Abstract
The understanding of stabilization mechanisms for ferrofluids (which are presented as fine dispersions of magnetic nanoparticles coated with surfactants) is an important favorable circumstance in the synthesis of highly stable magnetic colloids with specific properties. The presented work reviews principal results that were obtained in thorough investigations of ferrofluid’s stability regarding changes in the structure at nanoscale under various conditions, including the determination and analysis of the agglomeration regimes in biocompatible ferrofluids for biomedical applications. The structural features of the ferrocolloids and concomitant surfactant solutions were revealed and further analyzed principally relying on the data of Small-Angle Neutron Scattering (SANS). Thereby, for magnetic fluids prepared on the basis of nonpolar liquids (benzene, decalin) with magnetite nanoparticles covered by a single-layer shell of monocarboxylic acids, the studying of the effect of surfactant excess showed a tendency to a significant enhancement of the effective attraction between free (non-adsorbed) acid molecules. This explains the sharp and sudden loss of a ferrofluid’s stability that occurs because of the liquid crystal transition when exceeding some critical concentration of an acid. This transition depends strongly on an interparticle solvent-acid interaction and peculiarity of the different critical concentrations is for different solvents. For an aqueous ferrofluid (nanomagnetite stabilized with a double-layered shell of sodium oleate (SO)) that is used as a precursor for a biocompatible modification with polyethylene glycol (PEG), the fraction of micelles of non-adsorbed surfactant and its change under modification were found by SANS. The comparison with another kinds of water-based ferrocolloids showed the different rate of surfactant adsorption on magnetite particles surface depending on the surfactant type. The aggregate reorganization and its growth in the ferrofluid after ‘PEGylation’ were observed. In order to illuminate the possible influence of the micelle formation with free surfactants on this process in the presence of polymer, the SANS study was performed on mixed SO/PEG aqueous solutions. SANS results revealed drastic morphological and interacting changes of micelles due to addition of PEG. In particular, it was concluded the screening of the micelle interaction due to the formation of an effective PEG shell around micelles at high (about 10 vol%) concentration of the polymer.
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