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Evidence of Formation of Ammonium Perfluorononanoate/2H2O Multilamellar Vesicles: Morphological Analysis by Rheology and Rheo-2H NMR Experiments Luigi Coppola, Luigi Gentile,* Isabella Nicotera, Cesare Oliviero Rossi, and Giuseppe Antonio Ranieri Laboratory PC_SM “M. Terenzi, Department of Chemistry University of Calabria, Via P. Bucci, Cubo 14/D, 87036 Rende, Cosenza, Italy Received July 20, 2010. Revised Manuscript Received October 26, 2010 Rheology and rheo-2H NMR measurements are presented for 30 wt % ammonium perfluorononanoate (APFN)/2H2O mixture in the temperature range 20-70 °C. A first-order lamellar-to-nematic transition occurs at 42 °C, and a first-order nematic-to-isotropic transition occurs at 49 °C. Different rheological behaviors of the lamellar phase were observed with increasing the temperature. The lamellar structure at low temperature (LR-) has a clear gel-like viscoelasticity, while at high temperature the lamellar structure (LRþ) has a liquid-like response. In this study we have observed for the first time, along with the lamellar phase of a surfactant containing fluorinated fatty acid, the formation of multilamellar vesicles (MLVs) (“onions”) induced by shear. With the aid of nonlinear rheology and rheo-NMR techniques, onion formation was found to occur in both temperature regimes of the lamellar phase, but at different strain units. It is suggested that the lamellar phase consists of smectic structures in both LR- and LRþ, but with different percentages of defect density.
Introduction Surfactants (surface active molecules) play an important role in science with considerable implications for the environment and for industrial applications. The study of flow behaviors of surfactant aqueous solutions has become very important and has stimulated the development of rheological tools and models for probing the soft structures induced by the shear.1-13 Effects of the shear *To whom the correspondence should be addressed. E-mail: luigi.gentile@ unical.it. (1) Oettera, G.; Hoffmann, H. Ringing gels and their fascinating properties. Colloids Surf. 1989, 38(1), 225–250. (2) Coppola, L.; Gianferri, R.; Oliviero, C.; Ranieri, G. A. Rheology of a lyotropic mesophase through a stress-relaxation experiment. J. Colloid Interface Sci. 2003, 264(2), 554–557. (3) Montalvo, G.; Valiente, M.; Rodenas, E. Rheological properties of the L phase and the hexagonal, lamellar, and cubic liquid crystals of the CTAB/benzylalcohol/ water system. Langmuir 1996, 12(21), 5202–5208. (4) Gabriele, D.; Migliori, M.; Sanzo, R. D.; Oliviero Rossi, C.; Ruffolo, S. A.; de Cindio, B. Characterisation of dairy emulsions by NMR and rheological techniques. Food Hydrocolloids 2009, 23(3), 619–628. (5) Janeschitz-Kriegl, V. H. Polymer Melt Rheology and Flow Birefringence; Springer-Verlag: Berlin/Heildelberg/New York, 1983. (6) Fuller, G. G. Optical Rheometry of Complex Fluids; Oxford University Press: New York, 1995. (7) Nakatani, A. I.; Waldow, D. A.; Han, C. C. A rheometer with twodimensional area detection for light scattering studies of polymer melts and solutions. Rev. Sci. Instrum. 1992, 63(7), 3590–3598. (8) Plano, R. J.; Safinya, C. R.; Sirota, E. B.; Wenzel, L. J. X-ray Couette shear cell for nonequilibrium structural studies of complex fluids under flow. Rev. Sci. Instrum. 1993, 64(5), 1309–1318. (9) Kalus, J.; Neubauer, G.; Schmelzer, U. A new shear apparatus for small angle neutron scattering (SANS) measurements. Rev. Sci. Instrum. 1990, 61(11), 3384–3389. (10) Takahashi, Y.; Noda, M.; Naruse, M.; Kanaya, T.; Watanabe, H.; Kato, T.; Imai, M.; Matsushita, Y. Apparatus for small-angle neutron scattering and rheological measurements under sheared conditions. J. Soc. Rheol., Jpn. 2000, 28 (4), 187–191. (11) Nakatani, A. I.; Poliks, M. D.; Samulski, E. T. NMR investigation of chain deformation in sheared polymer fluids. Macromolecules 1990, 23(10), 2686–2692. (12) Callaghan, P. T.; Gil, A. M. Rheo-NMR of semidilute polyacrylamide in water. Macromolecules 2000, 33(11), 4116–4124. (13) Callaghan, P. T. Rheo NMR and shear banding. Rheol. Acta 2008, 47(3), 243–255.
