Structural transitions of cetyltrimethylammonium bromide micelles in aqueous media: Effect of additives

The effect of organic additives (n-octanol, n-octanoic acid, n-octylamine and n-octane) on the sphere-to-rod transitions in aqueous micellar solutions of cetyltrimethylammonium bromide (CTAB, 0.25 M) was examined by viscosity, conductance, sound velocity and small angle neutron scattering (SANS) measurements. Nonpolar additive n-octane solubilize in the core of micelles which retain sphericity, while additives with polar substituents like octanol, octanoic acid and octylamine forms the additive-CTAB mixed micelles which causes micellar growth. Octanol and octanoic acid is more effective in promoting the sphere-to-rod transition as compared to octylamine. These results are well supported by SANS measurements. The structural transformation is explained by hydrophobic interactions and electrostatic effects between the additives and CTAB micelle.