Rheology and Microstructural Transitions in the Lamellar Phase of a

Ctra. de Palos de la Frontera s/n, 21819 Huelva, Spain, Unilever Research Port Sunlight,. Bebington, Merseyside CH63 3JW, U.K., TEI of Kozani, Kila 50...
1 downloads 0 Views 497KB Size
Download PDF
Langmuir 2001, 17, 1331-1337

1331

Articles Rheology and Microstructural Transitions in the Lamellar Phase of a Cationic Surfactant P. Partal,† A. J. Kowalski,‡ D. Machin,‡ N. Kiratzis,§ M. G. Berni,| and C. J. Lawrence*,| Departamento de Ingenierı´a Quı´mica, Universidad de Huelva, Escuela Politecnica Superior, Ctra. de Palos de la Frontera s/n, 21819 Huelva, Spain, Unilever Research Port Sunlight, Bebington, Merseyside CH63 3JW, U.K., TEI of Kozani, Kila 50100, Kozani, Greece, and Department of Chemical Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BY, U.K. Received June 1, 2000. In Final Form: December 29, 2000 The rheological properties of mixtures of cationic surfactant (dialkyldimethylammonium chloride) and water are studied for a range of surfactant concentrations. In particular, we investigate rheological signatures of the conformational transformation of the system from lamellar sheets to lamellar vesicles under the action of shear. For a range of rheological test regimes, including transient, steady, and oscillatory shear, this transition is signaled by a critical stress or yield stress involving step changes in sample strain, viscosity, or dynamic moduli. However, only a portion of the sample undergoes the transition, resulting in lamellar sheets coexisting with vesicles. We further show that the transition is reversible in that ramping the stress down from above the critical stress value results in the disappearance of all of the vesicles. The final viscosity of the mixture can be higher or lower than that for the initial untreated materialspossibly reflecting an imbalance in the net generation or annealing of defects in the lamellar structure by the shear treatment. Oscillatory rheometry indicates that a minimum strain is also required to obtain a phase transition. For transient shear, this corresponds to a minimum residence time in the shear field. In general, the critical stresses increase with the surfactant concentration, though they are relatively constant at 15-20 Pa for concentrations between 20 and 40 wt %.

1. Introduction Knowledge of the rheological behavior of lamellar liquidcrystalline surfactant/water mixtures is of practical importance because of applications of such systems in the fields of detergents, cosmetics, pharmaceutical products, emulsions, and many other commercial fields.1 A detailed rheological characterization can provide practical input for the design of unit operations such as pumping, agitation, and mixing and is also important for quality control, for quantification of the system texture, and for optimization of formulations.2 When subjected to shear flows, lyotropic liquid crystals present complicated rheological responses,1,3-5 which depend on the rearrangement of the structure during the flow.6 Surfactants in solution are known to self-organize into many different structures, such as micelles and bilayers. In many systems surfactant bilayers become ordered and * To whom correspondence should be addressed. Tel: +44-207594-5622. Fax: +44-20-7594-5604. E-mail: [email protected]. † Universidad de Huelva. ‡ Unilever Research Port Sunlight. § TEI of Kozani. | Imperial College of Science, Technology and Medicine. (1) Franco, J. M.; Mun˜oz, J.; Gallegos, C. Langmuir 1995, 11, 669. (2) Cordobes, F.; Mun˜oz, J.; Gallegos, C. J. Colloid Interface Sci. 1997, 187, 401. (3) Oswald, P.; Allain, M. J. Colloid Interface Sci. 1988, 126, 45. (4) Matsumoto, T.; Heiuchi, T.; Horie, K. Colloid Polym. Sci. 1989, 267, 71. (5) Soltero, J. F. A.; Robles-Vazquez, O.; Puig, J. E.; Manero, O. J. Rheol. 1995, 39, 235. (6) Barnes, H. A. J. Non-Newtonian Fluid Mech. 1997, 70, 1.

