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Phase and Rheological Behavior of Surfactant/Novel Alkanolamide/Water Systems Carlos Rodriguez,† Durga P. Acharya,‡ Koheita Hattori,‡ Takaya Sakai,§ and Hironobu Kunieda*,‡ Escuela de Ingenierı´a Quı´mica, Universidad de Los Andes, Me´ rida, Venezuela, Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, and Materials Development Research Laboratories, Kao Corporation, 1334, Minato, Wakayama-shi, Wakayama 640-8580, Japan Received May 23, 2003. In Final Form: August 1, 2003 Phase diagrams of water; sodium dodecyl sulfate (SDS); and a new foam booster, alkanoyl-Nmethylethanolamide (C8, NMEA-8; C12, NMEA-12; C16, NMEA-16), were constructed at 25 °C. NMEA is hardly soluble in water: liquid-liquid phase separation occurs with NMEA-8, lamellar-phase formation occurs with NMEA-12, and solid precipitation occurs with NMEA-16. In the presence of a small amount of NMEA, the surfactant hexagonal phase (H1) is extended to the dilute region. In the aqueous micellar solution beyond the H1 phase, the viscosity dramatically increases and a viscoelastic solution is formed in NMEA-12 and NMEA-16 systems. The viscoelastic micellar solution formed in SDS-NMEA-12 systems follows the Maxwell model typical of wormlike micellar systems at low shear frequencies. In the SDSNMEA-16 system, a gel-like highly viscoelastic solution is formed in the maximum-viscosity region. Rheological measurements show that the ability of NMEA to induce micellar growth increases in the following order: NMEA-8 , NMEA-12 < NMEA-16. In agreement with this result, dynamic light scattering measurements show that with an increasing mixing fraction of NMEA-12 or NMEA-16 in SDS-NMEA systems the micellar size increases, leading to the formation of wormlike micelles.
Introduction Solutions of elongated micelles have attracted much interest because of their peculiar properties and potential applications as structured fluids. These solutions show rheological properties similar to those of polymers in a good solvent;1,2 the main difference is that micellar chains continuously break and recombine, and, therefore, they are referred to as “living polymers”.3 In charged micelles, there are two contributions to the energy: the end-cap energy that promotes micellar growth and a repulsive contribution due to charges along the micelle that favors the breaking of micelles.4 Hence, micellar growth occurs as a consequence of the reduction in the repulsion between surfactant headgroups, which can be induced by adding salts, strongly binding counterions, or cosurfactants.5 Three concentration-dependent regimes can be identified for micellar growth in charged systems:6 a dilute regime in which the micellar length increases slowly with concentration, a semidilute regime corresponding to rapid growth, and a concentrated regime in which, again, micelles grow only slightly with concentration. The * Author to whom correspondence should be addressed. E-mail:
[email protected]. Phone and fax: +81-45-339 4190. † Universidad de Los Andes. ‡ Yokohama National University. § Kao Corporation. (1) Hoffmann, H.; Rehage, H.; Schorr, W.; Thurn, H. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984. (2) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (3) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758. (4) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994, 10, 1714 (5) Evans, D. F.; Wennerstrom, H. The Colloidal Domain: Where Physics, Chemistry and Biology Meet; Wiley-VCH: New York, 1999. (6) Mackintosh, F. C.; Safran, S. A.; Pincus, P. A. Europhys. Lett. 1990, 12, 697.
transition between the dilute and the semidilute regimes corresponds to the overlap concentration in which the endcap energy equals the repulsive energy. At this concentration, a sharp increase in the viscosity is usually found. The local structure created by entanglement between elongated micelles is perturbed by shear, and viscoelastic behavior appears; namely, there is a superposition of the viscous and elastic forces.1 This behavior can be interpreted in terms of stress relaxation processes. The model of Cates7 considers two relaxation mechanisms: micelle breaking and reforming and reptation, the last related to curvilinear diffusion above the overlap concentration c*.8 When the relaxation time for breaking (τb) is shorter than the relaxation time for reptation (τr), the viscoelasticity of micellar solutions can be described by a simple Maxwell model2,9 at low frequencies, and the two-components of the complex elastic modulus G*(ω) ) G′(ω) + iG′′(ω) are given by10
G′(ω) )
ω2τ2 G0 1 + ω2τ2
(1)
G′′(ω) )
ωτ G0 1 + ω2τ2
(2)
where G′, G′′, and G0 are the storage, loss, and plateau moduli, respectively, ω is the frequency of oscillatory shear flow, and τ is the relaxation time, given by 1/ωc, where ωc is the frequency at which G′ ) G′′. If G′′ is plotted as a (7) Cates, M. E. Macromolecules 1987, 20, 2289. (8) De Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (9) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 5869. (10) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: Oxford, 1999.
