Stability of cobalt ferrite colloidal particles. Effect of pH and applied magnetic fields

An experimental investigation is described on the stability of cobalt ferri te colloidal spheres, by analyzing the time variation of the optical absorb ance of the suspensions as a function of pH and magnetic field strength. St ructural and chemical analysis of the particles suggest that they are compo sed of a mixed cobalt-iran ferrite and magnetite, with some excess oxygen, probably coming from adsorbed water. In order to consider all posible parti cle-particle interactions that might be responsible for the observed behavi or, the classical DLVO theory was extended to include magnetic dipole attra ctions. The electric double layer of the particles was characterized by ele ctrophoresis, and it was found that the ferrite colloids have an isoelectri c point (pH(iep), or pH of zero zeta potential, zeta) of congruent to 6.5. This is confirmed by stability measurements: the absolute value of the init ial slope of the absorbance-time curves shows a pronounced maximum around p H 7. Concerning the effect of a uniform magnetic field (applied in the dire ction of the gravitational field), the most significant feature found was t hat above congruent to 1 mT, and far particle concentrations larger than co ngruent to 0.7 g/L, the suspensions appear more stable the stronger the app lied held. Potential energy calculations, while explaining the lower stabil ity of the suspensions around pH(iep), show that increasing magnetic fields decrease indeed the potential barrier between the particles, but not enoug h to ensure irreversible aggregation. It is hence suggested that the observ ed stability behavior is due to a long-range structuration of the dispersed particles that form long chainlike aggregates extending almost to the whol e volume of the suspension. This may explain that the optical absorbance ta kes a longer time to decrease in the presence of a magnetic field applied i n vertical direction, and also that the final fall in turbidity occurs at a faster rate than in the absence of the field.