Dispersion characteristics of magnetic particles: Surface charge, surface adsorption and acid-base interaction behavior

Abstract

Electrophoretic mobility (EM) of magnetic oxide particles has been measured in aqueous as well as organic solvent media using a zeta meter and a mass transporter analyzer. EM has also been measured for the magnetic particles after treatment with surface active compounds and/or in the presence of a surfactant. The results show that the sign and magnitude of the EM values change depending on the type and polarity of the surfactants or surface-treating compounds. The sign reversal of the EM values when the medium is changed from water to organic solvent can be understood on the basis of acid-base interaction. The potential energy between particles calculated using the maximum available EM value in an organic solvent is far below the value required for dispersion stability. These results along with the calculation o magnetic potential energy show that the stability of these dispersions cannot be maintained by electrostatic potential alone. Treatment of these oxide particles with surface active compounds provide stable dispersions by a ‘stearic stabilization’ mechanism. Adsorption of surfactants such as organic phosphate compounds and binder resins such as nitrocellulose or polyurcthanes on magnetic oxide particles is due to acid-base interfacial interaction between oxide particles, organic macromolecules, and solvents. Adsorption of these macromolecules in multilayers seems to be a major cause for high dispersion viscosity and the poor magnetic properties of these dispersions on tapes. These results indicate flocculation of the particles, possibly by a bridging mechanism. Thus, surface treatment of oxide particles with proper compounds inhibits adsorption of the binder resins in multilayers, promotes better rheological properties of the formulated grinds, and improves magnetic performance of tapes coated with these grinds.