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rate on lamellar phase are widely investigated.14-17 Multilamellar vesicles (MLVs) are usually formed when a defective lamellar phase is subject to shear flow,18 and this shear-induced transition has received much attention.19-26 Shear-induced MLVs may be stable for a long time, but they do not correspond to the thermodynamic equilibrium structure.19 The development of rheo tools (X-ray, neutron, light scattering, and NMR) has allowed experimentalists to probe the fluid structure under flow-like MLV (14) Richtering, W. Rheology and shear induced structures in surfactant solutions. Curr. Opin. Colloid Interface Sci. 2001, 6(5-6), 446–450. (15) Mortensen, K. Structural studies of lamellar surfactant systems under shear. Curr. Opin. Colloid Interface Sci. 2001, 6(2), 140–145. (16) Butler, P. Shear induced structures and transformations in complex fluids. Curr. Opin. Colloid Interface Sci. 1999, 4(3), 214–221. (17) Berni, M. G.; Lawrence, C. J.; Machin, D. A review of the rheology of the lamellar phase in surfactant systems. Adv. Colloid Interface Sci. 2002, 98(2), 217– 243. (18) Diat, O.; Roux, D.; Nallet, F. Effect of shear on a lyotropic lamellar phase. J. Phys. II France 1993, 3(9), 1427–1452. (19) Gradzielski, M. Vesicles and vesicle gels - structure and dynamics of formation. J. Phys.: Condens. Matter 2003, 15(19), 655–697. (20) Bergenholtz, J.; Wagner, N. J. Formation of AOT/brine multilamellar vesicles. Langmuir 1996, 12(13), 3122–3126. (21) Medronho, B.; Shafaei, S.; Szopko, R.; Miguel, M. G.; Olsson, U.; Schmidt, C. Shear-induced transitions between a planar lamellar phase and multilamellar vesicles: Continuous versus discontinuous transformation. Langmuir 2008, 24(13), 6480–6486. (22) M€uller, S.; B€orschig, C.; Gronski, W.; Schmidt, C. Shear-induced states of orientation of the lamellar phase of C12E4/water. Langmuir 1999, 15(22), 7558– 7564. (23) Zipfel, J.; Nettesheim, F.; Lindner, P.; Le, T. D.; Olsson, U.; Richtering, W. Cylindrical intermediates in a shear-induced lamellar-to-vesicle transition. Europhys. Lett. 2001, 53(3), 335–341. (24) Medronho, B.; Fujii, S.; Richtering, W.; Miguel, M. G.; Olsson, U. Reversible size of shear-induced multi-lamellar vesicles. Colloid Polym. Sci. 2005, 284(3), 317–321. (25) Nettesheim, F.; Zipfel, J.; Olsson, U.; Renth, F.; Lindner, P.; Richtering, W. Pathway of the shear-induced transition between planar lamellae and multilamellar vesicles as studied by time-resolved scattering techniques. Langmuir 2003, 19(9), 3603–3618. (26) Fujii, S.; Koschoreck, S.; Lindner, P.; Richtering, W. Influence of a Triblock Copolymer on Phase Behavior and Shear-Induced Topologies of a Surfactant Lamellar Phase. Langmuir 2009, 25(10), 5476–5483.