stack, interspersed with aqueous layers, to form larger structures referred to as the lamellar phase, LR. The occurrence of the lamellar phase in aqueous surfactant systems is widespread7,8 and can take several forms. In dilute solution, the lamellar phase often exists as an aqueous dispersion. This is usually a dispersion of multilamellar vesicles, also called lamellar droplets, spherulites, liposomes, or “onions”,9,10 in an aqueous continuous phase. However, for some systems, application of sufficient mechanical energy (e.g., sonification) can convert vesicles into very small flat, disklike bilayer fragments.11-13 For more concentrated surfactant solutions, the lamellar phase commonly exists as a single phase having domains of oriented bilayer stacks, with different relative orientations.14 Under some conditions, such a continuous, “sheet” lamellar phase, which usually contains defects,14 may be transformed by shearing into multila(7) Ekwall, P. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; Vol. 1, p 1. (8) Laughlin, R. G. The Aqueous Phase Behaviour of Surfactants; Academic Press: New York, 1994. (9) van de Pas, J. C. A Study of the Physical Properties of Lamellar Liquid Crystalline Dispersions. Ph.D. Thesis, Rijksuniversiteit Groningen, Groningen, The Netherlands, 1993. (10) van der Linden, E.; Hogersvorst, W. T.; Lekkerker, H. N. W. Langmuir 1996, 12, 3127. (11) Pansu, R. B.; Arrio, B.; Roncin, J.; Faure, J. J. Phys. Chem. 1990, 94, 796. (12) Liu, L.; Pansu, R.; Roncin, J.; Faure, J.; Arai, T.; Tokumaru, K. J. Colloid Interface Sci. 1992, 148, 118. (13) Feitosa, E.; Brown, W. Langmuir 1997, 13, 4810. (14) La¨uger, J.; Weigel, R.; Berger, K.; Hiltrop, K.; Richtering, W. J. Colloid Interface Sci. 1996, 181, 521.

10.1021/la0007731 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/03/2001

1332

Langmuir, Vol. 17, No. 5, 2001

mellar vesicles.15 At higher concentrations, these multilamellar vesicles may be so closely packed that the outer layers deform to give a polyhedral structure similar to that of high internal phase ratio foams and emulsions. For some systems, application of higher degrees of shear has also been observed15 to disrupt the vesicles and align the material back into a sheetlike structure, albeit a more perfect one with fewer defects. The extent of mechanical shearing required to bring about transitions in the lamellar phase microstructure depends on the system and on the surfactant concentration. For a given system a “dynamic phase diagram” or “orientation diagram” relates critical shearing conditions to surfactant concentration. Such diagrams have been constructed for a few surfactant systems.16,17 Recent investigations of deformation-induced lamellar-phase transitions have included studies with anionic surfactant,18-20 with mixed anionic surfactant/un-ionized amphiphile,21 with mixed cationic/nonionic surfactants,22 and with polymeric surfactants.23 However, we are unaware of studies on cationic surfactant alone. Although creation of vesicles by steady shear has already been documented, the process of droplet formation from a continuous lamellar phase under transient deformations is not understood in detail. It has been shown to be complex and to depend on the applied shear stress and shear strain,20,23 surfactant ratio,21,22 and salt content.18 The latter has been interpreted in terms of salt effects on the elastic constants of lamellar bilayers.18 In addition to this complexity, in most industrial operations the deformation experienced by materials contains components which are strongly unsteady, transient, and oscillatory in nature. As a first step toward understanding the effect of such unsteady conditions on the microstructure, we have examined how classical rheological techniques can give insight into the response of the microstructure to welldefined time-varying shear fields. The particular aims of the study, which focused on a cationic surfactant/water system, were to investigate the lamellar-phase microstructural transitions, to identify rheological signatures of such transitions, and to construct an orientation diagram for the system. 2. Experiments The cationic surfactant was dialkyldimethylammonium chloride; two surfactant raw materials were used, namely, Varisoft TA100 (ex Witco-Sherex, alkyl ) C18) and Arquad 2HT (ex Akzo Chemical Inc., alkyl ) hardened tallow, typically 3% C14, 32% C16, 65% C18). The Arquad raw material comes in solvent (10% water and 15% alcohol); the alcohol was removed by evaporation at ca. 100 °C for 1 h and the remaining raw material composition determined by weight loss. The lamellar liquid-crystalline LR phase was obtained by blending the cationic surfactant with demineralized water in an oven at 80 °C for approximately 2 h and then leaving it at 80 °C for a further 75 h. The samples were then stored at 50-55 °C before use. The equilibrium phase (15) Roux, D.; Nallet, F.; Diat, O. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; ACS Symposium Series 578; American Chemical Society: Washington, DC, 1994; p 300. (16) Diat, O.; Roux, D.; Nallet, F. J. Phys. II 1993, 3, 1427. (17) Diat, O.; Roux, D.; Nallet, F. J. Phys. IV 1993, 3, 193. (18) Leon, A.; Bonn, D.; Meunier, J.; Al-Kahwaji, A.; Greffier, O.; Kellay, H. Phys. Rev. Lett. 2000, 84, 1335. (19) Bonn D.; Meunier, J.; Greffier, O.; Al-Kahwaji, A.; Kellay, H. Phys. Rev. E 1998, 58, 2115. (20) Bergenholtz, J.; Wagner, N. J. Langmuir 1996, 12, 3122. (21) Zipfel, J.; Berghausen, J.; Lindner, P.; Richtering, W. J. Phys. Chem. B 1999, 103, 2841. (22) Bergmeier, M.; Gradzielski, M.; Hoffmann, H.; Mortensen, K. J. Phys. Chem. B 1999, 103, 1605. (23) Schmidt, G.; Muller, S.; Schmidt, C.; Richtering, W. Rheol. Acta 1999, 38, 486.

