Aqueous and substantially crystalline
iron oxide nanoparticle dispersions and processes for making them are disclosed. The
nanoparticle size and size distribution width are advantageous for use in a fuel additive for catalytic reduction of
soot combustion in diesel particulate filters. Nanoparticles of the aqueous
colloid are transferred to a substantially non-polar liquid comprising a
carboxylic acid and one or more low-polarity solvents. The transfer is achieved by mixing the aqueous and substantially non-polar materials, forming an
emulsion, followed by a phase separation into a substantially
metal-free remnant polar phase and a substantially non-polar organic
colloid phase. A method for rapid and substantially complete transfer of non-agglomerated nanoparticles to the low polarity phase in the presence of an organic amine, and a rapid phase separation of the substantially non-polar
colloid from a remnant aqueous phase, are provided.