Two Routes to Vesicle Formation:  Metal−Ligand Complexation and Ionic Interactions

Two routes to vesicle formation were designed to prepare uni- and multilamellar vesicles in salt-free aqueous solutions of surfactants. The formation of a surfactant complex between a double-chain anionic surfactant with a divalent-metal ion as the counterion and a single-chain zwitterionic surfactant with the polar group of amine-oxide group is described for the first time as a powerful driving force for vesicle-phases constructed from salt-free mixtures of aqueous surfactant solutions. As a typical example, a Zn(2+)-induced charged complex fluid, vesicle-phase has been studied in aqueous mixtures of tetradecyldimethylamine oxide (C(14)DMAO) and zinc 2,2-dihydroperfluorooctanoate [Zn(OOCCH(2)C(6)F(13))(2)]. This ionically charged vesicle-phase formed due to surfactant complexation has interesting rheological properties and is not shielded by excess salts because there are no counterions in the solution. Such a vesicle-phase of surfactant complex is important for many applications; for example, the vesicle-phase was further used to produce in situ the vesicle-phase of the salt-free cationic/anionic (catanionic) surfactants, C(14)DMAOH(+)-(-)OOCCH(2)C(6)F(13). The salt-free catanionic vesicle-phase could be produced through injecting H(2)S gas into the C(14)DMAO/Zn(OOCCH(2)C(6)F(13))(2) vesicle-phase, because the zwitterionic surfactant C(14)DMAO can be charged by the H(+) released from H(2)S to become a cationic surfactant and Zn(2+) was precipitated as ZnS. After the ZnS precipitates were removed from C(14)DMAO/Zn(OOCCH(2)C(6)F(13))(2) solutions, the final mixed solution does not contain excess salts as do other cationic/anionic surfactant systems. Both the C(14)DMAO-Zn(OOCCH(2)C(6)F(13))(2) complex and the resulting catanionic C(14)DMAOH(+)-(-)OOCCH(2)C(6)F(13) solution are birefringent Lalpha-phase solutions that consist of uni- and multilamellar vesicles. Ring-shaped semiconductor ZnS materials with encapsulated ZnS precipitates and regular spherical ZnS particles were prepared, which resulted in a transition from vesicles composed of metal-ligand complexes to vesicles held together by ionic interactions in the salt-free aqueous systems. This strategy should provide a new method to prepare inorganic materials. The present routes to form vesicles solve a problem: how to prepare nanomaterials using surfactant self-assembly, with structure controlled not by the growing material, but by the phase behavior of the surfactants.