Colloidal Interactions in Low-polarity Dispersions of Permanently Magnetic Nanoplatelets

Le résumé fourni par la source

Ferrofluids based on permanently magnetic nanoplatelets are attractive functional materials. However, their preparation in low-polarity media is particularly challenging because magnetic dipole–dipole interactions strongly promote agglomeration while conventional electrostatic stabilisation is weakened. Here, low-polarity dispersions of barium hexaferrite nanoplatelets were prepared by functionalising the particles with ricinoleic acid (RA) and dispersing them in low-polarity solvents of different chemical character. The resulting dispersions exhibited markedly different colloidal stabilities, ranging from stable ferrofluids in chlorinated solvents to unstable dispersions in toluene and decalin. Infrared spectroscopy and thermogravimetric analysis confirmed successful functionalisation with RA and indicated the presence of both chemisorbed and loosely bound RA species. Electrokinetic measurements revealed a clear correlation between colloidal stability and the development of a surface charge. Based on these results, we propose that stable dispersions are stabilised by an electro-steric-solvation mechanism that enables spatial stabilisation of charge in low-polarity media. A fraction of the RA is chemisorbed on the nanoplatelet surface, while loosely associated RA species, together with locally ordered solvent molecules, are proposed to stabilise the negative countercharge, thereby maintaining its separation from the positively charged nanoplatelet surface. This charge separation generates a repulsive barrier that counterbalances magnetic dipole–dipole attraction and prevents agglomeration. These findings highlight charge stabilisation in low-polarity media as a key requirement for preparing stable ferrofluids based on permanently magnetic anisotropic nanoparticles.

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