The distinction of viscoelastic phases from entangled wormlike micelles and of densely packed multilamellar vesicles on the basis of rheological measurements |
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Authors: | Rami Abdel-Rahem Heinz Hoffmann |
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Institution: | 1. Al-Ahsa Teachers College, Qatar Street, P.O. Box 2313, Alhafouf 31982, Al-Ahsa, Kingdom of Saudi Arabia 2. Bayreuther Zentrum für Kolloide und Grenzfl?chen, Gottlieb-Keim-Str. 60, 95448, Bayreuth, Germany
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Abstract: | It is shown in this work how two viscoelastic surfactant systems that are both shear thinning but differ in their morphology can be distinguished on the basis of rheological measurements. The measurements were carried out on the novel surfactant system cetyltrimethylammonium 2-hydroxy-1-naphthoate. The phases in this system are produced by mixing cetyltrimethylammonium hydroxide and 2-hydroxy-1-naphthoic acid. With increasing counterion surfactant ratio X, the system has two viscoelastic regions that are separated by a two-phase region. It is shown by cryo-transmission electron microscopy and by small angle neutron scattering that the first viscoelastic region which exists between X=0.5 and X=0.75 contains wormlike micelles, while the second viscoelastic region that exists between X=0.9 and X=1.4 contains multilamellar vesicles. Both phases look alike, are highly viscoelastic, have similar storage modulus values, and are shear thinning. The phases and the properties of the phases for the studied system are very similar as the phases for the system CTA-3-hydroxy-2-naphthoate that has been studied before (see Hassan et al. Langmuir, 12:4350–4357, 1996; Horbaschek et al. J Colloid Interface Sci, 206:439–456, 1998). The two viscoelastic phases show the same shear-thinning behavior, but differ in other rheological results. The phases can most easily be distinguished with the help of normal stress measurements. The wormlike viscoelastic solutions show large normal stresses that give rise to a large Weissenberg effect while the vesicle phases show no Weissenberg effect. |
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Keywords: | Yield stress Normal stresses Rheology Surfactant Viscoelasticity Wormlike micelles |
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