Nanoparticle chain-like formation in electrical double-layered magnetic fluids evidenced by small-angle X-ray scattering

Small-angle X-ray scattering (SAXS) was performed on a series of Electric D ouble-Layered Magnetic Fluids (EDL-MF) composed of ferrite type-CoFe2O4, Mn Fe2O4, ZnFe2O4, NiFe2O4 and CuFe2O4-nanoparticles of different crystalline sizes (D-XR ranging from 40 to 139 Angstrom, as determined by X-ray diffrac tion), The information concerning the scattering objects uas obtained throu gh the analysis of the distance distribution function p(r) and of the size distribution function D(R), both retrieved from SAXS data. The results show that EDL-MF, in the absence of an applied magnetic field, are composed of small magnetic particle aggregates in solution. These agglomerates are elon gated in one direction (chain-like) with the longest dimension varying from 240 to 330 Angstrom. The cross-section size is of the order of D-XR The da ta also demonstrate that the maximum dimension of these aggregates is indep endent of the ferrite ta pe. On the other hand, the number of aggregated ma gnetic particles is nanopatrticle-size-dependent. Accordingly, larger ferri te-type nanoparticles as those with D-XR = 139 Angstrom form aggregates com posed of 2-3 magnetic particles, whereas smaller ones with D-XR congruent t o 40 Angstrom form agglomerates of about 6 magnetic particles in solution. As the nanoparticle size is reduced, it might increase the particle surface defects. Such occurrence would affect the particle surface charge density, which could reduce the electrostatic screening, favoring the agglomeration phenomenon.