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Rheology, Optical Microscopy, and Electron Microscopy of Cationic Surfactant Gels
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
180, 261–268 (1996)
0298
Rheology, Optical Microscopy, and Electron Microscopy of Cationic Surfactant Gels ALEXANDRE GOLDSZAL,* ,† ALEXANDER M. JAMIESON,‡ J. ADIN MANN, JR .,* ,‡ JOSEPH POLAK,§ AND CHARLES ROSENBLATT† ,‡ ,1 *Department of Chemical Engineering, †Department of Physics, ‡Department of Macromolecular Science, and §Department of Neurosciences, Case Western Reserve University, Cleveland, Ohio 44106 Received September 14, 1995; accepted December 11, 1995
A cationic surfactant mixture is studied in aqueous solution by rheological and imaging techniques. We find that the system becomes gel-like above a threshold surfactant concentration, as evidenced by the appearance of an apparent yield stress and a viscosity which is highly non-Newtonian. Hysteresis is observed in the shear stress vs shear rate measurements: the formation of shear-induced microstructures at high shear rates may cause shear thickening. Electron micrographs indicate three distinct structures in the sample: lamellae, large onion-like globules, and small nodules. These structures are discussed in the context of the rheological data. The density of the globular structures in sheared samples is observed to be larger than in unsheared samples. q 1996 Academic Press, Inc.
Key Words: surfactant gels; rheology; electron microscopy; complex fluids; micelles.
INTRODUCTION
Owing to their importance in both industry and basic science, surfactant molecules have been the subject of intense investigation for many years. In its most basic form a surfactant molecule possesses a polar hydrophilic head group and a nonpolar paraffinic tail. At sufficiently high concentrations, in an aqueous environment, surfactants can form a variety of structures and phases, including spherical and elongated micelles, lamellae, and hexagonal and cubic phases (1–4). Over the years, a battery of experimental tools has been used to investigate the structure of these lyotropic systems, with recent advances in imaging being among the most useful. Additionally, a renaissance in rheological studies of surfactant systems has provided important insights into not only the dynamics, but into the structure of these materials as well (5–9). The viscosity of micellar solutions composed of cationic surfactant molecules at concentrations close to the critical micelle concentration (CMC) is Newtonian in behavior. 1
To whom correspondence should be addressed.
Upon increasing the surfactant concentration, however, a variety of phase structures may appear. Initially, the system generally becomes significantly more viscous and then, at sufficiently high concentrations, it may become viscoelastic, behavior indicative of network formation. Under small stresses these concentrated lyotropic systems are primarily elastic, i.e., they exhibit an equilibrium modulus, although they flow above a limiting yield stress value (10). The rheological and mechanical properties of surfactant gels are directly linked to the structure of the network which is formed by the self associating molecules and to the various interactions between the micellar aggregates (2, 3). This gel-like behavior may be commercially important since concentrated surfactant solutions are frequently used in paints, cosmetics, and the food industry. In the detergent industry, on the other hand, surfactants are used for their surface active properties, and viscous concentrated formulations are often undesirable in practice. Although rheology provides an important analytic tool for our understanding of lyotropic systems at high concentration, it is by no means complete. A particularly useful and complementary approach was adopted several years ago by Zasadzinski (11), who related the network microstructure of a gel to its mechanical properties by using freeze fracture transmission electron microscope (TEM) images. The images allowed him to show, for example, that mechanical shear degrades the network by creating small domains of crosslinked gel separated by water. More recently, Hoffmann used the same approach (12–14), i.e., rheology coupled with TEM, to characterize a surfactant system composed of multilamellar vesicles which exhibits a yield stress. In this article we discuss the properties of a cationic surfactant system which, above a threshold concentration, forms a gel with a pronounced yield stress. We first examine the rheological properties as functions of both shear rate and time. Scanning electron micrographs are then presented to facilitate a comparison between the dynamic behavior and the structure of the surfactant gel. Finally, the scanning electron microscope (SEM) pictures are compared with polar-
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ized optical micrographs, providing further information about the gel structure. MATERIALS AND METHODS
The system which we have chosen to study is a mixture of cationic surfactants provided by Ceca S.A. and used without further purification. The surfactant mixture is composed of 62% w/w of the diester, quaternary ammonium salt C43H86O5N / , CH3OSO 30 ; 9% of the monoester C25H52O4N / ; and 8% of triester C61H120O6N / . The remaining 21% w/w consists of nonquaternized mono-, di-, and triester ammonium salts and nonquaternized mono-, di-, and triester, as well as a small amount of other side-products due to the synthesis. Samples were prepared by mixing the surfactant paste mixture, referred to as ‘‘Quaternary Ammonium Surfactant Mixture’’ (QASM), after preheating to 507C, with deionized water also at 507C. The effect of added salt was studied by adding small quantities of CaCl2 into the mixture during preparation. There are several reasons for using a complex mixture of surfactants. First, such a system may be of commercial interest, as it can exhibit a relatively low viscosity at relatively high surfactant concentrations (15% w/w). A product made of a pure quaternary ammonium diester is much more viscous and difficult to use, even at low surfactant concentration (15). Second, and most important, this particular mixture exhibits unusual and fascinating viscoelastic behavior. For rheological studies we used a Carri-Med CSL 50 Rheometer with an acrylic cone-plate device (diameter Å 4 cm, angle Å 17 58 * ). The CSL 50 is a controlled stress rheometer in which the stress is continuously varied while the flow rate is measured. We note that the shear rate was always varied linearly with time in our experiments. The different samples were directly poured onto the rheometer plate and all measurements were performed at 257C after a thermal equilibration period of 4 min. To prevent changes in composition during measurements, the sample was surrounded by a water ring seal. Optical microscopy studies were performed with an Olympus BHA polarizing transmission microscope; all measurements were performed at 257C. For the SEM studies we used a Jeol JSM 840A scanning electron microscope with a Polaron/Biorad Cryotrans system. Two different freezing techniques were used. In the first technique, a small amount of sample was plunged into a nitrogen slush and transferred under vacuum into the SEM. There it was fractured by a pick and etched at 0607C for 1 h. The use of a nitrogen slush reduces the ‘‘Leiden’’ frost effect in which a gaseous layer forms around the specimen, resulting in a more rapid freezing rate than can be achieved by plunging the sample into liquid nitrogen. After etching, the temperature of the sample was once again decreased to ˚ of 01907C and the sample was sputter coated with 400 A
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FIG. 1. Shear stress vs shear rate at various concentrations. The shear stress was increased linearly with time at a rate of 67 dyn cm02 min 01 . The total experimental time (in min) for each trace was therefore the maximum shear stress divided by 67 dyn cm02 min 01 .
