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In this paper, we compare the structure and the phase behavior of two kinds of magnetic fluids, also called ferrofluids. They are constituted of the same maghemite particles, the diameters of which lie around 8 nm, dispersed either in water or in cyclohexane. Both systems are constructed to get the same interparticle interactions and differ only through the nature of the repulsion. Repulsion is either electrostatic, due to the charges of citrate molecules adsorbed on the particles surface in water, or steric, due to the alkyl chains of adsorbed surfactants in cyclohexane. Small angle neutron scattering (SANS) experiments show that both systems are highly repulsive and that the structure factors are very similar. This is confirmed by stability measurements: the samples are stable if temperature is decreased and if a magnetic field is applied. If the repulsion is decreased by the addition of electrolyte in water or bad solvent in cyclohexane, a gas–liquid-like transition is observed in both systems. However, the standard electrostatic potential (Derjaguin–Landau–Verwey–Overbeek potential) fails to describe the electrostatic repulsion in the aqueous ferrofluid while the behavior of this system is very similar to the behavior of the sterically stabilized ferrofluid. This underestimate of the electrostatic repulsion is probably due to the finite size effects of the trivalent ions. The striking similarities in the structure and the behavior of both kinds of dispersions, despite their chemical differences, seems to be related to the presence, in both cases, of the adsorbed surface species which ensure the repulsion between particles. Moreover, this repulsion may be described by an effective Yukawa potential very similar in range and intensity in both systems.

We report a protocol that allowed us to fabricate nanoparticle aggregates from anionically coated 7 nm iron oxide nanocrystals and cationic-neutral block copolymers. The control of electrostatics resulted in the elaboration of spherical clusters or of highly persistent nanostructured rods, with lengths between 1 and 50 µm (see figure). The rods were shown to be superparamagnetic

spin-coating, the films were dried at 60 C for several hours. The spincoated films were always of excellent optical quality.GPC measurements were performed on a Waters 600 apparatus using THF as eluent towards polystyrene standards. NMR spectra were recorded on a Bruker 300 Avance. UV-vis spectra were recorded on a Perkin-Elmer Lambda 900, equipped with polarizing optics. AFM measurements were done on a Veeco Nanoscope II with a Si-tip (radius < 10 nm, force constant 2.8 N m ±1 ) in the tapping mode. STM measurements were performed on a Discoverer scanning tunneling microscope (Topometrix) along with an external pulse/function generator (8111A, Hewlett Packard), with negative sample bias. Tips were electrochemically etched from a Pt/Ir wire (80:20, diameter 0.2 mm) in an aqueous 2 N KOH/6 N NaCN solution.

A chemical core-shell strategy is developed here for the synthesis of ferrofluids based on nanoparticles of different ferrites with different mean sizes. A heterogeneity of chemical composition, associated with a superficial enrichment of iron, allows to obtain chemically stable ionic colloids. We propose here a coreshell model to describe the synthesized nanoparticles, which is tested by chemical and magnetic measurements performed at the various steps of the synthesis. The thickness of the superficial layer, rich in iron, is ranging between 0.4 and 1.3 nm, depending on the nanoparticle size and on the underlying ferrite. Its density is found close to that of maghemite, and its magnetization depends on the core ferrite. It is low with a cobalt ferrite core and larger for the three other ferrites investigated here (NiFe 2 O 4 , CuFe 2 O 4 , and ZnFe 2 O 4 ). Magnetic measurements prove that there is a strong redistribution of Zn 2+ ions inside the core of the synthesized nanoparticles based on ZnFe 2 O 4 .

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Small-angle neutron scattering is used to measure the two-dimensional diffraction pattern of a monophasic magnetic colloid, under an applied magnetic field. This dipolar system presents in zero field a fluidlike structure. It is well characterized by an interaction parameter K(0)(T) proportional to the second virial coefficient, which is here positive, expressing a repulsion of characteristic length kappa-10. Under the field a strong anisotropy is observed at the lowest q vectors. The length kappa-10 remains isotropic, but the interaction parameter K(T) becomes anisotropic due to the long-range dipolar interaction. However, the system remains stable, the interaction being repulsive in all directions. Thus we do not observe any chaining of the nanoparticles under magnetic field. On the contrary, the revealed structure of our anisotropic colloid is a lowering of the concentration fluctuations along the field while the fluidlike structure, observed without field, is roughly preserved perpendicularly to the field. It expresses a strong anisotropy of the Brownian motion of the nanoparticles in the solution under applied field.