Specific bilayer on the surface of water-based ferrofluids: Structure and particular persistence

The structure of sterically stabilized water-based ferrofluid in the vicinity of free interface with a gaseous atmosphere is studied by means of X-ray reflectometry and grazing incidence X-ray diffraction. Experimental data unambiguously show that in-depth distribution of the magnetic nano-particles and surfactant molecules in the ferrofluid layer close to the interface is essentially non-uniform. Particularly, bulk homogeneous liquid is covered with a relatively thick layer (about 200 A) with enhanced concentration of the surfactant and reduced concentration of the magnetic particles, which is consequently overlayed with a thin layer (about 40 A) with increased concentration of the particles. Having the aim to obtain more homogeneous and controllable ferrofluid surface, we tried to eliminate this specific bilayer in two different ways—mechanically and by means of long-term sedimentation of a sample in a dropping funnel under hexadecane. However, in both case the same surface structure but with slightly d...