Reverse micelles with spines: L2 phases of surfactant ion—polyion complex salts, n-alcohols and water investigated by rheology, NMR diffusion and SAXS measurements

Abstract

Homogeneous alcoholic isotropic solutions (L₂ phases) of “complex salts” of hexadecyltrimethylammonium neutralized by polyacrylate CTAPA_n (n = 30 or 6000), in the presence of water were examined at 25 °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 L₂ 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 (CTAPA₃₀) produced estimates of the aggregate lengths that were close to the extended length of one PA₃₀ 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.