Dispersions of magnetic nanoparticles in dense ionic fluids - Influence of water and of the solid/liquid interface on the colloidal and transport properties.

Abstract

Magnetic nanoparticles (NPs) dispersed in dense ionic fluids represent promising stimulus-responsive materials with applications in emerging thermoelectric technologies. This study takes advantage of the additive-free surface of NPs produced by the Massart method, which allows modification of the solid/liquid interface to transfer the NPs into dense ionic fluids. The significant role of residual water is also analyzed. We investigate here (γ-Fe2O3) and core@shell ferrite@maghemite NPs dispersed in two media: the deep eutectic solvent choline chloride–urea 1:2 (Reline-ChU) and the ionic liquid 1-ethyl-3-methylimidazolium bistriflimide (EMIM-TFSI). Structure and transport properties are analyzed using a combination of Small-Angle X-ray Scattering, Dynamic Light Scattering (DLS) and Forced Rayleigh Scattering (FRS), where applicable. Exploring the influence of particle size reveals phase separation for the largest NPs. With nanoparticles typically 9 nm in diameter, the interparticle interactions can be tuned through the combined effects of the surface coating, counterions, and solvent, whereas the nature of the nanoparticle core has only a limited influence. The impact of water is studied using a combination of direct (Karl Fischer titration, KF) and indirect (SAXS, DLS, FRS) techniques on the final dispersions or after water addition. In Reline-Chu, which is miscible with water but degrades above 353 K, adding 5 wt% water either increases or decreases repulsion between NPs depending on the nature of the NP/solvent interface. In EMIM TFSI, which exhibits limited water miscibility, KF titrations enable quantification of the residual water and its localization between the bulk and the interface. Temperature-dependent FRS measurements enable the determination of an activation energy (Ea), related to the surface hydrophilicity, and confirm the direct titrations: Ea is close to the value for water in core@shell particles after preparation and shifts to the value of the ionic liquid after heating, indicating water displacement. A systematic control of NP interfacial chemistry, particularly residual water, is crucial and enables tunable colloidal stability and transport properties essential for advanced applications in ionic fluid systems.