gold. The sample was then transferred to the cryostage and studied with the electron microscope. In the second technique, the samples were placed between two copper holders (referred to as ‘‘hats’’) in a spacer of thickness 40 mm and frozen by a jet of liquid propane at 01907C using a MF 7200 Gilkey–Staehelin propane jet freezer. This technique provides still higher rates of cooling than can be attained by plunging the sample into the cryogen: the propane jet velocity—approximately 100 m s 01 —provides more efficient heat exchange than can be attained by plunging. After freezing, the samples were transferred to the SEM chamber, where the sandwich was opened. This process created the desired fracture, which was then etched at 01007C for 15 min. The exposed surface was coated with gold (as above), and studied by CRYO-SEM. RESULTS AND DISCUSSION
Figure 1 shows the effect of surfactant concentration on the rheological behavior of the QASM system in water. For concentrations under 19% w/w the system appears liquidlike, in the sense that the shear stress varies smoothly with increasing shear rate. At a concentration of 19% and above, we observe an apparent yield stress effect, manifested by a sharp increase in the rate of deformation above a certain value. In addition, there is a subsequent nonmonotonic change in slope (i.e., a shoulder) in the stress vs rate curve. The first feature indicates that above 19% the system becomes gel-like and does not flow under small stresses. Above the yield stress, however, the system begins to flow more easily. At a slightly higher stress the shoulder in the figure indicates a discontinuous flow-induced increase in the apparent viscosity. The same behavior was found for ramp times ranging from 5 to 45 min. We assumed therefore that the shoulder is not caused by nonequilibrium measurements.
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FIG. 2. Shear stress vs shear rate for QASM 23% w/w. Trace a (- r- r-) is the first shear sweep: 5 min upward and 5 min downward with a 30 s pause at shear stress of 1000 dyn cm02 (see inset). The sweep profile of trace b ( ) was identical to and was made immediately after trace a. The sweep profile of trace c ( rrrrrrrr) was identical to and was made 1 h after trace b. The inset shows the sweep profile vs time.
Figure 2 shows shear loops which highlight the main rheological features of the system above a concentration of 19%. The first loop (trace a), performed on a fresh sample, reveals the presence of hysteresis, such that the system features an anti-thixotropic behavior. The shear stress was increased to its maximum value over a time period of 300 s, and then decreased to zero over the same period after a 30 s pause at the highest value (see inset, Fig. 2). Shear-induced structures have previously been reported for dilute cationic surfactant solutions (15) and, indeed, it can be inferred that new structures have been created in the QASM system during the first sweep by shear. Upon decreasing shear stress, the structures remained within the dispersion and the viscosity remained at higher values than those measured on increasing shear stress. Immediately after the first run we performed a second loop (trace b), where both the upward and downward traces follow quite closely the downward trace of the first run; hysteresis is nearly nonexistent, and no more structures are built. After waiting 1 h a third loop (trace c) was made (see inset) and again both the upward and downward traces follow quite closely the downward trace of the first run.
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There is no time or loop dependence once a first loop has been performed at high shear stress. This behavior is even more evident in Fig. 3. Here we made an initial loop (trace a), waited time t Å 2 h, and made a second loop (trace b) (see inset, Fig. 3). The behavior is similar to that observed in Fig. 2. We then reloaded the apparatus with a fresh sample, made an initial loop (again trace a), waited t Å 4 h, and made a second loop (trace c). Clearly the longer delay time results in no significant change of viscosity (measured at given shear rates). It appears that, after creating structures by performing a first loop, the rheological behavior approaches some limiting form, and no further significant changes take place. Figure 4 shows that the shear stress at which the shoulder in the shear loops occurs is actually a threshold value. For shear stress values maintained below this threshold we observe thixotropic behavior. However, when the shear stress is increased above this value the system switches from a thixotropic behavior to an anti-thixotropic behavior. This would seem to imply that a minimum shear stress value is needed to create the flow induced structures. It should be
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FIG. 3. Shear stress vs shear rate for QASM 23% w/w. Two samples were prepared. Sweep profiles are identical to those described in Fig. 2. Each ) was made at time t Å 2 h after a, and trace c ( rrrrrrrr) was made at t Å 4 h after a. The inset initial trace a (- r- r-) was identical. Trace b ( shows the profile of each pair of sweeps.