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function G′ in the so-called Cole-Cole plot, a semicircle is obtained for pure Maxwellian behavior. The complex viscosity |η*| in terms of G′ and G′′ is as follows:
|η*| )
(G′2 + G′′2)1/2 ω
(3)
For viscoelastic gels, the complex viscosity is related to the zero shear rate viscosity (η0) by11
|η*| )
η0 (1 + ω2τ2)1/2
(4)
This equation allows us to estimate the η0 by extrapolating the experimental points of viscosity to zero shear frequency in oscillatory-shear measurements. Viscoelastic properties are sometimes desired in applications such as health care products. Among other additives, alkanoylethanolamides are widely used in surfactants formulations as viscosity enhancers.12,13 However, only few studies have been reported on the properties of these mixed systems.14 Recently, we reported on the phase behavior and microstructure of a new kind of alkanoylethanolamide.15 The aim of this paper is to extend that study to other homologues, focusing on the relationship between phase diagrams, rheological behavior, and microstructure. Experimental Section Materials. A series of alkanoyl-N-methylethanolamides (Nhydroxyethyl-N-methylalkanamides, 99.4%) designated as NMEAn, where n is the alkanoyl chain length, were kindly supplied by Kao Corp., Japan. Sodium dodecyl sulfate (SDS, 98.9%) was also received from Kao Corp. Deionized (Millipore filtered) water was used for preparing the samples. Phase Behavior. For the study of the phase behavior, sealed ampules containing required amount of reagents were homogenized and left in a water bath at 25 °C for a few days (for the Wm phase) to several weeks (for the liquid-crystal phase) for equilibration. The liquid-crystal phases were identified by polarizing microscopy and from small-angle X-ray scattering performed on a small-angle scattering goniometer with a 15 kW Rigaku Rotating Anode generator (RINT 2500). Rheological Measurements. After mixing the components, the samples were left in a water bath for at least 24 h before the rheological measurements, which were performed in an ARES7 Test Station (Rheometric Scientific) at 25 °C using a Couette fixture with a 33.3-mm-long bob for samples of a low viscosity and a cone-plate fixture (diameter ) 25 mm, cone angle ) 0.04 rad) for viscous samples and gels. Dynamic rheological measurements were carried out in the linear viscoelastic regime. Dynamic Light Scattering (DLS). DLS measurements were performed on DLS-7000 equipment (Otsuka Electronics, Japan) with a 75-mW Ar laser source (488 nm) and a digital real-time correlator ALV-5000/EPP (ALV, Germany). Cylindrical cells of a 12-mm path length were used. Samples were filtered through 0.2-µm Millipore filters directly into the clean, dry cell. DLS measurements were carried out at 25 °C. Diffusion coefficients were calculated from the intensity correlation function using CONTIN analysis. (11) Fischer, P.; Rehage, H. Rheol. Acta 1997, 36, 13. (12) Barker, G. In Surfactants in Cosmetics; Rieger, M., Ed.; Marcel Dekker: New York, 1985. (13) Rieger, M. In Foams: Theory, Measurements and Applications; Prud’homme, R. K., Khan, S. A., Eds.; Marcel Dekker: New York, 1996. (14) Herb, C.; 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. (15) Rodrı´guez, C.; Sakai, T.; Fujiyama, R.; Kunieda, H. J. Colloid Interface Sci., submitted for publication.
Figure 1. Partial phase diagrams of water/SDS/NMEA-n systems at 25 °C. (a) n ) 8; (b) n ) 12, from ref (15); and (c) n ) 16. Wm, Om, and W are the micellar, reverse micellar, and excess water phases, respectively. LR and H1 are lamellar and hexagonal liquid crystals, respectively. Note that part c is shown in a different scale. The shaded area inside the Wm phase region is the region of the highly viscous micellar solution. Samples for rheological measurements were prepared by keeping the SDS concentration fixed and varying the NMEA concentration (composition indicated by circles) along the arrow originating from the SDS/water binary axis in the water-rich region.
