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Shear Rheology of Lyotropic Liquid Crystals: A Case Study Raffaele Mezzenga,*,† Cedric Meyer,† Colin Servais,† Alexandre I. Romoscanu,† Laurent Sagalowicz,† and Ryan C. Hayward‡ Nestle´ Research Center, Vers-Chez-Les-Blanc, 1000 Lausanne 26, Switzerland, and Department of Chemical Engineering, University of California, Santa Barbara, California 93106 Received December 10, 2004. In Final Form: February 1, 2005 We have investigated the rheological properties of lyotropic liquid crystals (LCs) formed by self-assembled neutral lipids and water, their relationship with the topology of the structure, and their dependence on temperature and water content. The phase diagram of a representative monoglyceride-water system, determined by combining cross-polarized optical microscopy and small-angle X-ray scattering (SAXS), included four structures: lamellar, hexagonal, gyroid bicontinuous cubic (Ia3d), and double diamond bicontinuous cubic (Pn3m), as well as several regions of two-phase coexistence of some of the above structures. Rheology in the linear viscoelastic regime revealed a specific signature that was characteristic of the topology of each structure considered. The order-order transitions lamellar-to-cubic and cubic-to-hexagonal, as well as the order-disorder transitions from each LC to an isotropic fluid, were easily identified by following the development of the storage and loss moduli, G′ and G′′, respectively. The viscoelastic properties of both bicontinuous cubic phases were shown to be strongly frequency-dependent, following a pseudoMaxwell behavior, with multiple relaxation times. Cubic-to-cubic transitions were nicely captured by scaling the longest relaxation time, τ, with either temperature or water volume fraction. Therefore, the set of the three main parameters used to establish the rheological behavior of the structure, that is, G′, G′′, and relaxation time, τ, constitutes a consistent ensemble to identify the structures of the liquid crystal. Finally, relaxation spectra, extracted for all liquid crystalline phases, allowed an additional possible identification criterion of the various structures considered.
1. Introduction Extensive research on lyotropic liquid crystals over the last few decades has allowed the phase diagrams, the most common structures, and the specific properties of these nanostructured materials to be established in great detail.1-5 Systems formed by lipids and water are a representative example of this category of materials and are of interest for a multitude of different applications such as dispersion technology, cosmetics, pharmaceutical products, encapsulation systems, and foods.6-13 Since different types of lipids result in slightly different phase diagrams, extensive efforts have been made in order * Corresponding author. E-mail: raffaele.mezzenga@ rdls.nestle.com. Phone: + 41 21 785 8078. Fax: + 41 21 785 8554. † Nestle ´ Research Center. ‡ University of California, Santa Barbara. (1) de Gennes, P. G. The Physics of Liquid Crystals; Oxford University Press: Oxford, England, 1974. (2) Donald, A. M.; Windle, A. H. Liquid Crystalline Polymers; Cambridge University Press: Cambridge, England, 1992. (3) Nagle, J. F.; Scott, H. L. Physics Today 1978, 10, 38. (4) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, England, 1998. (5) Chandrasekhar, S. Liquid Crystals; Cambridge University Press: Cambridge, England, 1992. (6) Krog, N. J. Food Emulsifiers and Their Chemical and Physical Properties. In Food Emulsions; Friberg, S. E., Larsson, K., Eds.; Marcel Dekker Inc.: New York, 1997; p 141. (7) Mariani, P.; Rustichelli, F.; Saturni, L.; Cardone, L. Eur. Biophys. J. 1999, 28, 294. (8) Larsson, K. Curr. Opin. Colloid Interface Sci. 2000, 5, 64. (9) Caboi, F.; Borne, J.; Nylander, T.; Khan, A.; Svendsen, A.; Patkar, S. Colloids Surf., B 2002, 26, 159. (10) de Campo, L.; Yaghmur, A.; Sagalowicz, L.; Leser, M. E.; Watzke, H.; Glatter, O. Langmuir 2004, 20, 5254. (11) Hemker, W. J. Am. Oil Chem. Soc. 1981, 58, 114. (12) Paash, S.; Schambil, F.; Schuwuger, M. J. Langmuir 1989, 5, 1344. (13) Kumar, T. N.; Sastry, Y. S. R.; Lakshminarayana, G. J. Am. Oil Chem. Soc. 1989, 66, 153.
