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Langmuir 2000, 16, 10564-10565
Lecithin Organogel as a Binary Blend of Monodisperse Polymer-like Micelles S. A. Mezzasalma,† G. J. M. Koper,†,‡ and Yu. A. Shchipunov*,§ Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, P.O. Box 9502, 2300 RA, Leiden, The Netherlands, Laboratory of Physical Chemistry, Delft University of Technology, Julianalaan, 136, 2628 BL, Delft, The Netherlands, and Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia Received June 20, 2000. In Final Form: August 24, 2000
Introduction The lecithin organogel is formed by adding trace amounts of water to a nonaqueous solution.1,2 The polar additive induces a transition of the initial reversed spherical micelles into lengthy cylindrical aggregates.3-6 The latter make up a temporal three-dimensional network just as polymer molecules do in semidilute solutions. Moreover, they demonstrate similar rheological behavior,7-9 for which reason they are often called “polymerlike” micelles. In general, the rheological behavior of lecithin organogels is well described within the framework of a Maxwell model with one single relaxation time.8,9 An exception has been recently established for a system containing enhanced amounts of water.10 It was also shown in this paper that the system possesses features which are inherent to binary blends of monodisperse polymers, in which case the rheological behavior can be described by a Maxwell model including two relaxation times.11-13 This approach is applied for the first time to the lecithin organogel in the present article. It is shown that at certain conditions the system of polymer-like micelles can be considered as a binary blend of entities having various lengths and relaxation times. * To whom correspondence should be addressed. c/o Prof. Dr. H. Hoffmann, Physical Chemistry I, Bayreuth University, 95440 Bayreuth, Germany. Tel: +49-921-552760. Fax: +49-921-552780. E-mail:
[email protected]. † Leiden University. ‡ Delft University of Technology. § Russian Academy of Sciences. (1) Scartazzini, R.; Luisi, P. L. J. Phys. Chem. 1988, 92, 829-833. (2) Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356-374. (3) Schurtenberger, P.; Magid, L. J.; Penfold, P.; Heenan, R. Langmuir 1990, 6, 1800-1803. (4) Schurtenberger, P.; Magid, L. J.; King, S. M.; Linder, P. J. Phys. Chem. 1991, 95, 4173-4176. (5) Schurtenberger, P.; Magid, L. J.; Lindner, P.; Luisi, P. L. Prog. Colloid Sci. 1992, 89, 274-277. (6) Jerke, G.; Pedersen, J. S.; Egelhaaf, S. U.; Schurtenberger, P. Phys. Rev. E 1997, 56, 5772-5788. (7) Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28, 372-381. (8) Shchipunov, Yu. A. Colloid J. 1995, 57, 556-560. (9) Shchipunov, Yu. A.; Hoffmann, H. Langmuir 1998, 14, 63506360. (10) Shchipunov, Yu. A.; Hoffmann, H. Langmuir 1999, 15, 71087110. (11) Struglinski, M. J.; Graessley, W. W. Macromolecules 1985, 18, 2630-2643. (12) Marin, G.; Montfort, J. P. In Rheology for Polymer Melt Processing; Piau, J. M., Agassant, J. F., Eds.; Elsevier Science Pub: New York, 1996; pp 95-139. (13) Montfort, J. P.; Marin, G.; Monge, P. Macromolecules 1984, 17, 1551.
Figure 1. The complex viscosity |η*|, storage modulus G′, and loss modulus G′′ vs oscillation frequency. An organogel consists of 16.3 wt % of lecithin and 3.0 (1.1 wt %) water molecules per lecithin molecule in n-decane. The temperature was 25.0 ( 0.1 °C. The experimental data are shown by points. The solid lines are the best fits in accordance with eq 1a-c for G01 ) 660 Pa and τ1 ) 1.4 s.
Figure 2. Frequency dependencies of the rheological parameters for an organogel with 17.4 wt % lecithin and 4.0 (1.6 wt %) water molecules per lecithin molecule in n-decane measured at 25.0 ( 0.1 °C. The points represent the experimental data, and the solid lines are the best fits in accordance with eq 2a-d for G01 ) 5 Pa, G02 ) 689 Pa, τ1 ) 1.9 s, and τ2 ) 0.02 s.
Materials and Methods Soybean lecithin, Epikuron 200, was used as supplied from Lukas Meyer, AG (Hamburg, Germany). n-Decane was of highpurity quality from Fluka. Doubly distilled water was prepared in the common manner. The homogeneous jellylike phases were made by dissolving appropriate amounts of lecithin and water in n-decane. They were allowed to stay at ambient temperature at least for 3 days to reach an equilibrium state. The procedure is detailed in previous publications.14,15 Rheological measurements were performed with a Bohlin CS10 stress-controlled rheometer. A cone-and-plate geometry with a diameter of 40 mm and a cone angle of 4° was used. The oscillatory frequency was varied from 0.001 to 10 Hz. The temperature was 25.0 ( 0.1 °C.
Experimental Results and Discussion Figures 1 and 2 present the frequency dependencies of the storage modulus (G′), the loss modulus (G′′), and the complex viscosity (η*) for organogels containing 16.5 and 17.4 wt % lecithin plus 3 (1.1 wt %) and 4 (1.6 wt %) water (14) Shchipunov, Yu. A.; Shumilina, E. V. Mater. Sci. Eng., C 1995, 3, 43-50. (15) Shchipunov, Yu. A.; Schmiedel, P. J. Colloid Interface Sci. 1996, 179, 201-206.
