Non-linear rheology and flow-induced transition of a lamellar-to-vesicle phase in ternary systems of alkyldimethyl oxide/alcohol/water

The time-dependent transformation of an ionically charged lamellar phase (L -alpha-phase) into a vesicle phase under the influence of shear is investig ated using rheological and conductivity measurements. The L-alpha-phase con sists of the zwitterionic surfactant tetradecyldimethylaminoxide (C(14)DMAO ), hexanol, oxalic acid and water. The experiments were carried out on the L-alpha-phase in a well defined state. It was prepared by a special route f rom the neighbouring L-3-phase that consists of 100 mM C(14)DMAO, 250 mM he xanol and 5 mM oxalic-diethylester (OEE). The OEE hydrolyses in the L-3-pha se to oxalic acid and ethanol. The result is a virgin L-alpha-phase which c onsists of stacked bilayers and which has not been exposed to shear. When t his low-viscous phase is subjected to shear it is transformed into a highly viscous vesicle phase. The transformation of the L-alpha-phase into vesicl es under constant shear was monitored by recording the viscosity and conduc tivity with time. It is observed that at least three different time constan ts can be distinguished in the transformation process. The conductivity pas ses through a minimum (tau(1)) in the direction of shear. The viscosity fir st passes through a minimum (tau(2)) and then over a maximum (tau(3)) It is concluded that tau(1) belongs to the complete alignment of the bilayer par allel to the wall, tau(2) to the beginning of the break-up of the bilayers to the vesicles and tau(3) to the complete transformation of the L-alpha- t o the vesicle phase. When the shear rate was varied, it was noted that the product of the time constants and shear is constant.