Structural studies of lamellar surfactant systems under shear

https://doi.org/10.1016/S1359-0294(01)00071-1Get rights and content

Abstract

Recent experimental studies on concentrated surfactant systems are reviewed. Particular attention is focused on the transformation from planar lamellar sheets to multilamellar vesicles. It is discussed whether both of these states are thermodynamic stable, or if the MLV is an artifact of shear induced factors. Recent studies includes the dependence on shear, and dependence on salt and cosurfactants, and thereby related lamellar defects. The review include moreover the demonstration that polymeric amphiphiles dramatically enhance the quality of classical surfactants.

Introduction

The selfassociation of amphiphilic molecules attracts significant attention within both basic and applied sciences [1]. A variety of structures can be formed, including micellar aggregates of various form and sizes and pseudo two-dimensional structures making the interface in oil/water systems and bilayer membranes in aqueous solutions. Recent studies include a variety of problems addressed, as for example living networks in micellar solutions [2], the structure of droplets in L2 microemulsions [3], coexistence of lamellar and L3 phases [4]•, thermal stability of micellar-type cubic phases [5] and critical behavior and shape transformation near the cloud point [6]•.

For many applications the so-called X̃-point of the ‘fish tail’ part of the phase diagram is a main parameter in the choice of an appropriate surfactant system. Strey and coworkers reported recently a systematic study of the phase behavior of ternary water–alkyl (CiEj) systems using different alkyl methacrylates and different surfactants [7]•. The microemulsion systems show all the same general patterns, however, the phase inversion temperature is highly dependent on the alkyl chain length of the oil, as the efficiency of the surfactant systematically depends on the alkyl chain length of both the surfactant and the oil. Incorporation of small amounts of amphiphilic block copolymers in systems of water, oil and non-ionic surfactant, dramatically increases in the volumes of oil and water, that can be solubilized in a bicontinuous microemulsion [8]•• and [9]••.

From work on the binary systems of CiEj surfactant and water Strey et al. demonstrated in early 1990 a dilute lamellar phase near the L3-phase [10]. The origin of this phase has been under discussion. The sequence of phases found in a trimethylsilane surfactant system resemble that known from water–non-ionic surfactant systems of the alkylpolyglycolether type, with two disconnected lamellar phases [11].

A large number of the recent reports have focused on dynamical properties like the relaxation processes and response to shear and stress. The dynamics of the L3 phase of C10E4 microemulsions was examined by Strey et al. [12]•. The relaxation after temperature and pressure jumps can be described in terms of a single relaxation time constant that varies systematically with concentration. Dynamic light scattering, however, shows only minor concentration dependence, proving that both concentration fluctuations and elastic shape deformations play important roles in the dynamics.

The viscosity of surfactant solutions is highly sensitive to the nanometer scale structure. Schosseler et al. made a time resolved structural study of the shear thickening phenomenom [13], showing evidence of shear induced phases of loosely connected network of long-lived micellar aggregates, which may account for the viscous properties. Richtering et al. have studied the evolution of birefringence in the hexagonal phase of polymeric surfactants applying shear. An alignment of the rod-like micelles was found along the flow direction [14]. Terry et al. examined the mesoscopic and molecular response of hexagonal mesophases to shear and extensional flows [15]. During shear an initial modest orientation of the surfactant rods along the flow direction appears, followed by a progressive further increase in alignment, corresponding to the progressive development of shear thinning. Berret et al. argue that the origin of the increase in viscosity in hexagonal systems is the same for concentrated and for dilute systems [16]. The authors speculate if shear might promote micellar growth.

Based on cryo-TEM Talmon et al. directly imaged the flow induced transition of vesicles to micelles [17]. When the dispersions are deformed and strained by rapid drainage of the excess from the specimens, the vesicles transform into an entangled network of thread-like micelles.

A significant fraction of the recent studies have focused on lamellar systems, both the classical lamellar Lα-phase of planar sheets and systems where the bilayer sheets fold to form closed structures as uni- or multi-layered lamellar vesicles.

The lamellar phases can be characterized according to the value of the average Gaussian curvature 〈K〉 [18]. Undulation instability of lamellar phases has been investigated theoretically by various groups [19], [20]. Auernhammer et al. made a hydrodynamic description of smectic A liquid crystals showing the origin of the undulation instability in layered systems under shear flow [19]. Shear leads to alignment of the layers parallel to the plates but with a tilt that causes the layers to have tendency to reduce their thickness and, above a critical shear rate, develop undulations.

