Morphological Transition of Oppositely Charged Calix[4]arene Surfactant Mixture

When a negatively charged sulfonic or a positively charged amine group is attached to calix[4]arene surfactants, denoted as SCal7 and QACal7, respectively, each system forms monodispersed spherical micelles with a defined aggregation number. We postulated that these defined monodisperse aggregations can be adjusted in terms of the coverage, that is, how efficiently can a spherical surface (, hydrophobic domain) be covered with identical caps (hydrophilic headgroup). We carried out small-angle X-ray scattering (SAXS), as well as titration calorimetry and dynamic light scattering (DLS), to observe the morphologies of the mixtures of these two components. The calorimetry showed a 1:1 stoichiometric exothermic reaction with a rather large binding constant. This means that, once the twin is formed from QACal7 and SCal7, these components hardly separate. SAXS showed that the spherical micelles changed to cylindrical ones and then to vesicle shapes as the composition reached a ratio of 1:1. At some intermediate compositions, mixtures of cylinders and plates (or spheres) were observed, which were confirmed by dynamic light scattering. We assume that the twin has a larger packing parameter than the individuals because of the cancellation of the charges between the sulfonic and the amine groups, and thus, the increase of the twin composition induces the entire morphology change, as expected from the packing parameter principle. In the present case, the coverage may be a secondary factor determining the morphology.