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

TL;DR

The paper introduces a new method to study how surfactant structures change under shear, combining rheology and electrochemical measurements.

Contribution

A novel rheo-impedance technique is developed to explore electrical properties during surfactant phase transitions.

Findings

BL-4SY forms stable vesicles without collapse due to uniform ethylene oxide chain length.

Rheo-impedance measurements showed resistance decreased during vesicle formation.

Low Na2SO4 concentrations caused higher resistance, suggesting ion-trapping by sulfate ions.

Abstract

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.

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