FIG. 4. Shear stress vs shear rate for QASM 23% w/w. Shear stress sweep ( ) of first sample was from 0 to 560 dyn cm02 in 5 min, followed by a 10 s pause, and a downward sweep to zero in 5 min. The second sample ( rrrrrrrr) was swept from 0 to 640 dyn cm02 in 5 min, followed by a 10 s pause, and a downward sweep to zero in 5 min.
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noted that the precise shape of the downward curve depends on the duration of the upward sweep, i.e., on the highest shear stress reached, the time required to reach this value, and the time spent at this value before decreasing the shear rate. This behavior has already been observed in double tail surfactant systems such as AOT [sodium bis-(2-ethylhexylsulfosuccinate)] –water or DDAB (didodecyldimethyl ammonium bromide) –water lamellar liquid crystals in a recent work by Soltero et al. (16). Figure 5 shows that the viscosity remains constant at a given shear rate once the flow induced structures have been created. Here, the shear stress was ramped from 0 to 1000 dyn cm02 in 5 min, at which time the shear stress was held constant at 1000 dyn cm02 , and the viscosity was monitored as a function of time. For approximately 50 min after reaching steady shear, the viscosity continued to increase, and then it remained constant with time. The presence of a yield stress suggests the existence of a network. Scanning electron microscopy studies reveal the morphological details of the structures formed by the surfac-
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FIG. 5. Viscosity vs time of QASM 23% w/w. Shear stress was swept from 0 to 1000 dyn cm02 in 5 min, and the viscosity measured at a constant shear stress of 1000 dyn cm02 as a function of time.
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FIG. 7. SEM micrograph of QASM 23% w/w prepared by nitrogen slush technique. The outsides or footprints of the largest structures are observed.
tant. Figure 6 is a freeze-fracture micrograph of 19% QASM, taken by the nitrogen slush technique, which reveals three kinds of structural features observed in these materials: large spherical particles having diameters Ç10 mm, small nodules having diameters Ç0.2 mm, and a network structure. The fracture process may expose in cross sections the internal organization of the largest structures, as observed in Fig. 6, or it may show the outside of the structure itself, or merely its footprint, as in Fig. 7. In Fig. 8 we show an optical microscope picture taken under crossed polarizers of 23% w/ w QASM at rest in which the location of the large spherical structures is apparent as birefringent ‘‘Maltese crosses.’’ The slush freezing technique may introduce artifacts such as ice crystal formation. So as to check the accuracy
of the micrograhs we used an additional approach, i.e., a propane jet technique in which the samples are vitrified. Figures 9, 10, and 11 show additional details of the network and the large structures in 23% w / w QASM, which were taken using the propane jet technique. The sheet-like structures, which are particularly evident in Fig. 9, suggest that at high surfactant concentrations a gel network is formed which consists of three-dimensionally interconnected lamellar bilayers. It is even more obvious in Fig. 10, where the right part of the sample has been removed by the fracture, revealing the sheet-like nature of the network. In addition, with this technique cross sections, footprints, or the
FIG. 6. SEM micrograph of QASM 19% w/w prepared by nitrogen slush technique. The figure shows the three different kinds of structures: onion-like globules, small nodules, and a network structure.
FIG. 8. Polarized optical micrograph of QASM 23% w/w showing the largest structures as birefringent crosses. Sample thickness was 25 mm. Bar corresponds to 50 mm.
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FIG. 9. SEM micrograph of QASM 23% w/w prepared by propane jet technique.
entire structures themselves can be observed, as shown in Fig. 11. The system appears to be more dense in the micrographs taken using the jet propane technique, but we have to bear in mind that the etching time was shorter and the etching temperature was lower than in the slush method, so it is not surprising to obtain samples which are less etched. We note that air bubbles, which are trapped during the preparation of the samples, never rise with time; this is further support for the presence of a gel-like network. The large spherical structures appear to be concentric multilamellar ensembles ( see Figs. 6 and 11 ) . It is well known that many surfactants which have a low solubility in water, especially long, double chained molecules such as the diester, do not form micelles. Instead they assemble in lamellae
FIG. 10. SEM micrograph of QASM 23% w/w prepared by propane jet technique, showing additional details of the network and the large structures.
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FIG. 11. SEM micrograph of QASM 23% w/w prepared by jet propane technique, showing additional details of the network and the large structures.
separated by aqueous layers; these lamellae in turn often self-assemble into multilayer structures like vesicles or liposomes ( 17 ) . Dubois and Zemb have shown that a single component double chain surfactant in water can form a dispersion in which lamellar crystallites ( spherulites ) are in equilibrium with an isotropic phase of lamellae ( 18 ) . Our SEM observations lead us to propose that the QASM system may feature a similar equilibrium between the birefringent onion-like spheres and an isotropic network made of interconnected bilayers. The apparent thickness of the lamellae is unexpectedly large: both freezing techniques yield values of 0.1 – 0.2 mm, and therefore the apparent layered structure must involve many bilayers. Future work involving the preparation of a replica and analysis with transmission electron microscopy will yield more insight into lamellar organization. Our observations suggest that the increase of yield stress with increasing surfactant concentration could be the result of more dense packing of the large spherical structures, as well as an increase in the number of interconnections within the network structure. Again, more work is needed to fully understand this phenomenon. The effect of shear on the microstructure has also been investigated through optical microscope pictures and CRYOSEM micrographs. Figures 12 and 13 show polarized optical microscope photographs of the 23% w/w sample before and 3 h after being sheared. The density of the birefringent structures in the sheared sample is clearly considerably larger than in the unsheared sample. This is confirmed by CRYOSEM (Fig. 14) which indicates that application of shear increases the density of globular structures. An analogous kind of rheological effect has previously been observed by Roux et al. (19) in a quaternary mixture of water–SDS– pentanol–dodecane. These authors showed that shear defor-
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FIG. 12. Polarized optical micrograph of QASM 23% w/w. Sample thickness was 25 mm. Bar corresponds to 100 mm.
