pubs.acs.org/Langmuir © 2010 American Chemical Society
Wormlike Micelles and Solution Properties of a C22-Tailed Amidosulfobetaine Surfactant Zonglin Chu,†,‡ Yujun Feng,*,† Xin Su,†,‡ and Yixiu Han†,‡ †
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China, and ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China Received December 4, 2009. Revised Manuscript Received April 16, 2010
Wormlike micelles have been observed and explained in a wide variety of different types of surfactants except sulfobetaine ones. Here, we first report branched worms formed by a C22-tailed amidosulfobetaine surfactant;3-(Nerucamidopropyl-N,N-dimethyl ammonium) propane sulfonate (EDAS). Increasing EDAS concentration in the semidilute region increases the viscosity by several orders of magnitude and forms viscoelastic micellar solution of entangled and branched worms. The intermicellar branching is proved by rheological methods and Cryo-TEM observation. Besides, the rheological experiments indicate that EDAS worms show some advantages such as low overlapping concentration, insensitive to inorganic salt, stable over the whole pH range.
Introduction Over the past few decades, there has been a great deal of interest in the aqueous self-assembly of surfactant molecules into threadlike or wormlike micelles.1-4 These cylindrical aggregates undergo reversible breakdown processes and in favorable cases can grow as long as few tens of micrometers.1,4,5 The entanglement of these assemblies imparts viscoelastic properties to the solution.4-7 Thus, a considerable amount of attention, both from a theoretical viewpoint and from industrial and technological applications, has been paid to these reversible rodlike systems.6,8 From a fundamental perspective, worms are of particular interest because they are models of “living polymers” that are constantly breaking down and reforming.9,10 On the practical side, the interest in these micelles is motivated by their applications as rheology modifiers,11 heat-transfer fluids,12 dragreduction agents,13,14 and personal care products.5 *To whom correspondence should be addressed. E-mail:
[email protected]. cn. Tel.: þ86 (28) 8523 6874. Fax: þ86 (28) 8523 6874.
(1) Hoffmann, H. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud'homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; p 2. (2) Zana, R., Kaler, E. W., Eds. Giant Micelles: Properties and Applications; Surfactant Series; CRC Press: Boca Raton, FL, 2007, Vol. 140. (3) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (4) Candau, S. J.; Oda, R. Colloids Surf., A 2001, 183-185, 5. (5) Yang, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 276. (6) Dreiss, C. A. Soft Matter 2007, 3, 956. (7) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869. (8) Acharya, D. P.; Kunieda, H. Adv. Colloid Interface Sci. 2006, 123-126, 401. (9) Cates, M. E. Macromolecules. 1987, 20, 2289. (10) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (11) Stukan, M. R.; Boek, E. S.; Padding, J. T.; Briels, W. J.; Crawshaw, J. P. Soft Matter 2008, 4, 870. (12) Ezrahi, S.; Tuval, E.; Aserin, A. Adv. Colloid Interface Sci. 2006, 128-130, 77. (13) Zakin, J. L.; Bewersdroff, H. W. Rev. Chem. Eng. 1998, 14, 253. (14) Qi, Y.; Zakin, J. L. Ind. Chem. Res. 2002, 41, 6326. (15) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1989, 5, 398. (16) Kern, F.; Zana, R.; Candau, S. J. Langmuir 1991, 7, 1344. (17) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 90, 5435. (18) Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456. (19) Tsuchiya, K.; Orihara, Y.; Kondo, Y.; Yoshino, N.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 12282. (20) Davies, T. S.; Ketner, A. M.; Raghavan, S. R. J. Am. Chem. Soc. 2006, 128, 6669. (21) Mu, J. H.; Li, G. Z.; Jia, X. L.; Wang, H. X.; Zhang, G. Y. J. Phys. Chem. B 2002, 106, 11685.
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These wormlike micelles have been observed and explained in a wide variety of surfactant types such as cationic,14-20 anionic,21 and nonionic22 species and mixed formulations such as cationic-anionic,23 ionic-nonionic,24 ionic-zwitterionic,25 and nonionic-zwitterionic1 systems. However, only a few studies have focused on worms formed by zwitterionic surfactants, most of which were carboxylic betaines26,27 or alkyl dimethylamine oxides.1,28,29 To the best of our knowledge, no report on sulfobetaine surfactant worms has appeared so far. In contrast to cationic or anionic surfactants whose properties depend on the counterion concentration,30 sulfobetaine amphiphilic molecules that behave like noncharged systems show some peculiar properties, including higher foam stability, better salt tolerance, and excellent lime soap dispersibility.31,32 Unlike carboxylic betaines that usually exhibit acid-base equilibrium and have narrow isoelectronic ranges,30,33-36 sulfobetaines retain their zwitterionic characteristic over the entire pH range.31,33,37 If they can also assemble into viscoelastic wormlike assemblies, it is expected that the self-organized aggregates are stable enough in the presence of inorganic species such as acid, base, or salt, which (22) Ericsson, C. A.; S€oderman, O.; Garamus, V. M.; Bergstr€om, M.; Ulvenlund, S. Langmuir 2005, 21, 1507. (23) Raghavan, S. R.; Fritz, G.; Kaler, E. W. Langmuir 2002, 18, 3797. (24) Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Langmuir 2003, 19, 9173. (25) Hoffmann, H.; Rauscher, A.; Gradzielski, M.; Schulz, S. F. Langmuir 1992, 8, 2140. (26) Kumar, R.; Kalur, G. C.; Ziserman, L.; Danino, D.; Raghavan, S. R. Langmuir 2007, 23, 12849. (27) Harrison, D.; Szule, R.; Fisch, M. R. J. Phys. Chem. B 1998, 102, 6487. (28) Ono, Y.; Shikata, T. J. Phys. Chem. B 2005, 109, 7412. (29) Hashimoto, K.; Imae, T. Langmuir 1991, 7, 1734. (30) Fisher, P.; Rehage, H.; Gr€uning, B. J. Phys. Chem. B 2002, 106, 11041. (31) Gonenne, A.; Ernst, R. Anal. Biochem. 1978, 87, 28. (32) Rosen, M. J. In Surfactants and Interfacial Phenomenon, 3rd ed.; John Wiley & Sons: New York, 2004; p 28. (33) Domingo, X. In Amphoteric Surfactants, 2nd ed.; Lomax, E. G., Ed.; Marcel Dekker: New York, 1996; p 75. (34) Weers, J. G.; Rathman, J. F.; Axe, F. U.; Crichlow, C. A.; Foland, L. D.; Scheuing, D. R.; Wiersema, R. J.; Zielske, A. G. Langmuir 1991, 7, 854. (35) Kato, K.; Kondo, H.; Morita, A.; Esumi, K.; Meguro, K. Colloid Polym. Sci. 1986, 264, 737. (36) Danov, K. D.; Kralchevska, S. D.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Langmuir 2004, 20, 5445. (37) Yoshimura, T.; Ichinokawa, T.; Kaji, M.; Esumi, K. Colloids Surf., A 2006, 273, 208.