Results and Discussion Phase Behavior of Water/SDS/Alkanolamide Systems. Figure 1 shows the partial phase diagrams of water/ SDS/NMEA systems. Only in the case of the dodecanoyl chain, a liquid-crystal phase (lamellar) appears in the binary system/NMEA. The octanoyl chain seems to be too flexible, and only liquid phases are found in aqueous
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Figure 2. Steady-shear-rate viscosity measurements of the 0.15 M SDS/NMEA-12 system with different concentrations of NMEA-12 at 25 °C. The NMEA-12 concentrations are (a) 0, (b) 0.08, (c) 0.16, (d) 0.20, (e) 0.25, (f) 0.29, (g) 0.34, and (h) 0.38 M.
mixtures whereas the hexadecanoyl chain is too rigid, and crystalline solid precipitates in water. As reported in our previous article, NMEA-12 is in the liquid state at room temperature (melting point ) 20 °C), in contrast to other homologues that show a much higher melting point. It is attributed to the presence of one additional methyl group in the molecule.15 Upon addition of SDS, lamellar and hexagonal liquid crystals are formed in all the systems. The ionic SDS increases the repulsion between headgroups in micelles, making the curvature more positive and inducing a lamellar-hexagonal phase transition. SDS also promotes the solubilization of the alkanolamide in the dilute region. The liquid-crystalline domain expands as the alkanoyl chain increases: in the case of NMEA-16, the hexagonal phase (H1) is formed at considerable low surfactant concentration (about 5 wt % SDS). Moreover, in the vicinity of this diluted H1 phase, a gel-like, isotropic phase was found. Because the H1 phase is formed by a packing of cylindrical micelles, it is possible that the micellar shape is similar in the adjacent isotropic solution. The mixing of SDS and alkanolamide allows for a balance of the charge on the surface of micelles and, hence, makes micellar growth possible. It can be observed in the phase diagrams for the NMEA-12 and NMEA-16 systems that the isotropic phase Wm is divided into two legs by an intruding hexagonal liquid-crystal region. This kind of phase behavior has been related to the existence of maxima in micelle relaxation times and viscosity.14 Rheological Behavior. To further investigate the properties of mixed surfactant solutions, steady- and oscillatory-shear viscosity measurements were performed in a dilute solution, keeping the SDS concentration fixed at 0.15 M (∼4.3 wt %) and changing the concentration of alkanolamide along the direction of the arrows in Figure 1. The results of the steady-shear-rate measurements are shown in Figures 2-3. Figure 2 shows the data for water/SDS/NMEA-12 systems. Samples with low NMEA-12 content show Newtonian behavior; namely, the viscosity is constant in the entire shear range. However, with an increasing NMEA-12 concentration, shear thinning appears at 0.20 M, which is typical of systems containing wormlike micelles.16-19 The decrease in the viscosity with the shear (16) Raghavan, S.; Kahler, E. Langmuir 2001, 17, 300. (17) Cappelaere, E.; Cressely, R. Colloid Polym. Sci. 1998, 276, 1050. (18) Kim, W.; Yang, S. J. Colloid Interface Sci. 2000, 232, 225.
Rodriguez et al.
Figure 3. Steady-shear-rate viscosity measurements of the 0.15 M SDS/NMEA-16 system with different concentrations of NMEA-16 at 25 °C. The NMEA-16 concentrations are (a) 0.08, (b) 0.16, (c) 0.18, (d) 0.20, and (e) 0.23 M.
Figure 4. Variation of the zero shear viscosity (η0) of 0.15 M SDS/NMEA systems as a function of NMEA concentrations at 25 °C.
rate might be attributed to structural changes in micellar entanglements16,20 The constant-viscosity plateau is changed when the alkanolamide content is increased from 0.20 to 0.29 M NMEA, and, however, Newtonian behavior is recovered above 0.29 M. Shear thinning is also observed in Figure 3 for water/ SDS/NMEA-16 systems above a certain NMEA-16 concentration (0.18 M). No viscosity plateau is found between 0.18 and 0.20 M NMEA-16, indicating highly nonNewtonian behavior. The change in the zero shear viscosity (η0) with the alkanolamide concentration is presented in Figure 4. The η0 values have been calculated by extrapolating the viscosity data at a low shear rate back to a zero shear rate in the steady- (for low-viscosity samples) or oscillatoryshear measurements (see eq 4). With increasing NMEA12 and NMEA-16 concentrations, the zero shear viscosity increases at first, attains a maximum, and then decreases, suggesting structural changes in the system with increasing NMEA concentration. However, with NMEA-8, no significant structural change occurs in the micellar shape and size, as suggested by a small increase in the viscosity over a wide span of NMEA concentrations. Samples with no added alkanolamide have a low viscosity, similar to water. Upon addition of NMEA-12 or NMEA(19) Lortie, F.; Boileau, S.; Bouteiller, L.; Chassenieux, C.; Deme, B.; Ducouret, G.; Jalabert, M.; Laupretre, F.; Terech, P. Langmuir 2002, 18, 7218. (20) Keller, S. L.; Boltenhagen, P.; Pine, D. J.; Zasadzinski, J. A. Phys. Rev. Lett. 1998, 80, 2725.