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formation.20-26 Usually the initial state of planar lamellae is followed by the intermediates with cylindrical symmetry (multilamellar cylinders (MLCs) or coherent stripe buckling of the lamellae).25,26 In the recent past, it has been observed that many surfactants, mixed with water in concentrated solutions, form lyotropic liquid crystals with a net macroscopic alignment in the presence of a magnetic field. In general, the characterization of their structural organizational state and dynamic properties is fundamental. Among these liquid crystal systems, one can find the ammonium perfluorononanoate (APFN)/2H2O system27,28 and similar systems containing fluorinated fatty acid salts with large counterions such as cesium or ammonium.29-31 In fact, perfluorinated fatty acids have been shown to form finite-sized discotic bilayers in nematic phases, with directors parallel to the magnetic field direction. Although there is still some controversy regarding the precise morphology of the lamellar phase32,33 small-angle X-ray scattering, deuterium NMR, and electrical conductivity have been used to show that the fundamental mesogenic particle in the nematic phase is the discrete discotic micelle.34 The phase diagram of APFN/2H2O system27,35,36 is characterized by the presence of two lyotropic phases (lamellar and nematic) coexisting with a wide isotropic solution. The nematic phase is of the type I, and the aggregates have a positive and diamagnetic anisotropy. Thus in the presence of a magnetic field, the aggregates align parallel to the direction of the field. The change of phase at the lamellar-nematic temperature has been ascribed to order-disorder transitions. A diffusion-NMR study has been conducted by some of us on several APFN/2H2O mixtures as a function of temperature.27,28 Here, the interpretations of the experimental results led to some first conclusions on the morphology of the two mesophases. The nematic phase consisting of disk-like micelles has a small diameter-to-thickness ratio, decreasing from ca. 8, at the lamellar-nematic transition, to 4 at the nematic-isotropic transition. For the lamellar smectic phases, two different models were assumed, namely, continuous lamellar planes of surfactants with water-filled holes, and separated circular disks arranged in layers. In any case, these defects covered about 25% of bilayer area, but no observation was made on the effect of the temperature increase.27 (27) Chidichimo, G.; Coppola, L.; Mesa, C. L.; Ranieri, G. A.; Saupe, A. Structure of the lamellar lyo-mesophase in water/ammonium perfluorononanoate mixtures: PFG NMR and 2H-NMR investigations. Chem. Phys. Lett. 1988, 145(1), 85–89. (28) Coppola, L. Ph.D Thesis, University of Calabria, 1986. (29) Photinos, P. J.; Saupe, A. The electric conductivity of the lamellar smectic, the micellar nematic, and the isotropic micellar solution of ammonium perfluorononanoate in water. J. Chem. Phys. 1986, 84(1), 517–521. (30) Photinos, P. J.; Saupe, A. Measurements of the conductivity and relaxation times for the micellar nematic phase of the system ammonium perfluorononanoate/H2O. J. Chem. Phys. 1986, 85(12), 7467–7471. (31) Boden, N.; Clements, J.; Jolley, K. W.; Parker, D.; Smith, M. H. Nematiclamellar tricritical behavior and structure of the lamellar phase in the ammonium pentadecafluorooctanoate (APFO)/water system. J. Chem. Phys. 1990, 93(12), 9096–9105. (32) Photinos, P.; Saupe, A. Slow relaxation effects at the second-order nematic to lamellar smectic phase transition in micellar liquid crystals. Phys. Rev. A 1990, 41(2), 954–959. (33) Boden, N.; Jolley, K. W. Interpretation of density and conductivity measurements in the liquid-crystal phases of the cesium pentadecafluorooctanoatewater system and its implication for the structure of the lamellar phase. Phys. Rev. A 1992, 45(12), 8751–8758. (34) Boden, N.; Corne, S. A.; Holmes, M. C.; Jackson, P. H.; Parker, D.; Jolley, K. W. Order-disorder transitions in solutions of discoid micelles. J. Phys. (Paris) 1986, 47(12), 2135–2144. (35) Fontell, K.; Lindman, B. Fluorocarbon surfactants. Phase equilibriums and phase structures in aqueous systems of a totally fluorinated fatty acid and some of its salts. J. Phys. Chem. 1983, 87(17), 3289–3297. (36) Reizlein, K.; Hoffmann, H. New lyotropic nematic liquid crystals. Prog. Colloid Polym. Sci. 1984, 69, 83–93.
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The present paper reports on accurate investigation of the APFN/2H2O mixture along an isoplethal path (30 wt % APFN) in the temperature range 20-70 °C by means of classical rheology and rheo-2H NMR. At this concentration, the mixture forms a nematic phase (N) consisting of disk-like aggregates between 42 and 49 °C. Below 42 °C it has a lamellar smectic phase (LR), consisting of a defective arrangement, and above 49 °C the mixture shows an isotropic phase (micellar phase, I). Two main goals can be identified for this study: (i) to probe the aggregate morphology at the lamellar-nematic transition, and (ii) to investigate the formation of shear-induced structures into the lamellar phase. In addition, there are two important features that positively characterize this study: first, only a few data concerning the rheological properties of nematic phases are available in literature; second, this paper pays particular attention to the correlation between the lamellar defects at equilibrium and MLV formation as a function of shear rate and temperature. Moreover, to our knowledge, only the work by Fujii et al.37 reports the formation of MLVs using a fluorinated surfactant, although with the addition of a nonionic surfactant.
