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J. Phys. Chem. B 1999, 103, 11218-11226
Flow Studies of a Surfactant Hexagonal Mesophase A. E. Terry,* J. A. Odell,* R. J. Nicol,† G. J. T. Tiddy,† and J. E. Wilson‡ H.H. Wills Physics Laboratory, UniVersity of Bristol, Tyndall AVenue, Bristol, BS8 1TL U.K., Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester, M60 1QD U.K., and UnileVer Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside, L63 3JW U.K. ReceiVed: June 2, 1999; In Final Form: October 6, 1999
We utilize real-time scattering techniques to examine the mesoscopic and molecular response of a nonionic surfactant hexagonal mesophase to shear and extensional flows. These results are correlated with the rheological response in simple shear. During shear there is an initial modest orientation of the surfactant rods along the flow direction. This is followed by a progressive further increase in alignment over the next 500 shear units. This process of rod alignment corresponds to the progressive development of shear thinning, which might be anticipated from molecular theories developed for liquid crystalline polymer systems (LCPs). It is surprising, however, that equilibrium requires such large strains, not seen in LCPs, possibly due to a unique feature of the surfactant mesophase: the ability to rupture under stress and reform. This long structure induction time has clear implications for conventional rheology, which may rarely reach an equilibrium response. By contrast, in an extensional flow the orientation is achieved much faster, over about two strain units, and is much more perfect (P2 ∼ 0.9). Threads spun are stable, which may point to extensional thickening. It is likely that in real flows, relevant to industrial applications, it is the response to extensional components of the flow (such as convergent channels, flow around obstacles, bifurcations) which dominates the behavior of the mesophase.
Introduction Surfactant molecules aggregate in solution to form micelles, sometimes many 100s of nm in size. At high concentrations these micelles become ordered to give a variety of liquid crystalline phases.1,2 The effect of flow upon these mesophases is of both practical and theoretical interest. The practical interest arises from the fact that in the manufacture and use of surfactant products, the materials will experience flow regimes combining both shear and extensional flow. The world market in surfactants is ca. 108 tonnes p.a., almost all of which experiences a mesomorphic state under flow, often several times. At present, the manufacturing process is often limited by product rheology. Theoretically, there is interest in the flow response of lyotropic mesophases because they are complex fluids with ordered arrays existing over a variety of length and time scales. In the past, theories that concentrate on rigid, inextensible systems have been suggested to explain the behavior of rod mesophases, principally with application to liquid crystalline polymers (LCPs). However, such theories do not address the fact that in surfactant systems the aggregates are able to break and reform during flow. The shear flow response therefore may not be explained merely in terms of the theories of, for example, Marrucci and Larson.3,4 Most research has been carried out on the shear flow response of lamellar phases, the most common structure. Early rheological studies report the occurrence of a yield stress followed by shear thinning behavior. In later work, particularly by Roux and coworkers,5 the sequence of morphological changes in the lamellar macro-domains under shear flow has been documented. For other phases, such as hexagonal and cubic structures, much less is known. Dimitrova et al.6 have reported rheological data for * Corresponding author address: H.H. Wills Physics Lab., University of Bristol, Tyndall Ave., Bristol, BS8 1TL U.K. † UMIST. ‡ Unilever Research Port Sunlight Laboratory.
