Effects of temperature and shear conditions on lamellar-to-onion transition in nonionic surfactant/water systems

Abstract

The effects of temperature and shear conditions on the lamellar-to-multilamellar vesicle (MLV) phase transition were elucidated using rheometry (Rheo)–small-angle light scattering (SALS) measurements for a system comprising C12E4.2 (a nonionic surfactant containing distributed ethylene oxide; 40 wt%) and water (60 wt%). Small-angle X-ray scattering (SAXS) measurements indicated that the bilayer d-spacing widened up to 30 °C, and further increasing the temperature caused the bilayer to fluctuate more easily. Rheo–SALS measurements indicated that the C12E4.2 viscosity increased, and a cloverleaf pattern appeared, as previously reported for other CnEm systems, indicating a phase transition. The decrease in vesicle size with increasing shear rate was slower than that reported for the non-distributed system. In a conventional dynamic phase diagram prepared in the shear-rate–temperature space, regions appeared in which temperatures and shear rates required for the phase transition were inconsistent. When a shear-stress-controlled flow was applied, the shear stress magnitude required for the phase transition increased continuously with increasing temperature. Furthermore, critical shear strains comparable to those measured for the shear rate control were obtained for different temperatures. These results suggest that the bilayer fluctuated more easily with increasing temperature, requiring more energy for the phase to transition, as indicated by the stress ramp measurements. Therefore, the temperature-dependent bilayer kinetics determined the conditions required for the phase transition, and shear stress magnitudes required for the phase transition must also be considered in CnEm systems.