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Formation of AOT/Brine Multilamellar Vesicles Johan Bergenholtz and Norman J. Wagner* Center for Molecular & Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received August 18, 1995. In Final Form: February 14, 1996X A rheo-optic study of the formation kinetics of the multilamellar vesicle (MLV) structure in the AOT/ brine system is reported. Controlled shear rate rheology and flow dichroism are used to identify a critical shear stress below which MLVs are formed and above which a transition to a biphasic state occurs. Controlled stress rheology with in situ flow small-angle light scattering shows that MLV formation occurs at a critical strain that is signaled by shear-thickening behavior and small-angle scattering typical of spherulites. Further, the applied shear stress controls both the MLV formation rate and the final MLV size. At high stresses shear-induced ordering of the MLVs is observed, similar to that seen in colloidal suspensions, as well as a transition to a biphasic state. The results are used to reinterpret previously published “nonequilibrium phase diagrams” and to test scaling models for the MLV size in the pure MLV phase.
Amphiphilic molecules, because of their solubility properties, aggregate into molecularly ordered structures, which often take the form of bilayers. When these bilayers curve and close, the resulting aggregates are called vesicles. This vesicle phase can consist of unilamellar vesicles or multi-lamellar vesicles (MLVs), the latter of which will be the subject of this study. When the amphiphilic molecules in addition are phospholipids, the vesicles are known as liposomes. Liposomes, due to their unique properties, e.g. encapsulation, permeability, and similarity to biological systems, have received considerable attention and are candidates for novel applications in biochemical catalysis, drug delivery, and cosmetics.1 The formulation of technologically useful applications of MLVs is complicated, however, by the fact that these are often formed under nonequilibrium conditions. Also, a reliable preparation procedure which gives reproducible MLV assays has been lacking. Recently however, it was discovered that lamellar phases of ionic surfactants in the absence of excess solvent can be driven to form an MLV structure through the application of a controlled shear rate.2,3 This procedure results in reproducible samples of relatively monodisperse MLVs. The advantage of this technique over more traditional ones, e.g. extrusion, sonication, and uncontrolled shearing in excess solvent, is that the MLV encapsulation capacity and size can be controlled, the former by adjusting the membrane spacing through dilution and the latter by controlling the flow field. The initial work on the AOT/brine2,3 (AOT is the sodium salt of di(2-ethylhexyl) sulfosuccinate) and the SDS/ pentanol/water/dodecane4-6 systems has resulted in the construction of “nonequilibrium phase diagrams” (structure vs shear rate), which are useful in that they allow identification of a processing window for a desired application. According to these, the MLV structure is observed intermediate between oriented lamellar states, as controlled by shear rate. In this study we focus on the AOT/brine system and use only the composition originally
investigated by Diat and Roux.2 They found that the initial lamellar phase is characterized by a pronounced defect texture, which is thought to be a factor in MLV formation.2 The final oriented lamellar state has been reported to be preceded by a biphasic state consisting of MLVs coexisting with oriented lamellae.4 They further report that the MLV size can be controlled through the applied shear rate γ˘ . A scaling relationship2,4,5 between MLV radius (RMLV) and shear rate has been proposed where RMLV ∝ γ˘ -1/2, which can be used to effect the detailed tailoring of the MLV structure. Note that this MLV structure is thermodynamically unstable; it is kinetically stable for long periods of time,2 enabling visual confirmation of the existence of this monodisperse MLV structure.2,4 In their work, Diat and Roux form the MLVs by shearing at a fixed shear rate until steady state is reached and the sample has the appearance of a homogeneous cream. They do not, however, study the kinetics of MLV formation. In this paper, we elucidate the dynamics of and conditions for MLV formation through a novel application of rheo-optic techniques, in which we correlate in situ (optically detected) structural changes with the transient rheological response. We find that the use of controlled shear rate rheometry does not provide a useful technique for obtaining a pure MLV structure; rather, the application of a controlled shear rate, given enough time, always results in a transition to a biphasic state. However, we find that controlled stress rheometry does provide a means for controlling MLV formation and size. In the following, we use controlled shear rate rheometry to identify a critical stress below which the desired MLV structure is formed. We then use controlled stress rheometry with simultaneous small-angle light scattering to correlate structural transitions with the applied stress, which leads to a reinterpretation of the nonequilibrium phase diagrams previously reported. At stresses above the critical stress we observe a transition to a highly ordered and apparently biphasic state. These results show unambiguously that shear stress, rather than shear rate, is the controlling rheological variable. We conclude by evaluating some theoretical scaling relationships for the MLV size.
* Author to whom correspondence should be sent. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, April 15, 1996.