a nonionic surfactant hexagonal phase, where the viscoelastic properties were studied. Schmidt7 has given an extensive account of hexagonal phase reorientation under shear. While these papers do give valuable descriptions of the behavior, there is at present no understanding of the link between surfactant chemical type, aggregate architecture and rheology, even at a qualitative level. Furthermore, and we feel vitally, the effect of extensional flow upon mesophases has been ignored. Over the past 30 years, the importance of surfactant chemical structure (headgroup size, hydrophobic group length and volume) in determining mesophase architecture has been elucidated using concepts such as alkyl chain “packing constraints” (see ref 2 and the references therein). Now one can deduce the mesophase behavior for complex systems containing novel single and mixed surfactants from these concepts (also using the library of data already in the literature), with at least semiquantitative accuracy. It is timely to extend this understanding to a consideration of kinetic processes such as mesophase formation, mixing phenomena, and rheology, all of which have an important influence in the user properties of surfactants and have implications for the behavior of natural surfactants in biomembranes. In this paper we address the flow properties of the hexagonal phase, in particular the importance of extensional flow over shear flow during processing of this phase. Later papers will describe results for other lyotropic mesophases. We have selected the series of nonionic surfactants with polyoxyethylene headgroups [CH3(CH2)n(OCH2CH2)mOH, CnEOm] for initial study because we have previous experience of their preparation and there already exists a wide range of data on their properties. In this paper we report on the behavior of a commercial surfactant, Imbentin Coco 6.5EO, with some data also for the pure surfactant C12E6. We investigate the way that the mesophase responds to an imposed flow field either by a change in the aggregate structure
10.1021/jp9917852 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/30/1999
Surfactant Hexagonal Mesophase Flow Studies
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TABLE 1: Ethoxylate Distribution of Imbentin Coco 6.5EO EO
mass area
area %
mol frac
mol %
mean EO
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
11073783 9522708 15808353 20324555 23879943 28277746 30415987 33321210 34408027 34003768 32291519 29893335 26224520 22837646 19178024 14884310 11593111 8477784 5656672 3515283 1929430 1084068 638318 211453 225541
2.64 2.27 3.77 4.84 5.69 6.74 7.25 7.94 8.20 8.10 7.69 7.12 6.25 5.44 4.57 3.55 2.76 2.02 1.35 0.84 0.46 0.26 0.15 0.05 0.05
0.0136 0.0096 0.0134 0.0149 0.0154 0.0163 0.0158 0.0150 0.0137 0.0121 0.0105 0.0087 0.0071 0.0056 0.0042 0.0031 0.0021 0.0014 0.001 0 0 0 0 0 0
6.82 4.78 6.69 7.44 7.70 8.14 7.92 7.91 7.51 6.87 6.07 5.25 4.33 3.55 2.82 2.08 1.54 1.07 0.68 0.41 0.21 0.16 0.07 0.02 0.02
6.96
Materials and Methods
TABLE 2: Chain Length Distribution of Imbentin Coco 6.5EO sample 1 C10 C12 C14 C16
sample 2 0.8% 71.7% 26.9% 0.6%
C10 C12 C14 C16
and relaxation of orientation within the materials may be investigated using X-ray diffraction, small-angle light scattering, or changes in birefringence. When combined with conventional rheological data, the relationship between mesophase properties, macroscopic rheological behavior, and microscopic structure can be probed for both model and commercial surfactants.
0.7% 73.0% 25.9% 0.4%
and/or dimensions, or by reorientation of the aggregate or domain alignment. To study the effects of flow, we have built various in-situ flow cells (elongational as well as shear), allowing real-time scattering studies to be performed during either shear or extensional flow. In this way, the development
Figure 1. Phase diagram of an aqueous solution of Imbentin C12EO6.5.
The commercial surfactant Imbentin Coco C12EO6.5 was used as received from Chemische Fabrik, Hedingen, Switzerland. The sample was dried in a vacuum oven for 24 h prior to dissolution in the desired amount of distilled water. The sample was mixed and allowed to cool ambiently to room temperature before centrifuging to remove any air bubbles. The ethoxylate and alkyl chain length distribution of the surfactant were measured by gas chromatography mass spectroscopy (GCMS), the mean ethoxylate distribution was 6.96 and the chain length varied between C10 and C16, with 73% of the sample being C12, (see Tables 1 and 2). The phase diagram of the surfactant with water, Figure 1, was constructed using polarized optical microscopy, differential scanning calorimetry (DSC), and X-ray diffraction. To investigate the effects of flow at room temperature, a 45 wt % surfactant composition was chosen, as this enabled effects resulting from proximity to phase boundaries to be ignored. It lies in the composition region where the hexagonal phase has the largest temperature range and is well away from phase boundaries with other mesophases. The shear and extensional flow response of the hexagonal phase were investigated using X-ray diffraction. During steady shear flow, in-situ X-ray measurements were made with a parallel plate shear cell, as shown schematically in Figure 2(a). A “sandwich” of boron nitride and Kapton film was used for the shearing plates of the cell and also acted as a window for the X-ray beam; both materials are highly transparent to X-rays, the boron nitride provided excellent mechanical rigidity while
11220 J. Phys. Chem. B, Vol. 103, No. 50, 1999
Figure 2. (a) Schematic of X-ray shear cell. (b) Schematic of X-ray extrusion cell.
