Vesicle-forming synthetic amphiphiles

The interactions of micelle forming nonionic surfactant, Polyoxyethylene Sorbitan Monolaurate (Tween 20), with spontaneously formed vesicle dispersions of Dimethyldioctadecylammonium bromide (DDOAB) have been investigated. DDOAB vesicles are solubilized in the Tween 20 micelles and undergo vesicle-to-micelle transition. Differential scanning calorimetry (DSC) measurements show that the addition of Tween 20 lowers the gel-to-liquid crystalline phase transition temperature (Tm) of DDOAB vesicles. The thermogram peak shifts to a lower temperature, broadens and ultimately disappears when the vesicle-to-micelle transition is complete. Fluorescence anisotropy and Turbidity data indicate that the vesicle-to-micelle transition takes place in three stages: the vesicle bilayer is first saturated with Tween 20, the bilayer then disintegrates, and finally a complete solubilization takes place, leading to the formation of mixed micelles. Dynamic light scattering study shows an initial increase of the average vesicle size upon the addition of Tween 20 up to the saturation point and then a decrease upon complete solubilization of vesicles into mixed micelles. The results have been confirmed by observing Transmission Electron Microscopy (TEM). TEM images show evidence of direct transformation of the vesicles to mixed micelles without the formation of intermediate aggregates.