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Vesicle and Tubule Formation in Catanionic Surfactant Mixture: Gemini Surfactant /Sodium Dodecyl Sulfate

Self-organization of surfactant molecules by noncovalent interactions to various aggregates is of great interest from both the fundamental and practical view-point. The surfactant’ s molecular architecture and the bulk phase tuning conditions are the cornerstones in designing the supramolecule with desired structure and properties. Vesicles and tubules are of particular interest due to their potential utility in medical applications, molecular reaction vessels, nanoscale transport systems, etc. Aqueous catanionic surfactant mixtures offer numerous possibilities to tailor the structure of aggregates by varying the molar ratio and/or concentration of oppositely charged surfactants ; i.e. by tuning the electrostatic interactions between the head groups at the surfactant aggregate/solution interface. The working hypothesis of this study was that folding to vesicles and/or wrapping into a tubule may be promoted if the electrostatic interactions in the head group region of elongated thread-like micelles are enhanced by addition of oppositely charged surfactant. To test this, the catanionic mixtures composed of thread-like micelle forming Gemini surfactant (ethanediyl-1, 2-bis(dimethyldodecylammonium bromide) and sodium dodecyl sulfate were studied by a variety of the conventional and modern techniques: tensiometry, conductometry, potentiometry, steady-state fluorescence, static and dynamic light scattering, electron spin resonance, phase contrast and confocal microscopy. Vesicles and tubules were indeed observed at the lower surfactant concentrations and the higher surfactant concentrations, respectively. The strong reduction in area per head group resulting from the ion pairing induces spontaneous formation of open and closed bilayers (vesicles). Wrapping of bilayers in a tubule is not common in catanionic surfactant mixtures. The appearance of tubules in investigated system can be ascribed to the interplay of specific molecular architecture of Gemini surfactant molecule and electrostatic effects. Vesicle and/or tubule tuned by the introduction of oppositely charged surfactants in various proportions might be of practical value.

catanionic surfactant; vesicles; tubules

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