Langmuir 2001, 17, 999-1008
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Nonionic Amphiphilic Bilayer Structures under Shear T. D. Le,†,* U. Olsson,† K. Mortensen,‡ J. Zipfel,§ and W. Richtering§ Center for Chem. and Chem. Eng., Phys. Chem. 1, Lund University, P.O. Box 124, S-22100 Lund, Sweden, Risø National Laboratory, Condensed Matter Phys. and Chem., P.O. Box 49, DK-4000 Roskilde, Denmark, and Institut fu¨ r Makromolekulare Chemie, Universita¨ t Freiburg, Stefan-Meier-Strasse 31, D-79104 Freiburg, Germany Received August 23, 2000. In Final Form: December 5, 2000 The effect of shear on the lamellar phase of amphiphilic systems has been widely studied in a variety of amphiphilic systems as a function of shear rate. In this investigation, we fixed the shear rate and performed temperature scan experiments on a two-component (C10E3-water) system. We observed a sequence of phases, from the low to high temperature range: multilamellar vesicle to planar lamellar to sponge phase. The shear-induced multilamellar vesicle (MLV) phase exhibited the most interesting behavior and is the main focus of the present study. Small-angle neutron and light scattering techniques were used to elucidate the microstructure, to determine the bilayer orientation, and to characterize the size of these structures. The interlamellar spacing was observed to be the same in the lamellar as in the MLV phase, and the MLV phase exhibited symmetrical scattering in the neutral and flow directions, indicating that the layers in the vesicles are spherically shaped at the selected shear rate (γ˘ ) 100 s-1). With all the information that we could gather for the planar lamellar and MLV phases, we used the framework of the elastic curvature energy model to describe qualitatively the stability of these bilayer structures formed at a given shear rate.
Introduction Amphiphilic bilayers made of surfactant, lipid, or polymer can rearrange to form bicontinuous structures (e.g., sponge or cubic), flatten into planar sheets (e.g., classical lamellar), or fold to form closed structures (e.g., uni- or multilayered vesicles). The latter structure has been given a number of descriptive names, such as onions, liposomes, spherulites, and multilamellar vesicles (MLV).1 Closed structures play an important role in biological and industrial processes. Natural or synthetic lipids, for example, have been used to produce liposome capsules for use in drug delivery, in medical imaging, or as model membrane structures for studying relationships between structures and functions of biological membranes.2 Surfactant or block copolymer in solvent can also form uni- or multilamellar vesicles, provided that mechanical energy is put into the system. Shearing, for instance, has been shown to produce relatively monodisperse sized MLV and can be used in technological processes.3,4 Motivated by the prospective use of these systems, researchers have performed a number of shear experiments on the lamellar phase to investigate the shear-induced formation of the multilamellar vesicle phase, defects,5 orientations,6,7 sizes,8 and transient states of these structures as a function of shear rate. These shearing experiments have provided insights into the required conditions for forming topologically different membrane structures and the means to test theoretical * To whom correspondence should be addressed. E-mail: Thao.
[email protected]. † Lund University. ‡ Risø National Laboratory. § Universita ¨ t Freiburg. (1) Rosoff, M. Vesicles; Schick, M. J., Fowkes, F. M., Eds.; Marcel Dekker: New York, 1996; Vol. 62, p 752. (2) Lasic, D. D. Liposomes: From Physics to Applications; Elsevier: Amsterdam, 1993. (3) Diat, O.; Roux, D. J. Phys. II 1993, 3, 9. (4) Gulik-Krzywicki, T.; Dedieu, J. C.; Roux, D.; Degert, C.; Laversanne, R. Langmuir 1996, 12, 4668.
