Rotational motion and lateral migration of surfactants in threadlike micelles

Molecular motion in a spherical and a threadlike micelle formed by cetyltrimethylammonium bromide (CTAB) and sodium p-toluenesulfonate (NapTS) in an aqueous solution was examined by fluorescence anisotropy using fluorescent probe molecules. The shape of micelles in the solution changes from spherical to long threadlike with the molar ratio of NapTS to CTAB. In a spherical micelle obtained at the ratio less than unity, a detergent ionic pair consisting of CTA(+) and Br- rotates around its long tail and migrates laterally along the micellar surface. On the other hand, in a threadlike micelle the Br- of the detergent ionic pair is replaced by pTS(-). The rotational relaxation time of the detergent ionic pair in the threadlike micelle is twice as long as that in the spherical micelle. Moreover, the lateral diffusion constant for the detergent ionic pair in the threadlike micelle is about a half of that in the spherical micelle. These results suggest that molecular motion in the threadlike micelle is slower than in a spherical micelle.