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Water System at Low Surfactant and Alcohol Concentrations
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
202, 232–237 (1998)
CS985404
Phase and Rheological Behavior of the Dodecyl Tetraethylene Glycol/Benzyl Alcohol/Water System at Low Surfactant and Alcohol Concentrations Gemma Montalvo, Elvira Rodenas, and Mercedes Valiente 1 Dpto. QuıB mica FıB sica, Universidad de Alcala´, E-28871 Madrid, Spain Received April 3, 1997; accepted January 6, 1998
The phase diagram of the ternary dodecyltetraethylene glycol (C12E4 )/benzyl alcohol/water system with a surfactant concentration of up to 0.2 M was determined. With an increase in the alcohol concentration, the sequence of the phases was Lal , Lah , L3 /Lah , L3 , and L3 /L. At low alcohol and surfactant concentrations, the anomalous lamellar phase L / a also appeared. Upon comparing the phase sequence of the C12E4 ternary system with that of medium chain alkanols, we concluded that benzyl alcohol stabilizes the lamellar phase. The rheological study allowed us to establish the boundary between the Lal and the Lah subregions. At intermediate alcohol concentration, a two-phase region (in which both phases coexist) formed in the phase diagram. There is a critical shear rate that induces the transition from extensive bilayers (Lah ) to vesicles (Lal ). q 1998 Academic Press Key Words: dodecyltetraethylene glycol; surfactant; lamellar phase; liquid crystal; phase behavior; rheology.
INTRODUCTION
It is well known that ionic surfactant/alcohol/water ternary systems form different phases such as the L1 and L2 isotropic phases and the lamellar La , hexagonal H1 , and cubic liquid crystalline phases (1). Binary systems of nonionic polyoxyethylene surfactants with water also form these mesophases (2). For dilute zwitterionic and nonionic surfactant solutions, other bilayer-type structures, L a/ , Lal , Lah , and L3 , with alcohol concentrations between the L1 and L2 micellar regions, were described. All these phases, except for the L a/ one, were characterized in alkyldimethylamine oxides/intermediate chain n-alcohols/water systems by freeze–fracture electron microscopy (3, 4). Jonstro¨mer and Strey (5) studied the effect of alcohols on the binary diagram water/dodecyl pentaethylene glycol (C12E5 ) and found different subregions in the lamellar phases with characteristics similar to the zwitterionic systems. The macroscopic appearance of these bilayer-type structures changes from phase 1
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L a/ (5) to phase L3 with an increase in the alcohol concentration at a fixed surfactant content. Lal and Lah are birefrigent, and lamellar phases, L a/ and L3 , are optically isotropic, with streaming birefrigence and located at, respectively, smaller and higher alcohol concentrations than the lamellar phase. The Lal found in zwitterionic systems as shown in the micrographs obtained with FF-TEM (6) is made up of vesicles. The Lah or dilute lamellar phase is made up of flexible bimolecular sheets of macroscopic dimensions. This is the typical lamellar phase structure. The microstructure of L a/ , however, has never been observed although Strey et al. (7) suggest that stable vesicles do exist. The streaming birefrigence of L3 shows a bicontinuous structure of bilayers that can be oriented in the shear direction to which the solution flows. This phase, because of its structure, is also known as the sponge phase (8). Due to its peculiar macroscopic appearance, in previous papers it was called an anomalous phase (9). L a/ is also called an anomalous lamellar phase (5, 10). L3 is very sensitive to ionic charge, and in ionic systems its formation requires the addition of salt. This phase appears in different systems such as those of n-alkyl polyglycol ethers/ water (9, 11–13), n-alkyldimethylaminoxide surfactant/ hexanol/water (3, 4, 6, 14), n-dodecylbetaine/pentanol/ water (15), and ionic surfactant/brine/water, which include AOT/NaCl/water (16–18), SDS/pentanol/brine/water (19), and CPyBr/hexanol/brine/water (20). The existence of the L3 phase has also been described from a theoretical point of view (21, 22). Medium chain alkanols acted as cosurfactants in all the systems studied, and the ternary C12E4 /hexanol/water system had been studied previously (5, 10). Systems with short chain alkanol or benzyl alcohol, which favor the transitions from direct to reverse micelles without phase separation in CTAB (1) have not been studied extensively. The objective of this manuscript is to establish the effects of benzyl alcohol on the formation of these kinds of bilayer-type structures with dodecyl tetraethylene glycol (C12E4 ) because benzyl alcohol favors the transition from direct to reverse micelles without phase separation in CTAB (23).