to explore all possible structures. In systems formed by monoglyceride and water, on which we will focus in the present paper, the extent of each specific phase region depends mostly on the length and degree of unsaturation of the hydrophobic chain.6,14-16 The most common types of liquid crystalline structures observed in monoglyceridewater systems are lamellar phases (LR and Lβ), the hexagonal columnar phase, and two types of bicontinuous cubic phases, the gyroid (Ia3d) and double diamond (Pn3m). Together with the latter two structures, a third cubic phase, the primitive (Im3m) phase, is occasionally observed.17,18 These cubic phases have been interpreted in the context of triply periodical minimal surfaces (TPMS), which has allowed a detailed explanation of their complex topology.19 On the other hand, lamellar and hexagonal structures have a simple morphology whose characteristic parameters can be extracted directly from small-angle X-ray scattering (SAXS) data and phase composition. Whereas several decades of investigation on the morphology of lyotropic liquid crystals have established reliable methods for identification and assessment of their structures, there have been few rheological studies on these complex fluids. Only recently has a coherent and uniform vision of the viscoelastic properties of these systems begun to emerge.20-28 To date, a specific rheo(14) Hyde, S. T.; Anderson, B.; Ericsson, Larsson, K. Z. Kristallogr. 1984, 168, 213. (15) Barfod, N. M.; Krog, N. J.; Buchheim, W. In Food Proteins. Part I: Structural and Functional Relationships; Kinsella, J. E., Ed.; American Oil Chemists Society: Champaign, IL, 1989; p 144. (16) Lutton, E. S.; Stauffer, C. E.; Martin, J. B.; Fehl, A. S. J. Colloid Interface Sci. 1969, 30, 283. (17) Esposito, E.; Eblovi, N.; Rasi, S.; Drechsler, M.; Di Gregorio, G. M.; Menegatti, E.; Cortesi, R. PharmSci 2003, 5, art. no. 30. (18) Angelova, A.; Ollivon, M.; Campitelli, A.; Bourgaux, C. Langmuir 2003, 19, 6928. (19) Scriven, L. E. Nature 1976, 263, 123. (20) Jones, J. L.; McLeish, T. C. B. Langmuir 1999, 15, 7495.
10.1021/la046964b CCC: $30.25 © 2005 American Chemical Society Published on Web 03/04/2005
Shear Rheology of Lyotropic Liquid Crystals
logical signature has not been established for all of the known self-assembled nanostructures in surfactantwater binary systems, and the viscoelastic and plastic mechanisms regulating their response to stress and deformation are still being debated. In particular, cubic phases are challenging systems for rheological studies, since their response to stress is the result of several relaxation mechanisms, as previously recognized in the literature.20-23 The presence of only two components in monoglyceride-water cubic phases should simplify the rheological response, as compared to bicontinuous cubic phases formed by two liquids and a surfactant, since separate motion of the interface and the hydrophobic phase is impossible. In the present work, we concentrate on a specific monoglyceride of relevance for technological applications and we investigate its phase diagram in the presence of water at various temperatures, extracting the characteristic parameters of different structures by combining SAXS data, simple topological models, and TPMS. Furthermore, we attempt to establish a rheological signature for all of the different liquid crystalline phases encountered in the phase diagram by considering the storage, G′, and loss, G′′, moduli and the longest relaxation time, τ, and we discuss the major mechanisms responsible for relaxation in the linear viscoelastic regime. 2. Experimental Section 2.1. Materials and Sample Preparation. Dimodan U/J was a generous gift from Danisco (Brabrand, Denmark) and was used as received. This commercial-grade form of monolinolein contains more than 98 wt % monoglyceride. The hydrocarbon tail consists predominantly of C18 chains (91%), distributed as follows: C18:2 (61.9%), C18:1 (24.9%), and C18:0 (4.2%) (where :2, :1, and :0 refer to the number of unsaturations); additional lipids are C16:0 chains (7.4%) and a residual amount of diglycerides (1.6%). The mixing of saturated and unsaturated chains lowers the melting temperature of the Lc phase, yielding a larger region of the bicontinuous cubic structures in the phase diagram. The same single batch was used for all the measurements of the present