Lamellar to Onion Transitions with Increasing Temperature under Shear Flow in a Nonionic Surfactant System Studied by Rheo-SAXS

Introduction In the past 15 years, much attention has been paid to the effects of shear flow on the structure of the lyotropic phase of surfactant systems, especially on the lamellar phase. Among them, the most striking result may be the transition from the lamellar phase to the "onion phase" where all the space is filled by multilamellar vesicles alone [1]. Although this type of transition has been reported both for various systems, transition mechanism has not yet been established. Recently, we have found the lamellar-to-onion transition with increasing temperature under shear flow in the lamellar phase of a nonionic surfactant C16E7/water system (CnEm is abbreviation of CnH2n+1(OC2H4)mOH) by using simultaneous measurements of shear stress and smallangle light scattering (Rheo-SALS) and small-angle Xray scattering (Rheo-SAXS). The lamellar-to-onion transition with decreasing temperature has been reported for several homologues which is explained in terms of increase (less negative) in the saddle-spray modulus of bilayers with the temperature elevation [2]. Our findings are apparently controversial with these results. In the present study, we investigate the transition processes in more detail and discuss the mechanisms of the transition.