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BEHAVIOR OF A C12E4 TERNARY SYSTEM
MATERIALS AND METHODS
The nonionic surfactant tetraethylene glycol monododecyl ether (C12E4 ) was purchased from Nikkol Ltd. Tokyo, and the benzyl alcohol (z.a.) was from Merck. Samples for the phase diagram were prepared by weighing the desired amounts of nonionic surfactant and benzyl alcohol. Pure water was added to obtain 10 mL of solution in calibrated glass tubes. The samples were kept in a water bath at 25.0 { 0.17C for a minimum of 6 months. To ensure phases in equilibrium phases, the samples were mixed and allowed to re-equilibrate several times. Reproduced results are assumed to reflect equilibrium behavior. Phase behavior was determined by visual observation between crossed polarizers and optical microscopy with a Laborlux S LEITZ. Rheological measurements were carried out at 25.0 { 0.17C by a Carri-Med CLS 100 controlled stress rheometer using a cone – plate configuration. The cone angle was 1 7, and its diameter was 4 cm. The temperature of the system was controlled, and a humidification chamber containing wet sponges was used to prevent the evaporation of the sample during the measurement. Data from both steady flow ( shear stress sweep mode ) and oscillatory experiments were collected. Determination of dynamic storage ( G * ) and loss ( G 9 ) moduli were always made in a domain of linear viscoelasticity. RESULTS AND DISCUSSION
Phase Diagram Figure 1 shows the phase diagram of the C12E4 /benzyl alcohol/water system for low surfactant and alcohol concentrations at 25.0 { 0.17C. The sequence of the phases with alcohol concentration, Lal , Lah , L3 /Lah , L3 , is similar to the one described in the C12E4 /hexanol/water system (10) for the same amount of nonionic surfactant. In Fig. 1, solid lines represent macroscopic phase separation and dotted dash lines represent phase separation by visual observation with crossed polarizers. The dashed lines in this figure represent our interpretation of the rheological behavior. The lamellar phase is optically anisotropic and its birefrigence increases with the alcohol concentration. From a macroscopic point of view, two different subregions can be delimited. The subregion at alcohol concentrations lower than 60 mM is turbid and weakly birefrigent. Its appearance is similar to the one found by Hoffmann et al. (3, 4, 6, 16) in alkyldimethylaminoxide/alcohol/water systems. It is termed Lal and characterized by vesicles. At a higher alcohol concentration, the lamellar phase samples are transparent and show strong birefrigent multidomains, as occurs in a typical lamellar phase built up from flexible bimolecular sheets of macroscopic dimensions, Lah . The boundary between both subregions (Lal and Lah ) is not clear because there is not an
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abrupt change in the optical properties, and for most of the systems there is no evidence of phase separation. In this system, however, those samples with a surfactant concentration of 200 mM and alcohol concentration ranging from 40 to 60 mM, separate into two macroscopic phases that can be visualized with crossed polarizers. At the top, the samples look like Lah with strong birefrigent multidomains, and at the bottom they look like Lal , turbid and weakly birefrigent. Both subregions also show a different appearance when observed through an optical microscope with polarizers. While the Lal phase is obviously birefrigent in a 15-mm diameter test tube, no birefrigence is visible between slide and cover slip. The Lah phase, however, shows different birefrigent domains with oily streaks. At surfactant concentrations below 100 mM and alcohol concentrations lower than 25 mM another different lamellar subregion appears. This phase, even though it is optically isotropic, has flow birefrigence. The samples in the L3 phase show similar characteristics but they are less viscous. In the binary phase diagram of C12E4 /water, had already been located (7) between 0 and 35 wt% surfactant content at 25.0 { 0.17C a phase with the same macroscopic properties, and called L a/ . This anomalous isotropic phase was also observed for alkyl polyethoxylates ternary systems within the typical lamellar phase (5). Adding alcohol to the lamellar phase causes the formation of an isotropic phase, L3 , which is translucent and shows flow birefrigence. This phase extends in a narrow range of alcohol concentration that increases with the amount of surfactant. By comparing it with the phase diagram of the C12E4 / hexanol/water system at 100 mM of surfactant concentration (10), we find that the lamellar phase, which covers Lal and Lah , extends to 120 mM hexanol. In the systems with benzyl alcohol it extends to 158 mM. This means that the aromatic group located in the bilayer stabilizes the lamellar structure. Moreover, the subregion L a/ does not appear in the system with hexanol. Rheological Behavior We studied the rheological behavior of the lamellar and L3 phases with flow and oscillation experiments. Rheological properties in lamellar systems are time-dependent. First, we analyzed the influence of preshear stress on the rheological behavior of the samples in the lamellar phase. To do this, we used different measures of preshear stress for a period of 10 min. If the applied preshear stress was lower than 0.01 Pa, there was no change in the flow curve. The 0.01 Pa preshear stress was applied for 10 min so that the same experimental conditions could be maintained before data collection. The effect of the measurement time on the data collection was also studied. Any change in the flow curves appears after 20 min of measurement time.
