Electrostatic Interactions between Barium Hexaferrite Nanoplatelets in Alcohol Suspensions

In a room-temperature liquid magnet, barium hexaferrite (BHF) nanoplatelets suspended in 1-butanol spontaneously order and form a ferromagnetic nematic phase. In such concentrated suspension, the nanoplatelets align in large macroscopic regions, forming magnetic domains. The key parameter for the suspension stability and the formation of the ferromagnetic nematic phase is electrostatic interaction, which can be influenced by the solvent and the concentration of surfactant, i.e., dodecylbenzenesulfonic acid (DBSA). In this study, we investigated electrostatic interactions of the DBSA-functionalized nanoplatelets' suspensions in different alcohols. We prepared suspensions in tert-butanol, 1-hexanol, 1-butanol, and 2-propanol and measured conductivity, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and electrophoretic mobility. SAXS results and electrophoretic mobility measurements confirmed the colloidal stability of the suspensions, which was not affected by the variation in concentration of added DBSA of the order of 1.3 mM. We showed that the dielectric constant of the solvent affects the surface charge, the strength of the electrostatic repulsion between the nanoplatelets, and the Debye screening length. The balance between the magnetic dipolar attraction and the electrostatic repulsion was proven to be essential for the ferromagnetic nematic phase formation.