Experimental Section Materials. APFN was prepared from the corresponding acid (Aldrich) which was dissolved in ethanol and neutralized by concentrated ammonia. Vacuum freeze-drying of the solution yields the APFN, which was purified twice by recrystallization from butanol-hexane mixtures. The final product has a melting temperature which is coincident with that of the other batch and with literature data.35 Ammonia, ethanol, butanol, and hexane were of analytical purity (Sigma Aldrich); water was deionized, doubly distilled, and degassed; D2O (99.7% isotropic enriched) was purchased by Sigma Aldrich. The sample was prepared by weighing proper amounts of each component in Pyrex glass tubes, which were centrifuged, sealed off, heated at 70 °C, centrifuged again, and allowed to stay at room temperature for a few days. A preliminary check of the sample was performed by visual inspection using crossed polarizers and a polarizing microscope. The textures of both lamellar and nematic mesophases are similar to those described by Rosevear.38 Methods. Rheology. Rheological measurements were conducted using a shear strain controlled rheometer RFS III (Rheometrics, USA) equipped with a Couette, cylinder geometry (external and inner radii of 17 and 16 mm, respectively). The temperature was controlled by a water circulator apparatus ((0.2 °C). To prevent errors due to evaporation, measuring geometries were surrounded by a solvent trap containing water. Two different kinds of experiments were carried out: (a) dynamic shear experiments and (b) steady shear experiments, where the viscosity has been measured in the time at a fixed shear rate. The small amplitude dynamic tests provided information on the linear viscoelastic behavior of materials through the determination of the complex shear modulus:39 0
G ¼ G0 ðωÞ þ iG0 ðωÞ
ð1Þ
where G0 (ω) is the in phase (or storage) component, and G00 (ω) is the out-of-phase (or loss) component. G0 (ω) is a measure of the reversible, elastic energy, while G00 (ω) represents the irreversible viscous dissipation of the mechanical energy. The applied strain amplitude for the viscoelastic measurements was reduced until the (37) Fujii, S.; Isojima, T.; Sasaki, N.; Kubota, K.; Nakata, M. Shear-induced structural transformation of pentaethylene glycol n-dodecyl ether and lithium perfluorooctane sulfonate mixed-surfactant lamellar solution. Colloid Polym. Sci. 2003, 281(5), 439–446. (38) Rosevear, F. B. Liquid crystals: The mesomorphic phases of surfactant compositions. J. Soc. Cosmet. Chem. 1968, 19, 581–594. (39) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed; Wiley: New York, 1980; p 672.
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linear response regime was reached. Such analysis was carried out by performing strain sweep tests in a frequency range between 0.100 and 15.9 Hz and in all the investigated temperature range. The temperature sweep measurements were conducted at frequency of 1 Hz and strain amplitude of 1% using a scan rate of 1 °C/min (time cure test). Steady shear experiments (nonlinear rheology) were performed by transient experiments (step rate tests). Controlling the velocity of the moving cylinder, the force exerted on the fluid was measured in a shear controlled rheometer. The macroscopic shear rate γ_ is defined as the ratio between the velocity over the gap, and the shear stress σ is defined as the macroscopic force divided by the surface. Under steady shear, non-Newtonian fluids exhibit a variety of nonlinear responses, including yield stress, shear-thinning and shearthickening. A wide variety of shear-thinning or shear-thickening fluids can be adequately described by a power law constitutive equation of the form η µ γ_
ðn - 1Þ
ð2Þ