the Kapton eliminated problems due to the porosity of the sintered boron nitride. The front plate, of diameter 22 mm, was rotated with an angular velocity ω while the back plate was stationary. This geometry means that the shear rate is γ˘ ) (rω/ t) where t is the sample thickness and r is the radius at which the X-ray beam intersects the sample. With the present design, shear rates of 1 s-1 to 100 s-1 are attainable. Following the application of a steady strain to the mesophase, a sequence of diffraction patterns were collected to monitor the change in the micellar orientation. The materials were also observed directly during shear in the polarizing optical microscope using a Linkam shearing hot stage of similar geometry. Extensional flows were produced by extrusion of a thread of the hexagonal phase into the X-ray beam. An extrusion cell, Figure 2(b), was built consisting of a stainless steel cylindrical chamber with a 1 mm diameter, 17 mm long extrusion die at the bottom. The hexagonal phase was loaded into the chamber and the chamber sealed. An overpressure of nitrogen gas was applied to the cell to extrude the sample, the pressure being regulated to alter the extrusion rate of the fiber. The extrudate was allowed to fall freely in atmosphere into a collecting dish. By careful positioning of the extrusion cell, the X-ray beam passed through the extruded thread. The height of the die exit above the beam was varied. In this way the effects of orientation within the die were examined close to the exit of the die, that
Terry et al. is, within 5 mm of the exit. By raising the cell, the effects of the amount of post-extrusion stress could also be examined within the fiber, up to 50 mm below the exit. Water loss might be a problem with extensional flow, but should result in a change in the d spacing. This was not observed (see below). Both synchrotron radiation and in-house X-ray facilities were employed for time-resolved X-ray diffraction studies of the development of orientation during either shear or extensional flow. The facilities used were stations 16.1 (λ ) 0.14 nm), the camera length ranges from 1 m to 6 m, which allows spatial distances of up to ∼3000 Å. Excellent signal/noise measurements are obtained because the detectors are able to accumulate the data. Station 8.2 (λ ) 0.154 nm) was also used at Daresbury Laboratory. A rotating anode X-ray generator (λ ) 0.1542 nm) coupled with a Siemens 2-D GADDS detector at Bristol University. Small-angle X-ray data were collected using an evacuated beam line at a sample to detector distance of 1 or 1.2 m. To follow transients upon the start up of shear, the data collection times were 10 s/frame, for the initial transients, which were then extended to 30 s/frame. The data were corrected for absorption and spatial distortions before calculation of the order parameters. The orientational order parameter P h 2 was calculated from the azimuthal intensity variation of the (100) signal to give a measure of the degree of micellar alignment, according to the closed form method suggested by Deutsche; P h 2 ) 0 means that there is no overall orientation, P h 2 ) 1 signifies perfect micellar alignment. The dynamic experiments reported here present problems for the evaluation of P h 2. Even in the best oriented patterns with sharp peaks on the equator there is considerable noise: the value of P h 2 is strongly affected by noise on the meridian. Of course we can subtract background scatter, but the random nature of the scatter results in some pixels being under- and others overcorrected. The problem can be alleviated to some degree by smoothing and weighting the equator more highly, but the values of P h 2 here should be regarded as indicative of the development of orientation rather than absolute values. In comparing the quality of orientation we have preferred to use the peak width at half-height (PW), which is relatively unaffected by noise. Rheological measurements were made using a Rheometrics RMS800 with a plate and cup attachment, so mirroring the parallel plate geometry of the shear cell. The cup allowed containment of the sample during shear. A saturated water vapor atmosphere was used to reduce the effects of solvent loss during the experiments. Steady strains of the same magnitude as those used within the shear cell were applied to the phase. It is necessary to define the starting condition of the sample for any rheological measurement since previous shear and thermal history will greatly influence any measurements made. As with some liquid crystalline polymers, it is possible to remove any orientation induced upon loading the cells by heating the mesophase to an “isotropic” solution, that is, one that does not exhibit liquid crystallinity, and then cooling to the desired temperature. For these surfactant solutions, it was found best to preheat the cells to 50 °C, load the samples at this elevated temperature, and allow them to cool ambiently to room temperature. It was confirmed that this procedure removed any orientation since by diffraction there was no residual orientation upon applying this temperature cycle to a previously oriented sample, and rheologically the sample returned back to the initial starting viscosity. Care must be taken to ensure that no concentration changes occur during this treatment.