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FIG. 1. Phase diagram for the C12E4 /benzyl alcohol/water system at 25.0 { 0.17C.
The flow curves for samples with [C12E4 ] Å 150 mM and different alcohol concentrations are given in Fig. 2. In all the samples, and for the complete range of shear rates, the viscosity decreases when the amount of alcohol increases. Three different kinds of curves related to the different microstructures of the lamellar phase can be seen.
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(i) For samples with [PhCH2OH] Å 14 and 60 mM, the viscosity values decrease for the whole range of shear rates and at low shear rates it exhibits a tendency to a constant zero-shear-rate viscosity, h0 . An Lal phase built up from vesicles could explain this rheological behavior. The vesicles, probably spherical, behave like a Newtonian fluid at
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FIG. 2. Viscosity as a function of the shear rate for samples with 150 mM C12E4 at 25.0 { 0.17C.
low shear rates with a constant viscosity value, h0 . When the shear rate increases, the vesicles can be packed closely together in the SDS/dodecane/pentanol/water system as described Panizza et al. (24). As a consequence the viscosity decreases. (ii) Samples with [PhCH2OH] Å 101 and 119 mM show a maximum viscosity value with a shear rate around 15 s 01 . Above the maximum, the viscosity values decrease to a constant infinite-shear-rate viscosity value, h` . (iii) The third kind of curve corresponds to samples with a higher alcohol content, [PhCH2OH] Å 161 and 195 mM, which behave like a plastic fluid such as a typical lamellar phase, the Lah phase. These solutions have the smaller viscosity values of the lamellar phase. The most interesting flow curves are the second type where a maximum appears. If we analyze this flow curve in more detail, we see that the maximum is located at a welldefined shear rate (Fig. 3). We can define the shear rate of the maximum as the critical shear rate involved with structural changes. At low shear rates, the sample behaves like a plastic fluid, such as the Lah phase, with extensive bilayers. With the increase of shear rates, some of these bilayers start to close, resulting in the formation of new vesicles that are more viscous than extended bilayers. Above the maximum, most of the bilayers form closed structures (vesicles) and behave in the same way as the Lal phase. When not applying any shear rate, both vesicles and extensive bilayers coexist (Lal / Lah phase) above the critical shear rate the extensive bilayers are oriented in multilayer vesicles, the Lal phase. The same kind of maximum value curves were observed
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for different samples with different compositions. All of these were located in the middle of the lamellar phase and at surfactant concentrations higher than 80 mM (represented by dashed lines in Fig. 1). When the preshear stress applied to all of these samples was increased until the shear stress corresponded to the maximum, the maxima disappeared and the flow curves were similar to the ones obtained for the lower alcohol concentration samples in the Lal subregion (Fig. 4). These samples change from transparent to turbid when the solution flows in the graduated glass tubes. Different structures induced by shearing are also described in the literature. The one most commonly described is the change from L3 to Lah when shearing (25–27, 3), but the formation of relatively monodispersed multilayer vesicles by shearing the lamellar phase in the system AOT/brine/ water (28) has also been observed by freeze-fracture electron microscopy. The viscosity at a fixed shear rate below the critical shear rate, gg Å 1 s 01 , vs alcohol concentration is given in Fig. 5. The viscosity values increase with the alcohol concentration until 50 mM (the Lal phase) because the vesicles grow and show a maximum value. This maximum is nearly independent of the amount of C12E4 in the sample. Once the vesicles are large enough they start deforming or even opening. This produces a decrease in the viscosity. At these concentrations, the vesicles of Lal coexist with the extensive bilayers of Lah . The viscosity at a shear rate close to the critical shear rate, gg Å 14 s 01 , vs alcohol concentration is given in Fig. 6. Above [C12E4 ] Å 100 mM, viscosity curves show two maxima with the alcohol concentration that could be related
FIG. 3. Viscosity as a function of the shear rate for samples in the middle of the lamellar phase at 25.0 { 0.17C.
