Structural evolution of reverse vesicles from a salt-free catanionic surfactant system in toluene

• The reverse vesicles in toluene are smaller compared to those in cyclohexane.
• The molecular bilayers have a compact organization and are highly curved.
• Small vesicles can fuse to form bigger ones with time.
• At elevated temperatures, the vesicles became “soft” before clearance.
• Cryo-TEM was used for the first time to detect reverse vesicles in toluene.

A detailed study of the reverse vesicles formed by a salt-free catanionic surfactant system in toluene was carried out by confocal fluorescence microscopy observations, dye-solubilizing tests, UV–vis measurements and cryo-TEM observations. When tetradecyltrimethyl ammonium laurate (TTAL) and lauric acid (LA) were mixed in the binary solution of water and toluene, reverse vesicular phase formed spontaneously. The reverse vesicular phase is quite stable and can be labeled with fluorescent dyes for subsequent confocal fluorescence microscopy observations. However, the reverse vesicles were found to have smaller sizes (<1 μm) and undergo structural evolutions in a much shorter time scale compared to those formed by the same surfactant mixture in cyclohexane, as shown in a previous report [19]. With extended observation time, interesting intermediate structures were observed including onions, sheets and cellular networks. The structural evolution pathways were deduced and the possibly influencing factors were discussed. Dye-solubilizing tests showed the ability of the reverse vesicles to accommodate dye molecules is smaller compared to those in cyclohexane. Besides, cryo-TEM observations were applied to probe the morphology of the reverse vesicles.

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