Surfactant Hexagonal Mesophase Flow Studies
J. Phys. Chem. B, Vol. 103, No. 50, 1999 11221
(a)
(b)
Figure 3. (a) X-ray diffraction pattern for unoriented C12EO6.5 (45 wt % surfactant). (b) Integration of intensity around χ, for the first reflection, showing angular independence.
Results A typical X-ray diffraction pattern recorded on an area detector, Figure 3, for the unoriented hexagonal mesophase of C12EO6.5 shows diffraction maxima at d spacings of 60, 35, and 30 Å, with only statistical variation in intensity for variation of the scattering angle from the symmetry axis, χ. These maxima correspond to the (100), (110), and (200) reflections respectively, as expected for the separation of micellar rods packed within a hexagonal lattice, with an interrod spacing of 69 Å. The absence
of χ angular structure indicates that we have achieved a polydomain texture where there is no global orientation of the rods; however, locally there may be regions or domains of high orientation. Shear Flow. The unoriented state was sheared at ∼1 s-1 and the material was observed to orient, that is, the (100) and (110) reflections lost their isotropic angular independence and each collapsed to two arcs centered on the equator, at χ ) ((π/2), Figure 4. This is consistent with the rods within the hexagonal phase aligning parallel to the velocity direction, V, without
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(a)
(b)
Figure 4. (a) X-ray diffraction pattern for C12EO6.5 (45 wt % surfactant) sheared at 1 s-1. (b) Integration of intensity around χ, for the first reflection, showing angular dependence.
disrupting the hexagonal packing. No changes in the positions of the diffraction maxima were observed. The degree of orientation rapidly increased upon starting shear, without observing an initial delay, that is, a low degree of orientation is observed in the diffraction pattern taken within the first 10 s of shear. It should be noted that P h 2 takes at least 500 shear units to reach a constant value of 0.84, Figure 5. The peak width at half-height for the best oriented material in shear was found to be 23°. At other shear rates examined from 0.1 to
10 s-1, the final steady-state degree of orientation obtained was the same. The amount of strain (number of shear units) required to achieve this final orientation was also constant, independent of applied shear rate. The comparison between P h 2 and the change in the viscosity of the mesophase at the same shear rate with time, Figure 6 shows that an initial sharp increase in the degree of global orientation was accompanied by a sharp decrease in the
Surfactant Hexagonal Mesophase Flow Studies
Figure 5. P h 2 vs time at a shear rate of 1 s-1.
J. Phys. Chem. B, Vol. 103, No. 50, 1999 11223 developed during shear. Comparing the two diffraction patterns, Figure 7 and Figure 5, it is clear that the orientation in the extruded fiber is much higher than for the sheared sample; however, as previously discussed, the decreased signal-to-noise ratio also acts to reduce the calculated value for P h 2. The extensional flow pattern has a peak width at half-height of 9.9° compared to 23° for the best oriented shear patterns. Interestingly, an increase in the extrusion rate decreases P h2 for the sample within the die, but the sample further down the fiber is at a constant orientation. No change in the position of the first reflection of the hexagonal phase (100) was observed during the experiment, within experimental inaccuracies due to the thread moving as it falls into a container, confirming that no water loss occurred. The second reflection (110) was present although its intensity is greatly reduced relative to the first reflection and it was only just above the noise in the system. Discussion
Figure 6. Viscosity vs time C12EO6.5 (45 wt % surfactant) at a shear rate of 1 s-1.
