title: Structure and rheology of reverse micelles in dipentaerythrityl tri-(12-hydroxystearate)/oil systems.
abstract: We report the formation of reverse rod-like micelles and their rheological properties in novel nonionic surfactant, dipentaerythrityl tri-(12-hydroxystearate) (designated as WO-6)/oil systems without external water addition. Small-angle X-ray scattering (SAXS) was used to investigate the structure of the micelles and their flow properties were studied by rheological measurements. We found that WO-6 spontaneously self-assembles into reverse micelles in a variety of organic solvents at ambient conditions, their structure depending on solvent molecular architecture, surfactant concentration, and temperature. Rod-like micelles with a maximum length of ca. 12 nm and a cross section diameter of ca. 2 nm were observed in cyclohexane. When cyclohexane was replaced with a linear chain octane, the length and the cross section diameter were simultaneously increased. With a further increase of hydrocarbon chain length of solvent oils from octane to hexadecane, the rod-like micelles grew axially, keeping the cross section diameter (ca. 3 nm) virtually constant. Increasing surfactant concentration also favored one-dimensional micellar growth. On the other hand, micelles shrunk with the rise of temperature, which is similar to a rod-to-sphere transition, and is essentially the opposite temperature dependence to that often observed in aqueous micellar systems. A structural picture drawn by SAXS is well supported by rheology; the relative (zero-shear) viscosity of the WO-6/oil systems was found to be markedly greater than that expected for a dispersion of spherical particles due to the elongated micellar structure, despite quantitative inconsistency with semi-empirically predicted values for rigid rod-like particles.
authors.affiliation: WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba Ibaraki, 305-0044, Japan. SHRESTHA.Lokkumar@nims.go.jp
authors.forename: Lok Kumar
authors.initials: LK
authors.lastname: Shrestha
authors.forename: Takaaki
authors.initials: T
authors.lastname: Sato
authors.forename: Rekha Goswami
authors.initials: RG
authors.lastname: Shrestha
authors.forename: Jonathan
authors.initials: J
authors.lastname: Hill
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Ariga
authors.forename: Kenji
authors.initials: K
authors.lastname: Aramaki