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FIG. 4. Viscosity as a function of the shear rate at 25.0 { 0.17C; preshear stress: 0.01 Pa (closed symbols) and 0.3 Pa (open symbols).
FIG. 6. Viscosity at 14 s 01 shear rate as a function of the alcohol concentration for different surfactant concentrations at 25.0 { 0.17C.
to changes in the structure of the lamellar phase. The first maximum corresponds to the maximum also observed at the smaller shear rate. The second maximum moves to higher benzyl alcohol contents with a nonionic surfactant concentration, and it only appears in the samples that show the maximum viscosity vs the shear rate in the flow curves. We can conclude that this second maximum is due to the increase
in viscosity produced by the formation of new vesicles during shearing. The samples of the L3 phase show a nearly Newtonian behavior (Fig. 7). At low shear rates, the viscosity is constant with a value three or four times that of water viscosity, but it decreases at higher shear rates when the random interconnected bilayers are oriented in the flow direction.
FIG. 5. Viscosity at 1 s 01 shear rate as a function of the alcohol concentration for different surfactant concentrations at 25.0 { 0.17C.
FIG. 7. Viscosity as a function of the shear rate for L3 samples at 25.0 { 0.17C.
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The viscoelastic properties of the system were studied with oscillatory experiments. In all lamellar phase samples, the storage moduli (G 9 ) are greater than the loss moduli (G * ). The loss modulus becomes smaller with the alcohol concentration and it is neglected in the L3 phase. The samples with a more elastic response correspond to Lal which is built up from vesicles. From all these results we can conclude that there is a good correlation between the proposed structures and the flow curves. The samples of the Lal subregion should be built up from vesicles, probably spherical, which are packed closely together when the shear rate increases. The samples of the Lah subregion behave as a plastic fluid as is well described in the literature for the typical lamellar phase. At the intermediate region both structures coexist and Lal changes to Lah under shear. ACKNOWLEDGMENTS This work has been financially supported by the CICYT through the PB92-1080 program and by the Universidad de Alcala´ through the 043/96 program, to whom the authors are grateful. G.M. also thanks the Ministerio de Educacio´n y Ciencia for the financial support. The authors thank Mrs. M. Heijnen for assistance with the preparation of the manuscript.
REFERENCES 1. Fontell, K., Khan, A., Lindstro¨m, b., Maciejewska, D., and Puag-Negern, S., Colloid Polym. Sci. 269, 727 (1991). 2. Mitchell, D. J., Tiddy, G. J. T., Warine, L., Bostock, M. P., and Mc Donald, M. P., J. Chem. Soc., Faraday Trans. 1, 79, 975 (1983). 3. Hoffmann, H., Thunig, C., and Valiente, M., Colloids Surf. 67, 223 (1992). 4. Platz, G., Thunig, C., and Hoffmann, H., Ber. Bunsenges. Phys. Chem. 96, 667 (1992). 5. Jonstro¨mer, M., and Strey, R., J. Phys. Chem. 96, 5993 (1992). 6. Hoffmann, H., Munkert, U., Thunig, C., and Valiente, M., J. Colloid Interface Sci. 163, 217 (1994).
