Effect of solubilized amines on the structural transition of cetyltrimethylammonium bromide micelles in aqueous potassium bromide

Rod‐shaped micelles were produced by mixing 0.1 M cetyltrimethylammonium bromide (CTAB) and 0.1 M KBr in aqueous solution. The effects of the addition of aliphaticn‐amines (C4, C6, C7 and C8) and temperature on the shape of micelles were studied by viscosity measurements. The viscosity data show that transition of rod‐shaped micelles to larger aggregates is induced by addition of higher amines (≥C6) up to a certain concentration; a further increase in concentration produced the opposite effect. Addition of C4‐amine induces only a rod‐to‐sphere transition. The data were interpreted in terms of solubilization/incorporation (decrease of micellar surface charge density) of amines inside the micelles and the nature of the effective solvent (water+amine). The latter effect dominated the change from larger aggregates to smaller micelles at higher concentrations of the added amine. Increasing the temperature produced effects similar to C4‐amine addition, namely, rod‐to‐sphere transition. Activation free energy (ΔG*) and enthalpy (ΔH*) were also computed from the temperature dependence of the viscosity. ΔG* and ΔH* values were higher for larger aggregates (long rods) than for smaller ones (spherical micelles) and ΔH* covered almost the total contribution to ΔG*.