mation can perturb the organization of lamellar phases with long-range order and create spherical multilayer vesicles which can roll over each other. Such an effect could explain the antithixotropic behavior observed in our system (Figs. 2 and 3): if we propose that globular structures are being created during shear, the viscosity of a dispersion of packed and charged onion-like vesicles is expected to be higher than a dilute one in equilibrium with a network made of interconnected lamellae. Figure 15 shows the strong effect that addition of salt has
FIG. 13. Polarized optical micrograph of QASM 23% w/w, taken 3 h after shear stress sweep. Shear stress sweep was from 0 to 1000 dyn cm02 in 5 min, followed by a 30 s pause, and a downward sweep to zero in 5 min. This figure shows the increased number density of the large structures after shearing (cf. Fig. 12). Sample thickness was 25 mm. Bar corresponds to 100 mm.
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FIG. 14. SEM micrograph of QASM 23% w/w prepared by jet propane technique 3 h after shear stress sweep. This figures shows an increase in number density of the spherical structures. Shear stress sweep was from 0 to 1000 dyn cm02 in 5 min, followed by a 30 s pause, and a downward sweep to zero in 5 min.
on the rheology of the QASM 19% system. From these results it is clear that electrostatic interactions within the microstructure play a major role in determining the yield stress. The data show that a minimum in yield stress occurs at a specific concentration Xc Ç 0.68% w/w of CaCl2 ; at salt concentrations above and below Xc the yield stress is larger. We note that Fig. 15 shows that the yield stress in these materials is not a true yield stress, since the initial slope of the shear stress vs shear rate curve is not infinite. Instead, what we refer to as the yield stress corresponds to the stress at which the rate of deformation of the sample
FIG. 15. Shear stress vs shear rate for QASM 19% w/w for different concentrations X of salt. Stress was ramped upward at 67 dyn cm02 min 01 . ), X Å 0.52% X Å 0 for trace a (- r- r-), X Å 0.35% w/w for trace b ( w/w for trace c ( rrrrrrrr), X Å 0.68% w/w for trace d (- rr- rr-), X Å 0.82 for trace e (- - - - - - -), and X Å 0.93% for trace f ( ÅÅÅÅ ).
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shows a sudden increase. Thus, upon increasing salt concentration, the viscoelastic properties of the system are first weakened. We speculate that this behavior occurs because electrostatic repulsions between the bilayers are screened out, thus facilitating shear. This effect has recently been observed with another quaternary ammonium salt system (tetradecyltrimethylammonium bromide) by Hoffmann et al. (20). As the salt concentration continues to increase, other forces such as dehydration effects become more dominant. The consequences of such forces are not well understood, although they would lead to changes in phase equilibria (17) which could in turn lead to an increase in yield stress. Again, these questions will be addressed in future studies.
CONCLUSION
The surfactant–water system we called QASM exhibits fascinating and rather complex rheological behavior. At sufficiently high surfactant concentration the sample becomes gel-like, with highly non-Newtonian viscosity and ultimately viscoelastic behavior. On applying a shear stress to a fresh sample, we find a nonmonotonically changing derivative ds / d(shear rate), indicating the formation of flow-induced structures. This behavior is most apparent in fresh (not previously sheared) samples, and becomes less obvious or vanishes completely in samples which have been sheared one or more times. We also observe that the apparent viscosity remains constant with time, once flow induced structures have been created by a first loop. Electron microscopy reveals a rich variety of structures, some of which may be easily related to the rheological data, and some of which are more complex. Clearly further work is needed to achieve a better understanding of these systems.
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ACKNOWLEDGMENTS The authors would like to acknowledge fruitful discussions with M. Bourrel and are indebted to Ceca S.A. and ELF Aquitaine for support of A.G. We also acknowledge support by the National Science Foundation under Grants DMR-9122227 and DMR-9502825.