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7. Olsson, U., Nakamura, K., Kunieda, H., and Strey, R., Langmuir 12, 3045 (1996). 8. Cates, M. E., Roux, D., Andelman, D., Milner, S. T., and Safran, S. A., Europhys. Lett. 5, 733 (1988). 9. Lang, J. C., and Morgan, R. D., J. Chem. Phys. 73, 5489 (1980). 10. Valiente, M., Colloids Surf. A: Physicochem. Eng. Aspects 105, 265 (1995). 11. Nilsson, P. G., and Lindman, B., J. Phys. Chem. 88, 4764 (1984). 12. Anderson, D., Wennerstro¨m, D., and Olsson, U., J. Phys. Chem. 93, 4243 (1989). 13. Mori, F., Lim, J. C., Raney, O. G., Elsik, C. M., and Miller, C. A., Colloids Surf. 40, 323 (1989). 14. Miller, C. A., Gradzielski, M., Hoffmann, H., Kra¨mer, U., and Thunig, C., Colloid Polym. Sci. 268, 1066 (1990). 15. Marignan, J., Gauthier-Fournier, F., Appell, J., Akoum, F., and Lang, J., J. Phys. Chem. 92, 440 (1988). 16. Miller, C. A., and Ghosh, O., Langmuir 2, 321 (1986). 17. Anderson, D., Wennerstro¨m, H., and Olsson, U., J. Phys. Chem. 93, 4243 (1989). 18. Balinov, B., Olsson, U., and So¨derman, O., J. Phys. Chem. 95, 593 (1992). 19. Gazeau, D., Bellocq, A. M., Roux, D., and Zemb, B., Prog. Colloid Polym. Sci. 79, 226 (1989). 20. Gomati, R., Appell, J., Bassereau, P., Marignan, J., and Porte, G., J. Phys. Chem. 91, 6203 (1987). 21. Gomper, G., Goos, J., and Kreus, M., Ber. Bunsenges Phys. Chem. 98, 501 (1994). 22. Gomper, G., and Schik, S., Phys. Rev. E 49(2), 1478 (1994). 23. Montalvo, G., Valiente, M., and Rodenas, E., J. Colloid Interface Sci. 172, 494 (1995). 24. Panizza, R., Roux, D., Vuillaume, V., Lu, C.-Y. D., and Cates, M. E., Langmuir 12, 248 (1996). 25. Hoffmann, H., Hofmann, S., Rauscher, A., and Kalus, J., Prog. Colloid Polym. Sci. 84, 24 (1991). 26. Hoffmann, H., Thunig, C., Munkert, U., Meyer, H. W., and Richter, W., Langmuir 8, 2629 (1992). 27. Mahjoub, H. F., McGrath, K. M., and Kle´man, M., Langmuir 12, 3131 (1996). 28. Gulik-Krzywicki, T., Dedieu, J. C., Roux, D., Degert, C., and Laversanne, R., Langmuir 12, 2668 (1996). 29. Paasch, S., Schambil, F., and Schwunger, M. J., Langmuir 5, 1344 (1989).
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202, 232–237 (1998)
CS985404
Phase and Rheological Behavior of the Dodecyl Tetraethylene Glycol/Benzyl Alcohol/Water System at Low Surfactant and Alcohol Concentrations Gemma Montalvo, Elvira Rodenas, and Mercedes Valiente 1 Dpto. QuıB mica FıB sica, Universidad de Alcala´, E-28871 Madrid, Spain Received April 3, 1997; accepted January 6, 1998
The phase diagram of the ternary dodecyltetraethylene glycol (C12E4 )/benzyl alcohol/water system with a surfactant concentration of up to 0.2 M was determined. With an increase in the alcohol concentration, the sequence of the phases was Lal , Lah , L3 /Lah , L3 , and L3 /L. At low alcohol and surfactant concentrations, the anomalous lamellar phase L / a also appeared. Upon comparing the phase sequence of the C12E4 ternary system with that of medium chain alkanols, we concluded that benzyl alcohol stabilizes the lamellar phase. The rheological study allowed us to establish the boundary between the Lal and the Lah subregions. At intermediate alcohol concentration, a two-phase region (in which both phases coexist) formed in the phase diagram. There is a critical shear rate that induces the transition from extensive bilayers (Lah ) to vesicles (Lal ). q 1998 Academic Press Key Words: dodecyltetraethylene glycol; surfactant; lamellar phase; liquid crystal; phase behavior; rheology.