where η is called the apparent macroscopic viscosity, and a Newtonian liquid has n = 1, while shear-thinning and shearthickening liquids have n > 1 and n < 1, respectively. 2 H NMR Spectroscopy. Rheo-2H NMR experiments of 2H2O were carried out using a cylindrical Couette cell (9 and 10 mm inner and outer radii, respectively). This cell is integrated into an NMR microimaging probe (25 mm) for a wide-bore superconducting magnet. The axis of the shear cell is aligned parallel to the external magnetic field. Shear is applied by rotating the outer cylinder with an external stepper-motor gearbox assembly mounted above the NMR magnet. The spectra were recorded with a Bruker Avance 300 (Bruker, Germany) operating at the deuterium resonance frequency of 46.073 MHz. Spectra were obtained in Fourier transformation (FT) mode of the signal following a single pulse, and 16 scans were accumulated for each spectrum. The temperature of the sample was controlled using an air-flow system. Under shear, the temperature of the sample remained constant within (0.4 °C. 2 H NMR probes the motionally averaged electric quadrupole couplings between the deuterium nuclei and the electric field gradients at the sites of the observed nuclei.40 The residual couplings begin from the anisotropy of the rotational motions of the water molecules and thus depend on the curvature of the hydrophobic/ hydrophilic interface.41-44 When 2H2O molecules experience a macroscopically anisotropic environment, the quadrupolar interaction has a nonzero average, leading to a splitting of the resonance. In the case of uniaxial symmetry, as for the LR phase, the 2H spectrum consists of a doublet with a frequency separation Δν.40,45 The deuterium NMR line shape depends on the distribution of director orientations. A simple doublet is observed only if the lamellar phase has been well aligned, for example, by the action of the magnetic field or by the application of shear. A disordered lamellar phase consisting of many domains of extended flat layers, whose orientations are isotropically distributed in space, gives rise to the characteristic line shape of a polycrystalline sample also known as a powder or Pake pattern. (40) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, 1961. (41) Douliez, J. P.; Bellocq, A. M.; Dufourc, E. J. Effect of vesicle size, polydispersity and multilayering on solid state 31P- and 2H-NMR spectra. J. Chim. Phys. Phys.-Chim. Biol. 1994, 91(6), 874–880. (42) Auguste, F.; Douliez, J.-P.; Bellocq, A.-M.; Dufourc, E. J.; Gulik-Krzywicki, T. Evidence for multilamellar vesicles in the lamellar phase of an electrostatic lyotropic ternary system. A solid state 2H-NMR and freeze fracture electron microscopy study. Langmuir 1997, 13(4), 666–672. (43) Baciu, M.; Olsson, U.; Leaver, M. S.; Holmes, M. C. 2H NMR evidence for the formation of random mesh phases in nonionic surfactant-water systems. J. Phys. Chem. B 2006, 110(16), 8184–8187. (44) Gotter, M.; Strey, R.; Olsson, U.; Wennerstr€om, H. Fusion and fission of fluid amphiphilic bilayers. Faraday Discuss. 2005, 129, 327–338. (45) Halle, B.; Wennerstr€om, H. Interpretation of magnetic resonance data from water nuclei in heterogeneous systems. J. Chem. Phys. 1981, 75(4), 1928–1943.
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Results and Discussion The phase diagram of the APFN/2H2O system was determined by ocular inspection, polarizing light microscopy, and by 2H NMR spectroscopy. Boundaries and phase equilibria are in agreement with the results described in the past by Chidichimo et al.27 and Coppola.28 In this study we are concerned with the mixture 30 wt % APFN/2H2O presenting a remarkable evolution with increasing temperature. A broad lamellar phase (LR) lies within an accessible experimental temperature range between 20 and 45 °C. This mesophase, reported by Chidichimo et al.27 as a defected structure, looks strongly translucent, exhibiting semifluid properties. It is optically anisotropic when viewed through crossed polarizers and shows focal-conic textures. The lamellar phase is in equilibrium with a nematic phase at 42 °C, which in turn is in equilibrium with a micellar solution at higher temperature (49 °C). Viscoelastic properties of the sample were analyzed by small amplitude oscillatory shear experiments at different temperatures. The dynamic rheology, usually determined though the frequency spectra, is a classical tool to investigate the material behavior and to probe microstructural features at equilibrium. The storage modulus (G0 ) and loss modulus, G00 , as a function of frequency are shown in Figure 1 for the 30 wt % APFN at different temperatures. In the light of these results, one can distinguish three main structural regimes within the temperature intervals investigated. First, in the temperature range 25-35 °C, the mixture behaves as a weak gel. G0 is slightly higher than G00 , and both moduli show a similar scaling with the frequency. This gel-like behavior recorded at low temperature is completely transformed upon heating the sample. Second, in the nematic phase, both moduli present quite similar values; third, at temperatures above 50 °C, the mixture shows a Maxwellian liquid-like behavior where G00 is higher than G0 . In order to better explore the structural changes induced by temperature, a temperature-sweep experiment was performed on the sample. In this experiment, the evolution of the storage and loss moduli is continuously monitored during a temperature ramp, at a constant heating rate (1 °C/min) and a frequency of 1 Hz (Figure 2). In the lower temperature range of the lamellar phase, the sample behaves as a gel-like material since the storage modulus (G0 ) is higher than the loss modulus (G00 ). At about 39 °C, a G0 -G00 crossover is observed, and both dynamic moduli decrease with increasing temperature with G00 > G0 . The crossover that is visible in the figure is at a temperature lower than the nematic transition temperature observed by differential scanning calorimetry (DSC). The sensitive decrease of dynamic moduli with the temperature increase accounts for the reduction of the smectic order in proximity to the lamellar-to-nematic transition. We can say that such an effect prevails on the increase of the defectivity of lamellae,27 and the result is a elasticity decrease with rising temperature. All these results states that the defective lamellar phase can be distinguished into two regions: (i) a low density defects phase at lower temperatures, LR- , and (ii) a high density defects phase at higher temperatures, LRþ. Figure 3 shows the transient viscosity for the mixture at 35 °C in LR- region after the inception of shear flow. For all shear rates reported in the Figure (0.1, 1.0, 4.0, and 10 s-1), the viscosity changes with time up to reaching a steady state, which is marked by a plateau in the curves (the plateaus have been shown just to their beginning). The time for reaching the steady state depends on the applied shear rate, and it takes longer time when the shear rate value is lower. The transient viscosity recorded at shear rates lower than 10 s-1 shows an evolution in agreement with the flow Langmuir 2010, 26(24), 19060–19065
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Figure 1. Frequency sweep tests in the linear strain region at different temperatures: (a) 30-40 °C and (b) 45-60 °C.
Figure 2. Time cure test at 1 Hz in the range 30-60 °C. The two vertical lines are the transition temperatures obtained by DSC. The inset shows a zoom in view of the LR region.
of a smectic lamellar systems as reported by Partal et al.46 The viscosity decrease together with increasing time and shear rate results in a shear-induced orientation of the lyotropic domains and leads to an aligned lamellar phase with the director perpendicular to the shear field.46 The transient viscosity recorded at 10 s-1 shows an increase with the time and reaches a plateau value higher than that obtained at lower shear. This trend seems to be reasonable with a gradual development of MLVs, as observed in some recent studies conducted on C10E3/water mixtures.21,23,24,47,50 It is important to mark (46) Partal, P.; Kowalski, A. J.; Machin, D.; Kiratzis, N.; Berni, M. G.; Lawrence, C. J. Rheology and microstructural transitions in the lamellar phase of a cationic surfactant. Langmuir 2001, 17(5), 1331–1337. (47) Oliviero, C.; Coppola, L.; Gianferri, R.; Nicotera, I.; Olsson, U. Dynamic phase diagram and onion formation in the system C10E3/D2O. Colloids Surf., A 2003, 228(1-3), 85–90. (48) Coppola, L.; Nicotera, I.; Oliviero, C.; Ranieri, G. A. Indirect detection of structural changes on the pluronic Pe 6200/H2O system by rheological measurements. Colloids Surf., A 2004, 245(1-3), 183–192. (49) Shahidzadeh, N.; Bonn, D.; Aguerre-Chariol, O.; Meunier, J. Large deformations of giant floppy vesicles in shear flow. Phys. Rev. Lett. 1998, 81(19), 4268–4271. (50) Koschoreck, S.; Fujii, S.; Lindner, P.; Richtering, W. Multilamellar vesicles (“onions”) under shear quench: Pathway of discontinuous size growth. Rheol. Acta 2009, 48(2), 231–240.
