Wet-dry cycle-mediated micelle-to-vesicle transition in sodium dodecylbenzenesulfonate solutions

Single-chain amphiphiles (SCAs) possess simple structures and are readily synthesized, and the vesicles derived from them exhibit excellent properties. However, except for some fatty acid-based systems, most SCAs tend to form micelles in aqueous solutions, making vesicle formation difficult and thus limiting their application potential. Herein, a wet-dry cycle method was used to induce a micelle-to-vesicle transition in a single-component aqueous solution of sodium dodecylbenzenesulfonate (SDBS), a typical anionic SCA. Specifically, a vesicular system was obtained by simply drying SDBS micelle solutions followed by rehydration. The so-obtained vesicles were characterized using dynamic light scattering (DLS), cryogenic transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and fluorescence spectroscopy, and their properties were determined. The effects of wet-dry cycle conditions and SDBS concentration on vesicle formation were also examined. The results revealed concentration-dependent aggregation behavior, with apparently pure vesicle (or vesicle-dominated) systems forming within a specific concentration. The vesicles exhibited good stability, and the permeation of OH^- across the SDBS vesicle membranes was well described by a first-order kinetic model. A possible mechanism for the wet-dry cycle-mediated vesicle formation is proposed: during dehydration of the SDBS micelle solution, bilayer sheets with interdigitated alkyl chains are formed, and upon rehydration, these sheets may transform into vesicles through self-closing of individual sheets and/or edge-to-edge merging of neighboring sheets. The highly interdigitated structure of the alkyl chains is likely the key factor underlying the formation and stability of the SCA vesicles.