Vesicle-to-Micelle Transformation in Systems Containing Dimeric Surfactants

The vesicle-to-micelle transformation has been investigated thus far in lipid + surfactant systems where the vesicle-forming lipid is chemically very different from the micelle-forming surfactant. The dimeric surfactants, alkanediyl-alpha, omega-bis(dimethyldodecylammonium bromide), are known to form vesicles when the alkanediyl spacer is long enough (for instance, spacer = eicosanediyl, referred to as 12-20-12) and spheroidal micelles for shorter spacers (spacer = decanediyl, referred to as 12-10-12). These surfactants together with the conventional surfactant dodecyltrimethylammonium bromide (DTAB) permitted us to study the transformation of the 12-20-12 vesicles into micelles on addition of the chemically similar micelle-forming DTAB and 12-10-12. Spectrophotometry (light absorbance measurements), video-enhanced light microscopy, and transmission electron microscopy at cryogenic temperatures (cryo-TEM) were used to study the transformation at different scales of aggregate size. Electrical conductivity, which probes the system at the atomic scale (free counterions), was also used. Absorbance measurements showed the transformation to occur between 1.8 and 2.8 wt% added surfactant at a constant 12-20-12 concentration of 1.4 wt%. Light microscopy showed the progressive solubilization of the larger vesicles. Cryo-TEM showed that the initial effect of DTAB addition was to reduce the size of the vesicles, whereas 12-10-12 addition resulted in the formation of multilamellar vesicles. Further additions of either surfactant reduced the size of the vesicles, then brought about the formation of spheroidal micelles until complete solubilization of the vesicles. The giant threadlike micelles seen in previous studies of vesicle-to-micelle transformation in lipid/surfactant systems were never observed with the systems investigated. The conductivity results also revealed differences in behavior on additions of DTAB and 12-10-12.