viscosity. However, the viscosity continued to exhibit shear thinning, although at a slower rate after 1000 shear units, which did not have a corresponding slow increase in P h 2 and did not reach a plateau in the time scale of this experiment. No relaxation was observed within this mesophase upon stopping shear at any degree of orientation. Results from the Linkam optical shear cell showed a uniform velocity field with a fine local texture corresponding to the liquid crystalline domain structure. Extensional Flow. Using the extrusion cell, stable fibers were spun from the C12EO6.5 hexagonal mesophase, which exhibits very little die swell. These fibers were able to support their own weight without apparently stretching and maintained their shape for at least 1 h after extrusion if solvent evaporation was minimized, by immersion in a saturated atmosphere. Real-time X-ray diffraction of the fiber during extrusion was performed using the rotating anode and area detector at Bristol. When the height of the extrusion die above the X-ray beam was low the orientation developed within the die was examined. This showed that the hexagonal rods aligned parallel to the long axis of the fiber, but that the degree of orientation induced by the extrusion through the die was relatively low at extrusion rates of 0.9 to 2.5 mm/s for a thread of diameter 1 mm. At slower extrusion times of 0.1 to 0.4 mm/s, the degree of induced orientation was higher. X-ray diffraction from further below the die showed very high degrees of orientation, Figure 7, significantly higher than those observed during shear. The orientation achieved within this fiber gives rise to an estimated P h 2 of 0.86, compared to 0.84
The shear flow response and hence the decrease in viscosity of the hexagonal mesophase of C12EO6.5 can be explained by the theories for the shear flow response of liquid crystalline polymers; either by the progressive alignment of rods parallel to the velocity direction, which was an extension of the DoiEdwards theory8,9 for the dynamic situation by Hess10 and Doi,11 or by the domain flow model of Onogi and Asada.12 Following the molecular theory of Doi-Edwards, in the liquid crystalline mesophase, the rods are aligned within domains. This cooperative alignment considerably reduces the probability of collisions between rods and hence the diffusivity of the rods is increased. As the shear flow imposes a global orientation within the mesophase, so the viscosity will reduce further as the rods align parallel to the velocity direction. This is in accord with the predictions of the molecular theories of Marrucci and Larson for the shear flow response of liquid crystalline polymers. The model of Onogi and Asada predicts three flow regimes within the shear flow response of a liquid crystalline mesophase. Taking a polydomain structure, they suggest that initially at low strains, shear thinning arises as domains flow past one another with no global reorientation of the domains into the velocity direction. As the strain rate is increased, the global orientation within the sample will begin to increase as the domains start to align with the velocity direction and the viscosity will plateau. At higher strain rates, shear thinning is resumed as the material transforms into a monodomain. Such molecular theories could form the basis for understanding the flow of surfactant mesophases, but there are important differences. In the case of LCPs the rods are essentially immutable. This is not the case for surfactant mesophases, where, under ambient conditions, molecules diffuse in and out of the rods and they form an equilibrium length under the influence of thermal fluctuations and the energy required for breakage. Under flow the rods can break if under sufficient stress and subsequently reform. This is likely to happen at entanglement points as envisaged in the molecular theories. Further, the surfactant rods are likely to be relatively flexible, which is not the case in the molecular theories of LCPs, normally treated as rigid and inextensible. One significant difference which arises between the response of surfactant mesophases and liquid crystalline polymers is the time scale required to achieve a steady-state orientation. The hexagonal mesophase of C12EO6.5 takes 100s of shear units, whereas in a lyotropic nematic mesophase this will be of the
11224 J. Phys. Chem. B, Vol. 103, No. 50, 1999
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(a)
(b)
Figure 7. (a) X-ray diffraction pattern for C12EO6.5 (45 wt % surfactant) extruded at a rate of 0.9 mm/s. This pattern was taken 40 mm below the exit of the die. (b) Integration around χ, for the first reflection, showing angular dependence.
