Stability of Cobalt Ferrite Colloidal Particles. Effect of pH and Applied Magnetic Fields
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- 作者：J. de Vicente ; A. V. Delgado ; R. C. Plaza ; J. D. G. Durá ; n ; and F. Gonzá ; lez-Caballero
- 刊名：Langmuir
- 出版年：2000
- 出版时间：October 17, 2000
- 年：2000
- 卷：16
- 期：21
- 页码：7954 - 7961
- 全文大小：155K
- 年卷期：v.16,no.21(October 17, 2000)
- ISSN：1520-5827
文摘
An experimental investigation is described on the stability of cobalt ferrite colloidal spheres, by analyzingthe time variation of the optical absorbance of the suspensions as a function of pH and magnetic fieldstrength. Structural and chemical analysis of the particles suggest that they are composed of a mixedcobalt-iron ferrite and magnetite, with some excess oxygen, probably coming from adsorbed water. Inorder to consider all posible particle-particle interactions that might be responsible for the observedbehavior, the classical DLVO theory was extended to include magnetic dipole attractions. The electricdouble layer of the particles was characterized by electrophoresis, and it was found that the ferrite colloidshave an isoelectric point (pHiep, or pH of zero zeta potential, 
) of 
6.5. This is confirmed by stabilitymeasurements: the absolute value of the initial slope of the absorbance-time curves shows a pronouncedmaximum around pH 7. Concerning the effect of a uniform magnetic field (applied in the direction of thegravitational field), the most significant feature found was that above 1 mT, and for particle concentrationslarger than 0.7 g/L, the suspensions appear more stable the stronger the applied field. Potential energycalculations, while explaining the lower stability of the suspensions around pHiep, show that increasingmagnetic fields decrease indeed the potential barrier between the particles, but not enough to ensureirreversible aggregation. It is hence suggested that the observed stability behavior is due to a long-rangestructuration of the dispersed particles that form long chainlike aggregates extending almost to the wholevolume of the suspension. This may explain that the optical absorbance takes a longer time to decreasein the presence of a magnetic field applied in vertical direction, and also that the final fall in turbidityoccurs at a faster rate than in the absence of the field.
) of 6.5. This is confirmed by stabilitymeasurements: the absolute value of the initial slope of the absorbance-time curves shows a pronouncedmaximum around pH 7. Concerning the effect of a uniform magnetic field (applied in the direction of thegravitational field), the most significant feature found was that above 1 mT, and for particle concentrationslarger than 0.7 g/L, the suspensions appear more stable the stronger the applied field. Potential energycalculations, while explaining the lower stability of the suspensions around pHiep, show that increasingmagnetic fields decrease indeed the potential barrier between the particles, but not enough to ensureirreversible aggregation. It is hence suggested that the observed stability behavior is due to a long-rangestructuration of the dispersed particles that form long chainlike aggregates extending almost to the wholevolume of the suspension. This may explain that the optical absorbance takes a longer time to decreasein the presence of a magnetic field applied in vertical direction, and also that the final fall in turbidityoccurs at a faster rate than in the absence of the field.