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Re-entrant Lamellar/Onion Transition with Varying Temperature under Shear Flow

2012; American Chemical Society; Volume: 29; Issue: 1 Linguagem: Inglês

10.1021/la3041665

1520-5827

Daijiro Sato, Kahoru Obara, Youhei Kawabata, Makio Iwahashi, Tadashi Kato,

Surfactants and Colloidal Systems

We have found for the first time the reentrant lamellar/onion (lamellar–onion–lamellar) transition with varying temperature under constant shear rate by using simultaneous measurements of shear stress and small-angle X-ray scattering (Rheo-SAXS) for a nonionic surfactant (C14E5)/water system, which exhibits the lamellar phase in a wide temperature range from 15–75 °C. The onion state exists in a closed region in the temperature–concentration diagram at a constant shear rate. Temperature dependence of the lamellar repeat distance (d) at rest has also been measured at several concentrations. It is shown that the increase of d with increasing temperature is necessary for the existence of the lower transition. We have investigated the change in the lamellar orientation in the lamellar-to-onion and onion-to-lamellar transition processes near the upper and lower transition temperatures. For all four kinds of transition processes, the following change in the lamellar orientation is observed; lamellar state (oriented to the velocity gradient direction) ↔ further enhancement of the orientation to the velocity gradient direction ↔ enhancement of the orientation to the neutral direction ↔ onion state.