Viscoelastic Properties of Worm-like Micelle of Aluminum Tristearylate/Decahydronaphthalene

It is well known that there are wide varieties of materials which form self-organized structure. When a sort of 3 dimensional network structure is formed, the system may possess viscoelastic properties similar to entangled polymers or gels depending on life time and strength of the structure. A typical example of low molecular weight system is wormlike micelles formed in aqueous solutions of surfactants and organic salts, such as cetyltrimethylammonium bromide and sodium salicylate (CTAB/NaSal). In this case, at least two different types of relaxation processes for network structure are known. That is, the main relaxation process for so called type II micelle is similar to entangled polymer systems, while that of type III is Maxwell type relaxation and the structure is called “phantom network”. There also exist surfactant systems which form wormlike micelles in organic solvents. Mixtures of aluminum tristearylate (C18Al) and non-polar organic solvents, known as metal soap, are typical example of them, which show gel-like behaviors. However, their viscoelastic properties and structure are less studied compared to wormlike micelles in aqueous solutions. It have been reported that the viscoelastic properties of C18Al/organic solvent systems strongly depend on sample preparation conditions and aging. Structural analysis by light scattering and small angle X-ray scattering may be somewhat difficult for this system due to the low contrast of refractive indices and electron densities between surfactants and solvents. These may be the reasons for scarce study. In a previous work, we examined effects of preparation conditions and aging at room temperature on the viscoelastic properties of C18Al/decahydronaphthalene (decalin) system. First, effects of dissolving temperature and heating time are examined. The system became more uniform at higher temperature. Therefore, the dissolving temperature was fixed at 130 °C, close to the boiling temperature of decalin. Dynamic viscoelastic measurements at 25 °C after 1 day room temperature aging revealed that the longest relaxation time becomes shorter and plateau region of storage modulus becomes narrower with increase of heating time at 130 °C. The mixture became low viscosity liquid after 36 hours heating. Thus, preparation condition was fixed at 130 °C, 2 hours mixing and the difference of viscoelastic properties with the concentration was examined. Viscoelastic behavior similar to entangled polymer systems was not observed for concentrations lower than 3 wt%, while uniform solution was not obtained at concentrations higher than 10 wt%. * To whom correspondence should be addressed. E-mail: [email protected], phone/fax: + 81-92-583-8822 Viscoelastic Properties of Worm-like Micelle of Aluminum Tristearylate/Decahydronaphthalene