order of 10s of shear units. Even comparing the shear flow response of the surfactant mesophase with that of a semiflexible liquid crystalline polymer solution, for example, aqueous (hydroxypropyl) cellulose solutions,13,14 a large difference still arises in the time required to achieve a steady-state response. The very long induction times to reach a final steady-state orientation and viscosity means that care must be taken when constructing a steady-state shear viscosity plot. It must be ensured that at each shear rate examined, a final constant value has indeed been achieved. In simple shear the hexagonal mesophase showed progressive orientation parallel to the velocity direction throughout. This is in agreement with several authors15,16 studying hexagonal phases of copolymers, but in contrast to reports based upon small angle
light scattering that suggest an initial orthogonal orientation corresponding to a log-rolling mode envisioned for LCPs.17 No relaxation was observed in these mesophases upon stopping shear. This is entirely consistent with the induced orientation depending only on the total applied strain, independent of strain-rate. The lack of relaxation contrasts with the behavior of LCPs which normally relax cooperatively through “banded structure”.3,4 Such relaxation may be entropic or due to the presence of defects. Certainly the entropic contribution would be much less in surfactant mesophases: it is also possible that, since rods can rupture and reform in the most stable configuration, that the defect density is also low. In an extensional flow, the degree of orientation achieved is much higher than for a comparable amount of shear strain.
Surfactant Hexagonal Mesophase Flow Studies However, it is only after being exposed to post-extrusion stresses that the highest orientations are induced. The actual degree of orientation induced upon passing through the extrusion die is very low. This may be because the rotational component of the shear flow, which will be present at the walls of the 17 mm long die, frustrates the orientation induced by extension into a 1 mm diameter die. The imposed post-extrusion stress as the fiber hangs under its own weight improves greatly the degree of orientation achieved within the fiber, P h 2’s achieved have been of the order of 0.9. The rods are aligned parallel to the fiber long axis. We estimate the strain of the fiber to be around 2. How the material is able to improve its orientation while not changing the hexagonal spacing d and with little apparent stretching the fiber must be explained. It is clear that an affine stretching of the system would require a reduction in interrod hexagonal spacing. It must be remembered that in a surfactant system the rodlike micelles are not truly inextensible, rather they are capable of fracture or scission during flow and of reformation.18 Once the stress experienced by a rod reaches a certain level, the rod will fracture; this shorter rod is then able to withstand larger stresses. The tension (T) in an isolated rod (length L, radius r) in extensional flow in a fluid of strain rate ˘ and viscosity ηs is given19 by
πηs˘ L2 2 ln(L/r) In order for the rod to break, this tension should exceed of order E/r, where E is the energy of rupture (of the order of a few kT) predicting T of order 8 × 10 -12 N. Combining these 2 relations and approximating the logarithmic term yields the equilibrium length (Le) as a function of strain rate
Le ≈
x
20E πrηs˘
Taking reasonable values for E (2kT), ηs (0.01 Pa.s), r (1 nm) and taking the strain rate as 10 s-1 yields an equilibrium length of around 25 µm. At the same time we can estimate the equilibrium stress during extrusion from the mass of the supported fiber, this predicts an average stress per rod of 0.5 × 10-13 N. This is significantly lower than the predicted rupture stress, but the stress will be borne very unevenly by the rods as a function of their length and disposition. We are presently unable to answer the question of whether the rods break or the liquid crystal disrupts in order to accommodate the elongation. The above calculations suggest that any reduction in length of rods at these strain rates will not be enough to affect the extensional viscosity, so that the filaments are probably extension thickening and hence spin stable fibers. It is interesting to note that the relative weakness of higher order reflections from the highly oriented stretched filament might point to a disruption of the long-range liquid crystalline order, consistent with “defect” structures permitting flow. We might speculate that the energy for breakage might be significantly lowered in this commercial system due to the known polydispersity, with rod scission preferentially occurring at large headgroups, which are expected to be energetically less stable. We propose in future works to investigate the pure surfactant system in extensional flow.