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Scheme 1. Chemical Structure of C22-Tailed Amidosulfobetaine Surfactant EDAS
is advantageous to both basic research and industrial applications in hostile environments. Experimental results30 from this ideal case of a salt-free environment may fit the theoretical predications very well. In practical applications, sulfobetaine surfactants tend to be very gentle on human skin and friendly to the environment because they are noncharged amphiphiles.26,30 Besides, the better salt tolerance may impart special applications to them, for instance, enhanced oil recovery from high-salinity oil reservoirs. Nevertheless, it seems that traditional sulfobetaine surfactants with a C16 or even shorter tail cannot form entangled threadlike micelles.6,26,34 The carbon atoms in the hydrophobic tails of most surfactants are normally between 8 and 16, with fewer over 18.26,38-42 Among long-chain surfactants whose hydrophobic chain is longer than C18, erucyl bis-(hydroxyethyl)methylammonium chloride (EHAC) and erucyl trimethylammonium chloride (ETAC) are frequently investigated.38,39 Compared with their shorter-chain counterparts, both of them exhibit unique rheological properties in the presence of hydrotropic additives because of the entanglement of long threadlike micelles.38,39 Recently, a long-chain zwitterionic surfactant, erucyl dimethyl amidopropyl betaine (EDAB), was also proven to show interesting rheological responses.26 However, whether their sulfobetaine counterparts could cluster into viscoelastic supermolecular worms is unknown for the lack of such surfactants to date. To obtain sulfobetaine worms, we recently synthesized a series of long-chain amidosulfobetaine surfactants,43 and we report here the properties of an aqueous solution and long, flexible wormlike micelles formed by one of them, C22-tailed amidosulfobetaine surfactant 3-(N-erucamidopropyl-N,N-dimethyl ammonium) propanesulfonate (EDAS, Scheme 1). The surface activities, rheological properties, and cryo-TEM observation of EDAS solutions will be presented.
Experimental Section Materials. Surfactant EDAS (molecular formula C30H60N2O4S; molecular weight 544.87) with a purity greater than 99.5% was synthesized previously.43 The NaCl used in the experiments was analytical grade, and the water was triply distilled by a quartz water-purification system. Samples were prepared by adding powderlike EDAS to a NaCl aqueous solution at the desired concentrations, followed by mild heat at 4050 °C and gentle agitation until the surfactant was solubilized completely; the samples were then left at room temperature for at least 2 days prior to measurements. For all sample solutions, the concentration of NaCl was fixed at 500 mM to ensure good solubility of the surfactant, and the pH was at the natural values (∼6.5), except when discussing the effects of NaCl and pH on wormlike micelle solutions. Surface Tension Measurements. Surface tension (γ) was measured with a Kr€ uss K100 tensiometer by the automatic model of the du No€ uy Ring technique at the desired temperatures ((0.01 °C), and a cover was used to minimize water evaporation. (38) Croce, V.; Cosgrove, T.; Maitland, G.; Hughes, T.; Karlsson, G. Langmuir 2003, 19, 8536. (39) Raghavan, S. R.; Edlund, H.; Kaler, E. W. Langmuir 2002, 18, 1056. (40) McGrady, J.; Laughlin, R. G. Synthesis 1984, 5, 426. (41) Brode, P. F., III. Langmuir 1988, 4, 176. (42) Hossain, M. K.; Rodriguez, C.; Kunieda, H. J. Oleo Sci. 2004, 53, 35. (43) Chu, Z.; Feng, Y. Synlett 2009, (16), 2655.
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A set of measurements to obtain equilibrium surface tension were taken until the change was less than 0.03 mN m-1 every 3 min.37 Rheology. Rheological measurements were made on a Physica MCR 301 (Anton Paar, Austria) rotational rheometer equipped with concentric cylinder geometry CC27 (ISO3219) with a measuring bob radius of 13.33 mm and a measuring cup radius of 14.46 mm. Samples were equilibrated at the temperature of interest for no less than 20 min prior to experiments. Dynamic frequency spectra were conducted in the linear viscoelastic regimes, as determined from dynamic stress-sweep measurements. All experiments were carried out using stress-controlled mode, and Cannon standard oil was used to calibrate the instrument before the measurements. The temperature was set to (0.1 °C accuracy by a Peltier temperature control device, and a solvent trap was used to minimize water evaporation during the measurements. Cryo-TEM. A Cryo-TEM observation of EDAS solution was carried out in a controlled-environment vitrification system. The climate chamber temperature was 25-28 °C, and the relative humidity was kept close to saturation to prevent evaporation from the sample during preparation. Five microliters of a 100 mM EDAS sample at room temperature was placed on a carboncoated holey film supported by a copper grid and gently blotted with filter paper to obtain a thin liquid film (20-400 nm) on the grid. The grid was quenched rapidly in liquid ethane at -180 °C and then transferred to liquid nitrogen (-196 °C) for storage. Then the vitrified specimen stored in liquid nitrogen was transferred to a JEM2010 cryo-microscope using a Gatan 626 cryoholder and its workstation. The acceleration voltage was 200 kV, and the working temperature was kept below -170 °C. The images were recorded digitally with a charge-coupled device camera (Gatan 832) under low-dose conditions with an underfocus of approximately 3 μm.