Experimental Section
Introduction
(1) Lasic, D. D. Liposomes; Elsevier: New York, 1993. (2) Diat, O.; Roux, D. J. Phys. II 1993, 3, 9. (3) Roux, D.; Diat, O. French patient number 92-04108. (4) Diat, O.; Roux, D.; Nallet, F. J. Phys. II 1993, 3, 1427. (5) Roux, D.; Nallet, F.; Diat, O. Europhys. Lett. 1993, 24, 53. (6) Diat, O.; Roux, D.; Nallet, F. J. Phys. IV 1993, C8, 193.
S0743-7463(95)00696-2 CCC: $12.00
Following Diat and Roux,2 17 wt % AOT (Sigma AG, 99% purity) was dissolved in brine (15 g of NaCl/L of water) to give a sample in the LR phase (AOT/brine phase diagrams can be found in refs 7 and 8). The controlled shear rate measurements were conducted using an RMS 800 (Rheometrics) with a cone-
© 1996 American Chemical Society
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Figure 1. Simultaneous small-angle light scattering and controlled stress rheometry apparatus.
Figure 3. Shear rate controlled rheometry: viscosity vs strain for shear rates of 7.5 s-1 < γ˘ < 100 s-1. From top to bottom along the right-hand axis: γ˘ ) 7.5, 10, 12.5, 15, 17.5, 20, 30, 100 s-1.
Figure 2. “Long-time” flow curve: viscosity vs shear rate from shear rate controlled rheometry. and-plate geometry. The flow dichroism, birefringence, and turbidity were measured using a Rheo-optic analyzer (ROA, Rheometrics). For details on this type of measurement, we refer to the book by Fuller.9 The controlled stress measurements were performed using a controlled stress (Bohlin CS-50) rheometer equipped with a custom glass Couette shear cell (C-14 DIN) of optical quality. These measurements were combined with in situ flow smallangle light scattering (FSALS). Figure 1 shows a schematic diagram of the CS/FSALS setup. A laser beam, polarized horizontally, is passed down the velocity gradient axis of the Couette cell and through an analyzer (oriented either horizontally (Hh) or vertically (Hv)), and the resulting two-dimensional scattering patterns are captured by a 0.5 in. CCD camera and frame-grabber. Additional details concerning this experimental apparatus can be found in ref 10.
Results Controlled Shear Rate Rheometry. We find in general that a true steady state is difficult to reach (if possible at all) by steady shearing. For comparison to previous work, however, we show in Figure 2 a “longtime” flow curve obtained from controlled shear rate rheometry. As seen, we find a shear-thinning behavior, characterized by η ∝ γ˘ -0.8. The same scaling for the viscosity over a similar range of shear rates was observed by Roux et al.5 for the SDS-based system. However, weaker shear thinning with an exponent of -0.55 has been observed in the same AOT system at higher surfactant and lower electrolyte concentration.11 To gain insight into the formation dynamics of the MLV structure, we focus on the transient rheological response of the material. Figure 3 shows viscosity as a function of strain (shear rate × time) for 7.5 s-1 < γ˘ < 100 s-1, where strain gives a measure of the amount of deformation the (7) Ghosh, O.; Miller, C. A. J. Phys. Chem. 1987, 91, 4528. (8) Skouri, M.; Marignan, J.; May, R. Colloid Polym. Sci. 1991, 269, 929. (9) Fuller, G. G. Optical Rheometry of Complex Fluids; Oxford: New York, 1995. (10) Walker, L. M. Rheo-Optic Studies of Liquid Crystal Polymers Under Flow. Ph.D. Thesis, University of Delaware, 1995. (11) van der Linden, E.; Hogervorst, W. T.; Lekkerkerker, H. N. W. Langmuir 1996, 12, 3127.