J. Phys. Chem. B, Vol. 103, No. 50, 1999 11225 The time taken to perfect the degree of orientation during extension is very short, requiring only a few strain units, which should be compared to the 100s of strain units required during shear flow. It is likely that, in real flow situations which contain both extensional and shear components, the molecular and rheological response of surfactant mesophases will be dominated by extensional components. We note here the well-established importance of extensional flows in orientation in liquid crystalline polymer systems.3,4 Much work remains to be done on the extensional behavior, to determine links between molecular orientation, mesoscopic structure, and extensional rheology. Obviously these results highlight the importance of postextrusion stress upon the surfactant mesophase during the processing of these materials and also, perhaps, the role of shear flow in frustrating the orientation developed during extension. If we take a simple case of filling a container by extruding a mesophase through a nozzle, the material at the bottom of the container will experience higher stresses and therefore be more highly oriented than the material at the top of the container. Most commercial systems for filling containers do not raise the nozzle as the container is filled in order to keep the height of the nozzle to the level of product constant. If these stresses were able to promote a phase change or to simply modify the viscosity, this could have serious complications for product uniformity and control, especially as the relaxation times within these mesophases are clearly extremely long. Conclusions Understanding the effect of extensional flow upon surfactant mesophases is vital in the manufacture and use of surfactant products. When comparing the influence of extensional and shear flow fields upon a hexagonal phase, we have shown that the shear flow has only a modest effect on the phase, requiring very large strains to modify the microstructural orientation. In contrast, an extensional flow field is able to induce very high orientations at only a few strain units. Extensional flow is therefore of greater importance than shear flow within this mesophase, a point we feel is neglected by many manufacturers who consider only the effect of shear flow upon their products. This is possibly because of the ease with which the effect of shear upon viscosity, for example, can be quantified. Acknowledgment. We are indebted to Mr. Richard Exley of Bristol University for his involvement in the design and construction of the shear cells and to the Soft Solids Program of the EPSRC for financial support. References and Notes (1) Small, D. M. The Handbook of Lipid Research - The Physical Chemistry of Lipids; Plenum Press: New York, 1988. (2) Fairhurst, C. E.; Fuller, S.; Gray, J.; Homes, M. C.; Tiddy, G. J. T. Lyotropic Surfactant Liquid Crystals in Handbook of Liquid Crystals”, Demus, D. et al., Eds.; Wiley-VCH: 1998, vol. 3, ch. VII. (3) Marrucci, G.; Maffetone, P. L. Macromolecules 1989, 22, 4076. (4) Larson, R. G. Macromolecules 1990, 23, 3983. (5) Diat, O.; Roux, D. J. Phys. II France 1993, 3, 9. (6) Dimitrova, G. T.; Tadros, Th. F.; Luckham, P. F. Langmuir 1995, 11, 1101-11. (7) Muller, S.; Fischer, P.; Schmidt, C. J. Phys. II France 1997, 7, 421. (8) Doi, M.; Edwards, S. F. J. Chem. Soc., Faraday Trans. 2 1978, 74, 560. (9) Doi, M.; Edwards, S. F. J. Chem. Soc., Faraday Trans. 2, 1978, 74, 919. (10) Hess, S. Z. Naturforsch. 1976, 31A, 1034. (11) Doi, M. J. Polym. Sci. Polym. Phys. 1981, 19, 229.
11226 J. Phys. Chem. B, Vol. 103, No. 50, 1999 (12) Onogi, S.; Asada, T. Rheology; Astarita, G., Marrucci, G., Nicolais, L., Eds.; Plenum Press: New York, 1980; Vol 1. (13) Keates, P.; Mitchell G. R.; Peuvrel-Disdier, E.; Riti, J. B.; Navard, P. J. Non-Newtonian Fluid Mech. 1994, 52, 197. (14) Terry, A. E. Ph.D. Thesis, University of Bristol, 1997. (15) Morrison, F. A.; Winter, H. Macromolecules 1989, 22, 3533.
Terry et al. (16) Tepe, T.; Schulz, M. F.; Zhao, J.; Tirrell, P.; Bates, F. S.; Mortensen, K. Almdal, K. Macromolecules 1995, 28, 3008. (17) Richtering, W.; Lauger, J.; Linemann, R. Langmuir 1994, 10, 4374. (18) Cates, M. E.; Candau, S. J. J. Phys.: Cond. Mater. 1990, 2, 6869. (19) Odell, J. A.; Taylor, M. A. Biopolymers 1994, 34, 1483.