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Figure 5. (a) Variation of the storage modulus G′ (circles), loss modulus G′′ (squares), and complex viscosity |η*| (triangles) as a function of the oscillatory shear frequency (ω) for the 0.15 M SDS/0.25 M NMEA-12 solution system at 25 °C. The lines show the best fitting to eqs 1, 2, and 4. (b) Cole-Cole plot of the values shown in part a.
16, a great increase in the zero shear viscosity is obtained. This increase is larger and sharper in the case of NMEA16: a concentration increase from 0.10 to 0.18 M causes a change in the viscosity of nearly 5 orders of magnitude. The most viscous samples in NMEA-16 systems (∼0.20 M NMEA-16) are gel-like, and they do not flow even when upside down (η0 > 1000 Pa‚s). Some of the samples are flow birefringent; that is, they are isotropic at rest but optically anisotropic when observed through crossed polarizers while some stress is applied, for example, by shaking the sample. Flow birefringence is typical for solutions containing wormlike micelles, and this phenomenon is attributed to a shear-induced first-order phasetransition isotropic solution-nematic liquid crystal.21,22 The concentration corresponding to a sharp increase in the viscosity is related to the overlap concentration c* at which the contact between polymerlike micelles starts. Above c*, entanglement occurs and a network is formed; therefore, the viscosity increases as a result of hindered diffusion between entangled micelles. Figure 4 suggests that the long-chain NMEA-16 is more prone to entanglement than NMEA-12. As mentioned previously, the viscosity decreases again at high alkanolamide concentrations. This decrease might be caused by changes in the structure of micelles, for example, breaking or branching;16 in the latter case, sliding of connections along the micelles can occur, which causes a decrease in the viscosity of the system.23,24 A maximum in the viscosity has also been related to a maximum in the micellar contour length.4,25 Further increase in the alkanolamide concentration causes liquid-crystal formation (21) Olmsted, P. D. Curr. Opin. Colloid Interface Sci. 1999, 4, 95. (22) Pujolle-Robic, C.; Olmsted, P. D.; Noirez, L. Europhys. Lett. 2002, 59, 364. (23) Drye, T. J.; Cates, M. E. J. Chem. Phys. 1992, 96, 1367. (24) Lequeux, F. Europhys. Lett. 1992, 19, 675. (25) Magid, L. J. J. Phys. Chem. 1998, 102, 4064.
Figure 6. Variation of G′, G′′, and |η*| as a function of ω for the 0.15 M SDS/NMEA-16 solution system at 25 °C for different NMEA-16 concentrations. (a) 0.19, (b) 0.20, and (c) 0.23 M. Notations are the same as those in Figure 5.
in the case of NMEA-12 or phase separation in the case of NMEA-16. Dynamic shear measurements were also carried out, and the results are presented in Figures 5 and 6. Figure 5 corresponds to the NMEA-12 sample with the highest viscosity. The storage modulus G′ is smaller than the loss modulus G′′ at low frequency, and the system behaves as a liquid. With an increasing frequency, G′ exceeds G′′, suggesting solidlike behavior. As can be seen in the fittings of Figure 5a and more clearly in the Cole-Cole plot (Figure 5b), this sample follows the Maxwell model, with a single relaxation time in the low-frequency region. This suggests that τb e τr.14 Values of ∼185 Pa‚s for the plateau modulus (G0) and ∼0.37 s for the relaxation time are obtained from eqs 1 and 2. The existence of G0 is a consequence of entanglements between wormlike aggregates. Experimental data for the complex viscosity also show a good fit to eq 4. At high frequency, G′′ deviates from Maxwell behavior. The deviation from Maxwell behavior observed in the Cole-Cole plot at high frequencies is probably caused by the Rouse mode of stress relaxation, which occurs at frequencies on the order of the inverse of the breaking time of micelles. Figure 6 shows the data for NMEA-16 systems. In the case of the 0.18 M NMEA-16 sample (Figure 6a), with increasing frequency, G′ increases more steeply than G′′, and this increase continues in the entire frequency range, with no plateau of G′ at the higher frequency. It suggests that there is no well-defined relaxation time (τb > τr), and, therefore, the behavior does not fit the Maxwell model.14
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Figure 7. DLS results showing the variation of the diffusion coefficient, D, with the scattering vector, q, for the (a) 0.15 M SDS/NMEA-12 and (b) 0.15 M SDS/NMEA-16 systems with different concentrations of NMEA at 25 °C.
