Stabilization of spherical nanoparticles of iron(III) hydroxides in aqueous solution by wormlike micelles.

HYPOTHESIS: The low Ksp value of Fe(OH)3 (3 × 10-38 at 298 K) explain the immediate coagulation when the pH of a solution of Fe(III) is adjusted to 7. However, stable dispersions of Fe(OH)3 can be formed when the pH is adjusted to 7 in the presence of wormlike micelles formed by cetyltrimethylammonium bromide and sodium salicylate. The formation of a structure containing Fe(OH)3 nanoparticles decorating wormlike micelles is responsible for the high stability of the dispersions. EXPERIMENTS: Fe(OH)3 nanoparticles were obtained by increasing the pH of solutions of cetyltrimethylammonium bromide and Fe(III), previously complexed with salicylate at pH 3. The interaction between nanoparticles and the chains of wormlike micelles was investigated by DLS, SAXS, TEM and Cryo-TEM. FINDINGS: DLS revealed higher scattering contrast and slower diffusion for wormlike micelles in the presence of nanoparticles. These results were interpreted as the decoration of the chains of wormlike micelles by nanoparticles of Fe(OH)3. A pearl-necklace model was successfully used to adjust SAXS curves, revealing nanoparticles with ∼3 nm of diameter, spaced ∼2 nm apart along the string. This result agrees with TEM and Cryo-TEM images. The formed structure prevents the coagulation of nanoparticles, assuring high stability to the dispersion.