Structural characterization of concentrated aqueous ferrofluids

•Long-term structural stability of concentrated aqueous ferrofluids is revealed.•Dilution of ferrofluids results in decrease in the aggregate size.•Difference in structure of ferrofluids was revealed depending on surfactant used. Structure characterizations of concentrated water-based magnetic fluids stabilized by oleic and lauric acids were done by means of small-angle neutron scattering (SANS) and complemented by dynamic light scattering and magnetization analysis. Stability of ferrofluids with time against sedimentation was concluded despite the presence of aggregates in solutions. Differences in the average sizes of individual polydisperse magnetite nanoparticles coated with surfactants and their aggregates were revealed. Structural parameters obtained from SANS have been confirmed by magnetogranulometry data. The comparative analysis of the aggregation and its change upon dilution is made basing on the data of SANS and dynamic light scattering techniques.