Zilman and Granek proposed a mechanism for onion formation based on the undulation instability [20]. It is argued that the flow generates an effective force that acts to reduce the excess area and thereby an effective lateral pressure. At low shear rates (γ̇) this pressure is balanced by the elastic restoring forces of the lamellar. Above a critical shear rate, the lamellar buckles into a harmonic shape modulation characterized by a stability limit, which is speculated to be the limit of where lamellar breaks up into the MLV L*α-phase. The critical shear rate γ̇c for the formation of onions is predicted to scale as γ̇c∼d−5/2s−1/2, which is consistent with experimental data.

Mang et al. argued that lamellar domains order in the direction of stress as a result of viscosity [21], [22]. Penfold and coworkers reported a related study applying both constant and oscillatory shear [23], [24]. For the lamellar phase of C16E6, two distinct orientations were identified. At low shear gradients the lamellae order parallel to the flow–vorticity plane, whereas at higher shear gradients the lamellae order parallel to the flow–shear gradient plane, analogous to findings in block copolymer melts. If the shear flow is continued the lamellar phase transforms to a highly ordered solution of monodisperse multilamellar vesicles [23].

Jonstromer and Strey found in 1992 that stirring in an aqueous system of CiEj in the lamellar phase produces an opalescent solution [25]. Scattering experiments have recently proven that this phase, labeled L*α, is a vesicle phase [26]•. In 1993 Roux and coworkers presented the pioneering orientation rheogram (γ̇, ϕ) describing the effect of shear on lyotropic lamellar phases, with γ̇ equal the shear rate and ϕ the volume fraction of bilayer membranes [27], [28], [29]. The studied system was a complex mixture of water, SDS, pentanol and dodecane, but more simple studies have shown similar behavior. Le and coworkes studied non-ionic CiEj systems in the attempt to investigate the lamellar-to-vesicle transformation with fewest possibly parameters, e.g. without adding salts or cosurfactants [30], [31], [32]. Fig. 1 shows the equilibrium diagram of C10E3/water as obtained at constant shear, showing the three phases L3, Lα and L*α.

Roux et al. found three different states of orientation separated with out-of-equilibrium transitions [35]. In the state at very low shear rates, the lamellar phase is, on average, oriented with the layers in the shear plane and a few dislocations remain in the direction of the flow. In the intermediate state, the layers organize themselves into monodisperse multilayer vesicles. The last state corresponds to the same orientation as the first one but with no dislocations in the flow direction. Qualitatively similar behavior is seen in the CiEj [31], but also mixed Lα- and L*α-phases are observed. The temperature induces Lα to L*α transformation showing remarkable reproducibility.

Marques argued that the majority of vesicles prepared from dispersions of amphiphiles in water are non-equilibrium structures [36]. Hoffmann, Gradzielsky and coworkers studied the Lα to L*α transformation, starting with a ‘virgin’ lamellar Lα-phase that has not been exposed to shear [37], [38], [39]. The Lα-phase was produced in a special route by protonating an L3-phase.

Richtering et al. have studied the shear dependence of multi-lammellar phases phase of polymeric surfactants [14], [33], [34]•. In polymeric PEO–PPO–PEO surfactant systems shear thickening occurs for low polymer concentration as a result of Lα to L*α transformation, while at high polymer concentrations, shear results in decrease in viscosity reflecting the transformation from parallel to perpendicular lamellar alignment [34]•.

The diameter D of the multi lamellar vesicles is fixed by a balance between the viscous and elastic stresses and varies as the inverse square root of the shear rate: Dγ̇−1/2. The size can be varied from a few microns to a tenth of a micron. Roux et al. found that the shear induced changes in size can be both continuous and discontinuous, depending on the variation in shear change [26]• and [35]. For the discontinuous chances, the onions break and free parts of the membranes.

Meyer et al. [40] investigated the steady-state rheological behaviour of lamellar phases and found in the MLV state that the stress varies as γ̇∼σ4.8.

Zipfel et al. focused on the influence of the degree and type of defects, which were controlled by varying the surfactant–cosurfactant ratio in a SDS/decanol/water system [41]•. It was shown that replacing SDS with decanol leads to a transition from a defective ribbon-like lamellar phase, for low decanol content, pore-like defect structure for intermediate decanol content and classical defect free lamellar phase for decanol content more than approximately 0.4. When exposed to shear, the lamellar phase reorient depending on the type of defect structure. In the ribbon structure of the lowest decanol content, the lamellar first align in parallel orientation, at medium shear rates perpendicular orientation is observed, a further increase in shear rate leads to reentrant to parallel alignment, a feature similar to that previously observed in block copolymer melts. The formation of vesicles was found in a limited regime of parallel orientation, as shown in Fig. 2.

