Thesis submitted 23-03-2012.pdf - University of Limpopo ...

The overall reaction may be written as follows:
Fe 2+ + 2Fe 3+ + 8OH -
Fe3O4 + 4H2O
(black colloidal particles)
According to the thermodynamics of this reaction, a complete precipitation
of Fe3O4 should be expected between pH 9 and 14, while maintaining a
molar ratio of Fe 3+ :Fe 2+ of 2:1 under non-oxidizing, oxygen (O2) free
environment, i.e inert environment. Otherwise, Fe3O4 might also be
oxidized as:
4Fe3O4 + O2 + 18H2O 12Fe (OH)3 (8)
This would critically affect the physical and chemical properties of the
nanosized magnetic nanoparticles. In order to prevent them from possible
oxidation in air as well as from agglomeration, Fe3O4 nanoparticles
produced by equation (1) are usually coated with organic or inorganic
molecules during the precipitation process. To control the reaction kinetics,
and therefore the oxidation speed of iron species, the synthesis of particles
must be done in an oxygen free environment by passing nitrogen gas.
Bubbling nitrogen gas through the solution not only removes critical oxygen
from the Fe3O4 but also reduces the particle size when compared with
methods without removing the O2. 171,172
3.3.6 Surface modifications of magnetic nanoparticles for biomedical
applications and their effect on stability and magnetization
In the preparation and storage of nanoparticles in colloidal form, the
stability of the colloid is of utmost importance. Ferro fluids are colloidal
suspensions of magnetic nanoparticles (Fe3O4 or Fe2O3), forming
magnetisable fluids that remain liquid in the most intense magnetic field
strength. As a result of their composition, the magnetic fluids (MFs)
possess a unique combination of fluidity and the capability to interact with
the magnetic fields. 158,173,174 In the absence of any surface coating, Fe3O4
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