Environmental Engineering Reference
In-Depth Information
7.2.3 Stabilization/Functioning of Magnetic Nanoparticles
Although within iron oxide magnetic nanoparticles synthesis there are significant
advances, maintenance of their stability over a longer time period, without their
agglomeration or precipitation, is a very important issue. Iron oxide magnetic
nanoparticles stability is required for almost any application, so that development
of effective strategies related to improvement of chemical stability is mandatory.
Agglomeration of magnetic nanoparticles is related to van der Waal
s and
'
magnetic forces. Van der Waal
s interaction occurs due to fluctuations of electron
orbitals from one particle, which induces oscillatory dipoles in a neighbor particle.
The simplest and direct method seems to be coating of Fe
3
O
4
magnetic
nanoparticles with an impenetrable cover, so that the oxygen cannot reach the
surface of magnetic particle.
Coating strategies applied to magnetic nanoparticles can be divided into two
main groups:
'
• Cover of magnetic nanoparticles with organic compounds, including surfactants
[
84
-
87
] and polymers [
88
-
91
];
• Coating of magnetic nanoparticles with inorganic compounds, including silica
gel [
92
-
94
], carbon [
95
,
96
], and precious metals (Au [
97
,
98
], Ag [
99
]).
Surfactants should have functional chemical moieties capable of interacting with
hydroxyl groups on the surface of preformed magnetite particles (hydrogen or
covalent bonds) and should be stable within media imposed by application fields.
In case of ferrofluids, the main factors providing their stability are: shape,
particle size, and chemical structure of the coating layer, responsible of compati-
bility with dispersion environment. There are three methods for impairing the
contact between magnetic nanoparticles and reduction of dipole-dipole interaction:
steric stabilization, electrostatic stabilization, and mix stabilization [
100
].
Generally, surfactants or polymers can be chemically anchored of physically
adsorbed on the surface of magnetic nanoparticles, in single layer or double layer,
thus creating repulsive forces in order to balance the van der Waal
s and magnetic
'
forces which are acting on the surface of magnetic nanoparticles.
Most usual functional groups which can bind on the surface of magnetite are
phosphates, sulfates, and carboxylates [
10
]. Carboxyl group of oleic acid
(CH
3
(CH
2
)
7
CH
¼
CH(CH
2
)
7
COOH) is involved in formation of hydrogen bonds
with hydroxyl groups from the surface of magnetite particles, and thus the coating
of particles and their stabilization against agglomeration. Magnetite particles cov-
ered with oleic acid are used for obtaining of magnetic fluids based on hydrocar-
bons [
101
,
102
].
Willis et al. [
103
] showed that degradation of oleic acid during thermal decom-
position, method used for obtaining iron oxide nano-crystallite, will result in
formation of high-quality
-Fe
2
O
3
nano-crystals.
Fauconnier et al. [
104
] investigated adsorption of citric and gluconic acid on
surface of maghemite particles for further use in biomedical application.
γ
