In-situ investigation of temperature induced agglomeration in non-polar magnetic nanoparticle dispersions by small angle X-ray scattering

Non-polar magnetic nanoparticles agglomerate upon cooling. This process is followed by in-situ small angle X-ray scattering to assess structural properties of the emerging agglomerates. On the length scale of a few particle diameters, no differences are found between the agglomerates of small (d = 12nm) and large (d = 22nm) nanoparticles. Hard-sphere like random packing with a local packing fraction of $\eta$ = 0.4 is seen. On larger length scales, small particle form compact superstructures, while large particles arrange into agglomerates that resemble chain-like structure in SAXS. This can be explained by directed magnetic dipole interactions that dominate larger particles, while isotropic van der Waals interaction governs the agglomeration of smaller particles.