Particle correlations in concentrated aqueous ferrofluids upon dilution by small-angle X-ray scattering

Small-angle X-ray scattering is applied to study particle correlations in concentrated (up to 7.5 vol% magnetite) aqueous magnetic fluids with double-layer coating by either oleic or lauric acids. The structure of aggregates in these fluids is analyzed when diluting samples with the goal of finding out a concentration threshold at which the aggregates can be considered weakly interacting independent formations. For dilute systems, suitable models fitting experimental scattering curves are proposed, which take into account the polydispersity of both particles and aggregates. It is shown that particle correlations found in aggregates are dependent on the surfactant coating.