work. Samples were prepared in the following manner. First, water and monoglyceride were inserted into vials with sealed caps and mixed by means of cyclic heating (up to 100 °C for a few seconds) and vortex-aided vibrations. Once homogeneous mixing was achieved, the vials were cooled to room temperature and opened, and capillary tubes were inserted. Filling of the capillaries was achieved by cyclic heating, vortex, and centrifugation of the vials. Finally, the tubes were cooled again to room temperature and opened, and the capillaries were extracted and sealed. For the rheometry measurements, the heated mixed dispersions were poured into a preheated instrument cup and the measurement fixture was directly closed and sealed as described below. Materials were then maintained at -20 °C for 1 h and then at the testing temperature for 40 min before the measurements started. 2.2. Methods. Density Measurements. Density measurements of Dimodan as a function of temperature were performed by evaluating the changes in volume of a constant mass of monoglyceride contained in a graduated capillary. The range of (21) Jones, J. L.; McLeish, T. C. B. Langmuir 1995, 11, 785. (22) Radiman, S.; Toprakcioglu, C.; McLeish, T. C. B. Langmuir 1994, 10, 61. (23) Montalvo G.; Valiente, M.; Rodenas, E. Langmuir 1996, 12, 5202. (24) Franco, J. M.; Munoz, J.; Gallegos, C. Langmuir 1995, 11, 669. (25) Moros, J. E.; Cordobe´s, F.; Gallegos, C.; Franco, J. M. J. Dispersion Sci. Technol. 2001, 22, 405. (26) Pitzalis, P.; Monduzzi, M.; Krog, N.; Larsson, H.; LjusbergWahren, H.; Nylander, T. Langmuir 2000, 16, 6358. (27) Kelarakis, A.; Castelletto, V.; Chaibundit, C.; Fundin, J.; Havredaki, V.; Hamley, I. W. Booth, C. Langmuir 2001, 17, 4232. (28) Messe´, L.; Corvasier, L.; Young, R. N.; Ryan, A. J. Langmuir 2002, 18, 2564.
Langmuir, Vol. 21, No. 8, 2005 3323 temperatures considered was 35-80 °C. On the basis of the measurements, the density of the monoglyceride was well fitted by Flip. ) 1.1214 - 0.0022602T + 9.5018 × 10-6T2, where Flip. is expressed in g/cm3 and T in °C. Reference tables were used for the density changes with the temperature of demineralized water. Cross-Polarized Visual Inspection and Optical Microscopy. A first visual inspection allowed a coarse phase diagram of the Dimodan-water system to be established on the basis of the different levels of birefringence of the liquid crystalline phases. Samples were placed between two polarizers crossed at 90° and were illuminated by a halogen light source. Phase transitions obtained by visual observation were confirmed by optical microscopy under cross-polarized light on a Zeiss Axioplan instrument. Small-Angle X-ray Scattering. Small-angle X-ray scattering was used to further refine the phase diagram obtained by crosspolarized visual inspection and optical microscopy, to identify regions where different phases coexist, and to extract the main parameters of the structures observed. Experiments were performed on 1.5 mm thick capillaries under X-rays of 1.54 Å wavelength, generated by a fine focus Rigaku rotating anode. Data were acquired on a high-precision pinhole SAXS camera. The sample-to-detector distance varied from 726 to 741 mm, depending on the experiment. Calibration of the sample-to-detector distance was performed using the layer spacing of the smectic liquid crystal phase in 8CB as a standard.29 Data for each test were acquired for 20 min. The time allowed for equilibrating samples at a given temperature depended on the temperature range, that is, 60 min at 35 °C, 30 min between 40 and 60 °C, and 15 min above 60 °C. Rheological Measurements. The rheological signatures of the various liquid crystalline phases were determined using a PaarPhysica MCR 500 rheometer in strain-controlled mode (direct strain oscillation). The measurement cell was a DIN concentric cylinder. The diameter of the inner cylinder was 17 mm, except for the analysis of the lamellar phase for which a 26.7 mm diameter double gap concentric cylinder was used. The temperature was adjusted using a Peltier system applied on the outer concentric cylinder. To minimize water loss by evaporation, a solvent trap system using concentric Teflon disks and a low-viscosity sealing oil were used. The contribution of the solvent trap to the overall torque measurement is below the threshold detection limit of the instrument (
