Structure and composition of the unilamellar vesicles (ULV) formed with catanionic surfactant of sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) are revealed using small-angle X-ray and neutron scattering (SAXS and SANS) and freeze-fracture-replication transmission electron microscopy (FFR-TEM). Imaging and scattering results consistently indicate a globular shape of the ULV, having a core size of 48 nm and a bilayer thickness of 32 Å; the bilayer comprises a central aliphatic shell of 24 Å, sandwiched by two headgroup shells, each of ca. 4 Å thickness. From the zero-angle scattering intensity ratio of the SAXS and SANS data, a CTAB/SDS composition ratio of 4:3 for the ULV bilayer is deduced. Poly(oxyethylene-b-oxypropylene-b-oxyethylene) (P123), a triblock copolymer, is found to intervene into the ULV bilayer of the catanionic surfactant in solution, leading to successively enlarged complex ULV, as P123 concentration in the solution increases. The ULV of SDS/CTAB is saturated with P123-loading after a critical concertation of 0.067 mM P123; thereafter, formation and proliferation of constant-sized, P123-based core-shell micelles are observed with increase of P123 concentration. A structural model of P123-embedded ULV of the catanionic surfactant is proposed. These results suggest a capacity and limits of using the catanionic ULV as molecule carriers.
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