Stability of Cobalt Ferrite Colloidal Particles. Effect of pH and Applied Magnetic Fields

An experimental investigation is described on the stability of cobalt ferrite colloidal spheres, by analyzing the time variation of the optical absorbance of the suspensions as a function of pH and magnetic field strength. Structural and chemical analysis of the particles suggest that they are composed of a mixed cobalt−iron ferrite and magnetite, with some excess oxygen, probably coming from adsorbed water. In order to consider all posible particle−particle interactions that might be responsible for the observed behavior, the classical DLVO theory was extended to include magnetic dipole attractions. The electric double layer of the particles was characterized by electrophoresis, and it was found that the ferrite colloids have an isoelectric point (pHiep, or pH of zero zeta potential, ζ) of ≅6.5. This is confirmed by stability measurements: the absolute value of the initial slope of the absorbance−time curves shows a pronounced maximum around pH 7. Concerning the effect of a uniform magnetic field (applied in the direction of the gravitational field), the most significant feature found was that above ≅1 mT, and for particle concentrations larger than ≅0.7 g/L, the suspensions appear more stable the stronger the applied field. Potential energy calculations, while explaining the lower stability of the suspensions around pHiep, show that increasing magnetic fields decrease indeed the potential barrier between the particles, but not enough to ensure irreversible aggregation. It is hence suggested that the observed stability behavior is due to a long-range structuration of the dispersed particles that form long chainlike aggregates extending almost to the whole volume of the suspension. This may explain that the optical absorbance takes a longer time to decrease in the presence of a magnetic field applied in vertical direction, and also that the final fall in turbidity occurs at a faster rate than in the absence of the field.