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 dimeth ylammonium bromide)) has been known to form threadlike micelles at relative ly low concentrations. We investigated the micellar growth of this surfacta nt in aqueous solutions by the much-improved cryo-TEM technique (transmissi on 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 diamete r than the cylindrical body of those micelles. These results lend support t o the theories, developed by various workers, that predicted these features . Some branching is observed at a surfactant concentration of 0.62 wt %. Ab ove 1 wt %, the elongated micelles show frequent branching. The electron mi crographs of the 1.5 wt % solution have the appearance of the saturated net work postulated by theory.