Rheo-Impedance Measurements of Lamellar–Vesicular Phase-Transition Behavior

In this study, we investigated the lamellar-to-vesicular phase transition of nonionic surfactants (BL-4.2 and BL-4SY) in concentrated aqueous solutions under shear flow using a newly developed rheo-impedance technique. Although conventional methods such as small-angle light scattering (SALS) have clarified macroscopic structural changes, the internal electrical properties during these transitions remain largely unexplored. Briefly, we simultaneously measured the viscosity and electrochemical impedance during shear-induced phase transitions and compared the results to SALS observations. For BL-4.2, scattering images revealed transitions from lamellar structures to vesicles, followed by structural collapse. In contrast, BL-4SY exhibited stable vesicle formation without collapse, likely because of its uniform ethylene oxide chain length. Further, rheo-impedance measurements showed a consistent decrease in resistance from approximately 1050 to 520 Ω during vesicle formation, and there was a greater decrease as the electrolyte concentration increased. The viscosity increased from ≈0.6 to 1.5 Pa·s, corresponding to the lamellar-to-vesicular transition, as confirmed by SALS. Interestingly, at low Na₂SO₄ concentrations (10^-3-10^-2 M), the resistance was 20-30% higher than that of the electrolyte-free sample, suggesting partial ion-trapping by sulfate ions at the surfactant termini, a phenomenon not observed for KCl. These findings demonstrate that rheo-impedance analysis can characterize both the structural evolution and ionic transport during surfactant phase transitions, offering new insights for the design and evaluation of dispersions and drug delivery systems.