REFERENCES 1. Hoffmann, H., ‘‘From Micellar Solutions to Liquid Crystalline Phases.’’ Verlag Chemie GmbH, Weinheim, 1984. 2. Tiddy, G. J. T., ‘‘Surfactant–Water Liquid Crystal Phases.’’ NorthHolland, Amsterdam, 1980. 3. Lindman, B., and Wennerstrom, H., ‘‘Topics in Current Chemistry,’’ Vol. 87. Spring–Verlag, Berlin, 1980. 4. Hagslatt, H., Ph.D. Thesis, Lund University, Sweden, 1993. 5. Cates, M. E., and Candau, S. J., J. Phys. Condens. Matter 2, 6869 (1990). 6. Diat, O., Roux, D., and Nallet, F., J. Phys. IV, Colloque C8, suppl. au. J. Phys. I 3, 193 (1993). 7. Candau, S. J., Khatory, A., Lequeux, F., and Kern, F., J. Phys. IV, Colloque C1, suppl. au. J. Phys. II 3, 197 (1993). 8. Wunderlich, A. M., and Brunn, P. O., Colloid Polym. Sci. 267, 627 (1989). 9. Hoffmann, H., Lobl, H., Rehage, H., and Wunderlich, I., Tenside Det. 22, 290 (1988). 10. Barnes, B. A., Hutton, J. F., and Walters, K., ‘‘An Introduction to Rheology.’’ Elsevier, Amsterdam, 1989. 11. Zasadzinski, J. A. N., Chu, A., and Prud’homme, R. K., Macromolecules 19, 2960 (1986). 12. Hoffmann, H., Munkert, U., Thunig, C., and Valiente, M., J. Colloid Interface Sci. 163, 217 (1994). 13. Hoffmann, H., and Rauscher, A., Colloid Polym. Sci. 271, 390 (1993). 14. Hoffman, H., and Rehage, H., Rheol. Acta 21, 561 (1982). 15. Wunderlich, I., Hoffmann, H., and Rehage, H., Rheol. Acta 26, 532 (1987). 16. Soltero, J. F. A., Robles-Vasquez, O., Puig, J. E., and Manero, O., J. Rheol. 39, 1, 235 (1995). 17. Myers, D., ‘‘Surfaces, Interfaces, and Colloids.’’ VCH Publishers, New York, 1991. 18. Dubois, M., and Zemb, Th., Langmuir 7, 1352 (1991). 19. Diat, O., Roux, D., and Nallet, F., J. Phys. II France 3, 1427 (1993). 20. Hoffmann, H., Thunig, C., Schmiedel, P., and Munkert, U., Langmuir 10, 3972 (1994).
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0298
Rheology, Optical Microscopy, and Electron Microscopy of Cationic Surfactant Gels ALEXANDRE GOLDSZAL,* ,† ALEXANDER M. JAMIESON,‡ J. ADIN MANN, JR .,* ,‡ JOSEPH POLAK,§ AND CHARLES ROSENBLATT† ,‡ ,1 *Department of Chemical Engineering, †Department of Physics, ‡Department of Macromolecular Science, and §Department of Neurosciences, Case Western Reserve University, Cleveland, Ohio 44106 Received September 14, 1995; accepted December 11, 1995
A cationic surfactant mixture is studied in aqueous solution by rheological and imaging techniques. We find that the system becomes gel-like above a threshold surfactant concentration, as evidenced by the appearance of an apparent yield stress and a viscosity which is highly non-Newtonian. Hysteresis is observed in the shear stress vs shear rate measurements: the formation of shear-induced microstructures at high shear rates may cause shear thickening. Electron micrographs indicate three distinct structures in the sample: lamellae, large onion-like globules, and small nodules. These structures are discussed in the context of the rheological data. The density of the globular structures in sheared samples is observed to be larger than in unsheared samples. q 1996 Academic Press, Inc.
Key Words: surfactant gels; rheology; electron microscopy; complex fluids; micelles.
INTRODUCTION
Owing to their importance in both industry and basic science, surfactant molecules have been the subject of intense investigation for many years. In its most basic form a surfactant molecule possesses a polar hydrophilic head group and a nonpolar paraffinic tail. At sufficiently high concentrations, in an aqueous environment, surfactants can form a variety of structures and phases, including spherical and elongated micelles, lamellae, and hexagonal and cubic phases (1–4). Over the years, a battery of experimental tools has been used to investigate the structure of these lyotropic systems, with recent advances in imaging being among the most useful. Additionally, a renaissance in rheological studies of surfactant systems has provided important insights into not only the dynamics, but into the structure of these materials as well (5–9). The viscosity of micellar solutions composed of cationic surfactant molecules at concentrations close to the critical micelle concentration (CMC) is Newtonian in behavior. 1
To whom correspondence should be addressed.
Upon increasing the surfactant concentration, however, a variety of phase structures may appear. Initially, the system generally becomes significantly more viscous and then, at sufficiently high concentrations, it may become viscoelastic, behavior indicative of network formation. Under small stresses these concentrated lyotropic systems are primarily elastic, i.e., they exhibit an equilibrium modulus, although they flow above a limiting yield stress value (10). The rheological and mechanical properties of surfactant gels are directly linked to the structure of the network which is formed by the self associating molecules and to the various interactions between the micellar aggregates (2, 3). This gel-like behavior may be commercially important since concentrated surfactant solutions are frequently used in paints, cosmetics, and the food industry. In the detergent industry, on the other hand, surfactants are used for their surface active properties, and viscous concentrated formulations are often undesirable in practice. Although rheology provides an important analytic tool for our understanding of lyotropic systems at high concentration, it is by no means complete. A particularly useful and complementary approach was adopted several years ago by Zasadzinski (11), who related the network microstructure of a gel to its mechanical properties by using freeze fracture transmission electron microscope (TEM) images. The images allowed him to show, for example, that mechanical shear degrades the network by creating small domains of crosslinked gel separated by water. More recently, Hoffmann used the same approach (12–14), i.e., rheology coupled with TEM, to characterize a surfactant system composed of multilamellar vesicles which exhibits a yield stress. In this article we discuss the properties of a cationic surfactant system which, above a threshold concentration, forms a gel with a pronounced yield stress. We first examine the rheological properties as functions of both shear rate and time. Scanning electron micrographs are then presented to facilitate a comparison between the dynamic behavior and the structure of the surfactant gel. Finally, the scanning electron microscope (SEM) pictures are compared with polar-
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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
coida
AP: Colloid
262
GOLDSZAL ET AL.