Results Solubility. The temperature at which the solubility of ionic surfactants rises sharply is known as the Krafft point (TK), where the solubility of the surfactant is normally equal to its cmc.34 Above TK, the surfactant is soluble in water independent of its concentration.33 The TK of the surfactant generally increases with lengthening its hydrophobic chain33 and decreases with the introduction of an unsaturated bond onto the tail.26 Tsujii et al.44 reported that propane sulfonate bearing a saturated C18 tail has a TK of 73.4 °C in pure water. In our work, C22-tailed surfactant EDAS could not be dissolved even in boiling water, which means that its TK is higher than 100 °C. However, though hours were spent to dissolve EDAS completely in a 500 mM NaCl solution at 40-50 °C, no precipitates were found in the final solutions regardless of the EDAS concentration even when the solutions were cooled to 0 °C, indicating that the TK of EDAS in 500 mM NaCl brine is below 0 °C. To ensure good solubility of EDAS, all of the samples studied in this work contain 500 mM NaCl unless specifically stated. Equilibrium Surface Tension and the cmc. When an automatic version of a tensiometer is used, sets of measurements would be taken until the surface tension reached the equilibrium value.37 It takes just a few minutes for shorter-chain surfactants to reach the equilibrium surface tension.41 However, a much longer time is needed for EDAS to reach the equilibrium value, especially when the testing concentration is lower than its cmc. This may be due to the fact that long-chain surfactants probably diffuse more slowly and adsorb less effectively at the air-water interface and soluble micelles may act as efficient transport species.41 Figure 1 shows that the surface tension of EDAS varies as a function of its (44) Tsujii, K.; Mino, J. J. Phys. Chem. 1978, 82, 1610.
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Article Table 1. Surface Activity and Micellization Properties of EDAS T (°C)
cmc (10-3 mM)
γcmc (mN m-1)
Amin (A˚2)
25 30 40 50 80
3.08 2.96 1.96 1.25 0.90
34.02 32.52 30.82 28.83 25.79
19.57 27.46 37.72 40.60 44.72
Figure 1. Surface tension plotted as a function of EDAS concentration at various temperatures.
concentration at different temperatures. No minimum in the surface tension curves is indicative of no surface-activate impurities in the samples.27 The extrapolation of linear curves from above and below the break point in the curve yields the cmc. As listed in Table 1, the cmc of EDAS decreases upon increasing temperature, ranging from 0.90 10-3 to 3.08 10-3 mM, which is close to that of EDAB (∼1 10-3 mM) reported previously.26 One can also find a monotonous decrease in the surface tension at the cmc (γcmc) but an increase in the Gibbs molecular area (Amin) for increasing temperature. Rheology. Figure 2 shows the variation of steady shear _ for EDAS solutions viscosity (η) as a function of shear rate (γ) at 25 °C. As can be seen, η of 0.75 mM EDAS is as small as that of water and maintains a constant value regardless of the shear rate, which corresponds to typical Newtonian flow behavior. For 2.5 mM EDAS, shear thinning occurs when γ_ exceeds ∼0.5 s-1, which can be taken as evidence of the presence of wormlike micelles that undergo a structural change, such as the alignment of long micelles at a high shear rate.6 With increasing EDAS concentration, the wormlike micelles become entangled into a transient network, thereby enhancing the viscoelasticity of the fluid and the critical γ_ at which shear thinning starts shifting to lower values. The extrapolation of viscosity to zero-shear rate in the steady-shear measurement yields the zero-shear viscosity, η0. However, for the 250 and 500 mM EDAS samples, η0 cannot be obtained because of the lack of a viscosity plateau at low shear rates. Nevertheless, one can deduce that η0 of such samples is very high. Therefore, with increasing EDAS concentration, η0 increases monotonously. This is different from the situation with shorter-chain surfactant systems. For the latter one, η0 usually passes through a maximum value1,4,7,16,45 that corresponds to the structural modification;the formation of branching points between the worms.4,6,46-48 Such branched joints can slide along the cylindrical body, allowing a faster, easier method of stress relaxation, and thereby result in the decrease in η0.4,6,46-48 It should be pointed out that the viscosity enhancements at high shear rates for the low EDAS concentration samples were not shown in Figure 2 because of Taylor instabilities. The Taylor vortex is due to the centrifugal forces and inertial effects caused by the mass of the fluid in concentric cylinder geometry. The critical shear rate of the Taylor instabilities increases with increasing (45) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (46) Khatory, A.; Kern, F.; Lequeux, F.; Appell, J.; Porte, G.; Morie, N.; Otta, A.; Urbach, W. Langmuir 1993, 9, 933. (47) Croce, V.; Cosgrove, T.; Driess, C. A.; King, S.; Maitland, G.; Hughes, T. Langmuir 2005, 21, 6762. (48) Sharma, S. C.; Shrestha, L. K.; Tsuchiya, K.; Sakai, K.; Sakai, H.; Abe, M. J. Phys. Chem. B 2009, 113, 3043.
