Vesicle Structure and Dynamics in a Mixed Surfactant System: Insights from Small Angle and Quasi Elastic Neutron Scattering

Abstract

This work investigates the internal structuring and molecular dynamics of an aqueous mixed surfactant system composed of the zwitterionic surfactant oleyl amidopropyl betaine (OAPB), and the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT). A key focus of the work is placed on understanding how temperature and ionic strength govern the self-assembly and dynamic behavior of these molecules in situ. Small angle neutron scattering (SANS), quasi elastic neutron scattering (QENS), dynamic light scattering (DLS), and selective deuteration were employed to probe the structure and dynamics across different length and time scales. SANS measurements demonstrate that the OAPB/AOT system undergoes a transition from ellipsoidal micelles to vesicles, driven by electrostatic screening from salt addition. However, increasing the system temperature was found to reverse this transition. In addition, contrast variation through selective deuteration of the surfactants provides insight into their spatial distribution within the vesicle structure. Modelling of the results suggests an asymmetric organization of OAPB and AOT within the vesicle bilayer, with OAPB partitioning preferentially to the inner layer. QENS measurements reveal vesicle Brownian motion, lateral diffusion of surfactants within the membrane and confined localized motions of hydrocarbon chains. These results show that molecular mobility increases with temperature, explaining the observed structural transitions. In addition, the confined motions help explain the small bilayer thicknesses of the vesicles observed by SANS, while the higher apparent immobile fraction of AOT compared to OAPB can be attributed to the asymmetric partitioning of the two surfactants within the bilayer. DLS measurements support the neutron scattering results by providing insight about translational diffusion over different timescales. Overall, this work highlights the capacity for using SANS and QENS as complementary techniques in gaining a deeper understanding of observed structural features and their relationship to the underlying molecular dynamics of soft matter systems.