Partal et al. diagram shows that there is a transition to dispersed lamellar vesicles upon dilution to about 4% surfactant and also a transition from the lamellar LR phase to a hydrated solid (with the Lβ form of the lamellar structure as a metastable state) on cooling below about 45-47 °C.24 Samples were prepared with surfactant concentrations from 10 to 85 wt %. Oscillatory and continuous shear rheological tests were conducted in controlled-stress rheometers (Carri-med CSL 100 and T.A. Instruments AR 1000) using a rough cone-plate geometry sensor system (40 mm diameter, 2° cone, and 60 µm gap). All samples had similar thermal and rheological history; following the thermal conditioning, samples were transferred directly from the oven to the rheometer surface with the minimum amount of agitation possible. All of the rheological measurements were carried out at 55 °C. A new freshly filled sample was used for each experiment. Transient flow measurements were made in two ways: (a) The stress or the shear rate was held constant, and measurements were made both of steady-state viscosities and of the approach to steady state, including creep experiments in the case of constant stress. The ranges of the applied stress and shear rate were 5-100 Pa and 0.12-1160 s-1, respectively. (b) Stress sweeps were applied in which the stress was increased exponentially with time. In one series of experiments, the stress was ramped to a maximum stress value in a time t1, then this stress value was held for a time t2, and finally the stress was ramped down to the minimum stress in a further time t1. The times t1 and t2 were varied from 300 to 3600 s. In another series of experiments, the stress was stepped and held at intermediate values for a specified time (dwell time) in each run. Dwell times of 1, 10, and 100 s/point were used. The maximum stresses were varied from 5 to 120 Pa. Exponential shear flow is a strong flow because the distance between material elements of the fluid increases exponentially with time. Dynamic oscillatory measurements were also made in two ways: i. Oscillatory shear was applied to one surfactant/water mixture for specific fixed values of frequency and stress amplitude outside the linear viscoelastic region. Changes in the (apparent) complex viscosity as a function of time were observed. These preconditioned samples were subsequently characterized by a frequency scan from about 0.009 24 to 92.4 rad/s, performed at a stress within the linear viscoelastic range. ii. Oscillatory stress amplitude sweeps were applied at three different frequencies; 0.1, 1, and 10 Hz. Changes in the apparent storage and loss moduli were observed as the stress amplitude was varied from 1 to 100 Pa. The viscosity at high shear rates and the effect of extensional deformation were studied in the capillary rheometer, a TA-HD texture analyzer from SMS (Surrey, U.K.). A tube of 2 mm diameter and 200 mm length was used. The lamellar-phase microstructural features corresponding to the different mechanical treatments of the samples were investigated by light microscopy and electron microscopy (EM). In the freeze-fracture EM replication technique, 1.5-2 mm droplets of a sample were placed in gold alloy support studs and frozen by rapid immersion in melting nitrogen. Replicas were prepared in a Balzers 400 freeze-fracture apparatus. The frozen samples were fractured and etched for 1 min at -100 °C and shadowed with Pt/C. Light microscopic observations were made with an Olympus BH2 microscope. Micrographs were obtained during or after shearing of the samples at up to about 80 s-1 in steady or oscillatory shear with a Linkam CSS450 shear cell attachment. Selected experimental results are presented below. Further results are presented as Supporting Information available from the authors and the publishers, as described at the end of the paper.

