Formation of spherical and rod-like micelles of cetyltrimethylammonium bromide in aqueous NaBr solutions

Abstract

Static light scattering from aqueous NaBr solutions of cetyltrimethylammonium bromide (CTAB) has been measured at 35°C with 488-nm laser light, and the molecular weight, radius of gyration, and the other parameters of its micelles have been evaluated. In water, spherical micelles are formed above a critical micelle concentration, 0.04 × 10 −2 g cm −3, and each micelle consists of 91 monomers of CTAB. In aqueous solutions having NaBr concentrations higher than 0.06 M, rod-like micelles are formed, besides spherical micelles. The molecular weight of rod-like micelles increases rapidly with increasing NaBr concentration. The rod-like micelle formed in 0.5 M NaBr has molecular weight as high as 3,470,000, and is considerably flexible. The persistence length of micelle is evaluated to be 44 nm. Electron micrographic observations have been carried out for specimens prepared from CTAB solutions in water and in 0.5 M NaBr at 35°C. A specimen prepared from aqueous solution of CTAB and negatively stained with uranyl acetate displays many globular images, which have a uniform diameter around 5–6 nm and can be assigned to spherical micelles. A specimen from 0.5 M NaBr solution shows tortuous thread-like images with a uniform diameter of 4.5–6 nm, which can be regarded as flexible rod-like micelles. Thus, the size and shape of spherical and rod-like micelles of CTAB observed by the light-scattering measurement are confirmed by electron microscopy.