models developed to describe the behavior of these structures under flow (e.g., elastic membrane properties, stability, etc.). Most of these theoretical models have in common the conceptual framework of Helfrich for describing the curvature energy of an incompressible amphiphilic membrane per unit area,9
Gc/A ) 2κb〈H2〉 + κjb〈K〉
(1)
where κb and κjb are the bending and saddle-splay moduli of the bilayer and H and K are the mean curvature and Gaussian curvature, respectively, evaluated at the bilayer midplane. The integral of the Gaussian curvature over a closed surface, according to the Gauss-Bonnet theorem, is topologically invariant;10 that is,
〈K〉 ) 4π(nc - nh)/A
(2)
where nc and nh are the number of disconnected components and handles, respectively. In other words, for a fixed area 〈K〉 changes only with the topology of the surface; that is, 〈K〉 > 0 for closed bilayers (e.g., vesicles), 〈K〉 ) 0 for planar bilayers (lamellar), and 〈K〉 < 0 for a multiply connected bilayer (e.g., sponge). With eqs 1 and 2, one can see that the minimum curvature energy for forming one vesicle is 4πκjb and for creating one handle from a planar bilayer is -4πκjb. Though the basis of these theories is the same, the physical picture describing the formation of these struc(5) Zipfel, J.; Berghausen, J.; Lindner, P.; Richtering, W. J. Phys. Chem. B 1999, 103, 2841. (6) Penfold, J.; Staples, E.; Lodhi, A. K.; Tucker, I.; Tiddy, G. J. T. J. Phys. Chem. B 1997, 101, 66. (7) Diat, O.; Roux, D.; Nallet, F. J. Phys. II 1993, 3, 1427. (8) Panizza, P.; Colin, A.; Coulon, C.; Roux, D. Eur. Phys. J. B 1998, 4, 65. (9) Helfrich, W. Z. Naturforsch. 1973, 28c, 693. (10) Hyde, S.; Andersson, S.; Larsson, K.; Blum, Z.; Landh, T.; Lidin, S.; Ninham, B. W. The Language of Shape (The Role of Curvature in Condensed Matter: Physics, Chemistry and Biology); Elsevier: Amsterdam, 1997.
10.1021/la001227a CCC: $20.00 © 2001 American Chemical Society Published on Web 01/26/2001
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Figure 1. Phase equilibria of the C10E3-H2O system adapted from ref 22. The phases shown are the lamellar (LR), reverse micellar (L2), and sponge (L3) phases.
tures varies greatly depending on the available data, the viewpoint, and the argument presented. The different case studies can be found in refs 11-17. The bulk of the data available in the literature covers mostly complex fluid membrane systems containing, for instance, salt (to screen the electrostatics) in charged systems, or alcohol (to lower the membrane rigidity and to modify the spontaneous curvature), or oil (to swell the bilayer). The effect arising from additive(s) coupled with input conditions, for example, shear, presents a more complex behavior in which case the individual contribution from each parameter may mask, or influence, one another. Experiments using a system with fewer components could simplify the situation. Nonionic surfactant systems containing n-CnH2n+1(OCH2CH2)mOH surfactant (abbreviated as CnEm) have been demonstrated to be sensitive to temperature changes. In a study on the effect of additives on a nonionic surfactant system, Jonstro¨mer and Strey18 found that stirring the sample at temperatures below the temperature of the lamellar phase produces an opalescent solution that is streaming birefringence and more viscous than the L3 phase. This region was labeled LR+ and, at the time, was suspected to be a vesicle phase. Experiments using freezefracture electron microscopy (FFEM),19 NMR,20 and scattering techniques21 have confirmed the existence of vesicles in the LR+ region in nonionic surfactant systems. The actual structure (single or multishelled vesicles), size, and distribution depend on the ability to control the stress applied on the sample system. Shown in Figure 1 is the equilibrium phase diagram of C10E3-water,22 where a broad lamellar phase (LR at low temperatures) and a narrow sponge phase (L3 at high (11) Safran, S. A.; Pincus, P. A.; Andelman, D.; MacKintosh, F. C. Phys. Rev. A 1991, 43, 1071. (12) Wennerstro¨m, H.; Anderson, D. M. Difference versus Gaussian Curvature Energies; Institute for Mathematics and Its Applications: Minneapolis, MN, 1991. (13) Simons, B. D.; Cates, M. E. J. Phys. II 1992, 2, 1439. (14) Herve´, P.; Roux, D.; Bellocq, A.-M.; Nallet, F.; Gulik-Krzywicki, T. J. Phys. II 1993, 3, 1255. (15) Helfrich, W. J. Phys.: Condens. Matter 1994, 6, A79. (16) Morse, D. C.; Milner, S. T. Phys. Rev. E 1995, 52, 5918. (17) van der Linden, E.; Hogervorst, W. T. Langmuir 1996, 12, 3127. (18) Jonstro¨mer, M.; Strey, R. J. Phys. Chem. 1992, 96, 5993. (19) Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 182. (20) Olsson, U.; Nakamura, K.; Kunieda, H.; Strey, R. Langmuir 1996, 12, 3045. (21) Mu¨ller, S.; Bo¨rschig, C.; Gronshi, W.; Schmidt, C. Langmuir 1999, 15, 7558. (22) Ali, A. A.; Mulley, B. A. J. Pharm. Pharmacol. 1978, 30, 205.