3. Rheological Features and Signatures of Transition 3.1. Transient Shear. Approach to Steady State. Figure 1a shows the transient viscosity buildup for 40% (24) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978, 82, 1710.

Rheology and Microstructural Transitions

Figure 1. Response of a 40% Arquad 2HT/water system under constant shear rate: (a) lower shear rates; (b) higher shear rates.

Arquad 2HT at fixed, low applied shear rates. There is a slight viscosity overshoot just visible in the very early stage of the measurement, which is a consequence of the viscoelasticity and yield behavior of the liquid-crystal phase. Once this peak in viscosity is passed, the lamellar phase is believed to be partially oriented in the direction of the shear field.16 This produces a decrease in viscosity because of a lower resistance to the flow, despite a great number of dislocations that move during the flow.3 This may be a reflection of bilayers sliding over one another after alignment of the lamellar phase. Hence, at low shear rates an equilibrium viscosity is reached after the viscosity decay. However, the viscosity increases slowly over longer periods of time which may be related to an increase in the concentration of defects. These are regions where the surfactant bilayers are highly curved, connecting adjacent layers, and so tend to resist the shear deformation of the structure.3 The oscillations recorded in Figure 1 appear to be real and may reflect, for example, large-scale alignment followed by fracture and reordering. At higher shear rates we see a different behavior (Figure 1b). The increase in viscosity with time is more marked and corresponds to a remarkable antithixotropic behavior. This is believed to reflect the gradual development of microstructural change, and electron microscopy revealed the presence of vesicles in the samples after the experiments (Figure 2a,b). The sample is not completely transformed into vesicles as has been observed with other surfactant systems, e.g., sodium dodecyl sulfate/pentanol/

Langmuir, Vol. 17, No. 5, 2001 1333

Figure 2. Electron micrographs of a 40% Arquad 2HT/water system after shearing at (a) 2 s-1 and (b) 200 s-1.

dodecane/water.16 These vesicles are believed to form because, as the shear rate increases, the sample is forced to deform more rapidly and the bilayer oscillations can no longer accommodate the flow, leading to an undulation instability16 and ultimately to the formation of vesicles. Lau¨ger et al.14 indicate that the kinetics of the structural changes are rather complex and the viscosity exhibits a complicated time dependence during formation of vesicles. 3.2. Steady-State Viscosities. Figure 3 presents the steady-state viscosities obtained from the constant shear rate (Figure 1) and the constant stress tests for 40% Arquad 2HT. As can be seen, the results from both tests are quite similar and show a step change in viscosity at a shear rate of between 3 and 4 s-1 or a stress of between 10 and 30 Pa. Electron micrographs indicate that there are no vesicles present after shearing at shear rates or stresses below these values, but vesicles are observed for higher applied shear rates or stresses. Hence, the viscosity jump indeed appears to be a signature of a microstructural transition, with shear leading to a structure including lamellar droplets (Figure 2), having a higher viscosity than the aligned sheet lamellar microstructure. Viscosities obtained in the capillary rheometer are also shown in Figure 3. The viscosities obtained are much lower than those obtained at the corresponding shear rate from the cone and plate geometry. This is probably due to the short residence time, less than 120 s, compared with the time required to reach steady state (of order 6000 s). Consequently, these values are not the equilibrium values. Nevertheless, we again see a step change in the viscosity between about 70 and 80 s-1 or, significantly, at about 30

1334

Langmuir, Vol. 17, No. 5, 2001

Partal et al.

