Environmental Engineering Reference
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Fig. 6.5 Zeta-potentials of APTES-silanized magnetic particles from water before and after
acid and base leaching
formed in toluene were more stable with an IEP shift from pH 9.0 to 8.5 as
compared to that from pH 8.5 to 5.5 for the particles silanized in water. This
may be related to the lower degree of protonation of amine groups in the films
formed from toluene. The protonated amine, attached to the surface through
electrostatic attraction, may be detached in acid solution as amine molecules are
more soluble at low pH.
Clearly, the direct silanation of APTES from water and toluene solutions on
g-Fe 2 O 3 is not suitable for engineering magnetic composite particles for envir-
onmental and biological applications. Since silica is more stable in acidic solu-
tions than iron oxide, an alternative approach is to coat the magnetic particles
with a thin silica film first, followed by conventional silanation on silica, to
improve the film stability and density of surface functionality.
6.4.3 A Novel Two-Step Silica Coating
The challenge for the applications of magnetic nanocomposites is to improve the
stability of magnetic particles against coagulation and leaching in an acidic
environment, withmaximized magnetization in an external magnetic field. Ultra-
thin silica films coated on nanosizemagnetic particles are of special interest due to
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