Liquid-gas transitions in charged colloidal dispersions:: Small-angle neutron scattering coupled with phase diagrams of magnetic fluids

We study here the phase behavior of colloidal dispersions, which are constituted of almost monodisperse gamma-Fe2O3 magnetic nanoparticles dispersed in water at pH 7. Those electrostatically stabilized dispersions present, if the interparticle repulsion is decreased, gas-liquid transitions, a phase behavior scarcely reported for such charged nanoparticles. Small-angle neutron scattering experiments prove that there is no chaining of the particles and that, from the point of view of interparticle interactions, increasing ionic strength (through addition of NaCl) is equivalent to decreasing temperature. A phenomenological expression of the osmotic pressure pi is determined and shown to be in agreement with direct pi measurements. The scattering experiments associated to optical microscopy determinations of phase diagrams lead to a general pi-Phi diagram. Gas-solid transitions may exist even if their observation is disturbed by metastability phenomena. The results are in qualitative agreement with theoretical predictions on charged colloids and in agreement with theoretical works on weak dipolar systems.