sample has undergone. From this figure it is apparent that the samples evolve with strain and never reach a well-defined steady state. Sample drying and other artifacts have been properly eliminated. The samples show strong thixotropy, terminating in a maximum, beyond which the viscosity collapses and then begins to weakly shear thicken. The data reported in Figure 2 correspond to viscosities recorded in this plateau regime (recorded after approximately 8000 strain units). Upon completion of the experiment the samples were observed to be turbid, as also observed in previous investigations.2 This effect was found to be very reproducible as long as fresh sample was used, although the exact strain value of the transition depended on loading. Once the sample has been sheared past the transition, however, it is irreparably altered (on a time scale of days). The important feature to recognize in the transient data shown in Figure 3 is that the observed viscosity maximum corresponds to a maximum in the shear stress. The stress at this peak, for all shear rates investigated at or above 7.5 s-1, is 27 ( 3 Pa. Below shear rates of 3 s-1 the sample never reaches this critical stress value. Thus, as the virgin sample is sheared at a constant rate the sample continuously thickens until a critical stress (σc) is reached. These transient rheological data strongly suggest that there is a microstructural buildup until a critical point, beyond which the microstructure initially formed can no longer sustain the applied deformation rate. To determine the microstructure responsible for this observed rheology, we complement the controlled shear rate rheology with a rheo-optic study in which we measure dichroism, birefringence, and turbidity under flow as functions of applied shear rate. The flow dichroism, because it is most sensitive to structures of the order of the wavelength of light, functions here as a probe of the anisotropy in the defect structure. The turbidity, which is related to the dichroism, tracks the total amount and size of the MLVs. The flow birefringence, on the other hand, is particularly sensitive to the lamellar orientation. Figure 4 shows data taken at a fixed shear rate of γ˘ ) 20 s-1. The dichroism (∆n′′) signal is found to track the viscosity closely, indicating that the MLVs are deformed due to the buildup of stress. Beyond the viscosity maximum the stress is relieved and the dichroism drops upon a transition to a new flow regime. This increase in dichroism suggests that the defect structure stretches into distorted MLVs, whereas the drop accompanying the reduction in viscosity signals a reduction in this distortion. This transition is signaled by a significant increase in turbidity. Similar results at similar strains hold for the other shear rates. Note that the small discrepancy in the
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Figure 4. Viscosity (η), flow dichroism (∆n′′), and percent change in turbidity as functions of strain at a fixed shear rate of 20 s-1.
Figure 6. Controlled stress rheology: viscosity vs strain for stresses (5 Pa < σ < 10 Pa) below the critical stress of 27 Pa. Also shown are FSALS patterns: (a) Hv scattering pattern observed just prior to shear-thickening regime; (b) Hh scattering pattern observed at critical strain; (c) Hh scattering pattern observed well into the shear-thickening regime. Note, the flow direction in the scattering patterns is along the horizontal and the vorticity is along the vertical.
Figure 5. Flow birefringence as a function of strain at fixed shear rate: from top to bottom γ˘ ) 50, 20, 10 s-1. Note, curves have been offset so that the zero-shear birefringence is zero.
strain value for the transition between techniques is due to both different measurement geometries and loading effects. Because these samples do not have a well-defined initial state, there are slight differences observed between loadings. The flow birefringence (∆n′), shown in Figure 5, is also seen to increase with increasing strain until the microstructural transition, whereupon it also drops. The strain value at this transition, to within the uncertainty introduced by different loadings and device geometries, is coincident with the drop in the shear viscosity observed after reaching the critical stress (see Figure 3). The birefringence was analyzed using the technique suggested by Zhang et al.12 for strongly birefringent samples. This technique enables the tracking of the actual retardance, which is directly related to the birefringence, as it increases through orders of π/2. The positive increase in the birefringence may result from form effects associated with the distorted MLV structure. As will be discussed, we interpret the subsequent drop in the birefringence to the appearance of the oriented lamellar structure in the biphasic regime. The noise seen at high strain values is due to the increase in turbidity and signals the biphasic state. Controlled Stress Rheometry (σ < σc). The controlled stress measurements were conducted simultaneously with small-angle light-scattering measurements. This construction provides a means by which MLV formation and size can both be controlled and quantitatively monitored as a function of rheological response. Figure 6 shows that at fixed stresses below the critical stress of 27 Pa the samples again show strong thixotropy, but first with strong shear thinning followed by shear thickening. In contrast to the controlled shear rate (12) Zhang, H.; Moldenaers, P.; Mewis, J. Rheol. Acta 1994, 33, 317.