In a study of surfactant system sodium oleate/octanol/water, Gradzielsky et al. have shown that addition of octanol to the surfactant solution for concentrations leads to the formation of small unilamellar vesicles [42], [43]•. Rouch et al. have shown that in a brine–surfactant system, under shear flow, lamellar and sponge phases coexist as a colloidal crystal of multilamellar vesicles immersed in the sponge matrix [4]•.

The influence on charge density in the Lα to L*α transformation was studied by Gradzielski et al. in the aqueous system TDMAO/TTABr/n-hexanol [44]. At low TTABr content, preferentially planar lamellae are formed, while at higher TTABr content, multilamellar vesicles appear. The effect of the shear is that first the planar lamellae are transformed into vesicles, while further increase of the shear rate causes vesicle shells to be stripped off until, at high shear rates, stable unilamellar vesicles are formed.

Hoffmann and coworkers studied the time-dependent transformation of an ionically charged lamellar Lα phase into a vesicle L*α phase under the influence of shear [37]. The ionically charged stacked bilayer was prepared by protonation of an L3 phase which thereby becomes unstable and transforms into a ‘virgin’ lamellar Lα-phase. In shear, this low-viscous Lα-phase is transformed into a highly viscous vesicle phase. The shear rate induced stacked lamellae to vesicles transformation is irreversible [38]•. In a study of cationic–anionic surfactant mixtures, Hoffmann et al. reported that the vesicle phases of this system have different macroscopic properties compared to the systems that are prepared from acid and cationic hydroxide [45]. In the latter situation, the vesicle phases contain no excess salt and the ionic charges on the vesicles are not shielded. As a consequence, the vesicular solutions are strongly viscoelastic and have a yield stress.

Leon et al. investigated the flow-structure properties and correlation between salinity and defect structures [46]. Low salinity leads to viscous L*α phase, whereas at high salinity, a low viscosity plane lamellar phase is found. Under shear, the latter shows, after a certain salinity dependent delay time, a sudden transition to a viscoelastic gel, indicating transition to the MLV phase.

In the phase diagram of the surfactant system sodium oleate/octanol/water discussed above, a very stiff gel phase has been found [42], [43]•. For sufficiently high octanol content unilamellar vesicles are formed that are so monodisperse that they are able to form a densely packed system with long-range order and with a shear modulus that is approximately 100 times higher than normally found for vesicle systems.

Three MLV phases have been identified in C12E4-vesicles [32]. At low and high temperatures near spherical vesicles are observed, while in the middle regime of the L*α-phase the vesicles face each other making a close packed ensemble of vesicles, with clear threefold (probably cubic) symmetry.

The discovery of the transformation from oriented planar lamellar Lα-phase to multi lamellar vesicle L*α-phase is the basis for making microcapsules. High entrapment of proteins has been documented in vesicles prepared by shearing a lamellar phase composed of soybean phosphatidylcholine, cholesterol and polyoxyethylene alcohol [48]. Gauffre and Roux present a spherulite system designed for pH-controlled encapsulation, using a zwitterionic soyabean lecithin mixed with a non-ionic surfactant [49]. The sensitivity to pH was achieved by the addition of fatty acids to the surfactant system.

Bernheim-Grosswasser et al. reproted use of multilamellar vesicles to encapsulate enzymes, specifically alkaline phosphatase [47]. Once encapsulated, the enzyme was shown to be unable to develop any enzymatic activity on its substrate, signifying absence of contact between enzyme and substrate. The whole enzymatic activity is recovered after destruction of the vesicles.

Roux et al. investigated in vivo the factors that affect multilamellar vesicles’ transport to the blood compartment after oral administration [48]. The vesicles are stabled in fetal calf serum in both acidic and basic buffers, but are partially lysed in bile salts.

Cates and coworkers studied various sized particles in MLV phases [50]. Depending on the sizes, volume fraction and the stage during the preparation process, three structurally distinct classes of composite were observed: onions, in which the particles are at the center of the onions; decorated onions, in which the particles decorate the polyhedral lattice of edges between onions, and onion/particle alloys. The latter are formed when the particles are added late in the shearing procedure, in which case the onions remain intact and the particles reside entirely in the interstitial regions between them.

Definitions

References (50)

  • G Cristobal et al.

    Phase separation in living micellar networks

    Physica A

    (1999)
  • C Rodriguez et al.

    Cubic-phase-based concentrated emulsions

    J. Colloid Interface Sci.

    (2000)
  • T.D Le et al.