ized optical micrographs, providing further information about the gel structure. MATERIALS AND METHODS
The system which we have chosen to study is a mixture of cationic surfactants provided by Ceca S.A. and used without further purification. The surfactant mixture is composed of 62% w/w of the diester, quaternary ammonium salt C43H86O5N / , CH3OSO 30 ; 9% of the monoester C25H52O4N / ; and 8% of triester C61H120O6N / . The remaining 21% w/w consists of nonquaternized mono-, di-, and triester ammonium salts and nonquaternized mono-, di-, and triester, as well as a small amount of other side-products due to the synthesis. Samples were prepared by mixing the surfactant paste mixture, referred to as ‘‘Quaternary Ammonium Surfactant Mixture’’ (QASM), after preheating to 507C, with deionized water also at 507C. The effect of added salt was studied by adding small quantities of CaCl2 into the mixture during preparation. There are several reasons for using a complex mixture of surfactants. First, such a system may be of commercial interest, as it can exhibit a relatively low viscosity at relatively high surfactant concentrations (15% w/w). A product made of a pure quaternary ammonium diester is much more viscous and difficult to use, even at low surfactant concentration (15). Second, and most important, this particular mixture exhibits unusual and fascinating viscoelastic behavior. For rheological studies we used a Carri-Med CSL 50 Rheometer with an acrylic cone-plate device (diameter Å 4 cm, angle Å 17 58 * ). The CSL 50 is a controlled stress rheometer in which the stress is continuously varied while the flow rate is measured. We note that the shear rate was always varied linearly with time in our experiments. The different samples were directly poured onto the rheometer plate and all measurements were performed at 257C after a thermal equilibration period of 4 min. To prevent changes in composition during measurements, the sample was surrounded by a water ring seal. Optical microscopy studies were performed with an Olympus BHA polarizing transmission microscope; all measurements were performed at 257C. For the SEM studies we used a Jeol JSM 840A scanning electron microscope with a Polaron/Biorad Cryotrans system. Two different freezing techniques were used. In the first technique, a small amount of sample was plunged into a nitrogen slush and transferred under vacuum into the SEM. There it was fractured by a pick and etched at 0607C for 1 h. The use of a nitrogen slush reduces the ‘‘Leiden’’ frost effect in which a gaseous layer forms around the specimen, resulting in a more rapid freezing rate than can be achieved by plunging the sample into liquid nitrogen. After etching, the temperature of the sample was once again decreased to ˚ of 01907C and the sample was sputter coated with 400 A
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FIG. 1. Shear stress vs shear rate at various concentrations. The shear stress was increased linearly with time at a rate of 67 dyn cm02 min 01 . The total experimental time (in min) for each trace was therefore the maximum shear stress divided by 67 dyn cm02 min 01 .
gold. The sample was then transferred to the cryostage and studied with the electron microscope. In the second technique, the samples were placed between two copper holders (referred to as ‘‘hats’’) in a spacer of thickness 40 mm and frozen by a jet of liquid propane at 01907C using a MF 7200 Gilkey–Staehelin propane jet freezer. This technique provides still higher rates of cooling than can be attained by plunging the sample into the cryogen: the propane jet velocity—approximately 100 m s 01 —provides more efficient heat exchange than can be attained by plunging. After freezing, the samples were transferred to the SEM chamber, where the sandwich was opened. This process created the desired fracture, which was then etched at 01007C for 15 min. The exposed surface was coated with gold (as above), and studied by CRYO-SEM. RESULTS AND DISCUSSION
Figure 1 shows the effect of surfactant concentration on the rheological behavior of the QASM system in water. For concentrations under 19% w/w the system appears liquidlike, in the sense that the shear stress varies smoothly with increasing shear rate. At a concentration of 19% and above, we observe an apparent yield stress effect, manifested by a sharp increase in the rate of deformation above a certain value. In addition, there is a subsequent nonmonotonic change in slope (i.e., a shoulder) in the stress vs rate curve. The first feature indicates that above 19% the system becomes gel-like and does not flow under small stresses. Above the yield stress, however, the system begins to flow more easily. At a slightly higher stress the shoulder in the figure indicates a discontinuous flow-induced increase in the apparent viscosity. The same behavior was found for ramp times ranging from 5 to 45 min. We assumed therefore that the shoulder is not caused by nonequilibrium measurements.
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FIG. 2. Shear stress vs shear rate for QASM 23% w/w. Trace a (- r- r-) is the first shear sweep: 5 min upward and 5 min downward with a 30 s pause at shear stress of 1000 dyn cm02 (see inset). The sweep profile of trace b ( ) was identical to and was made immediately after trace a. The sweep profile of trace c ( rrrrrrrr) was identical to and was made 1 h after trace b. The inset shows the sweep profile vs time.
Figure 2 shows shear loops which highlight the main rheological features of the system above a concentration of 19%. The first loop (trace a), performed on a fresh sample, reveals the presence of hysteresis, such that the system features an anti-thixotropic behavior. The shear stress was increased to its maximum value over a time period of 300 s, and then decreased to zero over the same period after a 30 s pause at the highest value (see inset, Fig. 2). Shear-induced structures have previously been reported for dilute cationic surfactant solutions (15) and, indeed, it can be inferred that new structures have been created in the QASM system during the first sweep by shear. Upon decreasing shear stress, the structures remained within the dispersion and the viscosity remained at higher values than those measured on increasing shear stress. Immediately after the first run we performed a second loop (trace b), where both the upward and downward traces follow quite closely the downward trace of the first run; hysteresis is nearly nonexistent, and no more structures are built. After waiting 1 h a third loop (trace c) was made (see inset) and again both the upward and downward traces follow quite closely the downward trace of the first run.