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Figure 2. Steady shear viscosity (η) plotted as a function of shear rate for various EDAS concentrations at 25 °C.
viscosity of the fluid and decreasing radii of the inner cylinder and the ratio of the radii of the outer cylinder to the inner one.49 For example, 0.75, 2.5, 10, and 15 mM EDAS show Taylor instabilities at critical shear rates of about 110, 150, 210, and 400 s-1, respectively, and such instabilities were not observed even at 1000 s-1 for the 100, 250, and 500 mM samples. It is also worth noting that the viscosity curves of the 100, 250, and 500 mM EDAS samples show obvious inflections that may be due to the formation of metastable branch structures in such samples. The flow behaviors reveal the difference between lowconcentration (Figure 3A) and high-concentration (Figure 3B) samples. For instance, the samples with concentration between 5 and 50 mM are shear-thinning fluids characterized by a Newtonian plateau at low shear stress, followed by a smooth decrease in the shear-thinning zone. On the contrary, for the 100, 250, or 500 mM EDAS sample, one observes an abrupt decrease in the viscosity at the critical shear stress, σp, and then a slight drop at higher shear stresses. σp values of 100, 250, and 500 mM EDAS are ∼10, 15, and 50 Pa, respectively. The abrupt decrease in the viscosity at σp can also find an explanation in the method of formation of the metastable branching of larger assemblies.50 The abrupt decrease in viscosity in the flow curves in Figure 3B are similar to those reported by Candau’s group50 and can be explained by shear banding behaviors.18,50,51 Figure 4 shows the shear stress as a function of shear rate for EDAS worm solutions. As can be seen, the shear stress of 100, 250, and 500 mM EDAS fluids manifests plateaus with values equal to σp. Cates and coworkers52 proposed a model based on the reptation mechanism for the relaxation of entanglements to elucidate this interesting behavior. In such a case, the shear stress as a function of the shear rate curve in homogeneous steady shear flow is multivalued and (49) Mezger, T. G. The Rheological Handbook: For Users of Rotational and Oscillatory Rheometers, 2nd revised ed.; Vincentz Network Gmbh & Co. KG: Hannover, Germany, 2006; Chapter 8, p 169. (50) Buhler, E.; Candau, S. J.; Kolomiets, E.; Lehn, J. M. Phys Rev. E 2007, 76, 061804. (51) Hassan, P. A.; Valaulikar, B. S.; Manohar, C.; Kern, F.; Bourdieu, L.; Candau, S. J. Langmuir 1996, 12, 4350. (52) Spenley, N. A.; Cates, M. E.; McLeish, T. C. B. Phys. Rev. Lett. 1993, 71, 939.
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Figure 3. Steady shear viscosity (η) plotted as a function of shear stress for low-concentration (A) and high-concentration (B) EDAS samples
at 25 °C.
Figure 4. Shear stress plotted as a function of shear rate for EDAS micellar solutions at 25 °C.
nonmonotonic. In other words, two branches;one at a low shear rate and the other at a high shear rate;are separated by a mechanically unstable regime. When the controlled shear enters into the unstable regime, the initially homogeneous flow will become nonmonotonic as characterized by the demixing of the fluid into two macroscopic bands. This interesting transition is usually regarded as shear banding behavior.18,50-52 Though the selection of the shear banding process and the corresponding σp shared by the two bands is still a matter of debate,50 shear banding behaviors have been demonstrated clearly in wormlike micellar systems by flow birefringence53 or NMR velocimetry54 experiments, and good agreement was found between the experimental data and the prediction of the model with the assumption of top jumping.18,50,51 The observed flow curves for 100, 250, and 500 mM EDAS in this work and a wormlike micellar solution reported by Candau et al.18,50,51 correspond to top jumping. Usually the shear banding instability could be detected clearly only when the surfactant concentration was at least 20 times higher than the overlapping concentration (C*).55,56 In this work, C* of EDAS is around 1 mM at 25 °C (later in this article); therefore, shear banding behaviors were not observed unambiguously for the 10, 25, and 50 mM EDAS samples. Nevertheless, the very slight increase in shear stress above a shear rate of ∼0.1 s-1 can also be regarded as shear banding.51 The shear banding behaviors of EDAS fluids imply the formation of transient (53) Makhloufi, R.; Decruppe, J. P.; Aı¨ t-Ali, A.; Cressely, R. Europhys. Lett. 1995, 32, 253. (54) Callaghan, P. T.; Cates, M. E.; Rofe, C. J.; Smeulders, J. B. A. F. J. Phys. II 1996, 6, 375. (55) Berret, J. F.; Roux, D. C.; Porte, G. J. Phys. II 1994, 4, 1261. (56) Grand, C.; Arrault, J.; Cates, M. E. J. Phys. II 1997, 7, 1071.
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Figure 5. Storage moduli (G0 , filled circles) and loss moduli (G”, open circles) varying as a function of angular frequency (ω) for various EDAS samples at 25 °C.
