Solubilization of a Water-Insoluble Dye in Aqueous Sodium Halide Solutions of Dodecylpyridinium Halides: Effects of Counterion Species of Ionic Micelles

Solubilization of Sudan Red B in aqueous micellar solutions of dodecylpyridinium chloride (DPC) in the presence of NaCl from 0 to 4.00 M and of dodecylpyridinium bromide (DPB) in the presence of NaBr from 0 to 6.00 M has been examined to investigate the effects of added salt, i.e., micelle size, shape, and structure, The DPC micelles have solubilization power, 2.08 × 10-3 mol per mole micellar DPC, over the range of 0 to 4.00 M NaCl. The constant solubilization power of DPC, independent of its micelle species, either porous at lower NaCl concentrations or spherical at higher NaCl concentrations, suggests that the solubilization locus of a spherical micelle locates near its polar or outer parts. The DPB micelles have solubilization power, 2.26 × 10-3 mol per mole micellar DPB, in the range from 0 to 3.12 M NaBr, where the formation of spherical micelles and their mutual repulsive or no mutual interaction in solution has been confirmed by light-scattering measurements. At higher NaBr concentrations where spherical micelles formed, the critical micelle concentrations are subject to mutual attractive interaction; at finite micelle concentrations, the solubilization power increases with increasing NaBr concentration, in such a way that it behaves characteristically as if rodlike micelles are formed.