Figure 5. Light micrograph: 54% Arquad 2HT after shearing at 60 s-1 in the shear cell.

Figure 3. Variation of steady-state viscosity with shear rate for 40% Arquad 2HT. Data obtained under different shear conditions.

Figure 6. Example of the hysteresis loop test: 40% Arquad 2HT (maximum stress 100 Pa).

Figure 4. Variation of viscosity with shear stress for 40% Arquad 2HT: examples of yield stress manifestation in steadystate experiments. Table 1. Types of Yield Behavior Observed in Steady-State Experiments type of experiment

Arquad 2HT raw material

Varisoft TA100 raw material

constant shear rate constant stress

types I and II type I

type II type III

Pa, which is very similar to the critical stress values obtained from steady shear experiments. This suggests that the transition from lamellar sheet to vesicles requires a minimum critical stress of between 10 and 30 Pa but that a significant total strain is required so that the time needed for the transition to occur can be substantially reduced by the use of high shear rates.

Data obtained from the constant stress portion of hysteresis loop experiments (described below in section 3.3) are also shown in Figure 3. They show behavior similar to that of the steady-state experiments, i.e., a step change in viscosity at an applied stress of about 30 Pa. The stress dependence of the viscosity is shown in Figure 4, where some of the viscosity data of Figure 3 are plotted against shear stress to give curve I. This indicates that the transition appears to occur around the onset of a “yield stress”, at about 30 Pa, where the viscosity rises significantly and then decreases markedly with further increases in the applied stress. Other patterns of behavior manifesting some kind of yield stress are also exemplified in Figure 4. The behavior shown by curve II was sometimes observed in constant shear rate experiments and indicates decreasing stress with a further increase in the shear rate immediately after the yield stress (in this case a maximum stress) is achieved. Behavior of types I and II has also been observed for the sodium dodecyl sulfate/decanol/ water lamellar-phase system.14 Curves of type III were observed only in constant stress experiments with compositions made from the Varisoft raw material; the

Rheology and Microstructural Transitions

Langmuir, Vol. 17, No. 5, 2001 1335

Figure 7. Electron micrographs for 40% Arquad 2HT after shear under different conditions: (a) stress ramp from 1 to 70 Pa in 15 min; (b) constant stress at 70 Pa for 60 min: (c) stress ramp from 70 to 1 Pa in 15 min, (d) from 1 to 20 Pa in 15 min, (e) from 1 to 70 Pa in 15 min (replicate of a), and (f) from 1 to 120 Pa in 15 min.

appearance of the yield stress was similar to that typically observed in non-Newtonian liquids, i.e., a precipitous drop in viscosity. In all cases, the transition appears to be discontinuous rather than gradual.25 Table 1 summarizes the patterns of behavior observed. Irrespective of the type of yield stress behavior prevailing, vesicles were observed only after passage beyond the yield point, i.e., after higher stresses and/or shear rates. An example of a light micrograph taken after shearing a composition beyond its yield stress in the shear cell is shown in Figure 5. Figure 5 is typical of many which indicate the formation of linear “trains” of defects, like strings of pearls, or indeed of onions, apparently along grain boundaries of the lamellar phase. These defects have the appearance of vesicles but could also be interpreted as focal conics. However, in all cases only a relatively small proportion of the lamellar phase appeared to convert to vesicles. Fewer vesicles were generally observed for samples made from Varisoft TA100 than from Arquad 2HT. (25) Roux, D.; Nallet, F.; Diat, O. Europhys. Lett. 1993, 24, 53.