Le et al.
temperatures) lie within an accessible experimental temperature range between 20 and 50 °C and cover a broad range of concentration, making the system suitable for temperature scan experiments. In a recent study,23 we used the C10E3-water system to demonstrate the possibility of studying the topological transformation of membrane structures using the small-angle neutron scattering (SANS) technique. This study is an extension of the former in that we make detailed measurements using SANS, rheology, and small-angle light scattering (SALS) techniques to elucidate the microstructure, to determine the bilayer orientation, to characterize the size of these structures, and to describe qualitatively the stability of these bilayer structures formed at a given shear rate using the elastic curvature energy framework. SANS and SALS are useful tools for in situ determination of structural properties, for example, geometric structure, size, number of layers, membrane fluidity, and interactions. Our main objective is to characterize the morphological transformation of surfactant bilayers under flow to better understand the driving force responsible for transforming lamellae to spherical multilamellar vesicles or to a multiply connected spongelike membrane. The former is the primary focus of the present study. Experimental Section Materials and Sample Preparation. Tri-ethyleneglycol mono-n-decyl ether, abbreviated as C10E3, was obtained from Nikko Chemicals Co. (Tokyo). The solvent D2O was obtained from Dr. Glaser AG (Basel). Deuterium oxide (D2O) was used to enhance the contrast between the sampling element and solvent. Both materials have a purity better than 99.8% and were used without further purification. All samples were prepared by weight, and the volume fraction of surfactant Φ was calculated using the following densities: 0.938 g/cm3 (C10E3) and 1.11 g/cm3 (D2O). In this binary system, an isotropic sponge (L3) phase exists at high temperatures and a lamellar (LR) phase exists at low temperatures. A gentle shake can cause vesicles to form in the sample, resulting in a drastic increase in viscosity. This shear thickening behavior poses a problem in concentrated samples because it can lead to inadequate mixing of the solution. To circumvent this problem, we prepared each sample by placing it into a thermostatic bath, raising the bath temperature until the L3 phase was reached (isotropically clear solution with low viscosity), mixing the sample, and allowing the sample to gradually cool to an LR phase at room temperature. Small-Angle Neutron Scattering (SANS). The major part of the SANS experiments were performed at Risø National Laboratory, Denmark. Two neutron wavelengths (λ ) 3.8 and 5.7 Å) and three sample-to-detector distances (1.5, 3, and 6 m) were used to cover a range of the scattering vector q, |q b| ) 4π sin(θ/2)/λ, between 0.002 and 0.3 Å-1. The resolution of the wavelength, ∆λ/λ, was 18%. The scattered intensity was recorded by a two-dimensional position sensitive area (128 pixel × 128 pixel) detector. The temperature of the sample was controlled within (0.2 °C by an external thermostatic water bath. A schematic illustration of the Couette cell and the scattering geometry is shown in Figure 2. Measurements were made using the through-view configuration; that is, the incident beam was parallel to the velocity gradient direction (y-axis) and the scattered intensity was monitored in the x-z plane (x is the flow direction and z is the neutral direction). The shear rate was calculated using24 γ˘ ) 2πΩ〈R〉(Ro - Ri), where Ω is the rotation speed, Ri ) 14.5 mm and Ro ) 15 mm are the inner and outer radii, and 〈R〉 ) (Ro + Ri)/2. In the setup, the outer cylinder (stator) was fixed while the inner cylinder (rotor) rotated and the neutron passed radially through the center of the cylinders yielding a total neutron path length of 2 times the annual gap (0.5 mm). (23) Le, T. D.; Olsson, U.; Mortensen, K. Physica B 2000, 276, 379. (24) Macosko, C. W. Rheology: Principles, Measurements, and Applications; VCH Pub., Inc.: New York, 1994.