intermicellar branching points between the worms, and such intermicellar branching was confirmed by the cryo-TEM observation (later in this article). With increasing EDAS concentration, the cross-links add increasing contributions to the entanglements to enhance the elasticity.50 The concentration dependence of the elasticity of EDAS worms was carried out by dynamic rheological experiments that were investigated by oscillatory-shear measurements. Figure 5 shows plots of the storage modulus (G0 ) and loss modulus (G00 ) as a function of the oscillatory shear frequency (ω) for EDAS solutions at 25 °C. For the 10 and 15 mM samples at frequencies below a critical value ωc, the elastic modulus G0 crosses over and drops below the viscous modulus G00 . In other words, the relaxation time τR (∼1/ωc) is finite for these samples. The sample’s response can be divided into two regimes on the basis of the relaxation time τR: on a timescale much longer than τR (ω , ωc), the response is viscous, whereas for a timescale much shorter than τR (ω . ωc), the response is elastic. The viscoelasticity of the solution is attributed to the entanglement of long worms to form a transient network. The increasing EDAS concentration increases the plateau modulus G0 (storage modulus at high frequencies) and decreases ωc. As a result, the ωc of 25 mM EDAS is too low to be determined and 100 and 250 mM EDAS show G0 exceeding G00 , where both moduli are independent of frequency. G0 values of 100 and 250 mM EDAS are ∼15 and 65 Pa, respectively. The two values follow the same order as those of σp, in accordance with the previously reported results.50 In the dilute regime, the average micellar length usually increases with surfactant concentration according to a simple Langmuir 2010, 26(11), 7783–7791
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Figure 6. Zero-shear viscosity η0 plotted as a function of EDAS concentration at various temperatures. The slopes give the powerlaw exponents.
power law with an exponent of ∼1/2,7 but when the surfactant concentration reaches a critical value (overlap concentration, C*), the wormlike micelles begin to entangle with each other to form larger supermolecular aggregates, which display remarkable viscoelastic behaviors.4,6,12,57 Therefore, the corresponding rheological properties of EDAS solutions show significant difference at concentration below and above C* (Figure 6). Below C*, η0 varies linearly according to the Einstein equation, η0 = ηwater(l þ KC), where K is on the order of unity; above C*, η0 increases drastically as ∼Cn by several orders of magnitude, where n is the power-law exponent.57 Micelles are fragile dynamic objects that are continually formed and destroyed by the addition and loss of surfactant monomers, and the rheological properties of their solutions are affected by the external environment such as surfactant concentration and temperature.10 When the temperature increases, the micelles may be destroyed by thermal fluctuations. For instance, η0 of 2.5 mM EDAS obviously decreases when the temperature is altered from medium to high, and such a change in 50 mM EDAS seems to be negligible. As a consequence, overlap concentration C* increases at high temperature. As is shown in Figure 6, C* values are around 1.0 mM at 25, 30, and 50 °C but increase to about 3.0 mM at 80 °C. Exponent n increases with increasing temperature, ranging from 2.79 to 3.79. The n value of 3.79 is close to theoretical predictions for entangled linear worms, which usually offer a value of ∼3.5;6-8,58,59 however, the concentration dependence of the zero-shear viscosity is smaller with an n value smaller than 3. The deviations can be interpreted with intermicellar branching in which both theoretical predictions59 and experimental results offer a value lower than 3.18,46 Branched worms, first speculated on by Porte et al.60 and then proposed on the basis of rheological methods,61 are now well confirmed via cryo-TEM observation.3 Shorter-chain surfactants in the presence of high-concentration counterions18,46 result in worm branching because of a screened electrostatic interaction. Thus EDAS, which behaves as an uncharged system,26,32 induces micellar branching even at a low salt concentration. It is also useful to study how plateau modulus G0 (the storage modulus at high frequency) varies with EDAS concentration. As shown in Figure 7, G0 also follows a power-law behavior with an exponent of 1.7 at 25 °C. This value is smaller than that (∼2.25) of entangled linear wormlike chains7 but very close to the values of (57) Berret, J. F.; Appell, J.; Porte, G. Langmuir 1993, 9, 2851. (58) Cates, M. E. J. Phys. 1988, 49, 1593. (59) Shchipunov, Y. A.; Hoffmann, H. Langmuir 1998, 14, 6350. (60) Porte, G.; Gomati, R.; Haitamy, O. E.; Appell, J.; Marignan, J. J. Phys. Chem. 1986, 90, 5746. (61) Appell, J.; Porte, G.; Khatory, A.; Kern, F.; Candau, S. J. J. Phys. II 1992, 2, 1045.
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Figure 7. Variation of the plateau modulus (G0) plotted vs EDAS concentration at 25 °C. The slope gives a power-law exponent of 1.7.
Figure 8. Zero-shear viscosity η0 plotted as a function of T-1 for 10 and 25 mM EDAS samples.
branched worms (∼1.8).18 Thus, the slighter concentration dependence of the plateau modulus in the case of EDAS solutions is also broadly consistent with the formation of long branched worms.18 The steady rheological behavior of 10 and 25 mM EDAS was examined at additional temperatures to study the temperature effect on the worms. Uncommon trends were found in the steady rheology as a function of temperature. The rheological properties showed an initial enhancement at lower temperatures, followed by a subsequent decrease at higher temperatures other than a progressive decrease. A similar result was also found in EHAC (a C22-tailed cationic surfactant) wormlike micellar solutions.62 At high temperature, an Arrhenius plot of η0 versus T -1 (Figure 8) shows the expected exponential decrease in η0 with increasing temperature, as given by the following equation63 η0 ¼ G0 AeEa =RT
ð1Þ
where T is the absolute temperature in K, Ea is the flow activation energy in J mol-1, R is 8.315 J mol K-1 (gas constant), and A is a pre-exponential factor. The straight lines fit η0 at high temperature very well and yield values for the activation energies. Ea values of 10 and 25 mM EDAS are 19.6 and 16.5 kJ mol-1, respectively. Both values are close to those of the branched wormlike micelle system,46 which further implies the presence of branched worms. (62) Raghavan, S. R.; Kaler, E. W. Langmuir 2001, 17, 300. (63) Candau, S. J.; Hirsch, E.; Zana, R.; Delsanti, M. Langmuir 1989, 5, 1225.
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Figure 9. Storage moduli (G0 , filled circles) and loss moduli (G00 , open circles) varying as a function of angular frequency (ω) for 25 mM EDAS at various temperatures. ωc increases with increasing temperature, but no variation was found in G0.
