Effects of micelle-to-vesicle transitions on the degree of counterion binding

Effects of micelle-to-vesicle transitions on the degree of counterion binding (β) were investigated on three systems. For the concentration-dependent micelle-to-vesicle transition in the didodecyldimethylammonium bromide (DDAB)/water system, in the region of coexistent micelles and vesicles, less than 3 mM, the β values increased significantly with DDAB concentration: β (0.07 mM) = 0.35 and β (3 mM) = 0.93. In the coexistent region, activities of the bromide ion, aBr, were almost independent of the DDAB concentration, suggesting the pseudo-phase nature of both micelles and vesicles. In the concentration-dependent vesicle-to-lamellar transition region above 5 mM, where multilamellar vesicles were prevailing, on the other hand, the β values were only little affected by this transition. This suggests that the increase in the layer number of DDAB multilamellar vesicles scarcely affects the β values. This was also supported by the fact that the destruction of multilamellar vesicles by ultrasonication did not change the β values. These results strongly suggest that the inner and outer monolayers of DDAB multilamellar vesicles are characterized by similar β values. The second system, cetyltrimethylammonium bromide (CTAB)/DDAB mixtures, showed composition- dependent transitions depending on the mole fraction of DDAB XDDAB: spherical micelles (0<XDDAB<0.2)→rodlike micelles (0.2<XDDAB<0.4) →vesicles (0.6<XDDAB<1). Values of β increased with both the sphere-to-rodlike micelle transition and the micelle-to-vesicle transition. The increase in the β value was larger for the latter than for the former transition. Little effects of ultrasonication on β was also observed for the multilamellar vesicles of CTAB/DDAB mixtures. In the third system of the mixtures of two single chain surfactants, protonated and non-protonated of oleyldimethylamine oxide, the dependence of β on the degree of ionization (α) can be well described with a theoretical curve, derived on the basis of the linear relation between the surface charge density and the α value, despite the occurrence of the micelle-to-vesicle transition.