INTRODUCTION
It is well known that ionic surfactant/alcohol/water ternary systems form different phases such as the L1 and L2 isotropic phases and the lamellar La , hexagonal H1 , and cubic liquid crystalline phases (1). Binary systems of nonionic polyoxyethylene surfactants with water also form these mesophases (2). For dilute zwitterionic and nonionic surfactant solutions, other bilayer-type structures, L a/ , Lal , Lah , and L3 , with alcohol concentrations between the L1 and L2 micellar regions, were described. All these phases, except for the L a/ one, were characterized in alkyldimethylamine oxides/intermediate chain n-alcohols/water systems by freeze–fracture electron microscopy (3, 4). Jonstro¨mer and Strey (5) studied the effect of alcohols on the binary diagram water/dodecyl pentaethylene glycol (C12E5 ) and found different subregions in the lamellar phases with characteristics similar to the zwitterionic systems. The macroscopic appearance of these bilayer-type structures changes from phase 1
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L a/ (5) to phase L3 with an increase in the alcohol concentration at a fixed surfactant content. Lal and Lah are birefrigent, and lamellar phases, L a/ and L3 , are optically isotropic, with streaming birefrigence and located at, respectively, smaller and higher alcohol concentrations than the lamellar phase. The Lal found in zwitterionic systems as shown in the micrographs obtained with FF-TEM (6) is made up of vesicles. The Lah or dilute lamellar phase is made up of flexible bimolecular sheets of macroscopic dimensions. This is the typical lamellar phase structure. The microstructure of L a/ , however, has never been observed although Strey et al. (7) suggest that stable vesicles do exist. The streaming birefrigence of L3 shows a bicontinuous structure of bilayers that can be oriented in the shear direction to which the solution flows. This phase, because of its structure, is also known as the sponge phase (8). Due to its peculiar macroscopic appearance, in previous papers it was called an anomalous phase (9). L a/ is also called an anomalous lamellar phase (5, 10). L3 is very sensitive to ionic charge, and in ionic systems its formation requires the addition of salt. This phase appears in different systems such as those of n-alkyl polyglycol ethers/ water (9, 11–13), n-alkyldimethylaminoxide surfactant/ hexanol/water (3, 4, 6, 14), n-dodecylbetaine/pentanol/ water (15), and ionic surfactant/brine/water, which include AOT/NaCl/water (16–18), SDS/pentanol/brine/water (19), and CPyBr/hexanol/brine/water (20). The existence of the L3 phase has also been described from a theoretical point of view (21, 22). Medium chain alkanols acted as cosurfactants in all the systems studied, and the ternary C12E4 /hexanol/water system had been studied previously (5, 10). Systems with short chain alkanol or benzyl alcohol, which favor the transitions from direct to reverse micelles without phase separation in CTAB (1) have not been studied extensively. The objective of this manuscript is to establish the effects of benzyl alcohol on the formation of these kinds of bilayer-type structures with dodecyl tetraethylene glycol (C12E4 ) because benzyl alcohol favors the transition from direct to reverse micelles without phase separation in CTAB (23).
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BEHAVIOR OF A C12E4 TERNARY SYSTEM
MATERIALS AND METHODS
The nonionic surfactant tetraethylene glycol monododecyl ether (C12E4 ) was purchased from Nikkol Ltd. Tokyo, and the benzyl alcohol (z.a.) was from Merck. Samples for the phase diagram were prepared by weighing the desired amounts of nonionic surfactant and benzyl alcohol. Pure water was added to obtain 10 mL of solution in calibrated glass tubes. The samples were kept in a water bath at 25.0 { 0.17C for a minimum of 6 months. To ensure phases in equilibrium phases, the samples were mixed and allowed to re-equilibrate several times. Reproduced results are assumed to reflect equilibrium behavior. Phase behavior was determined by visual observation between crossed polarizers and optical microscopy with a Laborlux S LEITZ. Rheological measurements were carried out at 25.0 { 0.17C by a Carri-Med CLS 100 controlled stress rheometer using a cone – plate configuration. The cone angle was 1 7, and its diameter was 4 cm. The temperature of the system was controlled, and a humidification chamber containing wet sponges was used to prevent the evaporation of the sample during the measurement. Data from both steady flow ( shear stress sweep mode ) and oscillatory experiments were collected. Determination of dynamic storage ( G * ) and loss ( G 9 ) moduli were always made in a domain of linear viscoelasticity. RESULTS AND DISCUSSION