Bilayer Structures under Shear
Langmuir, Vol. 17, No. 4, 2001 1001 performance of the Couette cell are described in detail in ref 26. A shear rate of 10 s-1 was used in these experiments. Small-Angle Light Scattering (SALS). The shear experiments using the SALS technique consisted of a Bohlin CVO-HR rheometer, a quartz glass 3° cone-and-plate shear cell, and a blue argon laser (λ ) 488 nm). The setup was configured for depolarized SALS measurements; that is, the analyzer (neutral direction) was aligned perpendicular to the polarizer (flow direction). The distance between the sample and screen provided a range of q-values. In the present study, the screen was fixed at one position, yielding q ) 3.35 µm-1 at the edge of the screen.
Results and Discussion
Figure 2. Schematic diagram of the SANS setup used in the shear experiments at Risø National Laboratory. The shear cell is that of a Couette design containing two concentric cylinders, the outer (stator) and the inner (rotor), separated by a 0.5 mm gap. The sample size was ca. 1.5 mL. The design and performance of the Couette cell are described in detail in ref 25. We used the SANS technique to map the formation of different phase structures under shear. To reduce the number of variables in the study, we fixed the shear rate at γ˘ ) 100 ( 5 s-1 and performed measurements while heating (or cooling) the sample between 10 °C and temperatures where we had observed an isotropic sponge (L3) phase in a phase equilibrium experiment. Knowing the stability range of the L3 phase in the phase equilibrium experiment, we set the maximum temperature of the experiment below the upper phase boundary to avoid demixing problems that can occur when stepping into the twophase region, L3 plus excess solvent. The time of the SANS measurements varied from sample to sample because of the experimental conditions, for instance, surfactant concentration, sample-to-detector distance, and wavelength of the neutron. Generally, the more dilute the sample the longer the time of measurement (e.g., 600 s per SANS 2-D recording for a 5% sample but only 120 s per recording for a 40% sample). Although the radial beam is important for detecting the lamellar to multilamellar vesicle transition (central focus of the present study), the tangential beam can be used to distinguish between parallel and perpendicular bilayer orientations. This is because in the radial beam position the lamellae with an orientation parallel to the walls of the shear cell do not contribute to the scattered intensity. In one set of experiments, measurements were performed on the D11 SANS instrument at the Institute Laue-Langevin (ILL) in Grenoble, where the instrument could be configured to measure both radial (qx-qz) and tangential (qy-qz) scattering planes. A q-range between 0.02 and 0.15 Å-1 was covered using λ ) 6 Å and a sample-to-detector distance equal to 2.5 m. The wavelength resolution was 9%. The shear cell assembly used at ILL is different from the one used in Risø in two respects, the rotation (from the outer cylinder) and the gap size (1 mm). As we will show, these subtle differences do not appear to influence the results. The shear experiment at ILL was made using both radial and tangential beam configurations. The radial (standard) position is as described above. In the tangential position, the beam passes through the sample along the velocity (x-axis) direction and data are collected in the qy-qz plane. In the tangential beam configuration, a rectangular opening of 0.3 mm × 15 mm was used and the 2-D SANS pattern is asymmetrical because of the path length difference as the beam traveled through the center of the gap. The design and (25) Mortensen, K.; Almdal, K.; Bates, F. S.; Koppi, K.; Tirrell, M.; Norde´n, B. Physica B 1995, 213-214, 682.
Phase Diagram at Constant Shear Rate. From the shearing experiments using SANS,27 we obtained a collection of 2-D spectra for six volume fractions of surfactant in the range 0.06 e Φ e 0.64. The data were collected while the sample was being heated (or cooled) in a Couette cell shearing at a rate of γ˘ ) 100 s-1. The applied strain (γ ) γ˘ t) values on the dilute and concentrated samples were in the range between 12 000 and 60 000 units per recorded scattering pattern, respectively. For each concentration, we recorded a few hundred 2-D spectra from which we find three characteristic scattering profiles at specific temperature intervals. Shown in Figure 3 are representative SANS spectra as obtained in the temperature interval between 37 and 48 °C. The scattering patterns were reversible in temperature. The example shown is for a Φ ) 0.44 sample, and the spectra show an isotropic scattering pattern at low temperatures (
