Cryo-TEM and NMR studies of solution microstructures of double-tailed surfactant systems: didodecyldimethylammonium hydroxide, acetate, and sulfate

Didodecyldimethylammonium hydroxide (DDAOH) and acetate (DDAAc) are easily soluble in water, forming isotropic solution phases (0-30 wt % DDAOH and 0-32 wt % DDAAc), whereas didodecyldimethylammonium sulfate (DDAS) is sparingly soluble (≈0.2 wt %) in water. All three surfactants form lamellar phases in water at high surfactant concentrations. Addition of dodecane to the lamellar dispersions of DDAS yields an isotropic solution phase in the water-rich part of the triangle. Cryo-TEM, NMR self-diffusion, and ¹H NMR transverse relaxation techniques have been employed to study aggregate structures in solutions for these surfactant systems. Cryo-TEM micrographs detect stable vesicles, and for solutions with OH^- and CH₃COO^- counterions, vesicles coexist with normal micelles within certain concentration ranges above which micelles are the only stable aggregates. Concentration-dependent self-diffusion coefficients measured by the PGSE NMR method show that the surfactant ions have a minimum in their self-diffusion coefficients with both OH^- and CH₃COO^- ions. Moreover, there is micellar growth in the dilute part of the solution regions. Molecular diffusion, i.e., the exchange of monomers between aggregates, becomes important in concentrated solutions. In the ternary solution with SO₄^2- as a counterion, near spherical oil-in-water-type droplets are formed at very high water contents, and with a decreasing water concentration at a constant molar ratio between oil and surfactant, a moderate swelling of the droplets with oil is observed. At low molar ratios between oil and surfactant and low water contents, the exchange of monomers between aggregates dominates the surfactant diffusion process.