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 s ame 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 r epulsion. 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 s cattering (SANS) experiments show that both systems are highly repulsive an d that the structure factors are very similar. This is confirmed by stabili ty 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 tr ansition is observed in both systems. However, the standard electrostatic p otential (Derjaguin-Landau-Verwey-Overbeek potential) fails to describe the electrostatic repulsion in the aqueous ferrofluid while the behavior of th is system is very similar to the behavior of the sterically stabilized ferr ofluid. This underestimate of the electrostatic repulsion is probably due t o the finite size effects of the trivalent ions. The striking similarities in the structure and the behavior of both kinds of dispersions, despite the ir chemical differences, seems to be related to the presence, in both cases , of the adsorbed surface species which ensure the repulsion between partic les. Moreover, this repulsion may be described by an effective Yukawa poten tial very similar in range and intensity in both systems. (C) 1999 American Institute of Physics. [S0021-9606(99)51336-6].