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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.

We analyze in this contribution the effect of aging on the electrokinetic properties of magnetite (Fe 3 O 4 ) and hematite (α-Fe 2 O 3 ). In both cases, high-purity commercial samples and monodisperse synthetic particles were studied. Commercial magnetite showed a rather erratic dependence of its electrophoretic mobility u e with the concentration of NaCl. Furthermore, sufficient concentrations of the latter were able to change the sign of the mobility. When KNO 3 solutions were used, although no such change was observed, no clear effect of [KNO 3 ] on the mobility was found, and, in addition, an intense aging effect was detected, as the mobility became increasingly positive in suspensions that were stored over 1 day. The picture was radically different with synthetic magnetite spheres, as the expected overall decrease of u e with either NaCl or KNO 3 concentration was measured. However, also in this case the aging effect was clearly observed: u e tended in this case to more negative values upon suspension storage, and a steady value of the mobility was reached only after 5 days in NaCl (and even longer in KNO 3 solutions). Because of the crystal structure similarities between magnetite and maghemite (γ -Fe 2 O 3 ), it has been shown that the final step of magnetite oxidation is maghemite. This is confirmed in the present study, as the mobility-pH trends of magnetite progressively approach those of maghemite after about 7 days of storage. Since hematite is chemically more stable than magnetite, our study focused in this case on the comparison between commercial and synthetic particles. The former showed a negative mobility at pH 5.5 under all conditions, suggesting an isoelectric point well below the value accepted for hematite (≥7). The effect of aging on commercial samples was again very significant, as u e decreased in absolute value, apparently without limit as the time since preparation was longer. In contrast, synthetic hematite showed a more predictable dependence on ionic strength, and more limited aging effects, as u e reached equilibrium values after around 5 days in NaCl; longer times were required in KNO 3 solutions. C 2002 Elsevier Science

Spherical and quite monodisperse particles (average diameter 60 ( 7 nm) of hematite (R-Fe2O3) were synthesized and then covered with a shell of yttrium oxide of variable thickness. A surface thermodynamic study was carried out for these core/shell colloidal particles, using two experimental techniques: contact angle measurements of selected liquids on glass slides uniformly covered by the material and determination of the penetration rates of liquids through thin layers of the solid. Using van Oss et al.'s model of interfacial interactions (van Oss, C. J. Interfacial Forces in Aqueous Media, Marcel Dekker: New York, 1994), the surface free energy, γS, of the particles was characterized in terms of its two components, Lifshitz-van der Waals, γ S LW , and acid-base, γ S AB . The latter is assumed to be the consequence of both the electrondonor, γ S -, and electron-acceptor, γ S + , characteristics of the solid surface. The efficiency of the coating of the hematite core by yttrium oxide is quantified through the comparison of γ S LW , γ S + , and γ S -values for pure hematite and pure Y2O3, with those of the composite particles. It is found that γ S LW does not depend significantly on the nature of the surface considered, ranging from 46 ( 1 mJ/m 2 for pure hematite to 51 ( 1 mJ/m 2 for pure Y2O3. Both the pure and composite particles show negligible electron-acceptor (γ S + )character. Unlike γ S + , the electron-donor parameter, γ S -, is very sensitive to the surface composition. For composite particles γ S -is closer to that of pure yttria than to hematite's. The shell appears to efficiently hide the interfacial interactions of R-Fe2O3, so that the particles are, in most practical respects, essentially yttria. The implications of this fact on the stability of hematite/yttria suspensions are discussed.