Nonconductive ferrofluids from permanently magnetic nanoplatelets hybridized with polar phosphonic ligands

Ferrofluids composed of permanently magnetic nanoplatelets of barium hexaferrite (BHF NPLs) represent a remarkable class of materials that can form a ferromagnetic liquid state at sufficiently high concentrations. To date, the surfactant dodecylbenzenesulfonic acid (DBSA) has demonstrated an excellent colloidal stability of the BHF NPLs in alcohols. However, a key limitation of DBSA is its labile adsorption onto the NPLs, meaning that the surface coverage by surfactant molecules is highly sensitive to various factors. In this study, we demonstrate that polar ligands based on phosphonic acids and phosphonate esters offer a viable alternative to DBSA, providing enhanced robustness and colloidal stability in low-polar solvents. Phosphonic 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. Due to the relatively low polarity of 1-hexanol, the ligands remain protonated and the ferrofluids have negligible electric conductivity and are suitable for the development of novel magneto-optic sensors that can operate under an electric field.