Stability of dispersions of colloidal nickel ferrite spheres

The stability of suspensions of nickel ferrite spheres is investigated for different compositions of the dispersion medium, in the absence and in the presence of an external magnetic field. The time-dependence of the optical absorbance of the suspensions is the quantity used for experimentally determining the stability. Two approaches have been used for the calculation of the interaction energy of the particles: first, the classical DLVO theory, in which the net interaction is considered as the superposition of electrostatic double-layer repulsion and van der Waals attraction; and second, the so-called extended-DLVO model, in which short-range hydrophobic (hydrophilic) attractions (repulsions) are also considered. Calculation of all these interactions required the determination of (i) the diffuse double-layer potential of the ferrite particles (that was approximated by the zeta potential, as deduced from electrophoresis measurements); (ii) the Hamaker constant of the particles in aqueous media; and (iii) the acid/base components of the surface free energy of the solids. The quantities in (ii) and (iii) above were obtained from contact-angle measurements for selected liquids on layers of the magnetic oxide. When a magnetic field is applied, another interaction, of magnetic origin, has to be accounted for. It was found that the suspensions are more stable the farther their pH is from the isoelectric point, and the lower the ionic strength of the medium is, in full agreement with the predictions of both the classical and extended-DLVO models. The application of the external magnetic field was found to provoke significant changes in the rate of variation of absorbance with time: The results are coherent with an increased velocity of particle aggregation due to magnetic attractions between the magnetized colloids. The DLVO theory (in any of its versions) including magnetic interactions can explain this behavior, since the calculated force between particles is more attractive when the field is present. (C) 2001 Academic Press.