3.3. Transient Shear-Stress Sweeps. Figure 6 presents an example of the data obtained in a full cycle of increasing, constant, and then decreasing stress. During the first stage, when stress is increasing, shear thinning behavior is observed; i.e., the viscosity decreases. However, in this first stage, the viscosity reaches a minimum and the shear rate reaches a maximum value (200 s-1); subsequently, the viscosity rises and the shear rate decreases. The sharp decrease in viscosity followed by a gradual rise is essentially a composite of the features seen for constant stress experiments. The minimum in the viscosity is taken to be an indication of a microstructural transformation. This transformation takes place between about 20 and 40 Pa. Indeed, as shown in Figure 7, vesicles are already observed at the end of the increasing-stress ramp but only if the final stress is sufficiently high. Furthermore, if the system is then conditioned for an hour at the maximum stress level, the viscosity continues to rise and the vesicles are still observed (Figure 7b). However, when the stress value is decreased, in the third stage, the vesicles vanish (Figure 7c). This indicates that

1336

Langmuir, Vol. 17, No. 5, 2001

Partal et al.

Figure 8. Magnitude of the apparent complex viscosity as a function of time for different fixed values of the oscillation frequency and maximum stress: 40% Arquad 2HT.

the phase transition is reversible under the action of shear, as has been found for other surfactant systems.14 3.4. Oscillatory Shear. Figure 8 shows data for 40% Arquad 2HT subjected to oscillatory shear at stresses outside the linear viscoelastic region. The complex viscosity is not well-defined for nonlinear oscillations, but the magnitude of the apparent complex viscosity serves as a useful measure of the overall rheological response. The curves show an increase in the apparent complex viscosity with time. Vesicles were observed in samples sheared at high stress (300 Pa) and at a frequency of 1 Hz (Figure 9a), so shear-induced microstructural transformation must occur under such conditions. Vesicles were also observed at 0.1 Hz at lower applied stress (50 Pa). Figure 8 shows a similar viscosity behavior in this case. However, shearinduced vesicle formation was reduced when the applied frequency was 10 Hz (Figure 9b), even with a high peak stress (200 Pa). In this case, Figure 8 shows a nearly constant, low apparent complex viscosity. The higher the frequency, the shorter the time for each oscillation and the smaller the associated deformation (strain). Hence, it would appear that the formation of vesicles requires a minimum deformation (or input energy) in addition to a minimum stress. 4. Construction of Orientation Diagrams

Figure 9. Electron micrographs of 40% Arquad 2HT after oscillatory shear: (a) 1 Hz, 300 Pa; (b) 10 Hz, 200 Pa.

Only one type of transition was observed in this study; in fact, this was a partial transition from a sheet lamellar structure to a mixture of vesicles and sheet lamellar structures. Orientation diagrams, therefore, take the form of plots of the critical stress required to generate vesicles against the surfactant concentration. The results from section 3 provide different signatures of this transition. In the steady-state experiments, the critical stress is marked by one of the three types of responses shown in Figure 4. Manifestations of critical stress in other experiments are shown in the Supporting Information described at the end of the paper. In creep tests, the critical stress marks the transition between very small strain values and the occurrence of large strain, often after a significant time delay. In slowly increasing exponential stress sweeps, the critical stress corresponds to a marked decrease in viscosity similar to the type III data in Figure 4. In (nonlinear) oscillatory stress tests, the critical stress amplitude is that where the apparent elastic modulus shows a dramatic decrease and crosses below the apparent

viscous modulus. The exponential stress sweeps and the oscillatory stress tests are inherently unsteady, and it is found that the critical stress increases with the unsteadiness of the flow (higher ramp rate or higher frequency). Figure 10 shows plots of the critical stress versus concentration for each series of experiments with the Varisoft TA100 raw material. Figure 10a indicates that there is a general increase in the critical stress as the surfactant concentration increases. This is to be expected and is consistent with other surfactant systems.16 The data exhibit significant scatter; nevertheless, it is possible to discern some trends. The critical stress appears to increase up to about 20% and above about 40% surfactant concentration but to be approximately constant at around 15-20 Pa between 20% and 40% surfactant concentration. With some exceptions, the general trend is also for creep and steady-state data (solid symbols) to give slightly lower critical stresses than slow stress sweeps or oscillatory data. This indicates that the critical stress for vesicle formation

