Use of nanoparticle concentration and magnetic fields to control the structures of superparamagnetic Fe3O4 nanoparticle Langmuir films

We investigated how structures formed by hydrophobic superparamagnetic Fe₃O₄ nanoparticles (NPs) at air-water interfaces could be changed, in order to achieve films with the desired structure. NP films were prepared on water using the Langmuir trough, and deposited onto glass substrates. The effect of the NP spreading solution concentration (C_NP) and the magnetic field strength on the NP aggregation were determined from surface pressure-area/particle isotherms and Atomic Force Microscope images. In the absence of a magnetic field, the NPs aggregated to form large and heterogeneous domains. The size and non-homogeneity of the domains increased as C_NP increased, a result explained by increasing attractive hydrophobic interactions. A magnetic field decreased the size and non-homogeneity of these domains, a result explained by a directional attractive magnetic force. The NP film structure depended on C_NP and the magnetic field strength. Structural changes by the magnetic field became more visible as C_NP was increased.