles might

have arisen from the solubilization sites of the oils and the effect of the solubilization on the interfacial curvature. Solubilization behavior of PFP in the aggregates of fluorinated surfactant can be understood from the phase behavior of C8F17EO10/PFP/water ternary system reported recently by one of us.37 When PFP is solubilized in a normal cylindrical aggregate of hexagonal phase in the C8F17EO10 system, the amphiphilic oil molecule penetrates the palisade layer but a large section of the hydrophobic chain of the long polymeric oil molecules forms a pool in the core of the cylinder, which is reflected in an increase in the head group area at the interface (as) and the radius of the hydrophobic cross section of the cylinders (rH) with increasing concentration of the solubilizate. PFD molecules are also known to form an oil pool in the hydrophobic core of the aggregates of fluorinated surfactant, as indicated by the transformation of biphasic Wm + H1 systems in water-fluorinate surfactant C8F17(CH2)2(OC2H4)9OH binary system to single cubic phase consisting of discrete globular micelles upon successive addition of PFD.53 To¨rnblom et al. studied the effect of different aliphatic hydrocarbons on the length of rodlike micelles in cationic surfactant system and found that the hydrocarbons with cyclic structure such as decalin and cyclohexane or short-chain alkane (n-hexane), when solubilized at low concentrations, increase the axial length at constant crosssectional radius of the rodlike aggregates. But with successive addition of the oil, the shortening of the axial length takes place accompanied by an increase in the radius and ultimately a rodsphere transition takes place.52 On the other hand, solubilization of long-chain hydrocarbons such as n-octane or higher alkanes, decrease the micellar length even in small amount. Hoffmann et al. obtained similar results in their studies on the effect of different oils, namely cyclohexane, toluene and decane on the growth of length of rodlike micelles in the systems of cationic surfactants in presence of salt.54,55 The results suggest that the oil molecules with flat and rigid structure or low molecular volume are solubilized at first in the region between the core and the interface up to an extent depending on the nature of oil and with successive addition, the solubilizates form a pool in the micellar core. The difference between the behavior of PFD and PFP observed in the present study may be understood in terms of their solubilization behavior in the fluorinated interior of the wormlike micelles. Nearly constant or slightly decreasing rheological parameters at a low concentration of PFD (Wo< 0.005) may be related to the solubilization of the oil in the vicinity of the micellar core. However, there is no indication of the PFD-induced micellar growth in the present system as it was observed in the hydrocarbon surfactant systems, probably due to a shift in the solubilization site closer to the core in the system of perfluorosurfactants having stiff hydrophobic chain, and with this the solubizations becomes less “penetrating” and more “swelling”. In fact, the solubilization site depends on various factors such as shape and size of the solubilizate molecule, geometrical constrains of packing, the free energy change associated with the change in interfacial area, entropy of mixing of solubilizate and hydrophobic chain of the surfactant, entropy change associated with the formation demixed region of oil pool in the micellar core,56,57 and estimation of the contribution of these factors in the present system is beyond the scope of the present work. In case of the solubilization of PFP, prediction of the oil solubilization on the interfacial curvature is more complicated,

(50) Kunieda, H.; Ozawa, K.; Huang. K. J. Phys. Chem. B 1998, 102, 831. (51) Kunieda, H.; Horii, M.; Koyama, M.; Sakamoto, K. J. Colloid Interface Sci. 2001, 236, 78. (52) Tornblom, M.; Henriksson, U. J. Phys. Chem. B 1997, 101, 6028. (53) Blin, J. L.; Ste´be, M. J. J. Phys. Chem. B 2004, 108, 11399.

(54) Bayer, O.; Hoffmann, H.; Ulbricht, W.; Thurn, H. AdV. Colloid Interface Sci. 1986, 26, 177. (55) Hoffmann, H.; Ulbricht, J. Colloid Interface Sci. 1989, 129, 388. (56) Aamodt, M.; Landgren, M.; Jo¨nsson, B. J. Phys. Chem. 1992, 96, 945. (57) Landgren, M.; Aamodt, M.; Jo¨nsson, B. J. Phys. Chem. 1992, 96, 950.

Figure 7. Variation of ηo (a) and Go, τR (b) as a function of the weight fraction of PFP (circles) or PFD (triangles) in total system, Wo, in 35%C8F17EO20 + C8F17EO1 system at W ) 0.087. The arrows in panel (a) show the values of Wo at which phase separation occur. The dynamic measurements are not shown for the same range of Wo as the viscosity measurements since crossover occurs outside the measurement range of the rheometer. Solid lines are for visual guide only.

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but the effect of the solubilization of a section of the oil molecule in the palisade layer outbalances the opposite effect of the formation of oil pool by the remaining hydrophobic section in the micellar core. Hence, the length of the wormlike micelles decrease with increasing concentration and the viscosity decreases monotonically. At higher concentration of PFD (up to Wo ≈ 0.025) the solubilization behavior is similar to that of PFP, and therefore, both oils show similar trend in the decay of rheological parameters. With successive addition of PFP, the viscosity decreases until it attains a lowest value, and judging from the viscosity value and concentration of surfactant in total system, this corresponds to the complete rod-sphere transformation. With further increase in oil up to phase separation, no structural change takes place except swelling of the spherical micelles. In case of PFD, however, the solubilization of the oil takes place as long as there are cylindrical micelles.

Summary Viscoelastic solution of wormlike micelle is formed in an aqueous solutions of highly hydrophilic nonionic fluorinated surfactant, perfluoroalkyl sulfonamide ethoxylate (C8F17EO20) at relatively high surfactant concentration when a hydrophobic amphiphile (cosurfactant) C8F17EO1 is added. Addition of C8F17EO1 reduces the interfacial curvature of the aggregates and induce one-dimensional micellar growth. With successive addition of C8F17EO1, the viscosity increases rapidly to form viscoelastic solutions, then decreases after the maximum, and ultimately, a phase separation occurs. The dynamic rheological behavior of

Sharma et al.

the viscoelastic solutions can be described by Maxwell model at low-shear frequency, which is typical behavior of wormlike micellar solution. The turning point in the viscosity curve is associated with the formation of micellar joints. Increasing surfactant or cosurfactant concentration in the mixed nonionic system increases the extent of one-dimensional micellar growth which is mainly attributed to the decrease in the spontaneous curvature of the aggregates and consequently a progressive increase in the energy cost for the formation of the hemispherical end caps of the aggregates. Addition of a small amount of fluorinated oils to the wormlike micellar solution disrupts the network structure and decreases the viscosity sharply. It is found that polymeric oil, PFP, decreases the viscosity more effectively than the perfluorodecalin, PFD. PFP which is solubilized in the core of the hydrophobic interior of the cylindrical aggregate increases the spontaneous interfacial curvature more effectively and stabilize the end caps. PFD, which is preferentially solubilized in the palisade layer shows a lower extent of viscosity drop. Acknowledgment. This work was supported by CREST of Japan Science and Technology Corporation. S. C. Sharma is thankful to the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan for the Monbukagakusho Scholarship. DPA thanks Japan Society for the Promotion of Science (JSPS) for financial support during the period of presented work. LA0700523