Rheology and Microstructural Transitions

Figure 10. Variation of critical stress with concentration (orientation diagrams) for the Varisoft TA100/water system: (a) data for four types of rheological tests; (b) influence of the relative time in the shear field.

is larger for a shorter residence time in the shear field. In Figure 10b, comparison is made of averaged data for “long times” (10 s dwell time in transient sweeps, or 0.1 Hz oscillatory sweeps) and “short times” (1 s dwell time, or 1 Hz) in the shear field for the two types of experiments. Despite significant scatter and some exceptions, longer time in the shear field is generally associated with a lower critical stress. There is also considerably more scatter in the data corresponding to shorter times, possibly indicating a greater influence of sample history effects on those data. 5. Conclusions The dialkyldimethylammonium chloride (Arquad 2HT and Varisoft TA100)/water systems exhibit a microstructural transition from lamellar sheets to lamellar vesicles under the action of shear. In all cases only a portion of the sample is observed to be vesicular, although vesicle

Langmuir, Vol. 17, No. 5, 2001 1337

formation appears to be particularly prevalent along lamellar-phase grain boundaries. The size of the vesicles is inversely proportional to the stress applied. For a given surfactant concentration, the critical stress for the transition to occur is always lowest in steady-state experiments. The critical stress increases with the degree of unsteadiness in an experiment. For a given stress, above the minimum (steady-state) critical stress, a minimum residence time in the shear field is necessary. The residence time necessary to achieve the transition can be much reduced by the use of high shear rate. Hence, there appears to be a minimum strain required for vesicle formation, in addition to a minimum critical stress. In oscillatory measurements, application of low frequencies, which correspond to larger deformations for a given stress amplitude, promotes the transition even at modest stress, while the transition may not occur at high frequency even with much higher peak stress. In transient shear this corresponds to a longer application time. The rate of increase in the shear stress and its maximum value are also shown to be important in promoting the phase transition. For a given maximum stress value, the transition occurs more readily for a lower ramp rate, again probably because of higher strains prevailing. The transition is reversible, with destruction of vesicles occurring on subsequent shearing at subcritical stresses. After shear treatment, the final product viscosity can be higher or lower than that of the original material. High shear treatment induces a large number of defects in the lamellar phase. Higher postshearing viscosities perhaps result from the persistence of a larger number of defects as a result of rapid or incomplete vesicle destruction, while lower viscosities perhaps arise because more of the defects are annealed during a slow stress reduction ramp. The critical stress increases with surfactant concentration, though between 20 and 40% surfactant concentration the critical stress appears to remain relatively constant at around 15-20 Pa. Arquad and Varisoft surfactant compositions behave similarly for surfactant concentrations below about 40%. At higher concentrations significantly higher critical stresses are observed in the case of Varisoft, which has a longer and more uniform chain length. Acknowledgment. Part of this work was jointly sponsored by the Universidad de Huelva and Unilever. M.G.B. is supported by EPSRC and Unilever. The authors gratefully acknowledge their financial support. Supporting Information Available: Further experimental results are presented as Supporting Information available from the authors and the publishers. Transient and steady-state results are presented for constant stress experiments, creep tests, and particle size determination. The effects of maximum stress and ramp time on the hysteresis loop tests are presented, as are exponential stress sweeps with different dwell times. Further results are given for the oscillatory tests, including the influence of large-amplitude oscillatory stress conditioning on the subsequent linear storage and loss moduli of the sample and oscillatory stress amplitude sweeps. Finally an orientation diagram comparing Varisoft and Arquad is presented. This material is available free of charge via the Internet at http://pubs.acs.org. LA0007731