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Figure 3. The viscosity as a function of time for fresh solutions (30 wt % APFN) at different shear rate values (0.1, 1.0, 4.0, and 10 s-1) at 35 °C. For each shear rate, the rheo-2H NMR correspond to the plateau region on viscosity.
that such a plateau value is not too high, because of the lamellae and MLV coexistence.47-49 Figure 3 also shows the 2H NMR spectra of 2H2O for the sample at different shear rates. These spectra correspond to steady state structures under shearing. Similar to viscosity measurements, the evolution of 2H NMR spectra at 10 s-1 reflects the gradual formation of MLVs in the system (coexistence of MLVs and planar lamellae). In fact, the Pake doublet of the planar lamellae turns into a broad peak as a consequence of the presence of MLVs.22,25,26,41,51,52 Figure 4 reports the 2H NMR spectra as a function of shear strain for the sample sheared at 10 s-1, at 35 °C . The initial spectrum is a powder pattern, which is typical of nonoriented lamellar structures, while it becomes a broad singlet, increasing the strain and confirming the transformation from planar lamellae, of the LRregion, to MLVs (at 35 °C and a constant shear rate of 10 s-1). (51) Lukaschek, M.; M€uller; Hasennindl, A.; Schmidt, C.; Grabowski, D. A. Lamellar lyomesophases under shear as studied by deuterium nuclear magnetic resonance. Colloid Polym. Sci. 1996, 274(1), 1–7. (52) Lutti, A.; Callaghan, P. T. Measurement of multilamellar onion dimensions under shear using frequency domain pulsed gradient NMR. J. Magn. Reson. 2007, 187(2), 251–257.
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Figure 4. Strain evolution of spectra during the MLV formation from planar layers at 35 °C and a constant shear rate of 10 s-1.
Figure 5. Storage modulus, G0 , and loss modulus, G00 as a function of the oscillatory frequency obtained before and after (1800 s) the transient viscosity measurement at 10 s-1 for the 30 wt % APFN, T = 35 °C.
Figure 6. The viscosity as a function of time for various solutions at different shear rate values (0.1, 1.0, 4.0, and 10 s-1) at 40 (a) and 45 °C (b). For each shear rate, the rheo-2H NMR corresponds to the plateau region on viscosity.
The broad singlet, after the 18 000 strain unit, can be due to a coexistence region of MLVs and planar lamellae. In Figure 5 we report the viscoelasticity of the sample at 35 °C, before and after the viscosity measurements. The storage modulus, G0 , before and after the shearing, is higher than the loss modulus, G00 , in the frequency range from 0.07 to 15 Hz. In addition, it is worth noting that the G0 dynamic modulus after the transient measurements becomes dominant. This is a characteristic feature of the MLV phase.53 Figure 6 reports the transient viscosity and 2H NMR spectra at different shear rates both for the LRþ region (40 °C) and for the nematic phase (at 45 °C). At 40 °C the 2H NMR spectra clearly show the transformation of lamellae into MLVs. The MLV formation begins at a shear rate of 4 s-1, while at a shear rate of 10 s-1 MLVs are formed with a smaller size, as suggested by the narrowing of the deuterium spectral line. As far as nematic phase is concerned, the 2H NMR line shape does not change with increasing shear rate, and the MLV formation can be excluded (Figure 6b). Quadrupolar splittings at the lamellar-to-nematic transition decreases with increasing the temperature and do not show any discontinuity.
At this point we may conclude with the following analysis. Considering the effect of shear on the lamellar phase, it is worth noting that the MLV formation occurs at ∼7200 strain units at 40 °C (Figure 6a, 4 s-1), while it occurs at ∼18 000 strain units at 35 °C (Figure 4, 10 s-1). This effect can be ascribed to an increase of defects (i.e., pores) in the lamellar phase on heating. Using water self-diffusion, Chidichimo et al.27 already demonstrated that the density of defects raised with increasing the temperature, and the defect area of layers reaches a value of ca. 25% at the lamellar-nematic transition. Similar results have been obtained by many researchers studing other lamellar-to-nematic phase transitions.54-58
(53) Panizza, P.; Roux, D.; Vuillaume, V.; Lu, C.-Y. D.; Cates, M. E. Viscoelasticity of the Onion Phase. Langmuir 1996, 12(2), 248–252.
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(54) Photinos, P.; Saupe, A. Measurements of electric conductivity and reorientation times for the cesium perfluorooctanoate-D2O micellar system. Phys. Rev. A 1991, 43(6), 2890–2896. (55) Buhler, E.; Mendes, E.; Boltenhagen, P.; Munch, J. P.; Zana, R.; Candau, S. J. Phase behavior of aqueous solutions of a dimeric surfactant. Langmuir 1997, 13(12), 3096–3102. (56) Fairhurst, C. E.; Holmes, M. C.; Leaver, M. S. Structure and morphology of the intermediate phase region in the nonionic surfactant C16EO6/Water System. Langmuir 1997, 13(19), 4964–4975. (57) Holmes, M. C.; Leaver, M. S.; Smith, A. M. Nematic and disrupted lamellar phases in cesium pentadecafluorooctanoate/2H2O: A small angle scattering study. Langmuir 1995, 11(1), 356–365. (58) Freyssingeas, E.; Martin, A.; Roux, D. Role of dislocation loops on the elastic constants of lyotropic lamellar phases. Eur. Phys. J. E 2005, 18(2), 219–230.
