Hydrotalcite-like Nanocrystals from water-in-oil microemulsions

A water-in-oil microemulsion made up of a cetyltrimethylammonium bromide/n-butanol/isooctane/MII and AlIII nitrate aqueous solution (MII = Mg or Ni or Zn) has been mixed with a microemulsion of the same composition but containing an ammonia solution instead of a metal nitrate solution. Collisions between the reverse micelles containing an MII and AlIII solution and those containing an NH3 solution form short-lived dimers that act as reaction vessels and control the nucleation and growth of MgAl, NiAl or ZnAl hydrotalcite-like compounds (HTlcs). The double water-in-oil microemulsion technique yields colloidal dispersions of nanoparticles whose size and shape were examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The colloidal particles were between 50 and 100 nm depending on the composition of the microemulsions (water-to-surfactant and oil-to-water molar ratios). The separation of the nanoparticles from the reaction medium produces a gel made up of surfactant-free particles. HTlc nanocrystals were recovered from the gel (reaction medium), washed, and dried. They were then studied by chemical and thermogravimetric analyses, X-ray powder diffraction, N2-adsorption and BET surface analysis and scanning electron microscopy (SEM). The AlIII/(MII + AlIII) molar ratio of the recovered HTlc nanocrystals was between 0.24 and 0.29, and the basal spacing was about 8 Å, a value in agreement with the presence of charge compensating bromide anions in the interlayer region. The proposed technique allows for HTlc nanocrystals to be obtained with a relatively high surface area with values ranging between 50 and 110 m2/g. The SEM analyses show that the nanoparticles are aggregated to give rise to complex spherical or rod-shaped microstructures.