Reverse micelles with spines: L-2 phases of surfactant ion-polyion complex salts, n-alcohols and water investigated by rheology, NMR diffusion and SAXS measurements

Homogeneous alcoholic isotropic solutions (L-2 phases) of "complex salts'' of hexadecyltrimethylammonium neutralized by polyacrylate CTAPA(n) (n = 30 or 6000), in the presence of water were examined at 25 degrees C by means of small-angle X-ray scattering (SAXS), rheology and NMR self-diffusion measurements for different n-alcohols (octanol, hexanol and butanol) and at varying water contents. The greater water solubility and the slower water self-diffusion coefficients in these L-2 phases, when compared to results with surfactant-free alcohols, suggested that these phases are composed of reverse aggregates with water in their cores. A comparatively rapid self-diffusion of the surfactant ion in butanol suggested a significant fraction of "free'' surfactant ions, dissociated from the reverse aggregates. Rheological data confirmed that the obtained viscoelastic properties were consistent with a system containing entangled elongated micelles, whose properties were controlled by the polyion chain length. NMR diffusion measurements for complex salt solutions with the shorter counter-polyion (CTAPA(30)) produced estimates of the aggregate lengths that were close to the extended length of one PA(30) chain. In summary, these results support the formation of aggregates composed by surfactant decorated polyion chains with a water core, whose properties are determined by the polyion chain length, behaving like reverse micelles with spines.