Sphere-to-cylinder transition in aqueous micellar solution of a dimeric (gemini) surfactant

The dimeric (gemini) surfactant 12-2-12 (dimethylene-1,2-bis(dodecyl dimethylammonium bromide)) has been known to form threadlike micelles at relatively low concentrations. We investigated the micellar growth of this surfactant in aqueous solutions by the much-improved cryo-TEM technique (transmission electron microscopy at cryogenic temperature) in the concentration range between 0.26 and 1.5 wt %. The digitally acquired electron micrographs of solutions, with concentrations up to about 1 wt %, show the coexistence of spheroidal micelles and long, threadlike micelles, the number and length of the latter increasing with concentration at the expense of the former. The micrographs show very few elongated micelles of intermediate sizes. Also, the endcaps of the elongated micelles can be seen to be of a larger diameter than the cylindrical body of those micelles. These results lend support to the theories, developed by various workers, that predicted these features. Some branching is observed at a surfactant concentration of 0.62 wt %. Above 1 wt %, the elongated micelles show frequent branching. The electron micrographs of the 1.5 wt % solution have the appearance of the saturated network postulated by theory.