experiment, we observe that the viscosity scales with strain for all stresses investigated below the critical stress. A rapidly decreasing viscosity with strain is initially obtained, followed by dramatic shear thickening at a critical strain value of a few thousand strain units. Also shown in Figure 6 is the evolution of the microstructure, as seen in the progression of FSALS patterns. Initially, no Hv and only isotropic Hh scattering is observed. Upon reaching the critical strain, a characteristic four-leaf-clover pattern in the Hv mode (Figure 6a) develops along with anisotropic Hh scattering in the form of bright spots aligned in the flow direction (Figure 6b). As the shear thickening proceeds, the length scale characteristic of the structure decreases (the FSALS patterns grow) and the spots in the Hh scattering evolve into a ring (Figure 6c), as documented in previous studies.2,4 All the structures persist for several days upon interruption of the applied stress. The observed scattering patterns are consistent with anisotropic scattering from spheres (anisotropy in radial and tangential polarizabilities) within the Rayleigh-Gans-Debye (RGD) theory and are seen to closely resemble patterns encountered in scattering from polymer spherulites.13 Four-leafclover patterns have recently been observed in the Hv scattering from sheared lamellar phases of water/tetraethylene glycol dodecyl ether (C12E5) and were similarly attributed to the presence of MLVs.14 To determine the MLV radius, we exploit the fact that the Hv scattering patterns show a maximum in the intensity along the 45° axis. This maximum, according to RGD theory, occurs at (4π/λ) RMLV sin θ/2 ) 4.1, where λ is the wavelength of the light in the medium and θ is the scattering angle. This is a well-known relationship and has been widely used to determine spherulite sizes in films of semicrystalline polymers.13 The scattering patterns were collected as a function of applied stress below the critical stress at large strain values where the viscosity is only a very weak function of strain (see Figure 6). Figure 7 shows that the measured MLV radius (13) Haudin, J. M. In Optical Properties of Polymers; Meeten, G. H., Ed.; Elsevier: New York, 1986. (14) Weigel, R.; La¨uger, J.; Richtering, W. Submitted to J. Phys. II.
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streak, and a ring (intermittently visible). As the vertical streak dominates the scattering under flow, we show in Figure 8b the scattering pattern obtained upon cessation of flow. The streak rapidly dissipates, leaving the sixfold symmetric pattern clearly visible. These scattering patterns are consistent with those recently reported by Diat et al.15 for the SDS-based system at high shear rates. This indicates that above the critical stress a structural transition to an ordered state occurs, similar to what is observed when systems of charge-stabilized spherical colloids are sheared.16 This state, however, is biphasic in nature, as the vertical streak in the Hh scattering alone surfives at even higher stresses. We interpret this to indicate a flow-aligned lamellar structure, consistent with other investigations.4 Figure 7. MLV radius as a function of applied stress below the critical stress as determined from FSALS. The error bars on the MLV radii reflect the broadness of the intensity maximum in the Hv scattering patterns. The line represents the equation RMLV ) 14.7σ-0.75 and is a best fit to the data.
Discussion From a force balance between a viscous force exerted by the flow field and an elastic bending force required to maintain a lamellar phase at size RMLV, Roux and coworkers2,4,5 proposed a dependence of the MLV size on shear rate as
RMLV ∝ (ηγ˘ )-1/2
(1)
and obtained good agreement with light-scattering data for shear rates ranging from 1 to 100 s-1. As seen in Figures 2 and 3 this system is highly non-Newtonian and there is thus an additional shear rate dependence implicit in the viscosity in the above scaling relation. For this reason and for the purpose of comparison with our controlled stress measurements, we rearrange the above expression, which then yields the following stress dependence5
RMLV ∝ σ-1/2
Figure 8. Controlled stress rheology: viscosity vs strain for a stress of 30 Pa, which is above the critical stress of 27 Pa. The sample was exposed to a stress below the critical stress prior to stepping above the critical stress. Also shown are FSALS patterns: (a) Hh scattering pattern observed after roughly 100 strain units of deformation (Pattern is a superposition of hexagonal spots, a vertical streak, and a ring); (b) Hh scattering pattern observed upon cessation of flow. (The sixfold symmetry is now visible.) Note, the flow direction in the scattering patterns is along the horizontal and the vorticity is along the vertical.