Figure 9 shows a temperature effect on the dynamic rheological behavior of the 25 mM EDAS samples. The critical frequency, ωc, changed little at lower temperature but obviously increased at high temperature, which was in line with its steady rheological response (Figure S3 in Supporting Information). These results also correspond to the destruction of the wormlike micelles at high temperature. However, G0 is almost constant (i.e., independent of temperature), which was usually found in wormlike micellar solutions.30 Cryo-TEM Observation of EDAS Solutions. Rheological results indicate that EDAS solutions show many feature of wormlike micelles, and it is helpful to use other complementary techniques to characterize the microstructure of the micelles. Cryo-TEM is a useful tool for obtaining such direct imaging of liquid or semiliquid specimens, and it can provide the nature and structures of the basic assemblies that make up the system.64 Because it could provide high-resolution direct images of the organized aggregates for a wide range of length scales from a few nanometers to several micrometers, it is helpful to confirm further the existence of long wormlike micelles in EDAS solutions. A cryo-TEM image of a 100 mM EDAS sample at room temperature is displayed in Figure 10. As can be seen, the diameter of the wormlike micelles ranges from 10 to 20 nm and the length is more than several micrometers. The long, flexible micelles entangle or even branch with each other to form larger aggregates of viscoelastic networks. This could exactly explain why the 100 mM sample shows obvious shear banding behavior. In a word, the cryo-TEM observation directly demonstrated the formation of long branched worms in the semidilute concentration regime of EDAS. Effect of NaCl on Rheological Behavior. In this part, the EDAS concentration is fixed at 25 mM and the concentration of NaCl is changed. Steady shear viscosity as a function of NaCl content was studied, and measurements were performed at 25 °C at their natural pH values. Steady rheological data indicates that the content of NaCl has no effect on EDAS worms at various shear rates (Figure 11). It is clear that the viscosity at a fixed shear rate is constant, independent of NaCl concentration (i.e., NaCl concentration does not affect the rheological properties of EDAS worms). Because the main effect of salt is believed to screen the electrostatic repulsion between surfactant headgroups and to increase the dimensional packing parameter of the micelles and (64) Danion, D.; Talmon, Y. In Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2006; p 253.
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thus favor growth in the micelle length and flexibility, it is not puzzling to understand the above result from the standpoint of the EDAS molecular structure. As a zwitterionic surfactant that contains both positively and negatively charged hydrophilic groups in the same molecule, EDAS is uncharged. Therefore, the electrostatic interaction between its headgroups is very weak, thus salt will have a negligible effect. The characteristic of insensitivity to NaCl or other inorganic salts is also found in carboxylic betaine EDAB, and it is argued that this feature is one of the important advantages of zwitterionic surfactant formulations as compared to cationic or anionic ones.26 Effect of pH on EDAS Worms. Unlike carboxylic betaine, which usually exhibits acid-base equilibrium,30,33-36 sulfobetaine surfactant EDAS can retain its zwitterionic characteristic over the entire pH range31,33,37 (i.e., the pH does not inspire the molecular structure transformation) and thus the pH may not affect the EDAS solution properties. Steady shear viscosity as a function of pH for 25 mM EDAS in 500 mM NaCl confirms such a presumption. As illustrated in Figure 12, the pH effect on the rheological properties of EDAS worms is negligible.
Discussion Solubility, cmc, and Area per Molecule. These parameters can be ascribed to three aspects of the molecular structure of C22tailed amidosulfobetaine surfactant EDAS: (a) it bears a long hydrophobic tail; (b) the hydrophobic chain is unsaturated; and (c) the hydrophilic headgroup is a sulfobetaine inner salt. Being aware of these points, it is not hard to understand the peculiarity of these parameters with respect to EDAS. It is well known that TK of the surfactant increases upon lengthening its hydrophobic tail.33 Thus one can easily understand the high TK (above 100 °C) of EDAS in pure water where the sulfobetaine headgroup is an inner salt and the positive and negative charges are attracted each other because of the electrostatic interactions. Thus, there is no free ion in its headgroup to carry out the ionic hydration, resulting in a decrease in solubility and an increase in TK. However, the positive and negative charges are separated because of the weakening of electrostatic interaction invoked by the added salt. There is strong ionic hydration in the separated charges, thus the solubility increases and the corresponding TK decreases. TK decreasing with the addition of inorganic salt is also found for other sulfobetaines,33,44 but the degree of decrease is not as large as that for EDAS. For example, TK of octadecylsulfobetaine at a concentration 2.0 mM is 73.4 °C in pure water and 33.6 °C in 1.0 M NaCl.33,44 Why does TK of EDAS decrease so sharply? The cis unsaturation and the resulting kink in the erucyl tail may explain such a significant reduction.26 As a consequence, EDAS solutions containing 500 mM NaCl are stable for months at room temperature, and no crystalline precipitate was found when cooled to 0 °C. The variation of the cmc with the hydrocarbon chain length (m) is often empirically described by the equation33,65 log cmc ¼ A - Bm
ð2Þ
where A and B are constants. For zwitterionic surfactants, the A and B values are normally close to 3 and 0.5, respectively.33 From this equation, one may deduce that the cmc of a C22-tailed zwitterionic surfactant has a magnitude of 10-5 mM.41 But why is the cmc of C22-tailed sulfobetaine surfactant EDAS on just the 10-3 mM scale? This may be due to the following reasons: (a) the (65) Klevens, H. B. J. Phys. Colloid Chem. 1948, 52, 130.