Phase Diagram Figure 1 shows the phase diagram of the C12E4 /benzyl alcohol/water system for low surfactant and alcohol concentrations at 25.0 { 0.17C. The sequence of the phases with alcohol concentration, Lal , Lah , L3 /Lah , L3 , is similar to the one described in the C12E4 /hexanol/water system (10) for the same amount of nonionic surfactant. In Fig. 1, solid lines represent macroscopic phase separation and dotted dash lines represent phase separation by visual observation with crossed polarizers. The dashed lines in this figure represent our interpretation of the rheological behavior. The lamellar phase is optically anisotropic and its birefrigence increases with the alcohol concentration. From a macroscopic point of view, two different subregions can be delimited. The subregion at alcohol concentrations lower than 60 mM is turbid and weakly birefrigent. Its appearance is similar to the one found by Hoffmann et al. (3, 4, 6, 16) in alkyldimethylaminoxide/alcohol/water systems. It is termed Lal and characterized by vesicles. At a higher alcohol concentration, the lamellar phase samples are transparent and show strong birefrigent multidomains, as occurs in a typical lamellar phase built up from flexible bimolecular sheets of macroscopic dimensions, Lah . The boundary between both subregions (Lal and Lah ) is not clear because there is not an
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abrupt change in the optical properties, and for most of the systems there is no evidence of phase separation. In this system, however, those samples with a surfactant concentration of 200 mM and alcohol concentration ranging from 40 to 60 mM, separate into two macroscopic phases that can be visualized with crossed polarizers. At the top, the samples look like Lah with strong birefrigent multidomains, and at the bottom they look like Lal , turbid and weakly birefrigent. Both subregions also show a different appearance when observed through an optical microscope with polarizers. While the Lal phase is obviously birefrigent in a 15-mm diameter test tube, no birefrigence is visible between slide and cover slip. The Lah phase, however, shows different birefrigent domains with oily streaks. At surfactant concentrations below 100 mM and alcohol concentrations lower than 25 mM another different lamellar subregion appears. This phase, even though it is optically isotropic, has flow birefrigence. The samples in the L3 phase show similar characteristics but they are less viscous. In the binary phase diagram of C12E4 /water, had already been located (7) between 0 and 35 wt% surfactant content at 25.0 { 0.17C a phase with the same macroscopic properties, and called L a/ . This anomalous isotropic phase was also observed for alkyl polyethoxylates ternary systems within the typical lamellar phase (5). Adding alcohol to the lamellar phase causes the formation of an isotropic phase, L3 , which is translucent and shows flow birefrigence. This phase extends in a narrow range of alcohol concentration that increases with the amount of surfactant. By comparing it with the phase diagram of the C12E4 / hexanol/water system at 100 mM of surfactant concentration (10), we find that the lamellar phase, which covers Lal and Lah , extends to 120 mM hexanol. In the systems with benzyl alcohol it extends to 158 mM. This means that the aromatic group located in the bilayer stabilizes the lamellar structure. Moreover, the subregion L a/ does not appear in the system with hexanol. Rheological Behavior We studied the rheological behavior of the lamellar and L3 phases with flow and oscillation experiments. Rheological properties in lamellar systems are time-dependent. First, we analyzed the influence of preshear stress on the rheological behavior of the samples in the lamellar phase. To do this, we used different measures of preshear stress for a period of 10 min. If the applied preshear stress was lower than 0.01 Pa, there was no change in the flow curve. The 0.01 Pa preshear stress was applied for 10 min so that the same experimental conditions could be maintained before data collection. The effect of the measurement time on the data collection was also studied. Any change in the flow curves appears after 20 min of measurement time.
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FIG. 1. Phase diagram for the C12E4 /benzyl alcohol/water system at 25.0 { 0.17C.
The flow curves for samples with [C12E4 ] Å 150 mM and different alcohol concentrations are given in Fig. 2. In all the samples, and for the complete range of shear rates, the viscosity decreases when the amount of alcohol increases. Three different kinds of curves related to the different microstructures of the lamellar phase can be seen.
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(i) For samples with [PhCH2OH] Å 14 and 60 mM, the viscosity values decrease for the whole range of shear rates and at low shear rates it exhibits a tendency to a constant zero-shear-rate viscosity, h0 . An Lal phase built up from vesicles could explain this rheological behavior. The vesicles, probably spherical, behave like a Newtonian fluid at
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FIG. 2. Viscosity as a function of the shear rate for samples with 150 mM C12E4 at 25.0 { 0.17C.
