pH effect on an ionic ferrofluid: Evidence of a thixotropic magnetic phase

We investigate the stability of aqueous colloidal dispersions of ionic magn etic nanoparticules as a function of pH. The charge of the gamma-Fe2O3 part icles is modified, and as a consequence, the interparticle interactions may be tuned through pH variations. Scanning the whole pH scale from acidic to alkaline medium, different states are observed for the dispersion: sol, th ixotropic gel, and flee (around the point of zero charge). Using the dynami c magnetooptical properties of the magnetic grains, we locally probe the so ls and the thixotropic gels. A steep increase of the characteristic time of birefringence relaxation, in the sol phase close to pH(Gel), marks a diver gence of the macroscopic viscosity at the gel point, compatible with a perc olation theory. A temporal study of the regeneration of shaken gels reveals that, in these thixotropic systems, the process is dominated by a cluster aggregation on a sample-spanning one, with an energy barrier to overcome, c ompatible with chemical measurements.