Physicochemical investigation of the solubilization of cobalt nitrate in sodium bis(2-ethylhexyl)sulfosuccinate reversed micelles

A wide investigation of the solubilization of the water-soluble salt Co(NO3)2 in water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane microemulsions and of some physicochemical properties of the Co(NO3)2/AOT/n-heptane and Co(NO3)2/AOT systems has been carried out. After saturation of water/AOT/n-heptane microemulsions with pure Co(NO3)2, the Co(NO3)2/AOT composites were prepared by complete evaporation under vacuum of the volatile components (water and apolar solvent) of the salt-containing microemulsions. It was observed that these composites could be totally dissolved in pure n-heptane, allowing the solubilization of a noticeable amount of Co(NO3)2 in a dry apolar organic solvent. By UV–vis–near-IR spectrophotometry, some information on the state of Co(NO3)2 within water-containing or dry AOT reversed micelles was acquired, whereas by small-angle X-ray scattering it the occurrence of small nanoparticles in the salt-containing dry AOT reversed micelles was ascertained. Surprisingly, the analysis of the X-ray diffraction spectra corroborated by UV–vis and X-ray photoelectron spectroscopy data of Co(NO3)2/AOT composites led to the hypothesis that nanoparticles are mainly constituted of sodium nitrate resulting from the metathesis reaction between AOT and Co(NO3)2. By transmission electron microscopy, information on the size and the size distribution of the nanoparticles in salt/AOT composites was gained.