NONCONDUCTIVE FERROFLUIDS FROM PERMANENTLY MAGNETIC NANOPLATELETS HYBRIDIZED WITH POLAR PHOSPHONIC LIGANDS

Stable ferrofluids of permanently magnetic nanoplatelets of barium hexaferrite (BHF NPLs) in 1-hexanol were obtained using phosphonic acid- and phosphonate ester-based polar ligands. These ligands with the different electron-withdrawing groups and alkyl chain lengths of the terminal chain and linker were synthesized. Their attachment to the surface of the BHF NPLs was studied for various conditions and followed by a combination of spectroscopic techniques, thermogravimetry, and electrokinetic measurements. The results confirmed the theoretically predicted surface condensation of the ligands onto the BHF NPLs surfaces at 120 °C in 1-hexanol, whereas at lower temperatures or in more polar solvents the ligands were mostly physisorbed. The NPL hybrids with chemisorbed ligands having surface densities of at least 0.4 molecules/nm2 formed stable ferrofluids in 1-hexanol. The hybridization of the BHF NPLs via the condensation reaction in 1-hexanol creates sufficient steric-solvation repulsion to overcome the magnetic dipolar attraction between the NPLs and stabilizes the NPLs in 1-hexanol. Due to the relatively low polarity of 1-hexanol, the ligands remain protonated and the ferrofluids have negligible electric conductivity. In addition, an increase in the saturation magnetization of such hybrid BHF NPLs was correlated with their superior sensitivity to a magnetic field compared to the core NPLs. The ferrofluids can be exploited for the development of novel magneto-optic sensors that can operate under an electric field.