10.1021/la000860t CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000
Notes
Langmuir, Vol. 16, No. 26, 2000 10565
molecules per lecithin molecule, respectively. They provide examples of various rheological behavior observed for the micellar systems. Figure 1 demonstrates the common case for systems consisting of polymer-like micelles.8,9,16,17 This is a singlemode relaxation process (hereinafter termed 1-M). It is described within the framework of the Maxwell model by the following equations:18
G′(ω) ) G01ω2τ12/(1 + ω2τ12)
(1a)
G′′(ω) ) G01ωτ1/(1 + ω2τ12)
(1b)
|η*(ω)| ) (G′2 + G′′2)1/2/ω
(1c)
where ω is the oscillation frequency in rad/s, G01 is the plateau modulus, and τ1 is the terminal relaxation time for the only mode present, mode 1. The solid lines in Figure 1 represent best fits to the experimental data shown by unfilled points. A deviation appears at high frequencies that is caused by a contribution of fast relaxation (Rouse) processes.19-21 After the critical water-to-lecithin ratio is reached, a jellylike phase is formed, to which the 1-M model is no longer applicable. An illustrative example is presented in Figure 2. The main feature of the considered frequency dependencies of rheological parameters is that there are intermediate plateaus in the curves. This enables one to think of a system including two relaxation processes. For this case, we adopted a Maxwell model with two parallel elements, thus extending eq 1a-c as the following:19
G′(ω) ) G01ω2τ12/(1 + ω2τ12) + G02ω2τ22/(1 + ω2τ22) (2a) G′′(ω) ) G01ωτ1/(1 + ω2τ12) + G02ωτ2/(1 + ω2τ22) (2b) η′(ω) )
∑k η0k/(1 + ω2τk2)
(2c)
∑k η0kτk/(1 + ω2τk2)
(2d)
η′′(ω) ) ω
with k ) 1, 2 and η*(ω) ) η′ - iη′′ and where a second plateau modulus G02 and a second relaxation time τ2 are introduced to take into account the presence of a second mode. This is called the two-mode (2-M) model. A standard best-fit procedure was used to mathematically describe the experimental data in Figures 1 and 2 (16) Kern, F.; Lemarechal, P.; Candau, S. J.; Cates, M. E. Langmuir 1992, 8, 437-440. (17) Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456-1464. (18) Ferry, J. D. Viscoelastic properties of polymers; John Wiley: New York, 1980. (19) Cates, M. E. Macromolecules 1987, 20, 2289-2296. (20) Turner, M. S.; Cates, M. E. J. Phys. (Paris) 1990, 51, 307-316. (21) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758-4767.
according to eqs 1 and 2, respectively. For any relaxation mode (i), coefficients corresponding to the characteristic time, zero-shear viscosity, and plateau modulus were constrained by using the fundamental relation18,22
τI ) η0i/G0I which is taken from the theory of rubber viscoelasticity. To have a better agreement in the low-frequency region for the 2-M case, we also constrained eq 2 to the following conditions: G01 < G02 and η01 < η02. The resulting curves obtained by the best-fitting procedure are shown by solid lines in Figures 1 and 2. As one can see, the agreement with the experimental data is quite satisfactory in both cases. This provides additional support for the initial idea of a possible 1-M to 2-M transition in the lecithin organogel. It is of interest to consider the obtained numerical results. The organogel containing three water molecules per lecithin molecule possesses a high viscosity of 890 Pa (Figure 1). The rest parameters, the plateau modulus G01 and the terminal relaxation time τ1, are 660 Pa and 1.4 s, respectively. This is a highly viscoelastic system with rather fast relaxation processes. With increasing water content, the viscosity undergoes a sharp decrease. It was found from the fitting procedure (Figure 2) that η01 ) 10 and η02 ) 15 Pa. As is obvious, the viscosities differ moderately. The distinction between the relaxation processes is mainly caused by the plateau moduli (G01 ) 5 and G02 ) 689 Pa). This provides the difference in the relaxation time (τ1 ) 1.9 and τ2 ) 0.02 s), ranging about 2 orders of magnitude. An important point is that the pairs of values for the plateau modulus and relaxation time (G01 ) 5 Pa and τ1 ) 1.9 s, G02 ) 689 Pa and τ2 ) 0.02 s) for the 2-M process do not correlate with that obtained at the smaller water-to-lecithin molar ratio (660 Pa and 1.4 s) when the 1-M process is in effect. This fact implies a significant change of lecithin organogel structure when transferring from the 1-M to the 2-M system. Thus, the theoretical consideration on the jellylike system composed of polymer-like lecithin micelles allows us to make the following main conclusions: (i) the lecithin organogel at large water-to-lecithin molar ratios demonstrates rheological behavior that obeys the Maxwell model and consists of two elements in parallel and, as a consequence, (ii) the micellar system might be represented as a binary blend of polymer-like micelles having various aggregation numbers. The difference should be significant because of a large ratio between the relaxation times τ1/τ2 that ranges 2 orders of magnitude. Acknowledgment. This work has been supported by the Marie-Curie TMR Contract No. ERBFMBICT-982918, under the EC and a collaborative linkage grant PST.CLG 975306 from NATO. We are indebted to Professor H. Hoffmann for support of this work and helpful discussions. LA000860T (22) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933-973.