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Figure 10. Cryo-TEM images of a 100 mM EDAS sample at room temperature. The images show well-developed networks made of branched wormlike micelles. The branch points and end caps of the worms are shown by white and black arrows, respectively. The circles are carbon-coated holes of the film used to support the sample. The scale bar in A is 500 nm, and that in B is 100 nm.
formation of long threadlike assemblies, which may significantly increase the viscoelasticity because of the entanglement of the long worms. The Gibbs molecular area, which reflects the area per headgroup, is also an important and useful parameter for surfactants, and it is determined with tensiometry data using66 Γcmc
1020 1 Dγ ¼ ¼ 2:303nRT Dðlog CÞ T NA Amin
Amin ¼ Figure 11. Steady shear viscosity plotted as a function of NaCl concentration for the 25 mM EDAS samples at their natural pH values at 25 °C. Data are chosen from steady rheological measurements, shown in Figure S4.
Figure 12. Steady shear viscosity plotted as a function of pH for 25 mM EDAS at 25 °C. The concentration of added NaCl is 500 mM. Data are chosen form steady rheological measurements, shown in Figure S5.
introduction of an unsaturated bond onto the hydrophobic tail; (b) surfactant oligomers or premicelles formed before the cmc; (c) long-chain surfactants diffusing more slowly and adsorbing less effectively at the air-water interface; and (d) the experimental limit for the measurement of cmc values.41 It is argued that the maximum reduction in the cmc is correlated with the formation of viscoelastic wormlike micelles.5 Therefore, the low cmc values of EDAS are favorable to the Langmuir 2010, 26(11), 7783–7791
2:303nRT 1020 Dγ NA Dðlog CÞ T
ð3Þ
ð4Þ
where Γcmc is the surface excess concentration in mol m2, n = 1 (for zwitterionic surfactants), NA = 6.023 1023 (Avogadro’s number), γ is the surface tension in N m-1, C is the concentration in mol L-1, and Amin is the Gibbs molecular area in A˚2. As depicted in Table 1, Amin of EDAS at lower temperatures (25 and 30 °C) was about 20-30 A˚2, which was lower than that of shorterchain zwitterionic surfactants (40-80 A˚2)33 and close to that of alkanols (20-22 A˚2).67 The small area per surfactant molecule favors an increase in the dimensional packing parameter14 and may be the origin of the formation of long, flexible EDAS worms. Unusual Rheological Behaviors Based on the Relaxation Mechanism. Though we have generally proven the formation of branched worms formed by long-chain amidosulfobetaine EDAS, the worms show some unusual behavior when compared with typical shorter-chain worms.1,4,7,16,45 For instance, η0 increases monotonously with surfactant concentration, and fluid with a high viscoelasticity and a very long relaxation time is formed for concentrated EDAS samples. This extraordinary behavior can be explained on the basis of relaxation time by the worm’s long hydrophobic tail. First, it is worthwhile to present a brief analysis of the model developed by Cates and co-workers9,68 on stress relaxation in ‘‘living polymers’’. Two dynamic regimes are considered, depending on the characteristic relaxation time for reptation, τrep, corresponding (66) Laschewsky, A.; Wattebled, L.; Arotcarena, M.; Habib-Jiwan, J. L.; Rakotoaly, R. H. Langmuir 2005, 21, 7170. (67) Shiao, S. Y.; Chhabra, V.; Patist, A.; Free, M. L.; Huibers, P. D. T.; Gregory, A.; Patel, S.; Shah, D. O. Adv. Colloid Interface Sci. 1998, 74, 1. (68) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758.
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Figure 13. Proposed micelle evolution with EDAS concentration at room temperature. With increasing EDAS concentration, the surfactant system changes from monomers to spherical micelles as well as to long flexible worms, entangled worms, and a well-developed network of branched worms.
The basic characteristic of the above model is that at low shear frequency (ω < ωc) the increases in G0 and G00 with ω follow G0 ≈ ω1 and G00 ≈ ω2, respectively. Another obvious feature is that G00 has a maximum value at ωc. According to these features, one can easily conclude that the dynamic spectrum of EDAS does not obey the Maxwell model (i.e., wormlike micellar samples of EDAS are non-Maxwellian fluids), which is very different from typical worm systems organized by shorter-chain surfactants.1,4,7,16,45 The differences between EDAS worms and typical shorterchain amphiphilic ones could be interpreted by the effect of the surfactant tail length on the basis of the two relaxation regimes. As mentioned above, the Maxwellian response of ‘‘living polymers’’ occurs in the fast-breaking limit, where τb , τrep. This is the common case for most worms formed by shorter-chain surfactants,1,4,7,16,45 and thus they are typical of Maxwellian flow.26 However, the situation is quite different for long-chain amphiphilic EDAS. More recently, Raghavan et al.26 proposed that a long hydrophobic tail results in a very long breaking time, which they attributed to the fact that a long-chain surfactant has poor solubility (high TK) and a low cmc, and such properties
significantly slow the exchange of monomers from one worm to another through an aqueous solution. Therefore, the assumption of τb , τrep is invalid for long-chain surfactant worm systems, or even τb > τrep is the real case. Therefore, EDAS worms are “unbreakable” and behave as a non-Maxwellian flow. Taking this into account, one easily understands the discrepancies between the shorter-chain worms and long-chain surfactant ones. For the former systems, the formation of branch points usually reduce η0 because the connections slide along the cylindrical micelle bodies and the networks of branched micelles are temporary networks; therefore, one observes a maximum value in the curve of the concentration dependence of η0. However, for the latter system such as EHAC,62 EDAB,26 or EDAS, the networks are unbreakable and may behave as ordinary polymer networks; hence, η0 grows monotonously with increasing EDAS concentration. Effect of EDAS Concentration on the Micelles and Rheological Properties. Figure 13 schematically summarizes the evolution of EDAS micelles with their concentration at room temperature according to our experimental results, along with complementary results from the literature. With increasing EDAS concentration, the surfactant system changes from monomers to spherical micelles as well as to long flexible worms, entangled linear worms, and a well-developed network of branched worms. When the EDAS concentration is above cmc (∼3 10-3 mM), spherical micelles or globular micelles are formed, and when the EDAS concentration is more than ∼10 times the cmc, long, flexible wormlike micelles are generated.4,70,71 If the EDAS concentration is increased to above C* (∼1 mM), then the long threadlike micelles become entangled, and if the EDAS concentration is increased further to greater than ∼10 C*, then the corresponding solutions show shear banding behavior because of the existence of micellar branching points.4,46-48 Comparison with Other C22-Tailed Worms. It is interesting to compare the wormlike micelles of EDAS with those of other C22-tailed surfactants such as EHAC, ETAC, and EDAB. Being a cationic surfactant, EHAC or ETAC does not form viscoelastic micellar solutions on its own,26,38,39 but does so upon addition of salts such as NaCl or other aromatic salts,16,38,39 and the rheological properties are affected dramatically by such salts.38,39 The worms of sulfobetaine surfactant EDAS also fall back on salts; however, salts lower the Krafft point but have no effect on the rheological properties. EDAB is also a C22-tailed zwitterionic surfactant, but its hydrophilic headgroup is carboxylic betaine, which ensures a lower Krafft point and good water solubility. Thus, it can form worms itself,26 and common inorganic salts have no effect on its rheological behavior. EDAS with a
(69) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: Oxford, U.K., 1999.