low shear rates with a constant viscosity value, h0 . When the shear rate increases, the vesicles can be packed closely together in the SDS/dodecane/pentanol/water system as described Panizza et al. (24). As a consequence the viscosity decreases. (ii) Samples with [PhCH2OH] Å 101 and 119 mM show a maximum viscosity value with a shear rate around 15 s 01 . Above the maximum, the viscosity values decrease to a constant infinite-shear-rate viscosity value, h` . (iii) The third kind of curve corresponds to samples with a higher alcohol content, [PhCH2OH] Å 161 and 195 mM, which behave like a plastic fluid such as a typical lamellar phase, the Lah phase. These solutions have the smaller viscosity values of the lamellar phase. The most interesting flow curves are the second type where a maximum appears. If we analyze this flow curve in more detail, we see that the maximum is located at a welldefined shear rate (Fig. 3). We can define the shear rate of the maximum as the critical shear rate involved with structural changes. At low shear rates, the sample behaves like a plastic fluid, such as the Lah phase, with extensive bilayers. With the increase of shear rates, some of these bilayers start to close, resulting in the formation of new vesicles that are more viscous than extended bilayers. Above the maximum, most of the bilayers form closed structures (vesicles) and behave in the same way as the Lal phase. When not applying any shear rate, both vesicles and extensive bilayers coexist (Lal / Lah phase) above the critical shear rate the extensive bilayers are oriented in multilayer vesicles, the Lal phase. The same kind of maximum value curves were observed
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for different samples with different compositions. All of these were located in the middle of the lamellar phase and at surfactant concentrations higher than 80 mM (represented by dashed lines in Fig. 1). When the preshear stress applied to all of these samples was increased until the shear stress corresponded to the maximum, the maxima disappeared and the flow curves were similar to the ones obtained for the lower alcohol concentration samples in the Lal subregion (Fig. 4). These samples change from transparent to turbid when the solution flows in the graduated glass tubes. Different structures induced by shearing are also described in the literature. The one most commonly described is the change from L3 to Lah when shearing (25–27, 3), but the formation of relatively monodispersed multilayer vesicles by shearing the lamellar phase in the system AOT/brine/ water (28) has also been observed by freeze-fracture electron microscopy. The viscosity at a fixed shear rate below the critical shear rate, gg Å 1 s 01 , vs alcohol concentration is given in Fig. 5. The viscosity values increase with the alcohol concentration until 50 mM (the Lal phase) because the vesicles grow and show a maximum value. This maximum is nearly independent of the amount of C12E4 in the sample. Once the vesicles are large enough they start deforming or even opening. This produces a decrease in the viscosity. At these concentrations, the vesicles of Lal coexist with the extensive bilayers of Lah . The viscosity at a shear rate close to the critical shear rate, gg Å 14 s 01 , vs alcohol concentration is given in Fig. 6. Above [C12E4 ] Å 100 mM, viscosity curves show two maxima with the alcohol concentration that could be related
FIG. 3. Viscosity as a function of the shear rate for samples in the middle of the lamellar phase at 25.0 { 0.17C.
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FIG. 4. Viscosity as a function of the shear rate at 25.0 { 0.17C; preshear stress: 0.01 Pa (closed symbols) and 0.3 Pa (open symbols).
FIG. 6. Viscosity at 14 s 01 shear rate as a function of the alcohol concentration for different surfactant concentrations at 25.0 { 0.17C.
to changes in the structure of the lamellar phase. The first maximum corresponds to the maximum also observed at the smaller shear rate. The second maximum moves to higher benzyl alcohol contents with a nonionic surfactant concentration, and it only appears in the samples that show the maximum viscosity vs the shear rate in the flow curves. We can conclude that this second maximum is due to the increase
in viscosity produced by the formation of new vesicles during shearing. The samples of the L3 phase show a nearly Newtonian behavior (Fig. 7). At low shear rates, the viscosity is constant with a value three or four times that of water viscosity, but it decreases at higher shear rates when the random interconnected bilayers are oriented in the flow direction.
FIG. 5. Viscosity at 1 s 01 shear rate as a function of the alcohol concentration for different surfactant concentrations at 25.0 { 0.17C.
FIG. 7. Viscosity as a function of the shear rate for L3 samples at 25.0 { 0.17C.
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The viscoelastic properties of the system were studied with oscillatory experiments. In all lamellar phase samples, the storage moduli (G 9 ) are greater than the loss moduli (G * ). The loss modulus becomes smaller with the alcohol concentration and it is neglected in the L3 phase. The samples with a more elastic response correspond to Lal which is built up from vesicles. From all these results we can conclude that there is a good correlation between the proposed structures and the flow curves. The samples of the Lal subregion should be built up from vesicles, probably spherical, which are packed closely together when the shear rate increases. The samples of the Lah subregion behave as a plastic fluid as is well described in the literature for the typical lamellar phase. At the intermediate region both structures coexist and Lal changes to Lah under shear. ACKNOWLEDGMENTS This work has been financially supported by the CICYT through the PB92-1080 program and by the Universidad de Alcala´ through the 043/96 program, to whom the authors are grateful. G.M. also thanks the Ministerio de Educacio´n y Ciencia for the financial support. The authors thank Mrs. M. Heijnen for assistance with the preparation of the manuscript.
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