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Coppola et al.
Article
plane, and the experimental exponent of -0.5 corresponds to the layer-displacement effect of planar lamellae and not to their destruction; (ii) the nematic micelles order themselves, under shear, in planes and flow by a layer-type displacement; and (iii) no shear-thinning behavior is observed at 40 °C because of the viscosity increase during MLV formation. Under the hypothesis that the lamellar LRþ and nematic phase are constituted by aggregates of similar morphology (i.e., discoidal micelles), the viscosity values at 45 °C would be markedly lower in comparison with the values measured at 40 °C. As a consequence, the important feature to be recognized in the data shown in Figure 6 is that different viscosity values are due at lamellar phase to MLV formation; therefore, the lamellar phase LRþ is constituted by structural units that are different from those of the nematic phase.
Conclusion Figure 7. Steady-state viscosity as a function of shear rate at selected temperatures (35, 40, and 45 °C). The arrow indicates the MLV formation at 4 s-1 at 35 °C, while the onions are completely formed at 40 °C.
Figure 7 turns out the steady state viscosity as a function of shear rate for the sample. Three temperatures are compared, corresponding to LR- (35 °C), LRþ (40 °C), and nematic (45 °C) regions. Examining with great attention the viscosity data, we noticed that the lamellar LR- region and the nematic phase show a similar shear-thinning behavior. This behavior is characterized by the power-law relation η µ γ_ -0.5, with the only exception of the last point being at shear rate of 10 s-1 for the LR- region. Such point, corresponding to the formation of MLVs, deviates sensitively from a straight-line. It is also interesting to note that the exponent of the power-law relation (ca. -0.5) observed at 35 and 45 °C is the same as the one observed by Bohlin59 and Fontell60 for the smectic lamellar phase of the 30% cetyltrimethylammonium bromide (CTAB)/l-hexanol/H2O system. Finally, no definite shear thinning behavior may be deducted for the change of the viscosity with increasing the shear rate for the lamellar LRþ region. In accordance with these results, we may conclude that (i) at 35 °C the lamellar aggregates orient parallel to the flow-vorticity (59) Bohlin, L. Coordination of structural units from flow measurements. J. Colloid Interface Sci. 1979, 69(1), 194–195. (60) Bohlin, L.; Fontell, K. Flow properties of lamellar liquid crystalline lipid-water systems. J. Colloid Interface Sci. 1978, 67(2), 272–283.
Langmuir 2010, 26(24), 19060–19065
In this paper we studied a 30 wt % APFN/2H2O mixture consisting of defected lamellae and where defect density increases with increasing temperature. Dynamic rheology showed a G0 -G00 crossover in the time cure test, occurring at a temperature lower than the lamellar-to-nematic transition temperature. Such crossover has been used to define two different structural regions inside the lamellar phase. We named these two regions as LR- and LRþ, namely, a low-temperature lamellar phase characterized by low density defects (at 35 °C) and a high-temperature lamellar phase characterized by high density defects (at 40 °C). The MLV formation was investigated by steady-state viscosity and rheo-2H NMR. The MLV formation (with lamellae coexistence) appears in LR- at a high strain unit (∼18000; shear rate 10 s-1) while it appears in LRþ at a low strain unit (∼7200; shear rate 4 s-1). The different defect percentages on the lamellar sheets at different temperatures should control the strain units of MLV transitions, which occur easily when the defect density is higher. The MLV formation on the lamellar phase of a fluorinated surfactant was observed for the first time. In keeping with these remarkable results, we may assume that the defective phase of APFN/2H2O system cannot be described by a picture of circular disks arranged in layers, because, in this case, the MLV formation is not probable. Then, one can suggest that this lamellar phase consists of smectic structures in both LR- and LRþ. Acknowledgment. Special thanks are due to prof. A. De Nino, University of Calabria, for the preparation of the APFN.
DOI: 10.1021/la102887e
19065
