Structural analogy between aqueous and oily magnetic fluids

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. (C) 1999 American Institute of Physics. [S0021-9606(99)51336-6].