    Topological transformation of a surfactant bilayer

    Physica B

    (2000)
  • O Freund et al.

    In vitro and in vivo stability of new multilamellar vesicles

    Life Sci.

    (2000)
  • H Mays et al.

    Microemulsions studied by scattering techniques

  • P.A Reynolds et al.

    High internal phase water-in-oil emulsions studied by small-angle neutron scattering

    J. Phys. Chem. B

    (2000)
  • G Cristobal et al.

    Shear-induced structural transitions in Newtonian non-Newtonian two-phase flow

    Phys. Rev. E

    (2000)
  • O Glatter et al.

    Non-ionic micelles near the critical point: micellar growth and attractive interaction

    Langmuir

    (2000)
  • O Lade et al.

    Polymerizable non-ionic microemulsions: phase behavior of H2O-n-alkyl methacrylate-n-alkyl poly(ethylene glycol) ether (CiEj)

    Langmuir

    (2000)
  • B Jakobs et al.

    Amphiphilic block copolymers as efficiency boosters for microemulsions

    Langmuir

    (1999)
  • H Endo et al.

    Membrane decoration by amphiphilic block copolymers in bicontinuous microemulsions

    Phys. Rev. Lett.

    (2000)
  • R Strey et al.

    Dilute lamellar and L3 phases in the binary water-C12E5 system

    J. Chem. Soc. Faraday Trans.

    (1990)
  • R Wagner et al.

    Phase behavior of binary water-trimethylsilane surfactant systems: origin of the dilute lamellar phase

    Langmuir

    (1999)
  • B Schwarz et al.

    Dynamics of the ‘sponge’ L3 phase

    Langmuir

    (2000)
  • R Oda et al.

    Schlosseler: time resolved small-angle neutron scattering study of shear thickening surfactant solutions after the cessation of flow

    Langmuir

    (2000)
  • G Schmidt et al.

    Rheo-optical investigations of lyotropic mesophases of polymeric surfactants

    Rheologica Acta

    (1999)
  • A.E Terry et al.

    Flow studies of a surfactant hexagonal mesophase

    J. Phys. Chem. B

    (1999)
  • R Gamez-Corrales et al.

    Shear-thickening dilute surfactant solutions: equilibrium structure as studied by small-angle neutron scattering

    Langmuir

    (1999)
  • Y Zheng et al.

    Cryo-TEM imaging the flow-induced transition from vesicles to threadlike micelles

    J. Phys. Chem. B

    (2000)
  • W Helfrich

    Lyotropic lamellar phases

    J. Phys. Condensed Matter

    (1994)
  • G.K Auernhammer et al.

    The undulation instability in layered systems under shear flow — a simple model

    Rheologica Acta

    (2000)
  • A.G Zilman et al.

    Undulation instability of lamellar phases under shear: a mechanism for onion formation?

    Eur. Phys. J. B

    (1999)
  • J Mang et al.

    Discotic micellar nematic and lamellar phases under shear-flow

    Europhys. Lett.

    (1994)
  • J.T Mang et al.

    Lyotropic liquid crystals under simple couette and oscillatory shear

    Molec. Cryst. Liquid Cryst. Sci. Technol. A

    (1997)
  • J Penfold et al.

    Shear-induced structures in concentrated surfactant micellar phases

    J. Appl. Cryst.

    (1997)
  • Cited by (50)

    • Shear-Induced Lamellar/Onion Transition in Surfactant Systems

      2018, Advances in Biomembranes and Lipid Self-Assembly
      Citation Excerpt :

      In the past three decades, much attention has been paid to the shear effects on self-assemblies composed of amphiphiles including surfactants and block copolymers. Even if restricted to the surfactant systems, many studies have been reported for micelles and lyotropic phases such as cubic, hexagonal, lamellar, and sponge phases [1–3]. Among them, the most striking result may be the transition from the lamellar phase to the “onion phase” where all the space is filled by multilamellar vesicles (MLVs) alone, which cannot be observed at rest (without shear) in usual.

    • Tuned, driven, and active soft matter

      2015, Physics Reports
      Citation Excerpt :

      In this way, their relative impact on the collective dynamic response can be modified and the overall appearance of the sample can be tuned. Other materials like common smectic (layered) low-molecular-weight liquid crystals [233,237–239], see Fig. 2(c), or lamellar phases of surfactant solutions [233,240–244] can similarly feature orientational effects due to their “sheet-like” structure. Reorientations under shear can also be observed in structural phases different from the lamellar one [204,205,226,227,245,246].

    View all citing articles on Scopus
    View full text