Complex fluids with a yield value; their microstructures and rheological properties

Abstract

ricevuto il 28 Ottobre 1994) Summary. --A general method is presented for the preparation of viscoelastic surfactant phases that consist of densely packed multilamellar vesicles in water. The phases form spontaneously when ionic surfactants are added to L=-or L3-phases from uncharged surfactants and cosurfactants. The dimensions and the structure of the vesicles were studied from FF-TEM micrographs for 100 mM surfactant solutions. The average diameter of the vesicles is in the range of 1 ~m, the interlamellar spacing is around 800/~. Under these conditions the vesicles are densely packed and cannot pass each other. The systems are highly viscoelastic and have a yield value. The storage and the loss moduli were determined by oscillating rheological measurements. The storage modulus was about one order of magnitude larger than the loss modulus and independent of frequency. Both parameters were determined as a function of the concentration and the chain length of the surfactant and cosurfactant, the charge density and ionic strength, the solubilization of hydrocarbon and temperature. The yield values and shear moduli increase linearly with the surfactant concentration. For constant surfactant concentration the moduli increase in a S-shaped form with the charge density and reach saturation for a mole fraction of about 7% of ionic surfactant. The storage moduli and yield values decrease with the addition of excess salt. The storage moduli depend strongly on the chain length of the surfactant. The values of the moduli are explained on the basis of a hard-sphere model in which the multilamellar vesicles are considered as hard-sphere particles. PACS 62.10 -Mechanical properties of liquids. PACS 01.30.Cc -Conference proceedings.

Novel Vesicles from Single-Chain Surfactants

“…In this respect, the phases of C 14 DMAO and Ca(DS) 2 behave in the same way as other charged multilamellar vesicle phases with different chemical compositions. 10 Classic L R -phases with extended bilayers behave very differently at a total concentration of 100 mM. The shear modulus of such phases is much lower than that for the present system.…”

“…The yield stress value of the phase can easily be recognized by the fact that small air bubbles that are dispersed in samples do not rise. In this respect, the phases of C 14 DMAO and Ca(DS) 2 behave in the same way as other charged multilamellar vesicle phases with different chemical compositions …”

“…These phases are highly viscoelastic and have a yield stress value even when the concentration is as low as 2% surfactant. 10 In the present work, we report the preparation of charged vesicle phases that are produced without the use of cosurfactants. Such phases are important for many applications where cosurfactants cannot be used.…”

“…Vesicle phases which are ionically charged and are not shielded by excess salt have interesting rheological properties. These phases are highly viscoelastic and have a yield stress value even when the concentration is as low as 2% surfactant …”

Gels Obtained by Colloidal Self-Assembly of Amphiphilic Molecules

“…An example for such densely packed MLV is given for the zwitterionic surfactant tetradecyl-dimethyl amine oxide (TDMAO) that by addition of a cosurfactant like hexanol or heptanol becomes transformed into a state of vesicles or lamellae. By protonation or substitution of the TDMAO by the cationic tetradecyltrimethyl ammonium bromide (TTAB) this system can be shifted further into the state of MLVs [50]. Already at a surfactant concentration of 100 mM (~2.5 wt %) the formation of MLVs results in pronounced elastic properties with a shear modulus G 0 in the range of 10–100 Pa and the formed systems even exhibit a yield stress.…”