decreases with increasing stress, which is consistent with simple models for such phenomena as droplet deformation.19 These results agree qualitatively with previous studies,2,4 although the previous work did not recognize that the controlling parameter is shear stress and not shear rate. Controlled Stress Rheometry (σ > σc). To investigate what transpires at stresses above the critical stress, we first presheared samples at a stress level below the critical stress until a relatively steady viscosity was obtained and a ring was present in the Hh scattering. Then, stepping above the critical stress, we observed, as shown in Figure 8, an initial decrease in the viscosity with increasing strain. As the viscosity reaches a minimum, a structural transition is observed through the FSALS pattern. Figure 8a shows that the ring, initially present in the Hh scattering, breaks into a superposition of patterns: spots with hexagonal symmetry, a vertical
(2)
Recently, van der Linden and Dro¨ge17 derived an expression for the effective interfacial tension of an MLV sphere in the presence of highly screened electrostatics between the surfactant layers. The system studied here is of relatively low viscosity and modulus compared to MLV phases in some mixed surfactant systems, consistent with the situation of highly screened inter-bilayer electrostatics.18 Therefore, to obtain an alternative scaling model to the above, we employ the expression given by van der Linden and Dro¨ge for the effective interfacial tension
γeff ) (KB)1/2 ) ((κ/d0)(9π2(kT)2d0/64κd4W))1/2
(3)
where K and B are bulk rigidity and compressibility moduli, given in terms of κ, the single layer bending modulus, d0, the repeat distance, and dW, the thickness of the water layers. Following Taylor19 we construct a force balance between an elastic force, γeffRMLV, and a 2 viscous force, σRMLV , which results in the following scaling relation (15) Diat, O.; Roux, D.; Nallet, F. Phys. Rev. E 1995, 51, 3296. (16) See, for instance: (a) Chen, L. B.; Zukoski, C. F.; Ackerson, B. J.; Hanley, H. J. M.; Straty, G. C.; Barker, J.; Glinka, C. J. Phys. Rev. Lett. 1992, 69, 688. (b) Laun, H. M.; Bung, R.; Hess, S.; Loose, W.; Hess, O.; Hahn, K.; Ha¨dicke, E.; Hingmann, R.; Schmidt, F.; Lindner, P. J. Rheol. 1992, 36, 743. (17) van der Linden, E.; Dro¨ge, J. H. M. Physica A 1993, 193, 439. (18) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972. (19) Taylor, G. I. Proc. R. Soc., A 1932, 138, 41.
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(4)
As seen in Figure 7 the data for the MLV radius lie intermediate between the two scaling models. We only have data for the MLV size in the pure homogeneous MLV phase for a limited range of applied shear stress. We did not obtain data at higher stresses because of the extreme shear thinning at start up, and at lower stresses it takes an inordinate amount of time to impart enough deformation on the sample for it to reach a near-steady state. It is, however, important to note that our measurements were recorded in the pure MLV “phase” while in the previous work the data were taken with a controlled shear rate device at high strain, resulting in a biphasic fluid. In recent experimental work by van der Linden et al.11 on a more concentrated system, the data for the MLV radius were found to be well represented by the RMLV ∝ σ-1 scaling. Finally, we combine the controlled shear rate and stress measurements to construct a “long-time” flow curve for this AOT/brine system and associate with it a nonequilibrium phase diagram. As seen in Figure 9, the controlled shear rate data fall outside the stress region containing the pure MLV structure, whereas with the controlled stress measurements we are able to control the state of the system. Below shear stresses of about 2 Pa we are unable to discern any MLV formation from the FSALS, so we draw here a tentative boundary between a defect ridden lamellar phase2 (I) and the MLV structure (II). At higher stress, we see a transition to an ordered but biphasic state (III + IV), and at even higher stresses a lamellar structure aligned in the flow direction is obtained (IV). The latter has been confirmed by our and previous flow-SANS measurements.4,6 We infer from our flow birefringence measurements that the orientation of the lamellae in this state is with the layer normal in the direction of the vorticity, as depicted in Figure 9. If the birefringence signal is dominated by the intrinsic birefringence of the AOT molecules, this orientation (with the AOT backbone oriented perpendicular to the direction of the incident light) would contribute negatively to the overall signal and, thus, would be consistent with the abrupt drop observed in the flow birefringence. This interpretation of the flow birefringence signal differs from that made in previous work.4 We emphasize that the appropriate abscissa in this nonequilibrium phase diagram is not the shear rate but instead the shear stress.
Figure 9. “Long-time” flow curve for the studied AOT/brine system: viscosity vs shear stress with data from (]) controlled shear rate rheometry and (0) controlled stress rheometry. Also shown is a proposed nonequilibrium phase diagram: (I) defect ridden lamellar phase; (II) MLV structure; (III) shear-ordered MLV structure; (IV) flow-aligned lamellar structure.
Conclusions We have used rheo-optic techniques to confirm that an MLV structure can be shear induced starting from a lamellar phase of AOT bilayers in brine. The MLV formation is signaled by a dramatic increase in viscosity and spherulite-type small-angle scattering, allowing for quantitative determination of the MLV size. The onset of MLV formation in controlled stress measurements requires a critical amount of deformation. MLV rheology, structure, and size are controlled by the applied shear stress rather than the applied shear rate. Exceeding a critical stress results in shear ordering, where MLVs organize in layers, and a turbid biphasic materials. The MLV radii obtained in the pure MLV “phase” decrease with increasing applied stress, which can be qualitatively understood as a balance between applied stress and the MLV elasticity. Acknowledgment. The authors wish to thank Dr. L. M. Walker for her assistance with the CS/FSALS measurements and Erik van der Linden for useful discussions. Funding from the National Science Foundation (Grant CTS-9158164) is gratefully acknowledged. LA950696N