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There is no time or loop dependence once a first loop has been performed at high shear stress. This behavior is even more evident in Fig. 3. Here we made an initial loop (trace a), waited time t Å 2 h, and made a second loop (trace b) (see inset, Fig. 3). The behavior is similar to that observed in Fig. 2. We then reloaded the apparatus with a fresh sample, made an initial loop (again trace a), waited t Å 4 h, and made a second loop (trace c). Clearly the longer delay time results in no significant change of viscosity (measured at given shear rates). It appears that, after creating structures by performing a first loop, the rheological behavior approaches some limiting form, and no further significant changes take place. Figure 4 shows that the shear stress at which the shoulder in the shear loops occurs is actually a threshold value. For shear stress values maintained below this threshold we observe thixotropic behavior. However, when the shear stress is increased above this value the system switches from a thixotropic behavior to an anti-thixotropic behavior. This would seem to imply that a minimum shear stress value is needed to create the flow induced structures. It should be
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FIG. 3. Shear stress vs shear rate for QASM 23% w/w. Two samples were prepared. Sweep profiles are identical to those described in Fig. 2. Each ) was made at time t Å 2 h after a, and trace c ( rrrrrrrr) was made at t Å 4 h after a. The inset initial trace a (- r- r-) was identical. Trace b ( shows the profile of each pair of sweeps.
FIG. 4. Shear stress vs shear rate for QASM 23% w/w. Shear stress sweep ( ) of first sample was from 0 to 560 dyn cm02 in 5 min, followed by a 10 s pause, and a downward sweep to zero in 5 min. The second sample ( rrrrrrrr) was swept from 0 to 640 dyn cm02 in 5 min, followed by a 10 s pause, and a downward sweep to zero in 5 min.
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noted that the precise shape of the downward curve depends on the duration of the upward sweep, i.e., on the highest shear stress reached, the time required to reach this value, and the time spent at this value before decreasing the shear rate. This behavior has already been observed in double tail surfactant systems such as AOT [sodium bis-(2-ethylhexylsulfosuccinate)] –water or DDAB (didodecyldimethyl ammonium bromide) –water lamellar liquid crystals in a recent work by Soltero et al. (16). Figure 5 shows that the viscosity remains constant at a given shear rate once the flow induced structures have been created. Here, the shear stress was ramped from 0 to 1000 dyn cm02 in 5 min, at which time the shear stress was held constant at 1000 dyn cm02 , and the viscosity was monitored as a function of time. For approximately 50 min after reaching steady shear, the viscosity continued to increase, and then it remained constant with time. The presence of a yield stress suggests the existence of a network. Scanning electron microscopy studies reveal the morphological details of the structures formed by the surfac-
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FIG. 5. Viscosity vs time of QASM 23% w/w. Shear stress was swept from 0 to 1000 dyn cm02 in 5 min, and the viscosity measured at a constant shear stress of 1000 dyn cm02 as a function of time.
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FIG. 7. SEM micrograph of QASM 23% w/w prepared by nitrogen slush technique. The outsides or footprints of the largest structures are observed.
tant. Figure 6 is a freeze-fracture micrograph of 19% QASM, taken by the nitrogen slush technique, which reveals three kinds of structural features observed in these materials: large spherical particles having diameters Ç10 mm, small nodules having diameters Ç0.2 mm, and a network structure. The fracture process may expose in cross sections the internal organization of the largest structures, as observed in Fig. 6, or it may show the outside of the structure itself, or merely its footprint, as in Fig. 7. In Fig. 8 we show an optical microscope picture taken under crossed polarizers of 23% w/ w QASM at rest in which the location of the large spherical structures is apparent as birefringent ‘‘Maltese crosses.’’ The slush freezing technique may introduce artifacts such as ice crystal formation. So as to check the accuracy
of the micrograhs we used an additional approach, i.e., a propane jet technique in which the samples are vitrified. Figures 9, 10, and 11 show additional details of the network and the large structures in 23% w / w QASM, which were taken using the propane jet technique. The sheet-like structures, which are particularly evident in Fig. 9, suggest that at high surfactant concentrations a gel network is formed which consists of three-dimensionally interconnected lamellar bilayers. It is even more obvious in Fig. 10, where the right part of the sample has been removed by the fracture, revealing the sheet-like nature of the network. In addition, with this technique cross sections, footprints, or the
FIG. 6. SEM micrograph of QASM 19% w/w prepared by nitrogen slush technique. The figure shows the three different kinds of structures: onion-like globules, small nodules, and a network structure.
FIG. 8. Polarized optical micrograph of QASM 23% w/w showing the largest structures as birefringent crosses. Sample thickness was 25 mm. Bar corresponds to 50 mm.
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FIG. 9. SEM micrograph of QASM 23% w/w prepared by propane jet technique.
entire structures themselves can be observed, as shown in Fig. 11. The system appears to be more dense in the micrographs taken using the jet propane technique, but we have to bear in mind that the etching time was shorter and the etching temperature was lower than in the slush method, so it is not surprising to obtain samples which are less etched. We note that air bubbles, which are trapped during the preparation of the samples, never rise with time; this is further support for the presence of a gel-like network. The large spherical structures appear to be concentric multilamellar ensembles ( see Figs. 6 and 11 ) . It is well known that many surfactants which have a low solubility in water, especially long, double chained molecules such as the diester, do not form micelles. Instead they assemble in lamellae
FIG. 10. SEM micrograph of QASM 23% w/w prepared by propane jet technique, showing additional details of the network and the large structures.