This is similar to some polymer systems where many configurational motions of the flexible chains contribute to the decay of shear stress and, therefore, have a continuous spectrum of relaxation processes, such as reptation and local motions of the chains (Rouse modes).3,19 Figure 6b corresponds to the sample with the highest viscosity. G′ > G′′ in the whole frequency range studied; namely, there is no crossover of G′ and G′′, suggesting very long relaxation times (low ωc values) and solidlike behavior in the wide range of frequencies. The gel-like behavior of this sample may be a consequence of the long relaxation times of wormlike micelles, associated with reptation.16 Figure 6c shows a behavior similar to that in Figure 6a; the viscous component G′′ prevails in the low-frequency range. Therefore, structural changes in micelles seem to occur when the surfactant concentration varies, as already shown in Figure 4. Again, no plateau is found for G′, and the behavior departs from the Maxwell model as a result of multiple relaxation times. It should be pointed out that for the NMEA-16 samples above 0.18 M NMEA, the values of the steady-state viscosity (Figure 3) and the complex viscosity (Figure 6) deviate from one another, indicating that a structure (network) is destroyed by shearing. On the other hand, there is an obvious variation of G′ with the NMEA-16 concentration. This can be correlated with changes in the number of effective chains between cross-links. The sample with highest viscosity (Figure 6b) seems also to show the longest relaxation time. DLS. To carry out a preliminary study on microscopic structural changes, DLS measurements were performed on some of the systems for different concentrations. As can be seen in Figure 7a, the diffusion coefficient (D) tends to decrease as the NMEA-12 concentration increases, suggesting an increase in the size of the aggregates. For concentrations below 0.12 M, the diffusion coefficient practically does not change with the scattering vector, q, indicating weak interactions between aggregates. This tendency is not clear for the 0.15 M SDS/NMEA-12 sample. DLS data for NMEA-16 systems are shown in Figure 7b. The diffusion coefficient first decreases with the NMEA-16 concentration and then tends to become constant; namely, micellar growth is masked by intermicellar
interactions.1,26 The existence of these interactions is also suggested by the change in the diffusion coefficient with the scattering vector for NMEA-16 concentrations above 0.15 M, which also indicates the existence of more than one relaxation time.27,28 Diffusion coefficients for the most viscous samples are of the same order of magnitude of that reported for other systems containing wormlike micelles.27 Conclusion Phase diagrams of water/SDS/NMEA-8, NMEA-12, and NMEA-16 show that NMEA-12 can form a lamellar (LR) phase in the NMEA/water binary system, whereas NMEA16 forms a solid precipitate and NMEA-8 shows a liquidliquid phase separation. Upon addition of SDS, lamellar and hexagonal (H1) liquid-crystal phases form in all the systems. The liquid-crystal region expands as the alkyl chain increases, and the H1 phase is formed at lower concentration of SDS. Upon adding a small amount of NMEA to the aqueous micellar solution of the SDS phase, the viscosity dramatically increases and a viscoelastic solution is formed in the NMEA-12 and NMEA-16 systems, whereas no such increase is observed in the NMEA-8 system. Oscillatory shear rheology of the viscoelastic solution of the SDS/NMEA-12 system can be described by the Maxwell model typical of wormlike micelles. Consistent with the rheological measurements, DLS also suggests that NMEA-16 is more efficient than NMEA-12 in inducing the micellar growth. Acknowledgment. The authors are grateful to Dr. Kenji Aramaki (Yokohama National University) for help in the experimental work and preparation of the manuscript. C.R. thanks Comisio´n de Intercambio Cientı´fico de la Universidad de Los Andes and FUNDACITE, Venezuela, for financial support during his stay at Yokohama National University. D.P.A. is thankful to Kathmandu University, Nepal, for providing study leave. LA0348923 (26) Nicoli, D. F.; Dorshow, R. B.; Bunton, C. A. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984. (27) Imae, T. J. Phys. Chem. 1990, 94, 5953. (28) Claire, K.; Pecora, R. J. Phys. Chem. B. 1997, 101, 746.