(70) Mackintosh, F. C.; Safran, S. A.; Pincus, P. A. Europhys. Lett. 1990, 12, 697. (71) Nusselder, J. J. H.; Engberts, J. B. F. N. J. Org. Chem. 1991, 56, 5522.
to the curvilinear diffusion of a chain with a mean length along a tube that is generated by entanglements with other chains and the reversible breaking time, τb, corresponding to the mean time required for a chain of mean length to break into two pieces: (1) τb > τrep: on a timescale comparable to τb, the socalled ‘‘living polymer’’ chains behave as ordinary “dead” polymers, dominated by reptation. In this case, terminal relaxation time τR equals τrep. (2) τb , τrep: This is the case of interest in which the long-time behavior of stress relaxation can be described by a single-exponential decay with the relaxation time given by τR ¼ ðτb τrep Þ1=2
ð5Þ
At low shear frequency, the behavior of the solution can be described by Maxwellian equations:69 G0 ðωÞ ¼
ω2 τ R 2 G0 1 þ ω2 τ R 2
ð6Þ
G}ðωÞ ¼
ωτR G0 1 þ ω2 τ R 2
ð7Þ
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sulfobetaine headgroup can retain its zwitterionic characteristic at all pH values,31,33,37 thus pH has a neglectable effect on the rheological properties of its worm solutions. However, as a carboxylic betaine, EDAB has a narrow isoelectric region.33,35,36 Therefore, one may deduce that worms of EDAB may not be as stable as those of EDAS at different pH values because of the molecular structure transformation from a zwitterionic surfactant to a cationic or anionic one.30,33-36 Comparison with other Sulfobetaine Surfactants. It is also instructive to compare EDAS with its shorter-chain counterparts. Though interest in wormlike micelles has been increasing in recent years, both from basic research and industrial applications, sulfobetaine surfactants worms have not been reported to date. This may be due to the fact that shorter-chain sulfobetaines cannot form wormlike micelles.34 In fact, we found that the relatively shorter-chain-saturated C18-tailed sulfobetaine surfactant could not form viscoelastic wormlike micelles even at a surfactant concentration of 250 mM. There are mainly two structural differences between them;the length of the hydrophobic tail and the introduction of an unsaturated bond into EDAS. However, it is still unknown as to which one plays a more important role between the two factors. The study of such a question is ongoing in our work.
per molecule, which favors the formation of long cylindrical worms that can significantly increase the viscoelasticity of the solutions. Increasing EDAS concentration in the semidilute region increases the viscosity by several orders of magnitude and forms viscoelastic micellar solution of entangled and branched worms. The intermicellar branching at a high EDAS concentration is proven by rheological methods and cryo-TEM observation. Compared with other typical worm systems, EDAS worms show many advantages such as low overlapping concentration, insensitivity to inorganic salt, and sufficient stability over the whole pH range. These unique properties of EDAS worms will make them suitable for potential applications in hostile environments, such as tertiary oil recovery in high-temperature, highsalinity oil reservoirs.
Conclusions
Acknowledgment. Financial support from the Chinese Academy of Sciences, the Sichuan Provincial Bureau of Science and Technology (2008GZ0004), and the open research fund program (200601) sponsored by the Key Laboratory for Colloid and Interface Chemistry of the State Education Ministry of Shandong University is greatly appreciated. We are also grateful to Dr. Qinfen Zhang and Dr. Kunpeng Li for performing cryo-TEM observations and to the reviewers for their critical comments and instructive suggestions, as well as to Dr. J€org L€auger and Mr. Phil Chen from Anton-Paar for their fruitful discussions on rheology.
Although viscoelastic wormlike micelles can be formed by a variety of surfactants, no report to date has focused on sulfobetaine surfactant worms. We first report in this work that C22tailed amidosulfobetaine surfactant EDAS can assemble into viscoelastic assemblies. Long-chain EDAS shows poor solubility in pure water, but the addition of inorganic salt NaCl can improve its solubility dramatically. EDAS has a low cmc and a small area
Supporting Information Available: Additional details for rheological measurements. Variation of surface tension versus measuring time for various EDAS concentrations. Steady shear viscosity (η) plotted as a function of shear rate for various EDAS samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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