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FIG. 11. SEM micrograph of QASM 23% w/w prepared by jet propane technique, showing additional details of the network and the large structures.
separated by aqueous layers; these lamellae in turn often self-assemble into multilayer structures like vesicles or liposomes ( 17 ) . Dubois and Zemb have shown that a single component double chain surfactant in water can form a dispersion in which lamellar crystallites ( spherulites ) are in equilibrium with an isotropic phase of lamellae ( 18 ) . Our SEM observations lead us to propose that the QASM system may feature a similar equilibrium between the birefringent onion-like spheres and an isotropic network made of interconnected bilayers. The apparent thickness of the lamellae is unexpectedly large: both freezing techniques yield values of 0.1 – 0.2 mm, and therefore the apparent layered structure must involve many bilayers. Future work involving the preparation of a replica and analysis with transmission electron microscopy will yield more insight into lamellar organization. Our observations suggest that the increase of yield stress with increasing surfactant concentration could be the result of more dense packing of the large spherical structures, as well as an increase in the number of interconnections within the network structure. Again, more work is needed to fully understand this phenomenon. The effect of shear on the microstructure has also been investigated through optical microscope pictures and CRYOSEM micrographs. Figures 12 and 13 show polarized optical microscope photographs of the 23% w/w sample before and 3 h after being sheared. The density of the birefringent structures in the sheared sample is clearly considerably larger than in the unsheared sample. This is confirmed by CRYOSEM (Fig. 14) which indicates that application of shear increases the density of globular structures. An analogous kind of rheological effect has previously been observed by Roux et al. (19) in a quaternary mixture of water–SDS– pentanol–dodecane. These authors showed that shear defor-
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FIG. 12. Polarized optical micrograph of QASM 23% w/w. Sample thickness was 25 mm. Bar corresponds to 100 mm.
mation can perturb the organization of lamellar phases with long-range order and create spherical multilayer vesicles which can roll over each other. Such an effect could explain the antithixotropic behavior observed in our system (Figs. 2 and 3): if we propose that globular structures are being created during shear, the viscosity of a dispersion of packed and charged onion-like vesicles is expected to be higher than a dilute one in equilibrium with a network made of interconnected lamellae. Figure 15 shows the strong effect that addition of salt has
FIG. 13. Polarized optical micrograph of QASM 23% w/w, taken 3 h after shear stress sweep. Shear stress sweep was from 0 to 1000 dyn cm02 in 5 min, followed by a 30 s pause, and a downward sweep to zero in 5 min. This figure shows the increased number density of the large structures after shearing (cf. Fig. 12). Sample thickness was 25 mm. Bar corresponds to 100 mm.
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FIG. 14. SEM micrograph of QASM 23% w/w prepared by jet propane technique 3 h after shear stress sweep. This figures shows an increase in number density of the spherical structures. Shear stress sweep was from 0 to 1000 dyn cm02 in 5 min, followed by a 30 s pause, and a downward sweep to zero in 5 min.
on the rheology of the QASM 19% system. From these results it is clear that electrostatic interactions within the microstructure play a major role in determining the yield stress. The data show that a minimum in yield stress occurs at a specific concentration Xc Ç 0.68% w/w of CaCl2 ; at salt concentrations above and below Xc the yield stress is larger. We note that Fig. 15 shows that the yield stress in these materials is not a true yield stress, since the initial slope of the shear stress vs shear rate curve is not infinite. Instead, what we refer to as the yield stress corresponds to the stress at which the rate of deformation of the sample
FIG. 15. Shear stress vs shear rate for QASM 19% w/w for different concentrations X of salt. Stress was ramped upward at 67 dyn cm02 min 01 . ), X Å 0.52% X Å 0 for trace a (- r- r-), X Å 0.35% w/w for trace b ( w/w for trace c ( rrrrrrrr), X Å 0.68% w/w for trace d (- rr- rr-), X Å 0.82 for trace e (- - - - - - -), and X Å 0.93% for trace f ( ÅÅÅÅ ).
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shows a sudden increase. Thus, upon increasing salt concentration, the viscoelastic properties of the system are first weakened. We speculate that this behavior occurs because electrostatic repulsions between the bilayers are screened out, thus facilitating shear. This effect has recently been observed with another quaternary ammonium salt system (tetradecyltrimethylammonium bromide) by Hoffmann et al. (20). As the salt concentration continues to increase, other forces such as dehydration effects become more dominant. The consequences of such forces are not well understood, although they would lead to changes in phase equilibria (17) which could in turn lead to an increase in yield stress. Again, these questions will be addressed in future studies.
CONCLUSION
The surfactant–water system we called QASM exhibits fascinating and rather complex rheological behavior. At sufficiently high surfactant concentration the sample becomes gel-like, with highly non-Newtonian viscosity and ultimately viscoelastic behavior. On applying a shear stress to a fresh sample, we find a nonmonotonically changing derivative ds / d(shear rate), indicating the formation of flow-induced structures. This behavior is most apparent in fresh (not previously sheared) samples, and becomes less obvious or vanishes completely in samples which have been sheared one or more times. We also observe that the apparent viscosity remains constant with time, once flow induced structures have been created by a first loop. Electron microscopy reveals a rich variety of structures, some of which may be easily related to the rheological data, and some of which are more complex. Clearly further work is needed to achieve a better understanding of these systems.
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ACKNOWLEDGMENTS The authors would like to acknowledge fruitful discussions with M. Bourrel and are indebted to Ceca S.A. and ELF Aquitaine for support of A.G. We also acknowledge support by the National Science Foundation under Grants DMR-9122227 and DMR-9502825.
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