Self organization in oleic acid-coated CoFe2O4 colloids: a SAXS study
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We report a structural study of magnetic colloids composed of CoFe2O4 nanoparticles (mean radii in the range 2–7 nm) synthesized by thermal decomposition of different high boiling temperature organic solvents in the presence of oleic acid and oleylamine, and subsequently re-suspended in hexane. Although the surfactant layer prevents permanent aggregation and precipitation of the disperse phase, competition between attractive interactions (i.e., dipolar and van der Waals) and repulsive steric interaction leads to self organization of the magnetic nanoparticles. Our small angle X-ray scattering results evidence the presence of distinctive self organized structures in the liquid colloid depending on the type of solvent used in the synthesis. A completely homogeneous dispersion is obtained for those colloids synthesized with benzyl-ether and octadecene. Bi-disperse systems, in which nanoclusters coexist with free nanoparticles, appear when phenyl-ether and trioctylamine are used. Chain-like structures are observed in a colloid containing the particles synthesized using phenyl-ether, while more compact 3D structures form in colloids prepared with particles synthesized with trioctylamine. The presented results have important implications in the design and selection of magnetic nanoparticles for those applications where the size dispersion determines the final efficiency of the material, such as magnetic fluid hyperthermia clinical therapy.
KeywordsMagnetic colloids Aggregation Dipolar interaction London–van der Waals interaction Small angle X-ray scattering
We thank financial support from: LNLS synchrotron, Campinas, SP, Brazil under proposals D11A-SAXS1-9293, CONICET (PIP 01111) and ANPCyT (PICT 00898) of Argentina and the Spanish Ministerio de Ciencia e Innovación (projects MAT2010-19326 and CONSOLIDER NANOBIOMED CS-27 2006). Valuable help of Dr. A. Ibarra on TEM analysis and Dr. Aldo Craievich on SAXS data analysis is deeply acknowledged.
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